Duplication models

Duplication models. Preparing for duplication. Farm supply.

1.  What is the purpose of duplication of models ?

2. Name the masses that  conduct duplication?

3. Technology of duplication of models.

4. Name the heat-resistant masses, their physical and chemical properties.

5. Name the methods of casting metallic framework of partial removable dentures.

6. Preparation of model to duplication.

7. Duplication of model.

8. Name the full-duplex masses, their physical and chemical properties.

9. Preparation of model with waxen framework of partial removable denture to casting.

10. Fitting of framework.

11. Methods of indemnification of contraction of metal.

12. Enumerate the clinical-laboratry steps that must be conducted at making of partial removable denture, beginning from the fitting of metallic framework.

 

1. For the receipt of heat-resistant model.

2. Hydrocolloid masses that under a temperature 85-90 C are melted on  “water bath”. It is the masses: H.P.Sosnin, О.І.Kruglyakova, mass of “Helin”.

3. Duplication of models is made in the special cuvette. On the bottom of cuvette we lay plasticine and strengthen a model for duplication, through the opening in a cuvette we poad warmed-up dublicate mass. After the mass becomes hard we open a cuvette  and take out a gypseous model, and into its place we poar heat-resistant mass. After consolidation  we should eliminate it and lay for 10 sec. in a boiling wax at a temperature 150 C.

4. We caame such heat-resistant masses: mass of “Bugelit”, “Selamin”, “Crystasyl”, mass of B.P.Panchoha, P.S.Flis, Marshalit, Quartz sand, Concentrated solution of etylselicsit, catalyst of dietanol 1% or 2%  acetone solution.

5. On a heat-resistant model, without heat-resistant models;

6. To fill next models with a warmed-up wax :

– to lay lining on a model from thin tape of wax or plaster in  the zone of saddle and arc;

– to inflict a ledge for retentional part of clasp of supporting tooth;

7.It is conducted in a cuvette for duplication. On the bottom of cuvette by plasticine a model registers for duplication, through the opening in a cuvette we poar full-duplex mass (Helin). After the mass becomes hard we open a cuvette  and take out a gypseous model, and into its place we poar heat-resistant mass. After consolidation we should eliminate it and lay for 10 sec. in a boiling wax at a temperature 150 C.

8. The full-duplex masses: is mass of H.P.Sosnin, 1961р;

it is mass of О.I.Kruhliakova, 1994р.

it is mass of  Telin, Leningrad plant of prosthetic materials.

All indicated masses are hydrocolloid. They are melted on water bath at – 85-90 С:

9. After a design on a heat-resistant model of waxen framework of  partial removable denture a heat-resistant cover (shirt) is formed . For this aim are used such materials of “Crystosyl”, “Silamin”. Powder is pulverized in a porcelain mortar and mixed with tetraetylselicate. Thus we get varnish  consistencies mass that inflict with a brush on a model of casting. Then we sprinkle it  with powder (this mass), after drying out inflict varnish (polish) mass. We do it 3-4 times. Preliminary we  connect  casting details to the construction. Casting is conducted in a muffle stove for the smelted models:

10. Fitting of construction of metallic partial removable denture is conducted at first on a model, and then in the mouth’ cavity.

11. On a heat-resistant model:

– by means of collapsible models;

– by means of compensative varnishes;

– by means of weak-willed selfhardening plastic.

12. Determination and fixing of central occlusion by means of waxen bite rollers:

raising of teeth;

verification of construction to prosthetic appliance;

gypsum of models in a cuvette;

filling of plastic;

pressing:

polymerization of plastic;

treatment of denture

polishing

fitting;

putting in operation (finish)

correction.

Agar has been used for a number of years in the preparation of elastic duplicating materials. These duplicating materials have been used mainly in partial denture prosthesis for the preparation of a duplicate refectory cast of the original stone cast of the patient’s mouth. Since moderately large quantities of duplicating materials are used in these procedures, the low cost and reversibility of the agar materials are a definite advantage.

The mercaptan and silicone rubber elastic impression materials have not been used extensively in duplicating procedures because of their high cost and the fact that these materials, once set, are irreversible. Recently, a plastic duplicating material has been developed which is a reversible polyvinyl chloride plastic gel. Because of its chemical and physical nature, this material has elastic properties somewhat different from the agar compounds.

The agar duplicating compounds are composed primarily of agar and water but may also contain borax to increase the strength, salts to accelerate the setting of the investment, and other ingredients. Cellulose fibers, waxes, and fillers, which may be found in impression compounds, generally are omitted from duplicating compounds, since the duplicating materials are often stored in the sol state for extended periods and these ingredients tend to separate under this condition of storage. Since the duplicating compounds may be stored at temperatures of 130°-150° F., their resistance to thermal degradation is an important property.

The basic composition of the agar impression compounds is much the same as for the agar duplicating compounds. The principal difference is that the impression compounds may contain 10-15 per cent agar, while the duplicating materials may contain only 5 per cent agar and, as just indicated, need not contain waxes, fillers, and other modifiers common to impression materials. In fact, agar impression materials have been used as duplicating compounds by simply diluting the impression material with an equal amount of water.

The properties of the agar impression compounds have been studied by numerous investigators. Skinner and Kern’ determined the linear-dimensional changes in air and subsequently in water of various agar impression compounds. They also measured the compressive properties of proportional limit, maximum flexibility, and crushing strength, as well as the plastic flow properties of these agar materials. Paffenbarger2 reported the compressive properties of strength, deformation, set, and stiffness of a number of agar compounds. The temperature of gel formation and the impression- taking qualities, as measured by the present A.D.A. master model for agar hydrocolloid materials, were also determined. Luster3 reported the effect of environmental factors of air, water, and inorganic sulfate solutions on the dimensional stability of agar gels. Phillips and co-workers4 also measured the dimensional changes of agar gels under various conditions.

The properties of elastic duplicating materials have received little attention, with only limited information available in the literature. Margetis and Hansen,5 however, recently studied the changes in agar duplicating materials on heating and storage by measuring changes in the pH and viscosity of the sols and the crushing times of the gels. These values indicated a gradual degradation of the agar material over a 32-day period when stored at 52° C. (125.6° F.).

DUPLICATION AND IMPRESSION MATERIALS STUDIED

Code Letter Product Supplier

Duplicating Compounds

A….. Copymaster Duplicating Material L. D. Caulk Co.

B….. Lastic Durallium Products Corp.

C….. Nobiloid Nobilium Products, Inc.

D …Su.r.gident Duplicating Compound Surgident, Ltd.

E….. Ticonium Special Dental Perfection Co.

F….. “Van R” Laboratory Duplicating Material “Van R” Dental Products

Impression Compounds

The present investigation represents a portion of a study being conducted on elastic duplicating materials and consists mainly of an initial comparison of the properties of duplicating materials with agar impression compounds. The physical properties of compressive strength, tear strength, stress-strain properties, per cent set, crushing time, impression qualities, and gelation temperature were measured. In addition, the properties of a number of experimental agar gels were determined, in order to examine the effect of the agar source and composition on physical properties.

Other phases of this study, to be reported at a later date, are the accuracy of duplicating materials, the effect of aging on duplicating compounds, and the visco-elastic behavior of these materials.

MATERIALS AND METHODS

The commercial duplicating materials investigated and the suppliers are listed in Table 1, with a code letter for each material. The materials are referred to by code letter because this study is an evaluation of the general properties of elastic duplieating materials, and a high or low value for any particular property does not imply that the material is a superior or inferior product. In fact, it is known that each of the materials is used routinely and gives acceptable results. All the duplicating compounds are hydrocolloid materials except material B, which is a reversible plastic gel.

Three hydrocolloid elastic impression materials were selected from the large number commercially available on the basis of general differences in physical properties. These products also are listed in Table 1 with a code letter.

Experimental agar gels were prepared from four different commercially available dental-grade agars* used in these investigations. Samples from different batches of these raw agars were obtained and the physical properties determined, in order to observe differences as a result of the source of supply. A dental-grade agar was also used to prepare experimental gels. Five lots of this type of agart taken over an 18- month period were examined.

Specimens used for the determination of compressive strength, stress-strain curves, per cent set, and crushing time were cylinders 3/4 inch high by ‘2 inch in diameter. These cylindrical specimens of agar impression or duplication compounds were prepared by liquefying them in boiling water and using specimen molds as described in A.D.A.for hydrocolloid impression material, section 4.4.5.1.6 The duplicating compounds were poured when the temperature was between 1400 and 150° F. The impression compounds were tempered according to the manufacturer’s directions.

Duplicating material B was poured at the storage temperature of 210° F. After pouring, the specimens were stored in a humidor at 100 per cent relative humidity for

1 hour until tested.

The tear specimens used had the shape of the ASTM tear-test die C (ASTM Test D624-54).7 The tear test has been used to evaluate rubber materials. The shape and dimensions of the specimens are indicated by the sketch in Figure 1. The mold for the tear specimens was prepared from a casting plastic, and the top and bottom of the mold were formed by glass plates. The specimens were prepared and stored as indicated for the compression specimens.

The dry strip agar was cut into small pieces, and the powdered forms were used asreceived. 10 gm. of agar plus 190 gm. of water were placed in half-pint mason jars. After the agar was thoroughly wetted, the sealed jar was placed in an autoclave for 15 minutes at 2650 F. After this time, the pressure was released and the agar sols allowed to cool to room temperature.

The compressive strength of the various materials was determined on the cylindrical specimens. The load required to rupture the specimen was determined, using an Instron Testing Machine. The rate of deformation during testing was varied from 1 to 20 inches/minute. It was observed that a maximum compressive strength resulted at a deformation rate of approximately 10 inches/minute, for which reason the majority of the testing was carried out at this deformation rate.

The stress-strain curve may also be obtained from the load and deformation data. The stress-strain curves were plotted from the data obtained at a deformation rate of 10 inches/minute. These curves permitted the determination of the stiffness, proportional limit, and per cent compression of the agar materials.

The tear strength of elastic duplicating and impression materials is a most important property because these materials must have sufficient strength that they may be removed from undercut areas and interproximal spaces without tearing. The load required to continue tearing the specimen was determined by using the Instron Tester. The load divided by the thickness of the specimen in inches was a measure of the tear strength and is reported in pounds per inch of notch or, more simply, pounds per inch. The per cent set after 12 per cent strain was determined on an instrument developed by Cresson.8 The instrument had a dial gauge sensitive to 0.001 inch, was mounted on a steady stand, and was equipped with a screw to allow a cylindrical specimen to be compressed the desired 12 per cent. The sample was compressed 12 per cent and the pressure maintained 1 minute, after which the sample was allowed to rest with no pressure for 1 minute, and the specimen height was measured. Knowing the original and final specimen length, the per cent set may be calculated.

A crushing-time test similar to that proposed by Margetis and Hansen5 was used. The modified compression-testing apparatus described by Paffenbarger2 was convenient for performing the crushing-time test. The cylindrical specimens were loaded with weights varying from 1,100 to 3,600 gm., and the time from the load application to the rupture of the specimen was determined in seconds. Variation in the load was necessary, since no single load could be found which would give reasonable crushing times for agar impression, duplication, and experimental materials.

The A.D.A. master model for testing hydrocolloid impression materials6 was used to give a general evaluation of the impression qualities of the duplicating materials. This master model has five posts, two of which are cylindrical, and the other three have varying degrees of undercut. A snap removal of the master model was carried out, and improved stone was used to prepare the duplicate model.

The gelation temperatures were measured according to the directions of the A.D.A. specification No. 11 for hydrocolloid impression materials.6 The test consists of inserting a metal tube into the impression material at various times as the material cools from a sol to a gel. The gel temperature is that temperature at which a clear impression of the inside and outside of the metal tube is obtained and none of the material clings to the walls of the tube. Stress-strain properties.-The stress-strain curves were determined for all the duplicating and impression compounds listed. The values for the initial slope of the straight-line portion of the curves and the proportional limit as indicated by flattening of the curve prior to fracture are listed in In addition, the ultimate per cent compressions also are reported in this table. The ultimate compressive strength also may be obtained from these curves, and this property will be discussed in the following section. Examples of the various types of stress-strain curves obtained are presented in for materials B, D, E, and I. Material (lb/in2) (lb/in2) (percent) The deformation rate was 10 inches/minute, with the result that the complete stress-strain curve for any of the samples was obtained in less than 3 seconds. On the basis of unpublished flow studies, the curves represented primarily the elastic behavior of the materials, and any viscous effect was neglected. Material I was an agar impression compound and, like the other impression compounds G and H, was stiffer and had a higher proportional limit than the agar duplicating compounds. These results would be expected, since the agar duplicating compounds are more dilute than the agar impression materials. Agar duplicating compounds A, D, and F were similar in their compressive properties, while compounds C and E had lower compressive characteristics.

This again would be anticipated because of the lower agar concentrations used in materials C and E. Material B was a plastic gel duplicating material and had compressive properties similar to agar duplicating compounds at low compression values. This material had considerably higher strength and ultimate compression values than did the agar types. It is important to point out that these high values are not required of a duplicating material. It may, however, affect the length of time over which the duplicating compound may be used before the physical properties deteriorate to such an extent that the material must be discarded.

The ultimate per cent compression of the agar duplicating compounds was generally lower than for the impression compounds, but no correlation existed between these values and the other compressive properties.

The shape of the stress-strain curves was different from those reported by Paffenbarger.

2 The initial portion of the curves in Figure 2 were straight lines, but between a strain of 0.1 and 0.2 inch/inch the curves began to rise. After this rise they flattened out and finally became discontinuous when the specimens ruptured. This shape may Strain, in/in be explained by the fact that the specimen cross-sectional area is increasing during the test or that the stiffness of the material increases with the deformation. The stressstrain curves reported by Paffenbarger2 were usually concave toward the strain axis. The differences in the results of the two studies may be reconciled by noting that the latter stress-strain curves were obtained by loading the specimens at the rate of 2.84 lb/in2/minute, and therefore, the tests required 10-17 minutes to complete. Considerably more viscous deformation could occur in this period of time compared to the 3 seconds or less required in the present study. Any flow taking place would result in the stress-strain curve being concave toward the strain axis. The curves shown, therefore, more nearly represent the behavior of the agar materials in service, since the impression is removed rapidly rather than slowly. Compressive strength.-The compressive strengths of the duplicating compounds are, those for the agar impression compounds, and those for the experimental agar gels. The compressive strengths, measured at a lbs/in2 8 deformation rate of 10 inches/minute, of the duplicating compounds generally ranged between 50 and 65 lb/square inch. Two exceptions to this generalization were compounds B and E. Compound B is a plastic gel and would be expected to have the much higher compressive strength of 130 lb/square inch, and agar compound E is diluted more than the other agar compounds and would be expected to have the lower strength of 37 lb/square inch.

The compressive strength of the agar impression compounds G, H, and I fall between 78 and 114 lb/square inch. These values are higher than for the duplicating compounds, since the agar concentration is two to three times higher than for the duplicating compounds.

The compressive strengths of experimental agar gels listed in Table 3 show considerable variation. The Portuguese, Spanish, and Japanese agar gels showed more variation between batches than did the American agar gels. The Spanish and Portuguese

PHYSICAL PROPERTIES OF 5 PER CENT COMMERCIAL AGAR GELS

Compres- Crushing Gel

Agar Batch sive Time Per Cent Tempera-

Strength (Seconds at Set ture,

(lb/in2) 870 gm/cm2) (°C.)

F12 3388 2166 21..46 36

American……… 3 40 19 2.6 37

4 32 13 2.5 36 t5 36 13 2.4 36

Portuguese …….. ft 60 359 1.5 36 12 52 134 1.6 36

Spanish f……….. f 70 337 1.6 36

)~2 53 53 2.3 36

Japanese..ft.27 7 1.7

21.6 1 3.9

agar gels were stronger than the Japanese and American, having compressive strengths from 52 to 70 lb/square inch. The American agar gels were intermediate in compressive strength, with the values from 32 to 40 lb/square inch, while the Japanese agar gels were the weakest, having values between 16 and 27 lb/square inch. The experimental agar gels contained approximately the same concentration of agar as did duplicating compound E.

Of principal interest is the stability of the properties of the American agar gels prepared from various batches of agar. The variability of the properties of agar from batch to batch has been recognized for a long time as being one of the main difficulties in the preparation of a consistent agar impression or duplicating compound. This fact is substantiated by the variation in the properties of the Portuguese, Spanish, and Japanese agar.

The reason for the choice of a deformation rate of 10 inches/minute may be seen by examination. The solid curves represent the relationship of the compressive strength to the deformation rate for three different dilutions of duplicating  compound A. The same types of curves have been obtained for the other agar duplicating and impression materials, although the maximum is not so pronounced in the case of the impression compounds. The dotted line indicates the per cent compression at fracture of the agar gels prepared by diluting material A with 0.5, 1, or 2 parts of water. All the values fell on a single curve, which showed that within this dilution range the percent compression was independent of the water to material ratio.

The curves for the compressive strength of agar gels versus deformation rate reached a maximum between 10 and 12 inches/minute. If the gels were deformed at low rates, such as 2 inches/minute, or at high rates, such as 20 inches/minute, considerably lower values for compressive strengths were obtained. A maximum was not observed for the per cent compression versus deformation rate, but the values increased between 2 and 10 inches/minute, after which a stable value was maintained. The deformation rate of 10 inches/minute was therefore selected because a maximum value was obtained at this rate and because the materials as used would be subjected to compression forces at approximately this rate.

The significance of selecting the proper deformation rate for evaluating compressive strength may be seen by comparing the values reported by Paffenbarger2 for agar impression materials with those listed in Figure 4. The values given by Paffenbarger2 ranged from 16 to 40 lb/square inch, while those reported in this study were between 78 and 114 lb/square inch. The former values are possibly lower because the deformation rate was much slower.

Tear strength.-Since one visual criterion for discarding a duplicating material is the tendency of the material to tear in the region of the interproximal spaces, the tear strength of duplicating compounds is a most important property. The tear specimens were tested in tension at a rate of 10 inches/minute, and the values obtained for the duplicating and impression compounds.

The order of the tear strengths of the duplicating and impression compounds is essentially the same as for their compressive strengths. The tear-strength test, however, appears to be somewhat more sensitive to differences in materials than the compressive- strength test. The tear strengths of agar duplicating compounds A, D, and F were between 2 and 3 lb/inch, while compounds C and E had lower values of about 1 lb/inch, and the plastic duplicating material had a higher value of over 6 lb/inch.

The tear strength of the agar impression compounds were approximately twice the value of the duplicating compounds, having values of 4-6 lb/inch.

In addition to the value of the tear strength, the rate at which a cast is removed from the duplication material is important. The effect of deformation rate on the tear strength of agar duplicating compounds A and D is presented. The results are similar to those obtained for the compressive strengths. The tear strength increased as the deformation rate increased from 1 to 10 inches/minute, after which the values appeared to decrease slightly, though not significantly. These results confirm the recommended procedure of snap removal of agar impressions.

Per cent set.-The per cent set after 12 per cent strain test was run by deforming the specimens 12 per cent for 1 minute, relieving the deformation, allowing a 1-minute rest period, and measuring the permanent deformation. The values for duplicating compounds, impression compounds, and experimental agar gels are listed and 4 and. The agar duplicating materials that had the higher compressive and tear strengths had per cent values of approximately 1.5. The weaker agar duplicating materials had per cent set values of 2.3-2.9. The plastic gel duplicating material gave a value of 2.6 per cent when evaluated according to this test procedure. There is some question whether this is a good test for materials of this type, since it recovers more slowly from a deformation than do the agar-type materials. A longer time of 5 minutes for recovery would be indicated when testing plastic gels. The stronger agar impression compounds had values of about 1.3 per cent, and little difference was observed between different products. All these values are less than 3 per cent, which is the limit suggested by the specification for agar impression materials. These data indicate that clinically these duplicating materials would function satisfactorily.

The per cent set values for the experimental agars listed in Table 3 were between 1.5 and 2.6, which is approximately the same range as that obtained for the commercial duplicating materials. One exception was the second batch of Japanese agar, which had a value of 3.9 per cent and, in general, appearal to be an inferior batch for dental purposes. The per cent set test does show large variations in agar gels but does not appear to be a particularly sensitive test. Large increases in the per cent set occur only when the material becomes excessively weak.

It should be pointed out that the per cent set will vary depending on the time allowed for the specimen to recover. The per cent set versus recovery time for duplicating compounds A, B, and E are shown in Figure 7. Material E is normally diluted 3 to 1, but the per cent set versus recovery time curves were determined at 3 to 1 and 1 to 1 dilution, in order to show the effect of agar concentration.

The change in per cent set with recovery time was very slight with duplicating compounds and, which were diluted 1 to 1 in their preparation. When material was used at a 3 to 1 dilution, the per cent set was higher at all comparable recovery times, and the change with time was more pronounced. The change in per cent set with time for the plastic duplicating compound was most pronounced, decreasing from 3.2 per cent at Y2 minute to 1.3 per cent at 10 minutes. Since 10 minutes usually elapses between withdrawing the master cast and pouring the refectory cast, the lower value of 1.3 per cent more nearly reflects the behavior of this material in service.

1.  What is the purpose of duplication of models ?

2. Name the masses that  conduct duplication?

3. Technology of duplication of models.

4. Name the heat-resistant masses, their physical and chemical properties.

5. Name the methods of casting metallic framework of partial removable dentures.

6. Preparation of model to duplication.

7. Duplication of model.

8. Name the full-duplex masses, their physical and chemical properties.

9. Preparation of model with waxen framework of partial removable denture to casting.

10. Fitting of framework.

11. Methods of indemnification of contraction of metal.

12. Enumerate the clinical-laboratry steps that must be conducted at making of partial removable denture, beginning from the fitting of metallic framework.

 

1. For the receipt of heat-resistant model.

2. Hydrocolloid masses that under a temperature 85-90 C are melted on

 “water bath”. It is the masses: H.P.Sosnin, О.І.Kruglyakova, mass of “Helin”.

3. Duplication of models is made in the special cuvette. On the bottom of cuvette welay plasticine and strengthen a model for duplication, through the opening ina cuvette we poad warmed-up dublicate mass. After the mass becomeshard we open a cuvette  and take out a gypseous model, and into its place we poar heat-resistant mass. After consolidation  we should eliminate it and lay for10 sec. in a boiling wax at a temperature 150 C.

4. We caame such heat-resistant masses: mass of “Bugelit”, “Selamin”,”Crystasyl”, mass of B.P.Panchoha, P.S.Flis, Marshalit, Quartz sand,Concentrated solution of etylselicsit, catalyst of dietanol 1% or 2%  acetone solution.

5. On a heat-resistant model, without heat-resistantmodels;

6. To fill next models with a warmed-up wax :

– to lay lining on a model from thin tape of wax or plaster in the zone of saddle and arc;

– to inflict a ledge for retentional part of clasp of supporting tooth;

7. It is conducted in a cuvette for duplication. On the bottom of cuvette by plasticinea model registers for duplication, through the opening in a cuvette we poarfull-duplex mass (Helin). After the mass becomeshard we open a cuvette  and take out a gypseous model, and into its place we poar heat-resistant mass. After consolidation we should eliminate it and lay for10 sec. in a boiling wax at a temperature 150 C.

8. The full-duplex masses: is mass of H.P.Sosnin, 1961р;it is mass of О.I.Kruhliakova, 1994р.it is mass of  Telin, Leningrad plant of prostheticmaterials. All indicated masses are hydrocolloid. They are melted on water bath at – 85-90 С: 9. After a design on a heat-resistant model of waxen framework of  partial removable denture a heat-resistant cover (shirt) is formed . For this aim are used such materials of “Crystosyl”, “Silamin”. Powder is pulverized in a porcelain mortar and mixed with tetraetylselicate. Thus we get varnish  consistencies mass that inflict with a brush on a model of casting. Then we sprinkle it  with powder (this mass), after drying out inflict varnish (polish) mass. We do it 3-4 times. Preliminary we  connect  casting details to the construction. Casting is conducted in a muffle stove for the smelted models:

10. Fitting of construction of metallic partial removable denture is conducted at first on a model, and then in the mouth’ cavity.

11. On a heat-resistant model:

– by means of collapsible models;

– by means of compensative varnishes;

– by means of weak-willed selfhardening plastic.

12. Determination and fixing of central occlusion by means of waxen bite rollers:raising of teeth;verification of construction to prosthetic appliance;

gypsum of models in a cuvette;

filling of plastic;

pressing:

polymerization of plastic;

treatment of denture

polishing

fitting;

putting in operation (finish)

correction.

Agar has been used for a number of years in the preparation of elastic duplicatingmaterials. These duplicating materials have been used mainly in partial denture prosthesisfor the preparation of a duplicate refectory cast of the original stone cast ofthe patient’s mouth. Since moderately large quantities of duplicating materials areused in these procedures, the low cost and reversibility of the agar materials are adefinite advantage.

The mercaptan and silicone rubber elastic impression materials have not been usedextensively in duplicating procedures because of their high cost and the fact that thesematerials, once set, are irreversible. Recently, a plastic duplicating material has beendeveloped which is a reversible polyvinyl chloride plastic gel. Because of its chemicaland physical nature, this material has elastic properties somewhat different from theagar compounds.

The agar duplicating compounds are composed primarily of agar and water but mayalso contain borax to increase the strength, salts to accelerate the setting of the investment,and other ingredients. Cellulose fibers, waxes, and fillers, which may be foundin impression compounds, generally are omitted from duplicating compounds, sincethe duplicating materials are often stored in the sol state for extended periods andthese ingredients tend to separate under this condition of storage. Since the duplicatingcompounds may be stored at temperatures of 130°-150° F., their resistanceto thermal degradation is an important property.

The basic composition of the agar impression compounds is much the same as forthe agar duplicating compounds. The principal difference is that the impression compoundsmay contain 10-15 per cent agar, while the duplicating materials may containonly 5 per cent agar and, as just indicated, need not contain waxes, fillers, and othermodifiers common to impression materials. In fact, agar impression materials havebeen used as duplicating compounds by simply diluting the impression material withan equal amount of water.

The properties of the agar impression compounds have been studied by numerousinvestigators. Skinner and Kern’ determined the linear-dimensional changes in air andsubsequently in water of various agar impression compounds. They also measured thecompressive properties of proportional limit, maximum flexibility, and crushingstrength, as well as the plastic flow properties of these agar materials. Paffenbarger2reported the compressive properties of strength, deformation, set, and stiffness of anumber of agar compounds. The temperature of gel formation and the impression-taking qualities, as measured by the present A.D.A. master model for agar hydrocolloidmaterials, were also determined. Luster3 reported the effect of environmentalfactors of air, water, and inorganic sulfate solutions on the dimensional stability ofagar gels. Phillips and co-workers4 also measured the dimensional changes of agar gelsunder various conditions.

The properties of elastic duplicating materials have received little attention, withonly limited information available in the literature. recently studied the changes in agar duplicating materials on heating and storage bymeasuring changes in the pH and viscosity of the sols and the crushing times of thegels. These values indicated a gradual degradation of the agar material over a 32-dayperiod when stored at 52° C. (125.6° F.).

DUPLICATION AND IMPRESSION MATERIALS

Duplicating Compounds

A…..Copymaster Duplicating Material L. D. Caulk Co.

B….. LasticDurallium Products Corp.

C….. NobiloidNobilium Products, Inc.

D …Su.r.gident Duplicating Compound Surgident, Ltd.

E….. Ticonium Special Dental Perfection Co.

F….. “Van R” Laboratory Duplicating Material “Van R” Dental Products

Impression Compounds

The present investigation represents a portion of a study being conducted on elasticduplicating materials and consists mainly of an initial comparison of the propertiesof duplicating materials with agar impression compounds. The physical properties ofcompressive strength, tear strength, stress-strain properties, per cent set, crushingtime, impression qualities, and gelation temperature were measured. In addition, theproperties of a number of experimental agar gels were determined, in order to examinethe effect of the agar source and composition on physical properties.

Other phases of this study, to be reported at a later date, are the accuracy ofduplicating materials, the effect of aging on duplicating compounds, and thevisco-elastic behavior of these materials.

MATERIALS AND METHODS

The commercial duplicating materials investigated and the suppliers are listed in, with a code letter for each material. The materials are referred to by codeletter because this study is an evaluation of the general properties of elastic duplieating materials, and a high or low value for any particular property does not implythat the material is a superior or inferior product. In fact, it is known that each ofthe materials is used routinely and gives acceptable results. All the duplicating compoundsare hydrocolloid materials except material B, which is a reversible plastic gel.Three hydrocolloid elastic impression materials were selected from the large numbercommercially available on the basis of general differences in physical properties. Theseproducts also are listed in Table 1 with a code letter.Experimental agar gels were prepared from four different commercially available dental-grade agars used in these investigations. Samples from different batchesof these raw agars were obtained and the physical properties determined, in order toobserve differences as a result of the source of supply. A dental-grade agar was alsoused to prepare experimental gels. Five lots of this type of agart taken over an 18-month period were examined.

Specimens used for the determination of compressive strength, stress-strain curves,per cent set, and crushing time were cylinders 3/4 inch high by ‘2 inch in diameter. Thesecylindrical specimens of agar impression or duplication compounds were prepared byliquefying them in boiling water and using specimen molds as described. specification for hydrocolloid impression material, section The duplicatingcompounds were poured when the temperature was between 1400 and 150° F.The impression compounds were tempered according to the manufacturer’s directions.Duplicating material B was poured at the storage temperature of 210° F. After pouring,the specimens were stored in a humidor at 100 per cent relative humidity for1 hour until tested.The tear specimens used had the shape of the ASTM tear-test die C (ASTM TestD624-54).7 The tear test has been used to evaluate rubber materials. The shape anddimensions of the specimens are indicated by the sketch in Figure 1. The mold for thetear specimens was prepared from a casting plastic, and the top and bottom of themold were formed by glass plates. The specimens were prepared and stored as indicatedfor the compression specimens.The dry strip agar was cut into small pieces, and the powdered forms were used asreceived. 10 gm. of agar plus 190 gm. of water were placed in half-pint mason jars. After the agar was thoroughly wetted, the sealed jar was placed in an autoclave for15 minutes at 2650 F. After this time, the pressure was released and the agar solsallowed to cool to room temperature.The compressive strength of the various materials was determined on the cylindricalspecimens. The load required to rupture the specimen was determined, using an InstronTesting Machine. The rate of deformation during testing was varied from 1 to 20inches/minute. It was observed that a maximum compressive strength resulted at adeformation rate of approximately 10 inches/minute, for which reason the majorityof the testing was carried out at this deformation rate.

The stress-strain curve may also be obtained from the load and deformation data.The stress-strain curves were plotted from the data obtained at a deformation rate of10 inches/minute. These curves permitted the determination of the stiffness, proportionallimit, and per cent compression of the agar materials.

The tear strength of elastic duplicating and impression materials is a most importantproperty because these materials must have sufficient strength that they may beremoved from undercut areas and interproximal spaces without tearing. The loadrequired to continue tearing the specimen was determined by using the Instron Tester.The load divided by the thickness of the specimen in inches was a measure of thetear strength and is reported in pounds per inch of notch or, more simply, poundsper inch.

The per cent set after 12 per cent strain was determined on an instrument developedby Cresson.8 The instrument had a dial gauge sensitive to 0.001 inch, was mountedon a steady stand, and was equipped with a screw to allow a cylindrical specimen tobe compressed the desired 12 per cent. The sample was compressed 12 per cent andthe pressure maintained 1 minute, after which the sample was allowed to rest withno pressure for 1 minute, and the specimen height was measured. Knowing the originaland final specimen length, the per cent set may be calculated.A crushing-time test similar to that proposed by Margetis and Hansen5 was used.The modified compression-testing apparatus described by Paffenbarger2 was convenientfor performing the crushing-time test. The cylindrical specimens were loaded withweights varying from 1,100 to 3,600 gm., and the time from the load application tothe rupture of the specimen was determined in seconds. Variation in the load wasnecessary, since no single load could be found which would give reasonable crushingtimes for agar impression, duplication, and experimental materials.The A.D.A. master model for testing hydrocolloid impression materials6 was usedto give a general evaluation of the impression qualities of the duplicating materials.This master model has five posts, two of which are cylindrical, and the other threehave varying degrees of undercut. A snap removal of the master model was carriedout, and improved stone was used to prepare the duplicate model.The gelation temperatures were measured according to the directions of the A.D.A.specification No. 11 for hydrocolloid impression materials.6 The test consists of insertinga metal tube into the impression material at various times as the material coolsfrom a sol to a gel. The gel temperature is that temperature at which a clear impressionof the inside and outside of the metal tube is obtained and none of the material clingsto the walls of the tube. Stress-strain properties.-The stress-strain curves were determined for all the duplicatingand impression compounds listed. The values for the initial slope of thestraight-line portion of the curves and the proportional limit as indicated by flatteningof the curve prior to fracture are listed. In addition, the ultimate per centcompressions also are reported in this table. The ultimate compressive strength alsomay be obtained from these curves, and this property will be discussed in the followingsection. Examples of the various types of stress-strain curves obtained are presentedfor materials B, D, E, and I.Material (lb/in2) (lb/in2) (percent)The deformation rate was 10 inches/minute, with the result that the completestress-strain curve for any of the samples was obtained in less than 3 seconds. Onthe basis of unpublished flow studies, the curves represented primarily the elastic behaviorof the materials, and any viscous effect was neglected. Material I was an agarimpression compound and, like the other impression compounds G and H, was stifferand had a higher proportional limit than the agar duplicating compounds. These resultswould be expected, since the agar duplicating compounds are more dilute than theagar impression materials. Agar duplicating compounds A, D, and F were similar intheir compressive properties, while compounds C and E had lower compressive characteristics.This again would be anticipated because of the lower agar concentrationsused in materials C and E. Material B was a plastic gel duplicating material and hadcompressive properties similar to agar duplicating compounds at low compressionvalues. This material had considerably higher strength and ultimate compressionvalues than did the agar types. It is important to point out that these high values are not required of a duplicating material. It may, however, affect the length of time overwhich the duplicating compound may be used before the physical properties deteriorateto such an extent that the material must be discarded.

The ultimate per cent compression of the agar duplicating compounds was generallylower than for the impression compounds, but no correlation existed between thesevalues and the other compressive properties.

The shape of the stress-strain curves was different from those reported by Paffenbarger.The initial portion of the curves were straight lines, but betweena strain of 0.1 and 0.2 inch/inch the curves began to rise. After this rise they flattenedout and finally became discontinuous when the specimens ruptured. This shape mayStrain, in/inbe explained by the fact that the specimen cross-sectional area is increasing duringthe test or that the stiffness of the material increases with the deformation. The stressstraincurves reported by Paffenbarger2 were usually concave toward the strain axis.The differences in the results of the two studies may be reconciled by noting that thelatter stress-strain curves were obtained by loading the specimens at the rate of 2.84lb/in2/minute, and therefore, the tests required 10-17 minutes to complete. Considerablymore viscous deformation could occur in this period of time compared to the3 seconds or less required in the present study. Any flow taking place would result inthe stress-strain curve being concave toward the strain axis. The curves, therefore, more nearly represent the behavior of the agar materials in service,since the impression is removed rapidly rather than slowly.

Compressive strength.-The compressive strengths of the duplicating compounds, those for the agar impression compounds, and thosefor the experimental agar gels. The compressive strengths, measured at albs/in28 deformation rate of 10 inches/minute, of the duplicating compounds generally rangedbetween 50 and 65 lb/square inch. Two exceptions to this generalization were compoundsB and E. Compound B is a plastic gel and would be expected to have themuch higher compressive strength of 130 lb/square inch, and agar compound E isdiluted more than the other agar compounds and would be expected to have the lowerstrength of 37 lb/square inch.The compressive strength of the agar impression compounds G, H, and I fall between78 and 114 lb/square inch. These values are higher than for the duplicatingcompounds, since the agar concentration is two to three times higher than for theduplicating compounds.The compressive strengths of experimental agar gels listed considerablevariation. The Portuguese, Spanish, and Japanese agar gels showed more variationbetween batches than did the American agar gels. The Spanish and Portuguese

PHYSICAL PROPERTIES OF 5 PER CENT COMMERCIAL AGAR GELS

Compres- Crushing Gel

Agar Batch sive Time Per Cent Tempera-

Strength (Seconds at Set ture,

(lb/in2) 870 gm/cm2) (°C.)

F12 3388 2166 21..46 36

American……… 3 40 19 2.6 37

4 32 13 2.5 36 t5 36 13 2.4 36

Portuguese ……..ft 60 359 1.5 36 12 52 134 1.6 36

Spanish f……….. f 70 337 1.6 36

)~2 53 53 2.3 36

Japanese..ft.27 7 1.7

21.6 1 3.9

agar gels were stronger than the Japanese and American, having compressive strengthsfrom 52 to 70 lb/square inch. The American agar gels were intermediate in compressivestrength, with the values from 32 to 40 lb/square inch, while the Japanese agargels were the weakest, having values between 16 and 27 lb/square inch. The experimentalagar gels contained approximately the same concentration of agar as did duplicatingcompound E.Of principal interest is the stability of the properties of the American agar gelsprepared from various batches of agar. The variability of the properties of agar frombatch to batch has been recognized for a long time as being one of the main difficultiesin the preparation of a consistent agar impression or duplicating compound. This factis substantiated by the variation in the properties of the Portuguese, Spanish, andJapanese agar.The reason for the choice of a deformation rate of 10 inches/minute may be seenby examination of. The solid curves represent the relationship of the compressivestrength to the deformation rate for three different dilutions of duplicating  compound A. The same types of curves have been obtained for the other agar duplicatingand impression materials, although the maximum is not so pronounced in thecase of the impression compounds. The dotted line indicates the per cent compressionat fracture of the agar gels prepared by diluting material A with 0.5, 1, or 2 partsof water. All the values fell on a single curve, which showed that within this dilutionrange the percent compression was independent of the water to material ratio.The curves for the compressive strength of agar gels versus deformation rate reacheda maximum between 10 and 12 inches/minute. If the gels were deformed at low rates,such as 2 inches/minute, or at high rates, such as 20 inches/minute, considerablylower values for compressive strengths were obtained. A maximum was not observedfor the per cent compression versus deformation rate, but the values increased between2 and 10 inches/minute, after which a stable value was maintained. The deformationrate of 10 inches/minute was therefore selected because a maximum valuewas obtained at this rate and because the materials as used would be subjected tocompression forces at approximately this rate.

The significance of selecting the proper deformation rate for evaluating compressive strength may be seen by comparing the values reported by Paffenbarger2 for agarimpression materials with those listed. The values given by Paffenbarger2ranged from 16 to 40 lb/square inch, while those reported in this study were between78 and 114 lb/square inch. The former values are possibly lower because the deformationrate was much slower.

Tear strength.-Since one visual criterion for discarding a duplicating material isthe tendency of the material to tear in the region of the interproximal spaces, the tearstrength of duplicating compounds is a most important property. The tear specimenswere tested in tension at a rate of 10 inches/minute, and the values obtained for the duplicating and impression. The order of the tear strengths of the duplicating and impression compounds isessentially the same as for their compressive strengths. The tear-strength test, however,appears to be somewhat more sensitive to differences in materials than the compressive-strength test. The tear strengths of agar duplicating compounds A, D, and Fwere between 2 and 3 lb/inch, while compounds C and E had lower values of about1 lb/inch, and the plastic duplicating material had a higher value of over 6 lb/inch.The tear strength of the agar impression compounds were approximately twice thevalue of the duplicating compounds, having values of 4-6 lb/inch.In addition to the value of the tear strength, the rate at which a cast is removedfrom the duplication material is important. The effect of deformation rate on the tearstrength of agar duplicating compounds A and D is presented.

The resultsare similar to those obtained for the compressive strengths. The tear strength increasedas the deformation rate increased from 1 to 10 inches/minute, after which the valuesappeared to decrease slightly, though not significantly. These results confirm therecommended procedure of snap removal of agar impressions.Per cent set.-The per cent set after 12 per cent strain test was run by deformingthe specimens 12 per cent for 1 minute, relieving the deformation, allowing a 1-minuterest period, and measuring the permanent deformation. The values for duplicatingcompounds, impression compounds, and experimental agar gels are listed and 4 and, respectively. The agar duplicating materials that had the highercompressive and tear strengths had per cent values of approximately 1.5. The weakeragar duplicating materials had per cent set values of 2.3-2.9. The plastic gel duplicatingmaterial gave a value of 2.6 per cent when evaluated according to this test procedure.There is some question whether this is a good test for materials of this type, sinceit recovers more slowly from a deformation than do the agar-type materials. A longertime of 5 minutes for recovery would be indicated when testing plastic gels. Thestronger agar impression compounds had values of about 1.3 per cent, and little differencewas observed between different products. All these values are less than 3 per cent,which is the limit suggested by the specification for agar impression materials. Thesedata indicate that clinically these duplicating materials would function satisfactorily.The per cent set values for the experimental agars listed were between 1.5and 2.6, which is approximately the same range as that obtained for the commercialduplicating materials. One exception was the second batch of Japanese agar, which hada value of 3.9 per cent and, in general, appearal to be an inferior batch for dental purposes. The per cent set test does show large variations in agar gels but does not appear tobe a particularly sensitive test. Large increases in the per cent set occur only when thematerial becomes excessively weak.It should be pointed out that the per cent set will vary depending on the time allowedfor the specimen to recover. The per cent set versus recovery time for duplicatingcompounds A, B, and E are shown in. Material is normally diluted 3 to 1,but the per cent set versus recovery time curves were determined at 3 to 1 and 1 to 1dilution, in order to show the effect of agar concentration.The change in per cent set with recovery time was very slight with duplicating compoundsA and E, which were diluted 1 to 1 in their preparation. When material E wasused at a 3 to 1 dilution, the per cent set was higher at all comparable recovery times,and the change with time was more pronounced. The change in per cent set with timefor the plastic duplicating compound was most pronounced, decreasing from 3.2 percent at Y2 minute to 1.3 per cent at 10 minutes. Since 10 minutes usually elapses betweenwithdrawing the master cast and pouring the refectory cast, the lower value of1.3 per cent more nearly reflects the behavior of this material in service.

Plastic is the general common term for a wide range of synthetic or semisynthetic organic amorphous solid materials suitable for the manufacture of industrial products. Plastics are typically polymers of high molecular weight, and may contain other substances to improve performance and/or reduce costs.

The word derives from the Greek πλαστικός (plastikos), “fit for molding”, from πλαστός (plastos) “molded”.It refers to their malleability, or plasticity during manufacture, that allows them to be cast, pressed, or extruded into an enormous variety of shapes—such as films, fibers, plates, tubes, bottles, boxes, and much more.

The common word “plastic” should not be confused with the technical adjective “plastic”, which is applied to any material which undergoes a permanent change of shape (a “plastic deformation”) when strained beyond a certain point. Aluminum, for instance, is “plastic” in this sense, but not “a plastic” in the common sense; while some plastics, in their finished forms, will break before deforming — and therefore are not “plastic” in the technical sense.

There are two types of plastics, thermoplastic and thermoset. Thermoplastics, if exposed to heat, will melt in two to seven minutes. Thermosets will keep their shape until they are charred and burnt. Some examples of thermoplastics are grocery bags, piano keys and some automobile parts. Examples of thermosets are kid’s dinner sets and circuit boards.

Plastics can be classified by their chemical structure, namely the molecular units that make up the polymer’s backbone and side chains. Some important groups in these classifications are the acrylics, polyesters, silicones, polyurethanes, and halogenated plastics. Plastics can also be classified by the chemical process used in their synthesis, e.g. ascondensation, polyaddition, cross-linking, etc.

Other classifications are based on qualities that are relevant for manufacturing or product design. Examples of such classes are the thermoplastic and thermoset, elastomer,structural, biodegradable, electrically conductive, etc. Plastics can also be ranked by various physical properties, such as density, tensile strength, glass transition temperature, resistance to various chemical products, etc.

Due to their relatively low cost, ease of manufacture, versatility, and imperviousness to water, plastics are used in an enormous and expanding range of products, from paper clips to spaceships. They have already displaced many traditional materials—such as wood, stone, horn and bone, leather, paper, metal, glass and ceramic—in most of their former uses.

The use of plastics is constrained chiefly by their organic chemistry, which seriously limits their hardness, density, and their ability to resist heat, organic solvents, oxidation, andionizing radiation. In particular, most plastics will melt or decompose when heated to a few hundred celsius. While plastics can be made electrically conductive to some extent, they are still no match for metals like copper or aluminum. Plastics are still too expensive to replace wood, concrete and ceramic in bulky items like ordinarybuildings, bridges, dams, pavement, railroad ties, etc

Common thermoplastics range from 20,000 to 500,000 in molecular mass, while thermosets are assumed to have infinite molecular weight. These chains are made up of many repeating molecular units, known as “repeat units”, derived from “monomers“; each polymer chain will have several thousand repeat units. The vast majority of plastics are composed of polymers of carbon and hydrogen alone or with oxygen, nitrogen, chlorine or sulfur in the backbone. (Some of commercial interests are silicon based.) The backbone is that part of the chain on the main “path” linking a large number of repeat units together. To vary the properties of plastics, both the repeat unit with different molecular groups “hanging” or “pendant” from the backbone, (usually they are “hung” as part of the monomers before linking monomers together to form the polymer chain). This customization by repeat unit’s molecular structure has allowed plastics to become such an indispensable part of twenty first-century life by fine tuning the properties of the polymer.

Some plastics are partially crystalline and partially amorphous in molecular structure, giving them both a melting point (the temperature at which the attractive intermolecular forcesare overcome) and one or more glass transitions (temperatures above which the extent of localized molecular flexibility is substantially increased). So-called semi-crystalline plastics include polyethylene, polypropylene, poly (vinyl chloride), polyamides (nylons), polyesters and some polyurethanes. Many plastics are completely amorphous, such as polystyrene and its copolymers, poly (methyl methacrylate), and all thermosets

Poly(methyl methacrylate)(PMMA) poly(methyl 2-methylpropenoate) is a thermoplastic and transparent plastic. Chemically, it is the synthetic polymer of methyl methacrylate. It is sold by the trade names Plexiglas, Vitroflex, Limacryl, R-Cast, Per-Clax, Perspex, Plazcryl, Acrylex, Acrylite, Acrylplast, Altuglas, Polycast, Oroglass, Optix andLucite and is commonly called acrylic glass, simply acrylic, perspex or plexiglas. Acrylic, or acrylic fiber, can also refer to polymers or copolymers containingpolyacrylonitrile. The material was developed in 1928 in various laboratories and was brought to market in 1933 by Rohm and Haas Company.

PMMA is often used as an alternative to glass, and in competition with polycarbonate (PC). It is often preferred because of its moderate properties, easy handling and processing, and low cost, but behaves in a brittle manner when loaded, especially under an impact force. To produce 1 kg of PMMA, about 2 kg of petroleum is needed. PMMA ignites at 460 °C and burns completely to form only carbon dioxide and water.

PMMA:

• has a density of 1,150–1,190 kg/m³. This is less than half the density of glass, and similar to that of other plastics.

• has a good impact strength higher than that of glass or polystyrene, but significantly lower than that of polycarbonate or engineering polymers. In the majority of applications, it will not shatter but instead breaks into large dull pieces.

• is softer and more easily scratched than glass. Scratch-resistant coatings (which may also have other functions) are often added to PMMA sheets.

• transmits up to 92% of visible light (3 mm thickness), and gives a reflection of about 4% from each of its surfaces on account of its refractive index of 1.4893 to 1.4899.

Skeletal structure of methyl methacrylate, the monomer that makes up PMMA

Structure of the PMMA polymer

filters ultraviolet (UV) light at wavelengths below about 300 nm. Some manufacturers add coatings or additives to PMMA to improve absorption in the 300–400 nm range.

allows infrared light of up to 2800 nm wavelength to pass. IR of longer wavelengths, up to 25,000 nm, are essentially blocked. Special formulations of colored PMMA exist to allow specific IR wavelengths to pass while blocking visible light (for remote control or heat sensor applications, for example).

has excellent environmental stability compared to other plastics such as polycarbonate, and is therefore often the material of choice for outdoors applications.

has poor resistance

PMMA has a good degree of compatibility with human tissue, and can be used for replacement intraocular lenses in the eye when the original lens has been removed in the treatment of cataracts. Historically, hard contact lenses were frequently made of this material. Soft contact lenses are often made of a related polymer, where acrylate monomers containing one or more hydroxyl groups make them hydrophilic.

In orthopaedics, PMMA bone cement is used to affix implants and to remodel lost bone. It is supplied as a powder with liquid methyl methacrylate (MMA). When mixed these yield dough-like cement that gradually hardens. Surgeons can judge the curing of the PMMA bone cement by pressing their thumb on it. Although PMMA is biologically compatible, MMA is considered to be an irritant and a possible carcinogen. PMMA has also been linked to cardiopulmonary events in the operating room due to hypotension. Bone cement acts like a grout and not so much like a glue in arthroplasty. Although sticky, it does not bond to either the bone or the implant, it primarily fills the spaces between the prosthesis and the bone preventing motion. A big disadvantage to this bone cement is that it heats to quite a high temperature while setting and because of this it kills the bone in the surrounding area. It has a Young’s modulus between cancellous bone and cortical bone. Thus it is a load sharing entity in the body not causing bone resorption.  

Dentures are often made of PMMA, and can be colour-matched to the patient’s teeth & gum tissue. In cosmetic surgery, tiny PMMA microspheres suspended in some biological fluid are injected under the skin to reduce wrinkles or scars permanently.

Preparation of the cast for duplication

On maxillary casts, check beadline

Beadline should be scribe into cast

1 mm wide and .5 mm deep

Used only on maxillary design

Ensure it is void free

Ensure it is free of wax and debris

Soak the master casts

Eliminates air from casts and replaces it with water so casts will not draw moisture from hydrocolloid and adhere to casts

Methods of soaking the casts

On their heels or with teeth down

In SDS (Saturated calcium sulfate dihydrate solution) water

Allow to soak in warm hygrobath 90°F for 30 mins before removing from water

Casts made from improved stone require a much longer soaking time

Purpose of Duplication – To produce two casts:

Refractory cast used for casting RPD frameworks

Heat resistant duplicate of a modified (blocked out and relieved) master cast Made from dental casting investment

Acts as the base for forming the RPD framework in wax and plastic

Duplicate master cast used for seating RPD frameworks

PREPARING MASTER CASTS FOR DUPLICATION

DUPLICATING MASTER CASTS

1.     Purpose of Blockout and Ledging

a.     Blockout: To eliminate undercut areas on a cast with blockout material

b.     Ledging: To control the amount of undercut the clasp tip engages

2.     Definition of partial denture relief

The adaptation of wax to the edentulous areas of a cast, before duplication, to create a raised area on the refractory cast

3.     Purpose of partial denture relief

a.     Acrylic retention

Allows a space for acrylic to flow around the denture base retention

b.     Internal finish lines

1.     Establishes a definite junction for metal framework and acrylic resin

2.     Located on the tissue side of RPD

3.     Used as a guide for placement of external finish line

c.      Relief of lingual bar and approach arms

1.     Eliminates the possibility of tissue ulceration due to framework rubbing during function

2.     Lingual bars when used in conjunction with distal extension defects, usually require relief

3.     Relief is rarely, if ever, indicated under palatal connectors or toothborne RPDs

4.     Equipment and Materials

a.     Blockout wax must have the following physical properties:

1.     Firm – must resist temperatures of at least 10°F in excess of the pouring temperature of the duplicating material

2.     Easy to carve

b.     Sheet casting wax

1.     Supplied in various gauges

2.     Pressure sensitive – do not compress the wax upon application, it results in an uneven layer of relief

3.     Adhesive – one side is tacky to adapt to cast

5.     Procedures for Blockout, Ledging and Relief Wax Adaptation

A.   Blockout all undercuts

1.     Cast remains on the survey table during the entire procedure

2.     Application of blockout wax and baseplate wax

a.     Below the survey line around the teeth

b.     Below the survey line of soft tissue undercuts within the RPD design

c.      Blockout gross undercuts outside of framework area, e.g., (anterior labial)

3.     Remove excess wax

a.     Warm the blockout tool slightly

b.     Remove excess blockout wax from the appropriate areas by moving blockout tool around teeth and tissue

c.      Ensure there is no wax above the survey line

B.   Ledging

1.     Locate desirable undercuts

2.     Expose the terminal 1/3 of the retentive clasp tip design from the blockout wax

a.     Use a roach carver to trim the wax at a right angle (90°) to the abutment tooth surface; do not scratch the cast

b.     Ledge should be free of debris

c.      Denture base retention relief

1.     Select the proper gauge wax

a.     Use 24 gauge adhesive wax unless directed otherwise

b.     The lower the gauge numbers, the thicker the wax

2.     Adapt and cut the wax

a.     Ensure the finish line is sharp and accurate

1.     Cut wax at a 90° angle to the tissue

2.     Do not bevel or taper the edge

b.     Cut the wax 1.5mm from the abutment teeth

D.   Cut the tissue stop

1.     Purpose of the tissue stop

To hold the retention area of the framework off of the tissue while packing forces are being applied

2.     Ensure the tissue stop is placed directly over the crest of the ridge preventing movement of the tissue stop in a lateral direction

3.     Cut 2mm by 2mm square

6.     Preparation of the cast for duplication

a.     On maxillary casts, check beadline

1.     Beadline should be scribe into cast

a.     1 mm wide and .5 mm deep

b.     Used only on maxillary design

2.     Ensure it is void free

3.     Ensure it is free of wax and debris

b.     Soak the master casts

1.     Eliminates air from casts and replaces it with water so casts will not draw moisture from hydrocolloid and adhere to casts

2.     Methods of soaking the casts

a.     On their heels or with teeth down

b.     In SDS (Saturated calcium sulfate dihydrate solution) water

c.      Allow to soak in warm hygrobath

1.     90°F for 30 mins before removing from water

2.     Casts made from improved stone require a much longer soaking time

7.     Purpose of Duplication – To produce two casts:

a.     Refractory cast used for casting RPD frameworks

1.     Heat resistant duplicate of a modified (blocked out and relieved) master cast

2.     Made from dental casting investment

3.     Acts as the base for forming the RPD framework in wax and plastic

b.     Duplicate master cast used for seating RPD frameworks

8.     Materials

a.     Hydrocolloid (agar type) Dupli-chrome – Reversible

b.     Red Strip investment

1.     Phosphate bound investment

2.     Contains crystobolite which facilitates expansion and enables the investment to withstand high heat

3.     Expansion factors: Molten metal shrinks 1.7% as it cools. This must be compensated by the expansion of the investment material

a.     Setting

1.     Expansion – 0.4%

2.     This occurs while the cast is setting up

b.     Hygroscopic

1.     Expansion – 0.3%

2.     Occurs as the investment sets- up in the hydrocolloid

c.      Thermal

1.     Expansion – 1.0%

2.     Occurs in the burn-out oven when the investment is subjected to high heat

4.     Important Rules

1.     Always use distilled water when mixing investment

2.     Use the manufacturer’s recommended water/powder ratio-more water/less expansion

3.     Be sure to roll the canister to mix all particles

9.     Procedures

a.     Autoduplicator: Used to breakdown and store hydrocolloid at a given temperature

Use manufacturer’s recommendations for temperature

1.     Set low temp at 50 degrees Celsius

2.     Set high temp at 90 degrees Celsius

3.     Fill duplicator with hydrocolloid

a.     Fill duplicator at least 1/2 full – material will burn if less than 1/2 full

4.     Start breakdown cycle (cookdown)

a.     Push cycle button on

1.     Temp will rise to high temp setting at 2 degrees per minute, then lower to low temp setting

2.     In 3 to 5 hours the hydrocolloid will be melted and cooled down to pouring temperature

b.     Clear the valve -Open the valve to drain off approximately 150 cc of hydrocolloid to clear the valve of lumps and debris

c.      Safety: Keep your fingers out of the unit when it is running. Use caution when pouring duplicating material, because it is hot

b.     Select a duplicating flask that allows at least 12mm clearance around the cast

c.      Remove the cast from the water bath

1.     Check the relief wax to make sure it has not lifted off cast

2.     Blow off excess water from the cast (DO NOT USE AIR HOSE)

d.     Place cast in duplicating flask

1.     Position cast on center of base

2.     Place body on base, ensure it is completely closed

e.      Fill flask with hydrocolloid

1.     Center the flask under the valve

2.     Open the valve slowly until the stream of hydrocolloid is about the diameter of a pencil

3.     Fill flask completely

4.     Close valve

f.       Cool the duplicating flask

1.     Place in cooling tray

a.     Ensure water level does not extend higher than top of the base

1.     Cast and base cool before the body

2.     The hydrocolloid solidifies toward cast for more accurate duplication

b.     Ensure the water remains below room temperature, but not less than 55 degrees F

c.      The water should circulate in the tank

2.     Length of cooling time

a.     Small flask – 30 minutes

b.     Large flask – 45 minutes

g.     Pour the refractory cast

1.     Water/powder ratio for redstripe investment

a.     For average size frameworks use 25cc distilled water to 200 grams investment

b.     For large horseshoe frameworks or cast metal bases uses 24cc distilled water to 200 grams investment

1.     For very large areas of metal, more expansion is needed

2.     Less water gives more expansion

2.     Retrieve master cast from hydrocolloid

a.     Pry off base with a knife

b.     Clean off any hydrocolloid covering the bottom of the master cast

c.      Using 2 knives, place them against the cast

d.     Pry master cast out of hydrocolloid by giving cast a quick upward snap

e.      Make sure the relief wax is still on the cast

3.     Mix water and investment

Vacuum mix investment for 30 seconds

4.     Fill the mold with investment

a.     Spot fill; do not flow the investment around the mold as is done for an alginate impression

b.     Hydrocolloid contains salts – filling impression like an alginate will distort the refractory cast

5.     Insert sprue former – too much pressure can distort the impression

a.     7-8mm from the major connector design

b.     the sprue provides a channel for the escape of gases and debris during burnout procedure

c.      The sprue provides a channel for molten metal to enter the cavity of investment mold during casting procedure

d.     Top of sprue cone should be parallel with the occlusal plane

e.      Fill the mold to the top

f.       Investment will not have proper setting expansion if it contacts the metal flask

g.     Allow the investment to set for 20- 35 minutes

6.     Recover refractory cast

a.     Always remove the hydrocolloid from the cast, never the reverse

1.     Cut the hydrocolloid in 4 areas

a.     Make cut at each cuspid area

b.     Make cut at each heel area

2.     Gently peel hydrocolloid away from refractory cast

b.     Remove sprue former

1.     Gently turn the sprue former until it comes loose from refractory cast

2.     Make every effort not to touch areas within the design on the cast

7.     Prepare hydrocolloid for reuse

a.     Rinse hydrocolloid under running water

b.     Remove all particles of investment, modeling clay and wax

c.      Store hydrocolloid in an airtight container until it’s ready for cookdown

d.     Replace the hydrocolloid after 200 duplications or as needed

8.     Prepare casts for dehydration

a.     Inspect for voids and bubbles in critical areas

b.     Inspect internal finish line for defects

c.      Smooth edge of sprue hole – eliminates the possibility of investment pick-up during the casting procedure

d.     Outline cast for trimming- make line approx. 6 mm away from extremities of design with a wax pencil

e.      Smooth base of refractory cast

f.       Trim heel area at 90 degree angle

g.     Trim the rest of cast at 45° degree angle

h.     Rinse off slurry

10.                       Purpose of Dehydration and Wax-dip

a.     Dehydration

1.     Prepares the refractory cast for wax dipping by removing moisture

2.     Preheat cast for wax dipping

b.     Wax dip

1.     Assures a smooth dense cast necessary for waxing procedures

2.     Eliminates soaking refractory casts prior to investing

a.     Unsealed casts would extract moisture from outer investment

b.     Refined Beeswax – used for wax dipping

c.      Dehydration and Wax-dip

1.     Dehydrating Oven

a.     Purpose: Used to dry refractory casts prior to sealing them with beeswax

b.     Safety: Take precautions for handling hot materials

2.     Beeswax heater

a.     Purpose: To melt refined beeswax and maintain the wax at a temperature between 280°F and 300°F

b.     Safety:

1.     Use carrier to immerse casts into the molten wax

2.     Do not drop casts into the molten wax, as splashing of hot wax can result in serious burns

3.     Avoid touching the exterior of the unit during and immediately after its operation

3.     Place refractory casts into dehydration oven at 180 degree – 200 degrees F for 1 hour

4.     Wax-dip in beeswax at 280° – 300° F

a.     Beeswax must be 100 degrees above dehydrating oven

b.     Place cast in hot wax using cast holder

5.     Cool casts after wax-dip

a.     Place wax-dipped casts on a paper towel on their heels

b.     Blow lightly on cast to prevent puddling

11.                       Make duplicate master casts, remove relief pads, duplicate using dental stone

DIGITAL DUPLICATION WITH GEOMAGIC DIGITAL DENTISTRY HELPS ATLANTIS COMPONENTS DELIVER PEARLY WHITES TO MAINSTREAM DENTISTS

For decades, dental implants have offered alternatives to dentures and bridges, improving people’s lives by giving them replacement teeth that look a bit more natural. But, despite the advances in prosthetic teeth, not everybody is smiling: Placing the implants can be a complicated, expensive, and lengthy process of trial and error for both the dentist and the patient. And, the results aren’t always aesthetically pleasing.Fortunately, Atlantis Components Inc. (Cambridge, Massachusetts), a custom dental products company, has begun to change the often-bleak scenario of “false teeth.” The company has enabled approximately 1,000 dentists around the country to provide implants that look as natural as real teeth in less time and at a lower cost. A key part of Atlantis’ process relies on digital duplication technology from Geomagic.

From Frustration to Innovation

A dental implant has three main parts: the fixture, which anchors the implant to the jawbone; the crown, which replaces the natural tooth; and the abutment, the part that screws into the fixture and holds the crown in place. Perhaps the most difficult stage of the procedure is fitting the abutment.

“It can be very time-consuming to fit an abutment,” says Tom Cole, Atlantis president. “For one thing, the mouth is a difficult place in which to work. And, depending on when the tooth was extracted or lost, there can be difficulties with the quality of bone in which to place the fixture.”

Because much of the bone might have shrunk over time, dentists often must angle the fixture in a patient’s mouth to anchor it into solid bone. These angles create problems in making the abutment conform to the natural shape of the patient’s other teeth. Abutments also can be difficult to seat on the implant the later in the process they are placed, resulting in entrapment of the gums before the crown can be put into place. Once the abutment is placed, numerous radiographs can be required to ensure that it is properly seated, resulting in more time and expense.

Frustrated with existing tooth implant-abutment methods, Dr. Julian Osorio, a Boston prosthodontist, formulated an abutment concept that would fit the exact geometry of the patient’s mouth. His goal was to create a process that would make implants accessible to mainstream dentistry. Osorio founded Atlantis Components in 1996, with a product that has changed implant dentistry — The Atlantis Permanent Healing Abutment.

Building a Better Abutment

To create his product, Dr. Osorio turned to the world of CAD/CAM technology and Geomagic Studio, which makes it possible to take output from 3D scanners and digitally duplicate the shape, textures, and color of real-world objects, including the cast of a patient’s mouth. Geomagic Studio models can be output directly to CAD software, stereolithography devices, or CNC manufacturing systems. They can also be automatically transformed to highly detailed, but very small, models that can be displayed efficiently on the Web.

“Geomagic allows us to obtain an accurate visualization of the cast of the mouth in the computer. This is very important in ensuring that the abutment has the proper fit,” says Cole, who is not only president, but also an engineer who helped develop the Atlantis abutment process. “We’re the first company to really make this process easy by taking advantage of computers and dental knowledge to design the abutment using the patient’s individual oral geometry.”

The Atlantis process begins with scanning a cast of the patient’s mouth provided by the dentist. Geomagic Studio is used to create surfaces from the scanned point cloud data. The surfaces are then imported into CAD software.

“The surface of the teeth is used as reference geometry for accurate modeling of our abutment. Surfacing the dental anatomy in Geomagic Studio allows us to modify CAD models in context, thus accurately producing an abutment that fits into the patient’s mouth with no further adjustments,” says Bethany Grant, Atlantis senior development engineer.

The template-based work flow feature of Studio increases productivity by enabling surface patch layouts to be reused, eliminating the repetitive task of creating new patch layouts for similar models.

“Exact Anatomic Duplicate”

Dr. Joseph Gian-Grasso, a Philadelphia dentist who has been in practice since 1973 and has been placing implants since 1983, has used the Atlantis abutment on about 50 patients in the last year or so. Dr. Gian-Grasso estimates that the Atlantis process saves him between two to six months’ time because the impression for the abutment can be made the same day the fixture is placed in the bone. Previously, Dr. Gian-Grasso had to wait for the implant to be integrated with the bone.

“If you’ve got a busy practice, time is money,” Gian-Grasso says. “Most patients really want the process completed expeditiously and expertly. The Atlantis abutment allows us to do that.”

In the earlier days of implant dentistry, Gian-Grasso said the abutments he used were more a one-size-fits-all, off-the-shelf type product. “In an ideal circumstance you would have an acceptable result, but with the Atlantis abutment, the result is much more predictable because it is customized to the exact fit of the patient’s mouth,” he says. “The abutment actually reproduces the exact anatomic duplicate of what a dentist would try to make manually.”

A Solution for Mainstream Dentistry

Using Geomagic Studio with CAD/CAM technology, what once took 10 visits for the implant patient now only takes three to five visits. Dr. Osorio’s goal of bringing implant technology to mainstream dentistry has become a reality, and the real winner is the patient, who walks away with a new smile that looks completely natural in far less time than ever before.

A dental refractory model material comprises as powdery components 5 to 20% by weight of a soluble phosphate, 5 to 20% by weight of magnesium oxide and 10 to 50% by weight of at least one selected from the group of alumina, zirconia, fused quartz, mullite, spinel and cordierite with the balance being crystalline quartz and crystobalite. The refractory model material also includes a liquid component such as a colloidal silica dispersion.

This application is a continuation of application Ser. No. 07/825,316, filed on Jan. 27, 1992, now abandoned, which is a continuation of application Ser. No. 07/560,274, filed on Jul. 30, 1990, now abandoned, which is a continuation of application Ser. No. 07/271,445, filed on Nov. 15, 1988, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a refractory model material for making dental porcelain restoratives by a building-up method. More specifically, the present invention concerns a dental refractory model material usable for dental porcelain restoratives such as porcelain laminated veneers, porcelain inlays or onlays and porcelain jacket crowns by taking an impression of formed teeth in the mouth with the use of a dental impression material, casting a blend of a powdery component with a colloidal silica dispersion into the impression as a slurry slush, which is then cured to prepare a refractory model, and building a finely divided porcelain slush directly upon the model, which is then placed in a furnace, etc. in its entirety to heat and burn the porcelain.

2. Statement of the Prior Art

Heretofore, refractory model materials based on phosphates and gypsum have been used as the refractory model materials for preparing dental porcelain restoratives such as porcelain laminated veneers. Soluble phosphates and magnesium oxide or gypsum have been used as the binding component therefor, and crystalline quartz and crystobalite as the aggregate component therefor. For the phosphate base refractory model materials, a colloidal silica dispersion has been used as a liquid component.

However, such conventional refractory model materials have had the following defects.

(1) Crystalline quartz and crystobalite incorporated as the main component of an aggregate undergo a sharp volume change due to crystal transformation in the vicinity of 573 200 model materials, it has thus been pointed out that since it is impossible to control sharp changes in the thermal expansion of crystalline quartz and crystobalite, crazing or cracking problems arise, when models are repeatedly burned.

(2) Sharp changes in the volume of the refractory model materials give rise to the distortion of built-up porcelain during burning, thus causing the porcelain to be cracked.

(3) Even though the mixing ratio of crystalline quartz and crystobalite used as the main component of the aggregate is varied, the conventional refractory model materials show only a slight variation in the rate of changes in heating. Thus, it is difficult to accommodate them to various types of commercially available porcelain having varied rates of changes in heating.

(4) When models are prepared and removed from impression materials, it is most likely that they may be fractured due to a low green strength of the refractory model materials.

(5) When a finely divided porcelain slush is condensed during building-up, it is most likely that the surfaces of models may be damaged by a condensing instrument by reason of a strength after burning with nothing placed thereon (hereinafter referred to as a post-burning strength) being low. The conventional model materials are also short of reasonable durability.

(6) In some cases, the conventional model materials may cause surface roughening of impressions, failing to provide smooth model surfaces. Thus, some difficulty is involved in precise dental manipulations.

SUMMARY OF THE INVENTION

An object of the present invention is to obtain a dental refractory model material which eliminates the aforesaid disadvantages of conventionally available refractory materials, and provides a dental refractory model which does not undergo any sharp change upon heated due to a volume change owing to the crystal transformation of an aggregate component, describes a curve of changes in heating approximating to that of the porcelain used, is not virtually subjected to cracking during repeated burning, and has a green strength and a post-burning strength which are so high that it is unlikely for the porcelain to be fractured and abraded. This makes it possible for dental technicians to prepare dental porcelain restoratives such as porcelain laminated veneers, porcelain inlays or onlays and porcelain jacket crowns in a sound manner without any failure such as cracking.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is characterized in that the following means are taken so as to solve the aforesaid problems and attain the aforesaid object. More specifically, crystalline quartz or crystobalite that is a main component of the aggregate of the conventional refractory model materials is partly or entirely substituted by an inorganic material which is used as an aggregate component in powdery components and shows no volume change upon heated due to crystal transformation, thereby obtaining a composition which reduces a sharp changes in heating of a refractory model material and, hence, describes a substantially linear curve changes in heating. As the inorganic materials showing no volume change due to crystal transformation by heating when heated from normal temperature to 1,000 least one of inorganics such as alumina, zirconia, fused quartz, mullite, spinel and cordierite which are found to show no sharp volume change due to crystal transformation and describe a substantially linear curve of changes in heating. Contents of such inorganics short of 10% and exceeding 90% are impractical, since it is difficult to limit a sharp volume change due to the crystal transformation of crystalline quartz and cristobalite in an amount of below 10%, whereas there is a lowering of strength in an amount of higher than 90% because the amount of magnesium oxide and a soluble phosphate to be mixed, which are main components of a binder, are correspondingly decreased. Thus, the content of the inorganic materials showing no volume change upon heated due to crystal transformation is limited to a range of 10% to 90%.

The binder serves to improve the green and post-burning strength of dental refractory model materials. However, a refractory model material having a magnesium oxide content of 5% or less is so poor in the green strength that it is likely to be fractured, when removed from an impression material. When the content of magnesium oxide to be mixed exceeds 20%, on the other hand, certain improvements in the green and post-burning strength are only achieved in spite of rises in the cost incurred. Thus, the content of magnesium oxide is limited to a range of 5% to 20%. Use of the soluble phosphate by 5% or less makes no attribution to increases in the green strength of a refractory model material, and is thus likely to be fractured, when removed from an impression material, whereas the use of the soluble phosphate by 20% or more gives rise to increases in the amount of contraction of a dental model after burned with nothing placed thereon, which thus becomes impractical. Accordingly, the content of the soluble phosphate is limited to a range of 5% to 20%.

A colloidal silica dispersion used as a blending liquid serves to improve the strength of a dental refractory model material, regulate the curing expansion thereof and make up for the burning contraction thereof during burning with nothing placed thereon. Thus, it is possible to adjust the strength and curing expansion of a dental refractory model material by varying the silica concentration of the colloidal silica dispersion. When the silica concentration is 10% or less, however, improvements in both strength and curing expansion are so reduced that a refractory model material may possibly be fractured, when removed from an impression material, and the burning contraction thereof during burning with nothing placed thereon cannot be made up for. When the silica concentration exceeds 40%, on the other hand, improvments in strength are not achieved probably because of the gelling of the colloidal silica dispersion occurring preferentially and there is also a rise in the cost. Hence, the silica concentration is limited to 10% at the minimum and 40% at the maximum, preferably to a range of 20% to 35%.

The following actions are attained by the means as mentioned above.

 In the refractory model materials according to the present invention, crystalline quartz and crystobalite that are the aggregate component of conventional refractory model materials are replaced by varied amounts of at least one of the inorganic materials selected from the group consisting of alumina, zirconia, fused quartz, mullite, spinel, cordierite, phorstelite, stellerite, silicon carbide, silicoitride, calcia and titanium oxide to reduce or substantially avoid a volume change attributable to the crystal transformation of crystalline quartz and crystobalite. Thus, there is no fear that the refractory model material may be cracked by repeated burning.

It is also very unlikely that when porcelain is built upon and burned on the refractory model material, the porcelain may be cracked due to a sharp volume change thereof.

The rate of changes in heating of the refractory model material after burning with nothing placed thereon can arbitrarily be varied by the proportion of the incorporated inorganic materials showing no volume change upon heated due to crystal transformation. Further, since alumina, zirconia, fused quartz, mullite, spinel, cordierite, phorstelite, stellerite, silicon carbide, silicoitride, calcia and titanium oxide, which are inorganic materials describing a substantially linear curve of changes in heating, are different from one another in the quantities of changes in heating, it is also possible to arbitrarily vary the rate of changes in heating of the refractory model material by varying the type of such inorganic materials. Thus, the refractory model material can be accommodated to commercially available porcelain products having various rates of changes in heating.

Of the aforesaid inorganic materials, alumina and zirconia have specific weights larger than those of crystallite quartz and crystobalite. Thus, when the refractory model material is kneaded into a slurried state, the proportion of the liquid component to be kneaded with the powdery component can be decreased, whereby the green strength of the refractory model material can be increased with no fear of fracturing, when a model is prepared and then removed from an impression material.

The post-burning strength of the refractory model material can also be so increased that, when condensing is effected during the building-up of procelain, it is unlikely that a model may be damaged on its surface by a condensing instrument. The refractory model material can also be so increased in durability that it undergoes little or no fracturing and abrading.

Since any surface roughening of an impression material can be substantially avoided by decreasing the proportion of the liquid component to be kneaded with the powdery component, it is possible to obtain a model surface so smooth that precise dental manipulations can be performed.

EXAMPLES

The present invention will now be explained in more detail with reference to the following examples which are given for the purpose of illustration alone and without any intention of limiting the present invention.

In the examples and comparative examples to follow, the powdery components used were weighed in the proportions as specified in a table to be given later, mixed together in a blender for 20 minutes, and were thereafter passed through an 100-mesh sieve.

In the preparation of samples, the proportion of the liquid component to be blended with the powdery component was determined by kneading 100 g of a powdery component sample with varied amounts of water in a room regulated to a temperature of 20 rpm for 60 seconds according to JIS T 6601 [Investment Materials for Dental Casting] with the use of a vaccum kneader ordinarily used for the kneading of dental investment materials, filling the thus kneaded refractory model material slush into a cylindrical mold of a metal and of 28 mm in inner diameter and 50 mm in height placed on a glass plate, gradually pulling up the mold two minutes after the initiation of kneading while leaving the refractory model material slush alone and measuring the maximum and minimum diameters of a portion of the refractory model material in contact with the glass plate in further one minute. Said determination of the proportion of the liquid component to be blended with the powdery component was made on the basis of the amount of water to be mixed which is defined by the standard consistency, as expressed in terms of an averaged measurement of 55 to 60 mm.

Crushing strength testing was performed according to the crushing strength test method of JIS T 6601. That is, a sample kneaded to the standard consistency was filled in a cylindrical mold of a metal and of 30 mm in inner diameter and 60 mm in height, in which it was cured to a degree sufficient to resist to handling. Afterwards, the sample was removed from within the mold, and was then allowed to stand at room temperature. Twenty-four (24) hours after the initiation of kneading, the sample was compressed at a compression rate of 1 mm/min. according to the compression testing method to obtain a crushing strength value of the refractory model material.

For the determination of the post-burning strength, a sample was prepared in a similar manner as mentioned above. The sample was subsequently heated from 700 C./min. in a dental electric furnace, maintained at that temperature for 10 minutes, cooled down to room temperature, and was finally tested according to the compression testing method.

For the determination of the rate of curing expansion, a sample kneaded to the standard consistency was put upon a wax paper laid down on the inner surface of a metal tray, as provided by the expansion-upon-solidification testing of JIS T 6601. The sample was flattened on the surface and provided with metal foil marks at an interval of 50 mm. Two (2) minutes after the initiation of kneading, a distance between the marks was measured and, 30 minutes after the initiation of kneading, that distance was again measured to determine the rate of expansion with respect to the original distance measurement. The measurement of the mark to mark distance was measured with a measuring machine having a precision of 1/100 mm or higher.

For the determination of the rate of changes in heating, a sample kneaded to the standard consistency was filled in a metallic cylindrical mold of 10 mm in inner diameter and 50 mm in height according to the thermal expansion testing of JIS T 6601, in which it was cured to a degree sufficient to resist to handling. The sample was removed from within the mold and, 1 hour after the initiation of kneading, it was heated from 700 electric furnace, maintained at 1000 cooled down to room temperature. Thereafter, the temperature of the furnace was increased by a measuring apparatus of fused quartz to about 1000 100 with respect to the original length.

Whether the refractory model material was cracked or not was measured in the following manner. A sample prepared in a similar manner as described in connection with the crushing strength testing was immersed and held in water until the sample did not give off air bubbles. The sample was sufficiently dried in front of an inlet of a dental electric furnace, in which it was then heated from 700 50 and was cooled down to room temperature. After this cylce was repeated five times, the sample was visually observed in terms of whether it was cracked or not.

For the determination of whether porcelain was cracked or not, a sample prepared in a similar manner as described in connection with the crushing strength testing was heated from 700 50 and was cooled down to room temperature. Afterwards, the sample was immersed in water and allowed to stand therein until it did not give off air bubbles. A slush of finely divided porcelain was built upon a side of the sample in the form of a layer of about 10 mm which was sufficiently dried in front of an inlet of a dental electric furnace, heated therein from 700 50 920 temperature. After this cycle was repeated three times, the sample was visually evaluated in terms of whether or not the porcelain was cracked.

For the determination of the surface smoothness of the model, a sample used for the cracking test of the refractory model material was used with a contact type surface-roughness measuring machine, and was measured in terms of an averaged value of ten points.