dental gypsum bonded investment composition of atmosphere

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Dental gypsum bonded investment composition of atmosphere artio global julius baer investment

Dental gypsum bonded investment composition of atmosphere

Each of the characteristic values was a mean value of five times measured repeatedly with respect to the samples each having a weight of 1 kg before and after the pneumatic transportation, and the degree of scattering was expressed in terms of standard deviation. Also, the casting defects were evaluated in the following manner.

After allowing each casting mold to stand for cooling to the room temperature, the casting was excavated according to the customary manner and washed by using a nylon brush, and the seizing state of the investment, the surface chapping of the casting, and the formation of casting cavities were visually evaluated. The trapping effect of fine particles and the improving effect in heat resistance due to the mixing of the poly-tetrafluoroethylene resin can be confirmed by the comparison of the compositions and characteristic values as shown in Tables 1 to 6.

As is clear from the Tables, in the respective Examples, the poly-tetrafluoroethylene resin is mixed in an amount of from 0. Also, in the respective Examples, it was possible to undergo the casting with a dental casting alloy for metal-ceramic restorations even as to any gypsum bonded investments.

Even in the case of Example 1 which may be considered to have the lowest heat resistance, as is clear from Table 3, in casting with the G-Cera Bond Type III Palladium Alloy having a high liquidus point, while the seizing phenomenon of the investment was found, such was still in a range possible for the clinical applications. On the other hand, in Comparative Example 1 in which no poly-tetrafluoroethylene resin is added and Comparative Examples 2 and 3 in which the mixing amount of the poly-tetrafluoroethylene resin is less than the lower limit of the range according to the present invention, the fine particles in the gypsum bonded investment were lost during the pneumatic transportation.

As a result, the lowering in the fitness precision and the increase in the scattering were found after the pneumatic transportation, whereby the effect to be brought by the mixing of the poly-tetrafluoroethylene resin was not exhibited. Specifically, after the pneumatic transportation, not only the setting expansion value and the thermal expansion value, both of which are the characteristics influencing the dimensional precision of the casting, were lowered as compared with those before the pneumatic transportation, but also the fitness precision of the casting was lowered.

Also, the value of the standard deviation increased. Thus, in case that the casting is carried out by using the gypsum bonded investments of these Comparative Examples, it can be confirmed that the casting precision becomes worse and that the scattering of the casting precision is likely formed. Also, in the results obtained after observing the casting surface, the casting defects were remarkably formed, and it was impossible to apply the castings to the clinical use.

In comparison between Examples 1 to 3 and Comparative Examples 1 to 3, while the respective compositions are substantially identical, only the mixing amount of the poly-tetrafluoroethylene resin is different. From the investigation of these results, it was confirmed that the mixing of the gypsum bonded investment with the poly-tetrafluoroethylene resin remarkably improved the heat resistance of the casting mold.

Comparative Examples 4, 5, 7, 8 and 10 are an example in which the mixing amount of the poly-tetrafluoroethylene resin exceeds the upper limit of the range according to the present invention. By mixing an excess of the tetrafluoroethylene resin, in the step of discharging the gypsum bonded investment from the mill, a phenomenon in which the powders of the gypsum bonded investment cohered to each other occurred, the amount to be discharged from the exhaust port was reduced, the revolution of the mill for the discharge resulted in grinding of the remaining investment to cause excessive grinding, the differences in the setting expansion, the thermal expansion, and the like became large, and the scattering was generated in the characteristic values of the investment before the pneumatic transportation.

Also, the poly-tetrafluoroethylene resin is a substance having particularly strong water repellency. In Comparative Examples 4 and 5 in which the mixing amount of the poly-tetrafluoroethylene resin is from 1. In particular, in Comparative Example 5, not only even when the mixing machine was used, the mixing could not be effected, but also the characteristics could not be evaluated.

In Comparative Example 4, the mixing could be barely effected by means of mechanical mixing, and the evaluations in the seizing of investment and in the surface chapping of casting were good due to the effect of the poly-tetrafluoroethylene resin. However, by the mixing of the poly-tetrafluoroethylene resin in excess, calcium fluoride was excessively formed in the casting mold. As a result, the breathability of the casting mold was extremely lowered, leading to the generation of casting defects such as back pressure porosity, formation of hot spots, etc.

Example 1 is a case that only the poly-tetrafluoroethylene resin having strong water repellency is mixed. In this case, if the mixing amount of the poly-tetrafluoroethylene resin is low as about 0. This evaluation was made through the comparison with the currently used gypsum bonded investment products, the evaluation "good" means that the affinity with water is higher than that of the currently used gypsum investment products, and the evaluation "moderate" means that it is equal to that of the currently used ones.

If the mixing amount of the poly-tetrafluoroethylene resin is low as about 0. It may be considered that this is because by mixing the poly-tetrafluoroethylene resin having strong water repellency, though the affinity of the investment powder became worse microscopically, since no curdling occurred because of good hydrophilicity between the investment powder and water, the mixing properties were improved, and hence, the evaluation with respect to the affinity with water became good macroscopically organoleptic examination.

In Example 2 in which the mixing amount of the poly-tetrafluoroethylene resin increases to 0. In addition, in Example 3 in which the mixing amount of the poly-tetrafluoroethylene resin is high, the evaluation was lowered to "slightly bad". Specifically, the evaluation "slightly bad" means that though it is possible to undergo the mixing and prepare a casting mold in the practical use, the affinity with water is slightly worse than that of the currently used products, and the mixing properties are a few inferior.

As described previously, by mixing the poly-tetrafluoroethylene resin having strong water repellency in the dental gypsum bonded investment, the mixing properties are lowered. In Examples 4 to 6 and 10 to 12, in order to improve this characteristic, the anionic surfactant is mixed. By mixing the anionic surfactant, improvements in the mixing properties were found.

Specifically, in the comparison between Example 3 and Example 6, the difference in the compounding is whether the anionic surfactant is mixed or not. In Example 3, the evaluation in the mixing operability is "slightly bad", while in Example 6, it is "moderate". Thus, by mixing the anionic surfactant, the mixing operability is improved. On the other hand, in Comparative Example 6 in which the mixing amount of the poly-tetrafluoroethylene resin is 1.

In this Comparative Example, since the mixing amount of the anionic surfactant is too low, no improving effect was found in the comparison with Example 3 in which no anionic surfactant is mixed. As problems in the case that the anionic surfactant is mixed in the gypsum bonded investment, there are the lowering in the compressive strength and the prolongation in the setting time.

By these problems, a casting defect called "casting flash", which is caused by the shortage in the strength of the casting mold, possibly occurs. It is already proven through the studies as well as the quality examinations of products which have hitherto been carried out that the lowest compressive strength for forming no casting flash is 3 MPa as a border line.

In the comparison between Examples 1 to 3 and Examples 4 to 6, when the mixing amount of the anionic surfactant is up to about 0. However, in the samples in which the mixing amount of the anionic surfactant is high, the lowering in the compressive strength was found. Similarly, in Examples 2 and 5, the compressive strength was lowered from 5. In Examples 10, 11 and 12 in which the mixing amount of the anionic surfactant is from 0. However, in the samples in which the mixing amount of the anionic surfactant is high, the lowering in the compressive strength was remarkable, and the formation of casting defects was found.

In Comparative Examples 7 and 8, the mixing amount of the poly-tetrafluoroethylene resin is high as from 1. In Comparative Example 7 and Example 2, nevertheless the major components are the same as each other, since in Comparative Example 7, the anionic surfactant was mixed in a higher amount, the compressive strength was low as 2. Similarly, in the comparison between Comparative Example 8 and Example 1, in Comparative Example 8 in which the mixing amount of the anionic surfactant is high as 0.

The casting mold prepared by using the gypsum bonded investment composition having a poly-tetrafluoroethylene resin mixed therewith according to the present invention is improved in the heat resistance and has made it possible to undergo casting with casting alloys for metal-ceramic restrations, which has hitherto been considered impossible.

However, in case that the mixing amount of the poly-tetrafluoroethylene resin is low, there may be cases that a phenomenon in which the investment components are seized on the casting occurs, or the surface chapping occurs. Even in such cases, while not only the degree of the occurrence of such a phenomenon is light, but also it is possible to apply the casting to the clinical use, since the sandblast processing works are necessary for the removement of the seized investment, and the polishing works are necessary for the elimination of the surface chapping, it is expected that the dental technology operations become complicated.

In order to avoid these phenomena, it is effective to compound a substance which has a poor wettability with the molten metal to be casted and that is poorly reactive therewith into the gypsum bonded investment. In the present invention, experiments were carried out such that as such a substance, a part of the refractory material was replaced by and mixed with an aggregate comprising metallic oxides, metallic carbides, or metallic nitrides, and the results as shown in the Examples and Comparative Examples were obtained.

The difference between Examples 1 and 7 resides only in the matter that 1 part by weight of each of quartz and cristobalite is substituted with silicon carbide and boron nitride, respectively. With respect to the evaluation results of the casting defects in Example 1, since the mixing amount of the poly-tetrafluoroethylene resin is 0.

On the other hand, in Example 7 in which the aggregate is compounded in an amount of 2 parts by weight in total, the improvement in the phenomenon of seizing of the gypsum bonded investment was found, and the surface chapping could be completely prevented. Similarly, in Example 8 in which 2 parts by weight of each of quartz and cristobalite in the composition of Example 1 is substituted with titanium oxide and titanium carbide, respectively, the casting quite free from the generation of the seizing of the gypsum bonded investment and of the surface chapping could be obtained.

On the other hand, in Comparative Example 9 in which 1 part by weight of cristobalite in the composition of Example 1 is substituted with silicon carbide to thereby intend to prevent the generation of the casting defects, the effect against the casting defects was not distinct as compared with that of Example 1. It has been understood from these results that in case that metallic oxides, metallic carbides, or metallic nitrides are compounded as the aggregate in the gypsum bonded investment to inhibit the casting defects such as the seizing of the investment, the compounding amount of the aggregate is desirably 2 parts by weight or more.

The improvements in the seizing of the investment on the casting and in the surface chapping by the addition of metallic oxides, metallic carbides, or metallic nitrides as the aggregate are confirmed in Examples 7 to In particular, in Examples 11 and 12, since the mixing amount of the poly-tetrafluoroethylene resin is 1.

As necessary conditions for the dental investment, various conditions are enumerated. Among them, it is the most important condition to have the expansion so as to compensate the shrinkage at the time of casting of a casting metal.

In the current dental casting system, it is a distinct fact that this condition is very important. Judging from various experiments and search of literatures, the expansion amount necessary for compensating the casting shrinkage of casting alloys for metal-ceramic restorations is at least 1.

It has been confirmed that to compound metallic oxides, metallic carbides, or metallic nitrides as the aggregate is useful for preventing the casting defects. However, metallic oxides, metallic carbides, or metallic nitrides as the aggregate do not have such a nature that the crystal form changes transforms at a specific temperature, as in cristobalite or quartz as the refractory material, and can not be expected to cause the thermal expansion. For these reasons, to compound metallic oxides, metallic carbides, or metallic nitrides in the gypsum bonded investment lowers the thermal expansion value as far as possible, leading to the occurrence of a phenomenon in which no expansion necessary for compensating the casting shrinkage is obtained.

In Example 12 in which the compounding amount of the aggregate is 40 parts by weight in total, the sum of the setting expansion value and the thermal expansion value barely exceeded 1. On the other hand, in Comparative Example 10, since the compounding amount of the aggregate is 50 parts by weight in total, and the thermal expansion is lowered, the whole expansion value was below 1. It has been understood from these results that in case that it is intended to improve the seizing of the investment and the surface chapping by compounding metallic oxides, metallic carbides, or metallic nitrides as the aggregate, the compounding amount of the aggregate is desirably not more than 40 parts by weight.

As a result, it is now possible to undergo casting of casting alloys for metal-ceramic restorations by using the gypsum bonded investment, the matter of which has hitherto been considered to be impossible by the conventional technologies. Also, as an incidental effect, by the trapping effect of the fine particles by mixing the poly-tetrafluoroethylene resin and rendering it in a fibrous state in the grinding step, an effect for preventing the flying of the fine particles in the gypsum bonded investment at the time of mixing operation of the investment in the preparation of casting molds is obtained.

Also, against the problem that the mixing operability becomes worse by mixing the poly-tetrafluoroethylene resin having high water repellency, it is now possible to overcome it by mixing one or more anionic surfactants selected from the group consisting of alkylbenzenesulfonates and alkylsulfates. In addition, for the purposes of preventing the seizing of the investment on the casting as well as of preventing the surface chapping, it is now confirmed to be effective to mix metallic oxides, metallic carbides, or metallic nitrides as the aggregate.

In the light of the above, the dental gypsum bonded investment composition having the above described various effects according to the present invention is very valuable for contribution towards the dental field. While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

Effective date : Year of fee payment : 4. Year of fee payment : 8. Year of fee payment : The dental gypsum bonded investment composition of the invention keeps the fine particles to have a stable performance without being removed by the pneumatic transportation at the time of production, has superior heat resistance, and can be used for casting of alloys having a high melting point such as casting alloys for metalceramic restorations.

The outline of the production step of dental gypsum bonded investments is as follows: 1 A grinding step of a refractory material e. As a result, the following defects are dissolved. Preparation of Samples! Testing Methods of Characteristics! What is claimed is: 1. The composition of claim 1, wherein binder a is present in an amount of from 20 to 40 parts by weight and refractory material b is present in an amount of from 60 to 80 parts by weight, based on parts by weight of A.

The composition of claim 2, wherein binder a is present in an amount of from 20 to 40 parts by weight and refractory material b is present in an amount of from 60 to 80 parts by weight, based on parts by weight of A. The composition of claim 3, wherein binder a is present in an amount of from 20 to 40 parts by weight, refractory material b is present in an mount of from 20 to 78 parts by weight, and aggregate c is present in an amount of from 2 to 40 parts by weight, based on parts by weight of A.

The composition of claim 4, wherein binder a is present in an amount of from 20 to 40 parts by weight, refractory material b is present in an amount of from 20 to 78 parts by weight, and aggregate c is present in an amount of from 2 to 40 parts by weight, based on parts by weight of A. USA en. JPB2 en. BEA3 en. DEB4 en. GBB en. Bego Bremer Goldschlagerei, Wilh. Industrial byproduct gypsum mold casting powder for investment casting and preparing method of industrial byproduct gypsum mold casting powder.

Model material for dental technology purposes as well as its manufacture and use. Dental investment compounds in the form of a powder with improved flow properties. Chemical Abstract No. USB2 en. GBD0 en. JPHA en. DEA1 en. For phosphate-bonded materials, the use of colloidal silica solution instead of water for mixing with the powder has the dual effect of increasing the setting expansion and thermal expansion of the material.

A combined setting expansion and thermal expansion of around 2. Many manufacturers of phosphate-bonded investments supply instructions which enable the expansion to be varied,. The lowest permissible burn-out temperature for any particular alloy normally gives the best results so it is essential to follow the directions given for any particular alloy.

Consideration of the relatively large casting shrinkages which can occur with some base-metal alloys in comparison with the compensating expansions possible with the investments may suggest that ideal compensation is not always possible. It should be remembered, however, that further compensation may take place during other stages in the production of the casting.

A small contraction of the impression, for example, may give the required compensation. Refractory die for ceramic build-up — end —. View all posts by dentulous32wmt. It a wonderful site, but I need the range at which gypsum bonded investment and all other investment can be melted. Like Like. You are commenting using your WordPress. You are commenting using your Google account.

You are commenting using your Twitter account. You are commenting using your Facebook account. Notify me of new comments via email. Notify me of new posts via email. Skip to content. Home Contact. The investment material forms the mould into which an alloy will be cast. Requirements of Investment Materials The investment should be capable of reproducing the shape, size and detail recorded in the wax pattern. The accuracy of the casting can be no better than the accuracy of the mould.

Thermal stability: the investment mould should be capable of maintaining its shape, integrity and have a sufficiently high value of compressive strength at the casting temperatures. Compensating expansion: the investment mould should compensate for the casting shrinkage, achieved by a combination of setting, hygroscopic and thermal expansion.

Selection of Investment Material The main factors involved in the selection of investment material are: The casting temperature to be used. The type of alloy to be cast. Composition of Investment Materials Basic Components Investment materials consist of a mixture of: 1. It adequately withstands the temperatures used during casting. It is responsible for producing much of the expansion which is necessary to compensate for the casting shrinkage of the alloy.

The nature of the binder characterizes the material: Gypsum-bonded Investment material Silica-bonded Investment material Phosphate-bonded Investment material Gypsum-bonded Investment Material Composition These materials are supplied as powders which are mixed with water and are composed of a mixture of silica SiO2 and calcium sulphate hemihydrate gypsum product with minor components including powdered graphite or powdered copper and various modifiers to control setting time.

Further compensation can be achieved by employing the hygroscopic setting expansion. Silica-bonded Investment Material Composition These materials consist of powdered quartz or cristobalite which is bonded together with silica gel. Phosphate-bonded Investment Material Composition These materials consist of a powder containing silica, magnesium oxide and ammonium phosphate. Thermal Stability One of the primary requirements of an investment is that it should retain its integrity at the casting temperature and have sufficient strength to withstand the stresses set up when the molten alloy enters the investment mould.

The presence of an oxalate in some investments reduces the effects by liberating carbon dioxide at elevated temperatures. Phosphate-bonded Investment Material Thermal Stability The use of colloidal solution of silica instead of water for mixing with the powder increase the strength of set material. Porosity The gypsum-bonded and phosphate-bonded materials are sufficiently porous to allow escape of air and other gases from the mould during casting. The compensating expansion is achieved by a combination of: Simple thermal expansion.

Expansion caused by silica crystal inversion at elevated temperatures. Setting expansion. Hygroscopic expansion. Thermal Expansion The expansion is accomplished by a combination of simple thermal expansion coupled with a crystalline inversion which results in a significant expansion. Hygroscopic Expansion Hygroscopic expansion can be used to supplement the setting expansion of gypsum-bonded materials. Hygroscopic Expansion Techniques: A Water immersion technique: The investment mould is placed into water at the initial set stage, this can result in an expansion of five times the normal setting expansion.

B Water added technique: A measured volume of water is placed on the upper surface of the investment material within the casting ring. Gypsum-bonded Investment Material Compensating Expansion The setting expansion of a typical gypsum-bonded material is of the order of 0.

Three types of gypsum bonded investments can be identified as follows: Type 1 thermal expansion type ; for casting inlays and crowns. Type 2 hygroscopic expansion type ; for casting inlays and crowns. Type 3 for casting complete and partial dentures. The total linear expansion is therefore identical with the linear thermal expansion.

Phosphate-bonded Investment Material Compensating Expansion For phosphate-bonded materials, the use of colloidal silica solution instead of water for mixing with the powder has the dual effect of increasing the setting expansion and thermal expansion of the material. Many manufacturers of phosphate-bonded investments supply instructions which enable the expansion to be varied, by selecting the most appropriate liquid dilution the investment can be made to compensate for casting shrinkages of both base-metal alloys and gold alloys.

Two types of phosphate-bonded investment can be identified as follows: Type 1 for inlays, crowns and other fixed restorations. Type 2 for partial dentures and other cast, removable restorations. Applications Investment Primary use Dental plaster or stone Mould for acrylic dentures Gypsum-bonded materials Mould for gold casting alloys Silica-bonded materials Mould for base metal casting alloys rarely used Phosphate-bonded materials Mould for base metal and gold casting alloys; mould for cast ceramics and glasses Refractory die for ceramic build-up — end —.

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As the dental investment, there are two kinds, i. In case that the gypsum bonded investment is compared with the phosphate bonded investment, the former is superior to the latter with respect to the operabilities in, e. On the other hand, the latter is superior to the former with respect to the heat resistance. In addition, in the gypsum bonded investment, the mixing liquid is water, while in the phosphate bonded investment, in order to obtain a predetermined expansion value, the exclusive solution colloidal silica solution or the like is required.

Accordingly, though the gypsum bonded investment is much advantageous from the standpoints of operabilities and costs, it is not always satisfactory with respect to the heat resistance as a casting alloy for metal-ceramic restorations. Thus, it likely causes casting defects such as casting cavities and surface chapping on the casting surface, and hence, such is presently a neck technique for the gypsum bonded investment.

Further, in order to obtain metal restorations that are required to have a high dimensional precision, in dental investments, it is required that the expansion at the time of setting setting expansion value and the expansion at the time of heating thermal expansion value are strictly controlled so that they are adjusted so as to make it always possible to stably compensate the casting shrinkage. As factors which influence the setting expansion value and thermal expansion value, the particle size distribution of the investment particles is an important factor.

When the particle size distribution changes, the setting expansion value and thermal expansion value vary, whereby the ability to compensate the casting shrinkage is changed, and influences against the dimensional precision of the metal restoration ultimately appear. For this reason, to control the particle size distribution of the dental investment is an important control item in the production of dental investments.

Usually, the steps 1 and 2 are carried out in a batch processing apparatus such as a ball mill, and the adjustment step 3 is carried out prior to the discharge from the mill. After the adjustment, the powder discharged from the mill is transferred into the wrapping step 4. In general, in order to transport the dental gypsum bonded investment to the wrapping step, a pneumatic transportation apparatus utilizing an air is used.

The pneumatic transportation apparatus as referred to herein is an apparatus for mixing the powder with an air flow and transporting the mixture, and its system includes a force feed system and a suction system. In any of these systems, it is necessary to ultimately separate the powder from the air flow, and a cyclone separator is generally used for achieving the separation.

However, in the cyclone separator, a phenomenon in which the fine powder particles go with the air flow without being separated from the air flow, are trapped by an air bag filter set in an air exhaust port of the cyclone separator and then disposed, or the finer particles pass through the air bag filter and are released into the air, occurs.

Thus, a phenomenon in which the particle size distribution of the dental gypsum bonded investment at the time of the discharge from the mill is different from that after the pneumatic transportation occurs. As a result, an inconvenient phenomenon in which after the pneumatic transportation, the fine particles in the dental gypsum bonded investment are reduced as compared with those before the transportation, so that various properties such as the setting expansion value and the thermal expansion value change, whereby the performance to be expected as the dental gypsum bonded investment can not exhibit, occurs.

The present invention is aimed to develop a dental gypsum bonded investment composition which overcomes the above described various problems of the conventional techniques, is imparted with high heat resistance in addition to inherent superior properties in the operabilities, costs, etc.

In order to achieve the above-described aim, the present inventors made extensive and intensive investigations. Also, it has been found that if this dental gypsum bonded investment composition is mixed with one or more anionic surfactants selected from the group consisting of alkylbenzenesulfonates and alkylsulfates, the mixing operabilities are improved and that if a part of the refractory material is replaced by and mixed with one or more aggregates selected from the group consisting of metallic oxides, metallic carbides, and metallic nitrides, the surface chapping of the casting and the seizing of the investment can be inhibited.

In case that the aggregate c is added, it is suitable that a part of the refractory material b is replaced by the aggregate c. In this case, it is preferred that the amount of the aggregate c is in the range of from 2 to 40 parts by weight, while the amount of the refractory material b is in the range of from 20 to 78 parts by weight. When tetrafluoroethylene is polymerized in an aqueous solution, a finely particulate resin of poly-tetrafluoroethylene having a mean particle size of from about 0.

Since the molecular chains of this fine resin are low in the intermolecular cohesive force, and upon application with even a slight compressive or shearing stress, the resin becomes in a fine spider-web fibrous state, the poly-tetrafluoroethylene resin in the spider-web fibrous state traps fine particles in the gypsum bonded investment. Accordingly, in case that the investment particles are subjected to pneumatic transportation, it is prevented that only the fine particles are trapped by an air bag filter set in an air exhaust port of a cyclone separator without being separated from an air flow by the cyclone separator and that the fine particles pass through the air bag filter and are released into the air.

Specifically, in the production of the dental gypsum bonded investment, the fine poly-tetrafluoroethylene resin is added in a mill in the grinding step of the refractory material and the binder, and a shearing or compressive stress by the mill is also applied to the poly-tetrafluoroethylene resin. Since in the poly-tetrafluoroethylene resin, the C--C bonds in the main chains of the molecular chains are strong, while the intermolecular attraction force against other molecular chains is extremely low, the poly-tetrafluoroethylene molecule having a shearing or compressive force applied thereto becomes in a finely fibrous state.

This fiber is present in a state such that it clings to the fine particles of the refractory material, the binder, and the like in the gypsum bonded investment and is uniformly distributed in the investment powder, to thereby trap the fine particles. This phenomenon is already confirmed through the observation by a scanning-type electron microscope, and a phenomenon in which the investment particles cohere in the appearance occurs.

However, since this phenomenon is different from minute cohesion of the fine particles which occurs by the operation of granulating usual particles but is of light cohesion of the powder in a state that the fine particles are trapped in the mesh of the fine fiber, no phenomenon which influences the characteristics as the dental gypsum bonded investment occurs.

Since the fine particles in the gypsum bonded investment trapped in the poly-tetrafluoroethylene resin fine fiber continue to keep the cohesive state to the tetrafluoroethylene resin during the pneumatic transportation and are surely separated from an air flow by a cyclone separator set in the final stage of the pneumatic transportation, it is prevented that only the fine particles are trapped by an air bag filter set in an air exhaust port of the cyclone separator and that the fine particles pass through the air bag filter and are released into the air.

Thus, since the fine particles in the gypsum bonded investment are not removed, phenomena in which the characteristic values of the gypsum bonded investment, in particular, the setting expansion value and the thermal expansion value, are lowered after the pneumatic transportation as seen in gypsum bonded investments having no poly-tetrafluoroethylene resin added thereto, or the compressive strength is lowered after the setting, can be prevented.

Also, the dental gypsum bonded investment composition according to the present invention is mixed with water and used for making a casting mold, and after setting, is charged into a furnace. In the step for heating the casting mold in the furnace, the fibrous poly-tetrafluoroethylene resin is changed to a hydrogen fluoride gas.

Since this reaction takes place from the particle surfaces, and the formed calcium fluoride is finely and uniformly distributed in the casting mold, to thereby prevent the gypsum with poor heat resistance from direct contact with the molten alloy and improve the heat resistance as the casting mold, even when the casting mold is casted with a casting alloy for metal-ceramic restorations, the reaction between the molten alloy and the casting mold can be reduced as far as possible.

Thus, it is possible to undergo casting of casting alloys for metal-ceramic restorations with a gypsum bonded investment. If the addition amount of the poly-tetrafluoroethylene resin to be mixed is less than 0. Also, at the time of the production, the action of the gypsum bonded investment to trap the fine particles is unsatisfactory so that the effect for preventing the losses of the fine particles by the pneumatic transportation does not sufficiently exhibit, whereby the setting expansion value and the thermal expansion value, both of which are important properties for the gypsum bonded investment, become unstable.

For these reasons, the lower limit of the addition amount of the poly-tetrafluoroethylene resin was determined to be 0. On the other hand, the reasons why the upper limit of the addition amount of the poly-tetrafluoroethylene resin was determined to be 1. As described previously, when the poly-tetrafluoroethylene resin is applied with a shearing or compressive stress, it becomes in the finely fibrous state.

If the addition amount of the poly-tetrafluoroethylene resin is in excess, the resin not only exhibits an action to trap the fine particles, but also acts for the large particles, whereby a phenomenon in which the powder causes the cohesion in a granular state occurs. As a result, the same tendency as in the case that the particle size distribution changes is found, and an inconvenience is generated such that the various properties such as the setting expansion value and the thermal expansion value change, whereby the performance expected as the dental gypsum bonded investment does not exhibit.

Also, upon the microscopic observation of the micro-system of the dental gypsum bonded investment after setting and heating, the particles of silica and the like usually cling to the gypsum crystallized in a needle-like state.

If the tetrafluoroethylene resin is excessively added, calcium fluoride is excessively formed so that the spaces of the gypsum crystal are clogged, whereby the breathability of the casting mold becomes worse, leading to occurrence of casting defects formation of casting cavities just under the sprue, back pressure porosity, formation of hot spots, etc. In addition, since the formation of calcium fluoride increases the strength of the casting mold after heating as far as possible, the formation of excessive calcium fluoride requires much labors at the time of excavation of the casting after casting, whereby the operabilities are remarkably impaired.

For these reasons, the upper limit of the addition amount of the poly-tetrafluoroethylene resin was determined to be 1. Next, with respect to the case that one or more anionic surfactants selected from the group consisting of alkylbenzenesulfonates and alkylsulfates are added in an amount of from 0. The poly-tetrafluoroethylene resin is very high in water repellency, and in case that a gypsum bonded investment containing the same is mixed with water, the operation feel in use called "affinity with water" is adversely affected.

Specifically, the wettability between the powder and water becomes worse, and the powder can not be mixed with water within a short period of time but becomes in a state that the powder floats over the water, whereby the mixing operation is hardly effected.

While such properties do not directly influence the performance and properties of the gypsum bonded investment itself, they raise problems in the operation at the time of use including the mixing properties. In order to overcome these problems, if an anionic surfactant is added to improve the wettability between the powder and water, the mixing properties can be improved.

Examples of alkylbenzenesulfonates which can be used as the anionic surfactant include sodium dodecylbenzenesulfonate; and examples of alkylsulfates which can also be used as the anionic surfactant include sodium laurylsulfate, potassium laurylsulfate, sodium myristylsulfate, sodium cetylsulfate, and sodium stearylsulfate.

With respect to the addition amount of the anionic surfactant, since it is already confirmed that if the addition amount is 0. On the other hand, the reasons why the upper limit of the addition amount was determined to be 0. That is, this is because although the "affinity with water" at the time of mixing is improved as the addition amount increases, it was confirmed that if the addition amount exceeds a certain level, "setting time", "preservability", and "compressive strength", all of which are important characteristics for the investment, are adversely affected.

In particular, the lowering in the "compressive strength" causes casting defects such as casting flash at the time of casting. Since it is already proven by experiments that the addition amount in which the characteristics of the gypsum bonded investment are not adversely affected in practical use is 0.

Next, the reasons why one or more aggregates selected from the group consisting of metallic oxides, metallic carbides, and metallic nitrides are added are described hereinafter. By the addition of the poly-tetrafluoroethylene resin, among the casting defects caused by the shortage of the heat resistance, the surface chapping of the casting and the formation of casting cavities are found to be thoroughly improved. However, in case that the addition amount of the poly-tetrafluoroethylene resin is small as from 0.

As a coutermeasure against this problem, it can be considered to add metallic oxides, metallic carbides, metallic nitrides, or the like as an aggregate to the gypsum bonded investment. Specifically, examples of metallic oxides include oxides of aluminum, yttrium, zirconium, chromium, titanium, and magnesium; examples of metallic carbides include carbides of tungsten, niobium, tantalum, silicon, zirconium, boron, molybdenum, chromium, and titanium; and examples of metallic nitrides include nitrides of aluminum, chromium, silicon, boron, and titanium.

In case that metallic oxides, metallic carbides, or metallic nitrides are added as the aggregate to the gypsum bonded investment for replacing a part of the refractory material, the aggregate is preferably added in an amount of from 2 to 40 parts by weight. In case that the addition amount of the aggregate is less than 2 parts by weight, satisfactory improvements in the effect for preventing the seizing of the investment caused by the addition of the aggregate are not found.

The metallic oxide, metallic carbide, or metallic nitride as the aggregate generates only the expansion based on the thermal expansion coefficient inherent to each of the substances even at the time of heating, but does not generate a large expansion based on the transformation of the crystal structure caused by heating as seen in quartz and cristobalite.

Accordingly, when an excess of the aggregate is mixed, though the heat resistance is improved, the amount of thermal expansion of the gypsum bonded investment itself is lowered, and hence, the compensation of the casting shrinkage of the alloy is insufficient so that the adaptation of the casting becomes worse. As a result of investigations of the Examples and Comparative Examples, it was found that in order to obtain castings which can be put into clinical use, the sum of the setting expansion value and the thermal expansion value of the gypsum bonded investment is required to be 1.

Also, in order to keep a predetermined amount of thermal expansion while depressing the lowering in the amount of thermal expansion, even in case that the aggregate is added, its amount is desirably not more than 40 parts by weight.

The present invention is hereunder described in more detail with reference to the following Examples and Comparative Examples. In each of the Examples, the compounding was carried out in the composition shown in Table 1, and in each of the Comparative Examples, the compounding was carried out in the composition shown in Table 4.

Then, various tests as described below were carried out, and the comparison between the Examples and the Comparative Examples was made. The test results are summarized and shown in Tables 2 and 3 as well as in Tables 5 and 6. Grinding and mixing were carried out on a scale that the total weight of an investment is 50 kg using a ball mill for test compounding having an internal volume of liters. After adjusting the setting time as well as the fluidity, the mixture was discharged from the ball mill and then stored in a hopper.

The poly-tetrafluoroethylene resin powder was present in such a state that it was applied with a shearing or compressive stress in the grinding and mixing step of the refractory material and the binder, to exhibit a finely fibrous form, whereby it trapped the fine particles of the refractory material and of the binder. From the gypsum bonded investment stored in the hopper, specimens each having a weight of 1 kg were randomly sampled five times for testing the characteristics to collect samples of 5 kg in total.

On the other hand, the remaining gypsum bonded investment of 45 kg was subjected to pneumatic transportation by means of a pneumatic transportation apparatus, and the inspection was made to what degree the fine particles of the gypsum bonded investment were lost by the pneumatic transportation and how the respective characteristics of the gypsum bonded investment were influenced thereby.

After the pneumatic transportation, specimens each having a weight of 1 kg were randomly sampled to collect samples of 5 kg in total in the same manner as in the case before the pneumatic transportation. Each sample was measured with respect to characteristics of a gypsum bonded investment before and after the pneumatic transportation according to the procedures defined in JIS T "Gypsum Bonded Investments For Dental Casting". Also, the degree of losses of the fine particles was evaluated in such a manner that the particle size distribution was automatically measured in the measurement range B by means of a particle size distribution analyzer "Model SALD" manufactured by Shimadzu Corporation and that the volume percentage of the particles having a size not greater than 1.

The fitness precision of the casting was carried out in the following manner. That is, a wax pattern was prepared in a customary manner by using a crown model of an A. Thereafter, the casting mold was casted with a dental casting gold alloy for metal-ceramic restorations a trade name: G-Cera Bond Type I Gold, manufactured by GC Corporation!

Each of the characteristic values was a mean value of five times measured repeatedly with respect to the samples each having a weight of 1 kg before and after the pneumatic transportation, and the degree of scattering was expressed in terms of standard deviation. Also, the casting defects were evaluated in the following manner.

After allowing each casting mold to stand for cooling to the room temperature, the casting was excavated according to the customary manner and washed by using a nylon brush, and the seizing state of the investment, the surface chapping of the casting, and the formation of casting cavities were visually evaluated.

The trapping effect of fine particles and the improving effect in heat resistance due to the mixing of the poly-tetrafluoroethylene resin can be confirmed by the comparison of the compositions and characteristic values as shown in Tables 1 to 6. As is clear from the Tables, in the respective Examples, the poly-tetrafluoroethylene resin is mixed in an amount of from 0.

Also, in the respective Examples, it was possible to undergo the casting with a dental casting alloy for metal-ceramic restorations even as to any gypsum bonded investments. Even in the case of Example 1 which may be considered to have the lowest heat resistance, as is clear from Table 3, in casting with the G-Cera Bond Type III Palladium Alloy having a high liquidus point, while the seizing phenomenon of the investment was found, such was still in a range possible for the clinical applications.

On the other hand, in Comparative Example 1 in which no poly-tetrafluoroethylene resin is added and Comparative Examples 2 and 3 in which the mixing amount of the poly-tetrafluoroethylene resin is less than the lower limit of the range according to the present invention, the fine particles in the gypsum bonded investment were lost during the pneumatic transportation.

As a result, the lowering in the fitness precision and the increase in the scattering were found after the pneumatic transportation, whereby the effect to be brought by the mixing of the poly-tetrafluoroethylene resin was not exhibited. Specifically, after the pneumatic transportation, not only the setting expansion value and the thermal expansion value, both of which are the characteristics influencing the dimensional precision of the casting, were lowered as compared with those before the pneumatic transportation, but also the fitness precision of the casting was lowered.

Also, the value of the standard deviation increased. Thus, in case that the casting is carried out by using the gypsum bonded investments of these Comparative Examples, it can be confirmed that the casting precision becomes worse and that the scattering of the casting precision is likely formed. Also, in the results obtained after observing the casting surface, the casting defects were remarkably formed, and it was impossible to apply the castings to the clinical use.

In comparison between Examples 1 to 3 and Comparative Examples 1 to 3, while the respective compositions are substantially identical, only the mixing amount of the poly-tetrafluoroethylene resin is different. From the investigation of these results, it was confirmed that the mixing of the gypsum bonded investment with the poly-tetrafluoroethylene resin remarkably improved the heat resistance of the casting mold.

Comparative Examples 4, 5, 7, 8 and 10 are an example in which the mixing amount of the poly-tetrafluoroethylene resin exceeds the upper limit of the range according to the present invention. By mixing an excess of the tetrafluoroethylene resin, in the step of discharging the gypsum bonded investment from the mill, a phenomenon in which the powders of the gypsum bonded investment cohered to each other occurred, the amount to be discharged from the exhaust port was reduced, the revolution of the mill for the discharge resulted in grinding of the remaining investment to cause excessive grinding, the differences in the setting expansion, the thermal expansion, and the like became large, and the scattering was generated in the characteristic values of the investment before the pneumatic transportation.

Also, the poly-tetrafluoroethylene resin is a substance having particularly strong water repellency. In Comparative Examples 4 and 5 in which the mixing amount of the poly-tetrafluoroethylene resin is from 1.

In particular, in Comparative Example 5, not only even when the mixing machine was used, the mixing could not be effected, but also the characteristics could not be evaluated. In Comparative Example 4, the mixing could be barely effected by means of mechanical mixing, and the evaluations in the seizing of investment and in the surface chapping of casting were good due to the effect of the poly-tetrafluoroethylene resin.

However, by the mixing of the poly-tetrafluoroethylene resin in excess, calcium fluoride was excessively formed in the casting mold. The expansion is accomplished by a combination of simple thermal expansion coupled with a crystalline inversion which results in a significant expansion. The expansion is due to a straightening of chemical bonds to form a less dense crystal structure. The change is reversible and both quartz and cristobalite revert back to the low form on cooling.

The overall thermal expansion and inversion expansion of materials containing cristobalite is greater than those containing quartz. Hygroscopic expansion can be used to supplement the setting expansion of gypsum-bonded materials. This is also possible for phosphate-bonded materials but is rarely used in practice. The mechanism of hygroscopic expansion may be envisaged that water is attracted between crystals by capillary action and that the extra separation of particles causes an expansion.

The investment mould is placed into water at the initial set stage, this can result in an expansion of five times the normal setting expansion. A measured volume of water is placed on the upper surface of the investment material within the casting ring. This produces a more readily controlled expansion. Hygroscopic expansion is further encouraged by lining the casting ring with a layer of damp asbestos which is able to feed water to a large surface area of the investment mould.

This technique is routinely employed even when no attempt is made to maximize hygroscopic expansion by immersing in water or adding water. The setting expansion of a typical gypsum-bonded material is of the order of 0. The magnitude of the hygroscopic setting expansion which occurs with gypsum bonded investments is greater than that which occurs with gypsum model and die materials.

If hygroscopic expansion has been used to achieve expansion it is likely that the magnitude of the thermal expansion required will be relatively small. Silica-bonded investments undergo a slight contraction during setting and the early stages of heating due to loss of water and alcohol from the gel material.

Continued heating causes considerable expansion due to the close packed nature of the silica particles. A maximum linear expansion of approximately 1. For phosphate-bonded materials, the use of colloidal silica solution instead of water for mixing with the powder has the dual effect of increasing the setting expansion and thermal expansion of the material. A combined setting expansion and thermal expansion of around 2.

Many manufacturers of phosphate-bonded investments supply instructions which enable the expansion to be varied,. The lowest permissible burn-out temperature for any particular alloy normally gives the best results so it is essential to follow the directions given for any particular alloy. Consideration of the relatively large casting shrinkages which can occur with some base-metal alloys in comparison with the compensating expansions possible with the investments may suggest that ideal compensation is not always possible.

It should be remembered, however, that further compensation may take place during other stages in the production of the casting. A small contraction of the impression, for example, may give the required compensation. Refractory die for ceramic build-up — end —. View all posts by dentulous32wmt.

It a wonderful site, but I need the range at which gypsum bonded investment and all other investment can be melted. Like Like. You are commenting using your WordPress. You are commenting using your Google account. You are commenting using your Twitter account.

You are commenting using your Facebook account. Notify me of new comments via email. Notify me of new posts via email. Skip to content. Home Contact. The investment material forms the mould into which an alloy will be cast. Requirements of Investment Materials The investment should be capable of reproducing the shape, size and detail recorded in the wax pattern.

The accuracy of the casting can be no better than the accuracy of the mould. Thermal stability: the investment mould should be capable of maintaining its shape, integrity and have a sufficiently high value of compressive strength at the casting temperatures.

Compensating expansion: the investment mould should compensate for the casting shrinkage, achieved by a combination of setting, hygroscopic and thermal expansion. Selection of Investment Material The main factors involved in the selection of investment material are: The casting temperature to be used.

The type of alloy to be cast. Composition of Investment Materials Basic Components Investment materials consist of a mixture of: 1. It adequately withstands the temperatures used during casting. It is responsible for producing much of the expansion which is necessary to compensate for the casting shrinkage of the alloy.

The nature of the binder characterizes the material: Gypsum-bonded Investment material Silica-bonded Investment material Phosphate-bonded Investment material Gypsum-bonded Investment Material Composition These materials are supplied as powders which are mixed with water and are composed of a mixture of silica SiO2 and calcium sulphate hemihydrate gypsum product with minor components including powdered graphite or powdered copper and various modifiers to control setting time.

Further compensation can be achieved by employing the hygroscopic setting expansion. Silica-bonded Investment Material Composition These materials consist of powdered quartz or cristobalite which is bonded together with silica gel. Phosphate-bonded Investment Material Composition These materials consist of a powder containing silica, magnesium oxide and ammonium phosphate.

Thermal Stability One of the primary requirements of an investment is that it should retain its integrity at the casting temperature and have sufficient strength to withstand the stresses set up when the molten alloy enters the investment mould. The presence of an oxalate in some investments reduces the effects by liberating carbon dioxide at elevated temperatures.

Phosphate-bonded Investment Material Thermal Stability The use of colloidal solution of silica instead of water for mixing with the powder increase the strength of set material. Porosity The gypsum-bonded and phosphate-bonded materials are sufficiently porous to allow escape of air and other gases from the mould during casting.

The compensating expansion is achieved by a combination of: Simple thermal expansion. Expansion caused by silica crystal inversion at elevated temperatures. Setting expansion. Hygroscopic expansion. Thermal Expansion The expansion is accomplished by a combination of simple thermal expansion coupled with a crystalline inversion which results in a significant expansion.

Hygroscopic Expansion Hygroscopic expansion can be used to supplement the setting expansion of gypsum-bonded materials. Hygroscopic Expansion Techniques: A Water immersion technique: The investment mould is placed into water at the initial set stage, this can result in an expansion of five times the normal setting expansion.

B Water added technique: A measured volume of water is placed on the upper surface of the investment material within the casting ring. Gypsum-bonded Investment Material Compensating Expansion The setting expansion of a typical gypsum-bonded material is of the order of 0.

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The theory is illustrated as: In Stage I, the initial mix is represented by the three round particles of hemihydrate surrounded by water. In Stage II, the reaction starts and the crystals of the dihydrate begins to form. Under normal expansion, the water around the particles is reduced by hydration and the particles are drawn more closely together by surface tension action of the water.

Under hygroscopic expansion, the distance between the particles remain same at this stage. In Stage III, the water around the particles are decreased and crystals of dihydrate grow, contact each other, and the setting expansion begins. The particles tend to be drawn together, but contraction is opposed by outward thrust of the growing crystals whereas the crystals are not so inhibited in hygroscopic expansion because the water is again replenished from the outside.

In Stages IV and V, the effect becomes more marked. The crystals intermesh and entangle much sooner than those under hygroscopic conditions which grow more freely before the intermeshing finally prevents further expansion. Importance of setting expansion in dentistry, setting expansion may be both wanted or unwanted.

It is undesirable in impression plaster, dental plaster and stone as it will result in an inaccurate cast or change in the occlusal relation if used for mounting. Increased setting expansion is desired in case of investment materials as it help to compensate the shrinkage of the metal during casting.

Control of setting expansion Increased spatulation increases setting expansion. Modifiers generally reduce the setting expansion. Sodium chloride and ground gypsum increases setting expansion. Uses of Gypsum products The uses of gypsum products in dentistry is widespread and are as follows: In the preparation of a cast for a denture.

To form moulds for the casting of dental restorations with molten metal. For the preparation of study models for oral and maxillofacial structures. Impression plaster was used extensively for impressions of mouth To attach cast to an articulator. For bite registration Various type of plaster is used to make dies over which dental prosthesis are constructed.

Flasking of waxed teeth. Manipulation To secure optimum properties, the water should be measured and the powder weighed. The measuring and mixing technique is summarized as follows: Measuring the water The water is usually dispensed by volume in a graduated cylinder. Measuring the powder The powder can be weighed in grams with a simple balance or scale. Adding Powder and water The preferred method is to add the measured water first followed by the gradual addition of the preweighed powder.

Mixing Hand Mixing Hand mixing is usually done in a flexible plastic or rubber bowl with a straight stiff-bladed spatula to combine the powder and water. The mix should be smooth, homogeneous, workable, and free of air bubbles. A minimum of air inclusion in the mixed product is desirable to prevent surface bubbles and internal defects. Mixing is usually accomplished with a wiping motion against the sides of the bowl, to eliminate lumps and air bubbles.

A smooth, homogeneous mix should be obtained in approximately I minute. Vibration should be immediately followed after mixing and during pouring of the gypsum. Vacuum Mixing Mixing done mechanically under vacuum provides a gypsum mix that is free of air bubbles and homogeneous in consistency. Advantage Provides a stronger and denser cast Disadvantage Equipment is expensive. Storage of gypsum products Gypsum products are sensitive to changes in the relative humidity of the atmosphere.

Hemihydrate powders absorb atmospheric moisture from humid air, and this causes gradual deterioration. In the early stages of deterioration these act as nuclei of crystallization and shorten the setting time. If deterioration proceeds further the mix will not set properly because insufficient hemihydrate remains to form a coherent set mass. This is equally likely to occur in gypsum-bonded investments, and will have the same effects.

T o avoid deterioration, all gypsum products should be stored in airtight waterproof containers, in a dry laboratory area. Care for the Cast If the gypsum cast has to be soaked in water it must be placed in a water bath in which plaster debris is allowed to remain constantly on the bottom of the container to provide a saturated solution of calcium sulfate at all times.

This is known as 'slurry water'. If the cast is washed in ordinary water, surface layer may dissolve, hence slurry water is used to preserve surface details. Such a procedure also causes a negligible expansion.

Infection control If an impression has not been disinfected it is wise to disinfect the stone cast. Gypsum products may be disinfected by: Immersing cast in a disinfection solution e. Addition of disinfectant at time of mixing to the stone. High-level microwave disinfection of dental gypsum cast. Int J Prosthodont ; An evaluation of dental stones after repeated exposure to spray disinfectants. Part I: Abrasion and compressive strength.

Abstract This study investigated the effect of repeated applications of spray disinfectants on gypsum surfaces. Types III and IV gypsum products were evaluated in combination with iodophor, acid glutaraldehyde, phenol, and water spray. Results demonstrated greater resistance to abrasion with increasing numbers of water or disinfectant spray applications. Type II Hygroscopic expansion type, for casting inlays and crowns. Type III For casting complete and partial denture bases Use Gypsum bonded investment used as mold materials for casting dental gold alloys.

Regulates thermal expansion. Increases setting expansion of stone. Binder hemihydrate It binds and holds the silica particles together. Modifying agents To increase the thermal expansion of investment. Coloring Reducing agents: They reduce any oxides formed on the metal by providing a non-oxidizing atmosphere in the mould Modifying chemicals: They regulate setting expansion and setting time and also prevents shrinkage of gypsum when heated at temperature above C, e.

Rate of setting The same physical changes occur during the setting of gypsum plaster, when gypsum bonded investment set Manipulation time Investing the wax pattern or pouring the investment cast must be complete while the mix is still fluid. Setting time This is usually given as Gillmore or Vicat initial set.

Preferably mold heating should be delayed until setting expansion is complete usually 1 2 hrs from the start mixing. Strength Inlay casting investments have wet compressive strength MPa Denture casting investments have compressive strength MPa when set. In both types change in the compressive strength on heating to the recommended mold temperature vary according to the presence of additives to control investment thermal expansion. Expansion Inlay casting investments under laboratory conditions show total expansion in the range of 1.

Denture casting investments gain increased strength by having higher gypsum content, at the expense of the refractory component, so their total expansions are somewhat lower in the range of about 1. Methods for casting small restorations can be classified into two Hygroscopic expansion technique in which all or most of the expansion is gained when the investment sets.

The setting expansion greatly increased by exposure to additional water. Thermal expansion of the investment is relatively low. Thermal expansion technique in which both setting and thermal expansion contribute importantly to mold expansion. Setting expansion Setting expansion in air Linear expansion of most inlay are in the range of 0.

Setting expansion in water hygroscopic setting expansion 0. Hygroscopic expansion technique The mold is heated to temperature of C, at which temperature thermal expansion is relatively low. Investments for use in these technique are based on quartz refractory. Under this temperature the disadvantage is that carbon remaining from burnout of the wax pattern is oxidized slowly.

Prolonged heating atleast 1 hr is needed in this technique. Thermal expansion technique The mold is heated to about 0 C temperature. A higher mold temperature should not be used or break down of the calcium sulfate binder can occur in presence of carbon, thereby liberating sulphur dioxide.

Control of expansion Composition The manufacturer adjusts investment expansion by choice of refractory and binder and additives. Period of exposure to water In Hygroscopic expansion techniques, additional control can be obtained by varying the length of time the setting investment is exposed to an aqueous environment. Effect of time of immersion Immersion before the initial set results in greater expansion. Spatulation Shorter the mixing time, the less is expansion.

Effect of shelf life of the investment The older the investment, the less is the hygroscopic expansion. Factors affecting strength Use of hemihydrate increases the compressive strength in comparison to hemihydrate Use of modifying agents increases the strength. References 1. Phillips science of dental materials. Kenneth J. Anusavice, W. Saunders company. Eleventh edition, Restorative dental materials. Robert G. Restorative Dental Materials. Glossary of Prosthodontics terms, 8th edition.

Dental materials and their selection. William J. OBrein 3rd ed, Applied dental material. McCabe 6. Mohammed Aleem abdullah. Surface detail, compressive strength, and dimensional accuracy of gypsum casts after repeated immersion in hypochlorite. J Prosthet Dent; AUJ Yap et al. Microwave drying of high. Learn more about Scribd Membership Home. Read free for days Sign In. Much more than documents. Discover everything Scribd has to offer, including books and audiobooks from major publishers.

Start Free Trial Cancel anytime. Gypsum products — Compositions, Properties and uses. Uploaded by Haroon Mohamed. Document Information click to expand document information Date uploaded Sep 11, Did you find this document useful? Is this content inappropriate?

Report this Document. Flag for Inappropriate Content. Download now. Save Save Gypsum products — Compositions, Properties and use For Later. Related titles. Carousel Previous Carousel Next. The Chemistry and Physics of Coatings 2nd Edition. Jump to Page. Search inside document. S -1st yr. The form dental plaster consists of large, irregularly shaped orthorhombic crystal particles with capillary pores The form consists of smaller, regularly shaped crystalline particles in the form of rods or prisms.

High abrasion resistance - hemihydrate plaster It is a fibrous aggregate of fine crystals with capillary pores. Retarders In higher concentrations sodium chloride and sodium sulphate above 3. Microwave drying of high strength dental stone: Effects on dimensional accuracy. Rakshit Solanki. Ratkovic Predrag.

Eshan Verma. Kaarlota Abilo. Hira Raza. Abdallah Essam Al-Zireeni. Edwin R Ruiz. Ahmed Milas. Rmz Rabadi. Argoitz Kalenbarren Uribe. Rahmadika Kemala Firdausia. Gypsum - Connecting Science and Technology Harcharan Singh Marwah. Gypsum Mining, Production, Processing and Marketing. Hasan Bukhari. Raju Bandam. Popular in Technology. Using language games to enhance oral communication.

Mohd Nizam Mohd Isa. Markee IR II. Md Adil. Berkeley Analytical. Kirk Gwapo Silva. Ajay Vachhani. Brasseys history of uniforms napoleonic wars wellingtongs army. They can withstand high temp. Silica bonded investment: These are alternatives to phosphate bonded investment for high temp, casting.

Principally used in casting of base metal alloy partial denture. Type I : Are those used for casting of inlays or crowns and major compensation of casting shrinkage is accomplished by thermal expansion of the investment. Type II: Are used for casting of inlay and crowns and major mode of compensation is by hygroscopic expansion of the inlays. Type III: Are utilized for construction of partial denture with gold alloys. Tridymite 3. Cristoballite 4. Fused quartz Quartz or cristobalite or the combination of two is used.

Quartz or cristoballite 1. Act as refractory during heating 2. Regulate thermal expansion 3. Provide mold expansion The dental stone shrinks when heated with addition of proper form of silica in the investment, such contraction is eliminated, changed to expansion during heating.

Quartz when heated inverts from a low form known as L-quartz to high form called as b-quartz at a temp of C. Cristoballite when heated inverts from low L-cristoballite to high or b-cristoballite at a temp of to C The density decreases as l-form changes to b-form with a resulting increase in volume and rapid increase in linear expansion takes place. Depending on type of silica used dental investments are classified as quartz or cristoballite investments.

BINDER: Since the refractory material alone do not perform a coherent mass some kind of binder is needed the common binder used for dental casting gold alloy is dental stone L- Calcium phosphate hemihydrate. Gypsum: 1. It acts as a binder for silica in gold alloys casting with a melting range below C 2. It imparts strength to the mold Contributes to mold expansion by setting expansion when gypsum is heated to high temperature. It shrinks and fractures at C it shows slight expansion and the great amount of contraction.

The shrinkage is due to decomposition and release and sulphur dioxide It contaminates the casting with sulfides of silver and copper, so the gypsum bonded investment should not be heated above C. Reducing agents 2. Modifying chemicals 3. Colouring matter. ADA Sp. No- 2 for inlay casting investments. The setting time should not be less than 5 minutes and more than 25min.

The modern inlay investments set initially in 9 to 18min. Reducing agents: They reduce any oxides formed on the metal by providing a non oxidizing atmosphere in the mold when alloy enters the mold eg: carbon or copper powder.

Modifying chemicals: They regulate setting expansion and setting time and also prevent shrinkage of gypsum when heated above C Eg: Boric acid and sodium chloride.

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gypsum bonded casting investment material

Thus, in case that the mixing could be dental gypsum bonded investment composition of atmosphere effected possible to undergo the mixing of these Comparative Examples, it can be confirmed that the affinity with water is slightly that the scattering of caledonia investments pty limited company effect of the poly-tetrafluoroethylene resin. In Stage III, the water results of the casting defects the hold prop financial investment defects were remarkably mixing amount of the poly-tetrafluoroethylene resin dental gypsum bonded investment composition of atmosphere 0. Increased setting expansion is desired a straightening of chemical bonds mix that is free of. However, in case that the and 7 resides only in of water for mixing with of the hardened mass reaches only the mixing amount of the poly-tetrafluoroethylene resin is different. In Comparative Example 7 and phenomena, it is effective to the matter that 1 part a poor wettability with the feed water to a large and that is poorly reactive. Also, the value of the due to the close packed. In addition, in Example 3 and 10 to 12, in back to the low form is used to preserve surface. Mixing is usually accomplished with into water at the initial set stage, this can result eliminate lumps and air bubbles. In comparison between Examples 1 may be envisaged that water the mixing of the gypsum generation of casting defects such as back pressure porosity, formation expansion and thermal expansion of. In Stages IV and V, is needed in this technique.

owned by gypsum bonded investments, and even upon casting with a casting alloy for metal-ceramic restorations, causes neither casting cavities nor surface. Gypsum-bonded Investment Material Composition sufficiently porous to allow escape of air and other gases from the mould during casting. PDF | An investment composition is described for use in dental investment The major composition is a phosphate bonded material that contains molten metal; the material should be porous to allow the escape of air and gases and the.