US3234607A - Method of forming an investment mold with potassium sulfate additive - Google Patents

Description

United States Patent 3,234,607 METHOD OF FORMING AN INVESTMENT MOLD WITH POTASSEUM SULFATE ADDITIVE Bengt G. Hammarlund, Alvsjo, Sweden, assignor to Bengt B. Edholm, Redondo Beach, Calif. N0 Drawing. Filed May 15, 1963, Ser. No. 280,704 4 Claims. (Cl. 22196) This invention relates to an investment composition and method of making an investment casting.

The composition of this invention and the novel method described is particularly utilized for the manufacture of refactory molds for casting metals such as gold alloy or other suitable casting material into inlays or other objects where the article cast must conform as precisely as possible in detail to the size and shape required, or the invention may be utilized wherever a precise investment casting is desired. It is particularly useful in a primary concern in the field of the manufacture of dental restorations to provide a suitable restoration for a tooth that has been damaged. A dentist normally proceeds to suitably shape the damaged tooth and then takes an impression thereof in a plastic material to provide a working model. On this model, a wax pattern is made which provides an exact replica of the required restoration. The wax pattern is used to form a mold which in turn is utilized to make a casting of a suitable metal alloy. This casting is used as the restoration. It is obvious that it is particularly desirable that the casting so provided fit the damaged tooth as precisely as possible and conform dimensi0n-wise to the tooth previous to damage thereof.

The procedure for making the precise metal alloy casting consists of first applying a warm wax to the working model. A small rod or sprue is inserted into the wax and then after chilling the wax pattern is removed by means of this sprue. The mold for the alloy is then formed by carefully placing a plastic material around the pattern. This material is generally known as the investment. This investment is then allowed to harden. The investment is then heated to burn out the Wax with the sprue having been removed. The investment is heated up to a temperature suitable for casting. And the metal in molten form is forced into the investment and allowed to cool forming the final restoration.

The metal normally used in most castings is a gold alloy which undergoes changes in volume with the change in temperature when passing either from the solid or liquid state.

The investment materials normally used comprise a binder material such as plaster of Paris and a filler matefial with various other inert or modified materials. The filler material is generally a form of silica such as ground silex and cristobalite, tridymite and vitreous silica all having the same chemical composition but differing widely in such properties as crystalline form, optical index of refraction, density or specific gravity and thermal expansion. The particular outstanding source for the above materials for making investments is ground silicia brick.

Prior to this invention the mold or investment was made, as previously mentioned, by forming the plastic material through mixing the binder and silica with water and utilizing this mixture to surround the wax casting. The mold is then allowed to set for a period of usually about 22 minutes in a water bath of about 104 F. This mold is then removed from the water and put into an oven which is initially at room temperature at the time the mold is inserted. The temperature is then raised gradually to 900 F.-1300 F. over approximately a two hour period. Previous experience has shown that it is essential to the production of a good mold to gradually raise the temperature of the oven to 1300 F. to avoid destruction of the mold by cracking or total disintegration. Thus, it can readily be appreciated, the necessity 3,234,607 Patented Feb. 15, 1966 for allowing the oven to gradually raise the temperature over approximately a two hour period, presents numerous problems to the individual preparing the investment. A given oven is not readily available throughout the working day for continuous use for it must be raised in temperature, as previously set forth, for a two hour period and then allowed to cool down again to room temperature before it is ready for use for another preparation for investment. As a result of this, the individual preparing the investments must plan his work so that he may insert several investments into the oven at one time, so as to most effectively utilize its capacity. Alternatively of course, numerous ovens may be utilized. However, the expense of doing so and of controlling this number is not a desirable solution. It should also be understood that the control of the rise in time to the desired temperature is critical and provides a possible stumbling block to the preparation of successful investments.

It is therefore an object of this invention to provide an investment composition and method of casting in which the residence time of the casting within the oven utilized is considerably less than that previously known.

It is another object of this invention to provide an investment casting method in which no gradual raising of the temperature of the oven utilized is required.

A still further object of this invention is to provide an investment composition and method of casting which provides for greater utilization of the oven needed in the process.

One further object of the invention is to provide an investment composition and casting method requiring a lower oven temperature and oven residence time than in previous methods and for previous compositions.

The above and other objects will become apparent from the following detailed description.

The investment composition of this invention comprises a silica material, a binder, and potassium sulfate solution sufiicient to make a paste suitable for casting. It has been discovered that the substitution of a potassium sulfate solution for the previously utilized water to form the casting base provides for the highly desirable properties of the novel investment composition described. The method for the preparation of a suitable investment utilizing this novel composition embraces a residence time of approximately 20 minutes in an oven maintained at about 1300 F., substantially reducing the set time previously known in the art.

It has been discovered that the potassium sulfate solution utilized should be at a particular concentration to accomplish the desired results. Thus, the process comprises the obtaining of such a solution. In order to do this, the potassium sulfate is dissolved in water, preferably distilled water, in a ratio of one gram of sulfate to 10 cc. of the water. The solution formed is shaken for a period of time to be sure that all of the sulfate has gone into solution. When it appears that the sulfate has completely gone into solution this solution is then diluted in the ratio of one part of the sulfate solution to nine parts of distilled water, for use in mixing with the solid other materials to form the paste for the investment. The solution of the potassium sulfate, is then mixed with the It has been found that l04 F. is a. particularly preferred temperature for this water bath. The material generally will not set up at temperatures in excess of 104 While at temperatures below this figure, one does not obtain the full expansion of the mold which is preferable for the desired results. Generally the period of time for the material to be submerged in the water bath ranges from two to seven minutes with the range from five to seven minutes being preferred. While successful investments have been made when the residence time in the bath is less than five minutes, these results have not been entirely consistent thus this preferred range of five to seven minutes is given.

After the material has set in the water bath, it is generally moved and placed in an oven which has a temperature not exceeding 1300 F. The investment is kept in the oven until it turns a dark red color which is due to a glow from the absorbed heat therein. This generally occurs in about fifteen to twenty minutes at the 1300 F. Though temperatures below 1300 can be utilized, such as for example 1100 F., the process would take considerably longer to transpire, thus it is not preferable though such may be accomplished. However, at temperatures much in excess of 1300 F. a sufii-cient investment cannot be successfully obtained because of improper expansion of the mold and possible breakdown of the investment material. The investment is then removed from the oven and the mold obtained is then utilized to form the restoration in a conventional manner.

As can now be seen from the novel composition and method of investment casting, the residence time of the mold in the water bath has been reduced from the state of the art 22 minutes to from 5 to 7 minutes. This water bath plays an important role in the process of preparing an investment casting in that it gives the neces sary preliminary expansion to the investment. This expansion is also known as hygroscopic expansion. Additionally, as can now be seen, through the utilization of the novel composition the casting residence time in the oven has been reduced from a previous two hours to approximately 15 to 20 minutes. The important factor of removing the need for slowly raising the temperature in the oven is apparent. As a result the oven or ovens utilized by an individual engaged in the practice of preparing such investments may all be maintained at 1300 F. constantly; thus the production of such investments may be continuous throughout the working day or week. No longer are so called batch processes for the preparation of these investments needed. This should substantially increase the capacity of one engaged in this practice.

To specifically illustrate by way of example, an investment prepared according to this invention, one gram of potassium sulfate was placed in 10 cc. of distilled Water in a normal test tube which was subjected to shaking until the potassium sulfate granules had disappeared and completely gone into solution; this solution was then diluted by dissolving 10 c. thereof in 9 cc. of distilled water. The solution was then added to a mixture comprised of 15 grams of plaster of Paris, 25 grams of cristobalite and 10 grams of tridymite. 15 cc. of the solution was stirred with the powder in a glass mixer until a smooth paste consistency was obtained. This paste was then placed around a wax pattern so as to obtain a suitable mold for casting. The paste and the Wax was then completely submerged from to 7 minutes in a water bath maintained at 104 F. Upon removal from the bath at the given time the material was then placed in an oven which had a temperature of 1300 The material was kept in the oven for 15 to 20 minutes at which time it turned a dark glowing red. After removal from the oven the investment was ready for utilization to form a restoration.

The potassium sulfate. solution of the invention may be utilized with various different binders as previously described, the binders not forming a novel part of the invention. In addition to the plaster of Paris previously set forth, other binders may be Keenes cement, Portlandv 4 cement, oxy salts cements, magnesium oxide cements, phosphoric acid cements, sodium silicate cements, and the like. Though, generally; plaster of Paris is to be preferred as the binder material. Usually the binder is present in amounts over 20% by weight of the powder composition.

While this invention has been described and illustrated in detail, it is to be clearly understood that this is intended by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the invention being limited only by the terms of the following claims.

I claim:

1. A method of preparing an investment casting of a pattern comprising forming an aqueous solution of potassium sulfate in the proportion of 1 gram of sulfate to 10 cc. of water,

diluting said aqueous sulfate solution by adding 9 parts of water to 1 part of said solution,

mixing silica and a binder which will set in the presence of Warm water with sufficient potassium sulfate solution to create a paste,

forming said paste about said pattern to provide said investment,

providing a water bath,

heating said bath to a temperature sufficient to cause said investment to set when submerged therein, submerging said investment in said bath,

maintaining said investment in said bath for a suflicient period of time to cause said investment to preset, removing said investment from said bath,

providing an oven,

heating said oven to a temperature sutficient to cause said investment to completely set,

placing said investment in said oven,

removing said investment from said oven when said investment turns to a dark red color.

2. The method of preparing an investment casting comprising forming an aqueous solution of potassium sulfate in the proportion of 1 gram of sulfate to 10 cc. of water,

diluting said aqueous sulfate solution by adding 9 parts of water to 1 part of said solution, mixing silica and a binder which will set in the presence of warm water with a sufficient amount of said diluted potassium sulfate solution to create a paste,

forming said paste about a pattern providing an investment,

submerging said investment in a water bath heated to a temperature sufficient to provide presetting of said investment,

subsequently placing said investment in a heated oven until said investment turns a dark red color.

3. The method of claim 2 wherein said water bath is maintained at a temperature not exceeding 104 F.

4. The method of claim 2 wherein said oven is maintained at a temperature not exceeding 1300 F.

References Cited by the Examiner UNITED STATES PATENTS 1,932,202 10/1933 Coleman et al 106-383 2,054,894 9/1936 Busch et al. 22l90.1 2,391,855 12/1945 Bean 22-193 2,513,212 6/1950 Saives 22193 2,529,835 11/1950 Dailey et al 22-193 FOREIGN PATENTS 164,669 10/1933 Germany.

OTHER REFERENCES Mellor, Modern Inorganic Chemistry, published 1939, pp. 15 O1 5 3.

MARCUS U. LYONS, Primary Examiner.

Claims (1)

1. A METHOD OF PREPARING AN INVESTMENT CASTING OF A PATTERN COMPRISING FORMING AN AQUEOUS SOLUTION OF POTASSIUM SULFATE IN THE PROPORTION OF 1 GRAM OF SULFATE TO 10 CC. OF WATER, DILUTING SAID AQUEOUS SULFATE SOLUTION BY ADDING 9 PARTS OF WATER TO 1 PART OF SAID SOLUTION, MIXING SILICA AND A BINDER WHICH WILL SET IN THE PRESENCE OF WARM WATER WITH SUFFICIENT POTASSIUM SULFATE SOLUTION TO CREATE A PASTE, FORMING SAID PASTE ABOUT SAID PATTERN TO PROVIDE SAID INVESTMENT, PROVIDING A WATER BATH, HEATING SAID BATH TO A TEMPERATURE SUFFICIENT TO CAUSE SAID INVESTMENT TO SET WHEN SUBMERGEN THEREIN, SUBMERGING SAID INVESTMENT IN SAID BATH, MAINTAINING SAID INVESTMENT IN SAID BATH FOR A SUFFICIENT PERIOD OF TIME TO CAUSE SAID INVESTMENT TO PRESET, REMOVING SAID INVESTMENT FROM SAID BATH PROVIDING AN OVEN, HEATING SAID OVEN TO A TEMPERATURE SUFFICIENT TO CAUSE SAID INVESTMENT TO COMPLETELY SET, PLACING SAID INVESTMENT IN SAID OVEN, REMOVING SAID INVESTMENT FROM SAID OVEN WHEN SAID INVESTMENT TURNS TO A DARK RED COLOR.