US2247588A - Investment composition - Google Patents

Description

PE/r CENT THERMAL EXPANSION July 1, 1941.

R. NEIMAN INVESTMENT COMPOSITION Filed April 9, 19 41 TEMPERATURE IN VEN TOR. ROBERT NE MAN Patented July 1, 1941 INVESTMENT COMPOSITION Robert Neiman,

Louisville, Ky.,

assignor to Edmund A. Steinbock, Louisville, Ky. Application April 9, 1941, Serial No. 387,704 (Cl. 22-188) 2 Claims.

This invention relates to investment compositions, that is, refractory compositions utilized in the making of molds for the casting of metallic objects which must conform as nearly as possible to a predetermined size, shape, and detail.

As an example of the use which would be made of this material reference is made to a dental casting, or filling, which is cast of a gold alloy or the like. As will be appreciated, this dental casting, or the like, must conform very accurately to the form and dimension of the cavity to be filled.

In the reconstruction of a tooth or a group of teeth, the dentist prepares the cavity in a skillful manner. fills this cavity with warm wax, carves wax down to desired shape, removes same, and attaches a sprue. This wax is then completely encased in a cementitious mass of a mixtureof investment with water. Upon setting to a hard mass, the practitioner proceeds to remove the sprue and places the mold in an oven or furnace. As this mold is heated the wax melts and then volatilizes, and upon continued heating the wax residues of carbon are gradually decomposed and volatilized, the latter process requiring a temperature in the neighborhood of 1100 F. Be-

tween this temperature and 1300 F. is the generally desired mold temperature for good casting. Upon reaching this temperature the mold is removed from the furnace and the alloy is cast into the cavity left by the disappearing wax pattern.

In this casting process the exact size of the desired inlay or thelike is not produced, due to the casting shrinkage of the alloy in solidifying and cooling to the solid state. This shrinkage is approximately 1.25% (linear) and obviously must be counteracted by a similar degree of expansion of the mold, on heating, in order to reproduce a casting of a predetermined dimension. Due to various conditions and modes of preparing the wax pattern the exact degree of mold expansion varies within the limits of 1% to 1.65%. The ad-' ditional expansion is achieved when the mold expands on setting in amounts varying from .1% to .6% depending on its composition. Best practice demands investments with thermal expansions of slightly better than 1% to approximately 1.25%.

From the above dissertation and from a practical knowledge of the subject there may be formulated a list of properties and niceties which the profession has long desired in a commercial investment. An investment meeting these desirable and necessary properties will give the skilled practitioner a simple, and sure method of making artistic and very accurate castings.

The investment should:

First-Have a thermal expansion (room temperature to approximately 1300 F.) in the neighborhood of 1.25% for a mix of painting or pouring consistency.

Second-Attain nearly all of this expansion at 1150 F. with very little additional expansion to 1300 F.

ThirdHave practically no variation in therand readily applied to every nook and crevice of the wax pattern and will not fall away from the wax due to its slippery nature.

Sixth-Possess enough setting expansion to make an accurate reproduction (about .1%) and then an additional amount of expansion to aid in compensating for shrinkages of wax pattern, when using the direct method, of .1 to 25%.

SeventhSet rapidly to a hard cementitious mass which will withstand high temperatures and rapid heating without cracking or distortion. I

Eighth-Retain this hardness for some time so that it may be used and handled, even the next day, if necessary, without undue danger of easy fracture of delicate parts.

NinthPossess adequate strength when hot to safely resist the sudden inrush of molten alloy when cast under usual pressure; and preferably to possess enough strength after quenching in water, to permit easy removal in toto rather than to disintegrate and leave a tenaciously adhering film on the casting.

TenthProvide a mold physically very smooth, and of such chemical makeup as to prevent undue oxidation and sulfurization of the embedded alby, and thus produce a smooth, dense, casting without discoloration.

Eleventh-Not change materially on standing or shipping due to the settling out. of unduly heavy constituents such as powdered heavy metals, necessitating packing under pressure. Also possessing constituents of a similar chemical and physical nature to give a homogeneous mass 'meets many of the above specifications.

when mixed with water and also to withstand storage under usual conditions without undue disintegration and separation of ingredients.

The principal ingredients of, what may be called simple investment compositions, as heretofore used, has been a mixture of plaster, known commercially as plaster of Paris, gypsum, stucco, hydrocal, alpha gypsum. and chemically as calcium sulfate (CaSO4. /2H2O) and a refractory filler or aggregate; The plaster in its usual form of the hemihydrate (CaSO4. /2HzO) or any other of the forms of calcium sulfate, hydrated or dehydrated, which set with water or watery solutions to a hard mass, generally acts as the binder. The filler or aggregate consists of any refractory material which aids the mold in withstanding the high temperatures. It is desirable to use a filler which will also aid in giving a high thermal expansion. The most desirable filler is silica in its three principal modifications; quartz, tridymite, and cristobalite. Quartz is the cheapest and most universally used. Obviously any other filler may be used that will contribute the desired properties, such as aluminum oxide, chromite, magnesium oxide, and other oxides as well as silicates, clays, and the like. To give the desired setting and hardening properties accelerators and retarders of a well-known and commercial nature are added. Small amounts of graphite, and sometimes a little (fraction of a percent) clay, or the like are added to aid in producing a smoother mold and thereby smoother castings.

It is well known that mixtures of plaster and silica will not give the desired thermal expansion nor satisfy many of the other specified requirements. In the endeavor to meet the desired specifications various ingredients have been added to the plaster and silica mixtures with varying degrees of success. Cristobalite, for example, when added in large amounts (50 to 70%) gives a material a high thermal expansion and This composition does not, however, meet, or meets but partially, the third, fourth, fifth, seventh, ninth, and tenth requirements. In a composition in which sodium (potassium or lithium) chloride is added the eighth, ninth, tenth and eleventh requirements are lacking. Metallic ingredients when added fail to meet the second, third, tenth and eleventh requirements. Boric acid has also been added to the basic composition but is required in good amounts such as 3 to 5% in order to give adequate expansion, but the composition is still lacking in the third, fifth, and

-tion should be readily apparent by reference to the following specification considered in conjunction with the accompanying drawing forming a part thereof, and it is understood that any modification may be made within the scope of the appended claims, without departing from or exceeding the spirit of the invention.

This application is a continuation in part of my application Serial No. 62,801, filed February '7, 1936, for Improvements in investments.

The drawing depicts a pair of graphs showing the expansion of an investment compound when heated under identical conditions, mixed with same proportion of water, and which compounds are respectively with and without one of the ingredients which may be added in the pursuance of my invention,

Simple investment compositions, as above not- I ed, are well known and have been used for many years. The general composition of these materials is from 20 to 50% plaster and to 50% silica. The increase in silica content produces an increase in expansion but a decrease in strength. The proportions may be varied even beyond these limits depending upon the exact technic used in producing the castings.

In the drawing the reference numeral Ill indicates the thermal expansion (and contraction) of a mixture of 44% plaster with 56% quartz and indicates a thermal expansion of .64% between room temperature (70 F.) and 1300 F.

This investment was made with a water to powder ratio (W/P) of .26, that is 26 grams of water to grams of the powder mixture. An increase of the W/P. causes a material decrease in the thermal expansion of this type of mixture.

It has been found that by the addition to the above simple investment compositions of small amounts of the chlorides of the alkaline earth metals, known in chemistry as the metals of the fourth group in qualitative analysis (Group IV) or the ammonium carbonate group, and including the chlorides or calcium, barium, strontium, and magnesium, that the expansion quality of the investment will be materially increased. As shown by the second curve in the graph and indicated by the reference numeral I I, the addition of 1.0% of any one of these chlorides for 1.0% of the quartz and maintaining the W/P- ratio constant at .26, will cause an expansion of this investment of 1.1%. Even small amounts of these chlorides will cause an appreciable increase in thermal expansion which continues to increase as the amount is increased. The upper limit of the amount of chlorides is perhaps 5% and even more depending upon the amount of plaster used and upon the desired expansion and strength.

. cific amount as this depends upon the other ingredients, but a very good product can be made by the inclusion of 1% as shown by the graph. An increase in expansion will result if the silica content is increased and vice versa. Variation in the W/P ratio, however, no longer afl'ects the thermal expansion so markedly as heretofore and will enable the practitioner who does not take advantage of the modern measuring devices to achieve a great measure of success nevertheless.

The addition of any of the chlorides of the alkaline earth metals will cause a greater increase in the thermal expansion of the simple investment compositions above noted, than the addition of a similar amount of any other substance heretofore known. Also these chlorides when added to the simple investment compositions will produce castings of greater smoothness and cleaner surfaces than heretofore possible. Similar results are noted if these chlorides are dissolved in the'water that is added to the usual simple investment compositions contioned chlorides in such a cristobalite investment,

the expansion of the investment may be raised to as high as 2% from room temperature of [300 F. Such an investment material is within the scope of my invention. Thus, it is within the scope of my invention to provide an investment composition containing from to 50% plaster and from 80 to 50% cristobalite.

While such an investment material may be desirable under certain conditions, it has some drawbacks, such as an unduly abrupt expansion upon subjection to the temperatures indicated and such as the necessity of using an unduly large amount of water for the mix with a consequent weakness of a mold made therefrom. Likewise, such an investment composition is relatively costly. However, I have found that by substituting certain amounts of quartz for the cristobalite, the abrupt expansion may be largely obviated, the required amount of water materially decreased and, at the same time, the cost of the investment composition may be materially reduced. Likewise, the resulting composition will give an increased mold strength.

It should be remembered that cristobalite has an inherent expansion quality of about 2%, quartz about 1.5% and trydimite about 1.3%, all from room temperature to about 1300 F. Therefore, other conditions being equal, the greater the percentage of cristobalite present, the greater will be the expansion due to the filler. Most any combination of these forms of silica may be used and it is not entirely necessary that these materials be used as distinct entities, since the grains of the siliceous material may contain a mixture of one or more of these forms of silica,

depending upon the degree of calcination. In these various forms of investment composition which contain cristobalite it is preferable that the total filler be present in an approximate range of 50 to 80% of the total investment composition.

Equal or very nearly equal results, as to properties, may be producedin these cristobalite investment compositions by the use of any one or more of the chlorides of group III or group II of qualitative chemical analysis, or by ammonium, rubidium or caesium. More specifically, it will be understood that by the use of any of the chlorides here described, the reduction in expansion due to the substitution of an appreciable quantity of quartz for cristobalite may be overcome or substantially minimized. For example, an investment material containing 70% cristo balite and 30% plaster will expand 1.25% from room temperature to 1300 F. (W/P=.38) while 69% cristobalite and 30% plaster plus 1% of one of the above chlorides such as ammonium chloride will expand 1.45% from room temperature to 1300 F. (W/P=.38). On the other hand, an investment material containing 30% cristobalite, 40% quartz and 30% plaster plus 1% of one of the above chlorides, such as ammonium chloride,

will expand 1.25% from room temperature to 1300 F. (W/P=.325).

From this it will be seen that by using a small percentage such as 1% of one of the abovementioned chlorides it will be possible to substitute quartz for 40% of the cristobalite without loss in expansion. This will also ameliorate the sudden or abrupt expansion which commonly occurs when cristobalite is used in a higher percentage. Furthermore, it will reduce the W/P ratio which will increase the strength of molds made from the investment. In addition, the appearance of castings made by the use of a mixture containing cristobalite, quartz, plaster and one of the chlorides mentioned in the percentages indicated will materially lower the cost of the investment material and will result in a markedly improved appearance of the castings produced therefrom. In these compositions containing cristobalite, I may use any one or more of the chlorides mentioned in percentages up to The above chlorides are, of course, of different costs. and of slightly different physical and chemical characteristics and while their effect on the expansion properties varies but slightly preference would of course be given to the least expensive and most refractory one of the group. Preference will at present be given, for these reasons, to the chloride of barium, followed in order by the chlorides of strontium, magnesium, and calcium. Use will therefore be made of barium chloride as an example and state that by the use of this material an investment can be made which will meet all of the necessary and desirable requirements set out above. Besides imparting marked increase in thermal expansion it forms a plastic mass with silica, plaster, and water, that is very smooth and creamy in its form for application to the wax, thus producing smooth and very desirable reproductions. This composition hardens to a cementitious mass which retains its strength for days, withstands rapid heating, and high temperatures. It does not adhere to castings and produces smooth, dense castings of such brightness that necessitate no pickling in acids, and subsequently very little bufling or grinding. In this last property of producing beautiful castings it is ideal and the casting produced is often as clean and bright as the original alloy, a property long desired in j investment compositions. Since this barium chloride salt is by nature similar'in density to the density of silica and plaster it Will not settle out on storage and will maintain a fine mixture for a long time.

It has also been found that a marked increase in expansion may be secured by the use of a comparable amount of the chlorides of groups III and II of qualitative analysis. The best results are obtained from the chlorides of group III metals in the following descending order: aluminum, chromium, nickel, iron, manganese, cobalt; and of the following descending order in group II; copper, tin, antimony, lead, 'bismuth, cadmium. It has also been found that the chlorides should the cost factor change materially due to economic and industrial changes, or should all of the desired requirements not be needed for certain types of castings, then the chlorides of the other groups will be of marked benefit in aiding the skilled reproduction of desired articles. The chlorides vary in the degree in which they increase the thermal expansion, reduce the setting expansion, and accelerate the setting time. From among the various chlorides it is therefore possible to use one or a combination of two or more as to give an investment possessing each of the above properties in the desired amount of degree. It may also be possible to secure the desired results by the use of other materials or minerals which will react to produce any of the desired chlorides. Thus hydrochloric acid has been found to increase the thermal expansion of silicaplaster investments nearly as much as the other chlorides, and this is no doubt due to the formation of one of the specified chlorides; namely, calcium chloride. Any other material containing chlorine which can be liberated to react with the plaster or other added ingredients may also produce the same beneficial results. The addition of the specified chlorides to investments containing other expanding ingredients will generally add its influence in aiding the increase in thermal expansion as well as adding its cleaning effect on the finished casting:

All of these are examples of the manner in which the specified chlorides may be added to an investment and should therefore be considered in the same light and spirit of the invention.

Throughout the specification and claims the terms plaster," binder," filler and invest.- ment are to be interpreted as including:

Plaster.-Shall be construed to include any of those of the precious metals. The usual investments contain to 50 per cent of binder and 50 to 80 per cent of filler. The types and combinations of filler and binder shall further include only those that will have their thermal expansion increased by the addition of the chlorides herein I set forth.

the forms of calcium sulfate, such as the exam- When definite examples of expansion values are given, the plaster used was in the form of alpha gypsum.

What is claimed is:

1. An investment composition for casting precious metals and their alloys, consisting principally of a siliceous refractory material and a calcium sulfate binder in such proportions as to give a dental investment composition and containing from .1 to 2.0 per cent aluminum chloride.

2. A dental investment composition containing more than per cent of a siliceous refractory material, a calcium sulfate binder, with the refractory material and the binder so proportioned as to give a dental investment composition and about 2 per cent of aluminum chloride.

'ROBERT NEIMAN.

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