US2247395A - Investment - Google Patents

Description

Jul 1, 1941.

Pmcmr TH RMAL EXPANSION R. NEIMAN INVESTMENT Filed Feb. 7, 1936 600 800 TEMPE/8A TU/Z E INVENT OR.

liaanzr A/E/MA/V ATTORNEYS' Patented July l 1941 INVESTMENT Robert Neiman, Louisville, Kym, assignor, by. mesne assi factoring Company, ration oil Ulric tions, that is refractory compositions utilized in the making of molds for the casting of metallic tents, to The Columbus Dental Manu- Colnnbus, Ulric, a corpo= Application February 7, 1936, Serial No. 62,801

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 some This wax is then completely encased in a cementitious mass of a mixture of 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. Between 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 \vax pattern.

In this casting process the exact size of the desired inlay or the like is not produced, due to the casting shrinkage of the alloy in solidifying and coolingto ,the solid state. This shrinkage is approximately-1.25% (linear) and obviously must be counteratted 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 additional 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 pracskilled practitioner a simple, and sure method of making artistic and very accurate castings.

The investment should:

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

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

Third.-.-Have practically no variation in thermal expansion when mixed with an amount of water from 7% above to 7% below, the amount prescribed by the manufacturer, a leaway of 14% approximately.

Fourth.--Expand at a fairly even rate throughout the entire heating range, the expansion curve never rising very much above the 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 detically straight line curve of the usual metallic container or ring of equal expansion.

FiftI .-'-Incorporate readily with water to a smooth creamy consistency which can be easily and readily applied to every nook and me ',ce 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%.

Seventh.-Set rapidly to a hard cementitious mass which will withstand high temperatures and rapid heating without cracking or distortion.

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.

Ninth-Possess adequate strength when hotto safely resist the sudden inrush of molten alloy when cast under usual pressure; and preferably to possess enough strengtht after quenching in water, to permit easy removal in toto rather than to disintegrate and leave a tenaciously'ad- EZeventh.-Not change materially on standing I orshipping due to the settling out of unduly heavy constitutents such as powdered heavy metals, necessitating packing under pressure.

sirable and necessary properties will give the '55 Also possessing constituents of a similar chemical ,Still lacking in the third,

and physical nature to give a homogeneous mass 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 5 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. H2O) and a refractory filler or aggregate. The plaster in its usual form, of the hemihydrate (CaSOL /ZHZO) 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. ex-

pansion. The most desirable filler is silica in its three principal modifications: Quartz, tridymite, and cristobalite. Quartz is the cheapest and mostuniversally used. Obviously any other filler may be used that will contribute the de-' sired 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 smother 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 40 degrees of success. cristobalite, for. example, when added in large amounts (50 to 70%) gives a material of high thermal expansion'and meets many of the above' specifications. 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,

flfth and tenth requirements.

It is, therefore, the object of this invention to provide the profession, and anyone desiringean but the composition. is

heated under identical conditions, mixed with same proportion of water, and which compounds are respectively with and without the added ingredient of this invention.

Simple investment compositions, as above noted, are well known and have been used for many years. The'general composition of these materials is from '20 to 50% plaster and 80 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 I 0 in dicates the thermal expansion (and contraction) of a mixture of 44% plaster with 56% silica and indicates a thermal expansion of .64% between room temperature (70 F.) to 1300 F. This investment was made with a water to powder ratio (W/P) of .26, that is 26 grams of water to 100 grams of the powder mixture. An increase of the W/P causes amaterial 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 stant at .26, will cause an expansion of this investment of 1.1%. Even small amountsof 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. It is not desired to limit the composition to the inclusion of any certain specific 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 increasedand vice versa. Variation in the W/P ratio, however, no longer affects 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.

artistic and accurate castingmade, principally, 60

by the disappearing wax pattern method, with' investment compositions which will meet the above requirements to a greater degree than anything now known or available.

.Other objects and advantages of this invention should be readily apparent by reference to the following specification considered in conjunction with the accompanying drawing forming a partthereof, and it is understood that any modification may be" made in the exact structural 7 details, there shown and described within the scope of the appended claims,- without departing from or exceeding the spirit of the invention.

The drawing depicts a pair of graphs showing the expansion of an investment compound when when added to the simple 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 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 consisting of dry ingredients since the resulting mixture is the same. Y

Investments having a thermal expansion of 2% may be fifade by using about cristobalite as the filler, the usual binder and about2% of any of these chlorides. Equal, or very nearly equal results, in the above properties, may be obtained by the use of any one of the other specified chlorides, namely, those of the metals of groups III and II, or ammonium, rubidium, and caesium.

The above chlorides are, of course, of diiferent costs, and of slightly difi'erent physical and chemical characteristics and while their effect on the expansion properties varies b slightly preference would of course be given to e 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 bufiing 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 investment compositions. Since this barium chloride salt is by nature similar in density to the density of silica and plaster it will notsettle the desired chlorides.

the rare rubidium and caesium also increase the expansion markedly.

While the chlorides of the alkaline earth metals produce the best results in meeting the desired requirements of an investment composition, it'is obvious that should these not be available, or should the cost factor change materially due to economic and industrial changes, or should 'all of the desired requirements not be neededfor certain types of castings, then the chloridesof the other groups will be of marked benefit in aiding the skilled' reproduction of desired articles. 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 has been found to increase the thermal expansion of silica-plaster 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 ments containing other expanding ingredients will generally add its influence in aiding the increase in thermal expansion as well as adding ,its cleaning efi'ect on'the finished casting.

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. results are obtained from the chlorides of group .The best I III metals in the following descending order,

{ill of these are examples of the manner in whidh'the specified chlorides may be added to ah investment and should therefore be considered in the same light and spirit of the invention.

What is claimed is:

1. An investment composition for casting prepious metals and their alloys, consisting principally of a. siliceous refractory material and a calcium sulfate binderin such proportions as to give a dental investment composition and containingfrom .1 to 2.0 per cent strontium chloride.

Thus hydrochloric acid

Images