US2204123A - Pattern for castings - Google Patents

Description

Patented June 11, 1940 UNITED STATES PATTERN roe CASTINGS Paul Francis Collins, Columbus, Ohio, assignor to The Columbus Dental Manufacturing Com pany, Columbus, hio, a corporation No Drawing.

8 Claims.

I My invention relates to patterns for castings. It has to do particularly with that type of casting wherein a mixture which is generally known as investment material and which is usually formed of siliceous refractory and usually with a binder, often also containing other materials, is formed and is caused to set around the pattern to produce a non-partible mold. It relates especially to a novel type of pattern which may *10 be dissipated by volatilization by the application of heat to the mold, cavity,

Molds of the type described are commonly used in certain classes of foundry work, dentistry,

is jewelry manufacturing, and similar arts. The patterns ordinarily require scraping, trimming, carving or like treatment after initial formation and before investment thereof and frequently require the addition of material thereto.

The material most commonlyused in making such patterns has been wax or of a wax-base and this is still used extensively in the dental and other arts. However, where castings are to be made to fit into or used in conjunction with other fabricated parts, wax patterns possess important draw-backs, such as non-uniform or excessive and contraction, which produce problems that increase in difficulty as the size of the pattern increases. Also such wax patterns do not lend themselves readily to necessary alterations such as those mentioned above. Likewise, hot wax additions or the removal thereof cause distortions and areas of unequal densities, which later result in variations in expansion under varying temperature conditions. In addition, wax patterns do not withstand handling or shipping except under extreme conditions of care, being soft and tending to distort in summer and becoming brittle and subject to fracture in the winter.

Some attempts have been made to use Celluloid, either with or without wax additions, but with little success. The use of a flame to soften or remove wax from Celluloid results in distortion of both and may ignite the Celluloid. Celluloid is also quite brittle and cannot be readily carved or trimmed. Synthetic resins have also been used in making patterns but such patterns are unduly subject to distortion, are comto leave a suitable casting bustiblc andhave other drawbacks. Those patterns of resin with which I am familiar cannot be machined to form a smooth non-fibrous surface.

Some attempt has been made to use low-fusing metals for patterns for large castings, these .tent in the specialty foundry art.

Application January 5, 1938, Serial No. 183,558

metals being designed for removal from the mold by melting and flowing therefrom. However, with small molds, the sprue holes are too small to permit of pouring certain molten metals therefrom. This is particularly true in the dental art and in the jewelry art and is true to some ex- Moreover, such metals as have been suggested, may, to some extent, form an oxide upon melting. This is especially so .when the temperature is ele- 1o vated substantially above the melting point, to facilitate pouring. This oxide may remain (to some extent at least) as an adherent coating upon the walls of the mold cavity and sprue holes to produce surface inaccuracies and imperfections in-the casting subsequently made in the mold. Another condition which tends to militate against the elimination of the said lowfusing metals by pouring is exemplified in the dental industry .wherein it is frequently necessary to utilize a multiplicity of sprues because of the complexity of design in order to insure drainage of the fused metal from all of the low cavity areas and to insure a perfect casting. Likewise, it is particularly important in those arts which utilize pressure or suction casting, to limit the cross section of the sprue holes, since melting of the casting metal takes place on top of the mold material itself and suchoperations rely on the surface tension of the molten metal to prevent its entry into the casting cavity until pressure or suction is actually applied. For casting metal (e. g. gold and dental alloys) 7 for dental restorations, the cross sectional area of the sprue holes is only a very small fraction of the cross sectional area of the article to be cast. Thus it is common to use round holes of about 0.04 to 0.05 inch in diameter, or other sprue holes of a similar cross sectional area.

However, similarly narrow slots are sometimes used and, as a matter of fact, any shape of hole may be used if some or all of its walls are close enough to each other to prevent flow of the casting metal into the casting cavity by gravity.

Thus, it will be seen that none of these materials has entirely fulfilled the requirements of the technicians or artisans in industries of the type indicated. Because of this, many of these technicians or artisans have resorted to the "practice of obtaining the wax in bulk and then extruding, casting, shaping, modelling, or carving it into required pattern designs and dimensions. This has been particularly true in dentistry where it is common practice to provide metal backing members which are designed to receive replaceable porcelain tooth facings, both having interfitting surfaces processed to very accurate dimensions. 'For reasons which are too numerous to discuss here, this practice of obtaining wax in bulk and extruding, casting, shaping, modelling or carving to required designs and dimensions has not always been satisfactory.

From the above, it will be seen that all of the prior art patterns for use in non-partible molds of the type indicated possess serious drawbacks which place severe limitationson industries of the kind described. In particular relation to dentistry, they place especially severe limitations on the casting of backing members so that it has been difficult to produce backing members which, as cast, fit accurately to tooth facings commonly available in the art and particularly on the interlocking tenon members which interfit accurately with the mortices and slots of such tooth facings. Aside from the drawbacks above enumerated in connection with prior art patterns of the type described, there are other serious drawbacks, such as difliculties of manufacturing ona production basis, diificulty in alteration, undue brittleness and tendency to distortion.

One of the objects of this invention is to provide a pattern for casting, particularly in nonpartible molds, which will be free from excessive contractions and expansions as well as from undesirable distortions.

Another object of this invention is to provide a pattern for casting which will be free of undue brittlenessand which will not be unduly combustible.

Another object of this invention is to provide a pattern for casting which can be readily dissipated by heat and preferably during the heat.

treatment of the mold, in industries where it is customary to subject the mold to heating operations.

Another object of this invention is to provide a pattern for casting which is'formed of such a material that it may readily be subjected to trimming, machining and other operations to give it the exact shape and dimensions desired so that the casting formed therefrom may have the exact shape and dimensions required for interfitting with the tooth facing and the hole and slot therein.

In its preferred form, my invention contemplates the provision of a pattern for the purposes indicated which may be dissipated by volatilization or sublimation when subjected to heat, so as to leaveno residue serious enough to materially afiect the details, surface characteristics or physical properties of the molded metal articles subsequently cast in the cavity formed by the pattern. Preferably, my pattern is of such a material that it will be volatilized during the heating of the mold. It is desirable, moreover,

that it be volatilized before suflicient oxidation occurs'to produce theundesirable residue abovementioned. 7

My invention in itspreferred form, further contemplates the provision of a pattern of such a material that it will be relatively rigid and susceptible to trimming, machining or other working with a facility substantially equal to those metals that are technically considered suitable for drawing and prefabrication. Likewise, it desirably comprises a pattern which, even though it is in thin section, will be of suflicientrigidity that it will permit the addingot pattern material, such as hot wax, metal of a lower melting point or other material, on one side thereof without warping or distortion, so that a composite pattern may thus be formed. It also contemplates the provision of such a pattern'that such an addition, or some portion of it, preferably may be removed 6 therefrom by subjection to a fiamewithout warping, distortion or ignition of the original pattern.

A specific application of my invention involves the provision .of a pattern formed in one piece to simulate the normal backing unit with its in- 10 terlocking tenon (commonly called post") made integral therewith and, if desired, to include the anatomical contour of the lingual or occlusal surfaces of the tooth while, at the same time, possessing such properties as will make it possible 15 to carve finer details or other alterations thereon. It will be understood by those familiar with the art that the backing patterns may be used singly or in a multiplicity of combinations depending upon the design of restoration to be constructed. 20

Preferably, my pattern is made of a metal that will readily volatilize with or without melting within the temperatures employed during the heat treatment of the mold preliminary to casting. It is, also preferable that my pattern be 25' made of a metal which will volatilize without leaving any residue serious enough to materially affect the details, surface characteristics or physical properties of the alloy to be cast, whether this residue results from the non-volatile character of the metal or from chemical reaction of the pattern material either with oxygen, sulphur or other substance.

The physical characteristics of the metal which I may use for my pattern material may vary con- 35 siderably. This is dependent somewhat upon whether the pattern is to be given its approximate final form by casting or whether it is to be cast in billet form and then reduced to final exact form by swaging, working, rolling, etc. In 40 the former case, the question of workability is not so important, whereas in the latter case workability is important.

Extensive tests have shown that those elements among the metals which show the most 5 promise for my particular purpose are cadmium, zinc, bismuth, arsenic, thallium, lithium and indium. Someof these metals may be used alone. However, my tests indicate that the best results are obtained by alloying two or more of these 50 metals. The most desirable alloys are cadmium and bismuth or cadmium and arsenic or cadmium and zinc. Many different formulas which would produce suitable patterns for the purposes indi cated above might be outlined. However, the 55 following will serve as several typical formulas which appear to be satisfactory for use as patterns having the desired characteristics outlined above. These alloys are also capable of being The above compositions merely serve as some 1 examples of those compositions which I have found to be satisfactory, to one degree or another, for the intended purpose. Much experimental work has been conducted, and certain alloys have been found to be quite satisfactory for the production of a pattern which will volatilize at tem peratures below 2200 F. without undesirable residue and which will also have adequateworkability to permit shaping to final exact form and dimensions by swaging, working, rolling, etc. Among these are alloys consisting of 83% cadmium and 17% zinc, alloys consisting of 99.75% cadmium and 0.25% bismuth and alloys consisting of 99.5% cadmium and 0.50% arsenic. I do not restrict the invention to these percentages.

As indicated in the above table, my pattern may be made entirely of cadmium, since this material will volatilize without undesirable residue and will possess adequate workability. However, it may be desirable that the cadmium be supplemented by a small percentage of a hardening material such as bismuth or arsenic, the bismuth not exceeding 1% and the arsenic. in amount not exceeding 3.5% for cold-workable alloys. Though zinc might be considered a hardening material, yet I have found that zinc may be used as a supplement to cadmium in percentages as high as 17% without creation of undesirable hardness.

apparently due to the altered nature of these materials when combined.

Though an alloy consisting of 83% cadmium and 17% zinc and an alloy consisting of 99% zinc and 1% cadmium, for example. are distinctly differe'nt alloys, nevertheless they both possess charpattern cavity. This residue may be in the formacteristics which render them suitable, under certain conditions, for use in the formation of my pattern and particularly a tooth backing, since they both volatilize within the desired temperature range without undesirable residue. However, as will be further explained, alloys of high zinc content possess some drawbacks, such as being lacking in good cold working properties and being somewhat detrimental to the making of good final castings. With these alloys having high percentages of zinc, or with pure zinc, it may be desirable to form the pattern at 250 F. to 300 F., and it may be preferable to die cast the alloy.

As indicated by the table above, zinc or cadmium alone may be used as the material from which my pattern is formed. The conditions under which zinc may be successfully employed as a pattern material may be best understood by the following descriptions. two types of investment or molding materials are employed. One type contains calcium sulphate as a binder; the other contains no calcium sulphate. The former type of investment material is rarely heated to temperatures exceeding 1600 F. whereas the latter type may be heated to temperatures approximating 2200 F. Zinc melts at 787 F. and boils at approximately 1665 F. When zinc is used as 'a'pattern material for use in association with molding mixtures of the type that are burned out at temperatures less than than 1600 F. there is usually. considerable residue remaining in the of pure zinc which was incompletely volatilized, zinc oxide which resulted through prolonged heating of the zinc at temperatures below its boiling point and from zinc sulphide which may be formed from the liberation of sulphur from the calcium sulphate type of investment composition. In the instance of those investments which contain no calcium sulphate, zinc is decidedly.

better for forming my patterns inasmuch as there In the dental art today is no opportunity for forming zinc sulphide. Further, the temperature at which zinc volatilizes is greatly exceeded by a burn out at 2200 F. or thereabouts and coupled with the rapidity with which the volatilization temperature of the zinc is reached in this mold prevents any appreciable formation of zinc oxide.

The suitability of zinc for a pattern material is further affected by the atmosphere that surrounds the mold during the entire heating period. Obviously, if this atmosphere is oxidizing in nature (as in the electric furnace) considerably more zinc oxide will be formed in the course of heating than if the surrounding atmosphere were reducing or mildly reducing (as in a gas fired furnace). In many instances the atmosphere is actually strongly reducing which greatly enunderstood that where the heating is to be conducted in a strongly reducing atmosphere, the metals, even if oxidizable in air, would not become oxidized. I

It should be understood that it is highly desirable that my pattern material is such that it will volatilize at temperatures below 2200 F. Without undesirable residue and, of course, each of the metals previously mentioned volatilizes in accordance with its own vapor pressure. Therefore, my invention includes any metallic pattern for nonpartible molds that will volatilize under 3000 F., and preferably under 2200 F., without undesirable residue, particularly any pattern formed of one 'liances the suitability of zinc as a pattern mateor more of the metals, cadmium, zinc, indium, I

bismuth, arsenic or lithium and such other hardening or softening elements as may be desirable.

In its more specific aspect, the question of workability, of the metals and alloys, is involved and in this aspect any metals and particularly any of the metals listed will fall within the scope of my invention if they are capable of being swaged, worked, rolled, machined, trimmed or otherwise shaped in a manner previously indicated.

Under certain conditions, it is desirable that my patterns be formed of such a metal or alloy that they will volatilize without undesirable residue at temperatures less than 1600 F. since the burning out of certain molds occurs below this temperature. Under other conditions, volatilization without undesirable residue need not occur at temperatures as low as 1600 F. but should preferably occur at temperatures below 2200 F. Thus, I have, in the table, given several examples of two types of alloys, viz., one group which will I .ing or distortion of the base material. Likewise,

such additions which have been applied to the base pattern may be partly or entirely removed by the application of heat or flame without dis-' tortion or warping of the base pattern.

Still another phase of my invention arises from the fact that I am the first to provide a pattern for use in the dental industry which is formed either entirely of a low-fusing metal or of a lowfusing metal supplemented by wax or some other low-fusing, normally solid material. The backings for interchangeable teeth which have been found most acceptable to the dental trade have comprised a main backing plate with a post thereon that is so formed as to permit it to interlock with the tooth. Such backings are shown in Patents Nos. 1,890,132, Dec. 6, 1932, and 1,953,- 075, April 3, 1934. The invention is also advantageous in the case of making any kinds of metal dental restorations or metal parts thereof, e. g.

.bars, clasps, cusps, crowns, bases, posts, teeth,

lugs. and attachments. The formation of a pattern for such a backing presents difficulties which are not encountered in most branches of the casting art and my pattern materials are of such a nature that the formation of such backing patterns are greatly simplified, so that I believe that it is new with meto form patterns of the type in question or patterns having a main backing plate with a post thereon, out of low-fusing metals.

Furthermore, my invention fills another longfelt want in the art in that it makes possible the formation of units embodying the backing proper with its posts and also with the anatomical carving on the rear surface thereof by one casting operation. .It may be explained that, in the formation of such unit by the present day practice, it is customary to first form the backing proper with the posts thereon (made of the final metal) and to then produce the anatomical carving on the rear surface thereof by casting metal against the backing which has already been embedded in the investment material. In some cases, the

, backing proper is first formed by casting and then a second casting operation is utilized for applying the metals embodying the anatomical carvings to the backing proper which has been embedded in the investment material. Thus, it will be seen that my invention makes possible. the forming of units of the type in question by one casting operation, whereas the prior art practice has required the initial formation by a second forming or casting operation. of this willbe obvious.

I do not limit the invention to those specific compositions set forth in the table. Where a metal is to'be used, it is within the scope of my invention to use a metal, metal alloy or article formed from; a'single powdered metal or from a plurality of metal powders initiallyv compressed together and subsequently sintered, which metal or alloy will volatilize either with or without melting in the course of the drying and burning out of the mold preparatory to casting and will leave substantially no residue that will be detrimental in any respect to the detail, surface characteristics,-density or other physical properties of the finished cast piece.

' Though, in the above discussion, I have somewhat stressed the importance of the fact that my pattern is formed of a material which will vola- The advantages tilize without leaving undesirable residue and which will be free from burning or warping at the temperatures of hot wax, as well as being relatively non-combustible, there are still other characteristics of my pattern material which are important. In certain modes of its application, the fact that my pattern material possesses such properties as to permit its being rolled, cut,

trimmed or otherwise machined to form precise,

dimensionally accurate units is quite important. In other instances, it is quite important that my pattern material possess such properties that it may be readily cast into desired configurations and that it will permit further carving or otherwise altering as required by common procedure in each respective art to which it is adapted.

Where I have used the terms sublime, sublimation, or volatilization, it will be understood that I intend these terms to mean any conversion of the pattern material into the vapor state with or without chemical change or melting, it being understood that any such actions will be within the scope of my invention,. providing they bring about removal of the pattern from the casting cavity without leaving any undesirable residue therein.

Where I have used the terms metal or metals in the claims of the application, I intend to include either a single metal or alloys or combinations thereof.

I have used the term non-partible mold" to cover an integral mold as distinguished from the typical founders two-piece mold.

It will be seen that my invention involves a number of different aspects and advantages which need not be reiterated here. Additional advantages will be made clear by the appended claims.

Having thus described my invention, what I claim is:

l. A pattern for use in casting metals and alloys in the dental and 'jewelry art in a onepiece mold provided with a sprue hole which is of a cross-sectional area only a minor fraction of the pattern, said pattern being formed of a rigid metal that will volatilize at a. temperature below 3000 F. so that the metal can be removed from the mold cavity by volatilization without any undesirable residue being left in the mold cavity.

2. A pattern for use in casting metals and alloys which is formed substantially entirely of cadmium.

3.-A pattern for use in casting metals and alloys having a cadmium base and a hardening metal selected'from the group consisting of bismuth, arsenic and zinc.

' 4. A pattern for use in casting metals and alloys comprising from 79.5 to 99.75 per cent cadmium, from .25 to 20.00 per cent of at least one of the elements selected from the group consisting of bismuth, arsenic and zinc.

5. A pattern for use in casting metals and alloys thereof in a non-partible mold provided with a sprue hole which is of a cross-sectional area only a. minor fraction of that of the pattern, said pattern being formed substantially entirely of zinc so that it can be volatiliied below 3000 F.

6. The method of forming a non-partible mold which comprises producing the cavity in the mold by forming the moldaround a. pattern which is constructed at least in part of a .rigid metal or and volatilizable at av temperaturebelow 3000 permitting the mold to set, and subjecting the mold containing said pattern to a temperature sufficiently high to volatilize the pattern without leaving any undesirable residue in the mold cavity.

7. The method of forming a non-partible mold which comprises producing the cavity in the mold with a sprue hole connected thereto which is of a cross-sectional area only a minor fraction of that of the pattern by forming the mold around a pattern which is constructed at least in part of a rigid metal or alloy with a sprue of proper size connected therewith, said pattern having predetermined form and dimensions and being volatilizable at a temperature below 3000 E, permitting the mold to set, and subjecting the mold containing said pattern to a temperature sufficiently high to volatilize the pattern without leaving any undesirable residue in the mold cavity.

8. The method of forming a non-partible mold which comprises producing a cavity in the mold by forming the mold around a pattern which is constructed partly of a rigid metal or alloy having predetermined form and dimensions and volatilizable at a temperature below 3000 F., the rigid metal part of said pattern having an addition member attached thereto which is formed of a material which is solid at atmospheric pressure and which material has a lower melting point than said base member, permitting the mold to set and subjecting the mold containing said pattern to a temperature sufficiently high to volatilize the pattern without leaving any undesirable residue in the mold cavity.

PAUL FRANCIS COLLINS.