US2362507A - Method and means for producing commercial castings - Google Patents

Description

' 1944- E. A. STEINBOCK EI'AL ,3

METHOD AND MEANS FOR PRODUCING COMMERCIAL CASTINGS Filed Oct. 27. 1942 A 2 4 Y X x Z 3 I z 2 41 V//////// Z INVENTOR. EuMu/vn A STEINBOCK ROBERT NEIMRN Patented Nov. 14, 1944 METHOD AND MEANS FOR PRODUCING COMMERCIAL CASTINGS Edmund A. Steinbeck and Robert Nciman. Louisville, Ohio; said Neiman assignor to said Steinbock Application October 27, 1942, Serial no. 463,560

15. Claims.

This invention relates to a method and means for prdducing commercial castings to substantially the desired size and more expeditiously than with the present methods.

It has been found in using plastic pattemsencased in investment compositions that with too large or heavy a section, in the burnout of the pattern, difliculty is encountered, in that the heating of the plastic material increases its bulk and causes cracking of the mold. By the present invention this fault is overcome, whereby plastic patterns may be employed for producing metallic castings which have relatively thick cross-sections.

It is, therefore, the principal object of the pres.. ent invention to provide a plastic pattern for making metallic castings wherein the mold or work, nor can they be economically produced in investment composition is not cracked during the dissipation of said pattern.

It is also the object of this invention to provide a pattern of the disappearing variety which is caused to disappear by heat and in which a clean, accurate mold cavity results.

Other objects and advantages of the present invention should be readily apparent by reference to the following specification considered'in con junction with the accompanying drawing forming a part thereof and it is to be understood that any modifications may be made in the exact structural details where shown and described, within the scope of the appended claims, without deview of a proposed part Fig. '6 is a view similar to Fig. 5, showing a modification in the sprue former.

Fig. 7 is a view similar to Fig. 4 showing a modification therein.

Fig. 8 is a vertical sectional view through a mold in'which is encased the pattern.

Fig; 9 is a view similar to Fig. 8 illustrating the same after the pattern and the spr'ue former have been eliminated.

Throughout the several views of the drawing,

similar reference characters are employed to denote the same or similar parts.

As was noted above; castings have been made in molds formed of cementitious materials and in quantities.

The usual wax patterns soften at approximately F; and actually melt into a fluid at approxi- V mately' F., while plastics seldom soften below F. and then do not melt but become viscous and remain so up to approximately 300 F." Molten waxes enter the pores of the mold and relieve pressure due to the expansion of the remaining portion of the pattern and as the water of the investment turns to steam, it helps to soften and remove the waxes from the mold. Plastics have a lower coefiicient of expansion than waxes, but remain solid over a much greater-temperature range and the actual total expansion of the plastics, while in solid form, is considerably greater than that of wax. Since plastics become viscous i rather than liquid, they have a tendency to ball up and do not dissipate into the pores of the investment.

This result was particularly noticed when thick wall patterns were employed, while relatively thin wall patterns did not and the use of thick sec- 7 tion patterns is precluded for the production of accurate objects. The limitations on section thickness depends considerably upon the actual design of the whole pattern, as well as on the size and design of the sprue holes through which the I I softened pattern makes its exodus. The nature of theinvestment, its property of elasticity, its strength and its expansion at low temperatures have a considerable effect in limiting the pattern size and thickness. In general, sections thicker than one-eighth inch are undesirable and, as

noted above,if of one-quarter inch will usually cause trouble, especially if the pattern is over an inch long and the sprue' is considerably smaller in cross section than the pattern.

Certain portions of dentures, such as backing patterns for slipped-on teeth, bars and pins for tubular teeth, have been made of plastics. These are all almost invariably used in conjunction with a matrix or support and also lead sprues of wax.

- The above noted evils relative to plastic patterns were not encountered because of the comparatively tiny dental plastic backings which are less than about .03 inch thick, and in almost all cases covered to a great extent by wax portions of the pattern. By a careful study, it was found that by reducing the patterns to substantially a, shell having the desired outline of the casting, mold cracking wasovercome; in other words, the pattern should be made of thin walls and this means hollow patterns.

By way of thin wall portions ,4 inch to M inch preferably, the dimculty of mold cracking and distortion is usually eliminated and in addition the time required to dissipate the pattern is greatly reduced because the pattern can collapse easily and is of sumciently low bulk to volatilize more rapidly.

Referring now to the drawing, the part illustrated in Figs. 1 and 2 comprises a small diameter cylinder l extending from a larger diameter cylinder ll, each of which is solid as shown in Fig. 2. To produce this part in a plastic and use same, difliculty would be encountered if the plastic were solid, as was pointed above.

The pattern illustrated in Figs. 3 and 4 have the cylinder 10' hollowed out as at [2 and similarly has the cylinder H hollowedout, as at Hi. It will be appreciated that to use the pattern as just described, the interior or hollow portions l2--| 3 would flll with the investment material when in the plastic state and encase the pattern. Therefore, the free end of the cylinder l I is countersunk to provide a seat H on which is mounted a disc l of plastic material. Upon the encasement of the complete pattern and the subjection thereof to heat, the walls of the cylinders l0 and H and the end caps l5 and 18 would collapse inwardly into the hollow portions l2-l3 and would not expand to distort and crack the mold.

It should be appreciated that it is necessary to form a sprue hole through which the molten metal is introduced to the mold cavity and this sprue former may be attached to any desired point on the pattern. As illustrated in Fig. 5, a solid sprue former I1 is employed, shown as attached to thecap l5. This sprue former may conveniently be'made of plastic material, wax or any other desired material, but in view of the ease of cementing plastic parts to one another by the use of the plastic solvent such as chloroform, a plastic sprue former can be conveniently used.

In Fig. 6, the sprue former "is shown as hollow, while the sprue former I! in Fig. 5 is shown as solid By using the hollow sprue former I 8, the sprue hole is not distorted upon the heating of the mold and furthermore a vent is provided to the mold to permit the escape of the air within the hollow portions l2-l3 of the pattern as soon as the wall to which it is attached fractures upon collapse.

In Fig. '1 a further modification is illustrated in that the cylinder of the pattern, instead of having integral therewith a solid end or cap I6, is provided with a seat i9 similar to the seat ll at its other end for the reception of a closure cap 20.

From the foregoing,- it will be noted that there has been provided a plastic pattern having thin walls with the ends closed to complete the exterior contour of the part which has a hollow interior- In use, and as seen in Fig. 8, the pattern II with a sprue former 22 of the form illustrated in Fig. 5 or in Fig. 6 secured thereto, is

aseauor disposed substantially centrally of a metallic or mold forming member or ring 23 with the space between the pattern and ring filled with the mold forming plastic 24. The upper end of the plastic is dished as at 25 to provide a crucible for the molten metal prior to its descent to the cavity. Upon heating, the pattern 2| and sprue former 22 are dissipated so that a mold as in Fig. 9 results, having a cavity 26 therein conforming to the exterior contour of the pattern.

As was pointed out in our co-pending application for Method and means for casting filed September 19, 1942, under Serial No. 458,984, the usual mold compositions for casting non-ferrous metals, such as aluminum and brass, comprises plaster and silica mixtures. Fibrous materials, such as talcymay be incorporated to increase the mold permeability and toughness. A typical mold material would consist of about 40% plaster and 60% comminuted silica. To prevent mold from cracking and also to provide expansion it is desirable to add small amounts of expanding agents which may be generally indicated as soluble inorganic halides and nitrates, such as disclosed in U. S. Patent No. 2,247,395 and pending application of Robert Neiman, Ser. No. 297,723. It has been found that the best results are obtainable with the investment compositions containing alkaline earth nitrates as the thermal expanding agents. These impart the desired expansion quantities, permitting heating of the mold to high temperatures and also decompose to leave a comparative inert material during the heating of the mold to about 900 F. which makes the mold suitable for casting brass.

The above investments are most suitable for non-ferrous metal casting and work best with the plastics that decompose at the low temperatures. For casting ferrous alloys and others of high melting point, investment compositions comprising principally silica and with a binder of an organic silicate such as ethyl silicate or magnesium phosphate and other phosphates are most suitable. The plastics requiring high decomposition temperatures may be used since the mold is usually heated to a red heat to receive the molten metal. Even then the lower decomposing piastics seem to give best results.

The use of disappearing patterns is old, primarily in the formation of dental restorations and jewelry, but these patterns have been formed of waxes of many varieties which have different characteristics and would be unsuitable for commercial work. Due to the complication in making hollow or sheil-like'patterns, it is practically impossible to use wax in the instant application; Waxes as heretofore known are brittle in cold weather, and pliable, soft and sticky in warm weather or upon handling, which is necessary in applying the closure caps to complete the exterior contour of the pattern. Even by the addition of proportions of resins, it has beenfound impossible to overcome entirely these undesirable features. Furthermore, since waxes are generally a combination of crystalline and amorphous ingredients, there is always a tendency tobonizes is not at a great disadvantage.

toosoit, or when strength is there, they are too brittle.

Another disadvantage of waxes is their high coefllcient of thermal expansion, a usual wax will have a coeflicient of expansion of about 15.0 at room'temperature whereas metlnrl methacrylate has a coemcient of only half as 'great.

Furthermore, waxes and resins, unless pure, leave-an undesirable ash residue upon burning out. Even when pure, they require temperatures above red heat to entirely decompose any carbonaceous residues. This is true also of most plastics. In plaster molds this property is very undesirable because the temperature required to decompose the pattern may be injurious to the plaster, even to the point of decomposing the same.

This disadvantage is overcome by the use of the newer plastics, including polyesters of acrylic and methacrylic acids. Mixtures of these, such as those including methyl methacrylate and polystyrene, permit-variations in physical properties, molding properties, stability on aging, and molding characteristics.

softening. then become liquid, and then gaseous, In some cases this is merely a depolymerizationprocess. Furthermore, these plastics are thermoplastic, and may be shaped as desired by heat- These materials do not carbonize upon heating, but merely go through a Where small quantities of patterns are desired for experimental work, itis possible to mold many of the resins and especially methyl methacrylate, for example, using plaster or dental stone. The liquid and powdered methyl methacrylate may be mixed to form a moldable plastic mass, and is very easy to form to shape. It is possible to form large patterns from standard size stock of sheet or rod plastics, as these can be machined, turned, assembled and glued into an infinite variety of forms.

Since plastics usually have more than the necessary strength for use as temporary patterns, it

is sometimes desirable or advisable to incorporate fillers. These may serve to reduce the cost and on some occasions, reduceshrinkage and promote pattern stability. These materials include carbon in various finely divided forms, and may include other carbonizable matter with low ash content. v

The use of lubricants and plasticizers may also impart advantages to plastic patterns or facilitate their manufacture. The use of plasticizers, such as phthalates or esters of sebacic acid, for example, promotes workability of plastics. Lubricants include, for example, stearates containing no non-volatile matter may help the molding ing, and retain such shape upon cooling. This process may be repeated and is reversible. It has been found that these plastics are decomposed and reduced to a gaseous state at temperature be low 900 F. Thermosetting plastics do not permit this handling.

Among the many plastics available may be listed the phenolic-aldehydic resins, Bakelite, for example; vinyl resins, styrene, hydroxy-carboxylic resins, such as glyptal and rezyl; cumarone and indene resins, cellulose nitrate, cellulose acetate, and cellulose others, such as ethyl cellulose, or mixtures of the aforementioned, sometimes including natural waxes and resins. Most of these are inferior to the polyesters of acrylic and methacrylic acid, but may be used where price considerations are important. Where it is necessary to raise the mold temperature quite high to receive the molten metal, then a plastic that car- Many of these resinous plastics may become sticky during the heating process and destroy the mold cavities by pulling particles of the surface away Some of these plastics are sufllcently water abv sorbing to cause distortion.

The part and pattern illustrated in the drawing is a relatively simple one and has bee "tilized merely for illustrative purposes. It will be appreciated that patterns having a much greater area and a far more complicated exterior contour can be formed on compression plastic casting machines. In this connection it is necessary that the machine be provided with plungers to form the hollow interior of the pattern and that caps,

such as 15 or 20, of an outline corresponding to the cross section of the hollow interior, be pro- .vided for closing the same. Plastics, especially when 'formed by compression molding, provide patterns of extreme accuracy and can be held to tolerances of .001 inch, sometimes better. This also applies where the patterns are machined from blocks or rods of plastic material. Plastic patterns may be made and stored under usual conditions without appreciable change in dimension or distortion.

properties by reducing adhesion to the mold walls during the molding cycle. Combinations of fillers, plasticizers or lubricants are possible with the plastic base material so long as the non-volatile matter is practically non-existent. I The advantages of plastics are best exemplified by the polyesters of the methacrylic acid type. A particular advantage of methyl methacrylate is-its low vaporization point, and the fact that its disappearance can be easily detected by the cessation of its distinctive, odoriierous decomposition products. It is also quite water repellant. a

. Another advantage is the fact that varioustypes and sizes of patterns may be assembled in the same mold. This again speeds up production and is possible because. the patterns remain stationary in the mold. Some plastics give ofi vaporization products that may be recovered as by-products. In certain cases it is desirable to heat the mold to 'a certain'point above the flash point of the plastic, and thus burn away the vapors. This eliminates the mold more quickly, and, in some cases, may be used to carbonize the cavity surface.

From the foregoing, it will be readily noted that the complete casting process of this invention comprises, particularly where a large number of similar castings is desired, the following ment and dissipate the pattern and sprue former. 4. The mold is then placed in a suitable casting machine and molten metal run into .the mold cavity.

5. After the mold has cooled and the metal 50 lidifled, the mold is-broken apart and the casting 4 recovered. Usually all that is necesary before placing the casting in use is removing the sprue left attached to the castings.

From the foregoing, it will be appreciated that there has been provided a method and means for making commercial castings in which disappearing plastic patterns are employed for producing castings of comparatively heavy or thick cross section and without damaging the mold during the distortion of the pattern and formation of the mold cavity. By this means, patterns of an infinite variety of sizes and shapes may be made. When the pattern is too large or complex to be made in one piece, a number of sections may be made singly and thence assembled into one pattern. Cores may be formed from the investment itself by providing hollow portions in the patterns and leaving them open to the atmosphere.

The use of thin walled, hollow patterns provides a double advantage in that it would enable a considerable saving of plastic, and even more important, it facilitates manufacture. Solid patterns of considerable cross section require unusual lengths of time, sometimes ten minutes, to cool in the forming machine before it can be ejected. Also the shrinkage is considerable, and

is not easily controlled. By making the same shape with thin walled sections,the same pattern may be ejected from the machine in a fraction forming portion formed of spaced apart walls of thin sections, whereby a hollow plastic pattern is formed with at least one end of the hollow communicating with the atmosphere, a plastic closure member secured to said open end and a sprue hole former secured to and projecting from the plastic closure member.

7. A pattern formed of plastic having the exterior conforming accurately to the exterior of the part to be produced and the mold cavity forming portion formed of spaced apart walls of thin sections, whereby a hollow plastic pattern is formed having the hollow interior communicating with the atmosphere and plastic closure members secured to the ends of the hollow interior of the pattern.

8. A pattern formed of plastic the ex- I terior conforming accurately to the exterior of 2. A pattern formed of plastic having the exterior conforming accurately to the exterior of the part to be produced and the mold cavity forming portion formed of spaced apart walls of thin sections and with hollow space between the inner surfaces of said walls whereby they, under heat, will collapse inwardly.

3. A pattern formed of plastic having the exterior conforming accurately to the exterior of the part to be produced and the mold cavity forming portion formed of spaced apart walls of thin sections, whereby a hollow, plastic pattern is formed with at least one end of the hollow communicating with the atmosphere, and a plastic closure member secured to said open end.

4. A pattern formed of plastic having the exterior conforming accurately to the exterior of the part to be produced and the mold cavity forming portion formed of spaced apart walls of thin sections, whereby a hollow, plastic pattern is formed with at least one end of the hollow communicating with the atmosphere, a plastic closure member secured to said open end and a sprue hole former secured to and projecting from the pattern.

5. A pattern formed of plastic having the exterior conforming accurately to the exterior of the-part to be produced andthe mold cavity forming portion formed of spaced apart walls of thin sections, whereby a hollow plastic pattern is formed with at least one end of the hollow communicating with the atmosphere, a

plastic closure member secured to said-open end,

and a sprue hole former made of plastic secured to and projecting from the pattern.

the part to be produced and the mold cavity forming portions formed of spaced apart walls of thin sections, whereby a hollow plastic pattern is formed having the hollow interior communicating with the atmosphere and plastic closure members secured to the ends of the hollow interior of the pattern, and a sprue hole former secured to and projecting from the pattern.

9. A pattern formed of plastic having the exterior conforming accurately to the exterior of the part to be produced and whose mold cavityforming portion is hollow so that the walls thereof, under heat, will collapse inwardly and a sprue hole former secured to and projecting from the pattern. 4

10. A pattern formed of plastic exterior conforming accurately to the exterior of the part to be produced and whose mold cavity forming portion is hollow so that the walls thereof, under heat, will collapse inwardly and a sprue hole former made of plastic secured to and projecting from the pattern.

11. A pattern formed of plastic having the exterior conforming accurately to the exterior of the part to be produced and whose mold cavity forming portion is hollow so that the walls thereof, under heat, will collapse inwardly and a hollow sprue hole former made of plasticsecured to and projecting from the pattern.

12. The method of producing solid metallic castings consisting in investing a plastic pattern whose mold cavity forming portion is hollow in an investment composition, allowing the mold to harden and subjecting the mold to heat to cause a softening of the plastic pattern, collapse of the walls of the pattern into the hollow interior thereof and finally a vaporization of the pattern.

13. The method of producing solid metallic castings consisting in forming a plastic reproduction of a polyester of methacrylic acid which has an exterior contour of the casting to be produced and a hollow mold cavity forming portion, attaching a sprue hole former to the pattern, investing the pattern and sprue hole former in a cementitious investment composition, allowing the investment to harden, and subjecting the hardened mold to heat to cause a softening of the plastic pattern, collapse of the walls of the pattern into the hollow interior thereof and finally a vaporization of the pattern.

14. The method of producing solid metallic castings consisting in forming a plastic reproduction of a methyl methacrylate which has an exterior contour of the casting to be produced and a hollow mold cavity forming portion, attaching a sprue hole former to the pattern, investing the pattern and sprue hole former in a cementitious investment composition, allowing the investment to harden, and subjecting the hardened mold to-heat to cause a softening of the plastic pattern, collapse of the walls of the pattern into the hollow interior thereof and finally a vaporization of the pattern.

15. The method oi producing solid metallic castings consisting in forming a plastic reproduction of polystyrene which has an exterior 10 contour of the casting 'to be produced and a hollow mold cavity forming portion, attaching a sprue hole former to the pattern, investing the

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