US3169288A

US3169288A - Coatings for patterns employed in cavityless casting process

Info

Publication number: US3169288A
Application number: US157245A
Authority: US
Inventors: John L Dewey
Original assignee: Dow Chemical Co
Current assignee: Dow Chemical Co
Priority date: 1961-12-15
Filing date: 1961-12-15
Publication date: 1965-02-16
Anticipated expiration: 1982-02-16

Description

United States Patent Ofitice Patented Feb. 16, 1965 Ser. N0. 157,245 22 iss This invention relates to an improved pattern for use in the preparation of foundry castings. ilore particularly, it relates to such patterns for use in the fabrication of castings by the so-called cavityless technique.

It has been proposed heretofore to prepare metal castings by embedding in a mold body a pattern formed from a material that is substantially completely volatile or combustible upon contact with the molten metal of a casting charge, providing the mold body with a pour opening and a vent opening, each communicating with the embedded pattern, and then pouring the casting charge into the pour opening. Contact between the entering molten metal and the pattern is effective to causethe rapid volatilization or decomposition of the latter so that it is completely destroyed, leaving behind a cavity in the mold body to be filled by the casting charge.

The above-described method of producing castings possess certain manifest advantages not to be found in the conventional molding practice which, for the casting of any but the most simple forms, requires rigorous attention to coring and the providing of proper parting lines in the mold. In the described method a casting of any degree of complexity can be produced Without regard to these factors. Parting lines need not be provided for, inasmuch as the casting may readily be formed in one piece. Elements of the structure of the casting conventionally provided by coring are readily produced by proper design of the destructible or fugitive pattern.

It is obvious that the fugitive pattern should possess as small a mass per unit volume as is possible, so that, for

a given material of its construction, the heat required for its destruction be minimal and the volume of the gaseous products of its volatilization be as small as possible. The heat is, of course, supplied by the casting charge and, if too great a quantity of heat be abstracted therefrom, localized solidification may occurwhich may result in the production of inferior castings.

Foamed, thermoplastic, resinous materials, such as polystyrene foam, are ideally suited to the production of these fugitive patterns. They possess the necessary strength to remain dimensionally stable during the embedment of the pattern in the mold body. The actual weight of material in the pattern is small. This, coupled with the volatility of the resinous foam, makes for rapid and complete burn-out of the pattern upon the pouring of the casting charge.

One major difficulty, however, attends the use of foamed thermoplastic resins. Advantageously, and usually, the pattern is fabricated from a solid block of such foamed resin, or component parts thereof are cut or carved from the foamed resin and are then assembled into the desired shape. Cutting through the foamed resin necessarily gives a surface that is rough and pitted, inasmuch as a multiplicity of open cells constitutes the said surface.

This surface pattern is transferred to the mold body in contact therewith and is thus duplicated on the surface of the casting.

it has been suggested that the surface of such a pattern be smoothed by the application of a hot iron. This expedient has only limited value, however, since it is virtually impossible to maintain the dimensions of the pattern by this technique. Furthermore, in the case of a pattern of complex shape, the difficulty of following the contours thereof with the smoothing iron are such that the method cannot successfully be employed.

It is therefore the principal object of the invention to provide a destructible pattern of a foamed, resinous thermoplastic material, the use of which in cavityless casting results in the production of castings having desirably smooth surfaces.

It is an additional object of the invention to provide a means of imparting smoothness to the surface of a destructible pattern of the type described without sacrifice of the dimensional accuracy thereof.

Other and additional objects of the invention will be evident from the description to follow.

According to the present invention, destructible patterns of foamed, thermoplastic resinous materials are given a smooth surface by the application of a coating material that is effective to fill up the open pores or pits of the surface of the pattern. As a result, that portion of the mold body which is in contact with the surface of the embedded destructible pattern does not assume the rough surface characteristics of the resinous foam but rather the smooth surface characteristics of the applied coating. The casting produced in the mold so prepared is accordingly possessed of a smooth surface and, as a consequence, requires the minimum of surface finishing. Maintenance of dimensional accuracy in the casting is readily realized.

Freferably, the pattern coating material should be substantially non-fugitive. structible on contact with the casting charge to give rise to any substantial volume of gaseous products of thermal decomposition or of volatilization, the rapid exit of which products from the mold body through the pour opening may result in sputtering of the molten metal being poured. Furthermore, excessive gas formation at the mold bodyrnctal interface is undesireable. Such gas formation, coupled with the cooling effect of volatilization of a fugitive coating material and heat absorption by the mold body, can cause surface defects in the casting by preventing intimate contact of the metal with the mold surface While localized solidification of the metal may be taking place.

The pattern coating material must be essentially nonadherent to the face of the mold body under the condi tions of high temperature prevailing during casting. A related requirement is that the coating should not retain its form when subjected to this high temperature. If these conditions are not met, the volatilization of the pattern will leave the inside Wall of the mold body faced with the coating material having a rough surface derived from the rough surface of the fugitive pattern.

That is, it should not be deing.

ate

. a V 7 All of these aforementioned desirable attributes'for.

a coating for fugitive patterns employed in cavityless cast ses trolyte s such as water soluble or dispersible salts of polying are possessed by the coatings of the present invention. V

The coating compositions of the present invention comprise admixtures of at least one pulverulentrefractory material, a binding agent for the aforesaid. refractory material and an aqueous dispersing medium therefor. Optionally, and advantageously, the compositions may also contain a dispersing agent. r

The refractory materials, of course, are the major constituents of the coating obtained from the coating compositions. They perform the principal function of filling the imperfections or voids in the pattern.surfacewith a material that will not melt or lead to slag formation during the casting operation. The binding agent serves to maintain the coating in a coherent form so that it will not be lost from the surface of'the pattern during the handling of the latter during its embedrnent in the mold body. i V

The dispersing agentis included in the composition to aid in obtaining uniform admixture of the other components in theaqueous dispersing medium, and to aid in maintaining the desired consistency'of the composition. 7 This consistency can be adjusted so that the product takes the form of a relatively fluid composition suitable for spray or brush application to the pattern, .or it maybe acrylic acid or polymethacrylic acid; polyacrylamide, polyaminoalkylacrylates, polyvinylpyridine and the like. A large number of these polyelectrolytes are known to the art and are employed to improve the handling characteristics of wetsand-clay ixtures in the preparation of foundry sand molds. I When an agent of this type is employed in the practice of the present invention, it imparts to the coating composition a desirable smoothness and consistency in addition to performing its function as a v l binding material in the dried coating.

The preparation of the coating compositions of the present invention, as well as the mode of their use, is

paste-like and adapted to application with 'a spatula, i

trowel or the like.

,After application of thecoating composition thereto,

the pattern may be allowed to stand at ambient temperature until substantial dehydration of the composition has taken place. Preferably, however, it maybe exposed to v an'elevated temperature to hasten the dehydration, which temperature necessarily is below that at which the pattern would-be adversely affected.

The dried coating is-smooth, hard and adherent to the pattern surface. It is of sufficient strength to withstand the stresses imposed upon it during embedment of the'p'attern inrthe mold body. In foundry practice, for example,

the coating remains intact during the sandramming operav tion'and-the sand at thesand-coatinginterface is readily" providedwith the smooth surface desirable for metal cast- When the coating material is subjected to the elevated temperature of the molten casting charge, the coating loses its cohesiveness. Becauseof the loss of its cohesivev ness,'the coatingassumes a'pulverulent form rathe'r tna n retaining the rough surface reflective of that of thenow destroyed pattern with which it was in contact. 1 much as its actualvolume is small, its presence exerts little if any measurable effect on the dimensions of the finished casting. The major portion of this pulverulentmaterial is swept out of the cavity through the vent openings by ing charge. p

A wide variety ofretractory materials may be employed in the preparation of the coating compositions of'the presentfinvention. ,Fireiclay, bentonite, silica, alumina and fine zirconsand are representative.- Advantageously,

bentonit e will constitutea portion of the refractory com-- ponent} being used in conjunction with a second refrac tory material. The use of bentonite aidsme'asurably in; Y dispersing thefother refractory material. in the aqueous' medium; Additionally, its presence in the composition described in the following non-limiting examples, where in all parts and percentages are given on a weight basis.

Example 1 i To'lOGQ parts of water there were added, with rapid stirring, 50 parts of bentonite followed by 1.5 parts of anhydrous sodium pyrophosphate. To the resultant dispersion there were slowly added 1.2 parts of polyacryl- I amide withcontinued stirring, after which 5t) parts of pulverulent fire clay were thoroughly admixed with the thick slurry obtained to give a stiff pasty mass suitable for application to a pattern surface by troweling.

Example 2 Following the procedure of Example 1, a relatively i fluid mixture was prepared from 1000 parts of water, parts of bento'nite, 40parts of pulverulent fire clay, 1.5

parts of sodium pmophosphate and"1.1 parts of polyacrylamide. To this mixture there were added, slowly and with rapid mixing, an additional 190 parts of jbentonite. An extremely thick, smooth paste,resulted. 7

Example Following the procedure of Example 1 a coating composition was prepafedfrom 1000 parts of water, 40 parts' "of bentonite, 140 parts of pulverulent fire clay, 1.5parts of sodium pyrophosphate and 1 part. of polyacrylamide.

'The resultant mixture was sufiiciently fluid for brush ap- 1 plication to the surfaces of a fugitive pattern.

yExa mp le 4 The coating composition of Example 2 was smoothly applied to the rough surfaces of a block of foamed polylnas- 1 jel'lt coating resulted.

styrene with a spatula. The composition was allowed to dry for 4 hours at F. A hard, smooth, adher- The thus .coat'ed block was embedded, according to known procedure, in a foundry sand mold body. A pour I the gaseous products of the thermal decomposition of the fugitive pattern resulting from the pouring of the cast-.

7 coated block of foamed polystyrene was employed as the lends"considerablev stability v thereto. in; general, the .7

bentonite will be employed in amount equal to. from about 8.0 percentto about 10.9 percent of the total weight ;of refractory material. Y

The binding agent may be chosenfrom the 'widerange of materials thatare known tojbe usefulin the .preparation of molds and ;c ores'of foundry sand compositions, such'as cereal, flour,- asphalti'cemulsions, thermoplastic A, resinous comop'sition'dispersions and the like. A- preferred type of binding agent" isrepresented by polyele'cacteristic of the pattern." 7

block of foamed PP i w th the coating composition of Example 3.

fugitive pattern possessed. the rough, pitted surfacechar Excinpled V The coating obtainedafter drying of thecomposition was smooth-and hard and the castingresult ng from the use i of the-coated block as a fugitive pattern was free fro'm fsurface roughness and pitting. i

v 1..Irl a process for casting of metals which comprises making a pattern mold with destructible patterns of What" is claimed is:

foamed,. thermoplasticj resinous; materials with surfaces correspondingfto' the desired casting embeddedl m said mold,'and the pouring'of' molten metal into sald mold the casting obtained when an unpolystyrene was coated, by brush whereby the foam pattern is destroyed and a metal casting is obtained, the improvement which comprises depositing on the surface of the pattern a coating of an aqueous suspension of an admixture of at least one pulverulent inorganic refractory material and a binding agent therefor and causing the removal of Water from the deposited Wet coating whereby to provide the pattern with a smooth, dry, adherent, cementitious coating.

2. The process of claim 1 wherein bentonite constitutes at least about 8 percent by Weight of the total Weight of 10 the pulverulerxt refractory material.

References (Zited in the file of this patent UN ITED STATES PATENTS Owen June 14, 1938 Myers Dec. 30, 1952 Shroyer Apr. 15, 1958 Bean June 3, 1958 Holbrook et a1 Jan. 1, 1963 FOREIGN PATENTS Great Britain Jan. 30, 1957

Claims

1. IN A PROCESS FOR CASTING OF METALS WHICH COMPRISES MAKING A PATTERN MOLD WITH DESTRUCTIBLE PATTERNS OF FOAMED, THERMOPLASTIC RESINOUS MATERIALS WITH SURFACES CORRESPONDING TO THE DESIRED CASTING EMBEDDED IN SAID MOLD, AND THE POURING OF MOLTEN METAL INTO SAID MOLD WHEREBY THE FOAM PATTERN IS DESTROYED AND A METAL CASTING IS OBTAINED, THE IMPROVEMENT WHICH COMPRISES DEPOSITING ON THE SURFACE OF THE PATTERN A COATING OF AN AQUEOUS SUSPENSION OF AN ADMISTURE OF AT LEAST ONE PULVERULENT INORGANIC REFRACTORY MATERIAL AND A BINDING AGENT THEREFOR AND CAUSING THE REMOVAL OF WATER FROM THE DEPOSITED WET COATING WHEREBY TO PROVIDE THE PATTERN WITH A SMOOTH, DRY, ADHERENT, CEMENTITIOUS COATING.