Patent Publication Number: US-4093770-A

Title: Coating for gasifiable foam patterns

Description:
This is a division of application Ser. No. 608,959, filed Aug. 29, 1975, now U.S. Pat. No. 4,010,791. 
    
    
     BACKGROUND OF THE INVENTION 
     Considerable attention has been devoted to the &#34;cavityless technique&#34; of casting. This technique employs a pattern formed from a material that is substantially completely volatile or combustible upon contact with molten metal. Accordingly, the mold body, typically of sand, is arranged with a pouring opening and a vent opening, each communicating with the embedded pattern; upon pouring the molten casting charge into the pouring opening, contact between the entering molten metal with the pattern will cause a rapid volatilization or decomposition of the pattern material so that it is completely destroyed, leaving behind a cavity in a mold body which is in turn filled by the casting charge. 
     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 embedment of the pattern in the sand mold body and the actual weight of the material is dramatically small. This, coupled with the volatility of the resinous foam, makes for a rapid and complete burn out of the pattern upon the pouring of the casting charge. 
     The pattern can be fabricated either from a solid block of such foam resin or by expanding resinous beads to the shape of a pattern die. Cutting blocks to share results in certain rough portions on the pattern surface. If the pattern is molded out of polystyrene beads through a conventional process of steaming and expanding the beads to form a solid mass, the beads tend to define a porous surface where the curved surfaces of separate beads meet. The full explanation for the formation of such porosity is not fully understood, although it is believed that the pressure of steam used to expand the beads, applies a uniform outward force with respect to each bead. But the outer die, in which the beads are expanded offers a continuous resistance surface along all of the beads. The beads are restrained at point contact with respect to each individual bead; the resistance surface does not act uniformly with respect to each bead. As a result, small depressions at the juncture between adjacent beads is produced. 
     It has been suggested by the prior art that such pattern surface be smoothed by the application of a hot element, such as an iron; this has proven to be of little value since it is impossible to maintain an accurate dimension for the pattern by the pressing technique. It is also been proposed by the prior art to use wax-like coatings which are meltable along with the fugitive pattern upon contact by the molten metal. This also has proven to be of little value because the wax-like coatings are difficult to maintain in an accurately smooth condition prior to casting and volatilization of the wax may occur in advance of volatilization of the pattern thereby retaining the problem as previously encountered. 
     In the earliest tests to solve the problem of a rough or imperfect surface of a casting formed by a foam pattern, the use of permeable washes were used; they were formed of a slurry of ceramic material consisting typically of pulverulent refractory material, an aqueous dispersing medium and a small amount of binding agent. Such slurries or washes did not prove entirely satisfactory since their purpose was to resist the molten metal and stay solid while the pattern was evaporated. This still does not produce an answer to the problem requiring an ultra-smooth surface since the refractory material, which remained after the pattern was volatilized, possessed the porous-type imprinted surface of the pattern. 
     Certain critical applications require that castings formed by this technique have an ultra-smooth surface devoid of any defect or undulations. For example, in the making of dynamically loaded elements, such as crankshafts or disc brake calibers, the presence of any slight crevice or pore will promote a site for fatigue fracture to start and will eventually reduce the fatigue life of said element. 
     Moreover, regardless of how smooth the pattern surface may eventually be made, refractory materials defining the mold cavity will have a particle size which imprints microscopic or small defect sites onto the casting surface since such casting must conform to the sand particle restraint. 
     Some mechanism must be found to provide an ultra-smooth surface for castings which are made by the cavityless method utilizing foamed fugitive patterns and sand and/or other molding media molds. 
     SUMMARY OF THE INVENTION 
     The primary object of this invention is to provide a destructible or fugitive pattern of foamed, resinous thermal plastic material, the use of which in a cavityless casting method will result in castings of unusually smooth surfaces. 
     Another object of this invention is to provide an improved method for making castings by the cavityless method, which method eliminates break down of the sand material constituting the mold for said technique upon contact by the molten metal, such break down being caused as a result of burning of said sand particles at the immediate contact with said molten metal. 
     Particular features pursuant to the above objects is the use of a composite coating on said foam pattern, the first portion of said coating consisting of a highly tacky, highly viscous adhesive substantially devoid of solid or refractory particles, the second portion of said coating consisting of refractory particles having a typical size in the range of 100-140 AFS and which particles are held in place by contact with said adhesive. 
    
    
     SUMMARY OF THE DRAWING 
     FIG. 1 is a highly enlarged schematic illustration of the macroscopic character of a portion each of the foam pattern, composite coating, and mold material consisting essentially of back-up sand and/or other molding medium. 
    
    
     DETAILED DESCRIPTION 
     According to the present invention, a destructible pattern A of foam thermoplastic resinous material is given a smooth surface 10 by the application of a composite coating B. One portion of the coating consists of a fugitive portion 11 which vaporizes upon contact with the molten material and the other portion 12 of the coating is comprised of extremely fine particles not effected by contact of the molten metal and therefore are not fugitive. The portions of the coating cooperate to align the small particles 12a of the nonfugitive portion into an extremely smooth flat arrangement. 
     The fugitive portion 11 of the coating is preferably comprised of a tacky adhesive, preferably an adhesive commonly referred to as synthemul, having a typical chemical composition consisting essentially of: acrylic resin having approximately 65% non-volatiles in solution with 100% normal propyl alcohol. Other adhesives can be employed provided they possess the following characteristics: a tackiness characterized by viscosity of 10,000 cps, an adhesive quality which will not chemically attack fugitive foam materials (such as polystyrenes), the adhesive is sprayable or capable of being deposited in an ultrathin coating no greater than 0.002 inches thick and the adhesive will flow to fill the pores or crevices of the foam material. In addition, the adhesive must be characterized by low gas and solid content. The gas content must be extremely low. When the adhesive is pryolized upon contact by the molten metal, it is important that there be no excessive generation of gases which migrate through the interstices of the mold material. Such interstices or channels must be utilized primarily by the vaporized foam pattern material for ecape; if excessive or additional gas is generated by the adhesive material, the ability of the porous molten material to release such gas would be burdened. The low solid contact requires that the adhesive be substantially devoid of any refractory materials. The ability to form the smooth surface is dependent upon this characteristic. 
     The nonfugitive portion 12 of the coating is preferably comprised of a carbon facing sand having a particle size in the range of a typical 100-140 AFS. Carbon facing sand is a by-product of petroleum manufacturing; it will not break down upon contact by molten metal (temperatures in excess of 2100° F), such as conventional sand. Other equivalent non-fugitive materials should possess the characteristics: do not break down at the temperatures of the moten metal upon contact therewith and thus become equivalent to a dust, and possess an extremely uniform fine particle size so that the particles can be aligned to form an ultra-smooth flat interface. It is important that the carbon facing sand or equivalent nonfugitive material of a fine nature be applied in ultra-low quantities, that is that the coating of the facing sand should be no greater than two particle diameters thickness. This insures that the concentration of facing sand will not be excessive and thereby cause problems upon re-use of the molding sand for subsequent molding operations. 
     The material from which the mold cavity 13 is formed is preferably unbonded sand 14 having a minimum particle size range of 20-40 AFS. Such molding material is identified herein as backup sand; it will impart microcrevices or pores if molten material is solidified directly in contact therewith. The unbonded sand can also be broken down slightly by a phenomenon called burn-in upon contact with the molten metal. This is avoided by the interposition of the carbon facing sand in a particularly unique aligned disposition as taught herein.