This process is economical, environmentally and ecologically acceptable, energy thrifty, versatile, and clean. A disadvantage of the V-process is the necessity of plated pattern equipment. The steps of the process are shown in Fig. 2.8.
2.3.3 Evaporative Pattern Casting Process
This process is also referred to as any of the following: expanded polystyrene (EPS) process, lostfoam process, expendable pattern casting, and lost pattern process. It is unique in that a mold and pattern must be produced for every casting. Evaporative-pattern casting is a manufacturing technique in which an expanded pattern is used during the casting process. Preforms of the parts to be cast are molded in expanded polystyrene; an aluminum master mold is used to create the pattern. Pre-expanded polystyrene beads are injected into the pattern mold. Steam expands the bead to bond the beads together and fill most of the space. As expandable polymers are steamed, the beads continue to bond and create a solid pattern. The mold is then cooled and opened, and the polystyrene pattern is removed. Bonding various individual segments of the mold together, using hot-melt adhesive, allows a complex shape of the pattern to be formed.
The individual patterns are assembled into a cluster around a sprue and then coated with a refractory compound. After the coating has dried, the foam pattern assembly is positioned on several inches of loose dry sand in a vented flask. Additional sand is then added while the flask is vibrated until the pattern assembly is completely compacted and embedded in sand. Finally, molten metal is poured into the pattern, the pattern vaporizes upon contact with the molten metal, which replaces it to form the casting. Gas formed from the vaporized pattern permeates through the coating on the pattern, the sand, and finally through the flask vents. The sequence in this casting process is illustrated in Fig. 2.9.
Fig. 2.9 Evaporative-pattern process: a) foam pattern of polystyrene is coated with refractory compound; b) coated foam pattern is placed in flask and sand is compacted around pattern; c) molten metal vaporizing the pattern and replacing it to form the casting.
A significant characteristic of the evaporative-pattern casting process is that the pattern need not be removed from the mold, no cores are needed, inexpensive flasks are satisfactory for the process, complex shapes can be cast, no binders or other additives are required for the sand, and machining can be eliminated. However, expensive tooling (i.e., a new pattern is needed for every casting) restricts the process to long-run casting.
Investment or lost-wax casting is primarily a precision method of casting metals to fabricate near-net-shaped metal parts from almost any alloy. Intricate shapes can be made with high accuracy. In addition, metals that are hard to machine or fabricate are good candidates for this process. This process is one of the oldest manufacturing processes and was developed by the ancient Egyptians some 4000 years ago.
Investment casting got its name from the fact that the pattern is invested (covered completely) with the refractory material. It can be used to make parts that cannot be easily produced by other manufacturing processes, such as turbine blades, and other components requiring complex, often thin-wall castings, for example aluminum structural parts having a wall dimension of less than 0.75 mm (0.03 in.).
The sequences involved in investment casting are shown in Fig. 2.10. A mechanical drawing of the part is the starting point of the process; the drawing illustrates an injection die in the desired shape. This die will be used to inject wax or a plastic such as polystyrene to create the pattern needed for investment casting. The patterns are attached to a central wax sprue, creating an assembly, or mold. The sprue contains the pouring cup from which the molten metal will be poured into the assembly.
Fig. 2.10 Steps in investment casting: a) wax patterns are produced by injection molding; b) the patterns are attached to sprue to form a pattern assembly (tree); c) pattern assembly is coated with a thin layer of refractory material; d) the mold is completed by covering coated tree with sufficient refractory material to make it rigid; e) the mold is held in an inverted position and dried in, the wax is melted out of the cavity; f) preheated mold is poured with molten metal; g) the mold is shaken out; h) casting is separated from the sprue.
The pattern assembly (tree) is then dipped into a slurry of very fine-grained silica and binders including water, ethyl silicate, or other refractory material, and allowed to dry. After this initial coating has dried, the final mold is achieved by repeatedly dipping the pattern tree into the refractory slurry until a shell of 6 to 8 mm (0.24 to 0.31 in.) has been applied.
The mold is allowed to air dry in an inverted position for about 8 to 10 hours to melt out the wax. The mold is then pre-heated to 800 to 1000°C (1474 to 1832°F) for 3 to 4 hours (depending on the metal to be cast) to drive off water, remove any residues of wax, and harden the binder.
Pouring into the preheated mold also ensures that the mold will fill completely. Pouring can be done using gravity or vacuum conditions. After the metal has solidified, the mold is broken up and the casting is removed. Most investment castings need some degree of post-casting machining to remove the sprue and runners and to improve the surface finish. The gate is ground off. Parts are also inspected to make sure they were cast properly, and if not, are either fixed or scrapped.
Investment casting produces exceedingly fine quality products made of all types of metals. It has special applications in fabricating very high-temperature metals such as superalloys for making gas turbine engine blades and nozzle guide vanes.
The variety of steels used and of parts cast has increased dramatically as designers and engineers have realized the potential of investment castings. The aerospace, armament, automotive, food, petrochemical, nuclear, textile, valve and pump, and other general engineering industries all use the technique.
Aluminium alloys are the most widely used nonferrous investment castings in the fields of electronics, avionics, aerospace, pump and valve applications, and military command equipment.
Titanium alloy investment castings are produced for static structural applications requiring metallurgical integrity with high fracture toughness.
Plaster mold casting is similar to sand molding except that plaster is substituted for sand.
Plaster of Paris, or simply “plaster,” is a type of building material based on calcium sulfate hemi-hydrate, nominally CaSO4 · 0.5H2O. It is created by heating gypsum to about 150°C (302°F). The reaction for the partial dehydration is
CaSO4 · 2H2O → CaSO4 · 0.5H2 + 1.5H2O (released as steam).
In plaster mold casting a plaster is mixed using talc, sand, sodium silicate, and water to form a slurry and to control contraction and setting time, reduce cracking, and increase strength. This slurry is poured over the polished surfaces of the pattern halves (usually plastic or metal) in a flask and allowed to set. The slurry sets in less
