•Heat from burning slows the casting-cooling rate, yielding a more machinable structure.
•Resin-bond strength allows smaller draft angles, deep draws, and built-in mold locators that prevent mold shift mismatch.
Disadvantages to the shell-mold process are:
•Since the tooling requires heat to cure the mold, pattern costs and pattern wear can be higher.
•Energy costs are higher than for other processes.
•Material costs are higher than those for greensand molding.
Tooling for shell molds is generally more expensive than for other processes because it is more precise and must resist heat and abrasion. Also, heat and resin binders are necessary. For these reasons, shell molding is most suitable for medium-to high-volume parts, where the manufacturer utilizes added value.
Tooling for shell molds is generally more expensive than for other processes because it is more precise and must resist heat and abrasion. Also, heat and resin binders are necessary. For these reasons, shell molding is most suitable for medium-to high-volume parts where the manufacturer utilizes added value.
The V-process, or vacuum molding, is one of the newest casting processes; in it, unbonded sand is held in place in the mold by a vacuum. In this process, a thin plastic film of 0.7 to 2.0 mm (0.03 to 0.08 in.) is heated and placed over a pattern. The softened film drops over the pattern with 26 to 52 kPa, (3.8 to 7.6 psi) and the vacuum tightly draws the film around the pattern. The flask is placed over the plastic-coated pattern and is filled with dry, unbonded, extremely fine sand and vibrated so that the sand tightly packs the pattern. The flask walls also create a vacuum chamber with the outlet shown in Fig. 2.8 at the right side of each illustration.
Another unheated sheet of plastic film is placed over the top of the sand in the flask, and the vacuum is applied to the flask. The vacuum hardens the sand so the pattern can be withdrawn. The other half of the mold is made the same way. After cores are put in place (if needed), the mold is closed. During pouring the mold is still under vacuum but the casting cavity is not. When the metal has solidified, the vacuum is turned off and the unbonded sand runs out freely, releasing a clean casting with zero draft, high-dimensional accuracy and with a 125 to 150 RMS surface finish.
Fig. 2.8 Steps in V-process: a) the pattern is placed on a hollow carrier plate; b) a heater softens the plastic film; c) the vacuum draws the plastic film tightly around pattern; d) the flask is placed on the film-coated pattern; e) the flask is filled with dry sand; f) the back of the mold is covered with unheated plastic film; g) the vacuum is released and the mold is stripped; h) the cope and drag assembly form a plastic-lined cavity; during pouring, molds are kept under vacuum; i) the vacuum is released and as the sand flows freely, the casting is freed; j) finished part.
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