to form the mold. The mold halves are extracted carefully from the pattern and then dried in an oven at a temperature range of 120 to 260°C (248 to 500°F) to remove moisture.
The mold halves are carefully assembled to form the mold cavity and are preheated to about 120°C (248°F). Prior to mold preparation the pattern is sprayed with a thin film of patting compound to prevent the mold from sticking to the pattern. Molten metal is then poured into mold. After the metal is solidified and cooled, the plaster mold is broken away ftom the finished casting. This process is normally used for nonferrous metals such as aluminum, zinc, or copper-based alloys. It cannot be used to cast ferrous material because the sulfur in gypsum slowly reacts with iron.
The minimum wall thickness of aluminum plaster castings typically is 1.5 mm (0.06 in.). Plaster molds have high reproducibility, permitting castings to be made with fine details and close tolerances. Because plaster molds have very low permeability, any gas produced during the solidification of the metal cannot escape, because mechanical properties and casting quality depend on alloy composition and foundry technique. Slow cooling due to the highly insulating nature of plaster molds tends to magnify solidification-related problems, and thus solidification must be controlled carefully to obtain good mechanical properties.
Casting sizes may range in weight from less than 30 grams (1 oz) to 7 kg (15 lb). The draft allowance is 0.5 to 1.0 degree. Good surface finish and dimensional accuracy, as well as the capability to make thin cross-sections, are advantages of plastermold casting.
The ceramic mold casting process, also called cope-and drag investment casting, uses a permanent pattern made of plastic, wood, or metal. To make the slurries for molding, fine-grained zircon (ZrSiO4), aluminum oxide, and fused silica are mixed with bonding agents and poured over the pattern, which has been placed in a flask. These slurries are comparable in composition to those used in investment casting. Like investment molds, ceramic molds are expendable. However, unlike the single-part molds obtained in investment castings, ceramic molds consist of a cope and drag setup.
Making a ceramic mold is similar to making a plaster mold in that the ceramic slurry is poured over the pattern. It hardens rapidly to the consistency of rubber; after that, the halves of the mold are removed from the pattern and reassembled. The volatiles are removed using a flame torch or in a low-temperature oven. The mold is then baked in a furnace at about 1000°C (1832°F). The mold is now capable of high-temperature pours. This ceramic molding can be used to cast ferrous and other high-temperature alloys, stainless steel, tool steels, and titanium. Its advantages (good accuracy and surface finish) are similar to those of plaster mold casting. A draft allowance of 1° is recommended. Parts made as a ceramic mold casting can be very small or up to a ton.
The process is expensive but can produce casting with fine detail and eliminate secondary machining operations.
2.4 PERMANENT MOLD CASTING PROCESSES
Permanent mold casting refers to all casting technologies in which the mold cavity is reused many times and is made of a metallic material or graphite. This is in contrast to other casting technologies, such as sand casting, investment casting, and others in which the mold is made of nonmetallic materials. Metal mold casting is the predominant way to manufacture shape castings. Specifically, about 90% of all aluminum castings produced are in metal molds, including gravity fed, low-pressure, and high-pressure die castings.
Permanent mold casting technologies are classified as gravity, pressure, squeeze, and specialized processes.
2.4.1 Basic Permanent Mold Casting
The permanent mold casting process is the production of castings by the pouring of molten metal into permanent metal molds using gravity or tilt pouring. These molds are commonly made of steel or cast iron, and their cores are made from metal or sand. Metal molds are constructed of two or more sections that are designed for easy, precise opening and closing. The cavity designs for these molds do not follow the same rules for shrinkage as do sand casting molds, due to the fact that the metal molds heat up and expand during the pour, so the cavity does not need to be expanded as much as in the sand castings.
Typical parts made by permanent mold casting are automobile pistons, cylinder heads, gears, and kitchenware. Parts that can be made economically generally weigh less than 25 kg (55 lb), although special castings weighing a few hundred kilograms have been made using this process. The cavity, with gating system included, is machined into the halves to provide accurate dimensions and good surface finish. Steps in the basic permanent mold casting process are shown in Fig. 2.11.
Fig. 2.11 Steps in basic permanent-mold casting: a) mold is preheated and coated; b) mold is closed and molten metal poured; c) mold is opened and casting is ejected; d) finished part.
In preparation for casting, the mold is first preheated at 150 to 260°C (302 to 500°F); then a refractory washer mold coating is brushed or sprayed onto those surfaces that will be in direct contact with the molten metal alloy. The proper operating temperature for each casting is set. Cores, if applicable, are inserted, and the mold is closed manually or mechanically. The alloy is heated at the pouring temperature and is poured into the mold through the gating system. Unlike expendable molds, permanent molds do not collapse, so the mold must be opened before appreciable cooling contraction occurs in order to prevent cracks from developing in the casting.
It is desirable and generally more economical to use permanent steel cores to form cavities in a permanent mold casting. When the casting has re-entrant surfaces or cavities from which one-piece permanent metal cores cannot be withdrawn, destructive cores made of sand, shell, plaster, and other materials are used. This process then is called semipermanent mold casting. Sectional steel cores are used in some instances.
Advantages of permanent mold casting include the facts that cast surfaces are generally smoother than sand castings, and closer dimensional tolerances can be maintained. Permanent mold castings usually have better mechanical properties than sand castings because solidification is more rapid and fill is more laminar.
This process is used mostly for aluminum, magnesium, copper alloys, and gray iron because of their generally lower melting points. The process is not economical for small production runs and intricate shapes because of the difficulty in removing the casting from the mold.
2.4.2 Vacuum Permanent Mold Casting
Vacuum permanent mold casting (not to be confused with vacuum molding) is similar to low-pressure permanent mold casting, except for the step of filling the mold. In this case, the molten metal is sucked upward into the mold by vacuum pump. A schematic illustration of the vacuum casting is shown in Fig. 2.12.
Fig. 2.12 Schematic illustration of the vacuum casting process a) before flow up of molten metal, and b) after flow up of molten metal into cavity.
The permanent mold is enclosed in an airtight bell housing. The housing has two openings: the sprue
