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 than 15 minutes 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.