2017-05-15 10:33:14

The process of Investment casting

The process is generally used for small castings, but has produced complete aircraft door frames, steel castings of up to 300 kg and Aluminum Sand Casting of up to 30 kg. It is generally more expensive per unit than Die Casting or Sand Casting, but with lower equipment cost. It can produce complicated shapes that would be difficult or impossible with Die Casting, yet like that process, it requires little surface finishing and only minor machining.

1. Investment: The ceramic mold, known as the investment, is produced by three repeating steps: coating, stuccoing, and hardening. The first step involves dipping the cluster into a slurry of fine refractory material and then letting any excess drain off, so a uniform surface is produced. This fine material is used first to give a smooth surface finish and reproduce fine details. In the second step, the cluster is stuccoed with a coarse ceramic particle, by dipping it into a fluidised bed, placing it in a rain sander, or by applying by hand. Finally, the coating is allowed to harden. These steps are repeated until the investment is the required thickness, which is usually 5 to 15 mm (0.2 to 0.6 in). Note that the first coatings are known as prime coats. An alternative to multiple dips is to place the cluster upside-down in a flask and then liquid investment material is poured into the flask. The flask is then vibrated to allow entrapped air to escape and help the investment material fill in all of the details.

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Stainless steel investment casting
Process: sodium silicate process(stainless steel casting)Material: ASTM440; SS304;SS316;SS316L;SS2343;ECT.Surface: mirror polishingUsed for: Engine blocks and manifolds, machine bases, gears

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Investment casting stainless steel
Process: sodium silicate process(stainless steel casting)Material: ASTM440;SS304;SS316;SS316L;SS2343;ECT.Surface: mirror polishingUsed for: Engine blocks and manifolds, machine bases

Common refractory materials used to create the investments are: silica, zircon, various aluminium silicates, and alumina. Silica is usually used in the fused silica form, but sometimes quartz is used because it is less expensive. Aluminium silicates are a mixture of alumina and silica, where commonly used mixtures have a alumina content from 42 to 72%; at 72% alumina the compound is known as mullite. During the primary coat(s), zircon-based refractories are commonly used, because zirconium is less likely to react with the molten metal.Other refractory materials that have been used include molochite and chamotte.Prior to silica, a mixture of plaster and ground up old molds (chamotte) was used.
The binders used to hold the refractory material in place include: ethyl silicate (alcohol-based and chemically set), colloidal silica (water-based, also known as silica sol, set by drying), sodium silicate, and a hybrid of these controlled for pH and viscosity.

 

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