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Investment casting

 

Process description

A mold is used to generate a wax pattern of the shape required. A refractory material zircon, then a ceramic slurry and finally a binder is used to coat the pattern which is slow fired in an oven to cure. The wax is melted out and the metal cast in the ceramic mold. The mold is then destroyed to remove the casting. Process often known as the ‘Lost Wax’ process.

Materials

All metals, including precious, refractory and reactive alloys (cast in vacuum).

Investment Casting

Process variations

  • Blends of resin, filler and wax used.
  • Use of thermoplastic resin instead of wax.
  • Ceramic and water soluble cores can be used.

Economic considerations

  • Production rates of up to 1000/h, depending on size.
  • Lead times usually several weeks, but can be shorter.
  • Slow process due to many steps in production. Cure time can be as high as 48 h.
  • Wax or plastic patterns can be injection molded for high production volumes.
  • Best suited to metals having high melting temperatures, and/or which difficult to machine or which have high cost. Material utilization high.
  • Some automation possible.
  • Pattern costs can be high for low quantities.
  • Ceramic and wax cores allow complex internal configurations to be produced, but increase the cost significantly.
  • A ‘tree’ of wax patterns enables many small castings to be handled together.
  • Most suitable for small batches (10–1000) using manual labor, but also high-volume production with automation.
  • Sometimes used for one-offs, especially in decorative work.
  • Tooling costs low to moderate, but dependent on complexity.
  • Equipment costs low to moderate (high when processing reactive materials).
  • Labor costs very high. Can be labor intensive as many operations involved.
  • Finishing costs low to moderate. Gates and feeders are removed by machining or grinding. As cast part typically cleaned by shot, bead or sand blasting.

Typical applications

  • Turbine blades
  • Machine tool parts
  • Aerospace components
  • Valve and pump casings
  • Pipe fittings
  • Automotive engine components
  • Decorative work, e.g. figurines
  • Optical instrument parts
  • Small arms parts
  • Gear blanks
  • Levers
  • Jewelry

Design aspects

  • Very complex castings with unusual internal configurations possible.
  • Wax pattern must be easily removable from its mold.
  • Complex shapes assembled from several simpler shapes.
  • Practical way of producing threads in hard to machine materials, or where thread design unusual.
  • Uniform sections preferred. Abrupt changes should be gradually blended in or designed out.
  • Avoid sharp corners.
  • Fillets should be as generous as possible.
  • Bosses and undercuts possible with added cost.
  • Inserts not possible, but integral rivets are.
  • Lettering possible, either in relief or inset.
  • Molded-in holes, both blind and through possible, but difficult.
  • Length to diameter ratio for blind holes typically 4:1.
  • Minimum hole = Ø0.5 mm.
  • Machining allowance usually between 0.25mm and 0.75 mm, depending on size.
  • Draft angle usually zero, but 0.5–1° desirable on long extended surfaces, or if mold cavity is deep.
  • Minimum section ranging from 1mm for aluminum alloys and steels, 2mm for copper alloys, but can be as low as 0.6mm for some applications.
  • Maximum section =75 mm.
  • Maximum dimension =1m.
  • Sizes ranging 0.5 g–100 kg in weight, but best for parts less than 5 kg.

Quality issues

  • Moderate porosity.
  • High strength castings can be produced.
  • Grain growth more pronounced in longer sections which may limit the toughness and fatigue life of the part.
  • Quality of casting to a large degree dependent upon the characteristics of wax.
  • Very good to excellent surface detail possible.
  • Surface roughness ranging 0.4–6.3 µm Ra.
  • Flatness tolerances typically ±0.13mm per 25 mm, but dependent on surface area.
  • Minimum angular tolerance =±0.5°.
  • A process capability chart showing the achievable dimensional tolerances is provided .
  • No parting line on casting.

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