Permanent Molding

Instead of using sand as the mold material, a metal is used as a mold. Typically cast iron or Meehanite (a dense cast iron) is used as the mold material and the cores are made from metal or sand. Cavity surfaces are coated with a thin layer of heat resistant material such as clay or sodium silicate.

The molds are pre-heated upto 200 ºC (392 ºF) before the metal is poured into the cavity. The cavity design for these molds does not follow the same rules for shrinkage as in 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. However, care has to be taken to ensure proper thermal balance, by using external water cooling or appropriate radiation techniques.

Permanent mold castings, while not as flexible as sand castings in allowing the use of different patterns (different part designs), lower the cost of producing a part. At a production run of 1000 or more parts, permanent mold castings produce a lower piece cost part. Of course, the break-even point depends on the complexity of the part. More complex parts being favored by the use of permanent molds.

Typical part sizes range from 50 g to 70 kg (1.5 ounces to 150 lb). Typical materials used are small and medium sized parts made from aluminum, magnesium and brass and their alloys.

Typical parts include gears, blades, props, hubs, splines, wheels, gear housings, pipe fittings, fuel injection housings, and automotive engine pistons.

Permanent Mold Castings

Molten metal is gravity poured into cast iron molds, or graphite molds coated with a ceramic mold wash. Cores can be metal, sand, sand shell, or other.

Molds open and castings are ejected. The Low Pressure Permanent Mold method pressure-pours up to 15 p.s.i.

Molten metal is gravity poured into cast iron molds, or graphite molds coated with a ceramic mold wash. Cores can be metal, sand, sand shell, or other. Molds open and castings are ejected. The Low Pressure Permanent Mold method pressure-pours up to 15 p.s.i.

Metals:
Aluminum, Zinc, some Brass, Bronze, H.C. Copper, Lead and Gray Iron.

Size Range:
Limitation mainly foundry capabilities. Aluminum and Copper base: ounces to 100 lbs. Ferrous: 60 lbs. max.

Tolerances:
Aluminum: Basic ± .015"
Add ± .002"/in.
If across parting line add
± .010" to ± .030" depending on size
Copper Base: Similar to Investment
Iron: ± .03" Basic.

Surface Finish:
Aluminum: 150-250RMS
Copper Base: 125-200RMS
Ferrous: 200-350RMS

Minimum Draft Requirements:
Non-Ferrous:
Outside: 2° Min (3° desirable)
Inside: 2° Min (4° desirable)
Ferrous: Outside: 1° Inside: 5°

Normal Minimum Section Thickness:
Aluminum: .100" for small areas, up to 3/16" or more for large areas.
Copper Base: .060"
Ferrous: 3/16" for small areas, 1/4" normal.

Illustration 4- Permanent Mold in action

Advantages:

Suitable for high volume casting
Quality of heavier casting improves with better use of tooling's and equipment
Casted products have better tensile strength and elongation than sand castings
Mass productions can be done is a single production run, which reduces the manufacturing cost
Products have excellent mechanical properties.

Disadvantages:

High tooling cost
Short mold life; higher the pouring temperature, shorter the mold life.
Limited to low-melting-point metals

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Permanent Mold Castings

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Last modified: November 07, 2011

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MPIF; and any and all other we did not mention.

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Last modified: March 09, 2012