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Technical Information: Plaster Mold Process

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What is Plaster Mold Casting?

Plaster mold casting, also called rubber plaster molding (RPM), is a method of producing aluminum or zinc castings by pouring liquid metal into plaster (gypsum) molds.

Step 1:  Model or Master Pattern

1. Constructed from customer drawing or CAD file.
2. Stereolithography, traditional hand crafted or machined.
3. Model is engineered to include:
     A) Metal shrinkage.
     B) Mold taper (if required)
     C) Machine stock (if required).
4. We can "clone" or adapt customer supplied model if requested.

Step 2:  Foundry Pattern Equipment

1. Negative molds are made from model.
2. Core plugs are made from negative molds.
3. A positive resin cope and drag pattern is now made from the negative molds.
4. Core boxes are made from the core plugs
5. Gating, runner system and flasks are added as necessary.
6. Duplicate sets of tooling can be made from the master negative.

Step 3:  Plaster Mold

1. A liquid plaster slurry is poured around the cope and drag pattern and into the core boxes.
2. The plaster mold is next removed from the cope and drag patterns.
3. The plaster mold and cores are then baked to remove moisture.

Step 4:  Pour Casting

1. Molten metal is prepared by degassing, and a spectrographic sample is taken to check the chemical analysis.
2. The molten metal is then poured into the assembled plaster mold.
3. The plaster is removed by mechanical knock-out and high pressure waterjet.
4. When the casting has cooled, the gates and risers are then removed.

Step 5:  Secondary Operations

1. The raw castings are inspected and serialized.
2. Castings may then require (per customer specifications):
     A) Heat treatment
     B) X-Ray
     C) Penetrant inspection
3. After finish inspection, casting is ready for:
     A) Machining
     B) Chemical film, chromate conversion, paint
         or special finishes
     C) Assembly
     D) Form-in-place gasketing.

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