RPM

Rubber Plaster Mold Casting

(AKA plaster mold casting)

What is RPM?

Rubber Plaster Molding is a method of producing aluminum or zinc castings by pouring liquid metal into plaster (gypsum) molds.

Rubber Plaster Molding is extremely versatile, and is capable of prototyping parts that are typically produced using die-casting, permanent-mold, investment casting or sand casting methods. Plaster Casting is excellent for rapid prototyping and short-run production. Die-cast surface finish and thin walled geometry are easily produced using plaster casting. The tooling used in the plaster casting process has the ability to release molds with undercuts or features with zero draft. Tooling can be inexpensively modified when compared to die-cast or investment cast tools for magnesium, aluminum and zinc alloys

How Does RPM Work?

Step 1:  Model or Master Pattern (positive)

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.      "Clone" or adapt customer supplied model

How Does RPM Work? (continued)

Step 2:  Rubber Mold Pattern (negative)

1.      Silicone molds (reusable masters) are made from the model (master pattern).

2.      Core plugs are made from negative molds.

Step 3:  Resin Mold Pattern (positive)

1.      A positive resin cope and drag and core segments are now made from the negative molds.  Core boxes are made from the core plugs. Gating, runner system and flasks are added as necessary.

2.      The resin mold is used repeatedly to produce each plaster mold. Tooling life varies, but generally yields 50 -200 plaster molds depending on part geometry.

1.      If the resin tooling wears out, new tooling can be quickly produced using the rubber masters described in step 2.

2.      Duplicate sets of tooling can be made from the master negative.

Step 4:  Plaster Mold (negative)

1.      A liquid plaster slurry is poured around the cope and drag pattern and into the core boxes.

2.      After the plaster is firm, the mold is removed from the cope and drag patterns. The plaster molds contain the negative part geometry. 

3.      The mold halves (cope/drag) and cores are carefully assembled into a complete mold.

4.      They are then baked for 1-3 days to remove all moisture content.

Step 5:  Metal Pouring (positive)

1.      Molten metal is prepared by degassing, and a spectrographic sample is taken to check the chemical analysis.

2.      The molten metal is then gravity poured into the assembled plaster mold.

3.      After the metal has solidified and the plaster has cooled, the plaster is carefully broken away by mechanical knock-out and high pressure waterjet to reveal the metal casting

How Does RPM Work? (continued)

Step 6:  Break-Out and Clean-Up:

The remaining plaster is removed via pressure washing and the part is then hand cleaned and detailed. During this process, gates and risers are removed and the part is prepped for machining.

Step 7:  Secondary Operations

1. The raw castings are inspected and serialized.
2. Castings may then be further inspected per customer request:
1. X-Ray
2. Penetrant inspection
3. After finish inspection, casting is ready for:
1. Machining
2. Heat treatment
3. Chemical film
4. Chromate conversion
5. Paint
6. Special finishes
7. Assembly
8. Form-in-place gasketing

What types of Materials can be Cast by RPM?

When should RPM be considered?

·         Low cost tooling is required to produce precision, highly complex shapes, and premium quality aluminum and zinc castings.

·         Good surface finish required.

·         Dimensional accuracy required.

·         Rapid prototyping for die-cast parts to reduce time to market and U.L. approval.

·         Economical short-run production (1-2000 pieces up to 5,000).

When should RPM be considered? (continued)

·         High volume applications of complex or unusual shapes.

·         For aesthetic applications where appearance is critical.

·         Bridge tooling for long lead-time production die cast & as backup tooling.

·         Require geometry with zero draft.

·         Wall thickness less than .040"/1.01mm.

·         Minimal residual stresses and distortion in castings.

·         Reduce time for machining and secondary operations.

RPM Technical References

Alloys:   All aluminum and zinc casting alloys to commercial and military specifications.

Size:  There is no size limitation but parts typically range within 2 Cu In. to 36 cu. in. and range in weight from 1 oz to 15 lb. 

Limitations: The process is limited to non-ferrous metals with pouring temperatures below 2,000 ºF - including aluminum, zinc casting alloys and some copper-based alloys.

Holes: Not economical to cast small holes (1/4 in. or less) unless odd shape or inaccessible areas for machining.

Draft: Typically 1/2 to 2 degrees.  Zero draft is possible in specified areas.

Corner radii and fillets:  As required, typically 1/16 in.

Mechanical properties: Tensile, yield & elongation per the appropriate commercial and military specifications. See separate technical sheet.

RPM Technical References (continued)

Tooling - pattern equipment:

• Loose pattern, to expedite for up to 20 pieces.
• Epoxy resin , usually up to 500 pieces.
• Metal, aluminum or brass, used to obtain best tolerance and quality.
• Rubber for quantities up to 1,000 pieces. Tooling can be duplicated easily from master tooling to expedite delivery or for higher volumes.

Typical Applications

1. Framework, cases, shells, enclosures, bases, housings, for  telecommunications, business machines, medical equipment, computers, automotive, aerospace, electronics and robotics.
2. Molds for plastics industry - rotational molds, vacuum form, expanded polystyrene molds, kirksite injection molds.

Ron Humphrey 425.450.0099

Humphrey Associates, Inc.

PO Box 700

Carlsborg, WA 98324-0700

rhumphrey@humphrey-associates.com