engineering fundamentals Rapid Tooling: Composite Epoxy Molding
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Composite Epoxy Molding
  • Mold Fabrication Time: 2-6 weeks.
  • Production Rate: cycle time of 3 to 15 minutes.
  • Accuracy: As good as the SLA pattern: ±0.005 inches (±0.127 mm).
  • Cost: $2000 (about half that of an Al soft tool) for a consumer product part of 5"x5"x3" (12.7x12.7x7.6 mm).
  • Advantages increase with the complexity of the molded part.
Composite tooling bridges the gap between silicone rubber soft molds and aluminum "soft tools". It has advantages in between the two RT methods:
  1. Composite tooling can use production materials (thermoset plastics).
  2. Less expensive than an aluminum tool.
  3. Lower lead time than aluminum tool.
  4. From one mold, 50-500 pieces can be produced.
  5. Part can be of moderate to high complexity.
Longevity of the Mold
Thermoplastic Resin Number of Shots
Investment Casting Wax 1000-10000
Polypropylene 400-4000
Polyethylene 400-4000
Polycarbonate 100-1100
PC/ABS Blends 100-1100
PBT 100-500
Nylon (filled with glass fiber) 300-3000
Nylon (virgin) 200-1200
ABS 300-3500
Acetal 300-3500

 
Since the consistency of the mold is hard, it cannot be distorted to ensure part release. Therefore, elimination of undercuts and the implementation of draft is necessary, as shown in the figure below. As can be seen in the figure, much detail is lost because of the need to make a non-distorting tool that releases the part.
 
 
Composite Molding Process Steps
  1. A master male pattern is made. The master can be made from any material that can tolerate epoxy, such as foam or wood. However, typically the master pattern is a stereolithography part. See the figure below.
  2. An RTV mold is made of the master pattern and a urethane male pattern is made from the mold. The reason for this is that in the epoxy mold fabrication process, the male pattern is typically destroyed. A parting line surface is added, as shown below.
  3. Epoxy and glass cloth are layered around the male pattern to form the mold and are cured successively in layers. The figure below shows the beginning of the "lay up" process for the top half of the mold.
  4. The following figure shows the completed lay up assembly for the top half of the mold.
  5. Curing of the mold epoxy can take from 0.5 to 40 hours. Aging the mold at room temperature increases the longevity of the mold.
  6. To structurally strengthen the mold and for fixturing, the epoxy lay up can be potted in a block of epoxy.
  7. After construction of the epoxy mold, steel inserts can be added for complex slides and coring and machining of the epoxy may need to be undertaken for example, in areas where sprues or gates could not be molded in.
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