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The How-To-Hootch: Ward's Workshop - Casting Resin Parts in an RTV Two-Part Mold

Sunday, October 2nd, 2011  by Keith Ward

  1. Open up a funnel-shaped passageway into the highest point of the mold cavity, using a sharp blade to make the opening as large as practicable. You may also need to cut an air escape passageway for some cavities to fill completely.
  2. Cut two pieces of .060 or heavier plastic to support the two sides of the RTV mold. (these are our mold support plates)Parting agent - baby powder works best for me. Use a soft brush to lightly coat everything inside the mold cavity, carefully blowing away any excess powder.
  3. Line up the mold locating points, and using the two mold support plates, rubber band the mold halves together. Use rubber bands that can hold the mold firmly together without distorting the RTV.  
  4. Resin - get the 3 to 7 minute pot-life type because air bubbles are your enemy in this casting process, and they need time to rise out of the pattern area before the resin starts setting up. (Alumalite has a pot life of 30 secs.)
  5. Mix resin 50/50 (measuring accuracy really counts here) into a plastic throwaway cup. Stir slowly to avoid as best as possible, too many air bubbles. (You just cannot get all of the bubbles out at this stage.)
  6. Pour the resin mix slowly into the mold passageway opening. When the mold is full, lightly squeeze the mold and "burp" the mold, hopefully getting the last air bubbles out of the pattern area.
  7. Let the resin cure for 45 minutes, and then remove the rubber bands and support plates. Carefully open the mold halves and inspect your new resin part!
  8. For repeat castings, dust interior of mold halves with more baby powder and make another resin casting.

The most reliable products I have used are:

Micromark                    # 82083 One-to-One Rapid Rubber RTV

Smooth-On Co.             # 321 Smooth-cast Resin (7 min. pot life) 

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