How to Use Fiberglass Resin Mother Molds

Use colored casting material or paint your mold after casting.

When casting three-dimensional objects, crafters often use a two-part mold comprised of a rigid outer structure and flexible inner lining. The exterior portion of the mold, which provides support during casting, is called the mother mold. Mold makers construct mother molds out of many material, but plaster bandages and fiberglass are commonly used because they are easy to shape, durable and inexpensive. This two-part mold structure is advantageous because it allows the caster to make accurate castings in a rigid base, and then dismantle the mold and peel the flexible lining away from the casting.

Things You'll Need

• Gloves
• Breathing mask
• Mother mold
• Lining
• Mold release spray
• Liquid rubber, parts A and B (or other casting material)
• Scale
• Disposable cups
• Stirrer
• Craft knife
• Towel
• Letter opener
• Cable ties
• Scissors

Instructions

One-Piece Mold

1. Put on gloves and a breathing mask. Work in a well-ventilated area.

2. Place the mother mold on a flat surface. Place the flexible lining inside of the mother mold. Spray the lining with mold release.

3. Measure out equal amounts of part A and part B in disposable plastic cups. Use a scale if the manufacturer measures proportions of parts A and B in weight.

4. Pour the two parts into a single cup and mix them together thoroughly. Scape the sides and bottom of the cup while you stir to make sure that all of the mixture is incorporated.

5. Pour the mixture slowly into the deepest part of the mold in a thin steady stream.

6. Wait 10 minutes to 12 hours for the mold to set. The setting time depends on the volume of the mold, the temperature and the type of rubber used.

7. Place a folded towel on your table. Flip the fiberglass mother mold over so that the rubber mold will fall out. Tap the sides of the mold against the towel to help the mold release. Use an object like a letter opener to help separate the mold from the fiberglass.

8. Wipe down the casting to remove the mold release agent.

Two-Piece Mold

1. Place the lining in the halves of the mold. Spray the mold with mold release. Hold the two halves together. Wrap a cable tie around the mold and pull it tight to keep the halves together. Add more cable ties to further secure the mold and prevent leaking.

2. Repeat Steps 3 through 6 from Section 1 to mix the rubber and pour it into the mother mold.

3. Place the mold on a padded surface. Cut off the cable ties and pull away the mother mold halves. Set the fiberglass mother mold pieces aside.

4. Peel the flexible lining away from the casting. Wipe down the casting to remove the mold release agent.

Tips & Warnings

• Estimate the volume of a mold by filling with with rice and then measuring the amount of rice. Use water instead of rice for a more accurate measurement.

• Use the above procedure to cast with plaster, rubber, plastic or clay.

• To make a hollow casting, for helmets or masks, brush the liquid casting material onto the surface of the mold. Apply multiple layers to increase the thickness and rigidity of the casting.

References

• "The Art & Craft of Ceramics"; Maria Dolors Ros I Frigola; 2006
• "The Essential Guide to Mold Making & Slip Casting"; Andrew Martin; 2007
• "Vertebrate Paleontological Techniques"; Patrick Leigg Et Al.; 2005