Mother-Molds

A rigid mother mold is used to hold the flexible rubber mold in place while casting. There are a wide variety of materials available to fabricate a "mother-mold" or rigid exterior shell for your mold: plaster with burlap, plaster gauze, Ultracal (or FGR) with fiberglass, Shell Shock and Plasti-Paste. Be sure to consider whether you will need to apply a release before the fabrication of the mother mold. If you have any questions, please give us a call on our Technical Support line, 212-367-7561.

• Plasti-Paste is a two-component fiber resin that holds a vertical surface without sagging and quickly cures to a strong, durable, lightweight plastic. It is suitable for use as a mother mold material or as an abrasion-resistant coating. The advantage Plasti-Paste has over conventional polyester mother molds is that it is easier to use and has no odor. It is also much lighter than comparable plaster mother molds. The pot life is 10 min. Demold is about 90 min. Plasti-Pastecan be troweled or brushed over the rubber mold to form a lightweight, super strong plastic support shell. Mix ratio is 3A:1B.

• Shell Shock “Shell” because because the plastic can be used to form a shell and “shock” in reference to this product’s good impact resistance vs. other products on the market.
  
  Applications - Shell Shock is ideal for making fast, lightweight rigid molds and for creating silicone appliances and effects (use as a replacement for ‘stone molds’). You can also brush a “gel coat” into a rubber mold and back it up with rigid foam, creating a highly detailed lightweight casting. This product can be brushed onto styrofoam (polystyrol) as an impact resistant coating that can be sanded, primed and painted (minimum 3 coats recommended).

  Shell Shock can also be used to make thin, light-weight rigid support shells (mother molds) to support brush-on rubber molds. It is mixed 1A:3B by volume and steadily develops thixotropy (becomes thick). Applied in thin sections, the mixture will hold a vertical surface. Like any brush-on material, if you apply too thick it will sag.

  Pot life for the fast is about 3 minutes which helps with thixotropy, but you have to be aware of your working time. The material will stick to itself, so you can quickly build 3 to 4 layers to a minimum recommended thickness of 3/8in. (1 cm) which is recommended for impact resistance. Time between coats is about 15 minutes depending on mass. Do not disturb the previous layer. Material cures in about 1 hour at room temperature (depending on mass). Gel time and cure time can be reduced with mild heat.

How Is Shell Shock Different From Plasti-Paste? Plasti-Paste is heavily filled with fibers which provide strength and make Plasti-Paste a good 'mother mold / support shell' material. The fibers make Plasti-Paste thick to the point where you can trowel the material onto surfaces without thickness limitation. Cured Plasti-Paste exudes an oil that makes it difficult to paint.

Shell Shock is a different product that chemically thickens and is best applied in thin layers. Applied more like a brush-on rubber (such as Rebound 25) Shell Shock wets out nicely on a variety of surfaces. When cured, it is harder and more rigid at Shore 85D (vs. 70D for Plasti-Paste). Shrinkage on Shell Shock is very low at .0006 in/in. (cm/cm). You can also sand and paint Shell Shock.