Silicon Rubber Mold Making â€” Part 4

Part 4 of the silicon rubber mold-making series. See the full mold-making series.

Frequently Asked Questions

What advanced mold making topics does Part 4 of the series cover? Part 4 of the silicon rubber mold making series covers advanced techniques for complex geometries, including multi-part molds for masters with undercuts in multiple directions, glove molds for extreme undercut situations, and brush-on silicone techniques for large masters where block pours are impractical. These techniques build on the two-part mold process covered in earlier parts.

What is a mother mold and when is it required? A mother mold is a rigid outer shell (fiberglass, plaster bandage, or urethane foam) that supports a thin silicone skin during casting. It is required for brush-applied silicone molds where the rubber is too thin to hold its shape independently, and for multi-part mold setups where silicone sections must be precisely supported and aligned during production casting.

How are brush-on silicone molds built for large masters? Brush-applied silicone (Smooth-On Rebound 25 or 40) is applied in thin layers directly on the master’s surface, with each layer allowed to gel before the next is applied. Three to four layers build up approximately 1/4 inch of silicone skin. A rigid mother mold is then built over the cured silicone to hold its shape during casting — fiberglass mother molds are strongest for production use.

How many parts can a complex silicone mold have? While two-part molds handle most prop building situations, three-part and four-part molds are used for masters with undercuts pointing in more than two directions. Multi-part molds require mold release between every cured silicone surface before pouring adjacent parts, and registration features at every parting line to ensure precise alignment of all parts during casting.