How To Make a 2 Piece Silicone Rubber Mold

This mold making tutorial video shows how to make a two piece silicone block mold using Mold Star 15. The finished mold can be used for casting resin, wax, foam, and many other materials.

The original model is a plastic sphere. The 2-part mold should be split along the horizontal axis.

Process Overview

1: One half of model is embedded in clay
2: The first half of the mold is poured and allowed to fully cure
3: The mold is turned over and clay is removed
4: Release agent is sprayed to prevent bonding
5: The second half of the mold is poured

The mold is allowed to fully cure prior to demolding.

Making the First Half

Build a mold box. Seal seams with hot glue. Secure with a mold strap. The model must first be partially embedded in clay. Begin by building a clay bed inside the box. Press the model into the clay bed until only the top half is exposed. Use more clay to fill in any open spaces. Level the clay surface to the model’s midpoint. Flatten and smooth the clay surface. Embed small acorn nuts in the clay; reason: creates registration keys between mold halves.

Mold Star 15
1:1 Low Viscosity Platinum Silicone

Dispense equal amounts 1A:1B by volume. Dispense parts A and B into mixing container. Mix parts A and B as directed. Slowly pour into one corner of the mold box. Pour 1/2 inch (1.25 cm) over model’s high point. Allow Mold Star 15 to cure.

Making the Second Half

Begin by removing the mold box. Turn the mold over and remove the clay. Do not remove the model from the mold. Carefully remove the acorn nuts without removing the model from the mold. Remove excess clay with a stiff brush. Trim flashing and then reassemble the mold box. Reseal seams with hot glue.

Ease Release 200 must be used to prevent fresh silicone from sticking to cured silicone.

Spray a light, even coating. Brush over all surfaces. Spray a second light, even coating. Create a funnel shaped pour spout with clay. Secure the clay funnel to the top of the model. Measure and mix second batch of Mold Star 15. Slowly pour into one corner of the mold box. Pour 1/2 inch (1.25 cm) over model’s high point. The clay funnel should remain partially exposed. Allow Mold Star 15 to cure.


Carefully remove the mold box and trim flashing. Remove clay funnel. Slowly peel back the cured rubber. Carefully remove the original model. Mold Star 15 reflects perfect detail, ready for casting.

Learn other mold making techniques:
* Brush-on mold –
* 1-piece block mold –
* Cavity pour mold –
* 2-piece core mold –

Make It Now! with Smooth-On

Post time: Dec-14-2016