This is Part 1 of the silicon rubber mold-making series originally published on this site. It covers the first phase of the process: selecting your materials and building the mold box before any rubber is mixed.

If you want the full process in one document, see the complete silicon rubber mold-making guide. This series goes deeper on each phase for those who want the detail.

Part 1 Overview

Before you mix a single gram of silicone, you need:

  1. The right silicone product for your application
  2. A mold box sized and sealed correctly
  3. A prepared master pattern that will release cleanly

Getting any of these wrong costs you the entire pour — silicone is not cheap, and a failed mold means starting over.

Choosing a Silicone Product

The silicone products most commonly used in prop hobbyist work come from Smooth-On and Polytek. Both sell through distributors and will ship directly to hobbyists.

Smooth-On MoldMax Series

MoldMax is the product line most B9 robot builders and prop makers rely on. Three options cover most use cases:

MoldMax 10 (Shore A 10) — Very soft and stretchy. Good for master patterns with significant undercuts because you can stretch the mold off. Tears more easily than firmer products. Best for small, detailed pieces you need to strip off intact.

MoldMax 20 (Shore A 20) — The most-used option for prop work. Enough body to hold shape without a mother mold shell on smaller pieces, still flexible enough to release complex shapes. Mix ratio 100A:10B by weight.

MoldMax 30 (Shore A 30) — Firmer. Preferred when you want a mold that holds its shape over many pulls without a supporting shell. Better for flat or gently curved pieces.

For the B9 robot parts and pieces like the devil head mold and Mystic Seer components, MoldMax 20 is the default choice.

Measuring Equipment

You must weigh both parts of the silicone on a digital gram scale. Do not try to measure by volume — the density difference between Part A and Part B means volume ratios are unreliable. A $15 kitchen scale with 1g accuracy is sufficient.

Use disposable mixing cups and sticks. Silicone is nearly impossible to clean out of containers once cured.

Building the Mold Box

The mold box contains the liquid silicone while it cures. It needs to be:

  • Rigid enough to hold its shape under the weight of liquid rubber
  • Sealed enough to prevent leaks at the corners and base
  • Easy to disassemble for demolding

LEGO Bricks

LEGO bricks became the standard mold box material in the prop community for good reason. They interlock precisely, create a relatively leak-resistant wall without glue, can be built to any footprint and height in 8mm increments, and are completely reusable.

Build your LEGO wall with at least 1 inch of clearance on all sides of the master and 1 inch minimum depth below the bottom of the master. This ensures adequate rubber thickness on all surfaces.

For the base plate, use a large LEGO baseplate and press the master down onto a thin layer of sulfur-free clay to seal the bottom edge. This prevents rubber from running under the master during the pour.

Foam Core Mold Boxes

For larger pieces, cut foamcore board to size and hot-glue the corners. Seal the interior corners with a bead of hot glue or clay before pouring. Foamcore walls can bow out under the weight of large rubber pours — reinforce with tape or clamps around the exterior.

Sealing the Base

For a one-part mold, the master sits on the base of the mold box with its flat side down. Press the master firmly against the base and seal the perimeter with a thin coil of clay to prevent rubber from sneaking underneath.

For a two-part mold, you’ll build a clay bed to the parting line — covered in Part 2.

Preparing the Master Pattern

The master must be sealed and released before the rubber goes in. This is non-negotiable for porous materials and strongly recommended for everything else.

Sealing

Porous materials — raw foam, unfired polymer clay, unsealed wood, cardboard — will absorb liquid silicone and bond to it, destroying both the mold and the master. Seal with:

  • Krylon Crystal Clear lacquer: 2–3 light coats, 30 minutes between coats. Fast and reliable for most surfaces.
  • Shellac (Zinsser Bulls Eye): Excellent for wood and plaster masters. Brush or spray.
  • Smooth-On Smooth-Prime: Two-part urethane primer that seals and adds hardness to soft masters.

Let the sealer cure fully — overnight is best. Warm-to-the-touch sealed surfaces can still off-gas and cause silicone inhibition.

Mold Release

Apply a light coat of petroleum jelly (Vaseline) or Ease Release 200 to the sealed master. Use a soft brush to work it into recesses. Wipe off excess — you want a thin film, not a thick coating that will fill in detail.

Platinum-cure silicones can release from many surfaces without mold release, but using it guarantees clean separation on the first pull and protects the master if anything goes wrong.

What’s Next

With your materials measured, mold box built, and master prepared, you’re ready for the pour. That process — mixing, pouring, and curing — is covered in Part 2. See the complete mold-making guide for the full picture, or continue to the resin casting guide to understand what the finished mold will be used for.