The devil head mold is one of the best practice projects for learning two-part silicone mold making. The piece has enough complexity — undercuts at the horns and chin, surface detail in the facial features — to teach the core technique without being so complex that a beginner’s first mold is guaranteed to fail.

This walkthrough covers the complete two-part mold process for a roughly fist-sized figurative head with undercuts. The same approach applies to any similar-scale piece with detail on all sides: small figures, helmet details, jewelry masters, and mechanical prop components.

Understanding Two-Part Molds

A one-part mold works when the master has a flat back — you pour rubber over it from one side, demold, and you have a cavity that replicates the front surface. Any piece with significant three-dimensional form on all sides requires a two-part mold: two separate rubber halves that mate at a parting line and enclose the entire master.

The devil head has no flat back, curved horns that create undercuts, and a chin that extends below the cheekline. A one-part mold would either trap the master or require tearing the rubber to release it. A two-part mold splits at the widest horizontal plane — typically through the ears — and each half releases cleanly.

Step 1: Determining the Parting Line

The parting line is the seam where the two mold halves meet. A good parting line:

  • Runs around the widest perimeter of the piece
  • Avoids cutting across prominent surface details
  • Minimizes undercuts on each half

For the devil head, the parting line runs horizontally through the sides of the head at approximately ear height — this is the widest point, and it means the horns (pointing up) fall cleanly in the top half, while the chin and neck (pointing down) fall in the bottom half. Each half has no significant undercuts.

Sketch the parting line on your master with a pencil or thin line of clay before committing.

Step 2: The Clay Bed

The clay bed is a layer of non-sulfur clay that supports the master at the parting line for the first pour. It fills the mold box to parting-line depth and creates the flat mating face of the first mold half.

Use sulfur-free clay — plasteline or Roma Plastilina work well. Sulfur-containing clays (many modeling clays) inhibit platinum-cure silicone and will prevent the rubber from curing at the clay contact surface.

  1. Press the clay into the mold box to roughly half the height of the master
  2. Set the master into the clay so it sinks to exactly the parting line
  3. Smooth the clay surface flush against the master with your finger or a clay tool
  4. The clay should conform perfectly to the master at the parting line — no gaps, no clay above the line

This clay surface will become the mating face between your two mold halves.

Step 3: Registration Keys

Registration keys are small bumps and divots pressed into the clay bed that create matching features in both mold halves. These keys lock the two halves together in exact alignment when casting.

Use a rounded tool (the back of a pen cap, a marble) to press 4–6 hemispherical divots into the clay surface around the master, staying 1/2 inch away from the master perimeter. The first rubber pour fills these divots, creating bumps. When the second pour is made against the first mold half, it creates corresponding divots that mate with the bumps.

Without registration keys, the two halves can shift and the cast piece will have a misaligned seam.

Step 4: First Pour

With the clay bed built and registration keys set, mix your silicone rubber and pour the first half.

Mix and pour following standard technique — see the complete mold-making guide for mixing ratios and pour technique. Pour from a height to help air bubbles escape. Fill to cover the exposed top half of the master by at least 1/2 inch.

Allow to cure fully (16–24 hours for most platinum-cure silicones at room temperature).

Step 5: Setting Up for the Second Pour

After the first half cures:

  1. Remove the mold box walls (if using LEGO, disassemble them)
  2. Flip the mold over — the clay bed is now on top
  3. Carefully remove all the clay, working it off with a clay tool without disturbing the cured rubber below
  4. Remove any clay residue from the master and the cured rubber with isopropyl alcohol on a cotton swab
  5. Rebuild the mold box walls around the assembly
  6. Apply a coat of mold release (Ease Release 200 or petroleum jelly) to the cured rubber face — this is critical, it prevents the second pour from bonding to the first half

Step 6: Second Pour

Mix a fresh batch of silicone and pour the second half, covering the exposed remaining half of the master by 1/2 inch.

Allow to cure fully.

Step 7: Demolding

Open the mold by flexing the two halves apart along the parting line. The registration keys will guide separation. Work slowly and flex both halves equally — don’t just peel one side.

Once the two halves are separated, flex each half to release the master. The undercuts that made a one-part mold impossible are now manageable because each half only encloses half the undercut.

Cleaning and Trimming

Trim any silicone flash at the parting line with scissors. Check the mold face of each half for defects — any clay residue that was left will show as a raised area on the mold face.

Test the registration keys by mating the two halves without a casting — they should lock together precisely with no movement at the parting line.

First Cast

Pour your first urethane resin casting with the two halves rubber-banded or clamped together. Fill from the pour gate at the bottom. The cast piece will have a seam line at the parting line — this gets sanded and filled during finishing.

The devil head mold technique scales directly to the larger-scale mold work needed for B9 robot components — the robot torso, collar, and radar section all use two-part molds of varying complexity.