Two-part molds handle most prop building situations. But some masters have geometry that can’t be split cleanly into two halves — or require hollow castings that two-part molds can’t produce. This guide covers the techniques for more complex mold configurations.

This is Part 3 of the silicone mold making series. Part 1 covers materials and mold box setup; the complete guide covers the standard two-part process.

When You Need More Than Two Parts

A two-part mold fails when:

  • The master has undercuts that point in three or more directions, making it impossible to define a single parting line that allows both halves to release
  • The master is nearly symmetrical in all axes (like a sphere) with no natural flat parting line
  • The casting requires hollow walls that can’t be achieved by back-filling a two-part mold

Signs you need a multi-part mold:

Draw imaginary lines from the deepest undercut points. If you can’t find a parting line that allows both mold halves to flex clear of all undercuts simultaneously, you need additional mold pieces.

Three-Part Block Molds

A three-part mold adds a third silicone piece — typically a side section, an undercut cap, or a separate base piece — to handle geometry that two parts can’t manage.

Example geometry: A figure with outstretched arms. A standard vertical split would lock the arms in both halves. A three-part mold splits vertically (left and right halves) with a separate bottom piece handling the base undercuts.

Construction sequence:

  1. Identify the three parting surfaces
  2. Build the clay bed for Part 1 (same as two-part mold process)
  3. Pour and cure Part 1
  4. Remove clay; build clay bed for Part 2 against the cured Part 1
  5. Apply mold release to Part 1 surface
  6. Pour and cure Part 2
  7. Remove remaining clay; apply mold release to both cured surfaces
  8. Pour and cure Part 3

Key discipline: Mold release between every cured silicone surface before pouring adjacent parts. Any missed surface will bond the parts together permanently.

Registration: Each part needs registration features that index it against the adjacent parts. Press hemispheres or small cylinders into the clay at each parting line to create matching keys in the silicone.

Glove Molds

For masters with extreme undercuts in all directions — a figure with deep undercutting everywhere — a block mold in any configuration won’t demold without tearing. A glove mold is the solution.

A glove mold builds up a thin, highly flexible silicone skin (Shore A 10 or softer) directly on the master’s surface. The skin is flexible enough to peel off the master and flex around cast pieces during demolding. Smooth-On’s silicone selector is a useful reference for comparing Shore hardness ratings across brushable silicone products.

Process:

  1. Mount the master on a base (this becomes the pour hole for casting)
  2. Brush-apply Shore A 10 silicone in thin layers, building up to 3/16"–1/4" thickness
  3. Allow each coat to gel before applying the next
  4. Build additional thickness at high-stress points (thin sections, sharp edges, parting line area)
  5. Create a rigid mother mold (plaster bandage or fiberglass) over the cured silicone skin to support it during casting

Demolding: The master is removed by carefully peeling the glove off — this is possible with Shore A 10 silicone because of its extreme flexibility. The same peeling action is used to demold cast pieces.

Limitation: Glove molds have short production life. The extreme flexibility that makes them work also makes them prone to tearing after repeated use. They’re appropriate for limited production runs (5–20 pieces) and aren’t suitable for high-volume casting.

Core Molds for Hollow Castings

Hollow castings — where the cast piece is a thin shell rather than a solid block — require a core. The core defines the interior surface and, combined with the outer mold, creates a consistent wall thickness throughout the casting.

Why hollow castings matter:

  • Dramatically lighter than solid casts — important for wearable props and large display pieces
  • Less resin consumption per piece
  • Faster cure (thinner sections cure more quickly and with less exothermic heat)
  • Required for pieces that need interior space (electronics, structural components)

Foam Core Method

A foam core placed inside the mold before casting:

  1. Cast a solid piece first; use it as the master for a foam core
  2. Or: shape foam directly to the interior profile
  3. Place foam core in the mold; pour resin around it
  4. The foam becomes permanently embedded (can’t be removed)

This produces a lightweight piece but the interior is closed foam rather than accessible hollow space. Appropriate for display parts; not appropriate for pieces needing interior access.

Rotational Casting

Pour a small amount of resin into the closed mold and rotate the mold continuously, coating all interior surfaces. As the resin gels, it builds up on the walls.

Technique: Pour approximately 20–30% of the mold’s cavity volume. Rotate the mold slowly in all axes — over, under, side to side — for the first 5–7 minutes of the pot life. Continue until the resin is gelled enough to hold the wall thickness.

Result: A hollow shell with wall thickness determined by resin volume and rotation duration. More resin or longer rotation = thicker walls.

This technique is covered in detail in the advanced casting techniques guide.

Two-Piece Core Mold

For precision hollow castings:

  1. Make a standard outer mold of the exterior surface
  2. Make a separate core mold of the interior profile (scaled down by the desired wall thickness)
  3. Pour the core first; demold
  4. Place the cured core inside the outer mold; pour resin between core and outer mold
  5. Demold and remove the core

Complex to set up but produces consistent, controllable wall thickness.

Mother Mold Design for Large Multi-Part Setups

Large or complex silicone molds need rigid support structures — mother molds — to hold their shape during casting. For multi-part molds, the mother mold must itself be multi-part, matching the silicone section geometry. The Wikipedia overview of molding processes provides helpful context on how mother mold design fits within the broader family of casting and molding techniques.

Materials:

  • Plaster bandage (Ultracal 30): Easy to work with; sets quickly; adequate strength for light-duty molds
  • Fiberglass: Strongest option; appropriate for high-production molds and large formats
  • Urethane rigid foam (poured behind the silicone): Quick and lightweight but lower precision

Alignment: Mother mold sections must register against each other precisely, or the silicone sections they support will be misaligned and cast pieces will show parting line offsets. Build registration features (bolts through flanges, or block key protrusions) into the mother mold sections at the design stage. Make: magazine’s moldmaking tutorials offer practical examples of mother mold construction across a range of project scales.

For practical guidance on the standard two-part mold process before progressing to these advanced techniques, see the complete silicone mold making guide and troubleshooting guide.

Frequently Asked Questions

When is a multi-part (three or more piece) mold required instead of a standard two-part mold? A two-part mold fails when the master has undercuts pointing in three or more directions making a single parting line impossible, when the master is nearly symmetrical in all axes with no natural flat parting line, or when the casting requires hollow walls that back-filling a two-part mold cannot achieve. The diagnostic test is drawing imaginary lines from the deepest undercut points to find whether any parting line allows both halves to flex clear simultaneously.

What is a glove mold and when is it used? A glove mold builds up a thin, highly flexible silicone skin (Shore A 10 or softer) directly on the master’s surface by brush-applying in thin layers to 3/16–1/4 inch thickness, backed by a rigid mother mold. It is used for masters with extreme undercuts in all directions where a block mold in any configuration would tear before releasing. Glove molds have short production life of 5–20 pieces due to the extreme flexibility that makes them work.

What are the three methods for producing hollow castings? The foam core method embeds foam permanently inside the casting for lightweight non-accessible pieces. Rotational casting pours a small resin amount (20–30% of cavity volume) into a closed mold and rotates it in all axes for 5–7 minutes while the resin gels against the walls. The two-piece core mold method uses a separate scaled-down core mold to precisely control wall thickness throughout the casting.

What registration discipline is critical in three-part mold construction? Mold release must be applied between every cured silicone surface before pouring the adjacent part — any missed surface will bond parts together permanently. Registration features (hemisphere or cylinder protrusions) must be pressed into the clay at each parting line to index adjacent parts. Each part needs its own registration system against both neighbors.

What materials are used for mother molds supporting large or complex silicone setups? Plaster bandage (Ultracal 30) is easy to work with and adequate for light-duty molds. Fiberglass is the strongest option for high-production molds and large formats. Urethane rigid foam poured behind the silicone is quick and lightweight but lower precision. All mother mold sections need registration features (bolts through flanges or block key protrusions) to align precisely against each other.

What is the advantage of rotational casting for large B9 robot components? Rotational casting produces a hollow shell with wall thickness determined by resin volume (approximately 20–30% of cavity volume) and rotation duration — more resin or longer rotation produces thicker walls. For large B9 components that would be impractically heavy if solid, this technique dramatically reduces weight while maintaining structural integrity through an even-thickness shell.