Making 3D Printed Moulds for Casting

Jackson B.

From Print to Production

3D printing and mould making are natural companions. The printer produces a high quality master pattern with precision and repeatability that would take skilled hand carving many hours. That master is used to make a silicone mould, which allows many identical parts to be made from different materials, such as casting resins, concrete, soap, chocolate, wax, low temperature bismuth alloys, or epoxy composites. The combination dramatically extends what a 3D printer can produce, enabling materials and quantities that are impossible with FDM alone.

This is particularly valuable for makers who sell products: a single printed master can produce a silicone mould that casts 50–200 identical parts before wearing out. The casting material can be far superior to FDM plastic in strength, flexibility, detail, or surface finish. And the per unit cost of casting drops dramatically compared to printing each item individually.

Preparing the Master Pattern

The master pattern is the 3D printed original from which the mould is made. Every surface detail, including layer lines, scratches, and primer brush marks, will be faithfully reproduced in the silicone mould and subsequently in every cast. This process demands the highest possible surface quality on the master. Use PLA or PLA+ at a 0.1 mm layer height, sand progressively to 1200 grit, apply 2–3 coats of filler primer, sand at 400 grit, and repeat until perfectly smooth. The time invested in a perfect master pays off across every cast made from the resulting mould.

Silk PLA has a particularly smooth natural surface that requires less post processing for a high quality master, making it ideal for small decorative items where hand sanding is tedious. Apply a release agent (petroleum jelly or a specific mould release spray) to the master surface before pouring silicone; this step prevents the silicone from bonding permanently to the plastic.

Designing Mould Boxes and Parting Lines

For simple convex objects (buttons, coins, flat decorative pieces), a one piece "open face" mould works: pour liquid silicone over the master in a box, let it cure, remove the master, and fill the resulting cavity with casting material. For objects with undercuts or complex geometry, a two piece mould is needed; the parting line (where the two mould halves meet) must be designed carefully to allow release without distorting the cast geometry. Print the mould box in PLA or PLA+ to the exact dimensions needed and print registration keys (small interlocking bumps and holes) that hold the mould halves in precise alignment during casting. This investment in careful mould design pays off immediately in production quality casts from the first pour.