The Problem a Mould Solves
A mould records a form as a negative to produce positives. The craft turns on one central question: how does the mould come off the original without destroying either?
Undercuts
An undercut is any part of a shape that widens after narrowing. It locks a rigid mould in place so it cannot pull away in a straight line.
Three strategies solve this problem:
- Destroy the mould to release the cast.
- Divide the mould into parts that draw away cleanly.
- Use a flexible material that stretches over the undercut.
The Typology
| Type | Made of | Reusable | Handles undercuts | Typical use |
|---|---|---|---|---|
| Waste mould | Rigid plaster | No, chipped away | By destruction | Single cast from clay |
| Piece mould | Rigid parts | Yes | By division | Multiples in plaster, resin |
| Flexible mould | Silicone, alginate | Yes (silicone) | By stretching | Complex forms, life casts |
| Mother mould | Rigid shell | Yes | Supports flexible layer | Floppy flexible liners |
How Mould Types Work
A waste mould produces a single cast from a clay model. Plaster is poured over the clay, the clay is dug out, and the positive is cast inside. The plaster is then chipped away to free the cast.
A piece mould offers reusability. The form is split into sections that pull away without locking. The pieces align using interlocking keys, though every seam leaves a mark on the cast.
A flexible mould stretches over undercuts easily. Because it lacks structural rigidity, it requires a rigid jacket called a mother mould to hold its shape during pouring.
Flexible Materials
- Alginate: Fast-setting and skin-safe for life casting. It shrinks within hours, making it strictly single-use.
- Silicone: Highly durable and reusable.
Silicone comes in two main types: condensation-cure and addition-cure. Addition-cure offers finer detail and lower shrinkage. However, its cure is inhibited by sulphur-bearing clay, latex, or certain resins.
Common pitfall: choosing the mould type based on budget rather than form. Always evaluate the undercuts first.