Injection moulding design basics for a better, cheaper mould

Why the part design decides the mould cost

The mould is made to match your part. A design that fights the process needs a more complex and more expensive mould, and makes more rejected parts. A few rules of thumb help a lot, and a good mould maker will suggest changes before cutting steel.

Rules of thumb

• Keep walls an even thickness. Thick and thin areas cool at different speeds, which causes warping and sink marks.
• Add draft, a slight taper on the walls in the direction the part comes out, usually one degree or more. Without it, parts stick and get scratched.
• Use ribs to add stiffness instead of thick walls. Keep a rib thinner than the wall it sits on, often around half to two thirds of it.
• Round the inside corners. Sharp corners concentrate stress and can crack, and they are harder for the plastic to fill.
• Avoid undercuts where you can. An undercut needs a side action or a slider in the mould, which adds cost.
• Think about where the plastic enters, the gate. It leaves a small mark, so place it where the mark does not matter.

Material choice matters too

Different plastics shrink by different amounts and suit different jobs. Tell the mould maker the grade you plan to use and the strength, heat and chemical conditions the part will face, so they can allow for shrinkage and pick the right steel.

Ask for a design review

Before ordering the mould, ask the mould maker to review your model. A short review often finds a change that saves weeks and a good sum of money.

Kinds of work

• Die making
• Mold making
• CNC machining
• Casting
• Fabrication