An injection-moulded part must be designed not only for its final shape but for filling, cooling and ejection from a rigid mould.

Important features include:

• draft so surfaces can withdraw from the mould;
• reasonably uniform wall thickness to reduce uneven cooling;
• ribs rather than unnecessarily thick solid sections when stiffness is needed;
• gate placement that supports balanced filling;
• ejector locations that can push the cooled part without damage.

Polymers contract as they cool. If a dimension in the mould is $L_m$ and an approximate linear moulding shrinkage is $s$, a first estimate of final dimension is

$$L_p\approx L_m(1-s).$$

For $L_m=100.0$ mm and $s=0.012$,

$$L_p\approx100(1-0.012)=98.8\ \mathrm{mm}.$$

Real shrinkage is not necessarily uniform or isotropic. Flow-induced molecular orientation, fibre reinforcement, local packing pressure and cooling rate can make different directions contract differently.

Uneven contraction creates warpage. A thick region that cools slowly can also produce sinks or internal voids because its interior contracts after the outer skin has stiffened.

Good injection-moulding design therefore tries to make filling and cooling predictable. The mould is intentionally shaped to produce the desired part after thermal contraction and ejection, not simply as an exact negative of the final CAD model.