Design for Moulding: Wall Thickness, Draft, Ribs, Gates and the Defects They Prevent

Why DFM happens before tooling

A mould is a precision steel investment. Once it is cut, the part’s geometry is fixed; every design weakness becomes a process compromise for the life of the tool. Design-for-moulding review is therefore the highest-leverage engineering activity in an appliance program.

The rules are well known. Applying them consistently, across every component in a product, is what distinguishes a manufacturable design.

Uniform wall thickness

Thick sections cool slower than thin ones, shrink more and produce sink marks on the opposite face, voids inside and warpage across the part. Walls should be as uniform as the function allows, thick sections cored out, and transitions made gradually rather than in steps.

Draft

Every surface parallel to the direction of mould opening needs draft so the part releases without dragging. Smooth surfaces need less; textured surfaces need more, in proportion to the texture depth. Zero-draft designs look clean in CAD and leave scuff marks on every part.

Ribs, bosses and fillets

Ribs add stiffness without adding wall thickness, but if they are too thick at the base they cause sink on the show surface. Screw bosses need a diameter matched to the fastener and a fillet at the base. Sharp internal corners concentrate stress and impede flow; fillets fix both.

• Rib thickness proportioned to the adjoining wall
• Rib height limited and drafted on both faces
• Boss outer diameter sized to the screw, cored to avoid sink
• Fillets at every rib and boss base

Gates, flow and weld lines

Where the material enters the cavity determines how it flows, where flow fronts meet (weld lines) and where the gate mark sits. Gates belong away from cosmetic faces and from areas that carry load, and should fill the part in a balanced way so that the last region to fill is vented.

Weld lines are weakest and most visible when flow fronts meet cold and at an angle. Gate placement, melt temperature and venting all reduce them, but the design decides whether they land on a handle root or a hidden rib.

Designing for assembly at the same time

Snap-fits, locating pins, screw bosses and alignment features are part of the mould too. Reviewing the assembly sequence alongside the part design ensures those features can be moulded without side actions where possible, and that the fixture that will hold the part during assembly has something to locate on.

This is the core argument for integrating engineering, tooling, moulding and assembly: the design review can include everyone who will live with the decision.