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1.3 Design for manufacturability

Design for manufacturability (DFM) means checking a part design against the rules that make it mouldable — before the mould is cut. For injection moulding the core rules are textbook material, stable for decades, and open. They are not secrets; they are the accumulated engineering common sense of the field, and any designer of moulded parts is expected to know them.

Each rule below links to a short glossary entry in the Glossary reference.

1.3.1 The core rules

  • Draft angle. Every wall parallel to the mould-opening direction must be slightly tapered (typically 1–3°) so the part slides off the steel instead of scraping. Zero-draft walls jam, scuff, or force the toolmaker into expensive mould mechanisms.
  • Uniform wall thickness. The single most important rule. Thick sections cool more slowly than thin ones, and the difference produces sink marks, internal voids, warpage and long cycle times. Good moulded parts are shells of near-constant thickness; stiffness comes from ribs, not from bulk material.
  • Rib ratios. A rib stiffens a wall without thickening it — but a rib that is too thick relative to the wall (a common rule of thumb keeps it around 50–60% of the wall thickness) creates a thick junction and a sink mark on the visible face opposite. The same caution applies to a boss: a solid raised mounting feature is a thick section, so it is cored out or blended to keep its effective thickness in check.
  • Undercut avoidance. An undercut is any feature that would trap the part in a two-half mould — a sideways hole, a snap hook, an internal lip. Each one demands a side action or lifter in the tool, adding cost, complexity and failure modes. Designers avoid undercuts where they can, or arrange for the feature to be formed by the natural opening of the mould.
  • Rounded corners: fillets and radii. Sharp internal corners concentrate stress and disrupt the flow of melt. Replacing them with generous fillets — each defined by its radius — improves both strength and fill, though the radius must be balanced against wall thickness so the corner does not itself become a thick section.
  • Gate and weld-line placement. Choose where the melt enters the part so that flow fronts meet — and knit their weld line — somewhere invisible and unstressed, and so the cavity fills completely before the melt freezes.

1.3.2 What these rules are — and where they stop

Notice what these rules have in common: they are geometry checks and ratios. A wall either has draft or it does not; a rib either respects the thickness ratio or it does not; a corner either has a radius or it is sharp. That makes them exactly the kind of knowledge a checklist — or an automated rules checker — can verify directly against the CAD model.

But the limits of the rules matter as much as the rules themselves. A rules checker can flag a missing draft angle or a rib that is too thick; it cannot judge whether this particular wall-thickness variation, on this geometry, in this material, will actually warp enough to matter. That judgment needs either a full mould-flow simulation (see 1.2) or the experience of an engineer who has seen similar parts fail. The rules catch the clear-cut violations cheaply and early; the harder, geometry-specific questions are what simulation and expertise exist to answer.

Together, the fundamentals in this chapter — the moulded part (1.1), the moulding process (1.2) and these design rules — form the baseline of the field: the way moulded parts have been designed, checked and made for decades. Everything else builds on this foundation.