Injection molding is brutally unforgiving of design mistakes — because once you've cut a steel mold, changing the part means changing the tool. Here are the five DFM errors that quietly blow up a molding budget, and how to catch each one before you commit to tooling.
The economics of molding are all upfront: the mold costs thousands, the parts cost pennies. That's great — until a design flaw forces a tooling revision, and suddenly you're paying to re-cut steel. Every mistake below is cheap to fix on the screen and expensive to fix in the mold.
1. Uneven wall thickness
This is the number-one cause of molding defects. Plastic shrinks as it cools, and thick sections cool slower than thin ones. When wall thickness varies across a part, those sections shrink at different rates and you get sink marks, warping, and internal stress.
The fix: keep walls as uniform as possible. Where you need a thick region for strength, core it out and add ribs instead of leaving a solid mass. Ribs should be about 40–60% of the wall thickness they attach to — thicker, and they cause sink marks on the opposite face.
2. Forgetting draft angles
A molded part has to eject from the mold, and perfectly vertical walls grip the steel like a suction cup. Without draft — a slight taper on vertical faces — parts scuff, stick, or need ejector pins that leave marks.
The fix: add at least 1–2° of draft to every face perpendicular to the parting line, and more for textured surfaces (a rough texture needs extra draft to release cleanly). Draft is nearly free to add in CAD and painful to add after the mold exists.
3. Designing in undercuts
An undercut is any feature that traps the part in the mold — a snap-fit lip, a side hole, a groove on an internal wall. The part can't simply pull straight out, so the mold needs side-actions or lifters: extra moving components that add real cost and complexity to the tool.
The fix: design undercuts out where you can. Sometimes a small geometry change — moving a hole to a face that pulls in the draw direction, or a "pass-through" window in the wall — eliminates a side-action entirely and shaves thousands off the tooling.
4. Ignoring the parting line
The parting line is where the two halves of the mold meet — and it always leaves a faint witness mark on the part. Designers who ignore it end up with a seam running across a cosmetic face, or a line that lands somewhere that hurts function.
The fix: decide early where the parting line should sit, and design so it falls on an edge or a hidden surface. Talk to your molder about it up front — its location drives draft direction, undercuts, and where the gate and ejector pins can go.
5. Over-tightening tolerances
Molded plastic shrinks, flexes, and moves with temperature and humidity. Trying to hold machined-metal tolerances on a molded plastic part is a recipe for high scrap and a very expensive mold. Precision in molding comes from an even more precise (and pricey) tool.
The fix: spec realistic molding tolerances, tighter only on the few features that truly mate, and let the rest follow standard molding capability. (Our companion guide, Tolerances 101, covers where to spend precision and where to save it.)
The pattern behind all five
Notice the theme: every one of these is a decision that's free to make correctly in CAD and costly to fix once steel is cut. That's exactly why a design review before tooling is the highest-ROI hour in the whole project.
Planning a molded part? Send us the model before you commit to a mold. Every quote includes a founder-reviewed DFM check — we'll flag walls, draft, undercuts, and the parting line while it's still just pixels.