Almost every moulding defect a buyer ever sees traces back to one of six design decisions, and all six are cheaper to get right in the model than to tune out on the machine. A design for manufacture review is a structured pass through them, and a designer who has done that pass privately will get a better tool quote before anyone else looks at the part.
Uniform wall thickness comes first
Plastic shrinks as it cools and thick sections cool last, so they shrink after their neighbours have set and pull the surface in. That is a sink mark, and no process adjustment removes it. Keep the wall constant, and where thickness must change, blend it over a distance rather than stepping it. Coring out thick sections is almost always better than accepting them.
Draft, ribs and bosses
Every face in the direction of draw needs draft, more if it is textured. Ribs stiffen a part far more efficiently than thickening the wall, and they must be a fraction of the wall thickness or they sink. Bosses need the same treatment, supported by ribs rather than made solid. These three rules between them remove most of the sink and warp a first design carries.
Radii, gates and weld lines
Sharp internal corners concentrate stress and restrict flow; a radius fixes both. Where the gate sits decides how the cavity fills, and wherever two flow fronts meet they form a weld line that is weaker and sometimes visible. If a part must be strong at a particular point, tell the toolmaker so the gate can be placed to keep the weld line away from it.
Undercuts and what they really cost
Anything that stops the part pulling straight off the core needs a mechanism: a side action, a lifter, a collapsing core or an unscrewing unit. Each adds design, components, maintenance and failure modes. Moving a snap feature to the parting line, or opening a window so a core can pass through, is often free in the design and expensive to skip.
Questions people ask about injection molding design guide
What is the single most important moulding design rule?
Uniform wall thickness. Most sink, warp and dimensional problems come from violating it, and no amount of process tuning fixes them afterwards.
How thick should a rib be?
A fraction of the adjoining wall, because a rib as thick as the wall creates a thick section behind it and sinks on the visible face. Height and draft matter too.
What is a weld line?
The seam where two flow fronts meet after passing around a hole or a core. It is weaker than the surrounding material and can be visible, so its position is a design consideration.
When should DFM review happen?
Before the tool is quoted, and ideally while the model is still being built. Every rule above is nearly free to apply early and expensive to retrofit.