Injection molding has one of the highest costs of late design changes of any manufacturing process. Once tooling is cut, design changes mean tool modifications, which cost time and money, and a substantial change can mean scrapping the tool entirely. The problems behind those changes almost always exist in the design before tooling starts, and they’re visible to someone who knows what to look for. That’s what a DFM review is for.
Wall thickness: the most common problem
Non-uniform wall thickness is the most frequent DFM issue in molded parts. When walls vary significantly, plastic cools at different rates in different sections. That differential cooling creates sink marks on cosmetic surfaces, warpage that takes the part out of tolerance, and internal stress that weakens the structure.
The fix is simple in design and expensive after tooling. Target uniform walls, typically between 0.060″ and 0.120″ depending on material. Where thick sections are structurally necessary, coring them out keeps walls uniform while preserving strength. The deliberate exception is a living hinge, where a thin section is the whole point.
Draft angle: why parts won’t release cleanly
Every vertical wall in a molded part needs draft, the slight taper on walls parallel to the direction of pull that lets the part release cleanly. Without it, the part drags on the tool surface, causing surface defects, tool wear, and inconsistent release across the run. Standard draft is 1° to 2° per side for most applications, and textured surfaces need more. Parts designed without draft in CAD don’t acquire it during tooling, so it gets caught in design review or becomes a problem after the tool is built.
Undercuts and how they drive tooling cost
An undercut is any feature that keeps the part from releasing straight out of the mold. Internal threads, side holes, clips, and some snap fits are common examples. They require side actions, lifters, or collapsible cores, each of which adds cost and complexity. The DFM question is whether the undercut is functionally necessary or a design artifact that can be eliminated, and it’s often the latter. We cover the options in designing for undercuts and the cost side in what drives mold cost.
What a drawing review catches before production
At Precision Expedited, every injection molding job gets a drawing review before it goes to a supplier. Wall thickness, draft, gate placement, sink risk, parting line location, and material selection all get evaluated, and a real person contacts you with DFM flags and risk callouts within 24 hours. The goal is to find problems before the tool is cut. For the full design rules and resin options, see our injection molding design guide and resin selection guide.
Frequently asked questions
Typically 0.060″ to 0.120″, depending on the resin. Uniformity matters more than the exact number, so core out thick sections instead of leaving solid mass.
1° to 2° per side covers most applications. Textured surfaces and deep features need more.
Often, especially when removing steel to add material to the part. Changes that add steel or move parting lines are harder and more expensive, and large changes can mean a new tool.
