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What does a DFM review check before an injection mould is made?
A DFM (design for manufacturing) review checks that a part can be moulded reliably and at a sensible cost before any steel is cut. For an injection moulded part it normally covers wall thickness, draft, ribs and corners, undercuts, gate and ejector positions, tolerances and surface finish, and it ends with a list of design changes or accepted risks.
Wikipedia describes DFM as designing a product so that it is easy and cheap to make, and notes the same principle across processes: the tighter the tolerance and the more complex the feature, the more it costs. For moulding, the review is usually done by the toolmaker or moulder from your 3D model, often with an automated moldability report and a fill simulation.
The checklist
| Check | What the reviewer looks for | Published rule of thumb |
|---|---|---|
| Wall thickness | Walls within the resin's range and as uniform as possible | Protolabs lists, for example, ABS at 0.045 to 0.140 in and polypropylene at 0.035 to 0.150 in |
| Draft | Taper on every face parallel to mould opening | Protolabs: at least 0.5° on vertical faces, more for shutoffs, and 5° or more for medium texture |
| Ribs and gussets | Thin enough not to create thick spots | Protolabs: no more than about 60 percent of the nominal wall |
| Corners | Radii where walls and ribs meet | Eastman: at least 0.5 mm, or 0.2 to 0.5 times the wall, whichever is greater |
| Undercuts | Features that stop straight release | Side actions, lifters, inserts or a redesign; stripped undercuts up to about 2 to 3 percent of diameter in thin walls (Eastman) |
| Gate | Where the part fills from and where the mark goes | Edge gates at 50 to 80 percent of the wall (Eastman) |
| Ejection | Where pins push and whether marks are acceptable | Pins on hidden, flat, well-supported faces |
| Tolerances | Whether drawing limits are achievable | Protolabs: about ±0.003 in machining tolerance plus at least ±0.002 in/in for the resin |
| Finish | Polish or texture on each face, and its effect on draft | Texture needs extra draft per depth of grain |
Why thickness comes first
Most moulding defects trace back to wall thickness. Thick areas cool last, so they sink or warp; thin areas may not fill. Protolabs' design guidance says small parts can mould well at about 1 mm (0.040 in), hand-sized parts are better around 2 mm (0.080 in), and thermoplastic moulding is generally stable between about 1.5 mm and 2.5 mm. Above roughly 3.8 mm (0.150 in), sections can start to sink on themselves. Coring out thick areas and replacing solid sections with ribs fixes many issues before they reach the mould.
How to prepare for a review
- Send a clean 3D model in a neutral format, plus a drawing showing critical dimensions and tolerances.
- State the resin, or at least the requirements it must meet, because wall, draft and shrink rules depend on it.
- Mark cosmetic faces and the finish or texture each face needs.
- Give the expected annual volume, since it changes the mould type, cavity count and runner choice.
- List the features you cannot change, so the reviewer works around them rather than suggesting their removal.
- Ask for the gate and ejector layout and any fill or warp simulation to be included in the review.
What you should get back
A useful review lists each issue, where it is on the model, why it matters, and a suggested change or the risk of leaving it. Expect items such as "add draft to these faces", "core out this boss", "this tolerance is tighter than the resin allows" or "this hole needs a side action". Agree each one in writing, update the model, and freeze the design before the mould is cut. Changes after that usually mean modifying steel.
The rules quoted here are published general guidance from specific suppliers. The review for your actual part, resin and mould takes priority.
Questions buyers ask
Who should do the DFM review?
Ideally the toolmaker who will build the mould, because the advice depends on how they build and run tools. An independent design engineer can review earlier, but the toolmaker's review is still needed.
Is DFM only for production moulds?
No. Prototype moulds benefit too, and many rapid-tooling suppliers provide an automated report with every quote. Fixing issues at the prototype stage avoids repeating them in production steel.
What if I disagree with a DFM comment?
Ask what happens if the feature stays as drawn: a cosmetic risk, a longer cycle, a more complex mould or a part that will not mould at all. Some risks are worth accepting; record the decision.
Does a DFM review include cost?
Often indirectly. Removing an undercut, loosening a tolerance or simplifying a finish all lower mould cost, and a good reviewer will point out those trade-offs.
Sources
- Wikipedia, Design for manufacturability
- Protolabs, Injection molding design guidelines
- Protolabs, Design stronger molded parts: ribs, gussets and materials
- Eastman, Keys to successful molding (PDF)