We review roughly 40 part files a month for DFM. The same seven patterns show up again and again — from startups and from tier-one suppliers alike. Each one has a cheap fix at design stage and an expensive fix after the steel is cut.
1. Undercuts that don't need to exist
Every undercut means a slider, a lifter or a side-action — typically $1,500–6,000 of tooling and a maintenance item for the life of the mould. Before accepting a slider, ask: can the clip move, can the hole be drilled after moulding, can the wall be split into two parts that snap together? On one recent automotive bracket, moving one snap-fit hook 6 mm to the B-side removed two sliders — the tool got 18% cheaper and 9 days shorter.
2. Wall thickness jumps
Going from a 2 mm wall to a 4 mm boss in one step creates a hot spot: sink on the visible side, voids inside, longer cooling. The fix is a gradual 3:1 taper or a cored pocket. Designers often thicken a wall "for strength" — usually a well-placed rib is stronger and cheaper to mould.
3. Missing or token draft angles
A 1° draft on a textured or deep wall is not enough. As a rule: 1.5° minimum on smooth surfaces, add 1° per 0.025 mm of texture depth. Under-drafted walls drag, scuff, and force the moulder to slow the ejection — every cycle, forever. Fixing it at design stage costs nothing.
4. Tolerances tighter than the process
A general ±0.05 mm callout on every dimension of an unfilled PP part is not a quality requirement — it's an invitation to a fight at T1. Shrinkage varies by flow direction and wall thickness. Specify tight tolerances only where the assembly needs them (boss centres, sealing faces) and let the rest be general. We flag these in the DFM report with the achievable numbers per material.
5. Gates placed by eyeball, not by flow
Gate position decides weld-line location, air traps, warpage direction and cosmetic defects. On a visible part, a gate on the show face means a visible mark forever. Moldflow in our DFM stage shows exactly where weld lines land — moving a gate 20 mm at design stage is free; doing it after T1 is not.
6. Cosmetic requirements vs. parting line reality
If the part has a class-A surface — appliance front, automotive trim — the parting line, ejector marks and gate mark must all be planned around it. "No visible marks on all faces" is not achievable; ask instead: which faces are cosmetic, which direction accepts the parting line, what finish (SPI grade) is actually required. That one conversation typically saves a week of polishing argument.
7. Sharp internal corners
A sharp inside corner is a stress riser in the part and a machining bottleneck in the steel insert — EDM burns it slowly and the corner can crack in service. A 0.5–1 mm radius costs nothing at design stage and can shave days off machining.
Cost impact at a glance
| Mistake | Typical cost if missed | Fix at design stage |
|---|---|---|
| Unnecessary undercut | $1.5k–6k per slider | Re-position or re-design feature |
| Wall jump | Sink, voids, slower cycle | 3:1 taper or cored pocket |
| No draft | Drag marks, slow ejection | 1.5° + texture allowance |
| Over-tight tolerance | Extra CMM hours, T2 loops | Tighten only functional dims |
| Bad gate | Weld line on show face | Moldflow gate study |
| Sharp corners | Slow EDM, cracked inserts | 0.5–1 mm radius |
Send us your 3D file before you commit to a design. Every RFQ at Mingfeng comes back with a written DFM opinion — free, within 48 hours, whether or not you place the order. Upload your part here, or message us on WhatsApp if you'd rather just ask a question.