Injection Molding DFM: 7 Questions Before Sending to a Mold Shop
A mold shop can build almost anything you send them — the question is how many revisions, re-cuts, and surprises it will take. Most of those surprises are predictable in CAD, before a single euro is spent on steel. These seven questions cover the design decisions that determine whether your part molds cleanly the first time.
Why Ask Before You Send
Changes to a plastic part get exponentially more expensive as it moves through the process. Editing a CAD model costs hours. Changing a quoted mold design costs days and money. Cutting steel and then finding a problem costs weeks and thousands. Answering these questions in the design phase is where DFM earns its keep.
| # | Question | What to Check |
|---|---|---|
| 1 | What is the wall thickness target, and is it consistent? | Uneven walls cool unevenly, causing sink marks, warpage, and residual stress. Aim for consistent nominal thickness; where it must change, transition gradually. |
| 2 | Is there draft on every vertical surface? | Faces parallel to the pull direction need draft to release from the mold. Ribs, bosses, and side walls all count — typically 0.5–2°, more for textured surfaces. |
| 3 | Where are the undercuts, and can the part eject? | Undercuts mean side actions or lifters, which add tooling cost and cycle time. Confirm which features truly need them and which can be re-oriented. |
| 4 | Which surfaces are cosmetic, and where will the parting line sit? | Parting lines, ejector marks, and gate vestiges land somewhere. Decide which faces must stay clean and make sure the tooling plan keeps marks off them. |
| 5 | What are the real critical tolerances? | Every tight tolerance adds cost and risk. Identify the dimensions that actually control fit, function, and assembly — and let everything else run loose. |
| 6 | How will the part be gated, and what marks are acceptable? | Gate location affects weld lines, filling, and visible marks. Choose it with the mold shop, based on the part's geometry and cosmetic requirements. |
| 7 | What material, shrinkage, and surface finish are specified? | Material choice drives shrinkage, draft needs, and tooling steel selection. Specify the grade, expected shrink, and surface finish (e.g., SPI/VDI) before quoting. |
What the Answers Tell the Mold Shop
A mold shop can work with incomplete information — but they will work with assumptions, and assumptions become change orders. When you send a part with wall thickness, draft, critical tolerances, and material already resolved, the quote is more accurate, the tooling review is shorter, and the first shot is far more likely to be close to the final part.
This is also where mold-fill simulation earns its place. If the part is complex, high-volume, or cosmetic-critical, simulating fill and cooling before tooling can catch short shots, weld lines, and air traps that a geometry review alone will miss. We use Moldflow-style analysis on parts where the stakes justify it.
Two more details belong in the same conversation, even if they are not design geometry: expected annual volume and material shrinkage. Volume determines whether a single-cavity tool, a multi-cavity tool, or a family mold makes sense — and it changes the economics of side actions and polish requirements. Shrinkage, which varies by material grade and wall thickness, determines the steel dimensions the mold shop cuts to, so a wrong assumption here means a wrong part, not a minor adjustment. State both explicitly before quoting.
When to Bring in an Engineer
If you can answer all seven questions confidently, your part is in good shape to go to quoting. If several answers are "we haven't decided" or "we assumed", a short DFM review before tooling is the cheapest insurance available. It is also the moment where an experienced engineer adds the most value: reviewing wall thickness strategy, draft, gate placement, and tolerance realism while changes are still free.
Apply the full checklist to your model with our DFM checklist, or request a focused DFM review before you send files to the mold shop.
Before You Cut Steel, Ask Us
Send the CAD, target material, and volume — we will review the part for molding and give you a straight answer on what to fix before quoting.
Request a DFM Review