What matters
- Choose material and a nominal wall before detailing ribs, bosses, and snaps.
- Design around a clear pull direction and add draft early.
- Put critical dimensions and acceptance requirements on a drawing; CAD alone rarely defines the part completely.
A practical design sequence
- Define function, environment, regulatory constraints, appearance, and expected volume.
- Choose a resin family and a realistic nominal wall range.
- Set the primary pull direction, parting strategy, and likely gate region.
- Create a uniform shell, then add ribs, bosses, clips, and required undercuts.
- Run DFM and, where risk justifies it, fill, pack, cool, and warp simulation.
- Tolerance only what function requires and define how it will be measured.
Geometry rules with a physical reason
Uniform walls cool more evenly. Draft reduces drag during ejection. Radii help flow and avoid sharp stress concentration. Ribs add stiffness without turning a wall into a slow-cooling mass. These are not styling rules; each one changes what the polymer does in the tool.
| Feature | Starting point | Check before release |
|---|---|---|
| Wall | Keep the nominal wall consistent | Material flow and cooling |
| Draft | Add on faces parallel to pull | Texture, depth, resin, steel |
| Ribs | Thinner than the adjacent wall | Sink, fill and stiffness |
| Bosses | Core the center; support with ribs | Screw load and sink |
| Corners | Use inside and outside radii | Wall continuity |
| Undercuts | Remove or make the mechanism deliberate | Ejection and maintenance |
Treat the gate and parting line as product features
The gate leaves evidence. So does the parting line, an ejector pin, and sometimes a weld line. Mark cosmetic surfaces and no-mark zones before the mold layout is fixed.
Gate position affects flow length, packing, fiber orientation, weld-line location, and warpage. A gate selected only because it is easy to machine can create a permanent part-quality problem.
What belongs in a quote-ready package
- A clean STEP model at the intended nominal geometry.
- A 2D drawing identifying datums, critical dimensions, tolerances, appearance surfaces, texture, and inspection notes.
- Resin grade or a clearly stated property requirement if the grade is open.
- Volume, tool-life expectation, assembly context, mating parts, and use environment.
- A revision identifier shared by every file and requirement document.
Use this as a design review, not a production release
These guides support early planning. The selected resin supplier, toolmaker, and molder still need to confirm the final geometry, process window, tolerances, safety factors, validation plan, and commercial assumptions for your application.