Plastic Part Design Guide.
A few fundamentals go a long way toward a part that molds well, performs, and costs less. Here's how to design for plastic manufacturing.
Design for the process
The best parts are designed with the manufacturing process in mind from the start. A handful of fundamentals — uniform walls, draft, generous radii, sensible tolerances — prevent the most common (and most expensive) problems down the line.
Use these as a starting point. When you’re ready, send the part over and we’ll review it against the specifics of your geometry, material, and process.
- Keep walls uniform. Even wall thickness reduces sink, warp, and internal stress.
- Add draft. A degree or two of draft on vertical faces eases release from the tool.
- Radius internal corners. Fillets cut stress concentrations and help material flow.
- Core out thick sections. Replace solid mass with ribs to keep strength without sink.
- Size ribs correctly. Keep ribs around 50–60% of the adjoining wall to avoid sink marks.
- Set realistic tolerances. Tighter-than-needed tolerances add cost — we’ll tell you what’s achievable.
By the numbers.
Starting points, not laws — resin and geometry shift them. We’ll confirm against your part.
| Feature | Guideline | Why |
|---|---|---|
| Nominal wall (molding) | 0.040″–0.160″, uniform within ~10% | Even cooling prevents sink and warp |
| Draft | 1° per side minimum; 3°+ on textured faces | Clean release without drag marks |
| Ribs | 50–60% of adjoining wall; height ≤ 3× wall | Stiffness without sink on the show face |
| Bosses | OD ≈ 2× hole diameter; gusset to walls | Screw retention without a thick mass |
| Internal radii | ≥ 0.5× wall (1× preferred) | Cuts stress risers; helps material flow |
| Thermoform draft | 2°–5°, more on deep draws | Sheet thins as it stretches into the tool |
| Tolerances | Commercial first; tighten only critical dims | Every needless tight dimension adds cost |
Design notes by process.
Injection molding
Uniform walls rule everything — sink, warp, and stress all trace back to thick sections. Gate placement drives cosmetics and weld-line location, so flag your show surfaces early. Snap fits and living hinges (PP, nylon) can eliminate hardware when designed in from the start. Undercuts are moldable with side actions — they work, but add tool cost, so keep them only where the function earns it.
Thermoforming
Tooling is one-sided, which means one precisely controlled surface — put the critical side against the tool. Use generous draft and radii, watch depth-of-draw (deep, narrow cavities thin the sheet), and plan trim lines early: trimming is part of the process, not an afterthought.
Rotational molding
Walls come out near-uniform by nature. Avoid large flat panels — they warp — and stiffen with kiss-offs or curvature instead. Keep parallel walls at least five wall-thicknesses apart so powder can flow between them, use big radii, and mold inserts in rather than machining them later.
Profile extrusion
The cross-section is constant, so all the design lives in it. Balance wall thickness across the profile to keep it stable out of the die, use hollows deliberately, and put holes, notches, and angle cuts in as downstream operations rather than fighting the die.
Fabrication
The fast path for prototypes and low volumes — and the only path for some large or one-off parts. Design to standard sheet and rod sizes, respect minimum bend radii in line bending, and call out edge finish — machined, flame-polished, or buffed — wherever it shows.
Have a design to review?
Send your files and we'll review manufacturability — and recommend the right process and material.