Once tooling is cut, design changes get expensive. Most of the price and risk on an injection-molded part is locked in by the time the steel hits the mill. The rules below are the same ones our tooling partners apply during DFM review.
Uniform wall thickness wins
The single most important rule is uniformity. Walls that vary by more than ~25 percent will create sink marks, warp, and inconsistent fill. Most resin families have a sweet spot between 1.5 mm and 3.5 mm.
| Resin | Recommended wall | Min/max wall |
|---|---|---|
| ABS | 2.0 - 2.5 mm | 1.0 - 4.0 mm |
| PC | 2.0 - 3.0 mm | 1.0 - 4.0 mm |
| PP | 1.0 - 2.5 mm | 0.6 - 3.0 mm |
| Nylon (PA6/PA66) | 1.5 - 2.5 mm | 0.8 - 3.0 mm |
| Glass-filled nylon | 1.0 - 2.5 mm | 0.8 - 3.0 mm |
Draft is non-negotiable
Every face along the pull direction must have draft, or the part will scrape during ejection and either drag or break. Aim for 1° per side as a minimum; textured surfaces typically need 1° per 0.025 mm of texture depth.
- Outside walls along the pull direction: ≥ 1°.
- Inside walls along the pull direction: ≥ 0.5° if shallow, ≥ 1° otherwise.
- Ribs: 0.25° each face for short ribs, 0.5° for tall ribs.
- Textured walls: add 1° per 0.025 mm of texture depth (consult MoldTech ID).
Ribs and bosses
Ribs are the cheapest way to add stiffness. Keep the rib base ≤ 60 percent of the parent wall and at least 1.5x the wall in height. Bosses for self-tapping screws should have an outer diameter of 2x the screw diameter and a rib gusset for cyclic loads.
Gating and runner strategy
Gate location dictates how the melt fills the cavity, where weld lines form, and how visible the gate scar is. Place the gate at the thickest section to ensure the cavity fills before freezing, and consider hot runners for any production run above ~5,000 cycles.
| Gate type | When to use | Visible scar? |
|---|---|---|
| Edge gate | Side feed, simple parts | Yes (small) |
| Submarine (tunnel) | Auto-shear, hidden gate | Minimal |
| Hot tip | Large flat surfaces | Slight pin |
| Direct sprue | Single cavity, large parts | Yes (large) |
Aluminum vs steel tooling
For runs under 10,000 parts, soft aluminum tooling is typically 50-65 percent cheaper and ships in 2-3 weeks. Once production crosses ~25,000 parts, hardened steel tooling pays for itself through faster cycles and longer life. The Uretyco quoting engine compares both based on volume forecast and material aggressiveness.
Aluminum molds can deliver enough parts to validate your market, then you migrate to steel once volumes justify it. Soft tooling also accommodates design changes far better.
Tolerances, finishes, and texture
Default tolerances follow DIN 16742 series Mp tolerance class for most engineering plastics. SPI surface finishes range from A1 (mirror) to D3 (rough sandblast). Anything outside these defaults should be called out on the drawing.
Files and what to upload
Upload the part STEP plus a drawing showing the parting line direction, gate location preference, surface finish callouts, critical dimensions, and the resin family. The platform will run mold-fill heuristics during analysis and surface any high-risk areas.
Related articles
Can't find what you need? Contact our team for expert assistance with your manufacturing project.