Injection Molding Design Guide
Draft, wall uniformity, ribs, shrinkage, gate/ejector marks and mold classes
Injection molding is the highest-volume plastic manufacturing process: molten polymer is forced under pressure into a steel or aluminum cavity, cooled and ejected. Tooling cost dominates at low volume, unit cost dominates at scale, and design decisions — draft, wall thickness, rib geometry, gate placement — are locked into the mold for the part life. Correct DFM here saves tens of thousands of dollars downstream.
1. Mold classes and tooling
Prototype and low-volume tooling use aluminum or P20 steel with Class 105–103 construction, delivering T1 samples in as fast as 5 working days. Production tools are hardened steel (Class 101–102), built for 100k–1M+ shots over 25–40 days.
Mold ownership transfers to the customer; the platform stores and maintains the tool, and mold cost is a one-time line item on the quote, amortized across your order volume in cost comparisons.
2. Draft and wall thickness
Ejection requires draft on faces parallel to the pull direction: 0.5–1° for shallow textures, 1–2° recommended, 2–3° for deep ribs and bosses. Zero-draft walls weld to the core and cause drag marks or part sticking.
Keep walls uniform, typically 1.0–4.0 mm, because thick sections cool slower, shrink more and create sink marks or voids. Ribs should be 0.5–0.6× the nominal wall (thicker ribs cause sinks) with height ≤ 3× wall; bosses need 0.5–0.6× wall thickness at their root and 0.5 mm minimum wall.
3. Shrinkage and tolerances
Every polymer shrinks as it cools; mold steel is dimensioned to the expected shrinkage: ABS ~0.4–0.7%, PP ~1.0–2.5%, PA66 ~1.2–1.8%, PC ~0.5–0.7%, POM ~1.8–2.2%. Shrinkage is not isotropic — flow direction shrinks less than cross-flow — so tight tolerances need gated and reinforced design.
Standard molded tolerance is roughly ±0.3% of dimension (±0.25 mm on 100 mm); precision grades with steel tools and process control hold ±0.05–0.1 mm. Gate and ejector marks are inherent: specify their location on non-visible surfaces to keep the cosmetic face clean.
4. Features that mold well
Undercuts need side actions (slides/lifters) that add tooling cost — avoid them on low-volume parts or design them into a straight-pull geometry. Inserts (threads, metal nuts) can be molded-in at added cycle cost.
Text and logos should be embossed (raised) rather than engraved when possible, ≥ 0.5 mm; texture (spark-eroded) on the cavity adds visual quality but requires draft ≥ 1.5–2°. Consider a flow simulation or experience-based gate layout for thin-wall, long-flow parts.
Quick reference
| Parameter | Typical value |
|---|---|
| Draft angle | 0.5–2° (1–2° standard; 2–3° deep ribs) |
| Wall thickness | 1.0–4.0 mm, uniform |
| Rib thickness | 0.5–0.6× wall; height ≤ 3× wall |
| Shrinkage range | ABS 0.4–0.7% · PP 1.0–2.5% · PA66 1.2–1.8% · PC 0.5–0.7% · POM 1.8–2.2% |
| Standard tolerance | ±0.3% of dimension |
| T1 (prototype tool) | as fast as 5 working days |
| Production tool | 25–40 days; Class 101 steel for 100k+ shots |
| Min text depth | 0.5 mm (embossed preferred) |
Sources & references
- ISO 294-4 — Determination of shrinkage
- DIN 16742 — Tolerances for plastic parts
- Xometry Learning Center — Injection Molding Design Guide (public engineering reference)