Injection Molding Design Guide
Design plastic parts that mold right, look great, and last
Injection molding can produce millions of identical plastic parts with complex geometry, excellent surface finish, and low per-unit cost. But mold tooling is expensive, and design mistakes found after tool steel is cut cost $5,000–$50,000 to fix. Getting the design right before tooling starts is critical. This guide covers the DFM rules that matter most.
Core Design Principles
Uniform Cooling
Uneven wall thickness causes differential shrinkage → warping, sink marks, and internal stress. Keep walls uniform.
Draft for Ejection
Plastic shrinks onto cores. Without draft, parts stick and get damaged during ejection.
Flow-Friendly Design
Molten plastic must flow from the gate to every corner. Thin sections, sharp corners, and long flow paths cause short shots.
Design for the Tool
Every feature has a tooling consequence. Undercuts need side actions. Texture needs draft. Think about how the mold opens.
DFM Rules & Guidelines
Wall Thickness
The single most important rule in injection molding design. Non-uniform walls cause sink marks, warping, and voids.
Keep walls between 1.0–3.5 mm. Maintain thickness within ±10% across the part. Transition gradually over 3× the thickness difference.
Walls thicker than 4 mm. Abrupt thickness changes. Thick-to-thin transitions at gates.
Draft Angles
Plastic shrinks during cooling, gripping the mold core. Draft enables clean ejection without scuffing or sticking.
1° minimum on all vertical surfaces. 1.5–2° is ideal. Add 1° per 0.025 mm texture depth. 0.5° minimum on shutoff surfaces.
Zero draft anywhere. Textured surfaces with less than 1° per 0.025 mm texture depth.
Ribs
Ribs add stiffness without increasing wall thickness. But ribs that are too thick cause sink marks on the opposite surface.
Rib base thickness: 50–60% of adjoining wall. Height: max 3× base thickness. Draft: 0.5–1.0° per side. Fillet radius at base: 0.25–0.5× wall thickness.
Ribs thicker than the wall. Ribs without draft. Intersecting ribs creating thick junctions.
Bosses
Bosses provide screw attachment points. They must be designed to avoid sink marks on the show surface.
OD: 2× screw diameter. Wall thickness: 60% of nominal wall. Connect to wall with ribs (60% of wall thickness). Fillet base radius 0.25× wall.
Bosses directly on a show surface without standoff. Boss walls thicker than nominal wall. No gusset ribs on tall bosses.
Undercuts & Side Actions
Features that prevent straight mold opening need side actions (slides/lifters), adding $2,000–$10,000+ per action.
Design to eliminate undercuts where possible. Use snap-fit designs that allow straight pull. If needed, keep side actions on one mold face.
Internal undercuts (require collapsible cores). More than 3–4 side actions per mold. Undercuts on multiple faces.
Gate Location
Where plastic enters the cavity affects appearance, strength, and warpage. Gate vestige is always visible.
Gate at the thickest section. Gate on a non-cosmetic surface. Edge or sub-gates for aesthetics. Pin gates for multi-cavity molds.
Gating into thin sections. Gates directly on show surfaces. Gates opposite thin ribs (causes jetting).
⚠️ Common Design Mistakes
- ⚠ Making walls too thick "for strength" — this causes sink marks, warping, and longer cycle times. Use ribs instead.
- ⚠ Forgetting draft on the inside of box/container shapes — cores are hardest to eject from.
- ⚠ Designing snap fits without calculating deflection — over-stressed snaps break; under-stressed ones don't hold.
- ⚠ Putting logos or text on draft-critical surfaces without adjusting — raised text needs more draft than recessed text.
- ⚠ Not discussing gate location with the mold maker — the gate affects everything: weld lines, flow marks, warpage.
💡 Pro Tips
- ▸ Coring out thick sections saves material, reduces cycle time, and eliminates sink marks — triple win.
- ▸ Use living hinges (PE/PP only) to combine two parts into one, eliminating a mold and assembly step.
- ▸ Add mouse-ear features at sharp corners in the mold to prevent warping on flat parts.
- ▸ Textured surfaces hide sink marks better than high-gloss — specify SPI-D or MT finishes on ribbed areas.
- ▸ For prototyping, consider aluminum tooling (3–5 week lead) vs. steel (6–10 weeks) — aluminum molds handle 10K+ shots.
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