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Part Design Guidelines for Injection Molding

Good injection molded parts begin with good design. The difference between a design that molds perfectly on the first try and one that requires three mold modifications and eight weeks of delays is often a few millimeters of wall thickness, a missing radius, or an insufficient draft angle.

These guidelines are distilled from decades of injection molding experience and are intended for design engineers, sourcing professionals, and anyone specifying plastic parts manufactured in China.

Core Design Principles

The fundamental rule: the part should be designed for the process, not the other way around.

1. Uniform Wall Thickness

This is the single most important rule. Aim for wall thickness variation ≤ 25% across the part.

Material	Recommended (mm)	Min (mm)
ABS	1.5-3.5	0.8
PC	1.5-4.0	1.0
PP	1.0-3.5	0.6
PA (Nylon)	1.0-3.0	0.6
POM (Acetal)	1.0-3.0	0.6
PMMA (Acrylic)	1.5-5.0	1.0
PC/ABS	1.5-3.5	0.8

2. Draft Angles

Minimum: 0.5° per side (textured), 1° (smooth)
Recommended: 1-3° per side
Deep ribs: 0.5-1° per side
Internal features: 0.5-1° per side minimum

3. Corner Radii

Sharp corners are stress concentrators and flow obstacles. Internal corner radius minimum = 25% of wall thickness. External should be 50-75% of wall.

4. Rib Design

Height: Max 3x wall thickness
Thickness: 50-60% of wall
Draft: 0.5-1° per side
Base radius: 25-50% of rib thickness
Spacing: At least 2x wall thickness

5. Boss Design

Wall thickness: 50-60% of wall
Height: Max 2.5x outer diameter
Draft: 0.5-1° per side
Base radius: Required — sharp base causes sink
Support: Add gussets for tall bosses

Buyer's Tip: Chinese mold makers will often accept a part design with non-optimal geometry to win the business, then request design changes after the mold is partially cut. The safest approach: have your design reviewed by an experienced mold designer before sending the RFQ. Include draft and radii notes on the 2D drawing — if only in the 3D model, they may be overlooked.

Design Rules Summary

Feature	Rule of Thumb	Critical for
Wall thickness	Uniform within 25%	All parts
Draft angle	1-3° per side	Ejection, finish
Internal radius	≥ 25% of wall	Strength, flow
Rib thickness	50-60% of wall	Sink marks
Boss wall	50-60% of wall	Sink marks, weld lines
Flow length/wall ratio	< 150:1 (amorphous)	Fillability
Undercut depth	< 5mm pick-out	Tool complexity

What This Means for Your Project: Invest time in DFM review before committing to a mold. Each guideline violated adds risk of mold modification costs ($500-2,000 per change) and schedule delays. Get wall thickness uniformity and draft angles right and 80% of moldability issues disappear.

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