How to Design a Plastic Part Correctly for Injection Moulding
Designing a plastic part for injection moulding requires understanding the manufacturing process from the very first sketch. A well-designed part moulds easily, costs less, and performs better. A poorly designed part creates expensive tooling, high scrap rates, and quality problems. This guide covers the essential design rules.
1. Uniform Wall Thickness
The number one rule in injection moulding design. Varying wall thickness causes differential cooling, leading to sink marks, warpage, and internal stresses. Target uniform thickness of 1.5-3mm for most applications. If thickness variation is unavoidable, transition gradually (3:1 taper ratio) and place thicker sections near the gate.
2. Draft Angles
Without draft, parts stick in the mould, causing ejection marks or damage. Minimum 1° per side for textured surfaces, 0.5° for polished surfaces. Deep ribs need more draft (2-3°). The deeper the feature, the more draft you need.
3. Rib Design
Ribs add stiffness without increasing wall thickness. Design rules: rib thickness = 50-70% of adjacent wall thickness, rib height = max 3x wall thickness, base radius = 25-50% of wall thickness. Violating these rules causes sink marks on the opposite surface.
4. Boss Design
Bosses for screw connections: outer diameter = 2x inner diameter, wall thickness = 60% of nominal wall, connect to walls via ribs rather than solid sections to prevent sink marks.
5. Undercuts and Core Pulls
Undercuts require side actions (slides, lifters) in the mould, adding cost and complexity. Design external undercuts as snap-fits where possible — they can often be released by flexing during ejection. Internal undercuts are more expensive to mould and should be avoided when possible.
6. Gate Location
The gate is where plastic enters the cavity. It affects fill pattern, weld line location, and surface quality. Place gates at the thickest section, away from cosmetic surfaces, and consider how the material will flow to fill the part completely.
7. Material Selection
Material choice affects everything: wall thickness minimums, draft requirements, tolerances, and mould design. Consult with your moulder early — we often suggest material changes that reduce cost while improving performance.
Common Mistakes to Avoid
Sharp internal corners (stress concentrators — always radius), non-uniform walls (sink marks and warpage), insufficient draft (ejection problems), oversized bosses (sink marks), and designing without consulting the moulder (expensive tooling modifications).
Frequently Asked Questions
At what stage should I involve the injection moulder?
As early as possible — ideally during concept design. A 15-minute DfM review at the concept stage can save weeks of redesign and thousands in tooling modifications later. Our initial DfM review is free.
Can you fix a design that was not originally intended for injection moulding?
Yes, we regularly adapt designs from 3D printing or CNC machining for injection moulding. Typical modifications: adding draft, adjusting wall thickness, replacing undercuts with snap-fits, and optimising rib/boss geometry.
Need Help?
Send us your 3D file (STEP, IGES, SolidWorks) and we provide a free DfM report within 48 hours: wall thickness analysis, draft check, gate location suggestion, material recommendation, and budget estimate.
