injection molding design rules
Injection Molding Design Rules
A moulded part is defined by the tool that makes it, and the tool is defined by the part. That loop is why the design rules exist: a wall that is 40% thicker than the one next to it creates a sink mark, and a feature without draft either drags or needs a side action that can double the tool cost.
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- Wall uniformity
- ±25% of nominal, not absolute
- Draft
- 1–2° minimum, 3° on textured faces
- Rib thickness
- 50–60% of the wall
- Tooling lead time
- 20–35 days for a simple tool
The cheapest moment to fix a moulding defect is on the drawing. After the tool is cut, every one of these rules turns into a welding job on hardened steel.
Specifications
| Wall uniformity | Within ±25% of nominal |
|---|---|
| Draft | 1–2° smooth · 3°+ textured |
| Rib thickness | 50–60% of wall, spacing ≥ 2 × wall |
| Boss wall | 50–60% of wall, height ≤ 3 × diameter |
| Tooling lead time | 20–35 days for a simple single-cavity tool |
| Typical tolerance | ±0.1 mm on moulded features, ±0.05 mm across parting lines after agreement |
Frequently asked questions
What is the most important injection molding design rule?
Uniform wall thickness. A wall that steps from 1.5 mm to 3 mm sinks on the visible face no matter how well the tool is made. Keep the wall within about ±25% of nominal and core out anything thicker.
How much draft angle does a molded part need?
One to two degrees on a smooth vertical face, and about three degrees on a textured face — texture grips the plastic as it releases. More draft on deep cores, because the drag is longer.
Why do my ribs cause sink marks?
Because a rib as thick as the wall creates a thick section where it meets the base, which cools slower and shrinks inward. Keep the rib at 50–60% of the wall thickness and space ribs at least two wall thicknesses apart.
What is a side action and why does it cost more?
A side action is a tool movement that releases a feature which cannot come out in the direction the mould opens — an undercut. It adds mechanism, cycle time and maintenance, so it is quoted as a tooling cost rather than absorbed.
Can snap fits be molded?
Yes, and they are common. A snap fit is a deliberate undercut, so it needs either a lifter or enough flexibility in the material to strip out of the tool. Small snaps are routine; large internal undercuts are where the tool cost jumps.
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