What matters
- Analyze undercuts against the chosen pull direction, not the screen view.
- A pass-through, parting-line change, or snap redesign can remove a mechanism.
- When an action is justified, define travel, support, timing, wear, and access for service.
Common release methods
| Method | Works well when | Watch for |
|---|---|---|
| Design out | Geometry can align with pull | Function and appearance |
| Pass-through core | An opening can be added behind the feature | Witness mark and sealing |
| Bump-off | Resin and geometry can flex safely | Strain, whitening, consistency |
| Lifter | Internal undercut has room for angled travel | Stroke, thin steel, ejection |
| Slide/core pull | External feature needs lateral release | Mold size, wear, witness line |
| Unscrewing core | A true molded thread is necessary | Cycle, drive and maintenance |
Try geometry before hardware
Move the feature to the parting line, open a window behind a latch, split the part, or turn a closed hole into a slot that can be formed by straight-pull steel. These changes can reduce mold cost and simplify maintenance, but they still need a product review for strength, sealing, appearance, and user access.
A flexible snap may release without a slide, but the resin's strain limit, feature length, draft, wall temperature, and repeated assembly load must all be considered. A bump-off should be validated, not assumed.
If a side action stays, design the surrounding part with it
- Provide enough travel to clear the feature before ejection.
- Keep sealing surfaces robust enough to resist flash and wear.
- Avoid fragile knife edges and inaccessible cooling around the action.
- Mark acceptable witness lines on appearance surfaces.
- Confirm sensing, sequence, lubrication, and service access for production tooling.
Why undercuts change an early quote
A generic geometry scanner can flag likely undercuts, but it cannot always infer the best parting line or the intended function. For a firm quote, show the assembly, identify keep-out surfaces, and tell the toolmaker which geometry can change. That turns an expensive assumption into a design decision.
Use this as a design review, not a production release
These guides support early planning. The selected resin supplier, toolmaker, and molder still need to confirm the final geometry, process window, tolerances, safety factors, validation plan, and commercial assumptions for your application.