QUICK ANSWER
An undercut obstructs removal along a chosen release direction. First review whether the feature or split can be changed; if not, select a mechanism that forms the feature and clears it through a verified movement sequence. Slider and lifter choices must include load support, available travel, cooling, maintenance and safe return, not just nominal undercut depth.
Engineering reference, not a material specification or machine-setting recipe. Validate decisions against the released drawing, resin-grade data and equipment instructions. Equipment work must follow the applicable safety procedures.
1. Identify an undercut relative to a movement
An undercut is not simply a hole that looks sideways in a screen view. Select the intended removal direction and identify which feature prevents the part from clearing its forming steel. The same geometry may be an undercut in one orientation and accessible in another. Review the complete assembled part model and include snap features, external grooves and internal returns.
Distinguish a true geometric lock from insufficient or incorrectly oriented draft. Correcting draft may solve a release concern, but it must preserve the functional dimensions. Conversely, a green draft-analysis result does not guarantee that a slide has enough travel or that a flexible feature can survive forced release. The release sequence needs a mechanical and material review.
2. Compare a design change with a moving mechanism
Start with the product function. Could a slot become open to an edge, could the split pass through the feature, or could a pass-through opening support a shutoff concept? Any change must be approved by the product owner. A simpler tool is not an improvement if the change weakens a required clip, compromises a seal or creates an unacceptable appearance.
If movement remains necessary, compare mechanisms in the full mold layout. Sliders, lifters, removable inserts and unscrewing arrangements solve different problems and are not interchangeable catalog labels. Evaluate the intended production duty and handling strategy. A hand-loaded insert may be feasible for one program but unacceptable for another because of labor, traceability, safety or repeatability requirements.
| Option | What it may address | Essential review |
|---|---|---|
| Reorientation or revised split | A feature becomes accessible to the main opening | Function, witness location, draft and remaining tool layout |
| Shutoff or pass-through feature | Approved geometry can be formed by mating steel | Support, wear, access and the new opening's acceptability |
| Slider | A forming component must retract before release | Load support, travel, retention and return sequence |
| Lifter | An internal feature needs clearance during ejection | Coupled movement, guidance, support and the part's motion |
| Dedicated thread or collapsible mechanism | A continuous or otherwise locked feature | Actual mechanism envelope, operation and service requirements |
3. Calculate travel using the correct coordinate definition
For a simple straight inclined lifter, if the ejector-plate movement is 30 mm along the main ejection direction and the guide angle is 10 degrees from that direction, the ideal lateral movement is 30 × tan(10 degrees), approximately 5.3 mm. This is an illustrative geometric relationship, not a recommended angle, stroke or permitted undercut. If the given stroke is measured along the inclined guide instead, the trigonometric relationship is different.
The theoretical lateral movement is not all available as useful clearance. Check the actual undercut extent, part motion, relevant tolerances and the mechanism supplier's requirements. Include the start and end positions as well as every intermediate position. A single open-position screenshot can miss a collision that occurs before the forming component has separated from the part.
4. Review support, retention and return
The forming component must resist the loads present while the cavity is filled and packed. Its actuation mechanism is not automatically its complete load-support arrangement. Review the backing and locking concept, contact, guide engagement and the steel remaining around the feature. Where a standard unit is used, apply its current documentation and obtain supplier review for conditions outside the stated limits.
Document what prevents movement at the wrong time and how the mechanism returns before the mold closes. Include ejector position, part retention, spring or actuator failure modes and the applicable machine interlocks. Cooling connections and lubricated or wearing interfaces must be reviewed for the actual resin, cleanliness and maintenance requirements. Never bypass a safety device to prove a movement concept.
5. Verify the sequence with a controlled acceptance plan
Before manufacture, release a motion study, sectioned mechanism drawing, supplier references and a list of critical clearances. Identify who approves the altered part geometry, if any. Do not assume that one moldmaker's prototype service limitations describe all production tooling, or that the availability of a standard component establishes its suitability for this application.
At tool verification and sampling, use the approved safe procedures and qualified personnel. Check travel, return, retention and release evidence before judging molded appearance. Inspect the undercut feature for damage or distortion and test its actual function on parts in the required condition. Preserve cavity identity and the mechanism revision so that a later repair or insert replacement can be checked against the accepted baseline.
PUT IT INTO PRACTICE
Engineering review checklist
Use these prompts for a project review. This page does not record or certify completion.
- The obstructing feature and removal direction are explicit.
- Product-approved simplification options were considered.
- Travel is defined in a named coordinate direction.
- Intermediate positions are checked for interference.
- Load support, guidance, retention and return are documented.
- Supplier limits and safe verification procedures are retained.
Questions engineers ask
Can a flexible plastic always be stripped over an undercut?
No. The geometry, material grade, temperature and required function govern the risk. Forced release needs specific engineering validation rather than an assumption based on the polymer family.
Does enough nominal stroke prove release?
No. The part and mechanism move together through intermediate positions. Useful clearance, support and the complete return sequence must also be verified.
Sources and application limits
This is independently written educational guidance. The linked manufacturer and engineering references support specific mechanisms or further study; they do not endorse PFMOLD or replace project approval. Supplier recommendations apply to their stated grades, tooling and equipment.
- Protolabs: Using Side-actions in Molding Design ↗
Primary manufacturer introduction to side actions and supported geometry; its service capabilities are not universal production-tool limits.
- Protolabs: Designing for Moldability — Complex Features ↗
Primary examples of undercut alternatives, including pass-through and shutoff concepts; no source illustrations are reproduced.
- DME / AHP Merkle: Mechanical Chain Lifters for Undercuts ↗
Primary component-supplier diagram supporting the distinction between ejection travel and lateral demolding movement. The example here is illustrative, not a component selection.