Injection mold sticking occurs when the molded part, runner, or sprue resists the intended release direction and remains attached to the wrong mold surface or mechanism. Sticking can cause drag marks, cracks, ejector marks, sprue damage, mold collisions, flash, and unplanned downtime.
Do not treat sticking as a single defect. Part sticking, runner sticking, and sprue sticking have different contact surfaces and often need different corrections.
Identify what is sticking
| Sticking location | Common contributors | First checks |
|---|---|---|
| Part on core or cavity | Draft, shrinkage, texture, undercut, polish, cooling, and ejection balance | Opening behavior, witness marks, short shots, and draft analysis |
| Runner or cold slug | Runner draft, rough steel, insufficient cold well, packing, and gate freeze | Runner geometry, surface finish, and release direction |
| Sprue in bushing | Sprue taper, nozzle contact, heat balance, packing, and burrs | Sprue surface, bushing fit, nozzle alignment, and temperature |
Root-cause troubleshooting
- Record which half of the mold retains the part or runner and whether the location changes with the cycle.
- Check draft, texture, parting-line shutoffs, undercuts, and core/cavity alignment.
- Compare part temperature, cooling time, packing, and shrinkage at the sticking location.
- Inspect steel polish, burrs, wear, vents, ejectors, pins, bushings, and return position.
- For sprues and runners, check taper, cold-slug wells, nozzle contact, and gate/runner balance.
- Make one controlled change and verify release, appearance, dimensions, and cycle stability.
Part-sticking solutions
- Add practical draft or change the pull direction where the design allows it.
- Reduce reverse draft, texture lock, deep ribs, and hidden undercuts.
- Balance cooling so the part does not remain too soft or shrink tightly onto the core.
- Review packing and gate freeze; over-packing can increase contact pressure and sticking.
- Improve polish, remove burrs, correct alignment, and repair worn steel.
- Balance ejector layout and force; do not simply increase ejection speed.
Sprue and runner sticking
Sprue sticking may relate to the sprue bushing, nozzle fit, taper, heat balance, or excessive packing. Runner sticking may relate to insufficient draft, a rough surface, a cold slug, an unsuitable runner transition, or a part that is packed too tightly into the runner system. Inspect the actual break location instead of changing only nozzle temperature.
What to avoid
Do not rely on mold-release spray as the permanent solution. It can contaminate surfaces, hide a design problem, and change downstream bonding or painting. Do not keep increasing ejection force when the part is clearly dragging; that can damage steel and parts. Solve the release direction, surface, cooling, and packing cause first.
Production checklist
Track mold half, material lot, cavity, cycle, cooling, packing, sticking location, defect appearance, and corrective action. The mold-making team can address steel, draft, runner, and ejection issues while the injection molding team confirms the process window. Contact Cavity Mold with photos and the location of the sticking.
Technical references
Eastman’s injection molding troubleshooting guide includes sprue-sticking checks. The Protolabs draft-angle guide explains how draft reduces scraping, sticking, cosmetic damage, and mold damage during ejection.
