Views: 0 Author: Site Editor Publish Time: 2026-04-27 Origin: Site
Few things are more frustrating on a production floor than parts that won't come out of the mold. You hear that telltale click — or worse, a crunch — and you know something has gone wrong.
Ejection difficulties (sticking parts, poor ejection, parts stuck on the core or cavity) are among the most common headaches in injection molding. But here's the good news: most ejection problems can be solved systematically.
Let me walk you through the five key areas to investigate.
This is the root cause solution. If a brand-new mold has ejection problems, you're almost certainly looking at a design issue.
Insufficient draft angle — This is the #1 culprit. Every surface in the direction of mold opening needs adequate draft.
General rule: 1-3 degrees per side
For highly polished or high-aspect-ratio features: at least 0.5 degrees
Ribs and bosses may need even more
Poor venting — Ever peeled a suction cup off a window? That vacuum effect happens inside molds too. When ejection creates negative pressure, the part sticks.
Solutions include:
Adding vent slots (typically 0.02-0.05mm deep)
Using vented ejector pins
Adding vent wells on the parting line
Surface finish issues — Polishing direction should always align with ejection direction. If polish marks run perpendicular to ejection, you've created microscopic undercuts.
Here's a useful trick: if parts stick to the cavity (the stationary side), polish the cavity smoother. If they stick properly to the core (moving side), consider light texturing to improve grip.
Poor ejector system design — Ask yourself these questions:
Are there enough ejector pins?
Are they located where resistance is highest?
Could a stripper plate or air-assist work better for deep parts?
When you can't modify the mold, process adjustments are your fastest, lowest-cost option.
Reduce holding pressure and time — Excessive packing is a major cause of sticking. The part gets over-compressed into every corner of the cavity.
Try this: Gradually reduce holding pressure step by step until you find the sweet spot — just enough to prevent sink marks, nothing more.
Check mold temperature
Parts sticking on core (moving side) : Lower core temperature. This makes the part shrink and grip the core more tightly.
Parts sticking on cavity (stationary side) : Raise cavity temperature OR lower core temperature. The part will stay on the colder side when the mold opens.
Adjust melt temperature and shot size
Symptom | Possible cause | Adjustment |
|---|---|---|
Overly sticky flow | Melt too hot | Reduce temperature |
Brittle, cracking parts | Melt too cold | Increase temperature |
Overfilled cavity | Excessive shot size | Reduce shot volume |
Extend cooling time — If parts aren't fully solidified, they'll be too weak to eject cleanly. Add 0.5-1 second of cooling time and test. Repeat until parts eject cleanly.
Even a perfectly designed mold will develop ejection problems over time due to wear and contamination.
Mold deposits (mold scale) — Release agents, pigments, and degraded polymer gradually build up on surfaces. This increases friction dramatically.
Solution: Clean the cavity and ejector pins regularly with proper mold cleaner.
Worn or stuck ejector pins — Over thousands of cycles, pins wear down or bend. They may not return fully or may move unevenly.
What to check:
Do all pins move freely?
Are any pins visibly worn or damaged?
Do pins return to the fully retracted position?
Blocked cooling channels — Uneven cooling creates uneven shrinkage. When one area cools faster than another, the part warps — and warped parts stick.
Check and descale your water lines regularly. Uniform mold temperature is your friend.
Sometimes the problem is sitting in your material hopper.
Insufficient additives — Some materials (like PA and POM) depend on internal lubricants or mold release agents. If you're using regrind, those additives may have been depleted.
Test: Run a batch of virgin material and see if the problem improves.
Moisture content too high — Hygroscopic materials (PA, PC, PET, ABS in some cases) degrade when wet. Degraded material becomes sticky and brittle.
The fix: Dry thoroughly according to the material spec sheet. For nylon, that typically means 80-100°C for 4-6 hours.
Material grade changed — Different grades of the same base polymer can have different flow characteristics and shrinkage rates. Did you recently switch suppliers or product lines?
When you need to finish a production run immediately, external mold release spray is your friend.
Choosing a spray:
Silicone-based: General purpose, but interferes with painting/adhesion
Non-silicone (fluorine-based) : Better for parts requiring secondary operations
A word of caution — Mold release treats the symptom, not the cause. Overuse contaminates the mold and leaves oily residues on parts. Always prioritize solving the root problem.
Here's a fast approach when you're standing at the machine:
Observe the failure mode
Stuck on core or cavity?
Part cracked during ejection or just stuck?
Try quick process tweaks (no tools needed)
Reduce holding pressure
Increase cooling time
Adjust mold temperature
Stop and inspect the mold
Check for deposits or damage
Verify all ejector pins move freely
Check material preparation
Was it properly dried?
Any recent material changes?
Plan for mold modification (if nothing else works)
Add draft angle
Improve venting
Rework ejector system
Ejection difficulties don't have to shut down your production. Most cases can be resolved by working systematically through these five areas.
Start with process adjustments — they're free and fast. Move to mold cleaning and maintenance if the problem persists. Only when those fail should you consider design modifications.
And remember: a clean, well-maintained mold with proper draft angle and venting will rarely give you ejection trouble.
