Ejection Deformation
Bending, cracks, fractures or sunken ejectors formed as the part comes out of the mold. Solved with the shrinkage management rule: low shrinkage for a part wrapping a core, high shrinkage at ribs.
Ejection deformation is the distortion of the part's shape as it separates from the mold. The difference from ejection scratches is that it is not the surface but **the geometry** that is damaged: the part bends, goes oval or one area collapses inward. The cause is either that the part has not yet hardened sufficiently or that the ejection force is applied unevenly.
How to Recognize It
- Cracks, fractures, overstretched areas, deeply sunken ejectors.
- Whitening/tearing in undercut areas ejected by force.
- If the part gets stuck in the mold, suspicion of residual pressure is high.
Physical Causes
- The ejection force is higher than the part can bear: shrinkage onto the core (sleeve/box parts) or ribs sticking to the wall.
- Ejection under residual pressure: the cavity pressure has flexed the mold, the part has solidified in the enlarged cavity and got stuck.
- Some materials adhere strongly to steel (TPEU, PUR, SEBS); with a mirror polish the ejection force for PP, PC and TPU INCREASES.
- In a process with dynamic heating, if the mold wall is >150°C, semi-crystalline materials tend to stick.
Solution Steps at the Machine
Apply the steps in order and one at a time: after each change, wait a few cycles and assess the result. If there is no improvement, undo the change and move on to the next step.
- GOLDEN RULE: For a part wrapping a core, LOWER the shrinkage (increase the holding pressure or reduce the cooling time). In the rib area, RAISE the shrinkage (reduce holding or increase cooling) — so the ribs come away from the wall.
- If there is residual pressure: early switchover, low holding, long cooling.
- For undercut deformation: REDUCE the cooling time (so the part comes out while more elastic), increase the ejector speed.
- For sunken ejectors, increase the cooling time.
Mold, Design and Material Solutions
- Align the polishing direction with the ejection direction; optimize the surface roughness for the material.
- Add core venting (breaks the vacuum effect); check the draft angles.
- For sticking materials use an anti-friction coating (note: some coatings increase the force — the layer must be chosen specifically).
- A mold release agent is only a temporary solution; consider air-assisted ejection. Release agents CONTAINING SILICONE must be removed from production entirely if the part is later to be coated, painted, printed or joined by bonding/2K — silicone transfers to the surface, permanently spoils adhesion and cannot be reliably removed by cleaning.
Which defect is it confused with?
A wrong diagnosis leads to turning the right parameter in the wrong direction. This defect can be confused with the ones below; the right-hand column gives the practical way to tell them apart.
| Confused with | How to tell them apart |
|---|
| Warpage | Warpage arises from unbalanced cooling and can develop within hours after the part leaves the mold. Ejection deformation forms at the moment of ejection. |
| Ejector Marks | An ejector mark is a local mark. With deformation the whole geometry of the part is affected. |
Frequently asked questions
Is lengthening the cooling time the only solution?
It is the fastest solution, but because it lengthens the cycle it should be a last resort. First try lowering the ejection speed, increasing the number of ejectors and checking the draft angle. With parts held by vacuum, insufficient venting also leads to deformation.
What should be done if the part sticks to the mold?
Sticking increases the ejection force and triggers deformation. The condition of the mold surface, the draft angle and the mold temperature must be assessed together. A soft part coming out of a hot mold both sticks and deforms easily.
Prepared by:Uğur Çamlıca· Teknik Part · Source: manufacturer documentation and field practice