Stress Whitening and Cracks
Whitening caused by excessive deformation or ejection under residual pressure, and cracks that appear over time.
Stress whitening is a light-colored area formed by the material structure opening up at micro level in the area where the part is strained; at an advanced stage it turns into a crack. It is mostly seen during ejection, at ejector areas and at sharp corners. The defect is not cosmetic — that area has been permanently weakened.
How to Recognize It
- Stretched areas whiten (around ejectors, undercut areas, film hinges).
- Stress cracks may appear DAYS-WEEKS after production (at the customer!).
- Contact with aggressive media (alkalis, grease) accelerates cracking.
Physical Causes
- The maximum permitted deformation of the material has been exceeded (external strain or warpage).
- Ejection under residual pressure: when the part comes out to atmosphere, the inner layers stretch the outer layers.
- Wetting/diffusion/swelling processes weaken the molecular bonds (media effect).
- The notch effect of sharp-edged metal inserts.
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.
- Lower the residual pressure: switch over to holding early, reduce the holding pressure/time, increase the cooling time.
- With semi-crystalline materials: try raising the mold and melt temperatures.
- Work film hinges while they are HOT.
Mold, Design and Material Solutions
- Stiffen the mold design (if deformation enlarges the cavity volume, the part gets stuck).
- Round off edges, avoid jumps in wall thickness.
- If the media are known, choose the material to suit them; prefer a high molecular weight.
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 |
|---|
| Moisture Streaks | A moisture streak is a glossy silvery line on a flat surface and has nothing to do with strain. Stress whitening is in the strained area and can be felt to the touch. |
| Ejection Deformation | With deformation the geometry is distorted. With stress whitening the shape is retained but the material has been damaged at that point. |
Frequently asked questions
Why does it concentrate at ejector areas?
Ejector pins apply force over a small area. If the part has not cooled enough or the ejector area is small, the stress per unit area exceeds the strength of the material. Increasing the number of ejectors or enlarging the contact area is a direct solution.
Is there a way to reduce internal stress?
Raising the mold temperature and balancing the cooling lower the internal stress. A part that solidifies quickly in a cold mold comes out with high internal stress and is prone to cracking later.
Prepared by:Uğur Çamlıca· Teknik Part · Source: manufacturer documentation and field practice