𝐓𝐨𝐝𝐚𝐲'𝐬 𝐊𝐍𝐎𝐖𝐋𝐄𝐃𝐆𝐄 𝐒𝐡𝐚𝐫𝐞 : 𝐓𝐡𝐞 𝐩𝐚𝐫𝐭 𝐜𝐚𝐦𝐞 𝐨𝐮𝐭 𝐨𝐟 𝐭𝐡𝐞 𝐦𝐨𝐥𝐝 𝐥𝐨𝐨𝐤𝐢𝐧𝐠 𝐩𝐞𝐫𝐟𝐞𝐜𝐭. 𝐓𝐡𝐞𝐧 𝐭𝐡𝐞 𝐞𝐣𝐞𝐜𝐭𝐨𝐫 𝐩𝐢𝐧𝐬 𝐩𝐮𝐬𝐡𝐞𝐝 𝐢𝐭 𝐨𝐮𝐭.

𝐓𝐨𝐝𝐚𝐲'𝐬 𝐊𝐍𝐎𝐖𝐋𝐄𝐃𝐆𝐄 𝐒𝐡𝐚𝐫𝐞

𝐓𝐡𝐞 𝐩𝐚𝐫𝐭 𝐜𝐚𝐦𝐞 𝐨𝐮𝐭 𝐨𝐟 𝐭𝐡𝐞 𝐦𝐨𝐥𝐝 𝐥𝐨𝐨𝐤𝐢𝐧𝐠 𝐩𝐞𝐫𝐟𝐞𝐜𝐭.

𝐓𝐡𝐞𝐧 𝐭𝐡𝐞 𝐞𝐣𝐞𝐜𝐭𝐨𝐫 𝐩𝐢𝐧𝐬 𝐩𝐮𝐬𝐡𝐞𝐝 𝐢𝐭 𝐨𝐮𝐭.

𝐖𝐡𝐢𝐭𝐞, 𝐜𝐡𝐚𝐥𝐤𝐲 𝐩𝐚𝐭𝐜𝐡𝐞𝐬 𝐚𝐩𝐩𝐞𝐚𝐫𝐞𝐝 𝐞𝐱𝐚𝐜𝐭𝐥𝐲 𝐰𝐡𝐞𝐫𝐞 𝐭𝐡𝐞 𝐩𝐢𝐧𝐬 𝐜𝐨𝐧𝐭𝐚𝐜𝐭𝐞𝐝 𝐭𝐡𝐞 𝐬𝐮𝐫𝐟𝐚𝐜𝐞.

That is stress whitening.

What actually happened?

The ejection force was too concentrated.

As the ejector pins pushed the part from the mold, the plastic at the contact point was stressed beyond its yield limit. The polymer chains formed microscopic crazes—tiny cracks that appear as a white, frosted patch on the surface.

That white patch is not just cosmetic.

It is the surface telling you that the material has experienced excessive mechanical stress.

Stress whitening commonly appears:

• Around ejector pin locations

• At sharp corners and edges

• Near stress concentration zones

• Around snap-fit features after assembly

Why it happens

① Excessive ejection force — ejector pins push too hard

② Overpacking the cavity — residual stress is locked into the part

③ Sharp corners and notches — stress concentrates at the geometry

④ Brittle material grade — low-impact PP crazes more easily

How to prevent it

✓ Increase ejector pin diameter to distribute force

✓ Add more ejector pins to reduce pressure per pin

✓ Reduce pack pressure to minimise residual stress

✓ Add corner radii to reduce stress concentration

✓ Select a tougher PP impact copolymer for better stress resistance

Stress whitening is not a cosmetic defect.

It is a craze—and a craze is a crack waiting to open.

Distribute the force.

Eliminate the stress.

Protect the surface.

💬 Have you experienced stress whitening on an automotive plastic part?

Was the root cause excessive ejection force, residual molding stress, or part geometry?

Share your experience in the comments.

source : IM Technologies