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Pitting corrosion on steel after molding acrylic plastics

I have been molding plastic parts such as PC, PP, etc using steel mold without any pitting corrosion. However, I found pitting corrosion in my steel mold when I use the mold to produce acrylic plastic parts through injection machine. By the way, the mold use to mold acrylic parts has not been used to mold other plastic parts. The steel is polished to give a polish finish on the acrylic parts. The mold is more than 1 year old. After polishing, the pitting reduces. However, after repeated injection molding, the pitting corrosion reappear. Even after numerous polishing, it does not help, the pitting remains. What is the cause?

David Tan HK
plastic industry - Singapore

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The pitting you have occurring is called the "orange peel effect". I have polished (fixed) many moulds with this effect. Start with a basic polish....no high speed buffs. Reduce the amount of buffing to a minimum. Do not press hard when buffing! Go back to paper if necessary, and go through the grades, back to the suitable micron finish required. I have heard that the die " breathes " every time it opens...the gases that are generated will also eat into pitted areas. When you have achieved the required finish you could always hard chrome the die surfaces. Polishing may be sped up using a high speed buff but in the long run a little bit of extra time in the foundation process will help during the production run.

By the way....I learned all this by reading and posting questions of my own by accessing this fantastic site. I apply the things I read at finishing.com to my own polishing and welding business.

Michael Hein
- Sydney, NSW, Australia

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Hello,

I agree 100% with the man in Australia. The pitting has following causes: 1-excessive speed upon the polishing buff 2-corrosion from released gases from plastic resins 3-both

I would only like to add that if you can get 0.0005 to 0.001" thick electroless nickel in your mold cavities, the corrosion resistance as well as the polishability of the plated surfaces will improve over chromium. Not so the scratch resistance. A plater with expertise in molds is a must.

Bye.

Guillermo Marrufo
Monterrey, NL, Mexico

Thank you for all your responses.

David Tan
- Singapore   
April 16, 2009