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topic 11575

Pitting in Mold Steel


I am working at TPO Injection Molding Company. I do have "pitting" problem in the cavity of the mold steel (P-20 large block), which replicate as small "bumps" on the molded TPO part.

My questions are:
1- What may cause these pits?(Source & Origin)
2- If it is "Steel Processing" failure, what the steel supplier should do to minimize or eliminate these pits.
3-If it is "Machining Process" failure, what are the remedies.

Thank you.

Anthony Theodore Z
- Windsor, Ontario, Canada


The nature of the pits could be from processing or could be defects in the metal.

You might be able to repair the pits most economically with brush plating of nickel if there are only a few pits.

Visit our Chemicals directory and our Jobshop directory, and search using the word "brush" to find companies that offer chemistry and services.

tom pullizzi portrait
Tom Pullizzi
Falls Township, Pennsylvania



There are two reasons for pitting in mold cavities.

1-Corrosion caused by the plastic during its molding cycle. We sometimes forget that the resins liberate different gases which combined with humidity in the air produce corrosive products. No matter how well you dry the granulate, every time the mold opens it "breathes" air.

2-Final polishing steps that depend on the moldmaker¥s ability and method. It is well known among professional mold polishers that if felt or cloth-like wheels used to polish cavities are used to remove excessive stock or with excessive pressure they can leave very tiny marks that subsequently develop as pits.

Remedy: professional repolishing of cavities and final nickel or chrome plate

Guillermo Marrufo
Monterrey, NL, Mexico

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