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Injection Mould Pitting Repair

I HAVE HAD SOME INJECTION MOULD TOOLS BADLY PITTED AFTER A FIRE. IS THERE A PROCESS TO REPAIR THESE HIGHLY POLISHED SURFACES WITHOUT THE NEED TO POLISH OUT THE PITTING FIRST.
CAN HARD CHROMING FILL IN THE PITTING OR WILL IT FOLLOW THE SURFACE CONTOURS? THE SURFACES IN QUESTION ARE ALSO SHUT OFF FACES, THIS IS WHY I don't WANT TO POLISH OUT THE PITTING FIRST.

NIGEL FARENDEN
TOOLMAKER - SUFFOLK ENGLAND
June 9, 2011

You may want to try electroless nickel coating. Depending on how deep your pits are, it may smooth them out and you can put a fairly thick coating on, then polish down the nickel coating. If your pits are much more than 0.3 mm you may be out of luck. Some electroless nickel coatings have a coefficient of friction lower than PTFE, so the coating would have the added benefit of better release. That may not be the answer but that's the first place I would look for a solution.

Chris Johnson
- South Jordan, Utah, USA
First of two simultaneous responses -- June 10, 2011

YOU CAN FILL THE PITTING WITH A TECHNIQUE CALLED BRUSH PLATING WITH NICKEL COBALT AND AFTER THAT YOU CAN POLISH THE MOLD

Ricardo Burstein
Bnei Berak, Israel
Second of two simultaneous responses -- June 11, 2011

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Brush plating technique is most commonly used to repair the defects in finished parts. The utility of this technique depends on how much damage your mould has experienced.

I agree with Chris Johnson. Electroless nickel coatings might be a useful choice as it enables a uniform coating formation on all areas.

Electroless nickel composite coatings would improve the life of the moulds.

T.S.N. Sankara Narayanan
- Chennai, Tamilnadu, India
(ed.note: The good doctor offers a fascinating blog, "Advancement in Science" )
June 13, 2011

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June 16, 2011

I agree with Mr. Narayanan. We do mold finishing and have both EN and brush plating in house. In the past, we have dealt with over ten molds that were caught in fires from two different customers that suffered catastrophes. In areas where you absolutely need to build-up thick layers, brush plating is the best way, specially mid-hard sulphamate after a proper strike of acid nickel. Electroless nickel, though very useful for intricate shapes and inherent uniformity, is too brittle and may crack and spall over parting line (shut-off) surfaces when thick, which is often the case in severely damaged molds. It will also be extremely difficult to stop-off in areas where you don't need metal. Brush plated sulphamate nickel is ductile, grindable, corrosion resistant and polishes to a superb luster. You can apply it just where you need it. On the other hand, no plating process will spontaneously fill-in pits. On the contrary, many plating chemical formulations tend to aggravate the problem, specially when applied over conventional tool steel.
If we can be of further help come back with more Q's.
Good luck,
G. Marrufo

Guillermo Marrufo
Monterrey, NL, Mexico