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"Issues and solutions in mirror polishing and restoring stainless steel molds"


Q. Sir;

I am wanting to remove scratches and restore brightness to my stainless steel molds that I use to cast cultured marble and solid surface materials. The molds get dull over time. Are there any special chemical, abrasives or polishers that you can direct me to?

Thank you,

Gary H [last name deleted for privacy by Editor]
- Massillon, Ohio


A. Hi Gary,

The first thing you need to do is to determine what the molds actually require in terms of topographical restoration before you start looking for equipment. In some operations I have had experience in, they required electropolishing; in others they required chrome plating; in others they required both; and in still others just satinizing (polishing with pumice) sufficed. Is it possible to characterize the required surface beyond "restore brightness"? Thanks.

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Need resistance to fluoropolymers


Q. My company builds molds for Fluoropolymers. We already use Stainless Steel for our tools. All our inserts to these tools are carbide. We would like to find a plating that would withstand Fluoropolymers (PSA, PFA, Tefzel 30% glass) that would be made from SS. Any comments would help.

R Posego
Plating for injection molding of Fluoropolymers - Meadville, Pennsylvania, US


A. Just in case there's a misunderstanding; you can plate over stainless steel but you cannot plate stainless steel as such, for it is an alloy with specific chemistry and various elements that cannot be plated together. To select a plating or coating for your molds you must identify what is the main problem. Is it corrosion, erosion, sliding wear? Also, do you have very tight tolerance? Then you can opt for such things as TiN ion plating, EN+hard or lubricant particles, spraying of ceramics or composite carbides, etc.

Guillermo Marrufo
Monterrey, NL, Mexico

Molds for epoxy tubes


Q. I am a production supervisor who's having trouble producing tube! We use 4.5 foot long stainless molds to cast epoxy tubes. Our tubes mirror exactly the i.d. of the mold. We spec a mirror gloss finish that is probably a #8 or better. We used to use carbon, and could attain & maintain finish with #320 to # 600 alum. oxide sandpaper.

Stainless gives us fits. We go up to # 1200 grit paper for 1 hour per mold and still have trouble attaining an acceptable finish; and also use diamond paste with varied results. Really tough to blend scratches. We mold opaque and translucent.

Chrome plating did not work.

Any input ?

Our biggest prob. size is .476 / .500 ... shows everything.


Dave B [last name deleted for privacy by Editor]
epoxy tube manufacturer - Rio Grande, New Jersey


A. Carbon steel can be hardened which would make the finish last longer for your application. By the same token, 300 series stainless is comparably soft and thus would be a victim of minor abuse. 400 series SS can be hardened and should be OK. Specialty (common) hardenable alloy like 17-4PH are typically better corrosion resistance than carbon steel and still can be equally hard.

"Chrome did not work" -- That does not make sense. Why didn't it work? If it was properly prepared, plated and polished, it should be superior. Did you try electroless nickel? It has plusses and minuses, but if properly done over properly prepared and appropriate substrate, it should also work. There is an outside possibility that electropolish could be an answer.

James Watts
- Navarre, Florida

Need better performance but no coating thickness


Q. I am wondering if there is anything that can be done to enhance mold component performance from a finishing standpoint. Conditions are 350 °F mold releases, elastomers, etc. The components are toleranced to ±.0002", so anything done cannot affect dimensions.

Daniel T [last name deleted for privacy by Editor]
manufacturing solutions - East Liverpool, Ohio


A. Daniel,
What exactly do you mean by finishing standpoint? Are you talking about shinier plastic surfaces, better part release, a seal problem that is producing parts that need to be finished? Let us know if we can help.

Guillermo Marrufo
Monterrey, NL, Mexico


Q. I was looking for any surface treatments that could enhance corrosion resistance in molding situations. The center rotates freely and I didn't know if there could be anything to produce a more superior product.

Daniel T [returning]
- Liverpool, Ohio


A. Look into TiN or similar coatings. They are very thin, super hard and somewhat corrosion resistant. They are applied under vacuum by Physical Vapor Deposition (PVD) techniques.

Guillermo Marrufo
Monterrey, NL, Mexico

Mirror polishing stainless steel for a mold insert to make an aircraft lens

March 7, 2010

Hi, my name is April and I work at an aviation supplier. We are trying to make a mold for a lens that goes in an aircraft instrument. Usually we send the mold insert out to get the "mirror finish" but it is very costly, and we have gotten a job to do quite a few of these lens, So we would like to do this process in house.
We have some SpeeFam machines that we lap and polish glass slugs/wedges on, and we have tried the same slurries & pads and method on the steel -- yet once we have reached the polish step it comes out with a orange peel look, yet extremely smooth , I don't think it will yield a great lens.
This has been our process so far:
4" x 4" stainless steel mold insert lapped until machine lines are gone, about (.005 lapped off) using slurry grit size 13 (I think).
I felt the table when we were lapping and the table was cool temperature and we used no pressure just the weight of the steel itself.
Next, polish with a pad and slurry (grit size 9, I think) that we use for polishing glass and it works for that method leaving no scratches or pits so we thought it would work on stainless steel ... well it kind of does, but it leaves an orange peel look but not quite, and orange peel feel and if you run your fingernail across it you can feel some ridges.
We tried for about two hours with no pressure, no luck and then we put a little pressure on for about two hours and finally you could see yourself in the finish but then we ran into the orange peel problem. So we decided to try the polish slurry on the lapping table; the lapping table is metal with no pad, used no pressure and let it run for about two hours. We got a better finish but we don't know if it will work -- our finish doesn't look like the finish of the mold inserts we have had sent off in the past to have done for us.

We are willing to buy a different slurry and pads; need to do this in house.
We have SpeedFam machines that have the four heads and they are manual.
We have hand tools to polish but we need a uniform polish throughout the whole mold insert, so we thought using our lapping and polishing machine it could be done, but our expertise in that department is glass not stainless steel.
Please help. Thanks, April

Addendum: Lapping #15 micron alumina oxide
polishing on a polyurethane pad with 1.5 micron cerium oxide grit

304 stainless steel

April M
- Hot Springs, Arkansas

March 11, 2010

A. Optional thought. Way in the past, we did some tests on polishing molds that were use to make glass bottles, using small vibratory machines and treated dry organic polishing media. It did a nice uniform job with not orange peel appearance; however, the end customer still needed to hand polish after this process so he decided not to use this method at all because that would have created another processing step. I think it probably would save time, but there are a lot a variables that would have to be tested.

tony kenton
AF Kenton


March 17, 2010

A. Hopefully when you placed your bid to get the job you considered the cost of sending the molds out. Mold polishing is more than a simple lapping operation. Hardly will you set a machine and let it do it unattended. Very much like jewelry, molds are always finished by artisans. Perhaps your best bet would be to find a qualified mold polisher that charges less.
Good luck,

Guillermo Marrufo
Monterrey, NL, Mexico

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