Wax or something to replicate surface roughness?

Conrad Hoffman · Aug 15, 2022

I have a very good surface profilometer, but it's a fixed unit and takes a moderately small and low mass sample. I also have a large aluminum plate, several feet square that I have to measure. Is there any accepted "stuff", like a wax or resin, that I can replicate the finish with, then measure it in the profilometer? I can scan relatively soft substances, but am not sure about a wax. Epoxy comes to mind, but I'd afraid of not being able to get it off cleanly, even with a mold release. It's important not to scratch or damage the plate. I know a silicone mold rubber will replicate the surface finish perfectly, but it's pretty soft. Ideas?

sfriedberg · Aug 15, 2022

Repro-rubber, the pourable sort, not the putty, should do a good job. Don't know the durometer off hand, so can't say how stiff it is compared to your silicone mold rubber.

dcsipo · Aug 15, 2022

casting plaster?

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Conrad Hoffman · Aug 15, 2022

Green machinable wax didn't work because it hardened before it could take a good impression. What worked, sort of, was hot glue. A dollop on a small aluminum block could be pressed into the surface and got a passable impression. It's subject to pinholes and imperfections, but it worked well enough to get the job done. This was a rough surface (Ra=3 µm) and I don't know if it would be good enough for a fine surface, but hey, on to the next job. Thanks for above suggestions and maybe I'll be better prepared the next time this comes up.

jccaclimber · Aug 15, 2022

Just a curiosity, but is your profilometer the contact or non-contact variety? I have a simple little Starrett one (contact), but found it to be useless on relatively smooth (optical components) polycarbonate and softer (Zeonex) materials due to the stylus plowing through the material rather than raising and lowering over each imperfection.

Conrad Hoffman · Aug 15, 2022

We're lucky enough to have a KLA/Tencor P-7 unit designed for inspecting semiconductor wafers and other things. I can control the stylus pressure down to about 1/2 milligram, so almost anything can be scanned. It will also do a 2-dimensional scan and give me an actual surface map. The downside is I can't put anything much bigger than a few inch cube inside, plus the stylus replacement cost is about $1500 if I crash one. https://www.kla.com/products/instruments/stylus-profilers/p-7

sfriedberg · Aug 15, 2022

Sweet device! Betcha don't let the interns use it in their first week.

Conrad Hoffman · Aug 15, 2022

Some people take right to it, others never. It requires a good sense of three dimensional space and knowing what the machine is going to do when you push a (on screen) button. Not that much different than running a CNC mill without crashing, though I think some CNC crashes are substantially more expensive!

rustyironism · Aug 15, 2022

Conrad Hoffman said:
Epoxy comes to mind, but I'd afraid of not being able to get it off cleanly, even with a mold release. It's important not to scratch or damage the plate. I know a silicone mold rubber will replicate the surface finish perfectly, but it's pretty soft. Ideas?

For what you are doing, I'd use urethane rather than silicone.
Half the price.
Up to about 85 durometer.
But if the surface is flat, not curved, and you can lift the plastic off without worrying about peeling off curves, plastic may be the way to go.

I use Freeman Repro plastic.
I use the Slow because I need time to fill molds, but it will find all imperfections in a surface down to pretty fine.
It is a real pain when fresh or off the shelf to stir, but a few minutes mixing makes a good product.
1 to 1 mix.

I needed to fill a large mold, so needed more time, so tried Smooth-on Smooth Cast 385.
It's a little thicker than the Repro, but a 20 minute pot life gives a little extra time.
but it is a 5 to 1 mix.

Can usually get away without vacuuming the pot with either.

Universal mold release absolutely releases without causing any damage.

Just sharing my experiences.

Mike

plastikdreams · Aug 15, 2022

Might want to look into rencast products.

memphisjed · Aug 16, 2022

Pourable Silicone Starter Kit Product Information

Everything You Need To Make a Silicone Block Mold

www.smooth-on.com

there is a reason smooth on is king in detail mold world.

Conrad Hoffman · Aug 16, 2022

Yes, love Smooth-On and that's the silicone I use, but the one I have is a bit soft for this application. At least I think it is, never having had the courage to scan it.

memphisjed · Aug 16, 2022

Conrad Hoffman said:
Yes, love Smooth-On and that's the silicone I use, but the one I have is a bit soft for this application. At least I think it is, never having had the courage to scan it.

Pour a smooth cast plastic into your silicone?I know you will loose some level of detail at each translation- not to much I think.

Their technical/sales people are pretty sharp too.

Conrad Hoffman · Aug 16, 2022

That's a good idea. I know I can replicate insane small surface finishes and it gives me a positive to work with.

Peter S · Aug 18, 2022

What about taking a sulfur cast? As used by toolmakers to check cavity detail etc. It doesn't shrink and is hard. To contain the sulfur, maybe cut a short piece of tube/pipe placed on the surface? Not sure if it will leak....

Sulfur can be remelted (indefinitely?), we kept it in a small open-topped can, a ladle would be better. Heat the container with a propane torch. Inevitably, you will overheat it and enjoy the resulting aroma :ack2:

I have read that you can buy sulfur from a garden centre.

Fulcircleny · Aug 18, 2022

Good morning Conrad,
in the past we have used this product.

Facsimile® Powder

Clone Any Part Quickly With Facsimile® Quick-Setting Compound. Facsimile® is a special plastic compound kit which is used for duplicating surface flats, roughness and for dimensional transfer of inaccessible locations. Makes Accurate Reproductions in 6 to 8 Minutes. Make duplicates for measuring

www.flexbar.com

Best of luck, Chris

M. Roberts · Dec 5, 2022

Conrad, I just came across this, don't know if you have a solution yet. There is a company by the name of "Testek" in Delaware that makes surface replication tape for the use to verify the surfaces of bridges prior to painting; that may be something that would work for you...

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