Why can't Black Plastics be Polished to High Gloss?

[Jmanm]

Pretty much all black plastics found on consumer appliances that I have tried to polish to a high gloss fails pretty bad, at best the results are inconsistent. Unlike many clear plastics, which are pretty easy.

 

I usually do a two step method by hand , 1- compound w/coarse foam pad, 2 - polish w/fine foam pad.

 

It's usually the polish process that causes visual flaws with any significant pressure & duration. You can't just do one pass to get the fine scratches out. Long before they come out, the appearance is ruined.

 

 

Finally, why is it that the plastic looses it's chemical resistance to isopropyl alcohol? And even watered down iso. Normally when new the black plastic will handle the isopropyl alchol, but when polished, the surface gets slightly damaged with what look like stains that don't come out.

 

 

thanks

[DrP]

I think it depends solely on what type of polymer the plastic is made from and the molecular weight and the degree of cross linking in the polymer - particularly with respect to it's solubility in IPA and other solvents.

[Sensei]

Polishing makes microscopic scale scratches.

Which under microscope would looks like mountains and valleys.

Photons are reflected and bounces from them in random directions.

You might try heating carefully with blowtorch to melt the outermost layer instead.

 

[Endy0816]

Are you working with CPET?

[Enthalpy]

Many plastics can be polished. For instance PMMA, for which it's common practice, with polishing kits available for use after glueing.

 

Most plastics can get a very smooth surface right from injection: PMMA, PC, all materials for O-rings, epoxies (they replicate mirors), silicone (I've seen structures 4µm wide with high definition just from casting over patterned silicon), and so on. Just think of CD and DVD, whose bits are lands <1µm long of different height, obtained by injection in a mould, or lamp reflectors.

 

Plastic polishing seems less easy than just smooth injection and than metal polishing. It is known that the finest paste or cloth deplaces metal atoms from hills to valleys. Maybe the finest paste still scratches polymers, but that's just a hypothesis.

[dimreepr]

Many plastics can be polished. For instance PMMA, for which it's common practice, with polishing kits available for use after glueing.

 

Most plastics can get a very smooth surface right from injection: PMMA, PC, all materials for O-rings, epoxies (they replicate mirors), silicone (I've seen structures 4µm wide with high definition just from casting over patterned silicon), and so on. Just think of CD and DVD, whose bits are lands <1µm long of different height, obtained by injection in a mould, or lamp reflectors.

 

Plastic polishing seems less easy than just smooth injection and than metal polishing. It is known that the finest paste or cloth deplaces metal atoms from hills to valleys. Maybe the finest paste still scratches polymers, but that's just a hypothesis.

 

 

As a former paint-sprayer, I'd say this is a little more than a hypothesis.

Pretty much all black plastics found on consumer appliances that I have tried to polish to a high gloss fails pretty bad, at best the results are inconsistent. Unlike many clear plastics, which are pretty easy.

 

I usually do a two step method by hand , 1- compound w/coarse foam pad, 2 - polish w/fine foam pad.

 

It's usually the polish process that causes visual flaws with any significant pressure & duration. You can't just do one pass to get the fine scratches out. Long before they come out, the appearance is ruined.

 

 

Finally, why is it that the plastic looses it's chemical resistance to isopropyl alcohol? And even watered down iso. Normally when new the black plastic will handle the isopropyl alchol, but when polished, the surface gets slightly damaged with what look like stains that don't come out.

 

 

thanks

 

 

Try spraying a two pack clear coat, then flat and polish.

[Jmanm]

Polishing makes microscopic scale scratches.

Which under microscope would looks like mountains and valleys.

Photons are reflected and bounces from them in random directions.

You might try heating carefully with blowtorch to melt the outermost layer instead.

 

Hi, I don't think a blowtorch would give a nice result over a large area. I did think about this, but most torches flame are small.

Many plastics can be polished. For instance PMMA, for which it's common practice, with polishing kits available for use after glueing.

 

Most plastics can get a very smooth surface right from injection: PMMA, PC, all materials for O-rings, epoxies (they replicate mirors), silicone (I've seen structures 4µm wide with high definition just from casting over patterned silicon), and so on. Just think of CD and DVD, whose bits are lands <1µm long of different height, obtained by injection in a mould, or lamp reflectors.

 

Plastic polishing seems less easy than just smooth injection and than metal polishing. It is known that the finest paste or cloth deplaces metal atoms from hills to valleys. Maybe the finest paste still scratches polymers, but that's just a hypothesis.

 

If only I could get to the level of a DVD disc, that is amazing how sharp & perfect.

 

No applicator exists to my knowledge that gives ultra fine scratches on soft plastic. Tissue is the best but on this black plastic it ruins the finish probably from too much friction, pressure & soo little cut ability like shaving with a blunt razor & causing irritation.

 

 

Are you working with CPET?

Don't know to be honest. It's got to be the worst cheap rubbish they use.

 

 

 

 

 

As a former paint-sprayer, I'd say this is a little more than a hypothesis.

 

 

Try spraying a two pack clear coat, then flat and polish.

You read my mind, I just thought about that as at least it will be durable.

Edited July 21, 2016 by Jmanm

[Endy0816]

I suspect material differences.

 

Crystallization makes the plastic more opaque, rigid and useful in higher temperature applications.

 

Black plastic polishes out there for autos. Looks to be along the same lines of Dimreepr's suggestion though.

[Enthalpy]

One difficulty to polish polymers is that their molecules use to be larger than the irregularities tolerated by optical polish, so if the polishing process rips off macromolecules, the surface keeps optically rough.

 

Also, polishing is often done using materials softer than the target, like cloth against metal, and softer than a polymer excludes every abrasive.

[StringJunky]

I think part of the reason is that a black surface gives the highest possible contrast ratio of light and dark areas, so, if it's not perfectly flat there will always be areas that scintillate reflected light, highlighting even the most microscopic irregularities in the the surface. Unless it's moulded in a perfectly blemish-free mould, which is flat from the beginning, you will only scratch the surface if you apply polishing techniques.

 

Perhaps Jeweller's Rouge might be worth trying and is not expensive. My grandad used to take the scratches out of glass and plastic with it; he did my watches a few times, which usually had plastic watchglasses, when I scratched them. I have some coming shortly and will have go scuffing a black plastic object and see what happens using it.

 

https://www.amazon.co.uk/gp/product/B00O3GVV2I/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

Edited August 9, 2016 by StringJunky

[dimreepr]

I think part of the reason is that a black surface gives the highest possible contrast ratio of light and dark areas, so, if it's not perfectly flat there will always be areas that scintillate reflected light, highlighting even the most microscopic irregularities in the the surface. Unless it's moulded in a perfectly blemish-free mould, which is flat from the beginning, you will only scratch the surface if you apply polishing techniques.

 

 

That would depend on the accuracy in the flatting phase.