Plastic machining accuracy & surface finish.

Question

Plastic machining accuracy & surface finish.

Vimalmechs (Mechanical)

(OP)

26 Jun 14 08:14

Hi,

I am reviewing a circular part, made by "polypropelene, homopolymer, natural".Which is having o-ring groove for vacuum sealing and oring groove finishing requirement is 16Ra Circular Lay. Most of the dimensions are in 3 decimal(inch)accuracy and +/-.005(for general feature location), +/-.003 for oring groove width.

1) i would like to know whether i can achieve 16Ra in plastic material ?. (i cant go with polishing process on the component because it result cross contamination on the part.)

2) what is the maximum finishing value can achieve by machining on plastic ?

3) what is the dimensional tolerance, we have to go for position the feature from edges, center to center of holes ?

4) what are all the secondary operations i can do on plastics to achieve the finishing value?

Thanks
Vimal VV

Timelord (Mechanical) · Answer 1 · 2014-06-26 11:14:07

Can you flame polish the groove? We used to do this with acrylic parts used in vacuum systems.

Timelord

CoryPad (Materials) · Answer 2 · 2014-06-26 11:34:45

RE: Plastic machining accuracy & surface finish.

CoryPad (Materials) 26 Jun 14 11:34

Solvent polishing is another technique

MikeHalloran (Mechanical) · Answer 3 · 2014-06-27 01:10:04

I don't think solvent polishing will affect PP.

You can get a decent finish by injection molding the stuff to finished size and shape.

If you're attached to starting with sheet or billet, you can probably add features by embossing with a hot iron, but you have to control the process tightly, and it will always be slow.

Mike Halloran
Pembroke Pines, FL, USA

Roserio (Mechanical) · Answer 4 · 2014-08-07 15:15:37

Dear Vimalmechs,

By now this info isn't probably new. Vapor polishing normally allows to achieve very low surface roughness, other methods such as cryogenic polishing are used to get well finished optical fibers. I am not sure what do you mean with cross contamination but for example vapor polishing is sometimes used to produce medical devices and there must be a way to go around the contamination issue.

Few months ago I was looking for companies working with vapor polishing and I found an USA company: ConnecticutPlastics they have some nice examples on their website.

Cheers,
Roserio

var10 (Mechanical) · Answer 5 · 2014-08-07 20:02:09

Have a look at din iso 2768 medium. That's what we use on our drawings for plastics.

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Plastic machining accuracy & surface finish.