Engineering Drawing: No Surface Finish

[kojiai]

I'm putting together some basic engineering drawings for a plastic injection molder. I was going through and indicating surface finishes when it dawned on me; I'm not sure how to indicate that no addition finish is necessary. There are several surfaces or parts that are not visible to the user, and don't need additional texture, just the machine finish of the cut mold is fine. Is there a standard to indicate this, or is it assumed to be the case unless otherwise indicated? How should I indicate this in my drawings? Thanks!

 

[KENAT]

We put a default 'unless otherwise specified' note on most of our drawings with a value well within the relevant process capability. I suggest you do similarly.

In terms of the drawing aspect you might want to ask over in the GD&T forum forum1103 . In terms of what default finish for plastic parts is, there are a couple of dedicated plastics forums.

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(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[mcgyvr]

Actually SPI (society of the plastics industry) and I'd suspect other standard bodies might have a standard too.
Just a few here
SPI Finish Guide Typical Applications
A-1 Grade #3 Diamond Lens / Mirror – requires 420 SS material
A-2 Grade #6 Diamond High Polish parts
A-3 Grade #15 Diamond High Polish parts
B-1 600 Grit paper Medium Polish parts
B-2 400 Grit paper Medium Polish
B-3 320 Grit paper Med – Low polish
C-1 600 Stone Low Polish parts
C-2 400 Stone Low Polish parts
C-3 320 Stone Low Polish parts
D-1 Dry Blast Glass Bead Satin finish
D-2 Dry Blast # 240 Oxide Dull Finish
D-3 Dry Blast # 24 Oxide Dull finish

Then molders might have their own "standard"

http://www.protolabs.com/injection-molding/fundamentals-of-molding/molding-surface-finishes

Personally for applications that aren't critical I avoid specifying anything.. I find that many times its better to under specify that over..
But I also discuss any molded parts with the molder during the quotation process and might ask them what they recommend too..
My one molder just likes that I put "light edm finish"

 

[MadMango]

SPI is a standard, for Society of Plastic Industry. Here's a link to a decent chart explaining the differences.

[URL unfurl="true"]http://cr4.globalspec.com/thread/19452[/url]



"Art without engineering is dreaming; Engineering without art is calculating."

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[JNieman]

We do Wire EDM, grinding, and machining, and we keep a default 125 or 250 Ra on the title block just to provide some form of boundary. Saves your butt if someone decides to re-bid a part with a rougher operation without informing you. For my situation, an example might be that we are performing a milled periphery and decide we want to get it waterjet or torch cut. If there's a certain allowable surface roughness, it will tie hands from delivery something 'surprising'.

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