Tolerances on plastic parts

[mechy20]

Hi,
I have very recently started working on plastic parts design. I would like to know what is a typical error in perpendicularity for a molded part. I am just trying to get an estimate and an educated guess from experts out there would really help.
Thanks

 

[bvanhiel]

Depends on the part geometry, the tool construction, the material, the processing and the part handling.

General tolerances are +/-.005" per inch, but you can hold about 1/3 of the shrink of the material on critical dimensions if you tune the tool and processing. Depending on the length of your perpendicular features this should give you an idea.

-b

 

[Theophilus]

Like bvanhiel said, it depends what you're doing. Material plays a huge part, since saying "plastic" is probably something like saying "metal"--maybe worse. If you provide more detail on your application, design, material, specific molding process (injected, blown), etc. you can probably get a more useful answer.

I design plastic widgets for a living, and normally don't consider molded parts as the ideal of precision--better to add sufficient "flex" to your design such that you can work with the relative fluctuation that molded plastic parts can provide.

The structure of your parts matters, too, since you have warping to consider when the part shrinks during cooling. There are several "rules of thumb" guides you can find all over the Internet as general ways to get things done. A little experience goes a long way in getting what you expect.



Jeff Mowry

http://www.industrialdesignhaus.com

Reason trumps all. And awe transcends reason.

 

[dgowans]

For tolerances on dimensions you can look at SPI (Society of the Plastics Industry) charts. I've got a copy of the Plastics Engineering Handbook published by SPI that has tolerance charts for a bunch of different materials. This data isn't a hard and fast rule on which you can base every design, but it's a great starting point.

The charts identify both standard commercial tolerances and fine tolerances. By consulting with a molder you should be able to establish tolerances that meet your design needs and are achievable by the molder.

 

[fcsuper]

Also, the processes used to form the plastics affect tolerances.

Matt
CAD Engineer/ECN Analyst
Silicon Valley, CA

http://sw.fcsuper.com/index.php

 

[mechy20]

Thanks for all your responses..