×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Tolerances on plastic parts

Tolerances on plastic parts

Tolerances on plastic parts

(OP)
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

RE: Tolerances on plastic parts

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

RE: Tolerances on plastic parts

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
www.industrialdesignhaus.com
Reason trumps all.  And awe transcends reason.

RE: Tolerances on plastic parts

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.

RE: Tolerances on plastic parts

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

Matt
CAD Engineer/ECN Analyst
Silicon Valley, CA
http://sw.fcsuper.com/index.php

RE: Tolerances on plastic parts

(OP)
Thanks for all your responses..

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources