Modeling for shrink

Postby finiteD » January 31st, 2013, 3:29 pm


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I am working on some casing designs to be an overmold for electronics and am wondering what process, or setup is taken to account for plastic injection shrink, as to make for a nice fit. This project is specific to solidworks modeling, although my assumption is that the approach would be synonymous.

FYI - I know shrink rates in/in for my plastics.

Thanks,
Rob
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Re: Modeling for shrink

Postby Travisimo » February 1st, 2013, 6:40 am

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Depending on whose molding the part, you can add tolerances on a drawing and they can 'tune' a mold to meet your requirements (temp/pressure/mold changes/etc..)

You can also design a part to be 'tool-safe' ... so you can remove material from the mold in key areas to adjust the fit, as it's harder to add material to a mold once it's been removed.

Re: Modeling for shrink

Postby mo-i » February 1st, 2013, 7:13 am

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Did you write what you mean, there, Travis? Or did you turn part of the prosecs arround?

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Re: Modeling for shrink

Postby nxakt » February 1st, 2013, 7:23 am

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MoldFlow software can help with the prediction. In my experience for extremely critical parts with odd sections and plastic with uneven shrink, sometimes a test mold was built first, sometimes not intentionally.

Re: Modeling for shrink

Postby Travisimo » February 1st, 2013, 8:09 am

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mo-i wrote:Did you write what you mean, there, Travis? Or did you turn part of the prosecs arround?

mo-i


I think that came out right - does it seem off? :(

Re: Modeling for shrink

Postby mo-i » February 1st, 2013, 8:41 am

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Well, for me the first part of your second sentence was misunderstandable:
You can also design a part to be 'tool-safe' ... so you can remove material from the mold in key areas to adjust the fit, as it's harder to add material to a mold once it's been removed.


What you mean by "tool-safe" is probably to have the material thickness =size of the CAD Model of the molded product on the minimum size for the first go at the tooling, to allow for further honing of the mold to optimize flow. As it is easier to
increase the size of a cavity slightly than viece versa. (You said that in the second part of that sentence.)

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Re: Modeling for shrink

Postby Travisimo » February 1st, 2013, 9:50 am

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you did a better job of explaining that ;-)... it took me a couple go's to get my poor description together in the first place (and it never actually got there!)

Re: Modeling for shrink

Postby iab » February 1st, 2013, 9:51 am


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I am not a tooling designer so I submit drawings with critical dims. It is up to the tooling designer to determine shrinkage (which I hear is quite significant when coming out of a pool) to hit the critical dims. I don't worry them doing their job. And of course, shrinkage is material specific.

Re: Modeling for shrink

Postby engio » February 1st, 2013, 10:54 am


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iab wrote:I am not a tooling designer so I submit drawings with critical dims. It is up to the tooling designer to determine shrinkage (which I hear is quite significant when coming out of a pool) to hit the critical dims. I don't worry them doing their job. And of course, shrinkage is material specific.


+1. Draw what you want to receive, specify material you want to use, let your tool maker do the rest.
Pro tip: add control dimensions the way you (or QC in the central warehouse) can easily and accurately control them with callipers, for example NOT edge to to the center of a hole but rather edge to edge.

Re: Modeling for shrink

Postby Cyberdemon » February 2nd, 2013, 1:48 am

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Yeah if you're going to any tooling house that can't give you a mold flow/DFM report when you go to tooling I'd be wary.

The tooling engineers should be able to take care of all of that, the critical dimensioning is a really just the way of covering your bases when you get parts back that are off and the tooling vendor refuses to fix it.


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