Vacuum Form tolerances 1

[LarryHup]

I'm developing a tolerance block for various commodities for my company. I've researched and met with suppliers to review our final format tolerance blocks.
In regards to Vacuum forming, does anyone have an angle tolerance recommendation? Machining, sheet metal and compression molding are pretty well defined and available.
I also have general tolerances for the standard angles for draft.
For example 3-5 degrees, 5 degrees whenever possible. But the tolerance for those angles are not defined.
Any input would be appreciated.

 

[MikeHalloran]

It's a waste of your time.

Vacuum forming vendors will readily promise to meet any tolerance you care to specify.

They won't deliver.

When you are desperate for the parts, you will adjust the dimensions and/or tolerances so as to accept whatever crap is submitted.


Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

Apart from what Mike says, which I certainly do not dissagree with, keep any spec only a little within the levels which are necessary to avoid problems. Over specifying just because it seems like it is better workmanship or whatever can be costly.

Regards
Pat
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[MikeHalloran]

I will try be a little more positive herein.

Keep in mind that vacuum formers typically use plywood tooling, and consider a router with a mechanical template to be a high tech tool.

I.e., they will be measuring everything with nothing more sophisticated than a tape rule. They might even be able to find a plastic protractor on site if pressed. That's the extent of their metrology, so adjust your expectations and your tolerances accordingly.

Which doesn't mean that you can't get a usable part from the process. You just have to accommodate quite a lot of imprecision and nonrepeatability.

If you study successful applications of vacuum forming, you will notice a lot of lap joints and hidden edges, self-drilling screws, and similar design features that don't require precise location or pre-location of anything.



Mike Halloran
Pembroke Pines, FL, USA

 

[LarryHup]

Mike and Pat,

thanks for your replies.
I'e worked with quite a few vacuum form molders and I understand your frustration with the process. I too have had my share.
But the process meets a need in some cases and we do go foward with designs that are sent to approved vendors.
I am just trying to come up with general tolerances that we can add to a title block and tolerance block. Knowing that we will need to work closely with the manufacturer to agree on certain tolerances that they are having a hard time keeping.
Do you have a format with mold tolerances, trim tolerances and angle tolerances called out?

 

[MikeHalloran]

No, I don't have a format.

Here's an idea: Ask your VF vendors to tell you, in writing, what tolerances they consider standard, and will agree to meet without extra charges. A simple email or fax response would do.

Maybe that will at least embarrass them into taking some responsibility for their work.


Mike Halloran
Pembroke Pines, FL, USA

 

[LarryHup]

Mike,

I'm not wasting my time. I have an assignment that "will" be completed. I will put a tolerance in for the angles. It may be changed later to meet the vendors capabilities, but it will be a general specification that will lead to the discussions with the vacuum form company. Do you have input on my question?
I'm requesting information from knowledgable sources within the trade for good, positive feedback.
Thanks again.

 

[HDS]

It all depends on the process, tool and material. There are design guides with some general information.

Mike likes to trash vacuum formers but you get what you pay for just like any other vendor. The process is capable of making precision parts and there are many vendors that do it well.

 

[MikeHalloran]

I don't like to trash anyone.

I have, however, had the misfortune of working for managers who insisted on always taking the lowest bid despite reasoned protestations and documented contraindications.

They, and hence we, therefore fell victim to vendors who took up vacuum forming after being run out of some other low-entry cost business.

My experience is certainly not representative of the best that vacuum forming can do, but it is very much representative of getting what you pay for.



Mike Halloran
Pembroke Pines, FL, USA

 

[ornerynorsk]

LOL, Mike uses the same vacuum former we do!

Seriously, vacuum forming is a very repeatable process when done well with a WELL DESIGNED PART! Your shrinkage and tolerance ranges are going to far exceed those of injection molding, but your best bet, as previously mentioned, is to get your molder to commit to realistic tolerances, and don't overspecify unnecessarily, as Pat mentioned.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.