Slightly OT: Vacuum Forming - any Guru's out there?

alphawave wrote in news:1192211970.505603.128460 @e9g2000prf.googlegroups.com:

I'm not a guru, and I didn't design it, but I do have a clamshell like you've described on my desk. It's approximately 8"x10" overall.

The two corners that engage have a "lock" or snap feature on both edges extending from the corners. So, there are four snap features on the clamshell.

Each of these features is .15" tall and .4" long, with .04" of engagement. Obviously they're holding a tighter tolerance than ±0.5mm on a small local feature like a snap.

MHill

Yes,

That one sounds similar to the one I have, and as you say the tolerances on those features look very tight.

Could it be they make them an interference fit and just rely on the compliance of the material to sort out any clashes

anyone in the know?

Kev

If I wanted some good data, I would go to the contract packaging guys who do this work.

The magazine has a list of suppliers and articles in their "Pharmaceutical & Medical Packaging News" magazine.

Bo

Kev,

I am in the thermforming industry. This probably won't help a lot, but the short answer is 'it depends'. The thicker the starting gage of material, the wider the tolerances. The bigger the part, the bigger the tolerances. And last of all, tighter tolerances can be held on the side of the part that touches the tool as opposed to the side that doesn't. We generally hold =B11/4" or larger - but, of course, our parts can exceed 8 feet in length.

Cheers Ed, The clamshell I'm looking at will be about 200mm long x 80mm wide x

25mm deep (max). The material will be 0.5mm thick. The sort of feature I'm looking at is the reverse taper sort of thing (dovetail) so that when the 2 halves of the clam shell are put together the reverse tapers lock together hence no welding is required. The clam shell I have seen appears to be a good tight fit - much closer than +/- 0.5 mm. I was wondering how this was achieved.

Regards,

Kev

If you want to see disposable clamshells with tight tolerances that wok, go to the supermarket in the prepackaged veggies and fruit.

I just opened the raspberries this morning and boy was that interference snap lock undercut tight. The Strawberry box is about

7-8" long x 4" wide x 2 inches deep and also has some good undercut snaps.

Bo

I know they work, I am interested in what tolerances they are manufactured to, if they are a deliberate interference fit and if so what the interference is.

Kev

When I've done these sort of "interference" joints in injection molded parts, I've gone out and bought similar parts, cut them apart and put them up on the comparator and measured them and then modeled them so I could analyze them better, and figure out how they were tooled and ejected.

I didn't find any company that was going to just blurt out their technical secrets of how they achieved their results, though there are clues in the photos in ads, articles, and packaging conventions.

The MD&M shows around the country each year are a good source of info and samples.

The show starts in a couple days, and registration is a nominal fee or free.

Exposition: October 22-23, 2008 Conference: October 21-23, 2008

Minneapolis Convention Center Minneapolis, Minnesota

I do not expect anyone to spill tech secrets - I just maybe thought that someone may have a link to "The boys book of vacuum forming" or some online resource that could help.

Kev.

I suspect that there might be a book on vacuum forming, but I haven't had reason to look for one.

My best bet would still be the people at the packaging area of the Minneapolis MD&M show starting today.

Given the dozens of vendors in one spot, it becomes very cost effective way to get the scoop on vacuum forming tolerances & undercuts.

Bo

Yeah, I agree - shame I'm in the UK though!

Kev