I am thermoforming a shelving unit out of 1/2inch material that is closed on 4 sides, with a one inch radius on all inside corners. I have created an MDF positive to form over with registration and I am having a problem with the material as it is closed on 3 sides of the mold. As it shrinks, it causes the corners to fracture not due to uneven heating but because the material has no where to go. I can not do an open-face mold as this will then cause an issue with keeping all four surfaces square and true. Do I have to now invent an incredible shrinking mold to compensate for the shrinkage?? Or do I have to contact NASA? I have this job where the deadline is creeping on me and I am going through a great expense of material and R@D. Any ideas and help would be most appreciated.
-Richard

Richard,
Per DuPont specifications, 1/2" thick material has a thermoforming bending dadius of 3". That may be some of your problem. Can you post a picture or drawing of what shape you a trying for?

Wow... 1" is tight.  I have had sucess with a narrow piece, but even then the bend created distortion.

What is the width of the unit?  I would have to agree with Bill, sounds like you are trying to do something outside the range of what has proven to work.

A drawing would be helpful, especially if Jon with Sterling surfaces takes a look at this thread. 

Lenny might know..but he will have to put down his Chinese Viper Venom Cocktail... to type.. :)

Richard:

Welcome to the fabnet. I'd like to see drawing of what it is you're trying to build.
At least a more detailed explanation.

Joe

I know I am pushing the boundaries here. I have been able to achieve a 1" radius with the 1/2" material using a strong block form and my vacuum press without any cracking or ripping, I have achieved this with no problems when I am bending over 2 surfaces. ie. a inverted backsplash, but my reall delema now is that I am trying to fold over the top / front and sides of the form . the shelving unit is 8" high/5' wide and 7" deep. the folds over each end of the form ,end up comprimised due to the shrinkage over the width of the material.So if you wher to imagen a box and I am folding over that box on 3 sides , when the material shrinks it has no wher to go but into the shape of the form wich has no give considering it is a solid form. If I wher to x the vacuum press and form into a negative i could have better results because the material can contract inside the form but this does not give me good results concerning the nice 1" radius I get from the vacuum press. I am almost thinking of putting springs placed within the mold to allow this contraction but am a bit worried that that initial gap will jeporadize the squareness of the form and make impressions on the good side of the material. I am using Cameo white so I have not noticed any discoloration within the material once seamed.
Sorry no pics as I have not learned yet how to post pictures.

I will try and get some pictures for you guys

Thanks so much for all your input

regards

Richard

Richard, any chance forming this in 2 pieces and putting them together?

Good point but I am really trying to get rid of the seaming aspect, I love a challenge but I suppose I have to wiegh the costs in figuring this one out.

I am going to try making a jig with the 10 mm shrinkage in length difference that I could perhaps take the formed piece out of the vacuum press befor completely cool and place within, A bit trickyconsidering these pieces end up haveing to line up exactly to a cabinet above that is 1500 mm and the client is very particular about seams and precision.

Okay, here are some pics with regards to my shrinkage issues.
Here is the form I am bending over in MDF.





This is the first test piece and what it should look like:






But the ends have sheared off due to shrinkage over the MDF form.

Posted By Kelsey Crisp on 04 Sep 2010 08:53 AM
Wow... 1" is tight.  I have had sucess with a narrow piece, but even then the bend created distortion.

What is the width of the unit?  I would have to agree with Bill, sounds like you are trying to do something outside the range of what has proven to work.

A drawing would be helpful, especially if Jon with Sterling surfaces takes a look at this thread. 

Lenny might know..but he will have to put down his Chinese Viper Venom Cocktail... to type.. :)

Richard,
 
Jon is the thermoforming guru here. I have done some thermoforming, but only to confirm the material limits after a formulation/process change, or when developing a new product.

From a theoretical standpoint this material expands quite a bit as it heats up (especially as it nears the glass transition temperature of the polymer). When it cools the material is trying to shrink against an impediment (mold). It cracked in the region of the inside radius exactly where one would expect it to crack.

Your idea of transferring the piece from the vacuum mold to a smaller mold while it cools has merit. Thanks for the photos; they were very helpful in seeing what was going on.

Kelsey,
 
FYI I’ve been in China for a week now, and haven’t touched a drop of snake wine. Thanks for reminding me, I will remedy that oversight later. I still have some in the cupboard.

BTW, I’m down to a gallon or two, of snake wine and I need to make some more. So I guess I’m off to the snake market for some more cobras. It has to sit for year or two before drinking to allow the venom to undergo complete re arrangement of the tertiary and super secondary structure (denaturation) in the presence of ethanol .

Venom chemistry is quite amazing and something that I find fascinating. Venom is a very complex mixture of proteins, some hemo-toxic and cytotoxic  (attacks blood and tissue)  some neurotoxic protieins  (attacks the nervous system mostly by blocking acetylcholine receptors) and some with multi valent metal ions surrounded by a complex maze of organics .

Generally snake venom isnt toxic if ingested, however if you had a cut inside your mouth, or a bleeding ulcer and were ingesting a very toxic  snake venom the outlook may not be good. One has to keep that in mind, because if you don’t let it sit long enough (to denature) and drink it too soon, the nectar of the Gods can be quite lethal, under those circumstances instead of the fine tonic it was meant to be.

Richard:

It seems to me like you are really over-thinking and over-working this. I can understand thermoforming the long edge, but you'll spend more time fitting the corners back in than you would if you would have stacked and shaped them.

You could pass the ends over a tablesaw, cutting several 3/8" deep kerfs. I'll bet these would bend without breaking easily. After cooling, fill the kerfs with adhesive.

I would also suggest sprinkling some of Lenny's Cobra Juice on your forms and ingest a fair quantity yourself. It may not make the thermorforming any better or easier, but you won't care anyway.

Good luck,

Joe

I'd be careful about the kerf cuts, as I think it might give a stress riser (or similar) point along each ridge. You also would have temperature absorption differences.

It might be better to dado out the whole area, beyond where the radius would start and stop. You should also anaeal (sp?) the sheets after you dado. This allows the material to heat, settle, and should remove some of the natural stresses inherent in the sheet.

This assumes the outside does not to be consistent.

Under the present circumstances, we would likely use Joe's method of stacking and shaping.

Oh! One other thing, are you using a reverse mold to cover it as it cools? We found that evening out the cooling rate as much as possible helped a lot, not only with warping, but with new stresses that can get formed into the material.

The uninsulated outside reacted worse than an insulated outside.

Determine the shrinkage and make another mold. After it cools for a couple minutes, move to the new mold. I don't believe that it will loose its shape.