[Len Smith]

We have been experiencing some cupping, or warping of sheet stock from one of the major suppliers of solid surface sheet.  Our shops are telling us that the bottom sheet or two usually seem to be worse.  I can’t figure out a good reason for that…it would seem that if anything, the weight of the top sheets would flatten the bottom sheets.

I’m starting to suspect stress in the sheets themselves, since they do seem to cup very slightly after being cut on a CNC (and prior to installation.)

Anyone else having cupping/warping issues?

I should probably add that we’re also seeing color shift in sequential sheets.

[Ken Dolph]

If the pallet is not solid, the bottom material may take the shape of the loaded pallet. If the material is acrylic based, there may be temperature fluctuation to take into account. Acrylic never stops realigning itself. If a sheet is raised in temperature and returned to the original temperature it is not the original size. The result is usually minimal and the “give” of the material usually is sufficient.

The magnitude of this effect is not the same along the length as it is along the width. I learned this on a project with the CIA where the tolerances were very tight. The heating caused by the routing of the pieces was enough to cup them. When heating the piece to flatten them the dimensions of the route changed. the pieces were longer in one direction and shorter in another.

I later experienced this in some fairly large plaques that we shipped air freight. The plaques were half inch with a one inch border. When they arrived the boxes were fine but the plaques looked like they were torn paper. The edges of the plaques were made from strips cut in the same orientation then laid up at 90 degrees angles. Once I realized what had happened, a minor change solved the problem.

I know that this is more information than anybody wants but maybe it will help.

[Len Smith]

Definitely not too much information!

We got a shipment of material one time that had stress built in to the sheet. The material was a major manufacturer, not a second tier. When we’d cut one inch off the edge for build up, the 1″ piece would curl like a banana. Very odd.

[Ken Dolph]

If that stress was only on the edge cuts it was probably induced during the cutting process at the plant.

DuPont and other companies use water cooled diamond saws to cut about 3/4″ off each side of the raw material. If the water stops for any reason the cut gets very hot. This will anneal the edge causing a shrinkage in the first 1/4″ to 1/2″ however the bulk of the sheet will not allow the movement until it is removed from the sheet. The annealing of the edges usually shows up as a slight rounding of the edge.

[Andy Graves]

I just cut seven sheets of Corian Rice Paper and it was the worst material I have used in years. The material was so bad I had to cut the seam edge of the apron on an angle to get it to fit tight to the deck.

The only reason we used it was because they were out of stock and the job had a deadline.

I have been dealing with this from Corian for some time but not this bad. Not sure the issue but what is the solution?

[Ken Dolph]

Andy,

Do you have any pictures?  What was the speed and feed?  I ask because I don’t see this often and with the tolerances on some of my products I should notice it more.  I would hate to see this happening to me in the future.

[Lenny E]

Hi Andy,

That sure sounds like residual stress to me. That’s stress built into a sheet. When you cut the sheet, it releases the previously “locked in” and the sheet is free to move  (twist, bend, cup) along the newly cut edge.

You can actually measure the stress using a strain gage (3 stacked gages in an array), a drill (to relieve the stress) and a wheatstone bridge (to measure the stress by rebalancing the circuit after the stress is relieved). I’ve done it before. I have come across sheets that had stress levels 1/2 of the ultimate material strength!!!!

Athough all material has some residual stress,  with Solid Surface its a manufacturing defect. If the sheet is manufactured correctly the residual stress will be so small its imperceptible.

I have seen it occur in both acrylic and polyblend, on continuous lines and in molded products.

[Andy Graves]

No, I don’t have pictures of these sheets. I have posted pics before of other Corian material.

I notice immediately when we load the CNC. The material just doesn’t lay flat. There are times when I have to flip the material face down to get it to work.

[Author]

Posts