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internal stress in sheets
Last Post 25 Nov 2008 01:38 PM by Lenny E. 13 Replies.
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Johnny C
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Andy
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| 21 Nov 2008 02:33 PM |
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Hey Johnny,
We used to cut the strips for a manufacturer on a panel saw. The two edge pieces would bow out in the middle as much as a 1/4" sometimes. Didn't happen on all the material, just select sheets and colors.
I haven't seen that much in a long time and we cut on a cnc.
Long time ago we had a Fountainhead counter snap in half as we were cutting out the sink opening.
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politefab
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| 21 Nov 2008 07:56 PM |
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i have seen similar stuff when i would take a sheet out of the storage area and it was cold as it warms up and expands it can do some funny stuff. i dont like cutting out sheets till they are about room temperature things seam to grow as they warm up not much but if its between walls it may be enough to cause a problem |
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Kelsey Crisp
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| 22 Nov 2008 07:29 AM |
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Johnny C.... do you check for cupped sheets? I cut a sheet that had a bow in it.. the 30" way... and as I was sawing it, the 4" rip on the panel saw was about 2" in the air after about 7-8 ft of rip. The other rips were good... really wild stuff we get. |
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The SS Tech Guy
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| 22 Nov 2008 09:00 AM |
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Lenny, please join in and correct me...
I had been told long ago that the cupping, curling, growing, etc issues were caused by incorrect curing process for the sheet allowing stresses in the sheet unevenly distributed. As a "full sheet", the stress was balanced. When cut, the stress which could have been concentrated in one section of the sheet, would no longer be balanced in the remaining now seperate pieces.
This does not solve the issue, I am just tryting to figure out if either my memory is correct or somebody was feeding me a line of ... 
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| Bill W.
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Lenny E
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| 22 Nov 2008 06:01 PM |
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Hi Bill,
Yes you are correct, the cupping or bowing growing, etc. when cutting is a result of uneven cure which can occur from many many sources- uneven mixing of componets especially catalyst and promoter, settling of filler making the top of the sheet resin rich, uneven heating, uneven cooling etc. etc.
If everything is done correctly bowing should be minimal when cut. Ive seen sheets do strange things when cut, Ive even seen them bow on 1 end, and cup on the other ?????
Bowing/cupping/growing, shortening, etc, - Its definitely a manufacturing defect.
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Lenny E
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| 22 Nov 2008 06:19 PM |
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Oh one more thing Bill, I almost forgot.
In sheet form the stress is locked into the sheet. When the sheet is cut the cut or free edge is able to move to relieve the stress, resulting in bowing and cupping as the force of that movement is transferred back through the sheet.
I used to test residual stress in sheets by a variety of methods. Sometimes the stress (in psi) was half the strength of the Sheet! Wowie Zowie!!!!! (thousands of pounds per square inch).
Now some fabricators press the warp down, and the proceed to seam. This isn’t the best idea, because that sheet is always pulling against the seam with as much as half its strength. So that seam is more prone to failure.
What makes the situation bad, is most manufacturers say, if you cut the sheet, you own it. But please show this stuff to your manufacturers, because it is a manufacturing defect and totally unacceptable to a fabricator or the end user (customer).
Heres a trick, cut the sheet larger than you need (if its a splash and you need 4 inches wide or high, cut it 5 or six inches wide), then when it bows, recut it to size. It wont bow as much the 2nd time. This is an ugly solution, that results in material waste, but better than nothing.
Make sure you take pix and send to your manufacturer. |
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Mike Gladstone
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| 22 Nov 2008 07:27 PM |
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Mike G
GCI Solid Surface Countertops |
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Lenny E
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| 22 Nov 2008 08:32 PM |
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Yeah, I know what you mean, manufacturers tend to telll that lie (cut once and its all gone..heh heh). If the second cut, is a few inches beyond the first cut, stress is released again introducing warp, cupping etc.
Ive seen cut edges (the strips) bow like a bananna. If cut right behind the first cut its much less, but cut further back, heh heh, its bow city again.
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Tom M
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| 23 Nov 2008 01:27 AM |
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The first time we ever tried thermoforming was dadoing the material at the correct spot to try to cove a splash, like you would do with laminate. It was 3/4" thick material, and in the 70's, when the stress stuff was heavily influencing the sheet characteristics.
It actually bent quite well, after we heated the crap out of it.
We put it on the horizontal panel saw to cut it, and when it reached the dado/bend, it cracked for several feet till the end and the splash simply fell off.
Scared the crap out of us when it went. |
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Kowboy
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| 23 Nov 2008 04:53 PM |
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I did a commercial Corian sill job about a decade ago and purchased all the sills pre-ripped. Every one had a huge hook in it over the twelve feet, a quarter inch minimum.
I thought they had their saw fence out of paralles to the blade, but I'll bet it was stressed material.
The overhang wasn't critical and I got a nice fat credit from my distributor for planing each one to fit on site.
Joe |
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Andy
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| 24 Nov 2008 06:36 PM |
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Funny you mention Window sills Joe. We did some sills for a child care place and made temps for each one. Fabricated them perfect and when we got to the job they were all bowed about 1/4". I had to refit every one of them. |
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The SS Tech Guy
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| 25 Nov 2008 08:16 AM |
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Lenny,
Another "legend" from the past...
"You can take the parts after they are cut and thermoform them back straight again and everything will be fine."
Any truth to that one? |
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| Bill W.
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Lenny E
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| 25 Nov 2008 01:38 PM |
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Actually there is some merit to that. Ive annealed warped sheets back to near flat in a heated press by placing them in there on a dwell cycle. They seem to stay pretty flat after cutting also.
The problem is you "age" the material (it thermally degrades a bit, slight color change etc.). And how many people have a large heated double platen press, or if they do, have the time and money (for power) to waste annealling a sheet that should already be flat and stress free? |
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