Acrylics, humidity, and warping
Acrylics, humidity, and warping
(OP)
A number of fellow associates are having issues with warping of colored acrylics, and we'd like to narrow down the exact cause (and solution). It is believed humidity is the issue, but unfortunately the effect does not appear to be transient.
The sheets are 4'x8' in size, and I believe are mostly extruded (I cannot say if any of the sheets are cast, though it may be possible with some colors). They are often ordered from suppliers cut down to 12"x24", and even over that small size have a bow in them of up to 1/2" edge-to-edge. We have ordered full sheets directly from the manufacturer, they show up straight as an arrow, and remain that way while sitting on the shelf... within 24 hours of removing the protective plastic the damn things are warped beyond use.
My question(s):
1) If humidity is the problem, how can we prevent it from warping the sheet (tests have been run in areas with dehumidifiers)?
2) Why does the warp occur? This happens with solid color sheets (it was initially believed the two-color sheets warped because one color absorbed more moisture), so shouldn't any expansion/contraction happen at equal rates on both sides? I assume if it's extruded there is a grain to the polymer chains, but that would be a length-versus-width change, not up-versus-down.
3) How can we avoid the warping in the first place? At least one has tried baking it in a drying oven, and it appears to work to a small degree, but it's not perfect, and few of us have such large ovens to play with.
The sheets are 4'x8' in size, and I believe are mostly extruded (I cannot say if any of the sheets are cast, though it may be possible with some colors). They are often ordered from suppliers cut down to 12"x24", and even over that small size have a bow in them of up to 1/2" edge-to-edge. We have ordered full sheets directly from the manufacturer, they show up straight as an arrow, and remain that way while sitting on the shelf... within 24 hours of removing the protective plastic the damn things are warped beyond use.
My question(s):
1) If humidity is the problem, how can we prevent it from warping the sheet (tests have been run in areas with dehumidifiers)?
2) Why does the warp occur? This happens with solid color sheets (it was initially believed the two-color sheets warped because one color absorbed more moisture), so shouldn't any expansion/contraction happen at equal rates on both sides? I assume if it's extruded there is a grain to the polymer chains, but that would be a length-versus-width change, not up-versus-down.
3) How can we avoid the warping in the first place? At least one has tried baking it in a drying oven, and it appears to work to a small degree, but it's not perfect, and few of us have such large ovens to play with.
Dan - Owner
http://www.Hi-TecDesigns.com 






RE: Acrylics, humidity, and warping
It is quite possibly residual stress from the extrusion process being relieved. You need a better supplier.
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
RE: Acrylics, humidity, and warping
Is the protective sheeting providing enough of a tensile force against residual manufacturing stress to keep the sheets straight?
I'll go through the communications between those of us who use the materials again and see if I have missed any details I didn't post here...
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Acrylics, humidity, and warping
I very much doubt the protective coating has that sort of strength.
I can't explain the timing.
As far as I know, almost all sign quality acrylic sheet is now extruded or what they call continuous cast which is a fancy name for extruded and final stage polymerised in one go.
Acrylic sheet is extruded onto a set of rollers and squeezed between the rollers to control surface finish and thickness. If continuous cast the final stage of polymerisation occurs on the roller.
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
RE: Acrylics, humidity, and warping
The full sheets (4'x8') were shipped in poly bags with desiccant. The sheets still in the poly bags remain dead straight. Only one side of the sheet is covered with the protective film. The bow appears to be worse in the direction perpendicular to the grain, along the 4' edge (I'm awaiting verification on this, and I'm assuming the sheet is extruded in 4' widths placing the grain along the 8' direction).
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Acrylics, humidity, and warping
I agree with Pat and Pud, water and any constraining effect of the protective layer are almost certainly not the explanation.
Chris DeArmitt PhD FRSC CChem
www.phantomplastics.com
Consultant to the plastics industry
RE: Acrylics, humidity, and warping
Chris DeArmitt PhD FRSC CChem
www.phantomplastics.com
Consultant to the plastics industry
RE: Acrylics, humidity, and warping
You may have missed my last comment about the poly-bagged sheets remaining flat until they're removed and allowed to sit for a day. It's only a few sheets, and shipping is via UPS/FedEx, so however they choose to throw them on the truck.
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Acrylics, humidity, and warping
This is especially perplexing (Perspexing?) as I saw a TV show where they made hugely thick PMMA tunnels so people could walk in them under the water at e.g. Sea World. They were massive parts but close tolerances were achieved.
Chris DeArmitt PhD FRSC CChem
www.phantomplastics.com
Consultant to the plastics industry
RE: Acrylics, humidity, and warping
OK, a wag: Does the protective film contain any plasticisers? Just wondering if plasticiser migration into/out of the sheet is causing surface swelling and hence warping?
(I did say it was a wag!)
H
www.tynevalleyplastics.co.uk
RE: Acrylics, humidity, and warping
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Acrylics, humidity, and warping
Protective cover then was paper and pressure sensitive adhesive. After that they went mostly to PE film covers.
If they now use PVC film it might contain plasticisers that might migrate and swell the acrylic, although I would consider that a real long shot.
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
RE: Acrylics, humidity, and warping
It sounds like you have low volumes. You made need a better distributor that can solve your problem. There should be someone in most major US cities supplying the sign shops with flat sheet to print on.
Are the sheets stored horizontally? Don't store them vertically.
Plascolite used to sell in small volumes but they might have grown to big for that.
RE: Acrylics, humidity, and warping
The product was direct from the manufacturer, not just distributors.
Dan - Owner

http://www.Hi-TecDesigns.com