Anyone having serious bowing issues with plastics?

[Dan Hintz]

The shearing process only introduces localized stress (<1-2"), so that won't warp an entire sheet. Unfortunately, this doesn't disprove a manufacturing issue... since yours came directly from the factory, there is no appreciable shelf-time to allow warping to happen, which may take weeks/months. If you can afford it, leave one of the new sheets sitting on the shelf for a couple of months and see if it warps... still not definitive, but one more data point.

[Scott Shepherd]

I'd bet money Mike is correct on this. I'm surprised coextruding 2 dissimilar materials ever comes out flat.

Sorry to burst your theory, but this material is ADA material (the material that came in from Rowmark) and it's a solid color. No lamination in this process.

It's 1/8" solid color all the way through.

[Scott Shepherd]

Here's the latest on this. They provided full sheets, 24" x 48" instead of quarter sheets. I took one out of the plastic it was shipped in (actually wrapped in plastic), and sat it on the floor. It curled as well. So it came time to use it and I put it in the laser to cut it into 12" x 24" pieces (who needs a stinkin' saw?), and I cut them.

What I noticed was the way I cut them was against the "grain" sticker that's on the protective film. So the box was the opposite direction. It meant the bow was over the 24" section, and it wasn't bad. Gravity and the vacuum table held it down fine, removing the bow. However, on the pieces cut from Johnson, they are cut with the "grain" sticker. This makes the bow run across the 12" side, and it's not enough weight, plus the bow is along the 24" width, so it's longer, making the bow stronger.

Attached is an image showing the way they cut it at Johnson and they way I cut it. They way I cut it makes it work, they way they cut it screws it up. On this piece, it had no grain, so it wouldn't matter, but if grain mattered, then it would be a problem to some degree.

[Dan Hintz]

Iiiiiiinteresting. I'm assuming this is extruded, right, due to the directional grain? Looks like it is having the same issues extruded acrylic sheets have... extruding polymer chains will always have issues that require special post-processing (or proper initial processing). Maybe their roller locations need a little tweaking to pre-stress in a slightly different location.

[Scott Shepherd]

But it's dead flat when it comes out of their factory. The sheets I have remaining in the plastic are still dead flat.

It's only when exposed to the air and humidity all around (it's got 2 of those drier packs in the bag they came in), they seem to have the edges curl.

I took a piece and removed the protective liner and it's sitting on the table, dead flat. It's stayed flat since I unwrapped it.

I do believe this is a humidity issue and the fact that one side is protected, one is not, which allows for uneven exposure. In this case, I believe Rowmark when they say it's the humidity causing it.

This product doesn't have any grain in it, it's a solid color, ADA material, but the plastic protective sheet on it has "grain this direction" so they can use it on any material they make.

[Dan Hintz]

I guess I didn't understand what you were saying. How long was the sheet out in the air before you saw a noticeable curl?

[Scott Shepherd]

I didn't pay too much attention to the time, but I can say it was 24 hours or so, maybe a little less.

[Dan Hintz]

But it's dead flat when it comes out of their factory. The sheets I have remaining in the plastic are still dead flat.

It's only when exposed to the air and humidity all around (it's got 2 of those drier packs in the bag they came in), they seem to have the edges curl.

I took a piece and removed the protective liner and it's sitting on the table, dead flat. It's stayed flat since I unwrapped it.

Steve,

I have a few questions in to some guys I really trust to know what's what (long-time, hands-on engineers in the plastics manufacturing field), but I wanted to clarify your earlier statements.

You said the full sheets remain dead flat as long as they are kept in the poly bags (with dessicant) they were shipped in.

Later you said you removed the protective liner from a sheet and it's sitting on the table dead flat... is this immediately after removing the liner, or is this after it has been sitting in open air for 24 hours? I'm guessing from earlier statements it's the former, but I wanted to clarify.

So far, one guy believes it's not a humidity issue, but a residual stress issue (something I mentioned here yesterday morning, so we're both thinking along the same lines).

It's a bit unclear, but I believe you are saying the bowing is worst perpendicular to the grain direction (i.e., along the 4' side of a 4'x8' sheet), correct?