Rigid PVC sheet wavy edges 1

[VincentB]

Hi everyone,

I'm new to this forum and hopefully it will help me to find some answers.
We try to produce PVC sheets for wall covering but we have an issue of wavy edges.
When the sheets come out the line and are stacked they are perfectly flat but after hours and sometimes days (it seems to depend of the ambient temperature), waves appear on the edges of the sheets. Sometimes more on one left side of the sheet but others, the same on both sides.
Do you have an idea of what could cause this?
I've posted a link for you to see a picture and have an idea of what I'm talking about.

Thanks for your answers.

Vincent

 

[Compositepro]

This can happen due to exposed edges absorbing moisture over time and expanding or due to stretching of the material at the edges during the slitting/cutting process. Initially, after slitting the edge material is restrained from moving by the adjacent material but creep caused by stress relaxation will result in wavy edges because the edges are now slightly longer than the rest of the sheet.

 

[berkshire]

Essentially a sheet that has more material in the center will have a bow or a dome , A sheet that has more material at the edges will have waves.
B.E.

You are judged not by what you know, but by what you can do.

 

[VincentB]

Compositepro,
We tried not to cut the edges online. we left the sheets with the edges to cool down but the waves appeared anyway...
But we did not consider the moisture factor.
Thanks for your answer

 

[VincentB]

Berkshire,
We have a device to control thickness across the sheet. I'm not saying that they are perfectly constant but I would say that thickness vary between 1.75mm and 1.9mm.
and the edges often are thiner than the center.
Thanks for your answer

 

[Compositepro]

uneven cooling can also cause such a problem. The edges cool faster than the rest of the sheet and take a set while the rest of the sheet is still cooling and shrinking.

 

[berkshire]

Vincent,
I am not talking thickness, I am talking linear distance. The same problem occurs when metal sheets are formed.
The makers of metal sheets get around this by stretching the sheet by differential roller speeds to even out the tensions across the sheet. Whether you can do this with PVC sheets I do not know.
B.E.

You are judged not by what you know, but by what you can do.