relaxing 5/16" thick sheet EPDM

[mecoman]

Hello,

We machine EPDM (55 durometer) in sheet form. It arrives in 9-12 foot long x 3ft wide coiled bundle. Once received the material is uncoiled to relax.

Once machined into strips 1/4x1x30" long+ lengths we store on shelves and wrap in coils and bag. We have found the coiling to be a bad idea.

We have discovered that they never lay flat. That is a curl exist for several inches on each end. The curl may rise anywhere from 1" to 6" off the flat surface. This then effects the length when wrapping around a cylinder. That is, it is not consistent between identical cut lengths to meet when wrapping around a cylinder. The circumference of the wrapped strip varies as the curl varies. It reminds me of the Joule effect, but no heat is applied.

Is there any advice as to helping induce the material to lie flat? Also, any technical explanation as to what may be happening? I am unsure of the temperature of material before it is wrapped in a bundle from our supplier.

We have tried heating to 200F for 1-1/2 hours, with little to no difference in curl. Considering increasing the temperature.

Thanks for any advice and let me know if you need additional information.

Cheers,
MECOMAN

 

[tom1953]

Is the material, as you receive it from the manufacturer in coiled bundles, totally flat? If you just cut that material into a 30" long length, would it lay flat? If it does, then there's something in your process that's putting a set into the rubber. If it doesn't lay flat as received, the problem lies with the material as manufactured.

The set may be a result of a less-than-optimally cured compound, with the material forming additional crosslinks while in the coiled state. If that's the case, the manufacturer may be able to give the compound a more complete cure (i.e., a longer cure, typically) so it will be less prone to keep the permanent set while in the coiled position. The compound's formulation may be able to be improved to give it better compression/tension set resistance, if that's an option, but that's something your supplier would have to do.

Hope this helps.

With the material you have right now that's curling on the ends, have you tried curling it the opposite way and giving it some heat?

Tom Jablonowski, TSE Industries, Inc.

http://www.tse-industries.com

 

[mkkessel]

Rubber will generally take a compression set in the same direction as it was calendared or rotary cured. If fabricated by cutting the 30" lengths across the width of the 36" roll stock, it may improve the tendency to set in that direction. (Although it may not produce the best yield, you may try this to check for improvement.)

Some manufacturers press cure rather than rotary cure which may improve the tendency to curl. This may be negated by storing and shipping in roll form where the material may pick up a slight memory set.

Finally, EPDM is available in peroxide cure as well as sulfur cure. One may have better relaxation properties over the other.

 

[mecoman]

Thanks All,

We do use sulfur cured.

We have been reverse rolling and applying heat. Still trying to determine if it doesn't return to curling.It does return a little, need to store longer to find results. We find the length changes( shortens after heating), still testing to determine if returns to original length.

We also found in discussing with supply that it is cured in rolled state, cannot be good. Makes me wonder if the molecular chains are cured at different lengths.

Unfortunately we have excessive inventory. I do like the idea of slicing the opposite direction, though it results in the eye rolling.

Thanks again for all your input.

Enjoy the Tofu,

Cheers,
MECOMAN