machined PVC with groove = bending/warping problems

[Gerard-E]

Hi all,

We are trying to machine a 1inch thick, grey PVC part that is 8in wide by 36in. It is a simple rectangle with a few 1/4in thick grooves that go about 1/2deep and run all the length of the part.

When we received the PVC sheet, it was slightly bent and made a banana in the length. We thought that once machined, since we would be removing material the bending would go away, but it actually made things worse. We are using an Amana plastic O flute cutting bit.
The groove machining was done perpendicularly to the sheet`s extrusion dimension. I wonder if this is somehow connected to the problem

-> Can anyone give some tips on how to flatten this back down? Heat? machine back side?

-> Does anyone have a clue of why this might be happening? and how to avoid this in the future?

Thanks for your help.
Cheers

Gerry

 

[3DDave]

The problem is there are trapped internal stresses, likely caused by non-uniform cooling when the material was extruded. The outer surface cools first and locks in a size while the core remains hot and pliable. When the inner material finally cools it leaves the outer surface in compression and the inner material in tension. Remove some of compression material from the outside and the tension material will curl it up.

The trick is to anneal the material while holding it flat - being a low conductivity plastic this is going to take a while. Since it is PVC it will require some consideration for fume control. Have you considered bonding multiple thinner sections together to make the required thickness? While these would have a similar, though much small scale problem, the fact they are in layers would cause the average stress to be more uniform through the thickness - the way plywood evens out the problem of grain and water sensitivity does for wood.

 

[John Donlan]

Perhaps if you started with a greatly oversized piece of extruded stock, so that you would have the allowance of a lot of material to remove when the internal stress has started relieving themselves.

Possibly another alternative would be if you could obtain PVC that was cast (not extruded) it would be fewer internal stresses, and your chances that the work piece May remain flat after machining would be enhanced.