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jdm2008315 (Mechanical) (OP)
6 Apr 12 13:39
I have a tube of acylic(about 12" long) and 2.5" ID, and the tue machined in half from the top of the tube to about 4" the bottom of the tube(so that 4" from the bottom you have the full cylinder and then the rest up is a half a cylinder(there is ahandle machined and bent, but that is the basics).  The edges are then flame treated.
The problem I am having is that when the customer uses this tube he cleans it with alcohol and it is cracking.  I do know that alcohol breaks down acrylic and causes crazing and cracking.  
However when I take the stock tube and simply soak it in alchol(for hours) it does not crack.
Since this is the case, I am lead to believe that the processing of the acrylic(machining) is causing it to become more brittle and more susceptable to breakage with use in alcohol.  What can I change with this process?  I have used a torch with less heat to flame treat the edges, I have even stopped flame treating them altogether.  Is there anything else I can do?  Will I have any better luck with lasercutting the acrylic?
In this case changing the material is not an option, in addition it is not an option to tell the customer they must keep the product away from alcohol.  Am I between a rock and a hard place?
swall (Materials)
6 Apr 12 16:28
Your cutting operation has caused residual stresses. Some are tensile stresses and these, along with the alcohol exposure, are causing environmental stress cracking. Corrective action involves getting rid of the tensile stresses via a thermal stress relief or avoiding the alcohol exposure.
jdm2008 (Industrial)
9 Apr 12 13:35
Will switching from a manual mill to a waterjet cutter make any difference?
 
btrueblood (Mechanical)
9 Apr 12 14:02
Don't know about posting a question like this in a forum devoted to metallurgy.  But it's a bit late for changing it now...

Some questions - is your raw stock cast or extruded?  Extruded stock is likely to have a fair bit of residual stress just from the forming process, consider switching to cast material if you aren't using it already.  Have you examined the cut edges after machining, and do you see any signs of cracking then, or likewise after flame treating the cut edges?  Is there evidence of overheating during cutting (molten blobs of material at cut edges)?  Try using coolant and/or sharper tools if so.

Yes, avoid cleaning with alcohol, a simple detergent wash would be a better idea.  Flame treating, while it produces a nice polished edge, is uncontrolled enough that you risk scorching the plastic, and might itself be a source of damage leading to cracks.  Better to simply sand/polish to deburr the cut.
swall (Materials)
9 Apr 12 14:52
jdm2008315--where is the cracking occurring? Is it just along the cut edges or is it all over?
jdm2008 (Industrial)
9 Apr 12 15:08
To swall

All over

To Btruehood

I have tried cast and that works fine(some cracking but it retains it structual integrity).  However this application is in production, so switching to cast will be very expensive.  I've tried getting rid of the flame treatment or using a cooler gun but that doesn't have much effect.  Unfortunately the alcohol cleaning method is a customer issue and compliance requirement so I don't have much control over that.  
I will try sharper tools.   
Helpful Member!  moltenmetal (Chemical)
10 Apr 12 7:39
There's nothing you can do to prevent an incompatible solvent from causing damage to the parent material.

All you can do is to delay the inevitable.

 
mfgenggear (Aerospace)
10 Apr 12 13:00
I agree with moltenmetal

mfgenggear

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