Machining Acrylic and Residual Stresses

[jdm2008]

I am machining a tube of acrylic. I am noticing that afterwords when putting the acrylic into contact with solvents(Alcohol in particular), is cracking at a very rapid rate. However when I place the stock material in contact with this same solvent it does not crack. What can I vary within the machining process to lessen the residual stress that is causing it to crack much more rapidly when exposed to alcohol.
Just use a sharper tool? I know I can switch to cast, but since this is a production issue that will be very expensive. Also a material change isn't an option for an unrelated reason. The

 

[MikeHalloran]

Use razor sharp HSS tools that have never been used on metal.
They should draw blood when you touch the edges.
Use kerosene for coolant, or at least use a different coolant than what you're using now.



Mike Halloran
Pembroke Pines, FL, USA

 

[TVP]

Sharper tools will help. Stress relieving (annealing) after machining is also a possibility. I don't have any info immediately available on proper temperature for acrylic, but Google would probably find something.

 

[mfgenggear]

I read some where that alcohol will induce cracking of glass that is stressed. embrittlement? Do not use alcohol to clean if possible.
surely there is some thermal treatment for stress relieving even if done with a heat gun.

I also agree with having razer sharp tools.
I believe there should be no coolant, You may want to do a test with no coolant. I use too get involved with screw machining plastic, if I remember right no coolant was used. look below for the recommended cutting fluids.

the machining data handbook list the following feeds & speeds

acrylic cast,or extruded. single point & box tools

depth of cut speed(FPM) feed carbide tool
.040 600 .005 c-2
.150 550 .010 c-2
.300 500 .012 c-2

recommended cutting fluids
0~dry
4~Emulsifiable oils
6~chemicals & synthetics

Mfgenggear

 

[Eltron]

I've noticed that I can avoid most crazing in acrylic if I keep the temperature of the stock material down during machining. Sharp tools, coolant (I use mineral oil), and the proper speeds will all help. Alcohol (IPA) will cause crazing as well. Part of the reason I use mineral oil as a coolant is that I can easily clean up parts with soap and water.

If you are running alcohol through the acrylic as part of your process then one thing I've found that helps is taking smaller bites out of the material. For example, if you are drilling a 1" hole through a thick acrylic plate make it in multiple steps rather than just hogging it out with a 1" drill bit. Those finish cuts makes a big difference.

Dan

http://www.eltronresearch.com

Dan's Blog

 

[tomwalz]

Might try a special plastics, cutting blade.

Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

 

[saberblue]

From a coolant point of view any thing with polar surfactants, oil, or chlorine (any halogen) can introduce stress into acrylics (they can will deplete the plasticizers in the part).

A.R. "Andy" Nelson
Engineering Consultant
anelson@arnengineering.com

 

[chicopee]

Contact with solvents can possibly permanently "fog" the acrylic tube.