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Re: Plastics question



Original poster: "Scott Hanson" <huil888@xxxxxxxxxxxx>

D.C. -

If you are using kerosene as a cutting fluid, your "annealing" process may not really be annealing anything, just evaporating the residual film of kerosene. The longer any hydrocarbon fluid stays in contact with the plastic, the greater the probability of crazing & cracking. The more quickly the hydrocarbon is washed off, or evaporates, the less tendency to crack.

I just finished up a failure analysis project on a cylindrical acrylic rod used as a lens used in a robotic machine-vision system. The rod had a tapped hole in each end, perpendicular to the long axis. The parts were machined, cleaned, inspected, packaged and shipped to the user. Although they were perfect at the time they were shipped, by the time they arrived at their destination several days later they were badly cracked and crazed, but only around the tapped holes. It turns out that the manufacturer was using a hydrocarbon-based soluble cutting oil, and his cleaning process was unable to remove the microscopic film of oil trapped in the threads. The additional 24-48 hours of contact with the plastic during shipping was enough to initiate cracking throughout the threads. When the machine shop changed to a special synthetic-oil based cutting fluid, the cracking problem immediately disappeared.

Regards,
Scott Hanson

----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Tuesday, January 31, 2006 8:36 PM
Subject: Re: Plastics question


Original poster: "D.C. Cox" <resonance@xxxxxxxxxx>

We have had it crack on a long 10 inch x 1 inch acrylic rod we used on a makeshift x-ray xmfr bushing.

I anneal in the oven now and never have cracking problems. Don't anneal and cracks may (emphasis on may) form.

Dr. Resonance


Tesla list wrote:

Original poster: "Antonio Carlos M. de Queiroz" <acmdq@xxxxxxxxxx>

Tesla list wrote:

Original poster: "D.C. Cox" <resonance@xxxxxxxxxx>
We have used both black delrin and black (smoked color) acrylic with no HV leakage or tracking. When you tap the acrylic for the fasteners be sure to use kerosine if in a lathe. Also gently (very gently) anneal the acyrlic after tapping in your oven to prevent the sharp tap edge points from expanding into cracks that will radiate to the edge of the acrylic and cause fastener failure.


I have never seen this happen. Drilling and tapping acrylic really needs
some lubricant, or the acrylic melts or cracks too easily. Annealing
acrylic is a temerity, as it shrinks and deforms with heat. The heat
developed by tapping (with lubrication) is probably enough.

Antonio Carlos M. de Queiroz

I have some tapping and drilling lubricant I got over 60 years ago; it's lasted a long time. Just rubbing the drill or tap with it (just poke into it) is satisfactory. Looks and smells like thick colorless shoe polish and is poured into a small cardboard tube about 1" diameter by about 4 inches long. Works beautifully for tapping anything I've tried it on, with the exception of stainless steel. Makes nice clean threads in acrylic. I wouldn't be surprised if paraffin wax or even bar soap would do as well and the soap would be easy to clean with water. Haven't tried it though. As Antonio says, tapping "dry" tends to be a most disappointing experience.

Ed