arcylic elongation or expansion??

[misc.garfield]

dear friends,

i am facing a problem currently - my part make of arclyic was dimension out!!

i measured the dimension each time after i CNC cutting the part. at the beginning, the dimension was in. however, after few days the dimension was out!! it became longer than the required size.

is this problem cause by the elongation of the arclyic?

if yes, what is the factor that cause the arclyic to elongate? high temperature? moisture absorption? or something else?

i will appreciate if anyone can share their experience

[Geof]

One possibility is a temperature difference between when you cut the part and when you measured it.

Another is that there were stresses in the acrylic and when you cut it this allowed the stresses to relax causing the part to distort.

When machining acrylic sometimes it is a good idea to stress relieve before machining and again after machining. Search on Google using 'annealing acrylic perpsex plexiglas' and you should find something.

Also try to always use cast acrylic when machining not extruded acrylic; the cast has less stress in it.

[ImanCarrot]

Totaly agree- extruded acrylic will bend like a bannana. It also has the impurities distributed about all inside the material as opposed to Cast where most of the impurities sink to the bottom.
Incidently, the thermal co-efficient of expansion for Acrylic is about 75, Iron 12 and Aluminium 24 so if a 4" iron bar will expand in length by 0.00047" for a temperature change of 10 deg C, then a similar Acrylic bar will expand roughly 6 times this (just under 3 thou).

If you're seeing a greater change than this over a similar temp change then something else is going on- as Geof says, the most likely culprit is internal stresses. Stick it in the oven at 90 deg C overnight before machining.

[Edit]You can see this stress effect by clamping a flat sheet of Acrylic on your mill and taking a half inch off the surface- un clamp it and stick a ruler side on to the surface- you'll see it bent (the back face will be bent too)[/Edit]

[misc.garfield]

then is it mean that moisture absorption of arcylic is not playing an important role in the arclyic elongation issue?

[ImanCarrot]

The honest answer is- I don't know.

However, I've never has a probelm with moisture absorbtion and I make lenses out of the stuff.

See here for more info and acrylic and moisture- it says it's totaly unaffected:

http://www.polyfab.biz/glossary.htm

[Geof]

I don't think acrylic absorbs moisture to any great amount. The reason I say this is I know that polycarbonate does; when you heat polycarbonate for vacuum forming, unless you have baked it first to drive out absorbed mositure first you will get bubbles. I have never heard that baking to remove absorbed moisture is needed for acrylic.