Extruded Acrylic/Plexi : BEWARE

[BHawthorne]

I learned this year in school of CTE and how it relates to materials. Maybe the extrusion process changes the material properties around the edges of the material in an uneven manner and CTE is effecting it? Maybe the compression around the edges are at a low CTE, while the unstressed center is a higher CTE and it sheers internally? Just making a random guess.

http://en.wikipedia.org/wiki/Coeffic...rmal_expansion

http://www.ridoutplastics.com/design.html

[BHawthorne]

I just found this via Google...

Acrylic solvent crazing: http://www.newton.dep.anl.gov/askasc.../chem00590.htm

[phil744]

Its caused by the heat being induced whilst cutting, basically the edges become so hot as the laser passes the edge (to approx 1mm in) becomes hard, where as the rest of the material is soft in comparison, so the edges are stressed, when you apply chemicals or solvents these cause the acrylic to soften cracking it.

Same applys to flame polish also, the heat blowlamp has the same effect as the heat from the laser, it's tempering the outer edges.

for any material that has been tempered the edges are always the weakest part, look at the glass in the side windows of your car, u can hit them with some severe force, they dont break, however get yourself a small screwdriver and tap the very edge its like a bomb going off, i dont reccomend u try this however, i did once, never again

The problem is worst on clear acrylic, white acrylics because of the pigments behave somewhat differently, infact the white acrylics ive used dont suffer from this problem.

Using clear cast acrylic supresses the issue, bit its not 100% perfect, cutting at a faster speed with air blow helps also, the trick is to minimise the ammount of heat you put into the piece whilst cutting, the less heat, the less cracks simple.

Annealing is the only way of making sure you dont have this problem, this is what i do, but always read the datasheet it tells you how to do it, for 6mm its generally around 90 degrees for 3 hours, cool down profile no more then 20 degrees per hour until piece reaches 45 degrees, but different acrylics require different time/temp.

Same applys for CNC milling clear acrylic, keep the cutting tool as cold as you can and you will never have this problem.

If you plan on using white(ish) acrylic (basically anything other than clear) you should be okay.

Also when laser cutting acrylic thickness plays a role also, the thicker the material the slower you have to cut the more heat you put into the piece, usually up to 3mm acrylic you can get away with it, anything over you will have this problem,