Extruded Acrylic/Plexi : BEWARE

[Hessel Oosten]

Extruded Acrylic/Plexi : BEWARE

Hi guys,

See pictures.

NO, this is not a story about my wife's sewing-work-table......:-)

BUT, have look to the ***clear acrylic plate*** on it.

After a few years the (previous perfect) edge got a few thousand crackles.

I presume this is/was ***extruded acryl*** (i.c.t. casted acryl).

The heat of the cutting gives (even after years) exagerating tension (this was already intrinsic in the material) and causes this kind of "problems".

I'm not really sure if this an important issue for our panels, but think about it !

Hessel

[Joe Cygan]

My guess would be the method used to radius the corners gave the plastic brittle characteristics. I have seen some companies use plastic router methods to shape edges before instead of a laser, perhaps this is the result.

Hessel did you make or cut the plastic?

[Hessel Oosten]

Hi Joe,

Spoke you shortly before isn't it ? ...:-)

I use a small cnc router and file the radius by hand.
Don't know how it is done in the plxi I showed in the pictures.

Hessel

[phil744]

Hey Chaps, thats nothing to do with the plastic being cast or extruded, whats happened there is after the plastic has been cut and the radiused edges applied that piece has been flame polished without annealing afterwards.

Hessel, if you have some thinners, or borrow the wifes nail varnsh removed put a smal ammount on a cloth and dab the edges of the acrylic, and watch the cracks grow, normally flame polished acrylic is okay but the edges are hard in comparison with the rest of the acrylic being soft, when chemicals are applied it softens the hard edges of the acrylic and the stress within causes it to crack, the problem is less obvious with casted acrylic but it will still happen, the problem is even less with white and solid coloured acrylic

[Hessel Oosten]

VERY usefull Phil !
Thanks.

Hessel

[Rob65]

Hey Chaps, thats nothing to do with the plastic being cast or extruded, whats happened there is after the plastic has been cut and the radiused edges applied that piece has been flame polished without annealing afterwards.

Hessel, if you have some thinners, or borrow the wifes nail varnsh removed put a smal ammount on a cloth and dab the edges of the acrylic, and watch the cracks grow,

Phil, I was told (by a local company selling plexi stuff) that this appears more/sooner on extruded acrylic than with the cast version.
In extruded acrylic, due to the continuous extrusion and cooling down of the extruded plastic, there are uneven tensions in the material. This can result in cracks in the material - specially when it is being glued (your thinner).

I guess you mean that one should not use thinner/solvents on flame polished acrylic to prevent cracks ? Maybe even not flame polishing at all ?

I have been told never to use extruded acrylic for (laser)cutting but I am no expert. I guess I'll find out; I've still got some extruded white material and within a few weeks the laser will arrive :rolleyes:

Regards,

Rob

[Hessel Oosten]

Good day Phil, Goedendag Rob (new to simming .... ?:-),

Here the effect Phil described.

http://www.youtube.com/watch?v=w8MEhbH6DK4&feature=related

Hessel

[Joe Cygan]

I Just poured alcohol on some laser engraved panels (clear & colored) and nothing happened to it. Must be what Phil is talking about with flame polishing edges.

Joe

[Rob65]

I Just poured alcohol on some laser engraved panels (clear & colored) and nothing happened to it. Must be what Phil is talking about with flame polishing edges.

Joe

After unpacking, cleaning, mounting and aligning of the laser equipment and installing of the software we got our laser working and I could not resist ...
Attached is an image of a test on some plexiglass from unknown origin but most likely this is XT (extruded) matte white plexi.
The result looked great but after pouring some alcohol (isopropanol) on it it did not take long to develop cracks in both the cut and the engravings.
The number 2 on the image has a height of about 9 mm.

Regards,

Rob

[Joe Cygan]

After unpacking, cleaning, mounting and aligning of the laser equipment and installing of the software we got our laser working and I could not resist ...
Attached is an image of a test on some plexiglass from unknown origin but most likely this is XT (extruded) matte white plexi.
The result looked great but after pouring some alcohol (isopropanol) on it it did not take long to develop cracks in both the cut and the engravings.
The number 2 on the image has a height of about 9 mm.

Regards,

Rob

This is certainly interesting. Wondering why yours cracked and my does not. What type of laser are you using? What type of plastic did you use?

Joe

[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/Coefficient_of_thermal_expansion

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

[BHawthorne]

I just found this via Google...

Acrylic solvent crazing: http://www.newton.dep.anl.gov/askasci/chem00/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,