Crazing question (Not CRAZY)

[Larry Bratton]

I have just started doing some things with acrylic. Today, I had occasion to vector an arch on a piece of extruded. I sent the file over and my helper ran it. She brought it up to my office when it finished and was amazed at the little vein like fractures on one part of the cut. I explaned that was called crazing and was a kind of stress crack.
My question and her's is this..is their any way to prevent it? Does cast acrylic do this also? Any way to predict that you might get it on a particular type of cut etc. Do you leave the protective paper on or take it off? (the paper was off of this) Any info would be appreciated.

[Joe Pelonio]

Ask your helper if she wiped it off with something.

I have been cutting cast and extruded every day for 3 years now and have never seen that except for when someone wiped the edges with a solvent.

If you vector a hunk and wipe it with denatured alcohol you can watch the cracks appear right before your eyes.

The heat from laser cutting (and also from flame polishing) will alter the acrylic at the molecular level and make it subject toi this problem. The answer is to clean with a non-solvent product, if needed, like windex or a citrus based product.

Oh, and some paints will do it too!

[Larry Bratton]

Ask your helper if she wiped it off with something.

I have been cutting cast and extruded every day for 3 years now and have never seen that except for when someone wiped the edges with a solvent.

If you vector a hunk and wipe it with denatured alcohol you can watch the cracks appear right before your eyes.

The heat from laser cutting (and also from flame polishing) will alter the acrylic at the molecular level and make it subject toi this problem. The answer is to clean with a non-solvent product, if needed, like windex or a citrus based product.

Oh, and some paints will do it too!
Joe, to my knowledge we don't even have anything in the shop except windex etc. We keep isopropl alcohol for cleaning various things, but for her to have even wiped it off would have been unusual unless I told her to and what to use.

[Wil Lambert]

I've had windex cause crazing on acrylic before. It only happened on one batch so I chalked it up to being a bad batch of plastic.

Wil

[Nancy Laird]

Do you leave the protective paper on or take it off? (the paper was off of this).

Larry, we've cut quite a bit of acrylic--some pretty intense curves--and never had a problem like this. We always leave the protective paper on, as that is what the cmanufacturer recommends. We buy our acrylic from Tap Plastics.

Nancy
(128 days)

[Larry Bratton]

Gosh, I posted this same thing in another forum and I got some totally different answers, so, I'm kinda confused.
One person here said OK to use citrus based cleaner, one in other forum said those were particularly bad.
One person here says Windex OK, one says no. Also alcohol will cause it.
One person in other forum says take paper off, it will cause scorching, one here says leave it on.
One person said that the problem only shows up in extruded and not cast. No one else replied to that question.

My plastic could have been bad also. I looked at the piece again and it actually had tiny fractures all along the curve where I cut it out.

Could it be that we don't actually know??? :confused: I think I will refrain from wiping edges until after cutting is done. Thanks for all the replies.

[Mike Null]

Larry

I have a small job that requires extruded and it occasionally crazes. I was buying the cheap stuff at the BORG but switched to buying from Johnson Plastics because it has a paper mask instead of plastic. I always cut acrylic with the mask on. If you are getting scorching I would guess tahat the power setting is too high.

The stuff I'm cutting is 1/16 or .060".

Alcohol is bad but I haven't seen any problem with orange cleaners.

[Rodne Gold]

Use cast , its far less prone to stress cracks than extruded as its a long polymer chain acrylic. Extruded is a short polymer chain and is manufactured a different way to cell cast.
Any solvent or even the fumes of some solvents can promote stress cracking. Heat is the cause of it all and the idea is to limit any heat affected zone , ie use settings that will cut and not heat up the pex , start by reducing your PPI value.
The worst thing about all this , is that you may cut a perfect piece , ship it and it can develop these stress cracks months after , especially if cleaned with a solvent or glued with a solvent type glue as the stress never releives. There is a way to stress releive perspex or acrylic and that is to heat it to 70c (extruded) or 80c (cast) and leave it in an oven at that temp for 1 hr per mm thickness and then let it air cool.
Your best immediate strategy is to use cast.

[Richard Rumancik]

Gosh, I posted this same thing in another forum and I got some totally different answers, so, I'm kinda confused.
One person here said OK to use citrus based cleaner, one in other forum said those were particularly bad.
One person here says Windex OK, one says no. Also alcohol will cause it.


Larry, there may be a lot of variables here. I'm sure everyone is trying to give accurate info pertaining to their own experience.

If there were no solvents or cleaners involved, and you still had crazing, then I don't know what caused it. My guess would be overheating during cutting - if it was cut at a higher energy level than needed (eg power too high or speed too low.)

You should NOT use ammonia based window cleaners (including Windex.) (Sorry Joe, but I think there's a consensus on this one. You might not have seen a problem, but it is generally NOT recommended.)

The only solvents I have seen recommended are hexane, aliphatic naphtha, or kerosene. I don't like the smell of kerosene. I have had success using Coleman camp fuel which I understand is a variation of aliphatic naphtha. Do experiments before using it on a project by applying it to a scrap with a laser-cut edge. Hexane is supposed to be available at paint/hardware stores but I have not attempted to source it. Be careful with the use of these solvents.

All other solvents including (but not limited to) acetone, gasoline, benzene, carbon tetrachloride, methylene chloride, lacquer thinner, ethyl and methyl alcohol are generally not recommended for use on acrylic.

Citrus cleaners: don't know if "citrus cleaner" defines a unique product. I have seen citrus cleaners with a long string of hydrocarbon ("solvent") additives. Some of these might be incompatible with acrylic. If I wanted to use a citrus product then I would ask the manufacturer of the acrylic for approval.

I tried "Goof-off 2" to clean acrylic. Goof-off 2 is marketed as "safe for most plastics". Well, it cracked my acrylic. But I won't say you should never use it on acrylic products, as my sample is very limited.

We can say that alcohol will cause cracking in ALL acrylics. Like Joe, I have seen it occur as I watched. But I recently did a test on a Rowmark Flexicolor acrylic and did not find a significant effect from alcohol. In other cases there may be more factors involved. Which brand of citrus cleaner? Which brand of acrylic? I know it may be frustrating but you might not find definitive published answers and have to do your own research and testing to find products that you can source, are comfortable using, and give the results that you want.

I wanted to try annealing on one of my projects but became disillusioned with it. You need a good oven (not a kitchen oven) with proper control and air movement inside. Then you need a good way to support the parts without leaving marks. Finally, you need to accept that the part will change dimensions. This third item gave me a problem as I was trying to make an accurate part and it shrunk unequally in the x and y axes. You might have better success.

You might try the manufacturer's website to see if they have technical support staff that can give you an idea of why you saw crazing when no chemical was applied.

[Richard Rumancik]

I think Rodne is probably on the right track with the use of cast acrylic. I prefer that option over annealing. Here is some marketing info on continuous-cast Lucite L. It is apparently a lower-cost option to cell-cast acrylic. I have bolded a few words below . . .


PRODUCT DESCRIPTION:

Acrylic sheet is a continuously processed product which exhibits outstanding optical properties, uniform thickness and a low stress level. Acrylic sheet is a lightweight, weather-resistant thermo plastic. It can be easily fabricated, formed, finished, cemented, painted and decorated.LUCITE® L continuous cast acrylic sheet, produced from methyl methacrylate monomer, offers a unique blend of superior performance properties. Its high molecular weight gives many of the benefits of cell-cast acrylic, such as excellent resistance to crazing and cracking, at an economical price. INEOS Acrylics' manufacturing process gives outstanding thickness uniformity vs. cell cast acrylic. LUCITE® cast acrylic sheet has exceptional freedom from warpage, cracks, scratches, voids, foreign matter and other defects which may affect appearance or serviceability.

Available with new thermoformable film-masking for easy processing and superior protection. LUCITE® L cast acrylic sheet can be heated, line-bent or laser-cut with the strong protective film-masking in place.

[Keith Outten]

I have been using Acrylite brand acrylic, both extruded and cast for the last four years. I clean all of my acrylic with Windex exclusively, and have never seen a stress crack. I use both Rustoleum and Krylon paints on acrylic, never had a problem using either one.

I can't say why this works for me, it just does.

.

[Joe Pelonio]

You should NOT use ammonia based window cleaners (including Windex.) (Sorry Joe, but I think there's a consensus on this one. You might not have seen a problem, but it is generally NOT recommended.)


I agree, and should have mentioned that. Only windex and other window cleaners that do NOT contain ammonia should be used (windex is available with or without).

[Brian Robison]

I cut a lot of acylic. Yes extruded is worse, the mirrors especialy. I try to never use any cleaners, cut with the mask on. I just did 60 mirrors with a plastic mask and it worked very nice. I was expecting to have trouble. Get your settings correct to just cut through the material. I use 1000 hertz.Cast acrylic is always much nicer to cut or engrave, a lot less oder. I've still never had good luck using a spray paint to color fill. I just use acrylic paints from Wal*Mart or acrylic enamel from the paint store and brush it on.
BTW, Windex=Bad, DNA= REALLY Bad.

[Larry Bratton]

If there were no solvents or cleaners involved, and you still had crazing, then I don't know what caused it. My guess would be overheating during cutting - if it was cut at a higher energy level than needed (eg power too high or speed too low.)

I ran it at 6 speed,100power and 5000 frequency-40w Epilog. These were Epilogs recommended speeds and feeds. However, my actual power may be over 40w. Quarter inch cut like butter. Next I do, I'll lower the frequency a bit and see what happens.

[Keith Outten]

I have always bought Windex by the gallon and then fill several spray bottles located all over my shop. I just checked the gallon jug and it contains Ammonia-D. I just finished cutting 15 acrylic signs, they were cut from 1/8" gold mirror and 1/8" black acrylic backers, all were cleaned with Windex from the same bottle...not one crack or visible damage to any edge. I used my Epilog Legend model 24, 35 watt laser engraver with a speed of 20% and 100% power. The gold mirror and the black acrylic were both made by Acrylite.

Why doesn't the acrylic I vector cut crack?
Is anyone else using Acrylite that has a cracking problem?

Our variables don't seem to be adding up to any kind of technical solution.

.

[Brian Robison]

Larry, if you can, try this. Bump up your speed until you JUST cut through the material. You'll probably go too far and then have to back off 1%. I get a "gooie" edge with the hrtz too high, try 1000 hertz.
I've played a lot with the acrylic and this is how I have the best results.

[Richard Rumancik]

I have been using Acrylite brand acrylic, both extruded and cast for the last four years. I clean all of my acrylic with Windex exclusively, and have never seen a stress crack. I use both Rustoleum and Krylon paints on acrylic, never had a problem using either one. I can't say why this works for me, it just does.

Keith - I just looked on the Cyro website and they have a chemical resistance chart. It says Acrylite is "resistant" to aqueous ammonia. But lots of references on the Internet will say not to use ammonia or Windex on acrylic. It appears Cyro has come up with a "fix" to make their product more resistant to ammonia. My apologies for misleading anyone.

The paints probably won't cause a problem in the main surface of the plastic sheet. The crazing tends to start on edges. It is possible that paint itself might be hiding any crazing that is occuring. (Or maybe Acrylite is resistant to these solvents too.) Crazing won't necessarily cause cracking unless it is further stressed. For a painted sign that just hangs, it might be fine.

If anyone wants to see crazing in action, cut out a piece of CLEAR acrylic with several small holes and square cutouts. Then hold it to the light and wipe it with isopropyl alcohol (70% or so). You will likely see crazing occur as you watch. (Unless you have an alcohol-resistant version.) You could try the test with ammonia as well to see if your brand is resistant.

Sharp corners will promote crazing as there are high residual stresses in the corners.

Unfortunately we likely won't find "absolutes" when it comes to compatibility of chemicals and plastics. I stopped using Windex because I damaged some parts using it. When I said you should NOT use ammonia I didn't know Acrylite was resistant to ammonia. I was generalizing based on numerous references I found some time ago. But manufacturers are always changing and improving products as well so information from a few years ago might be invalidated by new formulations.

[Keith Outten]

No problem Richard, I was just scratching my head trying to fiure out if we were dealing with variables that might be a solution to the problem.

I can certainly stop using Windex, even though it doesn't cause a problem with Acrylite you never know what brand of material someone may bring you to engrave or cut.

I have often used wet newspaper to clean glass and acrylic when I am suddenly out of a cleaner and it works well. Many people prefer wet newspaper for window cleaning because it never leaves streaks and it is certainly inexpensive.

.

[Rodne Gold]

There are different modifiers added by various manufacturers to impart different properties to acrylic , thus one type might work better than another. For example if you want to use extruded , Degussa's extruded is far less prone to stress cracking than any other we have used.
Easy test to see what you have in respect of cast or extruded if you don't know , is to burn a small strip , if it burns with a crackly flame and doesnt drip molten balls , its cast.

[Larry Bratton]

Just for the record..this particular piece of material was Acrylite.

I did discover something today, may not be related though. I got the supplier to cut this material for me. (We had not purchased a plastic cutting blade and I needed some specific sizes). He called me and told me he had the pieces ready. I asked about the drops..he said..OH, I suspect the shop trashed them..my reply was..I don't think so at 3 bucks a foot. So when we picked it up, I had this odd piece of Acrylite when the balance was Optix by Plasticoat. So when I needed to fab this template we just grabbed the Acrylite. It may have come out of their dumpster and could have been exposed to something there. No way to know.

Under any circumstances, this has been a great learning thread. I appreciate everyone's time in posting all this great input!