[GLCarlson]

A few months ago there was a discussion in another thread about coolants which cited a specific incompatibility between Kool-Mist and polycarbonate (Lexan etc). My enclosure is polycarbonate, and my coolant is often Kool-Mist. Stress cracking or embrittlement would defeat one of the reasons for an enclosure.

The claim that Kool-Mist causes stress cracks in polycarbonate can be tracked to a single repeatedly posted report. The manufacturer is unaware of any problems. If one checks the chemical compatibility of polycarbonate with the reported (MSDS) composition of the coolant, it's at least possible that one component of Kool-Mist (the alkanolamine) is causing trouble. At the dilution levels recommended, it seems unlikely- but it's possible in theory.

Theory is fine, but when a well-reasoned theory confronts a conflicting experimental result, the theory changes. A piece of polycarbonate has now been soaking in 25% Kool-Mist/75% water for 3 months at room temperature, exposed to normal shop lighting (in the shop, by a window). This is about 8x more concentrated than the use dilution (4 oz/gallon, about 3%). Don't know if the coolant is formula 77 or 78, it's not labelled as either (MSDS's are similar). At three months there is no change to the test sample of polycarbonate. Not sticky, no stress cracking visible, nothing visible under crossed polarizers, no edge degradation, doesn't seem less flexible (but breaking stress wasn't measured). In short, no change versus control.

So far, the report that Kool-Mist causes problems with polycarbonate is not substantiated by experimental results.

[LeeWay]

I think it may have been me that you saw posting results like that. I can tell you that it did indeed happen to more than one piece of polycarbonate. That said, there are different grades and surface coatings of polycarbonate and I use many different brands all the time. Just whatever my supplier ships. Once the film is removed, I can't tell what brand it might have been. I think I still have some of the last sheet that was affected. I'll check to see, but you could grab the edges with your fingers and snap it. It would crumble into a few pieces. I only checked on the edges and I'm petty sure that may be where it infiltrates from. There were also a couple holes drilled fairly close to the edges. That doesn't usually effect PC like it does acrylic. Acrylic is prone to cracking around drilled holes. Sharp bits and a counter sink will help prevent that in acrylic.

Also in an enclosure if the ends of a window or portal is sealed off with a rubber seal, it may never be affected.
I remember reading too that at least one other guy witnessed the same thing happening in his shop, so it ain't just me. It may very well be a rare occurrence when materials and situations are perfect. I haven't found the need to replicate the experiment yet, but I have plenty of both here and won't take much to try out.
I'll let you know how it goes either way. I'll try at least a couple brands and some of the known bad sheet if I can locate it.

[mike sr1]

Years ago we had a problem with lexan type machine guards, the operator found that a certain aerosol solvent would would destroy them in a matter of minutes, it would crack around the holes where it was bolted to the machine and fall off in a matter of a few minutes! Needles to say that brand of solvent was eliminated from stock immediately! I dont know what the solvent was but it was an electrical contact type cleaner. We had some less than stellar operators in those days......

I am going to make a lexan front guard for the Tormach, the one that came with it cracks easily.

[LeeWay]

The only other thing I remember using inside the enclosure was WD40 and some way oil. Both commonly used in machines and inside enclosures.

[GLCarlson]

I think it may have been me that you saw posting results like that.

I think you're right, but I didn't look up the past posting. My concern was whether I should quit using Kool-Mist or not. Right now, looks like not.

Couple more pieces of the puzzle. My KM is formula 78 (finally found that on the bottle), and the polycarbonate I'm using is Makrolon. The piece I've got soaking is about 1x2 inches, quarter inch material, no edge protection at all-and so far, no edge friability.

One other possibility is that this is an effect from a more concentrated residue. My experiment won't rule that out. Alkanolamines are not very volatile, and are fairly oily so they could remain in place after the water in the coolant evaporated. The chemical compatibility charts for polycarbonate are quite clear that these compounds are able to cause stress cracking.

My practice is to clean up coolant residue and wipe down the enclosure after a job, so that's not an issue for me, or anyway hasn't been yet.

It'd be nice to get this understood. Is it common, rare, specific to some brands of polycarbonate, conditions required, etc? Polycarbonate is all over as machine guards, and if it's easily vulnerable to degradation that is a worrisome safety issue.

I'm looking forward with great interest to what your tests show.

[Steve Seebold]

I made an enclosure for a lathe about 25 years ago and the straight parts of the Lexan remained in good shaoe with no cracks, even ariond the holes that were drilled for the handles, but the area where the cover had been heated and bent began to get hairline cracks almost immediately. It took about 3 months for the cover to fail completely and require replacement.

[mountaindew]

Hello all
I am new to cnc systems, but I do know my way around polymers.

Leeway nailed it by saying there are a half dozen brands that make the products and there are different grades and compositions.
Each one will act and perform sometimes very different. Some can be put on say a mill and machined to perfection others will gum up or distort.
What I think many people see is what we call crazing. The material is formed by a couple different processes. Extruded, cast and cross linked I think are the most common. When the material is formed by extrusion it will have stress formed into it. As a result it will craze around holes and some surfaces sometimes while you machine it others years later. Also products that contain alcohol can cause problems with crazing or damage. Extruded material is generally hard to work with because it is so stressed and it will be 70% or less of the cost of cast. Cast materials are more stable and machine much better, resist craze. Chemicals will still effect these materials mostly as noted above and products that contain methyl methacrylate would also cause major issues. That chemical is used to melt the material and bond it with same. Cast materials are still under stress but much less and for reference the material can be annealed and stress relieved with heat over time to lesson this also. Not that this applies to an enclosure made with the material.

Overall the material is very tough and chemical resistant.

hope this helps a little
Lane

Took time later to look at the makrolon material . and it looks to be one of the top brands / grades available and has been in use for machine enclosures for years. Like many products the manufacturer changes the composition of the material for reasons that are not always clear and that could effect performance also.... that point made this looks old and proven so imho you will find it will hold up well in the machine environment. Stuff might stop a bullet but it likes to be cleaned with cloth and dish soap. Another point if you leave a material dirty it can also effect performance and life span.

[Geof]

...... A piece of polycarbonate has now been soaking in 25% Kool-Mist/75% water for 3 months at room temperature, exposed to normal shop lighting (in the shop, by a window).....

Did you do anything to your test piece to induce stress into it before starting the test? It is not clear from your posts that you did do this and, of course, a necessary factor in testing for stress cracking is to have stress present in the test piece. Actually what you should have is a variety of pieces; some with holes drilled, some drilled and tapped, bent pieces, pieces with edges fire polished, etc. In addition the test should be carried out with samples fully submerged, exposed to splash and only exposed to vapors. With plain polycarbonate, that is NOT the stuff with the abrasion resistant surface coating, the only compound I have found that very quickly causes cracked around drilled holes is Loctite but it does not affect seem to affect unstressed polycarbonate.

In the case of abrasion resistant coated polycarbonate it is possible to get stress cracking even without (obvious) exposure to any solvents other than soapy water. I suspect this is because the coating itself is very brittle and tightly bonded to the polycarbonate surface so that microcracks in the coating propagate into the material underneath: This would be somewhat analogous to the decreased fatigue resistance in hard anodized aluminum alloys which have a very brittle layer on top of a far more ductile substrate.

[GLCarlson]

Did you do anything to your test piece to induce stress into it before starting the test?

No, I did not. My quick test was intended to do one thing: find out if prolonged exposure to relatively concentrated Kool Mist 78 was likely to be a problem for the Makrolon polycarbonate panels I had in my enclosure. Those panels are flat and only cut to size, so I used a small piece of flat material as I described. After 90 days immersion at RT, I see no effect. I'll let it cook a few more months.

I'll leave it to someone else to do a proper study. I'd love to see multiple brands of polycarbonate with various surface treatments and physical modifications (bending, holes, etc) exposed to a range of coolants (most have at least potentially problematic component) at various concentrations and physical conditions. It's probably several dozen cells, even with a fractional factorial design, and six months work to do properly. Or more.

I've got a good enough answer-I probably don't have a big immediate problem.

[Geof]

You probably don't have any problem.

But I thought I would put in these links in case anyone wants to see what has been discussed in the past on this topic.

http://www.cnczone.com/forums/bencht...tc1_lexan.html

http://www.cnczone.com/forums/glass_...ate_sheet.html

http://www.cnczone.com/forums/genera...l_windows.html