New Lexan Polycarbonate window cracking ?

[Don Davis 87]

Hello, I recently replaced a window in my machining center, It was perfect but after about a month I started to see cracks develop around the bolt holes that hold it in place, now they are getting long about 2" so it looks like spiders around the bolts which were only very lightly snugged, they didn't crack from being over torqued. The window is Lexan Polycarbonate and the coolant is Blaser Synergy 735. Has anyone else had this same issue?

Thanks, Don

 

[Milland]

Polycarbonate is very sensitive to attack from some chemicals, and stress (like drilled hole and the tightening of screws within them) can rapidly increase the likelihood of stress cracking. It's sort of an "unzipping" of the polymer chains.

Either use another clear plastic as a face shield, or try to make a perimeter frame of Al that captures a simple cut sheet, no holes.

 

[13engines]

You might try Acrylic. I've used it on multiple machines without troubles like you're having.

Not as bullet proof, (no pun intended) but bullet proof enough. Slightly less expensive too.

 

[Dan from Oakland]

Be aware that polycarb is reasonably shatter resistant if hit by a flying tool or part. Cast acrylic is somewhat brittle and may shatter if hit hard enough. While PC is prone to stress cracking when exposed to certain solvents, acrylic is much more forgiving.

Have you cleaned the new window with any solvents or alcohol that may explain the cracking? Your coolant may be to blame- if you have a scrap of PC laying around, drill a few holes in it and toss it in the coolant, and see what happens.

 

[gustafson]

petroleum solvents can do this to polycarb

 

[BT Fabrication]

I have windows I have made on my skid steer that have done the same. its stresses from flexing at my end. used 5/16" bolts and washers and all but one hole has the crackes all around it after it was hauled down the road. probably from the wind coming into the open cab, making it flex and crack all over.

 

[MaxPrairie]

Yeah. My windows are cracking too. Also the covers for the lights in the machine are cracked too. Doesn’t really bother me

 

[Scruffy887]

Be very careful when drilling the holes. You want a very controlled break through. Even at that you will see tiny cracks at the exit point, may need some magnification, but they are there. These are stress risers, cracks already started. Smooth countersink each side to get beyond them. Even if there are no holes you want very smooth sanded/semi polished edges. Saber saw a big no no.

 

[kustomizer]

I had this problem with a lathe I used to have, I made a bezel for inside the machine so I could make the window from polycarbonate without holes, it was just small enough to go inside the holes, silicone sealant then captured without holes. It lasted long enough to become all scratched up to where you couldn't see through it. I cut it in the VMC so the edges were smooth

 

[Tony Quiring]

Try drilling smaller holes first.

Then use step drill from both sides to increase size without tearing.

Get some metal tubing to make a sleeve that the screw fits through, make them just longer than material is thick, drill holes to this size.

Install with washers on both sides with maybe rubber gasket or o ring as cushion so the plastic is floating in the assembly and screws tight against the sleeve

Sent from my SM-G781V using Tapatalk

 

[Pathogen]

From the Way Back Machine when I was doing hospital stuff...

The flow meters on the oxygen tanks were made of Lexan, when cleaned with isopropyl alcohol (who hasn't used that for cleaning or disinfecting?), the float ball flow meter got crazed fractures and finally cracked.

Engineering always blamed the nurses, stressing if you want something broken give it to a nurse.

It wasn't long finding out the alcohol did it though.

 

[EndlessWaltz]

You need to order a special grade of Polycarb that is more chemical and impact resistant. You also have to predrill the holes carefully as to not induce stress. Do not wipe your windows with any solvent to clean them. Just water and a rag.

 

[CarbideBob]

Tiny cracks on the breakout of holes can and will grow fast.
They make differently shaped drill tips for poly.
Also back it up with wood when drilling as this cuts down on the exit problem.
Chamfering the hole after drilling also helps.
Bob

 

[706jim]

I used Lexan for chip guards on an open frame VMC. No holes to crack like in your window. And every time after a year or two the stuff got brittle and cracked. Maybe because of exposure to the coolant. So no easy solution that I can see to your problem.

 

[Milland]

I used Lexan for chip guards on an open frame VMC. No holes to crack like in your window. And every time after a year or two the stuff got brittle and cracked. Maybe because of exposure to the coolant. So no easy solution that I can see to your problem.

Polycarbonate is one of the more chemically "fragile" plastics that we machine. Concentrated stress speeds up the attack, but as you saw even simple exposure can do damage.

There are some hard-coated PC sheets, but very expensive. Smooth, mechanically polished edge will hold up better, as will using a coating (that itself doesn't attack PC) to protect cut surfaces.

I've used PETG sheets when I've wanted decent chemical resistance, clarity, and toughness, but it's not nearly as durable from a hardness standpoint.

Clear Easy-to-Form PETG Sheet

 

[guythatbrews]

Get some metal tubing to make a sleeve that the screw fits through, make them just longer than material is thick, drill holes to this size.

Install with washers on both sides with maybe rubber gasket or o ring as cushion so the plastic is floating in the assembly and screws tight against the sleeve

I like the idea of the slightly longer sleeves after careful drilling. We custome made some parts chutes out of polycarbonate and some holes cracking after 4 years or so. Vegetable oil based coolant so I suspect stress from bolt pressure. Sleeves will take care of it.

Okuma used to use laminated safety glass for windows. Maybe they still do. Cracked one in a crash. I think it was 2 (or 3?) sheets of safety glass with some kind of rubber in between. Cracked but held togethere. A local glass shop had the stuff and cut to size matching the big corner radii. It was held in place with rubber gasket with central spline. Example from grainger not sure if it cones with the spline.

If this continues to be a problem might use this stuff as long as your window has round corners or I guess you could miter the gasket and silicone the corners. Seems like the one we replaced had close to 4" radius so the gasket could wrap.

One problem with the glass tho. Ours was in a turning center and chips would sandblast and erode the gasket requiring replacement. We ran It hard two shifts and seems it would last a few years.

 

[cosmos_275]

cracks develop around the bolt holes that hold it in place, now they are getting long about 2" so it looks like spiders around the bolts

Loctite will do that to PC. Was there loctite on the bolts?

 

[DavidScott]

Not all polycarbonates are the same, so as EndlessWaltz said find one that is better suited to your needs. Some will disintegrate from hand lotion or skin oil.

 

[neilho]

Soft mounting, as mentioned numerous times here, is almost always good. Making holes oversize, inserting grommets and putting fasteners through is another technique.

 

[rk9268vc]

We use a lot of acrylic at work to make jigs and fixtures. When ever you drill it or cut it with a laser, always go back and heat the cut edges with a torch to slightly re-melt the exposed edges to prevent cracking. When cut, it seems that the exposed edge is super crack prone especially against solvents and alcohols. The re-melted edge is nolonger as prone.

I would try heating the poly carb up with a torch where ever it is drilled to close up all the small cracks from drilling. even laser cut has the same issue.