Polycarbonate cracking

[GVMOGUL]

Back in the 1970's when I first encountered Lexan polycarbonate, we used a lot of 1/8", 1/4" and 3/8" sheet to make machine guarding and enclosures. It was trmendously expensive then, but as I recall, virtually indestructable. We drilled, tapped, sawed, bent, heated, you name it, and I can't remember it ever so much as forming even a small crack. And I remember hitting it severely and repeatedly with a large hammer just trying to get it to crack. The stuff we get now cracks as soon as you look at it. The spec sheets for Tuffak and Lexan look similar as to impact and yield strength. Does anybody know what the story is with this material? I've called my distributor about different grades and he says the standard grade is the best. I know there are FRP types, but I don't think that's what we were getting 35 years ago.

 

[Demon3]

Polycarbonate is extremely tough as you mention. However, it is also extremely sensitive to liquids that come in contact with it and will become weakened and crack. This problem is well known and is called environmental stress cracking. It's the reason you can't put stickers on your motorbike helmet. It's made of polycarbonate and the adhesive in the stickers will ruin the impact resistance of the helmet.

Your polycarbonate is probably just fine initially but has been exposed to some unsuitable liquid. It may have been sprayed with some polish to clean it or been washed with detergent. Or some type of oil may have touched it. Look carefully at what is contacting your polycarbonate.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[Demon3]

One more point, the impact resistance of polycarbonate is a lot worse for thicker specimens than it is for thin ones. That's what the manufacturer always quote the impact resistance of thin samples. Not many people realise this.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[HDS]

To demonstrate this get a scrap of PC bend it. Then spray brake clean on it or wipe it with acetone. It will self destruct.

 

[Demon3]

Hi HDS,

I've done that too and it's pretty impressive. If you bend it first it explodes. Works on CDs too as they are made of polycarbonate.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[GVMOGUL]

I'm seeing what everybody is saying here about contact with liquids, and we do subject our usually 1/4" PC guards to a lot of liquids once on the floor in service in production areas. They do fall apart then in some cases. But I still see the material cracking a lot during initial, presumably dry fabrication from new stock. The guards are full of cracks before they even get to the hostile environment. I won't rule out that something is coming in contact with the PC during fabrication. I'll have to do some investigation to see if I can find some practice going on before or during the usual drilling/fabrication we do here to make these things.

 

[IRstuff]

Don't forget that losing recipes is rather trivial in the industry. Something that was known to be a good process 20 yrs ago can readily be lost because some newbie decided it was too expensive or too hard to do.

We did that once at a semiconductor company. Our EPROM yields were abysmal, and a corporate-wide tiger team was formed to investigate and solve the problem. I'm happy to report that I was the one to find an obscure IBM Technical Journal article from 15 yrs prior that showed EPROM oxides had to be grown at no less than 1100°C, but someone in our fab had tweaked our process down to 1050°C, possibly to prolong the life of the furnace tube, but it caused us 100x more lost money in production yield hits than the savings in not replacing the furnace tube.

It was an incredibly tiny 5% temperature change, but it killed the yield by a factor of 100.

TTFN

FAQ731-376

 

[Demon3]

Is there a cooling liquid used when drilling holes into the polycarbonate? Does some bright spark wipe them down with cleaner at some stage? Both of these would be enough to explain your cracks.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[Demon3]

I am fond of saying "anyone who knows anything about plastics is either dead or retired". It's very nearly true as the people who started the plastic revolution and who really understand the materials are mostly gone now. So we repeat the mistakes of the past. That included big companies as well as small.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[CoryPad]

CD's and DVD's use a very low molecular weight version of PC to enable fast molding into thin shapes, at the expense of mechanical properties. This is obvious since they fracture before much plastic deformation It is possible that the material you are purchasing now has low molecular weight, or was molded with too low of a mold temperature inducing low toughness, or you may have material that has been exposed to solvents, or ...

 

[patprimmer]

My money is on a cutting fluid containing a small percentage of aromatics impurities in the so called aliphatic oil that is used in the cutting fluid.

Part of the problem that Chris identifies is that some pointy haired 30 year old fresh out of business school fires all the burnt out old has beens that don't know the difference between an objective and a key success factor or a milepost or whatever they choose to label it this year.

Strange thing is those burnt out old has beens generally designed the processes these guys now think they know more about.

Don't get me wrong, there is a place for bright young recruits with lots of enthusiasm, but in this day and age, may need a lot more mentoring than they realise when they fire everyone capable of giving the mentoring.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[GVMOGUL]

After talking to our maintenance guys, I learned that they do routinely use tapping fluid while drilling and tapping polycarbonate here. At that point, we couldn't wait to try the brake cleaner test. We did and it was a real eye opener. Armed with this knowledge, I think we will have fewer guard failures from now on. Also, I can see it advisable to keep drilling and forming to a minimum on guard designs going into wet environments. As for my recollections of the old days, I believe we always worked Lexan dry back then. There was an old timer in that shop, who I suspect had advised us to do that. It pays to heed the warnings of those old guys. Thanks for all the help.

 

[Demon3]

Just be careful as the polycarbonate is ruined by contact with the drilling fluid whether or not cracks appear right then. That's the problem with the motorcycle helmets, they look fine until you're dead from a broken skull.

You might want to implement a moderate impact test during production to make sure they retain their impact resistance after drilling and forming.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry