Lexan Cracking 1

[Marmight]

I am having issues with drilled through holes in lexan forming radial cracks when compressed by a fastener. I am trying to diagnose the cause of the crack and am looking for advise! First time poster.

No cutting fluid, cleaning products or adhesives are applied to the lexan during the machining process except for the paper that comes adhered to it from the manufacturer. So chemical attack is ruled out.

No pilot holes or finishing steps are currently employed, are these effective methods for reducing cracking?

The steps we are currently planning on taking are as follows, are these likely solutions to the issue?

-Adding champers to the top of holes.

-Reducing heat generation by optimizing cutting speed, feed rate and the use of an air chiller.

-The through hole clearance might be too tight, which could be stressing the lexan during thermal cycling, so we are planning on expanding the through hole.

Any and all advise or suggestions are welcome!

 

[KENAT]

What's the surface finish of the holes like? Changes to your process to get a smoother finish may help prevent cracks. With a rough finish you have the start point for lots of cracks.

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[btrueblood]

Also, make sure you aren't leaving a raised burr at the hole edge.

 

[MacGyverS2000]

Flame polishing the holes might relieve any stress created by a rough finish

Dan - Owner

http://www.Hi-TecDesigns.com

 

[Marmight]

I am worried about flame polishing inducing stress in the component. Taken from the manufacturers fabrication guide:

"The most important factor to consider when drilling Lexan sheet products is the heat generated during the actual process. In order to produce a clean, well-finished hole that is stress-free, the heat generated must be kept to an absolute minimum.”

We are currently making changes to the drilling process with the intent of improving the final surface finish.

 

[patprimmer]

You almost certainly have solvent stress cracking.

You need a good smooth finish. I don't see a problem with flame polish. The problem with generated heat is it influence the drilling process, not the thermal shock.

You certainly need good clearance on the holes. Probably a lot more than you consider reasonable.

What are you using for a washer.

Some very common chemicals used everyday, like cleaners, insecticides, deodorant sprays, insect repellants, hand cream etc etc etc can cause solvent stress cracking in PC.

Also residual oil on the drill from previou use or even manufacturer of the drill can cause problems.

Plasticizers from PVC that may be used in wiring or as gaskets or as washers can will cause problems.

Regards
Pat
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[Medeski]

We had a problem with guard windows cracking around the installation holes as well. We increased to number of mounting holes to reduce surface pressure. I'm not sure, but I think Loctite may also cause problems. Also, we have spacers (bushings) in the holes to control the amount of compression.

Best,
Medeski

 

[MikeHalloran]

Loctite will definitely attack Lexan. ... even if only a brief exposure, even if wiped off immediately.



Mike Halloran
Pembroke Pines, FL, USA

 

[Marmight]

Thanks for all the input everyone!

So I changed the drilling process to drilling an undersized hole and then milling out to the proper diameter. This resulted in a greatly improved surface finish, so hopefully there will be less micro cracks to propagate. I also changed the fasteners to machine screws instead of self tapping sheet metal screws (going into Delrin). Now I am trying to figure out the correct torque to apply to the fasteners, anyone have any experience with this? (Stainless screws, Lexan thru hole, Delrin chassis, fastening an O-Ring seal from external pressure)

"You certainly need good clearance on the holes. Probably a lot more than you consider reasonable."

The clearance on the thru holes seems reasonable... can someone explain why my thru hole needs to be unreasonably large?

The Lexan comes from the manufacturer with a protective sheet/sticker that uses some kind of adhesive. What are the odds that the adhesive used by the manufacturer of the Lexan could be chemically attacking the material?

As part of a test, I assembled a unit (with the new manufacturing methods) and brutally overtightened the fastener. No visible cracks propagating from the thru hole (which was the previous failure mode) however I did get some cracking on the upper face. Pic below:

http://imgur.com/7ghCe

Is this normal behaviour for Lexan, or is this further evidence that the Lexan is under chemical attack from somewhere? I repeat again, the sample from the above picture was very overtightened.

 

[Demon3]

Adhesives do attack polycarbonate. That's why you can't put stickers on motorcycle helmets. It weakens them so that they look fine but have no impact resistance.

The probability that your PC supplier has used unsuitable adhesive is low. They are well aware of the problems.

Chris DeArmitt PhD FRSC CChem

 

[patprimmer]

If you are using Lexan brand sheet, it is inconceivable that Sabic got it wrong with the adhesive.

It is normal, but undesirable for PC to crack from the stress of the bolt heads.

The reason for large clearance on the holes is so the PC can float to allow for thermal expansion as PC has about ten times the expansion rate of steel.

To avoid potential cracks you need soft washers made from an elastomer that contains no plasticisers at all. Things like EVA, PU or TPE or Santoprene types of materials so the seal/retention is both chemically compatible and flexible.

The washers and screws typically provided specifically to fasten down PC roofing profiles area good example.

Regards
Pat
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[patprimmer]

Re solvent stress crack.

All aromatic hydrocarbons and all chlorinated hydrocarbons cause the stress cracking even when present in very small quantities. Even if previously exposed, residual solvent that has been absorbed can be a problem.

Certainly residual lubricants left on screws and drills can be a problem if not cleaned off and dried well before contact with the PC.

Regards
Pat
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