securing lexan

[driveline]

I have an application where lexan sheets are mounted with bolts through drilled holes. The sheets are used as protective screens for machine operators so they are subjected to severe impacts on occasion. And occasionally the sheets crack after impact along the drilled holes. I can't assure that the holes will be prepped correctly, without any machine marks. I was toying with the idea of using a shoulder washer bonded to the lexan through oversized holes. Will the washer prevent the stress from reaching the drilled holes?

 

[TheAnswerGuy]

driveline,

Yes, it will help to relieve much of the stress especially if the metal washer is bonded to the "Lexan" right after the hole is drilled.

This may not solve your problem of breaking, since other factors that you may not have listed in your post, may be the cause.

But you are off in the right direction

 

[plastx]

Another concern with LEXAN and other polycarbonates is chemical attack. If the mounting bolts have oils, aggresive greases or other chemicals in contact with the PC this will cause cracking and crazing especially if bolted tight against the sheet. A shoulder fixture might help if the shoulder protrudes enough that the mounting bolts tighten to the shoulder instead of the PC. Totally eliminating chemicals in this environment might be tough.

Mike

http://www.plastxpi.com

 

[patprimmer]

The chemicals that do most damage are chlorinated hydrocarbons and aromatic hydrocarbons. Aromatics are most often the cause of problems because they are often only present as impurities in other chemicals, or in small, often undeclared quantities.

Regards
pat

 

[ornerynorsk]

We grommet all holes in lexan guarding with rubber grommets. Adds some impact protection and a little "give" for tolerance problems.

 

[driveline]

ornerynorsk,
My intial research led me to try using grommets on the first prototype. After a weathering test, the lexan was removed and there were small cracks forming at the edges of the drilled holes. I summised that the grommets didn't allow enough movement for normal thermal expansion. I may have compressed the grommets too much to eliminate the noise from the steel to lexan contact. I resorted to a bonded sealing washer which eliminated noise and cracks. I'm trying the shoulder washer idea as soon as I find a suitable glue.

 

[patprimmer]

Grommets work if done correctly, but take care that the compound the grommet is made from does not contain any plasticisers that effect polycarbonate (note Lexan is just one brand name for polycarbonate)

Regards
pat

 

[ornerynorsk]

Hey driveline,

Try reaming the holes after drilling, and use a vargus twist-deburr tool to round out the hole edges. Sounds stupid, I know, but if you end up with a ragged-edged drilled hole, it leaves a great situation for crack propagation. Good luck.

 

[patprimmer]

You can also polish the edges of the drilled hole with a flame

Regards
pat