Cracking plexiglass 2

[superdupertrooper]

I have a problem where I have a plexiglass sign that is bolted to an aluminum box (where there are lights inside), and the plexiglass cracks in cold weather. The plexiglass is screwed to the aluminum through four small bolt holes that are near the corners of the sign. In addition an aluminum strip is fastened to the top of the plexiglass panel, and a piece of silicon weather stripping sits between the plexiglass and the box.

I was wondering if the problem could be avoided if I changed the material of the spacer, or if I used a different material for the sign.

 

[CoryPad]

Plexiglass is a trademark for acrylic material. Acrylic is brittle and tends to crack at fastener locations. You could reduce this problem by switching to a more ductile material like polycarbonate (e.g. Lexan).

Regards,

Cory

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[nigelt]

...and I bet all is fine until the temperature drops?

I'd offer that your issue is another example of the question posed in my thread 334-107496 "ABS in low temp service".
There is strain on the plexi material where it compresses your weather stripping locally under each screw. As the temp drops, the effective stress 'rating' of the matl gets poorer, resulting in your case, as in mine, in failure!
Switching to polycarb should help, as acrylic - a brittle polymer - becomes more brittle in low temp. PC is simply a stronger matl.
I'd suggest you look into getting rid of the screws altogether and checking out 3M VHB adhesive foam tapes. That can waterproof for you, and after 24 hrs, they are absolutely stuck on. They work! In that case you can stay with your plexiglass.

"The ideal client is one possessed of great good sense and perfect judgement;
that is to say, one who agrees entirely with the designer at all times and in every respect."
Fenwick Williams, Naval Architect.

 

[alezjo]

This seems to be a typical “stress cracking” phenomenon. Amorphous polymers (like PMMA - plexiglass), during the extrusion/injection moulding phase, freeze their internal stresses more than crystalline ones do. Sometimes these tensions are released/raised when carrying out holes, ... (any mechanical tooling) thus creating cracks inside the part.
Other times these tensions are released by low molecular substances (like, for example, solvents used in additives) which enter the little cracks and open it, thus creating an even greater crack.
I wonder whether the silicon could cause these stress cracking.
Sorry for my English, I’m Italian...
Good luck
Ale

 

[patprimmer]

Plastics in general have about ten times the co-efficient of expansion as metals in general.

Without looking up actual numbers, you can be sure that the acrylic or polycarbonate will have at least four times the expansion of aluminium.

This different rate of expansion can result in stress being applied to the joints, creating stress cracks. These will be further encourages by interference fit of the bolts or the bolts compressing the plastic. It will be even further encouraged by chemicals that are inclined to attack the plastic, as they will attack the stressed areas first.

Notches and stresses from drilling the holes and cutting the sheet can also be a problem. Correct sharpening and feed rate on tools can help, as can annealing or flame polishing.

Silicone rubber can emit acid fumes on setting. Acrylic only has moderate resistance to acids.

Polycarbonate can be attacked by many common solvents, found in things like paints, glues, cosmetics, foods, aerosols, cleaning agents, PVC, Rubber, lubricants and fuels

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
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[RikuY]

As it is a sign, you might try keeping the materials the same and using a different fastening system. I am thinking much smaller diameter bolts and bigger washers (rubber lined if warranted) to keep the sign in place. Use the friction of the fasteners and let the material move around the connection.

Just a thought...

 

[plasgears]

My fellow aircraft home builders are beginning to avoid fasteners when assembling plexi windshields and canopies. The swing is to flexible adhesives. Check with plexi sources.