window plastic

about a year ago I replaced all the window and port plastic on my boat. I contacted industrial plastics and sought there recomendations.they recomended 6mm cast acrylic for windows and 8mm for hatches. it is in a fairly dark tint. in the past silicon and black 4200 and mastic has been used as sealants. the acrylic is screwed to the outside of the hull. 5/16 holes with 1/4 screws. the sealants have allways leaked and never held to the plastic. this time i used sikaflex 295uv with primer. boy does this stuff stick. well the seal remains intact but one by one the windows are cracking from top to bottom mostly starting at screw holes but some not,due to different expansion rates to the glass/foam they are sealed to. when cold there is about a 1mm crack. the crack closes when hot. industrial plastics say this is the product used worldwide as an industry standard. I figure that i replaced the acrylic in bright carribean sun and the plastic was expanded when the sealant set, maybe thats the problem. I asked about polycarbonate. they could not give me an answer. my thoughts are it seems softer so it may not be as brittle. also what about thicker. 10mm is the next size in polycarbonate. what do you think.
bazza

 

Plastic needs to float in it's mounting..

If the plastic was fitted so the fastenings were sort of snug in the holes then when the plastic MOVES it sets up a lot of strain against the fastenings and the cracks usually start in the fastening holes.

The holes need to be a good bit bigger than the fastening and a washer on top under the fastening head will hold the plastic while letting it move about as it expands/contracts.

 

all holes were drilled oversize and have clearance to screws with washers. still cracked. some cracks are not even at screw holes.
bazza

 

Secured to tightly?

I have ALWAYS heard that float was critical. Even if the fastening holes were oversize, if the plastic were pulled down tight on a "THIN" bed of glue like bedding compound it could prevent expansion/contraction and promote the stresses that would cause cracks to develop.

pictures?