67King

Installed a rear hatch glass. It was a bit oversized, so I had to cut it down, and stress it to make it fit in the frame. I noticed that there seems to be some small spider web like cracking eminating from the screws that are holding the sides down.

I know the stuff is formed by heat. Can I take a heat gun to the curved section to soften the stuff to hopefully relieve some of the stress? Is there anything else I can do to prolong the life of teh glass?

Lou L

Wow - Just had the Lexan Hatch installed on mine this weekend also! Had to cut it down a little to fit the hatch also. I had a glass place install it - I don't see any cracks, but I'm going to add 2 support straps because it does bow a little in the middle. My front lexan has those little cracks by the screws and I've never done anthing with them - it seems to be fine.

333pg333

btw, what did you guys start with, a stock glass hatch and break the glass out? If so, how was that?

stownsen914

I wouldn't say I know a lot about Lexan, but I've found out a few things the hard way ... it's flexible stuff, but it doesn't like to be pinched (i.e. by a bolt/nut). It will crack if you tighten bolts on it. It seems to work better if you do "finger tight" and use nylock nuts or thread locker. A support or two down the middle of large unsupported areas is a good idea too. To relieve the stresses you're describing may require trimming, refitting, etc. I haven't tried heating to do that. I believe Lexan is sensitive to heating, and may get brittle if heated too much.

Scott

67King

Yes. I tried to keep the glass in tact, but that didnt' work. I ended up using a torch with a rosebud tip to get some of the last of the stuff out.

My biggest problem is that the glass was oversized, almost as if it weren't supposed to be used with the frame. It was wider than the glass, and popped out a little on teh sides, where the glass turns down.

ninjabones

No expert here either... actually a setion in this month's Grassroots Magazine on various motorsports materials. They state that Lexan also has the tendency to contract/expand with temperature changes... best to overisize your holes to allow for some movement or you will get cracking at the mounting holes (somewhat like what you are describing).

kurt M

Polycarbonate (PC) can be thermoformed but only at hight temps and close to the melt/boil point. PC is hydrophilic and the water it holds will boil bubbling the material if you do not heat it properly. The pros heat the entire part in an oven to de-gas prior to the final forming. I use 2 heat guns to slowly and evenly heat the material for a good while then zero in on the area I need to bend or form. It can be done but you need to get the hang of it before torching a part you spent time making. You can see the thermal condition of the PC as you are heating it. The surface refraction effect fades away as the PC becomes soft and plastic, clearer in a way and ready to form. Watch from an angle while heating. Start slow and heat soak. Large areas are hard to form cleanly unless oven heated. The temps are far higher than with other thermoforming materials. Skin burns are easy.

As noted when using bolts or screws to fasten it you need to leave room around the fastener for movement. The larger the span the more movement for a given sheet thickness is to be expected. I like to use flat plastic or rubber washers to make a sandwich when mounting it to window frames. you can't crank dow on the bolts the PC has to be able to move a little. I use soft washers under the PC and a slippery nylon type washer between the fastener head and the PC. Do not deform the PC with the bolt head or it will crack. Do not use countersunk fasteners. They will end up cracking. Simply put PC will crack if stressed enough and left in the stressed position. Methods such as pop rivets tend to make small sharp stress points that end up stress cracking at some point. Good rule of thumb. Set the fastener then look at the surface of the plastic from the side using florescent light. If you can see any deflection or dimpling of the PC made by the fastener you have a stress riser that is a ticking clock for a crack.



The car in the avatar has a 1/4 Shields brand PC windshield (good stuff!) and 1/8 uncoated everywhere else. The windshield and back "glass" were set using bolts and washers but the small side windows were set with pop rivets. The back window is made from flat un-formed stock with the sides being held by an aluminum strip to press and support the PC along the entire stressed edge. One of the flat stock simple shape sides has a crack in it already. I expected this but used pops anyway as they presented a clean simple install of a cheap part.

Uncoated PC is also attacked by most hydrocarbons. There is more to working PC than than the little bit I noted. It is great stuff if used properly.

Lou L

King - thats the exact same situation I had - it was a little too wide - I wonder if we bought it from the same place??? The windshield place did not break out my glass hatch - 333 - I started with the stock hatch also! My front windshield was from Shields and it has been great - no complaints at all there!

67King

Mine was blemished, so I got it for about half price. There is a horizontal ripple in the middle of it. The guy who sells them doesn't do glass work, he had his installed. But, there are two styles - one is "cut to fit," the other is oversized. I'vebeen told (by a different guy) that the oversized ones are typically made to eliminate the hatch frame.

Kurt, , THANK YOU! very good info. Will definately have to change the way I have it mounted.