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gwsg · Posted: Wed Dec 18, 2002 4:25 pm Post subject:

OK, I have just weighed up some sample glass and 6mm weighs 15kgs/m2, 4mm weighs 10kg/m2. If the hatch glass was 6mm thick it would weigh 55lbs tops. If it was 4mm thick it would weigh 37lbs top. I dont have a vernier scale on my thumb but my pinch test estimates 4mm too.

The hatch is made of tempered glass and cannot be cut, even with a diamond saw, because as soon as you break the surface tension it will shatter into a million pieces like Cathys did. The good thing is that tempered glass is often used for the window in cast iron wood heaters which means it should withstand the heating of the plastic. When bending glass it is usually slumped concave but this may only be because its easier to line up with the mold I dont know. An oven preferably or a series of electric strip heaters is the way to go because you need to heat the whole thing evenly although if the finished product is not quite accurate a heat gun might be handy for minor adjustments when installing into the frame.

I would not recommend polycarbonate for street cars because it will scratch from the slightest amount of dust during washing and certainly from a windscreen wiper if fitted.

I hope Zuffen can get his scales home because I'm leaning to the notion that the 67lbs includes frame, wiper motor, wiper blades, rubbers and struts. Or maybe I just stuffed up my arithmetic.
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My 80 924 loves me.

Zuffen · Posted: Wed Dec 18, 2002 4:36 pm Post subject:

Is there a way to de-temper the glass?

The wiper hole is drilled after the tempering? How is that done?

What if the glass is scoured front and back and then cut?
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Bob Dodd - 924turbo@cox.net
931 1982, 944 1982 euro, 924S 1988SE, 93 968 tip 06 Silver Cayenne S, 06 Black Cayenne S

I have Way too many cars, parts for the 931,944 and 951

Peter_in_AU · Joined: 29 Jul 2001 Posts: 2743 Location: Sydney, Australia

gwsg · Posted: Wed Dec 18, 2002 6:06 pm Post subject:

The wiper hole and all the edge work is done before the tempering.

When tempering, the hot glass is cooled very quickly by blowing on chilled air cooling the outside of the glass much quicker than the inside. This sets up an extremely high tension between the inside of the glass and the surface [remember glass is a super cooled liquid and as such is stabilised only by surface tension this is why annealed glass breaks so easily after scoring]. I know from sandblasting patterns on tempered glass that if you try to deep etch over about 0.25mm the glass will explode because you have released the tension.

Scoring on both sides of the glass is done to cut laminated glass and I am only assuming the hatch is tempered because of Cathy's description. If the hatch glass is laminated [2 sheets of glass with a plastic membrane between] then it can be cut or drilled. The problem with laminate is it does noy have the heat resistance of tempered and would have to be heated and cooled VERY slowly and evenly.

It is theoretically possible to detemper glass but it would have to be heated to over 550 celsius [above the annealing range] and then slowly cooled and annealed. It would be easier and less risk to make a fibreglass mould.

I still think slumping polycarbonate in the existing glass would work. I would start with the cheapest, non UV resistant acrylic and see how it fits, if OK go for a more expensive material. Just looking at how the glass fits into the frame on my car its certainly not perfect hard against the frame at the top and about 6mm clear at the bottom.

If you can pull it off, I am sure there would be some guys on this board who would want one. Not for me though.
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My 80 924 loves me.

gwsg · Posted: Wed Dec 18, 2002 6:28 pm Post subject:

Go to this site: http://www.alternativeplastics.net.au/ and look in the November newsletter for a basic comparison between the qualities of perspex and polycarbonate then dig deeper for more info.

I use highest grade polycarbonate for vandal protection on stained glass windows but instruct my guys to not even brush off any saw cut chips that may have attached to the screen through static electricity. Experience has shown this stuff scratches very easily.

I must get back to work, Christmas is looming.

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My 80 924 loves me.

Peter_in_AU · Joined: 29 Jul 2001 Posts: 2743 Location: Sydney, Australia

ahhh now this will teach me to check my facts in fields that I am not an expert in.

Here I was talking polycarbonate and thinking about acrylic as far as the scratch-resistance goes.

924RACR · Posted: Wed Dec 18, 2002 10:58 pm Post subject:

I'm the one who'd posted the 67 lbs number, and yes it is from a company's info. Agreed, they may be talking about ditching struts and wiper and 3rd brake light and such. Maybe even the rubber spoiler.

I also agree, I think it'd be easier to use the stock glass as is to form a close enough approximation of the hatch glass to be useful.
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'79 924 #77 SCCA H Prod racecar
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924_fan · Joined: 03 Nov 2002 Posts: 123 Location: Lompoc, CA; USA

I seem to remember a company in Pennsylvania (Apex?) offering the hatch for about half of the price as GT or Spektr. But their web page does not list it anymore.
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Skip Denton
1979 Sebring

Lizard · Posted: Thu Dec 19, 2002 3:49 am Post subject:

with the light weight hatches do you have to have those 2 ribs running down it as you see on alot of these cars that have the light weight hatches
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924RACR · Posted: Thu Dec 19, 2002 5:04 am Post subject:

There's usually inner braces on the inside of a lightweight windshield (I think they're called Earnhardt bars in circle-track) to brace in case a tire/rim combo hits the windshield, to protect the driver. You often have external straps that hold the windshield (plastic or glass) on the outside of the car to keep it there in case of impact - keep in mind that cars are designed so that the windshield will pop out in one piece usually in an accident, IIRC.

As for the rear hatch, there won't be a need to internally brace it, because I can't really see external loads ever putting the driver at risk. Rather, you'd expect to see the external window retention straps that were discussed above to keep it from falling out. Usually just thin strips of aluminium pinned at each end.
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

gwsg · Posted: Thu Dec 19, 2002 11:06 am Post subject:

If any of you guys are planning to have a go at it for your race car, I would check with your sanctioning body's safety regulations as there may be an issue here. When you bend double a sheet of polycarbonate it will deform and fatigue along the bend line but will not break, acrylic on the other hand will reach a point where it breaks leaving glass-like sharp edges.

It's said that 6mm polycarbonate will withstand a shotgun blast from 10 feet. This could be a very handy back window for some forum members.

Plaster moulds are used one-off in glass slumping to 800 celsius but are cracked afterwards. They should be reusable for slumping plastic if you are not comfortable using fibreglass but you would need to reinforce it with wire mesh for a mould the size of the hatch.

When cutting the plastic you will need new super sharp blades for whatever tool you use: jigsaw, cicular saw, router. I dont use a jigsaw any more because of the risk of thermal cracking due to friction [it doesn't happen every time]. Whatever tool you use the plastic should fly out as chips, if the chips are starting to melt together back off and reassess what you are doing. Any tool that works on wood wiil work on plastic so long as it is sharp.

Peter, if your going for a 10 minute drive to your local plastic people ask them is there a need for a resist to stop the plastic sticking to the mould or is it possible to bend the material with its paper covering still on as this would protect the material from scratching when trimming it to size.

Good Luck, Geoff
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My 80 924 loves me.

Peter_in_AU · Joined: 29 Jul 2001 Posts: 2743 Location: Sydney, Australia

OK, good tip. I'll ask.
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1979 924 (Gone to a better place)
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1982 924 (As featured on Wikipedia)

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924RACR · Posted: Thu Dec 19, 2002 11:56 am Post subject:

I'll post some weights just measured in a separate thread, but FYI the glass alone weighs 46 lbs by my measure.
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

CBass · Joined: 03 Nov 2002 Posts: 2807 Location: Vancouver, Canada

So do we have a consensus on the best material to use? I could go check with my friend who can get the stuff at cost.

81turbo · Joined: 03 Nov 2002 Posts: 1065 Location: Oakland, CA

Are you planning on making the light weight hatch for your Aerostar? Seems pretty useless. Maybe your hitting on a new aftermarket.