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Re: [EVDL] windscreens from plastic

I remember seeing a website (the wissel? somewhere from the Human
Powered Vehicle Site) on this but don't know about finding a link, the
guys were doing this and selling the parts.

The took two plates and in one they cut an oval opening and the other a
threaded hole for an air line. This had clear holes drilled every inch
around the oval opening and threaded holes in the air plate. I think
they made the plates out of aluminum ($$, but productive and safe)

They made a 4 walled chamber with those fireplace panels you get from
home depot and but a 2 burner coleman camp stove in the middle with a
straight pide out the wall for to the gas bottle. They placed a 1" steel
plate across both burners.

They drilled a hole to look through at the appropriate height and pot a
line on the opposite side.

To make a unit they sandwiched a piece of Vivak (tradename of sheet
PTEG, same material used to make plastic coke bottles)

They turned on the stove and closed the ends of the box. When the Vivak
got soft and started to sag, the operator opened the air valve which
blew a bubble.
They did this in steps. Let it warm until it shows sag, apply a little
air, repeat.
When it got to the line, they turned off the stove and opened the ends
of the box while watching and maintaining the air pressure needed to
keep it's shape(as the air cools it contracts and may allow the bubble
to distort, they made up for this pressure reduction manually)

In high school, I made parabolic reflectors for strobelights (it was
1980 after all)
We had a wooden mold with holes thru it and a vacuum pump on the back, a
platen above the mold with clamps and electric strip heaters.
In this setup we clamped a piece of acrylic sheet, turned on the heaters
and when the material started to sag pulled a vacuum.
In this project timeing was everything. Pull vacuum to soon and the
parabolic would only pull 1/2 way and the heat no longer could re-melt
the center that is furthest away without burning the edges. Wait to long
and it would loose it's taffyness and tear a hole as the heavy center
ripped away from the edges that thinned to fast from being to soft and
pulled on. I think using polycarbonate or the PETG would make a more
consistent process than acrylic.

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