[Greenbuilding] Tweaking a band sealer

[nick pine]

Marcus Sadler and his crew have made 34 4'x8' glazing panels with 2
10 mil HP92W polycarbonate films with a butyl rubber tape seal in
a 2x2 sandwich with a Schraeder tire valve. They are gently inflated
with propane now for leak testing with a sensitive gas leak detector.
They will be inflated with welding argon soon. Each panel required
about 12 person-hours of labor...  

We hope to reduce the labor content with a $400 tabletop Chinese
band-sealing "fusing machine" with a 500 watt heater and a digital
temperature control and 2 Teflon belts with a speed control:
http://www.simplesealers.com/fr900-horizontal-heat-sealer.html

It's intended for sealing plastic pouches which pass between 2 offset
rollers. The 1/4" Teflon belts drag the edges of 2 plastic films
between 2 1/2"x1/2"x4" brass heater bars with a 0-300C bang-bang 
temperature controller (polycarbonate melts at 267C.) Then the films
pass between 2 brass cooler bars about 3" long and 2 more unpowered
compression rollers with an adjustable spring. 

Sometimes we get good results, with an airtight seal that cannot be
pulled apart, but the results vary a lot. The DC motor stopped once
in a while and the machine went dead and wouldn't start again until
we turned the power switch off and on. And it slowed when the heater
turned on and the voltage at the end of the tiny power cord wires
dropped from 120 to 114 V, so we rewired the cord and the rest of
the machine and fixed a few bad solder joints.

Now it seems that most of the variability comes from the cooler bars,
which are not required for polycarbonate. They are only heated when
hot plastic film passes between them, so their temperature increases
during long seams (we would like to make 8 foot seams.) We plan to
replace them with another $39 set of heater bars and another $25
temperature controller, which seems doable, although it requires
shortening the new heater bars, which are hollow brass boxes with
uncoated wirewound resistors inside. 

The films also tend to pucker a lot between the 1/2" seam and 
the edges about an inch further outboard, and the seam smears along
its length. Powering some of the unpowered pairs of rollers and
making them pinch the films at right angles instead of offsetting
them might make for a nicer seam.

Nick