[Beowulf] materials for air shroud?

mathog mathog at caltech.edu
Tue Sep 13 13:05:30 PDT 2011


FYI,

Some sheets of 1/16" polypropylene were obtained from smallparts.com.  
(This is very similar or identical
to the case plastic used on DLT and other larger tape cartridges.)  So 
far this material is
relatively easy to work with.

Plus:
1.  Can be cut with a (big) pair of scissors.
2.  Can be bent by hand
3.  Melting point >130C
4.  Can be welded with high temperature hot glue (not verified)
5.  Rigid enough for a duct (but not for anything load bearing)

Minus:
1.  Holds a static charge.  (Probably not more than any other plastic 
though.)
2.  Few adhesives stick to it.  Generic brown masking tape holds pretty 
well.

Some details

I tried bending a sample two ways.  First, just putting it on the edge 
of a desk and then
folding it.  That worked but it turned white and thinned somewhat at 
the bend, and it
slowly opened back up again from 90 to about 100 degrees. That edge was 
then melted by
gently rubbing it with the barrel of a soldering iron.  (Find the point 
just below where
it smokes).  After remelting the edge was once again clear and the 
angle stayed firmly at 90.
Another 90 degree bend was made by first heating the flat plastic on 
both sides with the
soldering iron barrel and then bending.  That edge turned out a little 
better, the thickness
of the plastic through the turn was very close to that of the flat 
parts.  Hard to get it to
just the right temperature though, so there was some smoke.  Either 
method made a good enough
90 degree bend for an air shroud.

Finally, I tried gluing two pieces at right angles using a high melt 
hot glue.  The hot glue gun
claims to run at 395F, and the glue stick was nothing special, just 
generic high temperature
hot melt.  Mixed results.  After it cooled and was allowed to set 
overnight I tried to
tear the two pieces apart by hand, pulling in opposite directions, and 
they held together.
However, I was able to snap the pieces apart by folding it at the 
junction.  (Applying quite
a lot of torque to the junction.)  The glue completely let go of the 
top of the "T", all of it
stayed on the vertical part.  At this point it was easy to peel the 
rest of the glue off.
Seems like the bonding was good perpendicular to the surface, but 
pretty weak
parallel to it. If the piece wasn't physically abused it would likely 
hold together in an
air shroud.  I had read somewhere that the hot glue melts the 
polypropylene so that it was
effectively a weld, but that is not how it turned out with this glue.  
Neither piece of plastic
was distorted where the glue had been, so clearly not melted.  There 
are specialty hot melts made
of polyethylene or polypropylene, and those may actually weld this 
material.

Regards,

David Mathog
mathog at caltech.edu
Manager, Sequence Analysis Facility, Biology Division, Caltech


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