[Beowulf] materials for air shroud?

Prentice Bisbal prentice at ias.edu
Wed Sep 14 06:02:21 PDT 2011


Instead of a soldering iron, have you tried using just an old clothing
iron? That should have a more adjustable temperature setting. And once
it's heated up, you can fold it like origami paper. (I challenge this
list to produce an origami crane this way!)

I would keep a large bowl of cold water nearby, so that after you make
your folds, you can dunk your finished product in the cold water to
thermoset the plastic.

For cutting the plastic, I highly recommend these Wiss MultiMaster
multi-purpose shears:

http://www.cooperhandtools.com/brands/CF_Files/model_detail.cfm?upc=037103126481

I bought them to cut up a rug, thinking I'd never use them again. They
are now probably one of the most used tools in my toolbox. They cut
through everything: sheetmetal, hardware cloth, zip-ties, obnoxious
plastic packaging - you name it! Since they have a compound action, you
don't need to use a lot of force, which makes it easier to control your
cuts. I think I paid $12-$15 at Lowe's for mine, and they're worth every
penny.

On 09/13/2011 04:05 PM, mathog wrote:
> 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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