Saturday, June 17, 2006


Testing assumptions about polymer melts...

The first assumption that I tested vis a vis the proposed filament method was whether I could melt polymer bits in a pot.

The short answer is that you can, if you put the pot in an oven where it gets heated evenly. The leftmost sample is caprolactone whilst the right is HDPE. The HDPE sample was brownish because the bits used in the melt were cut with a rusty saw. It makes for a nice marbled appearance. The caprolactone was heated for 30 minutes at 130 degrees Celsius while the HDPE was also heated for 30 minutes but at 160 degrees.

The corelle ware cup at the top right was the crucible for the melts. Corelle ware has the advantage that plastics no not adhere to it.

While I was at Santa Barbara yesterday and today I asked myself whether if you simply powered a capillary viscometer instead of putting a weighted piston on top of the polymer melt if you couldn't efficiently make filament.

The numbers seem to work. A polymer melt can easily be done in a cylinder with internal dimensions of 50 mm diameter x 100 mm height. That much melt, converted to filament, would let the Mk II operate for about 70 hours at 4 mm/sec.

I am going to go look at plumbing and piping parts this weekend and cobble something together. :-)

I melted various plastics in my oven a month or two ago.
HDPE seems to oxidize. It is easy to get it to melt but it doesn't liquify to the point where the bubbles come out. Heat it too much (in air) anf the clear melt looks brown but on cooling this is all on the surface.
The bubbles come to the surface pretty well if you leave it in the oven long enough. :-)
I left it quite a while. Most of the bubbles did come out but the plastic was chocolate brown on the surface.
Even one small bubble will ruin a filament.
Polystrene and poly-prop had similar problems but not as bad.
The only one that worked well was LDPE.
PET turned white because it reacts with water.
One obvious difference in what we've done is I use 2 ltr milk bottles as my source of HDPE. They are well washed and de-labeled and cut with clean scissors. If you used a hacksaw you probably aren't using bottles.
Bigger chunks will settle better than the flakes I have. All the plastics may have additives.
I turned my piece and it machines well. It looks like really good stuff.
One common way to degas a viscous liquid is to pull a vacuum on it several times. This might also help to pull additives/free monomers out of the plastic, if they're volatile, and could even help keep the PET from being degraded. Does anyone have a setup that would allow them to pull the air out of their crucible? It could even be a hand pump, since really low pressure isn't necessary.
That wouldn't work in my case because they aren't all true bubbles. There are open celled pockets. The flakes melt easily but it is hard to get beyond a "chewing gum" stage. The limp bits of plastic just want to stick to the sides and each other and not form one puddle.
I do have a couple of vacuum pumps.
For filament it may be possible to heat the plastic near the extruder more than the suface.
On the PET front I read stuff which said the commercial systems run high temp desicators for several hours to remove the water.
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