Sunday, January 07, 2007


Extruding polypropylene

My son got back from his holiday with his mum and gran in Texas last night so this morning I got him out of bed with a good breakfast and then got him to film the extrusion and demo of extruded HPP (homopolypropylene).

The extrusion is taking place using my low thermal inertia extruder design with a 0.5 mm orifice and producing a 0.8 mm polymer thread. It is being run at 12 v and 0.95 amps. I was able to measure the temperature of the copper collar braised to the back of the extruder at 90 degrees Celsius during this exercise. I can make the HPP extrude while the collar is as cool as 70 degrees. I am hand feeding 3 mm HPP filament using pliers to grip the filament. The filament is a bit slick to hold onto with fingertips alone.

You can see a video of the extrusion here. In the video clip you can see the extrusion oscillating laterally at times. This occurs when I put too much pressure on the HPP filament. When I stop putting the little pressure that I am on the filament (estimated at about half a kilogram) extrusion stops immediately. I have yet to detect any dribbling whatsoever with HPP.

HPP is a robust engineering plastic. It handles bending and fatigue extremely well. A short video of it's behaviour demonstrates this characteristic.

Where might we get our hands on more of this, preferably in spool form? Mmm, actual plastic extruder. Much quieter than the dremel scream I'm starting to get used to
You can get it from pretty much any supplier of plastic welding rod. You can also get it on spools. No problem. $5-10/lb depending on who you buy from from what I've seen.

Yeah, Dremels are loud. I also wonder how many hours you can run one continuously before it overheats and goes to Dremel heaven.
Between all the testing it's been going at least 5 hours now, with no problems.

Of course, with the dremel tool repstrap you also get lots of sawdust that can theoretically clog up the dremel, too, but the air tends to be drawn from the back of the tool and eject it out the front, which actually helps somewhat with keeping it clean.
You need to keep a rough log of how many hours you've run your Dremel so that eventually we'll get a handle on it's mean time to failure.

I'm doing the same thing with these solarbotics gearmotors I'm using. The old 6 v motors that Solarbotics was using could do three months under load as a MTTF. The 12 v motors that I'm using now are supposed to be built a lot more solidly and are supposed to last a lot longer.
Is there a difference between the polypropylene used here and the stuff they make margarine tubs out of (recycling symbol is "PP")?
It's pretty much the same stuff.
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