Saturday, 1 September 2007

Caught in the act

You may remember that I reported a ribbon of swarf coming out of the side of my extruder :-

It wasn't obvious to me how this was formed. When I stripped it down today I caught it in the act :-

It appears that when the threaded rod cuts into the plastic it displaces a corrugated ribbon of material sideways. This remains attached to the filament at the leeward side and the ridges formed by the thread remain joined to each other by very thin webs. As it progresses down the pump it gets separated from the filament, presumably where it enters the barrel, and finds its way out through the side. I think the root cause is that when polymers like HDPE are stretched the long molecules get aligned length ways and it becomes very strong even though it is very thin.


  1. Interesting! Mine hasn't done that. When I get the spring tension too tight the threaded rod just grinds off the side of the filament into dust.

    I'm wondering. The HDPE that I'm using has white pigment in it and isn't translucent like yours. It looks like toothpaste when it extrudes.

    What temperature are you heating your extruder barrel to? Is it insulated? How many amps are you putting into it and what voltage?

    At a guess, I'd estimate that you're running your extruder barrel too cool.

  2. Strange, mine grinds the side into dust when I have the tension too low.

    My HDPE is completely transparent when it comes out the nozzle and returns to translucent one or two centimeters below. I have not tried landing it yet.

    My default temperature is 200C measured with an IR thermometer pointing at the JB Weld. I achieve that with just over six watts I think. The heater is 12V into about 8.6R so about 17W. It only takes 9W to get to 250C, which is has high as I have let it go. It has plenty of power in reserve giving a fast warm up time of 90 seconds. Significantly more power is needed when it extruding quickly, I have yet to quantify that.

    I will publish some more detailed figures over a range of temperatures and flow rates when I have hooked it up to the Python script.

    The heater barrel is not insulated at the moment but it is quite short, only about 30mm is exposed, and most of that is covered by JB-WELD.

    The official design is now 6R with insulation but they waste a couple of volts with a Darlington so they sill get about the same power as me. I think the insulation is needed when you have a fan blowing on the workpiece. I don't know if that is required yet for HDPE.

  3. What size extrusion orifice are you using?

  4. Interesting. We certainly don't get that with polycaprolactone. At Bath we haven't had enough experience with HDPE yet to tell how that goes.

    I wonder if the friction between the HDPE and the back of the extruder is too high? Would it be possible to hit it with some spray-on PTFE and see if that helps?

  5. Yes I was worried about the friction being higher with my metal filled resin material, and initialy it certainly was. After milling it I sanded with fine emery, polished it with metal polish and then sprayed it with PTFE. It is actually quite slippery now but under the enourmouse spring force there is probably still a lot of friction. I think the main resistance is in the barrel though. My HDPE is a little oversized and oval in places so I drilled it 3.2, but I think 3.3 would have been better.

    When I set the spring tension less it will extrude for a while and then it jams and strips the thread, presumably when it gets to an oversized section of filament.

    With my springs almost fully compressed it runs reliably but is cutting deep into the filament as you can see. Still there is no sign of the motor clutch slipping like Forrest reported, the HDPE always gives way first.