Friday, May 15, 2009

 

Small Melt Zones Look Good


The difference in operation of the no-lathe extruder with different barrel lengths and a heatsink leads me to suspect that the main factor in reducing friction is to reduce the size of the zone where the plastic is actually melting.

As you can see in the photos I've chopped an old CPU heatsink (easily salvaged or made from copper/aluminium stock) into 10mm slices, and clamped a pair around the barrel. There is a recess in the middle of the two halves made with a drill 0.5mm smaller than the barrel's external diameter. I have experimented with moving it up and down the barrel. The barrel above the heatsink does nothing useful in this setup, but might be useful for guiding the filament later.


The heatsink stops the plastic melting on entry, keeping the top part of the barrel under 60C. I've rewound the heater to start the coil as close as possible to the nozzle, and to keep the windings close together. Kapton tape is great for this!

Best results definitely seem to be had with a small, powerful heater ([email protected] max, running at about 60%) and the heatsink placed as close as possible to the heater windings. This is much more successful than my earlier experiments with heatsinks.

The extruder works continuously, or in bursts, and I checked that it will restart after cooling. It still needs no lathe!

Vik :v)

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Comments:
Nice work.
I'm going to dig out my old CPU heatsink box and build one too.
 
A heatsink might make the old granule extruder design viable.
 
This comment has been removed by the author.
 
You don't need a heatsink anywhere near that big. I kept the heated zone of my old 60 mm extruder barrel down to 20 mm at the business end. A simple brass mounting plate at the other end of 25x50 mm kept the temperature at that end at 80 C.
 
Yes but ABS starts to go soft at 80C, so it would swell and fill the hole length of your barrel.

Did you try any ABS and leave it resting long enough to jam?

My guess is that it would jam like the soldering iron I tested and like the stainless steel barrel I tried.

The transition zone has to be only a few millimeters in my experience, or else made from PTFE. No other combination has worked reliably for me.
 
"Yes but ABS starts to go soft at 80C"

I was using HDPE.

"Did you try any ABS and leave it resting long enough to jam?"

I ran some ABS through it, but my polymer pump at the time was right at the edge of being able to supply the force necessary for ABS and polypropylene.

There may have been a jamming issue, mind. With the much more powerful pump that you've confected with the GKM-17 hack, however, I don't think that is going to be much of an issue in future, hopefully. :-)

"The transition zone has to be only a few millimeters in my experience, or else made from PTFE."

As I mentioned earlier, I'm wondering whether your problem with the transition zone is made better with the rather slick inside surface of the 5/32 inch copper tubing that I'm using for an extruder barrel.

It's certainly going to be rather fun to find out in the next few weeks. :-D
 
ABS requires far less force than HDPE to extrude, so if you found it took more then I suspect you were suffering from the effects of the long transistion.

I don't know if the inside of a copper pipe is any more sliperry than stainless steel.

The other thing that makes a huge difference is having a taper in the transition zone.
 
"ABS requires far less force than HDPE to extrude, so if you found it took more then I suspect you were suffering from the effects of the long transistion."

You may well be correct, however, keep in mind that I was operating my extruder barrels at much lower temperatures {~185-190 C} than you subsequently discovered were optimum. That may have made a difference, too. :-)
 
Yes ABS gets very hard to extrude below 195C. I think HDPE has a more gradual change with temperature.
 
Once I finish repstrap baby this weekend.
I will mill the plastic drive block then turn the peek heat barier on the lathe.
Then using the longer 0.5mm copper welding tips from B&Q £3.42 I will wind the insulated nichrome directly on to the welding tip cover it with the DX high temp tape and give it a go.
 
i never thought about using mig welding tips before. thats not a bad thought.
 
I wondered if gas jets might also work as nozzles?

Hmm, I need someone to look over my gas stove anyway. One of the jets needs replacing 'cos it's not big enough ... I Might have an opportunity here :)

Vik :v)
 
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