Sunday, 2 March 2008

Extruder spits out its dummy

My extruder's heater barrel jumped out of the PTFE insulator so I am back to where I was two months ago with nothing extruded except some test filament and a couple of rafts.

I drilled out the nozzle aperture to 0.5mm to reduce the pressure in the PTFE. I ran the extruder for a while at different flow rates and monitored the motor duty cycle and measured the filament diameter before and after I drilled it. Here is how the motor duty cycle varies with flow rate with different hole sizes:-

Assuming the point on its own is measurement error rather than a weird anomaly, then the torque required is proportional to flow rate plus a constant for mechanical friction, as I had discovered before. Surprisingly, reducing the hole diameter 40% and thus its area by 64% only increases the torque about 5%, which is hard to rationalise.

This is how the filament diameter varies with flow rate for the two hole sizes :-

As I found before with a 0.5mm hole, the die swell is pretty much proportional to flow rate plus a constant explained by there needing to be a minimum pressure before the HDPE flows. With the smaller hole the die swell is greater, as expected, but it levels off as the pressure increases. Presumably there is a limit to how much the plastic can compress and expand. I expect that the 0.5mm hole curve would level off as well at higher flow rates. The die swell as a percentage is about the same at the start of the graph for the 0.3mm hole as it is at the end of the 0.5mm hole's curve.

The die swell I get from the 0.5mm hole is less than it was from my previous nozzle. I think that is because the hole is now shorter.

Other things I have noticed with the refurbished extruder is that the overrun is much worse. I.e. after switching off, the filament continues to flow for longer. Perhaps this is the downside of a shorter outlet hole or perhaps for some reason the amount of molten plastic in the extruder is now greater. On the positive side the problem of modulated filament width, that was due to my pump screw bearing lands being eccentric, is now solved. The raft I managed to make (left) is a lot neater than the last raft the old extruder made (right).

Note that I have boosted the contrast, they are actually both white.

Another thing I learned was that the PTFE is ~0.5mm longer at 200°C than it is at room temperature, so I have to calibrate the z-axis while it is hot. I hadn't noticed this before but I checked the thermal expansion coefficient and this figure is in the right ballpark. The brass nozzle expansion is an order of magnitude less.

So that was it for the new extruder as the heater barrel jumped several threads on the PTFE insulator and the nozzle buried itself into the bed, which is now starting to look like the surface of the moon. The reason? Well the thread is not stripped but it is now 1.3mm too big all the way along. This is despite the fact that the outside of the PTFE tube was constrained by a copper pipe. You can see this from the HDPE left on the heater nozzle :-

The PTFE is in a far worse state after less than one hour use than the previous one which lasted hundreds of hours.

The PTFE is from the same rod and machined in exactly the same way. The pressure in the system, if anything would be less than before because the hole was the same size but not as deep. The only differences are the heater is closer to the PTFE and I had a copper pipe over the end to stop it expanding. Somehow the inside expanded uniformly, while the outside was constrained. The only explanation I can come up with is that it got too hot and melted. I was only running at 220°C when it happened whereas the old nozzle was used at 240°C. It is closer to the heater but as the brass runs inside it I can't see that would have much effect. The copper pipe on the outside may have made it a bit hotter but I am at loss to explain this dramatic failure.

1 comment:

  1. Looks like a very neat 'raft'. I keep meaning to look at reprap stuff, and this has jolted my interest again. I found your site from Hackaday.

    Nice to see another blogging engineer in the north west - I'm in Bolton.