Back up and running?

I rebuilt my extruder again and this time it lasted long enough to complete a test object so hopefully I can finish my research into HDPE FDM before moving on to PCL and ABS.

I replaced the 12mm diameter PTFE barrel with the recommended 16mm. Rather than make a new clamp I turned down the top end to 12mm.



I also replaced the woven insulation I was using with PTFE insulation. This is good for 250°C, which is fine for the thermistor but still a bit low for the heater. With this heater I brought out the nichrome tails which probably get hotter than the covered part of the winding. I think I prefer the way I have made my other heaters, which is to put the connections to the copper wires under the heater insulation. That way the copper wires never get any hotter than the body of the heater.

I put a pipe clip round the end of the PTFE to compress it against the screw thread. In an attempt to find out why my previous PTFE barrel deformed so much I made some temperature measurements with a different thermocouple to the one I used before, just in case it was faulty.

These are the temperatures I get with my software set to 200°C :-



The control of the nozzle temperature is very good, +/- 1°C. The other measurements show just how good an insulator PTFE is compared to the soapstone I mentioned in my previous article.

I think my problems stem from the fact that the heater barrel is quite a bit hotter than the nozzle. With the fan on, cooling the nozzle, the temperature difference will be even higher. It must have reached the point where PTFE starts to melt. I will try extruding without the fan from now on as I think that is what causes the PTFE and J-B Weld to give up. I might need inter layer pauses.

Compared to my first attempt at the extruder I have made the following improvements :-

• The steel cable for the flexible drive is now the recommended 3mm rather than 2.5mm.
• The drive screw has been replaced with one that has correctly centered bearing lands. This completely fixes the modulated filament I was getting before.
• The springs are much stronger which means I don't need to tighten them as far, making assembly quicker.
• The lock nuts on the studding have been replaced with plates which also make assembly and disassembly easier.
• The PTFE barrel is now the recommended 16mm rather than 12mm.
• The PTFE barrel is pinned into the clamp rather than relying on friction alone.
• The heater barrel is held into the PTFE with a pipe clip.
• The nozzle is now removable and has a shallower and tapered exit hole.
• The thermistor is closer to the heater so my on off control cycles much faster and keeps within +/- 1°C compared to +/-3°C with my one piece nozzle.

So far I am finding that without the fan I need to extrude slower to get the same results I was getting before.

Cerastil and soap stone

As a bit of light relief from continually repairing my extruder I decided to have a play with the Cerastil H-115 high temperature cement that I have bought.



The minimum quantity that I could buy was 1Kg, which cost aver £100 including shipping and VAT. You only need a couple of grams to make an extruder heater so it is actually cheaper than things like J-B Weld.

It is labeled as a hazardous substance and comes with a material safety data sheet which says it can't be disposed of in domestic waste and must not enter the sewage system. The hazardous components are identified as potassium silicate and sodium fluorosilicate. When I looked them up on the web I found that the former is added to growing medium and in cosmetics and the latter is one of the chemicals added to water for fluoridation. So they don't seem very hazardous but I suppose it's a matter of concentration.

I am assuming that once it has been cured, by the addition of a little water, that it is then no more hazardous that a ceramic potted resistor like this :-



We are no longer allowed to put electronics in domestic waste in the UK but you can just take it to the local tip.

I masked a brass heater barrel and applied a thin layer.



I left it to set for 24 hours and then wound it with two strands of 0.1mm nichrome twisted together. That gives me just 110mm for 8Ω, to keep the heater short. I attached copper wires with high temperature solder and then put a thicker layer of Cerastil over the top. I then left it another 24 hours to cure.



It looks a bit lumpy because of the solder joints underneath.

I mounted it in an insulator that I turned from soapstone and ran it for a few hours at ~290°C.



The bottom of the soapstone barrel got to about 120°C. After the test the Cerastil looked exactly the same, unlike J-B Weld which goes very dark. The soapstone did discolour though at the hot end.



So where has this experiment taken me on my quest to make a durable extruder that covers the full range of thermoplastics? Well I will definitely be using Cerastil from now on as it seems the perfect adhesive for potting heaters, not surprisingly as that is what it is designed for. It is a high temperature adhesive that is a good electrical insulator and a good thermal conductor. I am not sure I can recommend it for the RepRap project though because it is very specialist and not widely available

I am also not sure about the soapstone. I was surprised it changed colour but I don't know if it matters or not. It looks like it would need to be twice as long, or have a heatsink at the cool end. I am also a bit worried about its strength.

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.