HDPE + PU

Because my test objects are less warped while they are still attached to the polypropylene bed, I had the idea of filling them with something that sets hard to freeze them in that shape. That would also allow me to use a sparse fill pattern, which speeds up the FDM build time, but still get a strong object.

I needed something that was not too viscous so that it would flow in between the mesh of the fill pattern and would set hard.



Polyurethane was recommended to me because it has the consistency of milk before it sets and is strong enough to cast parts for Darwin. I bought some Smooth-Cast 300 which has a pot life of 3 minutes after it has been mixed, and cures in 15 minutes. I choose a fairly fast setting one because it gets hot while curing and I hoped it would soften the HDPE to relieve the stress. It only seems to get to about 50°C though so I don't think that it has much effect in that way.

This is the equipment I used :-



I know the internal volume of my objects pretty accurately so I measure out the required amount of plastic using separate labeled syringes for the two components. I mix it in a small pot before filling a third syringe to inject it. The syringes and pot are made out of polypropylene, which polyurethane does not stick to, so they can all be reused. I haven't found a way of unblocking the needles though.

I made a 50% filled object and drilled a hole the diameter of the needle in the middle that allowed the needle to go to the bottom. I also drilled a small riser hole at each end to let air out. Obviously, with cleverer software these holes could be made during the FDM phase.

The first attempt was a complete failure because the needle blocked when the object was only about 50% filled. Here is a cross section :-



For my second attempt I used a thicker needle, 1mm OD rather than 0.8mm :-



The object filled OK, but just as it became full the plastic in the needle set suddenly but I carried on pushing. The needle popped off the end of the syringe and PU sprayed all over the place. It was a good job I was wearing goggles and gloves but I should also have been wearing long sleeves, a mask and a hat! Fortunately PU does not stick to much, only untreated wood, skin and hair! Where it gets on your skin it burns slightly. Because it is transparent before it sets it is very hard to see where it has gone but when it sets it turns opaque white so it becomes obvious.



It actually sprayed around one quarter of the room. I even got some on my lips which I didn't notice until I tried eating.

What seems to happen is that if you subject the liquid plastic to pressure it accelerates the curing, which increases the temperature and pressure creating a positive feedback effect which makes it set suddenly in the needle. I only had two needles and they were now blocked so I did the remainder of my experiments using just the nozzle of the syringe into a bigger hole in the object.



The ideal solution is probably a very big needle that locks onto the syringe. It doesn't need to be sharp but the 45° slant at the end is handy because it stops the end being blocked if you press it against the bottom of the object.

Results
I left the objects on the bed overnight to make sure the PU was fully cured even though it sets in 15 minutes. The first object I made had a 50% fill and warped 0.36mm compared with 0.47mm without the PU injection.



Thinking that 50% fill leaves the PU fairly weak, I did another test at 25% fill. That gave 0.24mm warp, the lowest figure I have achieved yet for this shape.



I also tried a 100 x 10 x 20 mm test with 20% fill ratio. That gave about 50% less warping compared to the version without PU.



Conclusions
A useful technique for reducing warping and reducing the build time of FDM objects. The main disadvantage is that FDM is one of the cleanest and safest fabrication techniques whereas injecting PU is messy and somewhat dangerous unless you wear protective gear.

I was disappointed not to get rid of the warping completely. Instead of alternating the horizontal and vertical fill patterns, several layers of one followed by several layers of the other might make the PU lattice stronger. Raising the PU to 50°C for a few hours is supposed to harden it further, so I could try removing the bed and putting it in a very low oven for a while. I have a Peltier effect 12V beer fridge which can be reversed and used as an oven, so that would be ideal.

Using a harder plastic like epoxy might work better but it may be too viscous to inject. I believe heating it reduces viscosity.

Reducing HDPE warping feels much like banging ones head against the wall so I will try PCL and ABS next for some light relief.

Infill and warping

Now that I can create blocks with different infill densities I decided to experiment to see what effect it has on HDPE warping.

I have been using a standard test shape and a jig made of three nails to make comparative measurements.



I measure from the middle nail to the base with a pair of digital calipers and subtract the distance to a rule placed across the nails. The figure I get is an average of the amount each end warps upwards. Not very precise because the base is warped the other way as well.

The block is 40 x 10 x 20mm because you need about 40mm length before the warping becomes big enough to measure and 20mm height is about where things start to straighten out. Bigger shapes warp more but obviously take a lot longer to make. Each one of these takes about an hour including making the raft, extruding the block, separating it from the base and measuring it.

The block is held flat while it is stuck to the bed of the machine by the raft. It warps when I remove it. I have only recently noticed that it warps even more when left overnight, so some of my previous tests are not that accurate. For example I was quite pleased when I first produced this extruder sized block :-



But here it is again photographed some days later :-



Not easy to compare because of the angle but the uplift at each end probably increased from about 0.5mm to 1mm. It implies to me that HDPE creeps when under prolonged strain, not a very good engineering property. That is the main reason PTFE fails in the extruder.

I made the test blocks with different infill densities and left them overnight before measuring them :-



Here are the results: -

Density	Warp
20%	0.44 mm
25%	0.79 mm
33%	0.47 mm
50%	0.47 mm
100%	0.53 mm

The 33% value looks totally anomalous but that is because I tried a thicker base. Its base is 3mm of 100% fill including the raft, whereas all the other tests begin the sparse fill on the first layer above the raft.

I also tried 1mm filament 50% fill which gave 0.42mm warp showing that not stretching the filament does not give any improvement.

Conclusions: well sparser fill reduces the warping slightly. A thicker base, rather than resisting warping, actually contributes to it. I must point out that once you get less than 50% fill the object is considerably weaker than a solid block.

Finally here is a longer example, which illustrates how warping gets worse the larger the object is. This is 100 x 10 x 20mm with 20% fill. The first time I made it it lifted the raft away from the base. I got round that by increasing the raft temperature by 10°C to get a stronger weld. It was then quite hard work removing it and it caused some damage to the PP bed.



The 40mm section in the middle is only warped by 0.19mm but the ends are well over 1mm. That shows that you cannot compensate for the warping with a crowned bed because it is not a constant curvature. One could probably scan the shape of the base and lay down support material with the inverse curve. I expect it would then pull itself flat.

In my next experiment I will try filling the sparse blocks with polyurethane two part thermoset plastic.

Hot maths

As the power lost through the stainless steel barrel in my previous post seemed very low I decided to calculate it as a sanity check. I ignored the heat lost from the barrel by convection and radiation. They may be significant now but when I insulate it they shouldn't be.

The outside diameter of the tube is 6.35mm and the bore is 3.6mm, so that gives a cross sectional area of 2.15x10^-5m². Its length is 0.05m. The temperature difference over that length is 240°C-50°C = 190°C. The conductivity of stainless steel is 17 W/mK. So the heat flow is 17 x 190 x 2.15x10^-5 / 0.05 = 1.39W. That means it isn't very much compared to the total power required, so that matches my observation.

The amount of heat flowing into the heat sink is therefore 1.39W and it raises its temperature by 30°C, so the heat sink would have to be 22 °C/W. It was just a scrap one I had laying about so I don't have a spec but it is only 70 x 25 x 20mm so that seems in the right ball park.

Sanity checked!

A high temperature extruder?

The standard RepRap extruder can't quite handle the temperatures for HDPE for very long. I have found a high temperature replacement for J-B Weld. The main weak point remaining is the PTFE thermal barrier. PTFE is an excellent thermal insulator but it is not very strong mechanically. It also expands by about 0.5mm at 225°C. Worse than that it seems to slowly creep the more I use it, which makes a mockery of my z axis calibration. Since I got it working again I have re-calibrated it four times and each time it has grown: 0.3mm, 0.2mm, 0.15mm and 0.3mm. I.e. it is now 0.95mm longer than when I built it and a further 0.5mm when it is hot.

I have come to realise that stainless steel is quite a poor conductor of heat compared to other metals:-

Stainless Steel	Brass	Aluminium	Copper
17 W/mK	109	250	400

I bought some stainless steel pipes on eBay that have an outside diameter of 6.4mm and an inside diameter of about 3.5mm. I cut a 50mm length, tapped it and screwed in into a medium sized heatsink. I tapped the other end and screwed in my experimental high temperature heater. I applied heatsink compound to both threads.



I put a thermocouple in the heater and adjusted the power to get 240°C inside the brass part of the barrel. That only required 7.3W. I put another thermocouple at the top of the stainless steel barrel and that only reached 50°C.



Although this is just a lash up, it looks really promising. I can get the temperature even lower by using a CPU heatsink or a small fan. I will make a nozzle out of aluminium or copper with a built in heater and thermistor.

Not only will this stand temperatures up to the limit of the thermistor, which is 300C, but it is also much more rigid and does not change in length significantly with temperature. It should also reduce the amount of molten plastic because of the thermal gradient down the SS barrel. That should give less extruder overrun.

Filling in

I have been experimenting with various infill patterns. Here is a 40 x 10mm block made with 0.5mm filament at 50% fill: -



For simplicity I used alternating horizontal and vertical lines rather than diagonal. The layer height is 0.4mm so the width is about 0.6mm and so are the gaps. A couple of things that weren't obvious to me at the beginning were: -

The first and last lines of the fill must be adjacent to the outline so that the U turns on the alternate layer above have something to rest on, otherwise they curl upwards or downwards and don't bond to the outer skin. That means adjusting the gaps slightly to make the overall width correct. When the fill is 100% I adjust the filament width slightly to exactly fill the interior. Easy enough with a rectangular object but probably not with an irregular polygon.

The fill lines probably should line up with the those two layers below so that the intersections form a solid column of filament from top to bottom, otherwise some sag may be expected. Again trivial for rectangles but could get tricky to generalise.

Here is 33% fill, i.e. the gaps are about twice the filament width: -



This is 25%. Notice how, although the filament is laid down in a perfect square wave, when it shrinks it pulls itself to the first harmonic. A physical low pass filter!



And here is 20%: -



I found that when putting a lid over the top it struggled with an infill this sparse, so I settled on 25% as the limit for making closed boxes.

All the above are done with filament stretched to 0.5mm. When extruding through a 0.5mm orifice, left to its own devices the filament would be about 1mm due to die swell. I decided to try the same pattern with 1mm filament, i.e. with no stretching: -



As you can see the filament holds the square wave better but what is not obvious is that without stretching it sags a bit in the gaps where it is not supported from below. So some stretching is beneficial, when it comes to spanning voids, but it does increase corner cutting.

As I mentioned before, with my old nozzle, I could extrude 0.5mm filament at 16mm/s. This is what happens with the new one which has an exit hole which is too shallow: -



One unfortunate characteristic of FDM is that errors tend to be cumulative. What I mean by that is if, for example, the U turn of the zig zag fails to bond to the outer wall then that causes the next layer to have nothing to rest on, so that fails as well. The defect then propagates all the way up the object. With 100% fill, any errors tend to have less effect on the layers above.

Rather than slow down my experiments I decided to go to 0.75mm filament at 7mm/s until I make a new nozzle. Here is a 50% fill: -



I also added a bit of overlap between the fill and the outline at the u-turns to get a better bond.

So does the infill density affect warping? I made several test blocks and it looks like the answer is not much. However, I have come to realise that the warping takes hours to fully develop after the object is removed from the base so I will leave them overnight before attempting to make measurements.

The pros and cons of nozzles

HydraRaptor seems to be running reliably again, touch wood. I did have one scare when it started making noises like a machine gun when I had left it running unattended. It turned out that the shaft encoder code wheel on the extruder motor had fallen off. That caused the firmware to think it was far behind and so it applied maximum power in an attempt to catch up, which caused the GM3 gearmotor's torque liming clutch to slip. I added it to the list of sanity checks to put in my extruder firmware :-

• If the shaft position gets more than, say, half a turn behind then give up.
• If the thermistor resistance is too high then the thermistor is open circuit so turn the heater off.
• If the thermistor resistance is too low then the thermistor is short circuit.
• If the heater has been on for more than 5 seconds and the temperature has not risen then the heater is open circuit.
• If the heater has been off for 5 seconds and the temperature has not dropped then panic, the transistor is short circuit.

All these checks are necessary for safe unattended operation in my opinion.

The solution to the code wheel problem was to extend the shaft of the GM3 with a piece of brass rod :-



I have managed to perform quite a lot of tests with HDPE and it is clear that the new acorn nut nozzle behaves quite differently to the previous one piece design.

The original nozzle looked like this and had a 0.5mm hole that was about 0.6mm deep: -



The new nozzle is made from an acorn nut turned to a point. I also has a 0.5mm hole, but it is tapered at about 45° so the the part of the hole that is 0.5mm diameter is very thin :-



The differences this seems to make are: -

1. The die swell, i.e. the amount the filament expands from the hole diameter, is a little less.
   
2. The amount of filament that extrudes after the motor is switched off has increased quite a lot. The excess is wiped from the nozzle, but by the time the head has moved from the brush back to the workpiece, a few more mm have leaked out making for a messy line start. I think this is because the shorter exit hole makes it easier for the plastic to escape.
3. If I move the head quickly with the extruder off, then the filament snaps. It quite often leaves a blob that sticks to the workpiece. With the longer hole it stretched to a long thin string rather than snapping.
4. I used to be able to lay down 0.5mm filament at 16mm/s by stretching, but now I can only do this reliably at 8mm/s as the filament has a tendency to snap. I think it is too easy to pull it from the new shaped hole.
   

When stretching the filament it has a greater tendency to cut corners. I think this is mainly due to not running the fan, but may also be because the nozzle is too pointy. A wider nose will help to push the corners down.

I can't run the fan because the heat loss from the bigger nozzle causes the heater to work harder, raising the temperature of the barrel above the point where the PTFE distorts. I need to insulate the nozzle so I will try making a new one similar to this one with a PTFE cover over it.

Here is about where I am at with extrusion quality: -



This is a rectangular block about the size of the extruder pump (60 x 20 x 15mm), with a 50% fill. I forgot to put a top surface on it but that is perfectly possible. It was extruded at 220°C (measured at the nozzle) with filament stretched to 0.75mm at 7mm/s. The layer height is 0.6mm and the pitch is 0.9mm. Some warping still evident but it has come a long way from my first attempts.

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.

Too much pressure?

Well my rebuilt extruder didn't last long enough to even make a raft!

I calibrated the Z origin yesterday but when I started extruding today the nozzle ploughed into the polypropylene bed. Thinking I had made some mistake I calibrated it again and it did the same thing. The PTFE barrel can no longer slip in the clamp because it is pinned. The heater barrel can not slip out of its thread because I have a metal ring around the PTFE to stop it swelling. What seems to have happened is that the PTFE barrel has elongated.



It has also bent somewhat. The last PTFE barrel did not elongate significantly but it did swell. I can only think that because I have reduced the nozzle aperture from 0.5mm to 0.3mm the pressure has increased. I didn't notice much change in the motor current though.

I am not sure what to do now. I could make a thicker PTFE barrel but I will have to make a new clamp, which means converting my machine back to a milling machine, or I could drill the hole out to 0.5mm.

A 0.5mm hole gave me ~1.2mm filament which I stretched to 0.5mm. The 0.3mm hole gave me 0.77mm filament at the same extrusion volume rate. It still needed stretching to meet my 0.5mm target. Also it has the disadvantage that the maximum size I can extrude without stretching is now 0.77mm rather than 1.2mm. I didn't get chance to find out what effect less stretching had if any.

The sleeving I used is already looking sad so I ordered some PTFE sleeving to replace it.

Coming back together at last

One of the things that irritated me about the extruder was the time it took to open the pump and close it again. To make it slightly less fiddly I replaced the lock nuts with a pair of threaded steel plates :-



I used Forrest's recipe for cooling fluid while drilling these. It worked a treat, I had no idea you needed cooling when drilling steel to prevent the drill from being burnt.

The springs I used first time around that came from an old CD player weren't quite strong enough but I found some better ones in the hinges of an A3 scanner I dismantled recently. Six of these held the lid open.



Here they are installed on the machine :-



I should be extruding again tomorrow!

Thermal thoughts

I attached the wires of my new heater with some small crimps that I cut from connector pins like this:-



to just leave the crimp part :-



I also soldered them with lead free solder which melts at about 220°C rather than the tin lead solder that I used last time, which melts at 183°C. It could do with being higher still so I might see if I can buy some high temperature solder (301°C) for the next one I make.



Rather than insulate with the recommended heat shrink sleeving, which is only rated for 125°C, I used some high temperature woven sleeving. Even that seems to discolour at 240°C. PTFE sleeving might be better.



I pushed the wires from the thermistor into a two pin connector so that I can easily remove the nozzle.

I had to make a new mounting bracket because when I added the pipe around the PTFE insulator to stop it expanding it made it too big to go through the hole in my previous bracket, which also doubles as my spindle mount when milling.

The bit of aluminium box section I cut it from has been waiting for a new life since it appeared in a prototype electronic photo booth on Tomorrow's World in the early nineties.

I calibrated the thermistor by putting a thermocouple into a second hole in the nozzle with some silicon grease. I also put a second thermocouple inside the barrel. I then put a range of voltages across the heater and waited for it to reach thermal equilibrium before reading the temperatures and the resistance of the thermistor.



Notice how dark the J-B Weld has gone when briefly heated to 240°C.

I made the new heater resistance 7.1Ω compared to 8.6Ω last time because I guessed the acorn nut nozzle would have a larger surface area and hence greater heat loss. In fact it has a remarkably similar temperature power curve to my last nozzle.


Note that the difference between the internal and external temperatures is as much as 25°C. The PTFE tape I used to seal the nozzle may not be helping here. I also measured the outside temperature of the J-B Weld using an IR thermometer which read 242°C when the inside was 245°C and the nozzle was 220°C.

After calibrating the thermistor it seems to track the thermocouple within about 5°C.