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Part 06 - Hot End

The actual business end of the printer is the hot end. This is comprised of two machined blocks of aluminium and a few components such as a pneumatic connector for the Bowden tube, a barrel connector which goes between the two blocks, allowing filament to pass into the lower block, which is the melt chamber. A small brass nozzle goes into the bottom, which is where the melted filament finally give us our 3D printed goodness. Assembly is straightforward enough, although everything needs to be fitted very well together to prevent molten leakage out of the assembly during operation. Asides from the standard plumbers PTFE tape on the joints, the way described in the instructions to achieve a proper fitting of the nozzle itself, is to heat the lower block with a blow torch to around 300c, and then tighten the brass fitting - the theory being that the aluminium will expand more than the brass, allowing another one or two degrees of rotation. How about almost a full turn....it just kept going! I can confirm that it's 'bloody tight' now though!

Once everything has cooled down, it's time to attach the hot assembly to the fan. Getting the PTFE backing off the fan is a bit fiddly, as you need to keep it intact to use on the SMD heatsinks later, but I found a wide razor blade did the trick. This was the first time I'd needed to use the supplied heatsink grease - in case you've never used the stuff before, it's horrible, same as the stuff used on computer CPUs - my advise is wear gloves and have rags on hand. And be prepared to clean all your tools. And the cat.

In this shot, you can see the top PTFE heat shield, two bolts used to hand the hot end onto the Z axis, and the pneumatic Bowden hose connector in the middle.

And finally, with the rear & bottom shields in place. 

Next step is to install the power resistor - this is quite a neat solution, and I take my hat off to eMaker for coming up with this - I've seen other designs that use complex heating elements or even wound nichrome wire to provide the hot end, but this is an efficient, lightweight and highly controllable solution. The power resistor used here is a 6.8 ohm, 3W which only seems to be available in the US - Digikey have them though, and deliver to the UK for the usual fee - the part number is RWMA-6.8CT-ND in case you need spares, and the unit cost is 68 cents. When I installed mine, it was loose in the 5mm hole, so I wrapped a single layer of Kapton around it - this made it very snug - so snug that I suspect the drill will be coming out if it ever needs to be changed!

Of course, the hot end needs some monitoring love in the shape of a thermistor - I used the 'proper' method to assemble this one, as the version used in my heated bed wouldn't work in the hot end - that I could see anyway. The assembly was fiddly, and I was being very over cautious with my handling of the thermister as I know how fragile they are, but it all went together fine. The only modification I made was to make the connections further away from the bead than the 2mm recommended. My reasoning for this decision was that having the joins so close to the bead would put unnecessary strain on the device. The only mistake I made was to use the standard black PTFE heatshrink on the bead - I realized whilst shrinking it that I should have used the clear high temperature stuff...so I over heated deliberatly the incorrect heatshrink, and then let it cool - this made it brittle enough to crumble off, and I could replace it with the proper stuff. Yes, I did test the device afterwards, and got a nice steady 102k reading, so I guess no harm done.

Of course, putting the thermistor into it's hole means first filling up the hole with more thermal grease. Mind the cat. Long story...but don't let it put you off - the thermistor won't give accurate readings without it. In terms of wiring up the molex connector, as with the heated bed you can choose whichever sockets you want - just remember which is which. If you forget, test the pair with your multimeter - the thermistor at room temperature should read between 80k and 110k, and the resistor between 6.8 ohm and 7.2 ohm. If you've wired up the fan, it'll give you another reading, but I haven't actually metered it.

And here it is attached to my printer in all it's glory! Well I was pleased, anyway!