This modification is intended to improve heat transfer to the nozzle an allow better heater core insulation for lower power heating and better temperature control.
I am adapting the extruder head to fit to a CNC mill to increase the accuracy and build envelope available to me, so whilst I've had my extruder running on the bench, I'm still working on adapting the extruder controller to allow it to be driven by the CNC controller (Siemens Sinumerik)
UPDATE: I've got my TechZone Extruder controller re-coded and running my extruder nozzle now. I've measured a warm-up cycle at 3'44", with the heater running a continuous 28W. Some of my theoretical power has disappeared somehow - not sure why
The heater core is drilled to accept wirewound resistors as shown. I used 4 10ohm ceramic wirewound resistors (RS part no 485-2669)
The resistors are glued into the core using a calcium carbonate-filled epoxy resin mix.
I had some spare CPU paste, so I used it when installing the nozzle to increase the heat transfer properties. I also counter-bored the block to allow some insulation around the filament tube to reduce heat transfer up the tube - I'm not sure whether this is worthwhile to do though...
After installing the thermistor, the resistor leads are poked through short lengths of insulation tape each side and the remainder is wrapped in a length of tape running the other axis with a hole punched in it to allow the nozzle to poke through. All the tape is held on with Kapton tape.
The resistor wires are brought together and soldered to the power supply cables on the outside of the insulation. This has the advantage that the solder connections are not subject to as much temperature as on the original design.