Binary Construct

It turns out a single resistor wasn't enough to drive the extruder, so following some ideas from Makerbot I decided to create a double resistor heater. I wanted to reuse parts that I already had as much as possible, so I started with a standard heater barrel. The heater block is machined from a single piece of brass.

Here are the build instructions.
Materials Required:

• Heater Barrel
• Nozzle
• Double Resistor Heater Block
• PTFE Washer
• Heatsink
• Hybrid Thermal Barrier
• 2x Resistors
• Heatsink Epoxy
• Heatsink Grease

Mix up some heatsink epoxy and coat the resistors with it. This will hold them inside the heater block permanently, so be sure to place them exactly where you want them.

Once the epoxy sets, 5 minutes or so, apply a drop of heatsink grease to the heater barrel where the block will sit. This will help heat conduction from the block to the barrel. Screw the block onto the barrel. I used a bit of fiberglass sleeving to insulate the resistor wires. If you don't have any you can use PTFE tubing or any insulator that can handle 250C. 

Attach the thermistor or thermocouple to the nozzle and wrap a bit of Kapton to keep it in place.

Slide the PTFE washer onto the barrel. Put another drop of heatsink grease on the barrel, then thread the heatsink on.

Thread the barrel into the thermal barrier, then put it back on the RepRap.

The resistors I used are 5ohms each, and in parallel this is 2.5 ohms. The power draw is a little too much for the extruder board, so I added in a 40 amp automotive relay to drive them.

And, it works!