The heater in my last design has two layers of Cerastil that take 24 hours each to cure and the bobbin takes some time to machine. Attaching the wires and winding the coil is quite fiddly. Looking for a short cut I wondered if we could use power resistors. I had this one lying around to play with.
Unfortunately it is only rated for operation up to 200°C. In fact the datasheet says "It is essential that the maximum hot spot temperature of 220°C is not exceeded". Curious to see why, I put enough voltage across it to heat it up to 240°C. That turned out to be about 75W. It seemed quite happy at that temperature for several hours.
It is too big really for an extruder so I bought some smaller 10 Watt ones for £1.42 each.
These are only rated for 165°C but what the heck. I heated a 6.8Ω one to 300°C. At about 180°C it produces a little smoke but that soon goes. At 280°C it starts to smell bad, but at 240°C it seems happy. I left one powered up for a few days. The writing disappears and the connection tags oxidise, but its resistance is stable.
To make a heater I cut a 19mm x 19mm x 8mm block of aluminium from a bar, drilled a 5mm hole through it and tapped it to M6 to fit a heater barrel. The mounting holes of the resistor are big enough for M2.5 but there is not enough room for the head, or a nut. M2 is a bit weedy so instead I used 8BA bolts. These need a 2.8mm hole for tapping. A simpler solution would be to just file a flat on the head of an M2.5 bolt, drill a clearance hole and use a nut on the other side.
Here is the full assembly: -
I put heatsink compound on the M6 thread and under the resistor. I attached tinned copper wires with 300°C solder and insulated them with PTFE sleeving.
I have run the assembly for a couple of days and it held up. I am loath to recommend something which is unsound engineering, but it does seem a simple and robust solution as long it lasts a reasonable amount of time, say 1000 hours. Replacement is easy because the most time consuming thing is making the block which is reusable. I expect there might be some matching crimp connections to avoid the high temperature solder.
Quite a lot of heat is lost from the large surface area so some insulation would be a good idea.