Extruding at a rather low 200 °C with that pair of 25 W cartridge heaters in the 14 °C Basement Laboratory Machine Shop Wing, the heaters exhibit fairly consistent timing:
- 47-49 seconds OFF
- 59-66 seconds ON
So, in round numbers, a 50 W heater has a 56% duty cycle with a 111 second period, with the temperature varying ±2 °C around the setpoint. The head has a fairly substantial ceramic wool insulation blanket, not the stock ceramic tape wrap, so your results will be different. One side of the build chamber is open to the ambient air, so it’s not as warm in there as usual.
The average dissipation of 28 W is half of the original MK5 head’s 58.8 W dissipation, which agrees reasonably well with what other folks have reported for the duty cycle of stock MK5 heads. More insulation is better, but a substantial fraction now escapes up the Thermal Riser tube, so doubling the blanket thickness might not be worth the bulk.
A single 40 W heater would run at 70% duty cycle. The only downside of lower power is a longer delay from power-on to extruding; a lower stuck-on overheat temperature seems like a Better Thing.
I’m deliberately using a relatively low extrusion temperature to explore the lower bounds of what’s practical. I think another 10 °C would improve the thread’s stickiness; right now some spots seem not so well glued together..