Insulating the heater block... more data!

I recently posted on the topic of Insulating the Heater Block and a few days later received some great data (graphs) from Alzibiff, a Mendel90 owner and keen RepRaper. Alan (Alzibiff) was in the process of insulating his J-Head heater block with some silicone tape, as per Nopheads design improvements, when he kindly captured some before & after data. This is great data in many ways. I hope I can do its interpretation justice...

{By the way, Alan is the proud owner of that 'pin-up' of 3D printers, the Black Dibond one with the Christmas Tree, which famously featured on the cover of the Mendel90 build manual for some time!)

The target temperature for all graphs was the same, 220 Deg C. The power resistor in the J-Head v5b, running on 12v, had no difficulty bringing the heater block up to temperature in all tests. The significant observable differences between graphs was in the average % power draw, the blue line along the lower (green) graph in each case. Alan has two fans on his x-carriage, one ducted under-carriage PEEK cooling fan (same as this), which I'll call the "Upper Fan". He has a standard M90 work cooling fan, which I'll call the "Work Fan". The latter fan duct was unmodified, no insulation cooling hole (another of Nopheads mods.)

Small Tech Note: I've checked that the power draw of the fans do not impact on the power graph. It only shows % power draw by the heater.

Figure 1. No (heater block) Insulation, Upper Fan ON, Work Fan OFF. 45% power to maintain temp.

Figure 1

Figure 2. No (heater block) Insulation, Upper Fan ON, Work Fan ON. 55% power to maintain temp. This is interesting because it's suggesting there is extra power needed to maintain temperature when the Work Fan is on. There must be air drift from the downward facing cooling fan that is impacting on the heater block, causing it to draw more power to maintain temperature.

Figure 2

Figure 3. Heater block insulation fitted (Silicone Tape), Upper Fan ON, then Work Fan ON at the '31' mark, so both fans on for the latter half of the graph. 38% power (Upper Fan ON), 48% power (both ON), approximately.The power draw to maintain temperature after the insulation is fitted to the heater block is nicely reduced. The insulation is doing it's job. But, once the Work Fan cuts in the power usage increases again slightly. This suggests that cool air drift from the work fan is cooling the heater block somehow. I expect the heater block would benefit from some insulation on it's base also, to further improve the insulation, but the nozzle probably doesn't protrude enough to permit this with the present j-head design.

Figure 3

Figure 4 is probably the most impressive. It shows the insulated heater block in operation with both fans OFF. To me it's the most impressive because the power draw (green graph) is barely above 25%. It shows that the silicone tape is a very good insulator, and really outperforms my skinny Teflon jacket. I must add that this particular test (Fans off) did unease Alan, and he didn't run it very long as he has had his share of extruder jams in the past.

Figure 4

Quick Conclusions:

The silicone tape is an excellent insulator, and an enhancement worth considering.

Indirect cool air (air drift) from fans has a greater impact on the power required to maintain hot-end temperatures than one might think, due to the cooling effect of such air.

Thanks to Alan for the graphs. Hope folks found it of interest. Questions and comments welcome as always.

Regards,

NumberSix



Extruder / heater testing...the less scientific way!

Over the past few evenings I've been 'playing with' my newly assembled extruder. I've read many other blogs and studied the RepRap.org wiki, searching for extruder related posts, but there's been no substitute for trying things out myself! I suppose I could have bought more of the components and availed of the collective knowledge gathered by many, and probably been printing away by now, but for me it's also somewhat about the journey!

So... I got my heater/thermistor and stepper motor connected up, after this little interlude, and mounted the assembled unit on a bracket, not on my repstrap, and started testing. I even wired up my little fan. I started up RepSnapper and 'commanded' the heater to commence heating, 40, 50, 60... 80... 100Deg C, on it went. I got brave and punched in 180Deg C, but coax it as I might it would not heat above 140Deg C! What was wrong?

The heatsink is just too efficient it seems, and even with the fan switched off the maximum temperature I could achieve was 180Deg C. I'm sure some experts out there could have seen this coming, and probably also what happened next. Still itching to see what would happen I fed some filament into it (3mm PLA). Not satisfied with hand turning the wheel I kicked the stepper into action and in went the PLA, into the heater. I got a tiny purge of plastic from my newly drilled .4mm hole before the whole thing just stopped feeding. Reversing was also futile. There it stopped to await it's first autopsy! :-)

Reading back through some of the extruder related articles made a lot more sense now. A short as possible transition zone from hot to cool is good. See the guru Nophead's writings on the extruder subject here, and the benefit of a PTFE lining reducing upward heat migration, and smoothing the path downwards, is also manditory you'd feel if you study Adrian Bowyer's most excellent Universal Mini Extruder design. But flying in the face of the need for any PEEK or PTFE, and the long journey from feeder to heater, is the UP! Extruder design, with simple metal pipe linking 'hot-end' to 'cold-end', and not a special plastic in sight.

So where was I going wrong? Examination of the jamed extruder revealed that the idler bearing that applies pressure on the filament so it's gripped by the nobbed feeder spindle, had completely squashed the filament. See photo below.

In switching off the fan to allow the temperature to rise in the heater, the temperature also rose in the unlined feeder tube, causing the PLA filament to soften and be deformed by the idle bearing lateral pressure. The raised temperature in the feeder tube also cause the PLA to deform, expand and jam.

To remove the jammed PLA i had to heat the dismantled assembly slightly with a hot-air gun and the plastic bits pulled right out.

Determined to continue testing the hot-end I next separated it from the heatsink completely, insulated the shaft in with some glass rope and held it in a small vice. (see photo). There was no difficulty in reaching temperatures a high as 220Dec C in this situation. (12v supply to 6ohm resistor in the heater block).

While I'm not sure of the accuracy of current temperature feedback to RepSnapper I can expect it's a pretty good guideline indicator of temperature. Here's what the RepSnapper temperature control/feedback fields look like. You can also see the manual extruder speed/feed control buttons just below the temperature section. There is a 'heater on' green light also on the Gen6 board which is very handy.

And... by manually feeding some PLA I got a nice free flowing extrusion. (see photo below). It took only minimul pressure to feed the filament. The extrusion did curl as it emerged and I did have to pull it straight just to prevent it sticking to the nozzle, but over all a satisfactory result!

Conclusions:

My heatsink is too large for heat output capability of the resistor/voltage.

I either need a more powerful heater, like the UP! has... 24v 80W heat probe, or I need a better thermal barrier between the hot and cold ends of the extruder. That's back to PEEK/PTFE type design.

Some positive points... the extruder stepper, which I salvaged off and old 5.25" floppy drive, the old photocopier cog wheels, the nobbed drive shaft and the idler pressure bearing all worked very well as a feed mechanism!

Thanks for viewing... comments and questions welcome!