Friday, October 4, 2013

Installing and PID tuning new J-Head Extruder

I recently received my new J-Head V Extruder Nozzle. Hotends.com are also now selling a 15mm long 30W cartridge, which fits the J-Head nicely. This is good since the fan ducting on the Mendel90 surrounds the heater block, and it wouldn't fit very well, if at all, with a larger heater cartridge. The 15mm cartridge is neater, and the currently popular 20mm 40W cartridges I've seen around. I also like that it draws slightly less power, at 30W, as I'm always concious of the risk of overheating connectors, wires or even some component a controller board.
The bits and pieces came well packaged. It included a heat resistor, but I plan to use the cartridge. It's good to have the heat resistor as a spare. It has a 100K thermistor, and since the last time I ordered, now includes suitably sized PTFE sleeves for insulating the thermistor and resistor wires. All the heater cartridges seem to sell with wires pre-crimped on and very well insulated, which is great.

The cartridge heater was a close fit to the hole in the J-Head brass heater block, and with one or two wraps of tinfoil it pushed snugly into place. (Cartridge wrapped in tinfoil and partially fitted, in photo above.)

I fitted the little PTFE sleeves to the thermistor, soldered some wires on and insulated the joints with heat-shrink. I inserted the thermistor into the hole in the heater-block, and surrounded it with some car exhaust putty. The wires from both the thermistor and heater cartridge where then zip-tied to the barrel (photo below.).
I thought about using car exhaust putty to secure the heater also, but the wiring is quite stiff, and it's a good fit to the block so it doesn't seem to be drifting. The one reservation I have about the cartridge arrangement is the bend in the wires. I was careful not to kink the very stiff wires, and hopefully they will be fine since they are secured well to the peek barrel. We'll see how it goes.
The J-Head Nozzle is secured to the extruder with three M3 bolts, M4 washers and M3 star washers to ensure nothing vibrates loose. This is Nophead's design, an excellent method of securing the nozzle. The M4 washers sit against the shoulder of the groove in the PEEK barrel and lock it firmly into the perfectly sized hole in the base of the extruder. It's not going anywhere!

I left the exhaust putty to dry for a few hours, connected the thermistor and heater into my connector board, plugged in my ribbon cable and gave the block a few hours at 100 Deg C to bed dry out the putty completely.


PID Tuning
A purchased printer kit, such as Nophead's Mendel90 will have pre-established PID values in the firmware, which will have been set for the characteristics of the extruder. The purchased M90 kit ships with a power resistor (at time of posting), but since I've chosen to try out a cartridge heater in the J-Head nozzle I've retuned the Marling firmware PID values using an auto-tuning feature. The auto-tuning is called on using an M303 Gcode command which is manually sent to the controller from the host software. If the command is executed on it's own it sets a target temperature of 150 Deg C, but to calibrate for different target temperatures the S parameter is added, e.g. M303 S230. You may want to obtain separate PID values for different target temperatures (different materials). 

Once the PID tuning has completed its process it will return a set of constants for you to note and enter.
The PID values can be later sent via a Start-Gcode, written to EPROM if supported, or hardcoded to the firmware. For hardcoding, the values are entered in the Configuration.h file, the firmware compiled and downloaded to the controller.
e.g.
// J-Head Mk V with cartridge heater. IOS 20131002
    #define  DEFAULT_Kp 27.95
    #define  DEFAULT_Ki 4.22  
    #define  DEFAULT_Kd 46.25

Repetier Host has a convenient temperature plotting capability which illustrates what's happening as the auto-tuning runs. You will see it tuning (pict below) to a target temperature of 220 Dec C, although I later repeated it to a target temperature of 230 for ABS, and at 185 for PLA, noting the value set for each.. The nozzle should be allowed to cool completely before repeating a calibration.

The graph shows the calibration process in action. The power is first applied fully, then as the target temperature is reached it is cut, then applied in cycles, reducing a little each time. The power is shown as % over time in Green, and Temperature plotted in Red. The concluding values and finishing message is also shown in the picture below.

After writing the new PID values to the firmware I set a target temperature and turned the heater on. The graph below nicely illustrates the rate of heating, climbing quickly on full power to well over 200, then PID Control cuts in within 10 Deg of the target, set in this firmware line "#define PID_FUNCTIONAL_RANGE 10",  and from there you see the temperature curve climbing again but quickly smoothing out to the target temperature. Also visible is how the power delivery settles to modulate at a much reduced level, maintaining the target temperature nicely (green graph below.). The red curve drops down when the heater is turned off.
Finally, for comparison purposes I generated the same graph on my older printer, with it's Gen6 Controller, J-Head IV Extruder and 5.6 ohm power resistor. See picture below.
There are a number of aspects to compare, the rate of temperature rise and time to reach target temperature, the draw on power, the quality of PID control. There are explanations for the difference in each comparision, but one major conclusion... my new set-up is significantly better in all respects and I look forward to printing with it!

As always, thanks for viewing!
NumberSix

3 comments:

  1. All interesting stuff as ever but one question comes to mind - what led to your decision to go with the JHead Mk5b? Did you consider using one of the later design all metal hotends such as that offered by E3D and if so, what was it that turned you away from them? I only ask as you clearly did a lot of thinking before going ahead with this project - the Azteeg X3 Controller for example is certainly not mainstream (yet) as far as I am aware.
    I am with you when it comes to heater cartridges, 40W seems rather over the top to me although if the wiring - and associated connectors - can take the current, I guess that it would enable higher temperatures .... should there ever be a need of course!
    Alan

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  2. Hi Alan,
    There’s so much to say about hotends I could do an entire post! :-) But, for now I’ll respond to your question. I chose the j-head over an all-metal hotend because it’s proven to work excellently with PLA and ABS, and is the best fit for the M90 x-carriage (bar one incident caused by manufacturing quality failure.). For me, the all-metal hotend only comes into its own if you are planning to print at higher temperatures (e.g. polycarbonate). It seems PTFE/PEEK got a bad name when the materials themselves were not to blame. They were used poorly, resulting in hotend failures, leaks, and general loss of confidence in nozzles mentioning peek/ptfe.

    There seems to be a flocking to ‘all meal’, as though it were better in all respects, but there’s no guarantee of that. “all metal” is too loose a grouping for these hotends since they are very different if compared. Compare the Prusa hotend to the E3D for instance. The former has stainless steel path all the way through, while the E3D has aluminium followed by stainless steel, followed by brass. Quite different. The j-head has PTFE followed by brass.

    Comparing J-Head to E3D directly for a second, assuming you don’t have a very high-temperature need… the j-head has less joints on the route the filament takes to the nozzle, has arguably a less resistant path, and doesn’t need active cooling of the insulator. In fact the E3D doesn’t have an insulator is has an active “heat dissipater”, the fan cooled aluminium finned cylinder. BTW, my fan on the j-head is to disrupt conductive air flow from the heatblock. The j-head would survive if the fan failed, and Nophead would say you don’t need it at all, but the E3D would fail without it’s fan.

    I compared a lot of the “all metal” hotends out there to the j-head and as a starting point for my M90 the j-head wins.  It doesn’t mean I won’t try one of the others at some stage, but only if I am trying out plastics requiring higher temperatures. I haven’t turned away from them as such.

    I did experiment with building my own all metal hot-ends over two years ago, but the precision of machining and internal finishing of stainless steel was beyond my equipment or experience. A polished internal finish on the ss is critical to them all working well. It makes me appreciate what any of the folks that have built these hotends have gone through. Good quality-control and failure analysis will be key also to their success. I know Brian at hotends.com does that.

    Hope that gives you a flavour of the logic governing my choice of hot-end.
    Appreciate your questions and comments.
    By the way, I’m also using the Helios heated bed, which I must post about soon. I don’t think either it or the Azteeg X3 is distributed in Europe, but I expect we will see a lot more of them, if and when ‘Royco’ gets his supply and distribution ramped up.

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  3. Correction: For "conductive" read "convective" in paragraph 3 above! Same mix-up again! :-)

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