1 Topic by riki_74

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Topic: A really special Paste Extruder

I am adapting a paste extruder (thingiverse:26047) for a quite particular task: extruding an hydrogel as a bioscaffold for cell colture. This would be a preliminary evaluation for an eventula more complex biological research program. I am hacking my solidoodle 3 for this and I am using the temperature sensor and the heating resistor of the hotend to set temperature of the syringe needle that I am using for the extrusion (37 °C - biologic extrusion temperature). At the same time I would like to mantain the body oif the syringe at 37° C. Do you have any suggestion how to hack the bed heating system and using it with a resistor in a similar manner to the hotend. I know that I should upgrade to a RAMPS for this , but at this point it is a really no budget project and I would do it without any cost.

http://www.thingiverse.com/thing:26047

2 Reply by Tomek

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Re: A really special Paste Extruder

Yeah. no problem.  But you will want to either calibrate the thermistor or possibly use a different thermistor. I don't know for certain, but I suspect the thermistor table is poorly calibrated for lower temps.


Other than that, you just need to wrap your syringe in aluminum foil, then kapton, then nichrome wire, then kapton, then aluminum foil, then any tape.  You probably want the nichrome to be >3.6ohms, since that's the resistance they use for the hot-end (i think? something like that.) and you probably need much less power. Guestimate = 8ohms. That's 1.6 ft of nichrome 29 gauge btw, so 29-30 gauge will probably be good, depending how big your syringe is.  You also will want to turn PWM-bed on in the firmware (its bang on-off by default I think)

3 Reply by riki_74

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Re: A really special Paste Extruder

Really thanks Tomek. Actually I tried some wiring yesterday with a spare  heating cartridge. I activated the PID for the heatedbed. It works quite well. The only problem that I faced it is the really first cycle of heating approaching to 36°C. This equally happens to the gauge and syringe. At the first cycle the temperature highly overcomes the target of 36° arriving to 45-50°. Then it goes down and stabilizes quite well on 36°C. Anyway, disappointing me, the temperature of 50° is not really polite for eukariotic cells. Do you have any suggestion how tuning the thermistor table for low temperature. I tried by firmware to low the power of heating (scaling down from 256 to 16) but without any improvement on the first cycle overrun, it affected only the slope of the increasing temperature but not the target.

4 Reply by elmoret

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Re: A really special Paste Extruder

You need to autotune the PID controller. Covered many times here.

Filastruder - the robust, inexpensive filament extruder
E3Dv6 All Metal Hotend - the premier 3D printing hotend

5 Reply by Tomek

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Re: A really special Paste Extruder

elmoret wrote:

You need to autotune the PID controller. Covered many times here.

  In general, this. However, I would say tuning the *hot end* pid has been covered many times, but I don't actually recall the bed PID being covered many times. It's presumably a similar Gcode command though.

The gcode for autotuning the bed PID will give you a P,I, and D value that you then need to change on your firmware, either by uploading the code again with the values edited in the sketch, or through your eeprom if that is available.


After getting the variables I assume you'll find all the values slightly shifted down, and if you're still overshooting, drop the P a bit 10-30%.

6 Reply by ronsii

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Re: A really special Paste Extruder

I believe the bed pid's were covered in a few threads... Adrian covered it at least once smile

7 Reply by riki_74

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Re: A really special Paste Extruder

Thank you . Yes the autotune command is similar for the bed. It is considered as a second hotend (M303 E-1 S36 C8). Anyway I  can not solve the overshooting problem. I tried to decrease the P component, but nothing , the temperature goes upto 45°. The autotuning seems really poor at low temperature. The strange thing is this happens just at the first cycle than the temperature is pretty stable on 36°.

8 Reply by ronsii

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Re: A really special Paste Extruder

Hey riki_74, How do you have the thermistor mounted? in relation to the nichrome.

9 Reply by riki_74

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Re: A really special Paste Extruder

Actually it is a common hotend. I drilled it to make the needle pass through. I am curious to know if you get the same with your hot ends if you set a 36°

10 Reply by riki_74

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Re: A really special Paste Extruder

Sorry to be clear:I get the same result with the syringe and with the needle heating system. That on the needle is a drilled hotend

11 Reply by ronsii

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Re: A really special Paste Extruder

Oh, sorry I misunderstood smile mounting the thermistor closer to the heating element will lower overshoot but then you will have to retune the other pid's, it will also lengthen the time required to get to the target temp.

12 Reply by riki_74

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Re: A really special Paste Extruder

I can not solve the problem of overshooting . I tried autotuning and and manual lowering of P. No way I have a bad overshoot  up to 50°C. Any suggestion ?

13 Reply by Tomek

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Re: A really special Paste Extruder

What is the resistance of your heater? I would up your resistance so that there is less power available to the system.  Alternatively, set your first temp at 29C, then when that stabilizes, set to 37C.

14 Reply by riki_74

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Re: A really special Paste Extruder

Ok , after a day of attempts I obtained some results. if somebody else will ever need to work around 36°C these are my advices :

1) use autotuning
2) reduce the duty cycle (#define MAX_BED_POWER 48 // limits duty cycle to bed; 256=full current)  ; for my setting 48 is good
3) REALLY IMPORTANT : use temperature table 1 (#define TEMP_SENSOR_BED 1) ; the default table 6 is completley inconsistent for temperatures around 40°C

At this point the needle hotend is perfect. the heating of the syringe(bed heating system) has still 2-3 °C of oscillation , but I suppose that is due to the structure of the heating jacket for the syringe with the temperature sensor not really close to the thermistor.
Probably I should split the thermistor. So, I do not understand nothing in electronics : the two new thermistor should be in parallel or serial and with  doubled or halved ohm?