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Hot End Temperature Not Reading Accurately

Posted by redhatman 
Hot End Temperature Not Reading Accurately
July 22, 2014 08:53AM
Hello RepRap Community!

The excitement is real, I have finally began building a 3D printer. However, after tackling many issues, I still have at least one left before doing a test print.

My current setup is:
-Prusa i2
-MK2 Heatbed
-550W Modified Computer Power Supply
-Sanguinololu Board (I believe 1.2 or 1.3)
-Marlin Firmware
-Geared Extruder
-Prusa Nozzle Hotend, 0.4 tip (stainless steel, see attached picture) .... or - [prusanozzle.org]
-3mm PLA Filament
-Arduino 0022
-Pronterface



The issue
I am having trouble getting the nozzle hot enough, and reading the correct temperature on Pronterface. When measuring the temperature with a meat thermometer (capable of going up to 300C), the measured temperature always seems to read around 30 degrees lower than what is displayed in Pronterface. Also, when touching PLA filament to the head when hot, it melts, but very slow. The extruder struggles to push out molten PLA. The highest temperature I have been able to reach is 240C when measured with a thermometer on the heating block of the extruder nozzle, and PLA still doesn't seem to melt like I would imagine.

Attempted Solutions/What's been checked
-Measured output voltage to be 12 volts on the heater cartridge wires
-Measured E-Temp Output to be 5 volts
-Measured resistance of the E-Temp sensor (EPCOS 100k) at ambient temp of 21.11 C to be ~100K ohms, and in boiling water 100C = 5.3K ohms
-Measured resistance heater cartridge at ambient and in boiling water to be steady around 4 ohms
-Changed Marlin Firmware Thermister Table values between options 1 and 6, and doing the PID autotune for each
-Changed PID Max from 255 to 128, as well as max temp to 300C (per manufacturer website setup instructions)
-Measured Resistance of resistors on board (with no voltage present) to be ~4.5K, pull-up resistors --Although 4.5 is not the desired 4.7k, the bed seems to be reading accurately (20C at ambient temp on pronterface).
-Tried two different batches of black pla, as well as some purple PLA (to make sure it wasn't the material at fault)

Question
The manufacturers website claims the stainless steel doesn't conduct heat as well, so to extrude PLA you will have to run around 220-240C with a fan pointed at the tip. However, even with an actual temp of 240C, the PLA still doesn't seem to melt very easily when touched to the tip. I was able to find a A595 offset calibration in the firmware, and tried messing with it, but I am not using an A595, so I would assume that's why this option did not correct the faulty reading.Overall, I am wondering if anyone has experience or suggestions for getting this nozzle to work? Would I be better off with a simple cheap nozzle?

Thank you for all of your time and efforts, all suggestions are welcome and much appreciated!
-Pat


- "Project Locus" - Mostly all printed parts, Corexy, Bowden Setup, 12x12 heatbed, ~1 cubic foot build volume
- SeeMeCNC Rostock Max V2
Re: Hot End Temperature Not Reading Accurately
July 23, 2014 04:11AM
The meat thermometer probably does not show correct surface temperature, being round and surface flat etc.

The thermistor table in firmware contains one adc column and one temperature column. The firmware reads the adc value and translates it to temperature by the use of that thermistor table. You can calibrate the thermistor table manually. You can take each row, and from the table that is uploaded in firmware, set the temperature in the row, to trick the system to reach the corresponding adc value. Then you measure that point with a real life temperature tool (preferably a multimeter with a termocouple), and write that real value down for each row. Then you replace the old table temperatures with your own temperatures and do the firmware upload again. And double check etc.
Re: Hot End Temperature Not Reading Accurately
July 23, 2014 08:36AM
Quote
NoobMan
The meat thermometer probably does not show correct surface temperature, being round and surface flat etc.

The thermistor table in firmware contains one adc column and one temperature column. The firmware reads the adc value and translates it to temperature by the use of that thermistor table. You can calibrate the thermistor table manually. You can take each row, and from the table that is uploaded in firmware, set the temperature in the row, to trick the system to reach the corresponding adc value. Then you measure that point with a real life temperature tool (preferably a multimeter with a termocouple), and write that real value down for each row. Then you replace the old table temperatures with your own temperatures and do the firmware upload again. And double check etc.

Thank you for the reply! I will definitely try playing around with the table values, I had looked at that, but became confused once I started looking into generating a table based on the resistor specs, oversampler, etc.. I'll give it a shot and post back with results.

As for the meat thermometer, unfortunately it's all I have to measure temperatures that high. Attempting to have the most accuracy, the tip was laid "flat" for the most contact area on the heated block. I have been to several electric supply stores and nobody seems to have thermistors (short of buying a whole new multimeter), which I have 80 miles from here.



I guess my next question would be, assuming the accuracy is at least within +/- 15C, shouldn't the PLA still be melting more than just slightly? Regardless of the reading, what characteristics should I notice when the filament is touched to a "hot enough" tip?

I appreciate all of the advice!
Thank you,
-Pat


- "Project Locus" - Mostly all printed parts, Corexy, Bowden Setup, 12x12 heatbed, ~1 cubic foot build volume
- SeeMeCNC Rostock Max V2
Re: Hot End Temperature Not Reading Accurately
July 23, 2014 12:38PM
What about the fan? I can't see any in the picture. If you have a general one that blows over the heatcore, that's not actually a good setup.
Did you check that the set screw holding the stainless steel barrel is tight? Eventually, unscrew it, take the barrel out and check if the surfaces are clean and there is nothing impeding the thermal transfer. Make sure both surfaces are clean and smooth.
Re: Hot End Temperature Not Reading Accurately
July 23, 2014 02:09PM
I think your extruder "having trouble pushing the filament" may not be related to temperature. Perhaps the filament path has an inner diameter too small (should be at least 3.2mm to allow for filament swelling before melting). Or perhaps the surface of this "inner cylinder" is rough so the filament just gets hard to push, in which case you could try putting the finest kind of sandpaper through and by the use of a rotating tool you can try give it some "luster" (e.g. shiny inner surface). Or perhaps the nozzle orrifice got something inside blocking the nozzle, in which case it may only need a cleaning. Or perhaps something else. But anyway a filament "hard to push" is not just about temperature. In your case ~210 and more should already be sufficient extruder to work.

You can take out the pressure bearing and just push the filament by hand, and see if it is hard to push by hand at working temperature. I think this is a very useful test, and if it feels the filament is too hard to push through then its a problem. Because you already have an geared extruder, if it has trouble pushing the filament then you probably have this (normally this can be done even un-geared). So if that is an issue, filament "hard to push" perhaps you can investigate the above possibilities.

Dunno about "slightly melting when touching a hot surface", because this is bit subjective expression - it could be interpreted quite differently by any of us.
Re: Hot End Temperature Not Reading Accurately
July 23, 2014 02:21PM
UPDATE:

After adjusting the thermistor tables as NoobMan suggested (thanks again), I was able to toy with it and get the hotend reading correctly on pronterface. To avoid screwing up the calibration of the heatbed, I made a copy of the table #1, renamed it, and assigned the hotend to it.

As far as extruding PLA, I have also got that working. There have been a few stupid assembly errors in terms of the extruder binding up (bought it used), but have managed to get the printer going decent.

However, the lowest successful temperature/speed has been (240 C, 30 mm/min) which still seems high. I can get a nice looking stream of PLA, without curling at the tip, at around (250 C, 30 mm/min).

Quote
3Dmaker4U
What about the fan? I can't see any in the picture. If you have a general one that blows over the heatcore, that's not actually a good setup.
Did you check that the set screw holding the stainless steel barrel is tight? Eventually, unscrew it, take the barrel out and check if the surfaces are clean and there is nothing impeding the thermal transfer. Make sure both surfaces are clean and smooth.

No fan on the hot end or extruder area as of now, the only fan on the printer is one mounted for the board... However, the manufacturers website for the hotend says







I guess I am confused as to why a fan is recommended at one of the lower temperature applications, but not the higher temp ones?


- "Project Locus" - Mostly all printed parts, Corexy, Bowden Setup, 12x12 heatbed, ~1 cubic foot build volume
- SeeMeCNC Rostock Max V2
Re: Hot End Temperature Not Reading Accurately
July 23, 2014 05:10PM
Where did you get the thermistor that is in the hot end you have? If you can find the correct table for it (or calculate it) then you'll get much better results.

You may find that the calibration is still off (eg: 240 when it's say 200) and that your temp readings are not as accurate as you'd like.
Re: Hot End Temperature Not Reading Accurately
July 24, 2014 01:23AM
You mentioned that you bought the extruder used. Maybe the guy before used it for abs, and was not fully cleaned, so it needs 240c to melt those abs bits in order to extrude, otherwise at lower temp the abs might block the nozzle orrifice. This could be a plausible scenario or explanation, if it needs a real 240c for extrusion. You mentioned you use black and purple pla, if it would of been clear transparent pla maybe it would of showed some impurities inside. Or you can perhaps try ask the previous owner if it printed abs with it.

Also i would try to give some insulation to hotend. My personal preferred method is to cover the hotend in a high temp silicon. This can be bought at gas stations or auto parts shop and it is "silicon gasket" for motor block or evacuation galleries and it is stable to temps up to 400C and is cheap, and has fairly good adhesion to metal surfaces. It aslo makes some bad smell first time its heated up, but after that it gets cured and its fine and odorless afterwards.
Re: Hot End Temperature Not Reading Accurately
July 24, 2014 09:45AM
Quote
Cefiar
Where did you get the thermistor that is in the hot end you have? If you can find the correct table for it (or calculate it) then you'll get much better results.

You may find that the calibration is still off (eg: 240 when it's say 200) and that your temp readings are not as accurate as you'd like.

Cefiar, I was using the correct thermistor table according to the manufacturer specs, (#1 on Marlin for EPCOS 100K). I used the same table for the heatbed, which is accurate. I ended up making a copy of the table 1, assigning a new variable to it, and using the new (modified table 1) for the hotend, and the standard #1 table for the bed.


Quote
NoobMan
You mentioned that you bought the extruder used. Maybe the guy before used it for abs, and was not fully cleaned, so it needs 240c to melt those abs bits in order to extrude, otherwise at lower temp the abs might block the nozzle orrifice. This could be a plausible scenario or explanation, if it needs a real 240c for extrusion. You mentioned you use black and purple pla, if it would of been clear transparent pla maybe it would of showed some impurities inside. Or you can perhaps try ask the previous owner if it printed abs with it.

Also i would try to give some insulation to hotend. My personal preferred method is to cover the hotend in a high temp silicon. This can be bought at gas stations or auto parts shop and it is "silicon gasket" for motor block or evacuation galleries and it is stable to temps up to 400C and is cheap, and has fairly good adhesion to metal surfaces. It aslo makes some bad smell first time its heated up, but after that it gets cured and its fine and odorless afterwards.


NoobMan, that is definitely another possibility. I'm not sure if the extruder was ever "used". As far as I am concerned, the guy was building the printer, and was about halfway through assembling everything when he sold it. Is there a good way to ensure the nozzle is cleaned entirely? I have checked the hotend heating block set screw at both ambient temp and operating temp to ensure it is tight (just incase the heater block is expanding and losing contact area). I have noticed when taking the temperature at various points, that it does indeed loose heat fast in relation to distance from the hotend which tells me it is poor at conducting heat. The base plate by the extruder is a tolerable temperature where I'm not worried about it melting the components (52C on the base plate, while extruding at 240C).

The high temp silicone sounds like a great idea, I may have to try that. Yesterday when extruding successfully around 240C, I kept the thermometer on the hotend block to monitor the temps. It did not seem like it was losing heat as suspected since it maintained the desired temperature, but that doesn't mean it wasn't internally.

As far as a fan, should I look into mounting one near the nozzle? I am still confused as to why the manufacturer says "PLA 220-240C, fan is highly suggested", whereas other materials are recommended to be extruded at up to 280C, without a fan. What exactly am I worried about where I need a fan?

If a new, nicer working hot end isn't much, I may just opt for that. As I said, I have been able to extrude material now, but the temps/requirement for a fan seems odd considering I don't see that on other setups accomplishing the same thing.

All of the feedback and support is awesome, and is greatly appreciated! Thank you!
-Pat

Edited 1 time(s). Last edit at 07/24/2014 09:51AM by redhatman.

- "Project Locus" - Mostly all printed parts, Corexy, Bowden Setup, 12x12 heatbed, ~1 cubic foot build volume
- SeeMeCNC Rostock Max V2
Re: Hot End Temperature Not Reading Accurately
July 25, 2014 12:48AM
There's a bunch of tables for EPCOS 100k thermistors, and that's because while they may all be made y EPCOS, and may all be 100k max resistance, various models have various beta curves.

I've got a bunch of EPCOS 100k thermistors that work fine on table #7, and read too low when using table #1. I'd look through the other tables and see if any are "close" to what you've created. You might find that your thermistor actually matches one of those tables instead.
Re: Hot End Temperature Not Reading Accurately
May 08, 2015 02:58PM
We have an I3v Prusa 3D printer; been using it about 6 months. Recently a new problem cropped up – the thermistor temperature is reading 50 degrees higher than the hot end actually is. When the LCD reads 200 degrees C the hot end is actually 155, as shown by 2 meat thermometers. We have tried 2 thermistors and 2 heating elements with no change so we know those components are OK. Before this the hot end temperature readings were always correct.

Other posters have suggested changing the table in the firmware. We could, but we’d rather fix the actual problem. Anyone had this experience and been able to correct it? Any guesses as to what may have happened?

Please share your thoughts…
Re: Hot End Temperature Not Reading Accurately
July 22, 2017 02:48PM
Hi
I am having issues get the right temp reading I have a Pursa I3 but have no tec spec I have been try to change the temperature table but do not no which set to change how do I work this out

Garrath
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