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Controlling heat distribution in plastic extruder barrel

Posted by gstrooder 
Controlling heat distribution in plastic extruder barrel
July 24, 2016 12:09AM
Hi reprap community - thanks for putting together such an excellent resource in this forum.

I've been working on a plastic / filament extruder build for the past few months to do large scale robotic 3d printing and have come up against an issue with uneven heating of the extruder's barrel. We are using a couple of cheap REX C100 PIDs with K type thermocouples to control 75mm band heaters. We've tried to invest in decent quality heaters - they are probably middle of the range compared to some of the ones you can find on ebay. The control loop itself seems to be OK as the PIDs will manage to oscillate +- 5 degrees around the measured temperature at the thermocouple - we've checked this with another temperature probe and the read out is correct. Unfortunately on the inside of the barrel the temperature seems to spike at the centre of the heater band by about +70 degrees which is way too much deviation to safely extrude plastic. I've made a little diagram that is a guestimate of the temperature profile down the barrel.

I was wondering if anyone had come up against similar issues building their own extruders, or been able to find good ways to distribute heat from the heater bands? The barrel we are using is 5mm thick tool steel - and I am very surprised to find such a huge variation in temperature. Cheers.
Attachments:
open | download - heater_issue.jpg (114.3 KB)
Re: Controlling heat distribution in plastic extruder barrel
July 24, 2016 11:10AM
Quote
gstrooder
The control loop itself seems to be OK as the PIDs will manage to oscillate +- 5 degrees around the measured temperature at the thermocouple
5? not .5? 5 is way to much, either you have issues with the cooling near the sensor or the PID was not correctly tuned or some changes were made and you forgot to tune the PID again.


Quote
gstrooder
Unfortunately on the inside of the barrel the temperature seems to spike at the centre of the heater band by about +70 degrees which is way too much deviation to safely extrude plastic
That peak is only important if the plastic is really slowly extruded, that said I don't really care about the temp near the band heaters, For me the nozzle temp. is the most important temp.

Just my 2cents grinning smiley
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