<p>If I set temperature say 220 °C, printer heats up to it and it only varies +/- 0.5 °C under non operating condition. But if I start a print, there's a shift of +/- 15 °C.</p>
<p>I've already auto tuned PID parameters and when I run <code>M503</code>, the printer shows the updated PID values.</p>
<p><a href="https://i.stack.imgur.com/w8SCE.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/w8SCE.png" alt="Octoprint temperature graph showing hotend temperature drops" /></a></p>
<p>What could be the reason?</p>
<p>Electronics details:</p>
<ul>
<li>Firmware used:  Marlin</li>
<li>Controller board: Printrboard rev D</li>
</ul>

[{'answer_id': 6052, 'author': 'profesor79', 'author_id': 9730, 'author_profile': 'https://3dprinting.stackexchange.com/users/9730', 'pm_score': 3, 'selected': False, 'text': '<p>There could be a number of reasons for that behaviour:</p>\n\n<p>Please check following items:</p>\n\n<ol>\n<li>part blower/fan cools down the nozzle - stop the fan</li>\n<li>material is extruded at high speed and takes the heat</li>\n<li>loose thermistor (when the move occurs as it moves a bit internally) </li>\n<li>the power supply unit voltage varies - so temperature reading varies as well - measure voltage and see if there are significant drops</li>\n<li>nozzle heater and thermistor cables can be loose as well (check screw terminals)</li>\n<li>bed heater connection cables having not a good connection (check screw terminals)</li>\n</ol>\n'}, {'answer_id': 6053, 'author': '0scar', 'author_id': 5740, 'author_profile': 'https://3dprinting.stackexchange.com/users/5740', 'pm_score': 2, 'selected': False, 'text': '<p>Dropping hot end temperatures can be caused by the filament cooler blowing too much cold air onto the nozzle, or a too high of a print speed so that the nozzle is cooled by the filament (in combination with a heating element that cannot heat that fast to compensate).</p>\n\n<p>To rule out the PSU you could try measuring the voltage of the PSU before and after powering the heat bed, if it drops dramatically when the heat bed starts change the PSU.</p>\n'}, {'answer_id': 6062, 'author': 'AterLux', 'author_id': 10875, 'author_profile': 'https://3dprinting.stackexchange.com/users/10875', 'pm_score': 0, 'selected': False, 'text': '<p>Can you also show a "power output" graph (or something like that, which show PWD duty cycle output to the hot end)?</p>\n\n<p>Have you noted any relation between temperature raise and fall and the printing position?</p>\n\n<p>Looking at graph I can only suggest the heating is suddenly stops and then the same sudden resumes.\nIt looks like you have a loose wire connection to the heater. When head is starting to move, it makes disturbance for the wiring, so heater occasionally stops to work.\nThere are usual two weak points: first it is where cable is connected to the board. Check is connector heated? Are any noises/sparks from there?\nAnother part: it is just at the point where wires going into the hot end. If wires are not secured to the printing head, then during printing those place experiences a lot of bending, which makes it weaker over the time.</p>\n'}, {'answer_id': 13925, 'author': 'Tomas Mora', 'author_id': 21664, 'author_profile': 'https://3dprinting.stackexchange.com/users/21664', 'pm_score': 0, 'selected': False, 'text': "<p>Hello I have had the same issue, fluctuating thermistor temperature, all of them. Discarding the board, and accessories setup (loose the heater, thermistor and so on) the problem was on the PSU. When I have opened the PSU and touched gently all the parts  then closed again and connected it to the printer: temperature readings were stable egain, however just for a time.</p>\n<p>Fluctuations of temperature due to heating have in general low frequencies due to the heat transport phenomena in material such as plastic and metals especially if the amount of metal material is of the order of 100 grams.</p>\n<p>Changes over the time with supplied power of about 500 watt over 100 grams of metal and plastic flow of a fewer grams per minute, seldom, cant cause temporal fluctuations  in frequencies of the order of electrical fluctuations. It's possible but not in 3D printing case.</p>\n"}, {'answer_id': 14075, 'author': 'Dani Díaz', 'author_id': 21880, 'author_profile': 'https://3dprinting.stackexchange.com/users/21880', 'pm_score': 1, 'selected': False, 'text': '<p>My problem was like yours and after several test I found bad PID tunning in heated bed disturb hotend temperature process.</p>\n<p>Before heated bed PID tunning:\n<a href="https://i.stack.imgur.com/XH6RC.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/XH6RC.png" alt="enter image description here" /></a></p>\n<p>When I fixed PID values for heated bed everything works like a charm :)</p>\n'}]
['https://3dprinting.stackexchange.com/questions/6051', 'https://3dprinting.stackexchange.com', 'https://3dprinting.stackexchange.com/users/10670/']