I got an Anycubic Predator last month, and after resolving a few mechanical problems, I was able to get it printing decently well. The only significant modification I've made so far is a set of 8-diode TL Smoothers, and I'm now mostly operating it via Octoprint.

However, during the last few prints, I've noticed the temperature dropping midway through the print. It warms up and cools down fine, but for some reason it's not able to sustain the temperature throughout the print.

temp graph
In this case, the print started out at the correct temperature (200 °C), held that temp for around 2 hours, then it dropped to a lower temp (174 °C). It eventually went back up to the target temp, then dropped again 5 minutes later. I tried manually adjusting it to see if that could fix it, but no luck.

After this print completed, I restarted it to show how it is easily able to reach the target temp and hold it at the start of the print:

temp graph 2

Any tips on diagnosing and resolving this issue?

  • $\begingroup$ Welcome to 3dPrinting.SE! In the 2nd image, it looks as though it is being commanded to drop the temps down to zero ... your line for "Target T" and Target Bed" show it dropping at ~(-23)mins, which in turn allows your extruder and bed temps to drop, until they are commanded to bring the temps up again at ~(-9)mins for bed and ~(-8)mins for extruder. is this what you are trying to explain? You'd need to see why this is being commanded to do so, but it appears the temps are following exactly along the lines of what it is being told to do. $\endgroup$ Jan 19, 2020 at 17:44
  • $\begingroup$ @Pᴀᴜʟsᴛᴇʀ2 What you're describing is correct, but not the issue I'm having. I allowed the print to complete (since it was almost done by the time I noticed the issue). I cleaned the bed and let it cool to close to room temp and restarted it. The range ~(-10) min on is how it should look -- I Included that image to show that this does not appear to be the same problem described here. The problem is the drop in temps shown in the first image. $\endgroup$
    – p.s.w.g
    Jan 19, 2020 at 18:00
  • $\begingroup$ That makes sense then for the 2nd image. In the first image is it just the heating element isn't keeping up with the print requirements? $\endgroup$ Jan 19, 2020 at 18:15
  • $\begingroup$ @Pᴀᴜʟsᴛᴇʀ2 Correct. It heats up correctly at first, maintains temp for a while, then at some point, it drops to around 170~180°C. $\endgroup$
    – p.s.w.g
    Jan 19, 2020 at 18:21
  • 1
    $\begingroup$ If you can, add some tinfoil over the heater block to prevent air from the coolend cooling fan. $\endgroup$
    – Trish
    Jan 19, 2020 at 18:27

3 Answers 3


Safety First

Let's look at the graphs. First: you should swap firmware for one that has Thermal Runaway, as, as it is, running about 15 minutes with 28 K less than the printer is ordered to work at is a clear indication that there is no Thermal runaway protection in place - it should have tripped over that long ago! But there is more!


But this graph and the lack of Thermal Runaway Protection also are typical for printers that have a design flaw: If the airflow from the part cooling fans or the coldend-cooling fan (that's the fan that always runs) brushes over the heater block, it cools it. This limits the achievable temperature.

Luckily, such is easily remedied in one of several ways:

  • Changing the airducts for ones that does not hit the heater block
  • Adding a silicone sock around the heater block
  • Kapton-tape and ceramic wool can be used to make a heater-sock too
  • Adding an air-shield in the shape of a bit of tinfoil can redirect the airflow away from the heater block, but make positively sure it is mounted Fire-Safe and can't be lost into the print!

The rise times are quicker than the fall times, which is not what I expected. Maybe a fan is turning on and off, but I'd expect to see the heater struggling to heat up. On both graphs, the rise times look like good heating and not much interference.

It seems to me that power to the hotend is the problem, but what can cause a drop in power but not to zero (as in the first graph)? Assuming that the file and the software are OK, then it's hardware: the MOSFET and the subsequent tracks and connections become the most likely suspects. So, I'd check all the connections between hotend and control board, even undoing and reconnecting. If you have a logging voltmeter, you could try that to see if the heater voltage changes when it shouldn't. If you have an oscilloscope, see the input pulses into the MOSFET to see if changes happen at the same time you see something weird with temperatures. MOSFETs can fail in peculiar ways, so consider replacement - it's not the cost of the MOSFET that's the issue; it's all the fiddling around to actually do it.

  • 1
    $\begingroup$ Thanks for the suggestion, but in my case, it turned out to be the fan issue Trish described (combined with the fact that the hotend had slipped slightly out of position, causing the heat block to make contact with the effector body and thus giving it a much larger surface area through which to radiate heat). Hopefully this idea about the MOSFETs can help someone else though. $\endgroup$
    – p.s.w.g
    Feb 2, 2020 at 12:40

Based on your picture from Octoprint I can assume that you may have the wrong heater. Why? Because heating hotend to 215 takes quite a lot of time (3 minutes) in your case. If you have an appropriate heater it should take around 1 minute

You need to check the resistance of the heater and then calculate the power based on voltage.

Current = Voltage / measured resistance

Power = Current * voltage

For a good working hotend, you need to have at least a 35 W to 40 W heater.

Some shops sell 40 W heaters but these heaters are for 24 V systems, so in your case, if you have 12 V system it may be the case why the temperature drops because the heater will work like it has 10 W of power

But even if you swap the heater, you need to be sure that your power supply/board will be able to deliver appropriate current without damaging itself - MOSFET/SSR (solid state relay) could be helpful sometimes.

As someone mentioned in a different response it's good to have cooling protection like silicone socks or any other protection material.


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