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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?

Trish
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p.s.w.g
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  • 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. – Pᴀᴜʟsᴛᴇʀ2 Jan 19 '20 at 17:44
  • @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](https://3dprinting.stackexchange.com/q/7720/19846). The problem is the drop in temps shown in the first image. – p.s.w.g Jan 19 '20 at 18:00
  • 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? – Pᴀᴜʟsᴛᴇʀ2 Jan 19 '20 at 18:15
  • @Pᴀᴜʟsᴛᴇʀ2 Correct. It heats up correctly at first, maintains temp for a while, then at some point, it drops to around 170~180°C. – p.s.w.g Jan 19 '20 at 18:21
  • Have you altered the flow rate? Does air from either fan brush over the heater block? – Trish Jan 19 '20 at 18:22
  • @Trish Flow for this model is at 100%. This model has two part cooling fans, and the factory duct isn't great; one of them could be blowing on the heat block. I can try a print without part cooling to see if that helps. – p.s.w.g Jan 19 '20 at 18:25
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    If you can, add some tinfoil over the heater block to prevent air from the coolend cooling fan. – Trish Jan 19 '20 at 18:27
  • @Trish Thanks, I will try both of those ideas. This seems to happen well into a print so it may be a while before I can post the results. – p.s.w.g Jan 19 '20 at 18:29
  • As I look at the graphs, I realize: your printer comes without Thermal Runaway Protection! – Trish Jan 19 '20 at 18:43
  • @Trish This is definitely it. I started a print without part cooling fans and allowed it to run for nearly an hour -- it had no problem keeping at the target temp throughout that time. I then tried turning the fans on by manually through octoprint, and immediately saw the temp start dropping; turned them off again, and it returned to the target temp quickly. Thank you so much for the pointers. I can look into the aluminum foil idea, but I'll probably want to invest in silicon socks at some point. Feel free to write this up as an answer, and I'll accept it. – p.s.w.g Jan 19 '20 at 20:16
  • And yes, I agree that this printer doesn't have Thermal Runaway Protection, and if this were supported, it would likely have notified me of a problem sooner. I am not sure if the firmware supports it and it's just turned off by default, or if there's no support for it all. I'll investigate this further as well. – p.s.w.g Jan 19 '20 at 20:22
  • Done! You could very easily use Marlin with the right size settings. But that would be an excellent question: how to set up my custom Marlin 2.0? – Trish Jan 19 '20 at 23:16

2 Answers2

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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!

Problem

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!
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Trish
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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.

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    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. – p.s.w.g Feb 02 '20 at 12:40