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We're printing on a WASP 3MT pellet extruder with PLA. To save time, we're leaving the hotend at 160 °C between prints but realized that the print quality varies, from one print to the another, when using the same G-code file.

Could it be that leaving the temperature at 160 °C constantly creates more fluid PLA and therefore affects the following print?

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  • $\begingroup$ Leaving the temp high for 5 minutes is one thing; for an hour is another. Can you be more specific? $\endgroup$ – Carl Witthoft Dec 12 '18 at 18:49
  • $\begingroup$ Hey Frank , Your not very specific on what do you mean by print quality varying ? Could you describe what exactly is different every time ? is it dimensions or surface texture , layer height or something else ? . Other than a small clog in the extruder due to carbonising, there shouldn't really be much of a problem. PLA is quite robust that way. $\endgroup$ – Axel Fernandes Dec 12 '18 at 19:30
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    $\begingroup$ While leaving the temp at 160 °C in between prints (an average of 30 mins between prints and sending prints that take about 15mins continuously for several hours) i am observing these in the following prints : 1. over-extrusion 2. A slight colour change from transparent to yellowish 3. bubbles in the PLA. We're doing tests on spatial printing. To allow the material to cool faster in mid-air we are printing at a lower temp. of 160°C. $\endgroup$ – Frank Quek Dec 12 '18 at 22:33
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PLA starts to change its properties at above its glass transition temperature of 60-65 °C, if stored there too long. Keeping it at 160°C, close to the melting temperature (173-178 °C) can degrade the material relatively rapidly. During an extrusion, this is usually mitigated by filling fresh material into the melt while the older material gets extruded, but keeping the machine stagnant at the high temperature has not only creep the heat up from the designated melt zone (thus preheating material that should not be preheated yet) but also can damage the material deep in the melt zone. Together with this possible material degradation, the preheated material melts faster and might overshoot the aimed at print temperature until the normal temperature is reached again. Both effects can lead to reduced print quality in the lower layers.

The time saving from keeping the filament heated up for an extended period of time is, in my opinion, not worth the quality reduction that can come from using non-uniform material. You pay more in lost prints than you save in time for heating up the head.

If it is impossible to not keep the hotend heated between prints, it might be advisable try these:

  • Starting every print with a larger purging operation might get rid of degraded material. It would be as simple as extruding some quantity of material before starting the actual print at the edge of the build plate.
  • Aside from purging, it might be a good idea to reduce the hold temperature from 160 °C to a lower temperature, allowing the melt to partly solidify and keep the heat creep in check.
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With a bottoming value of being printable at 180 °C you are basically cooking the filament at 160 °C. If done a prolonged time, carbonization will take place as the more volatile parts will slowly be gassed out and part from the melt first.

When you start extruding after a certain time, you will need to get rid of the goo inside, this can lead to (partial) clogs and inconsistent printing. You should either lower it much further or not leaving it on at all.

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I leave the extruder working at 190°C between printings up to for 20 minutes while I made the change; this only provoques that the nozzle gets empty; nothing gets burned or clogged. Once to start againg just feed a little more of filament manualy or waits to the skirt do the cleaning and refilling the nozzle.

Normally at this temperature (190°C) the nozzle gets empty in the next 2 minutes due normal melting flow. I can assure that your printings will be like water drops.

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    $\begingroup$ you use a filament bases printer - they use a pellet based system that melts a considerably larger amount $\endgroup$ – Trish Dec 12 '18 at 20:45
  • $\begingroup$ @Trish The process is the same to clean a lyman filament extruder. Also in the flow meters is needed allowing that all old material flows fully to proceed with a new one. $\endgroup$ – Fernando Baltazar Dec 13 '18 at 0:30
  • $\begingroup$ @Trish I think Fernando's saying that if you completely clean out the hotend, then you don't have a problem. That's true but pretty much useless :-( $\endgroup$ – Carl Witthoft Dec 13 '18 at 14:16
  • $\begingroup$ @CarlWitthoft :D Nice!, not having problems is useless. Normally is not a full cleaning due is emptying by melting flow until you feed more material, if the time is larger then the emptying also is larger and could be totally empty on the next charge. $\endgroup$ – Fernando Baltazar Dec 14 '18 at 5:06

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