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simmonsstummer

Raising speed needs raising flowing?

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No. Instead raising speed might need raising temperature.

The tiny .4mm hole in the nozzle can only let a certain amount of filament out. If you raise the speed of any axis Marlin is smart and increases the movement of extruder also. But the extruder motor is often not strong enough. So for faster speeds you might have to do thinner layer heights, or hotter nozzle temp (hot PLA flows like honey. Cold PLA flows like toothpaste).

Here is a test I did:

http://umforum.ultimaker.com/index.php?/topic/3418-um2-extrusion-rates/

 

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260C can cause PLA to possibly boil a bit and/or get gunky and cause clogs.

240C is much safer. I often print at 240C when I don't care about quality and want to print 100mm/sec.

210-220C is recommended as you tend to get better quality but you have to print slower.

When I want extra high quality I print maybe 190C.

Every color of filament is different. But usually only different by 5-10C.

 

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260C can cause PLA to possibly boil a bit and/or get gunky and cause clogs.

240C is much safer. I often print at 240C when I don't care about quality and want to print 100mm/sec.

210-220C is recommended as you tend to get better quality but you have to print slower.

When I want extra high quality I print maybe 190C.

Every color of filament is different. But usually only different by 5-10C.

 

This is interesting, I'm wondering what I'm doing differently (or incorrectly);

I'm printing 0.2mm layer heights at 50 mm/s, and have to run my PLA (the light blue Ultimaker PLA that came with my UM2) at 240-245 degC in order to get minimal (if any) under-extrusion. The finish, at least on the few parts I've done, looks to mine eyes be to very uniform and smooth, excluding the layer start/stop seam. Ambient is between 65 and 75 degF (18.3-23.9 degC).

 

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The UM2 behaves rather differently than the original Ultimaker, with regards to temperature. This is because it has a very very small/short heating block.

The faster you print, in terms of mm³/s, the less and less time the plastic spends in the heater before it is extruded. On a UM2, with it very short heating zone, this means that when printing fast the plastic has very little time to heat up.

The behavior of the plastic depends on the temperature that the plastic is at when it is extruded. When printing slowly, the plastic has time to reach equilibrium with the heater block. Lets say that you are using a plastic that extrudes best at 220ºC. So that is what you set your heater to, and if you are printing slowly with the heater set at 220ºC, then the plastic that comes out of the nozzle is at roughly 220ºC.

But if you increase the volumetric speed of printing (by increasing the linear print speed, or extruding a wider bead, or increasing the layer height), then the plastic is passing through the heater faster, and so it is at a lower temperature by the time it extrudes. So, while the heater is still set at 220, and the block is still at 220, the plastic itself has only heated to maybe 210 before it extrudes. Since it is at a lower temperature, it flows less well, and is harder to extrude.

If you print fast enough, the temperature may fall so low - perhaps to 190, as an example, that the plastic is sufficiently viscous that you cannot put enough pressure on it with the extruder motor to get it to squeeze out fast enough. As a result you get under extrusion, or even stripping of the filament and a failed print.

The way to get around this - to some extent - is to raise the temperature of the heater. If you set the heater to, say, 260, then when that plastic is being forced quickly through the head, it might at least make it to 220 by the time it is extruded.

The major problem with this is that it only works as long as the extrusion rate is high and constant. As soon as the print head slows down for a detailed part with lots of short segments, or the print slows down to allow for the necessary minimum layer cooling time, the plastic is suddenly spending much longer in the super-hot print head, and getting heated up much hotter than it was. This can lead to much more oozing and blobbing - most PLA's heated up to a true 250 become very runny - and in extreme cases, to boiling and breakdown of the plastic, leading to head blockages.

In theory, this would be ok, if the temperature could adjust dynamically to manage the temperature of the extruded plastic. But it simply isn't done, and I don't think it can be - because even a 'small' hot zone still takes quite a long time to heat up and cool down, so it can't respond fast enough to changing needs that might vary tens of degrees in just a few seconds.

Which is why I think that a short melt zone on a print head is the wrong way to solve the problem. It does give good results, but at the expense of needing to print more slowly, and not really offering a viable way to scale up to larger/faster prints. Some of the results I've gotten on my UM2 have been stunning, but they have all been the longest-ever prints I've done, by quite some margin. For things where quality isn't at such a premium, the stronger extruder and longer melt zone of the UM1 might be a better option.

 

This is interesting, I'm wondering what I'm doing differently (or incorrectly);

I'm printing 0.2mm layer heights at 50 mm/s, and have to run my PLA (the light blue Ultimaker PLA that came with my UM2) at 240-245 degC in order to get minimal (if any) under-extrusion. The finish, at least on the few parts I've done, looks to mine eyes be to very uniform and smooth, excluding the layer start/stop seam. Ambient is between 65 and 75 degF (18.3-23.9 degC).

 

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