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Cura printing top speed change

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this is posted as well on google, sorry for the cross

Sorry if this has been answered before but a quick search didn't turn anything up.

 

I've not used Cura for a while but wanted to give it a test to see how things were going. Normally I print with smaller layers at slower speeds with a flow rate of about 4 Cu mm/s, but some of the tests were faster with thicker layers with flow rate of around 7 Cu mm/s resulting in under extrusion, but only for the top layers. II'm testing with 30mm X 10mm X 5mm cube. The print speed looked a bit odd and found in the gcode that the first layer is correct at around 20mm/s, as expected speed ramps up in the next layers to around 50% of the requested speed and the print continues at that speed. Problem comes at the last layers where it suddenly jumps to full requested speed to finish the top and the print is under extruded. Speed settings for the infill outer/inner shell are set to 0, so should default to print speed.

 

What has changed, what am I missing? Thanks for any help

 

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I guess you use a low percentage infill and thus for most of the layers the minimum layer time (expert settings) is hit and the print speed is slowed down.

However, The underextrusion at the end when jumping back to nominal speed should only be a few mm long. What temperature are printing at?

 

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I didn't think it was the mlt because the layer time was less, but you were correct. it seems the mlt is not a fixed border (cannot remember if it was that way before) when going from a layer time that is less than the mlt time, but print speed is gradually adjusted down. It seems that if the layer time goes longer than the mlt it is not gradual, it just move from adjusted to full speed. is that the case?

print temp is 145 with XT.

thanks for the help

 

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I didn't think it was the mlt because the layer time was less, but you were correct. it seems the mlt is not a fixed border (cannot remember if it was that way before) when going from a layer time that is less than the mlt time, but print speed is gradually adjusted down. It seems that if the layer time goes longer than the mlt it is not gradual, it just move from adjusted to full speed. is that the case?

print temp is 145 with XT.

thanks for the help

 

There is another influence: the minimum print speed, it overrules the minimum layer time. I can't remember if in such a case the missing time is compensated on other layers; I would have expected the minimum layer time to be a sharp threshold.

245°C is ok afaik with this year's XT, but maybe 250°C is worth trying?

 

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If you want to experiment, try my speedrefactor branch of CuraEngine on github. It does always smooth out extruder speed changes gradually (during the layer, not only on layer change). It works both for the first layer, and for sudden layer time changes.

It helps for some objects. Here is an extreme one: (left: old code, which suddenly slows down, right: speedrefactor branch)

smooth-speed-small.jpg

It works well, and I still use this branch myself for printing. But I think I have broken dual-extruder support. Also, it does not help as much as I hoped, so I'm not pusing for upstream integration.

(Note: the XY speed will still jump after the first layer. But it is now calculated such that the E speed does not jump.)

 

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Thanks all for the replies, but this was a combination of poor test object choice and rough count of the current layer time. my bad.

But, it did show me that you can have a large, sudden change in the speed due to the mlt time setting. For smaller objects when the print shifts from printing the perimeter and infill to top surface, the sudden change in flow rate can make weak layer.

 

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