Why are the print times almost the same when I double the speed?
 

Why are the print times almost the same when I double the speed?  

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thomas.m83
(@thomas-m83)
Eminent Member

Hi,

I switched to a new hotend yesterday and wanted to do some speed tests today.

So my first attempt was about 20% faster. No problems. Then about 40% faster. No problems.

But I noticed the total printing time almost doesn´t change, how can that be?

Now I wanted to crank the values up to 200mm/s for some serious test but Slicer spits out the same total time.

What  am I missing?

Posted : 12/07/2020 5:30 pm
Neophyl
(@neophyl)
Noble Member

Several factors if you are using Prusa Slicer.   Firstly is Acceleration. The printer must accelerate to get to those speeds and that is limited by the acceleration settings, which are determined by how fast your actual hardware can keep up. Smaller models or complex shapes don’t have long enough runs to reach the tops speeds.
Secondly it’s highly likely you are running into the Max Volumetric limit. This is a value you can configure that defines the maximum throughput of plastic your hotels can actually keep up with. The MVS acts as a top limit no matter what speeds you set. If you try and print faster than your extruder can lay down plastic you will gets skips which is why Slicer has a mvs setting. 

Posted : 12/07/2020 7:38 pm
thomas.m83
(@thomas-m83)
Eminent Member

Hmm okay, the max volumetric speed is set to 0 by default at the print settings and another value in the filament settings at 8mm³ per second.

That is about 3,3mm filament per second. Not sure how to calculate this accurately however this should be enough for about 24mm/s at 0,2 layer height.

However increasing that to 16 doesn´t give any advantage in the calculation.

Sigh, looks like that is more complicated than I thought.

Posted : 12/07/2020 7:57 pm
Neophyl
(@neophyl)
Noble Member

When you slice a object look through the various options on the preview. One of them is volumetric flow, it’s easy to tell if you are topping out due to mvs with that view.   Usually you only get near it for stuff like infill on the default profiles. 
in slicer where you have 2 values configured in different areas it uses the lowest value. The volumetric rate is usually defined in the filament settings and you must be set to petg for a value of 8. 
Might want to wait until Bobstro spots this thread as he has done a lot of experimenting with mvs in slicer. 

Posted : 12/07/2020 9:17 pm
thomas.m83
(@thomas-m83)
Eminent Member

Thanks, this is really helpful. However in my test it is at max 4.945mm³/s even with all the speed settings increased to 200.

Funny thing, it is at max 5.075 with standard settings.

So instead of increasing the speed it seems to go down.

 

Edit: I just tried a double-speed setting and now the max flow is at 4.569

This doesn´t make any sense to me 🙁

This post was modified 4 weeks ago by thomas.m83
Posted : 13/07/2020 7:44 am
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