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3dnerd

UM2 physics question

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Dear all,

I am still not really clear ybout the relationship between flow rate, print speed and nozzle size.

Can someone shed a light on it for me?

Here some question I have:

1.

Does the feeder motor turn always at same speed for a single print?

What I mean is if the filament gets pushed through the bowden always at same speed? Or are there some paths in the gcode where the filament gets faster / slower extruded?

2.

If I increase the flow-rate does that mean the feeder motor turns faster?

3.

When do we need to increase / decrease flow rate?

4.

When do we need increase / decrease printspeed?

So basically I don't know the relationship between feeder speed, flow rate, printspeed and layer height.

Thanks for any light :wink:

 

 

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The feeder turns according to a combination of all your settings. from your filament diameter to your print speed to your nozzle diameter. It's all calculated based on putting down the right amount of material. It will turn slower when you print slower, or if you want less material extruded.

So your first layer would turn slow (since it's usually around 20mm/s) and then when your print reaches full speed (usually around 50mm/s it will turn accordingly faster to ensure that the right amount of material is coming out of the nozzle for how fast it is going. There is no constant set speed in which your extruder turns it's all relative to everything else.

If you increase flow rate, the extruder will turn faster.

The flow rate is a percentage, if you're under extruding for some reason, you can increase flow rate on the fly. Or decrease if you're over extruding. I wouldn't recommend this though, usually it means something else is off like your filament diameter or bed level. It's better to get a proper start than a bandaid solution fix like flow rate adjustments.

You adjust print speed according to your precision needs. Slower is usually better, faster means you'll get your part faster. 50mm/s usually is a good place to be good a good looking print. You don't need to adjust the flow rate manually if you adjust your print speed this is all pre-calculated.

 

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Plus a couple of points. If you are printing something that gets narrower as you reach the top. e.g. a church spire, Cura or whatever slicer you are using will slow the print speed to maintain the minimum layer print time you have set.

Assuming you have no blockages or feed path problems then if you under extrude you either need to increase your extruder temp. or lower you print speed because the filament has too high a viscosity for the printer to feed it though the nozzle at the required speed.

If your model has overhangs then often you will get a better surface finish if you print slow (and fans on 100%)

 

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Hi guys,

thanks for reply, it is a bit more clear now.

1.

What I still not understand is one should not increase flow rate if underextrusion happens. Instead the recommened way is t increase print temperature.

This I don't understand. If I 'just' increase temperature and feeder is at same speed, then no additional material comes out of the nozzle. The amount of material is same. The material is just hotter.

So why increasing temperature works against underextrusion?

2.

If I want to print thicker layers then I should reduce printspeed.

But if I reduce printspeed then feeder speed is also reduced (as Valcrow mentioned in his 1st paragraph).

If both is reduced then the amount of material coming out of the nozzle is also reduced. In my understanding then I don't have enough material for a thicker layer.

What is wrong in my thinking?

 

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1.

The primary reason for underextrusion is too much (back-) pressure in the nozzle. The possible extruded volume per second is limited by the nozzle diameter. The feeder will skip back, if the pressure becomes too high. Increasing the flow rate would increase the pressure any further... That's why the general advise is to increase the temperature instead. Higher temperature leads to a better flow and reduced pressure (and automatically: increased extrusion volume).

Only if the diameter of the material is much less than expected, increasing the flow rate would help to avoid underextrusion. But this circumstance is very rare compared to the former.

2.

Of course the amount of extruded material is higher for thicker layers. This is a part of the calculations of the slicer software. But - as stated above - the volume is limited, that's why you can't reach the same printing speed as with thinner layers.

Don't worry too much. Most of this stuff is very logical...

 

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I think Tinkergnome meant to say "... that's why you can't reach the same printing speed with thicker layers." :)

Just so you do not understand. You can print .100 and .300 layers at the same speed - to a point, which is the maximum flow your printer can generate. Compared with .100, .300 is 3 times the quantity to be extruded at any given time. So in that context your maximum print speed at .100 (i.e up to your maximum flow rate) will be faster than your maximum speed at .300.

What you need to understand is the relationship between the amount of filament you want to extrude(your layer height), how fast you want to print (your print speed), and the viscosity of the filament (your extruder temperature). As a general rule the more filament you want to deposit and the faster you want to go, then the hotter your extruder needs to be. I say general rule because you can probably print say .100 at 30mm/s and .100 at 40mm/s with the same temperature and get decent prints from both sets of settings. Push that speed up to say 60mm/s and you will almost certainly need to increase the temperature to compensate or else get under extrusion.

And just to make things fun, you will find eventually (when it goes wrong!) that certain filaments may need different temperatures to print at the same thickness and speed.

 

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Hi guys,

thanks for making it clear now.

I just wonder if Cura calculates all those things why it does not adjust nozzle temperature and print speed according to the choosen lyer thickness? Why I have to reduce prints speed manually?

 

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Well the first simple answer is that Cura does not know what material you are using and nozzle temperature ranges differ between different materials. As I said earlier I think, even if you are using say PLA from the same manufacturer there will be some colours that will benefit from a different temp; when I say "benefit" I mean you will get a better result.

As for print speed, well that is your decision. It plays a major part in the quality of result. YOU rather than Cura may decide that for the first print of an item you just want to get a general view of how the part, or a specific section of the part, will come out. In which case fine quality is of no concern and you may be happy to print at say 60mm/ or faster. So once you have verified that the top of the Eiffel tower is coming out OK, you can start it again at 20mm/s for top quality and go away for the weekend :)

"Why I have to reduce prints speed manually". I am not sure if I understand your question fully correctly. Apart from the 1st layer where you normally want to go slower and Cura lets you set that separate speed, you do not normally need to change the print speed. Yes sometimes maybe, I have had models in the past where I have wanted to slow down somewhere vertically in the print, but for me that has been rare and you can do it manually. Simply3D has a layer height architecture which gives you lots of flexibility on changing settings at multiple layer heights; but it costs 150 bucks, Cura is free. There are a couple of add-ins for Cura that let you change at a particular layer height, whether or not they include a speed change I do not know; one is TweakAtZ and I forget the name of the other one.

 

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And your setting differs on various micro level as well. On a cold day I usually turn up my head temperature by a few degrees so I won't suffer random under-extrusion attacks. Even different colours of the same material from the same brand may result in a +-5 degree difference. Same material from different brands may cause an even bigger +-20 degree difference.

 

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