Any developers in here? Does slowing speed increase extrusion width?

[jeevesme 0]

I am getting under extrusion only during infill. If I decrease only the infill speed by half, will it make the infill lines thicker?

[DidierKlein 729]

Hi and welcome,

 

It will not make the infill lines thicker, but if you are having underextrusion on infill then reducing the infill speed makes a lot of sense.

 

The result of slowing speed will reduce the underextrusion you have (if the underextrusion is caused by speed and not something else) so in a way it will make the infill lines thicker, but in theory, with the initial speed the line thickness should be the same (if what i say makes sense :P)

 

What speed and what layer height is it? (And what is the material printed and temperature?)

[jeevesme 0]

60mms .2 layers PLA 205

 

Does Cura adjust the extrusion rate if I change just the infill speed then? If it does then why even have a separate option to just change the infill speed?

[gr5 2,172]

So cura creates gcodes which are coordinates and speeds in X,Y,Z,E notation.  If your line width is 0.4mm for example and your Z isn't moving and your layer height is 0.2mm and the distance to the next point is say 10mm then cura multiplies these 3 numbers and calculates the volume of the cuboid: 0.2 X 0.4 X 10.  Then it increase the E position by the exact amount to extrude this amount of filament.

 

THIS IGNORES THE SPEED.  In other words - when cura calculates the E value - the extrusion amount - it ignores the speed.

 

In addition the gcodes include an F value for "feedrate" which is how fast the X and Y axes move.  Marlin - the firmware on UMO, UM2, UM3 - tries to achieve this speed (but doesn't always get to that speed if the distance is short and it can't accelerate up to that speed).  Usually paths longer than 2mm are enough to get up to full speed on a UMO,UM2 (UM3 is a bit less acceleration so it takes a little longer to get up to high speeds).

 

Anyway back to the point - print speed does not affect the amount of extrusion in the gcodes.  However in practice, the faster the feeder is moving, the more pressure you are likely to have in the print head, which means more back pressure on the feeder, which means it can slip a bit.  it's quite common for the feeder to slip 10% during typical printing speeds.

 

Also there is a delay between when you change speed and when you get equilibrium pressure in the nozzle and the nozzle is printing properly at the new speed.  So if infill is printed faster then you get underextruded infill at first and then it over extrudes when it goes back to the shell.  For this reason I usually set ALL the printing speeds to the same value in Cura.  But most people don't care much about quality of infill or inner shells and it *does* save a lot of time to print infill faster.

 

[yellowshark 153]

2 hours ago, jeevesme said:

60mms .2 layers PLA 205

 

Does Cura adjust the extrusion rate if I change just the infill speed then? If it does then why even have a separate option to just change the infill speed?

If let's say you print everything at 40mm/s the Cura will calculate and deliver the correct pressure in the extrusion system to achieve the print. If you on a subsequent print then increase the speed of infill to 60mm/s Cura will then increase the pressure at the start of the infill segment to cope with the extra speed; at the end of the infill segment Cura will reduce the pressure back to that need for 40mm/s. Please note that whilst I say "Cura will", it may be the firmware that does this based in instructions given by Cura - in the end the same thing from a user perspective. Now the change in pressure, either way, is not instantaneous and can cause artefacts to be printed, which when moving to the infill is not important because one cannot see the infill. But when moving from infill to perimeters if it happens it can be seen. This is why I, and as @gr5 says, many people will print at the same speed for everything to avoid changes in pressure. I will only consider printing infill at a faster speed when the model is either large with a large horizontal surface area, or if surface artefacts are not an issue, perhaps when doing a very early prototype.

 

Also you can ameliorate surface artefacts from the change of speed from infill to perimeters by printing a couple of more perimeters and/or printing internal  perimeters first.