PET Top layer gaps

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Joined: Mon Apr 23, 2018 10:37 pm

PET Top layer gaps

Postby Truktruk » Mon Apr 23, 2018 10:54 pm

Ive been tuning the our settings to get good results printing some Novamaker PETG on a N2+ at work. Ive made leaps and bounds but im still having problems with the top layers not bridging for shit an getting terribly gummed up in places. I understand this is the nature of PET and I intend to add a bit more infill (its not printing well either) but Im looking for some input to point me in the right direction to get the top layers printing better.
Heres what im at now.

Hot end------------240C
Infill Speed--------50mm/s
Infill Flow rate----80%
Top Layers--------4
Top Speed---------30mm/s
Top Flowrate------60%

FWIW Ive got retraction enabled and the bottom/walls print great.

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Re: PET Top layer gaps

Postby Jetguy » Tue Apr 24, 2018 2:24 am

Why on earth anything less than 100% flowrate?
Right there is your massive failure.
Top Flowrate------60% And you wonder why it's under extruding????

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Joined: Tue Mar 22, 2016 1:40 am
Location: In a van, down by the river

Re: PET Top layer gaps

Postby Jetguy » Tue Apr 24, 2018 2:56 am

Let's go back to basics 101 of 3D printing and slicing and what flowrate is, what and how extrusion volume is calculated, and what you as the user are expected to know and understand about this.

So you have an STL and you slice it.
In doing so you give the slicer the following values
Layer height
Nozzle diameter and extrusion width (although extrusion width is the number actually used)
Filament diameter

With those basic values, it's going to create the individual line segments of gcode line by line, layer by layer.
A gcode segment is also a logical volume of space.
You set the layer height, so that's the height
You set the layer width (extrusion width)
Then the segment has an XY (because it could be diagonal) length distance
So, the slicer has volume of that segment because it has length, width, and height of that line of extrusion
You also gave the slicer the filament diameter and that should match your actual measured filament diameter
From those numbers, again, we know the segment has a logical volume. The filament going into the extruder of a known diameter is a cylinder of infinite length. So what happens is the slicer takes the volume and the filament diameter to get the E extrusion length of filament to be pushed into the extruder to equal the volume of plastic for that line segment.

Flowrate is simply a multiplier of final value of that E length.
So if theoretical is exactly what it should be (you measured filament diameter, that's what the slicer used, your extruder and e steps per mm is spot on and not slipping or skipping steps) then you get a perfect print output of correct volume.
Anything less than 100% flowrate is the extruder being told to put less than the ideal amount of filament.
Again, what flowrate is modifying is the theoretical ideal linear distance of filament pushed into the extruder nozzle. "Ideal" is 100% anything less is less filament, anything more than 100% is more filament.

Reasons why that might be "logical":
#1 is a very old carryover of using flowrate to make up for the difference in E steps per mm as gripped by the drive gear.
In other words, depending on how hard or soft the plastic filament is, the drive gear teeth bite into the surface differently. That depth determines the effective rolling distance of how much filament is pushed per rotation of the motor. ABS was the standard filament back in 2010-2011 timeframe and PLA was the new experimental filament. As such, Makerbot started the 94% flowrate convention for PLA because PLA is a harder filament and the drive gear did not get the same tooth depth in the side of filament and so more PLA was pushed per rotation, so 6% reduction was applied.
#2 minor, often under 5% ratios (both over or under 100%) could be fine tuning a filament error compensation (minor steps per mm error in the extruder feeder and/or minor error of actual filament diameter VS input diameter at time of slicing).

So beyond 10% deviation from 100% flowrate- it means you done messed up Ay-Ay-ron
you done messed up A_Aron.jpg

Again, normal compensating factors like minor error in measuring filament diameter or just that roll is that far out, or say for whatever reason minor variation in effective diameter of the drive gear due to filament surface hardness and tooth profile. These should be under 10% and hopefully not more than 5%

So effectively, what is happening is:
Your 80% flowrate on infill is leaving infill under extruded by 20% and every layer adds up and so infill may not be as high as indicated by layer height (the stacking effect of under extruded layers). But then the icing on the cake 60% flowrate is 40% under extrusion on roof layers?? Sure, this stretches the noodle of plastic across the span by definition of what under extruding is (pushing less plastic and moving the nozzle the same distance) it's going to stretch that line of extrusion. However, it's also likely to break or simply not bond at the ends and certainly creates a thin under extruded adjacent threads that do not fill the space. Again, yes, in the past, some slicers might reduce flowrate on the very first layer of a bridge or roof infill to create tight threads across the space, and then after that first layer, then crank up and go full normal flowrate. But in that case, it was in the 5% range, not 40% less.

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