Why did I have to set my extrusion rate to 120%?

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Why did I have to set my extrusion rate to 120%?

Postby daveculp » 2018-Mar-Sat-22-Mar

Why did I need to set my extrusion rate to 120%?

TLDR: Printrbot Simple Metal looks like it was underextruding. Played with lots of hardware and software settings noting worked other than upping the extrusion rate to 120%. That seems high, what could cause it?

I had noticed that my Printrbot Simple Metal looked like it was underextruding. The prints were coming out OK but it was definitely underextruding. I thought I possibly had a partially clogged nozzle. I am actually giving this printer away to my brother and his two girls so tonight I did the following:

1. I just replaced the entire extruder instead of just the nozzle. I wanted my brother to get the printer with a brand new extruder and nozzle.

2. replaced the weak part cooling fan and duct with a blower fan and better duct.

I then went to print a test cube and set the Z offset. I got it set and the first layer was coming out good but every layer above it was thin, despite a new extruder it was still underextruding.

I tried the following:

1. Turned extraction off - I thought it was extracting too far. Not it, still underextruding.

2. Played with printing temps, printing at low and high temps - Nothing.

3. Various other slicer settings - none of which helped.

4. Played with varying the motor current to the extruder - No help.

Finally I started upping the extrusion rate. At 120% it finally looks good although I could probably bump it up a little bit.

120% seems high. What could cause this?
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Why did I have to set my extrusion rate to 120%?

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Re: Why did I have to set my extrusion rate to 120%?

Postby cbxbiker61 » 2018-Mar-Sun-01-Mar

#if defined(EXTRUDER_GEARHEAD)
#define EXTRUDER_STEPS_PER_MM 127
#elif defined(EXTRUDER_OLD)
#define EXTRUDER_STEPS_PER_MM 96

I'd say you changed to the gearhead extruder and didn't change the Marlin config.
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Re: Why did I have to set my extrusion rate to 120%?

Postby RetireeJay » 2018-Mar-Sun-06-Mar

While the X, Y, and Z axes have definite calibration figures defined by the hardware used - mm per tooth on X and Y, mm per thread on Z, the Extruder calibration is a bit more empirical.

The goal is to extrude the right Volume of plastic to match the ideal volume for the trace being laid down. Now, Volume is the multiplication of the Area of the filament going into the extruder times the Distance of filament that went in.

You calibrate the Distance by pulling the filament straight up above the extruder and making a mark at, say, 50mm above. Then use the "manual" controls to call for 30mm of filament to be extruded. If the filament doesn't move exactly 30mm, then you need to correct the steps per millimeter and try again until it does move 30mm. For example, if it move 20mm, then you know your steps per mm is too low. You'd multiply it by (30/20) and try again. if it was previously 100, you'd set it to 150. You set it in the printer with the M92 command: M92 E150. Once you have the calibration set correctly, it's very important to save this calibration in EEPROM with the M500 command. That way, your printer will remember this calibration after you turn it off and turn it back on again. Usually this number is pretty stable, even when you change filaments; set it once and forget it.

The other part of the calculation is Area. Actually the slicer takes care of this part, and it uses the filament diameter to calculate the area. If you have digital calipers, you can measure the true diameter of your filament, which is probably not exactly equal to the purchase specification. In fact, the filament is probably slightly oval. So you measure both the maximum and minimum "diameter" on a little piece of filament and use the average as your specified diameter in your slicer. Ideally you do this for every spool of filament. But if you have a good filament supplier, that may not be necessary.
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Re: Why did I have to set my extrusion rate to 120%?

Postby daveculp » 2018-Mar-Sun-12-Mar

Thanks for the help guys. I calibrated the extruder this morning and it seems to be working. I do have the gearhead extruder which I bought as an upgrade when I bought the printer around two years ago. I did reset the steps per mm on it when I bought it. I wonder what would cause it to be so far off all of a sudden - could the stepper be going bad?
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Re: Why did I have to set my extrusion rate to 120%?

Postby RetireeJay » 2018-Mar-Sun-13-Mar

Stepper motors are extremely simple inside - just copper coils and iron, plus bearings on each end of the shaft. It's extremely rare for one to fail. If you were able to get a good repeatable calibration, then there's nothing wrong with your motor.

Some slicer "tuning" files have a line in the startup code which changes some of the parameters that actually should be constant and defined in EEPROM; if you used a sliced file with an M92 Exx followed by an M500, then your extruder calibration got changed - perhaps without you really being aware of it.

If a stepper motor is really "going bad" you'd hear it as grinding, clicking, or some kind of unusual noise because of mechanical wear and/or misalignment. If you remove it from the printer, a good motor has 200 magnetic "detents" as you turn it, but a bad one would have actual mechanical interference, grinding bearings, etc. Also, a bad stepper would not be consistent, so you would not be able to get a good calibration at all.
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Printrbot Plus operational January 2013
Brass threaded rods (5/16" X 18) & nuts for Z axis
GT2 belts & pulleys
Cable chain to reduce probability of fatigue failure in wires
E3D V5 Hot End, 0.4mm nozzle, also 0.8 and 0.25 in use occasionally
PB fan mount + 40mm fan -- using printed mount adapter, not the E3D supplied fan
Injection molded extruder gears
Optical Z "endstop" (custom designed and built)
Have used many pounds of T-Glase filament. Now also doing some work with Ninjaflex SemiFlex
Print on glass with Scotch Craft Stick or other glue stick
"My next printer is..." Prusa i3 MK3
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Re: Why did I have to set my extrusion rate to 120%?

Postby Mooselake » 2018-Mar-Sun-13-Mar

Unlikely the stepper is going bad. Extruder calibration isn't like the other axes where the movement is defined by the belt pulley size and step angle. Extruders also need to take into account the cross sectional area of the filament (rarely exactly 1.75 or 3.0, and if you're lucky it's consistent and round, not variable and oval), how far the extruder teeth bite into the filament, and likely the phase of the moon and how loud the coyotes howl.

The most accurate method is to measure the filament diameter (in multiple places and angles around the material, then average it), mark a carefully measured length (100mm is handy), then extrude that length and compare it to your mark. Adjust your calibration numbers (either steps/mm or whatever your slicer uses for a flow adjustment), remark, and re-extrude. After this my pre-production beta geared extruder was much more consistent than the previous hobbed gear version, likely because of a consistent bite, and most times good enough. With the previous hobber it would often vary by spool and need adjustment (I wrote the numbers on scrap paper and stuck them in the bag with the spool). For respectable name filament it'll stay the same for the whole spool, for the cheap stuff it can become an ongoing nuisance best solved with the weekly trash pickup. Once you get that down you can refine it by printing a single layer object and measuring the thickness, and compare it to what you specified. Tweak again until the width is what you wanted.

It's also not unusual to need to vary the print temperature by spool, even within the same type of plastic. PLA is a general description of a range of plastic mixes (like ABS) and colorants. That's another show, but basically hotter has better layer adhesion, cooler has less slump and better print definition, and the specific temperature can vary per spool and by the ambient air temperature. Add that number to the spool note, too.

In real life, most of the time you get it close and try not to mess with it, prints can be pretty forgiving. When I worked for a NASA contractor in the pre-PC dark ages they really did say "Good enough for Government work", although usually as good enough for a g job. Perfect is the enemy of good?
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Re: Why did I have to set my extrusion rate to 120%?

Postby daveculp » 2018-Mar-Sun-16-Mar

RetireeJay wrote:
Some slicer "tuning" files have a line in the startup code which changes some of the parameters that actually should be constant and defined in EEPROM; if you used a sliced file with an M92 Exx followed by an M500, then your extruder calibration got changed - perhaps without you really being aware of it.


This may be what happened, I have been experimenting with a bunch of different slicers lately.
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