EZStruder kickback on Rostock Max V3

[dmpalmer]

On my new from-kit Rostock Max V3, I am getting bad EZStruder kickback.

This is where the EZstruder stepper and gears tries to push the PLA down the Bowden tube, and the motor suddenly jumps back about 30 degrees. (This is not a software commanded retraction in the GCode.)

As a result, much less PLA is extruded than is expected based on the number of stepper pulses sent to the extruder motor. The prints end up more void than plastic.

I am trying to print at 60 mm/sec (vs advertised value of 100 mm/s max on SeeMeCNC's website). Slowing things down by a factor of 2 seems to reduce (but does not eliminate) the amount of kicking.

This seems to be a long standing problem even before the current generation of HE280. e.g.
http://forum.seemecnc.com/viewtopic.php?t=4732

What is the current recommendation on how to fix it, especially with the current generation of Rambo, Rostock, EZStruder, and hot end?

[mhackney]

What are your slicer's retract settings? retract length and retract speed

Can you easily push filament through the hotend (at temperature) by disconnecting the Bowden tube at the hotend side and pushing the filament by hand to look for excess friction or snagging?

Can you easily push filament though the bowden tube by hand by disconnecting it at both the hotend and ezrstruder?

Can the system extrude filament "in air" without issue? I.E. use the extruder controls in Matter Control, Repeater, OctoPrint or whatever controller you are using. Extrude several 100 mm at 10mm/s to test.

Have you checked that the cog in the ezrstruder is properly aligned as per the assembly guide using a short piece of filament?

Have you verified the current setting in firmware was not inadvertently lowered?

[dmpalmer]

It kicks when not extruding.

When I have the temperature at 210 (using Excelvan PLA labelled as 190-220 C) and the ezstruder motor is unpowered, I can turn the knob about half a turn before it it stops due to back force from the fillament. (When I let go it springs back quite a bit.)

Pushing into the air at 10 mm/s doesn't cause problems. I can crank the speed up a lot and then I get the kickback behavior.

The EZstruder looks and feels aligned, and it doesn't seem to have a problem with gripping. (It grips so well that the back force on the filament can jump the motor backwards.)

I didn't touch the current values. configuration.h says:
#define MOTOR_CURRENT {140,140,140,130,0} // Values 0-255 (RAMBO 135 = ~0.75A, 185 = ~1A)

With a nominal speed of 60 mm/s, if I set the temperature to 210 C I get kickback until I drop down to 30% of the nominal speed.
If I set the temperature to 220 I get no kickback at 85% nominal, and only a little at 100% nominal. Going to 225 I get no kickback at 100% nominal.

Might my problem be a miscalibrated thermister (it seems to give good temperatures when cooled)? What about filament that wasn't QC'd to have a good melting temperature?

What is an unsafe temperature for PLA?

[Polygonhell]

Did you check the actual temperature of the hotend with a thermocouple.
If you can't push it through relatively easily by hand, the temperature is most likely too low.
I've seen NTC thermistors misread by as much as 30C at 220 degrees when using the Beta value from the spec sheet.

[dmpalmer]

I don't have a thermocouple. I guess that's my next tool to buy. (Any recommendations for a reasonably good cheap one?)

If the thermistor were off by so much at 220 C, wouldn't you expect more than a few C in error at room temperature? My bed and nozzle read the same value to about half a C, and in agreement with another thermometer.

[Penged]

I bought this one and it has done well for me.
https://www.amazon.com/gp/product/B00FF ... UTF8&psc=1

[Polygonhell]

I don't have a thermocouple. I guess that's my next tool to buy. (Any recommendations for a reasonably good cheap one?)

If the thermistor were off by so much at 220 C, wouldn't you expect more than a few C in error at room temperature? My bed and nozzle read the same value to about half a C, and in agreement with another thermometer.

No the thermistor curves are fixed at 25C @ 100KOhms give or take and then there is a quadratic curve that approximates the mapping from resistance to temperature. The curve is dictated by a "Beta" value, and that value is usually in the data sheet, but it's usually tuned to be accurate near room temperature, not at 200+C

[IMBoring25]

The equation in question is b = Ln(R t1/ Rt2) / (1/T1 – 1/T2), with the Temperatures expressed in Kelvin (Degrees C + 273.15).

Both the resistance at 25 C (298.15 K) and the Beta value have to be correct for the equation to reasonably-accurately model the behavior of the thermistor across the working temperature range. If you can get two actual temperatures and the actual resistance of the thermistor at those temperatures you can calculate the beta value to put into the firmware.

[dmpalmer]

I got a thermocouple and according to it, the thermistor is actually reading low. When I set the extruder temperature in the 200-220 range, and the thermistor says that it is at the target, the thermocouple is indicating 6 degrees hotter. So running the PLA at indicated 225 (which doesn't have any extruder kickback even when printing at 60 mm/s) the actual temperature (in the same cavity as the thermistor) is 231.

What is the 'don't let the PLA get above this unless you have very good insurance' temperature?

[IMBoring25]

I don't recall hearing anything about PLA doing anything nasty at higher temperatures, but 230 is already pretty absurd for it. 230 should give easy extrusion by hand even if it's mislabeled ABS (and if it IS mislabeled ABS, that does emit some nasty stuff as the temperatures go higher than 250-260).

Some fundamentals...Is your hot end fan turning and is it turning the direction that blows air across the hot end instead of the direction that would draw air across it? Have you inspected the condition of the nozzle? Is the hot end assembled exactly per the instructions? What retraction settings are you using? If you heat the hot end, push filament into it, cool it just below the melting point, and pull it back out, is there a thick spot in the filament?