Mk3 Extruder Upgrade to Mk3S Problems please help  

Active Member

I have been upgrading my Mk3 to Mk3s all went well till I started to fit the Print cooling fan and shroud, I cant seem to get the fan to sit in the shroud so I can tighten the 18/20mm securing bolt (which is the 4the screw in Hot End Cooling Fan) with out the fan buckling, if I tighten the fan in with the two diagonal 3mm bolts then fan shroud wont fit as the picture below shows

The Fan unit is my original print fan it only sits into the shroud on 3 faces the 4th is a but joint is this correct

my last picture shows when I bolt the fan shroud into position how the Fan is out of alignment with the mounting brackets.

The components have all gone together like Prusa components do, Like clockwork but these don't and If I force them together the fan shroud buckles under the force,,, this cant be wright..

My other dilemma is I'm waiting for the MMU2s upgrade and I'm now not sure if I want to fit this as the room it takes up is more than I have so probably will sell it before its even used.. any suggestions on this as well would be appreciated..

Posted : 23/03/2019 2:11 pm
Active Member

Hi. It appears that I have the same problem. The angle of the fan shroud where you insert the fan, when you tighten the bolt, is not the same as the angle on which the fan is mounted on the extruder cover.
I will try to combine the fan shroud, the extruder cover and the print fan support in a drawing program and see where it goes wrong.

Posted : 31/03/2019 6:26 pm
Eminent Member

Same here, I did the best I could to make them match up, but it is quite infuriating.

Posted : 01/04/2019 4:41 pm
Active Member

I made my MK3S upgrade few days ago. It took some effort to place the fan inside the fan shroud properly but in the end it fits OK.

Posted : 01/04/2019 11:45 pm
Doug Bailey
Eminent Member

I just completed my upgrade from MK3 to MK3S extruder. Two issues cropped up:

1. The X-Axis seems to be calibrated incorrectly. The printer "homes" to the right point and the setup (finding calibration points) is perfect. But when it starts a print, it seems to take a quick 1cm jog to the left, and then starts the bed leveling from a new "home" that is right at the edge of the bed. This looks like a firmware problem of some kind to me.

2. Using Slic3r and the standard PLA profile for 0.4mm nozzel, 0.15mm "Quality" - the only one I tested - the printer gets through layer 1 and then the fan turns on 100%. Extruder temperature dives from 215C to 192C and then climbs slowly back to 196C before the printer shuts down with a "Thermal Runaway" message. It's easy to fix - turn max fan down to 50%. Then the temperature dive is just to 199C and it climbs slowly back to 215C - the printer keeps working. I'm not sure why this happens - perhaps a hardware problem (heater lousy?) or perhaps some kind of heater control loop problem.

I'd appreciate help and insights - or just shared pain if anyone else is seeing the same thing.

Posted : 07/04/2019 7:25 am
Doug Bailey
Eminent Member

And one more thing - the old nylon filament from the extruder to the control box to stiffen the cables is way too short for the new textile cover and does not plug into the hole on the printer controller enclosure.

Posted : 07/04/2019 7:27 am
Doug Bailey
Eminent Member

Actually, after looking at the design I think that there's probably a design error with the new fan shroud. The blast of cold air is directed right at the hot end where the thermistor is located - which is a long way from the heater. I'm speculating, but I think that the cold air is hitting the thermistor and cooling it down too much which triggers the Thermal Runaway error. The heater is on the other side of the aluminum block and it takes a few seconds for it to ramp up to the new heat delivery level required by the fan. Probably we're going to need a new extruder fan shroud design.

Posted : 07/04/2019 8:03 am
Grandpa Joe
Eminent Member

I am also having "Thermal Runaway" errors. They started recently. I have an MK2.5 and broke a wire on my Thermistor. While replacing the Thermistor, I also broke a wire to the heater cartridge. ( not being careful enough I guess.).

Anyway, after all the replacements, I now have a "Thermal Runaway" problem. I have made sure that both the heater cartridge and the thermistor are tight and secure in their spots. Both are plugged in to board.

It also occurs when the first layer is printed and the second layer is starting. The fan jumps to 256 and temp drops enough to trigger the "Thermal Runaway". If I am quick enough to get to settings and turn fan setting down to about 70, the temp climbs back up, stabilizes, and the print continues.

I tried starting out with the fan set to 70 ( is this a speed or a % of maximum? - if it is a %, how does mine jump to 256 ? - does that mean 256% ? )
Anyway - I set fan to 70, started my print. At the end of the first layer, the fan again jumps to 256 and if not turned back down manually, "Thermal Runaway" hits. I was able to turn it down quickly enough to continue to print. Later on it again jumped to 256. I was able to turn it down - this time to 60 - and continue. The third time it happened, I was unable to catch it in time and the print stopped.

If I can't set the fan and have it stay at the setting, how do I control the fan speed ?

Posted : 07/04/2019 11:34 pm
Doug Bailey
Eminent Member

I fixed my issue in Slic3r by turning the fan speed max to 50% for the PLA profile. This fixes getting through a print, but I still think that the new MK3S extrude fan shroud design is poor. Firstly, as someone pointed out, it does not fit well, and secondly it directs a blast of cold air right at the thermistor - the heater can't keep up!

Posted : 08/04/2019 7:08 am
Honorable Member

And one more thing - the old nylon filament from the extruder to the control box to stiffen the cables is way too short for the new textile cover and does not plug into the hole on the printer controller enclosure.

You have to twist the textile sleeve a bunch of times and it get's short enough for the filament to properly stick out enough for the Einsy box

Posted : 13/04/2019 7:01 am
New Member

I am have the same thermal runaway error on a new i3 MK3S. The fan is on full and the temperature drops and can’t seem to recover ending in a Thermal runaway. This has only happened on larger prints with a large base. Usually on the second layer.

Posted : 20/04/2019 2:55 am
Active Member

Hi there...

Is it possible, due to the torque when tightening the screw, that the fan shroud drifts slightly upwards, thereby cooling the heatblock ?


Posted : 28/04/2019 3:07 pm
Estimable Member

I recommend a silicone sock for the heatblock.

Posted : 28/04/2019 3:10 pm
Milos V.
Honorable Member

Common problem many times discussed. Silicon sock is best solution. If you do not like it, you have to reduce the fan speed manualy in menu or in Slic3r.


Posted : 28/04/2019 3:24 pm

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