Prusa Mini Hotend occasionally crashes in to PEI and ruins the surface
 

Prusa Mini Hotend occasionally crashes in to PEI and ruins the surface  

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Bonjipoo
(@ben-9)
Active Member

I’ve had my Mini for a while now. All the Z heights are calibrated and printing perfect first layers. I have one file I print everyday and all the code works fine however....

Occassionally on the very first print of the day the printer crashes into PEI and drags the hotend across the surface marking it and damaging it forever. I’m now on my third PEI sheet and PRUSA not sure what the issue is. 

After selecting print and doing a seemingly normal bed mesh level the printer starts at z height zero. The extruded clicks because it can’t push any plastic out. Obviously the hot end in contact with the PEI damages it leaving a molten dent in the surface. 

PRUSA tell me that the MINDA probe senses differently depending on the temperatures but surely this shouldn’t happen to this degree. 

Anyone have any ideas what could be at fault. It’s intermittent so you never know when it’s going to do it. 

Best Answer by joan.t:

The Minda is like the first generation Pinda, Both have the thermal sensitivity. 

With my Mk2 I always start with my extruder about 60mm above the build plate, and this seems to mitigate the issue for me.

you may find that you need to do a live z calibration starting from 60mm to begin with... 

you are trying to get the printer into a regular cycle... to overcome outside influences

if you start mesh bed levelling from near the printbed, because that is where your last print, ended, (and your last print was a live Z test...) you are likely to get different results, compared to starting mesh bed levelling from the height your last Tree Frog or taller

I hope this helps

Joan

 

Posted : 13/09/2020 12:02 pm
joan.t
(@joantabb)
Veteran Member Moderator

The Minda is like the first generation Pinda, Both have the thermal sensitivity. 

With my Mk2 I always start with my extruder about 60mm above the build plate, and this seems to mitigate the issue for me.

you may find that you need to do a live z calibration starting from 60mm to begin with... 

you are trying to get the printer into a regular cycle... to overcome outside influences

if you start mesh bed levelling from near the printbed, because that is where your last print, ended, (and your last print was a live Z test...) you are likely to get different results, compared to starting mesh bed levelling from the height your last Tree Frog or taller

I hope this helps

Joan

 

I try to make safe suggestions,You should understand the context and ensure you are happy that they are safe before attempting to apply my suggestions, what you do, is YOUR responsibility. Location Halifax UK...
Posted : 13/09/2020 6:20 pm
Karl Herbert
(@karl-herbert)
Famed Member
Posted by: @ben-9

I’ve had my Mini for a while now. All the Z heights are calibrated and printing perfect first layers. I have one file I print everyday and all the code works fine however....

Occassionally on the very first print of the day the printer crashes into PEI and drags the hotend across the surface marking it and damaging it forever. I’m now on my third PEI sheet and PRUSA not sure what the issue is. 

After selecting print and doing a seemingly normal bed mesh level the printer starts at z height zero. The extruded clicks because it can’t push any plastic out. Obviously the hot end in contact with the PEI damages it leaving a molten dent in the surface. 

PRUSA tell me that the MINDA probe senses differently depending on the temperatures but surely this shouldn’t happen to this degree. 

Anyone have any ideas what could be at fault. It’s intermittent so you never know when it’s going to do it. 

This can happen, however. To avoid consequential damage, I have constructed a Z-axis stop that can prevent such collisions:

Z-protection

 

Statt zu klagen, dass wir nicht alles haben, was wir wollen, sollten wir lieber dankbar sein, dass wir nicht alles bekommen, was wir verdienen....
Posted : 24/09/2020 8:43 am
bobcousins
(@bobcousins)
Reputable Member
Posted by: @karl-herbert
Posted by: @ben-9

I’ve had my Mini for a while now. All the Z heights are calibrated and printing perfect first layers. I have one file I print everyday and all the code works fine however....

Occassionally on the very first print of the day the printer crashes into PEI and drags the hotend across the surface marking it and damaging it forever. I’m now on my third PEI sheet and PRUSA not sure what the issue is. 

After selecting print and doing a seemingly normal bed mesh level the printer starts at z height zero. The extruded clicks because it can’t push any plastic out. Obviously the hot end in contact with the PEI damages it leaving a molten dent in the surface. 

PRUSA tell me that the MINDA probe senses differently depending on the temperatures but surely this shouldn’t happen to this degree. 

Anyone have any ideas what could be at fault. It’s intermittent so you never know when it’s going to do it. 

This can happen, however. To avoid consequential damage, I have constructed a Z-axis stop that can prevent such collisions:

Z-protection

 

That's a great mod! Maybe it should be standard...

Note that the MINDA has a temperature compensation problem (by design), but it is more or less linear (hotter = lower). BUT there are a number of MINDA sensors that suffer a more random variation, these are faulty (by manufacture). If you can persuade Prusa support you have one of the faulty ones, they will replace it.

I've realised that quite a few of the people reporting Z issues may have a faulty sensor of the second kind (manufacturing fault).

Reference : https://forum.prusaprinters.org/forum/hardware-firmware-and-software-help/analysed-problems-with-minda-probe/

Posted : 24/09/2020 1:06 pm
Bonjipoo
(@ben-9)
Active Member

@joantabb


So I have to thank you on this. 

I set the gcode to lift a good 50mm after a print and for some reason unknown to me it appears to have resolved the issue. 

It’s worth mentioning that this problem occurred while printing thin models of approximately 7mm. Maybe the Z needs to start above a certain height for the sensors etc to be ready. 

Posted : 28/09/2020 3:39 pm
Karl Herbert
(@karl-herbert)
Famed Member

@ben-9

To prevent Z-crashes, I have mounted a buffer. In case the sensor fails or any other error occurs, the nozzle or the printing bed cannot be damaged.

The buffer is made of two parts. The upper part is exchangeable and must be adjusted individually for each printer.

 
Statt zu klagen, dass wir nicht alles haben, was wir wollen, sollten wir lieber dankbar sein, dass wir nicht alles bekommen, was wir verdienen....
Posted : 28/09/2020 7:22 pm
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