Brand new prusa mini: Z needs to adjust on every new print.
 

Brand new prusa mini: Z needs to adjust on every new print.  

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Simon
(@simon-10)
Active Member

Hi,

3 days ago I received my brand new Prusa Mini with upgraded parts. I already have 2 other printers but its my first prusa.

I made the first layer calibration and ended up with Z Offset -1.2 mm on the PEI sheet. After one succesful print I switched the sheet to the powder coated and had to adjust Z offset to -1,5 mm (the powder coated sheet is ~0,3 mm thiner).  The first print looks very well.

But on the second print, the nozzle starts far to high. And I have to go down to -1.6mm. It always goes on.

Now I'm at -1,9 mm and I'm not sure whats going on.

Everything is very stiff and nothing wobbles. The prints overall looks very good and constant

What can cause the problem? Maybe a problem with the sensor?

 

Thanks for your help

Posted : 21/03/2020 1:44 pm
Area51
(@area51)
Honorable Member

Take a look on this thread on common Mini issues it could be that the three grub screws in the cooler is loose.

Have a look at my designs on Thingiverse or on PrusaPrinters ;)...
Posted : 22/03/2020 1:27 am
Rick
(@rick-2)
Trusted Member
Posted by: @area51

Take a look on this thread on common Mini issues it could be that the three grub screws in the cooler is loose.

You would think that loose grub screws would allow the nozzle to slip down closer to the bed. That hasn’t been my experience. We keep having to dial in more negative offset which would mean the nozzle is essentially moving higher after a print, not down. It acts more like the Minda is triggering higher which would make the nozzle first layer higher. 

Posted : 22/03/2020 2:05 am
Tracy
(@tracy)
Trusted Member
Posted by: @rick-m12
Posted by: @area51

Take a look on this thread on common Mini issues it could be that the three grub screws in the cooler is loose.

You would think that loose grub screws would allow the nozzle to slip down closer to the bed. That hasn’t been my experience. We keep having to dial in more negative offset which would mean the nozzle is essentially moving higher after a print, not down. It acts more like the Minda is triggering higher which would make the nozzle first layer higher. 

I'm experiencing the exact same problem.

Posted : 22/03/2020 11:12 am
Simon
(@simon-10)
Active Member

Thanks for reply.

All screws are tight. Nothing is loose.

Today I startet a print and I starts on the right high. Very strange...

Posted : 22/03/2020 11:50 am
Area51
(@area51)
Honorable Member

It could be that the nozzle gets pushed up, when printing. But if the grub screws are tight, this is not the problem.

I have trouble with the Minda getting warmed up on the first print and the following prints have wrong z-height (MDL). If I let the printer (Minda) cool down between prints the z-height is more consistent.

I added this line:

{if layer_z < max_print_height}G1 Z{z_offset+max(100, min(layer_z+30, max_print_height))}{endif} ; Move print head up

in End-Gcode to always have the printer head move at least 100 mm over the print bed after each print to keep the Minda cool.

Have a look at my designs on Thingiverse or on PrusaPrinters ;)...
Posted : 22/03/2020 12:20 pm
Oxygen liked
Simon
(@simon-10)
Active Member

@area51

I think that isnt the problem. I startet the different prints with many hours between. And I always completely turn of the printer after finished the print.

Posted : 22/03/2020 3:21 pm
Rick
(@rick-2)
Trusted Member

@area51

I have this problem on the first print of the day. Heat may make it worse but I don’t think that is the root cause. 

Posted : 22/03/2020 4:19 pm
Area51
(@area51)
Honorable Member

I have no other suggestions.

The failure must be mechanical, thermal, electronic or software. As I hear it thermal failure is not the main cause, that leaves the other three.

Have a look at my designs on Thingiverse or on PrusaPrinters ;)...
Posted : 22/03/2020 5:13 pm
Benji
(@benji)
Estimable Member

My workaround is to always preheat the Mini to PLA temps for a minimum of 5 minutes before starting a print from a cold start.

You can take a look at some results from a few tests here: https://docs.google.com/spreadsheets/d/1n3qKVI80dKMmc-eSm6O3HZD5gDbHZ-0_29ghAAEJTq8/edit?usp=sharing

Prusa Mini Silicone Bed Leveling Mod: https://bbbenji.github.io/PMSBLM/...
Posted : 22/03/2020 6:01 pm
Oxygen
(@oxygen)
Reputable Member

OK, seems fair but where do you park your head (do you pre-heat the MINDA) by going home (leaves the head near the bed)

Mini with FW:4.2.1 + Bondtech Heatbreak...
Posted : 22/03/2020 6:20 pm
Benji
(@benji)
Estimable Member

@oxygen

That's right. I keep the extruder close to the bed to get the MINDA nice and toasty. 

Prusa Mini Silicone Bed Leveling Mod: https://bbbenji.github.io/PMSBLM/...
Posted : 22/03/2020 6:47 pm
Simon
(@simon-10)
Active Member

I think preheating Minda is more a workaround than permanent solution. But I will also investigate if that fixes the issue....

Posted : 22/03/2020 6:54 pm
Rick
(@rick-2)
Trusted Member

If it's important for the Minda to be warmed before running mesh leveling then the firmware should home the z axis first (x and y would also need to be homed) before warming the hot end and bed. As I recall this is one difference between the Mini and the MK3S. I think the MK3S sensor (Pinda) is temperature compensated. 

Posted : 22/03/2020 11:46 pm
Rick
(@rick-2)
Trusted Member

I just ran an unscientific yet useful test on the Mini.  The printer has been off all day since I haven used it since last night. (Yes, I know, the flogging is scheduled for 10pm. Bring your own wip.)  

I turned the printer on an immediately ran a first layer calibration. The head was about 25mm above the bed when I started. When that first calibration was done I pulled off the printed filament then started a second calibration run. The logic here is that he probe should be warmed up a bit from the first calibration. Again I pulled off the printed filament. 

There was an obvious difference between the two calibration runs. Note that I did not make any changes to my z level during either calibration run. The first calibration filament was not "squished" as much as I would like and came off easily. The second calibration run looked much better and it took more effort to get the filament off the bed. So, that may lend credence to the theory that warming the probe may make some difference.

That's a test that anyone can do for themselves. I'd be interested to see if anyone else sees the same result. It will require you to leave your printer off for an extended period so be prepared for a group flogging when you announce your results.

Good Printing!

 

P.S. I'm going to do the same test on the MK3S to see if I get a similar result on that printer.

Posted : 23/03/2020 12:04 am
Area51
(@area51)
Honorable Member

I experience the same difference on my Mini and that was the reason for raising the Minda after every print.
My MK3s is far more tolerant to Pinda temperature changes.

Have a look at my designs on Thingiverse or on PrusaPrinters ;)...
Posted : 23/03/2020 2:47 am
Rick
(@rick-2)
Trusted Member

I saw no difference in the two back to back tests on the MK3S so this appears to be specific to the Mini.

Posted : 23/03/2020 3:19 am
Simon
(@simon-10)
Active Member

Hi, I also did this test.

I startet with the value from my last print -1,86mm Z-Offset.

The printer was cold, last prints a few hours ago.

I startet the calibration with same Prusament and powder sheet.

1. Calibration = -1,86mm was far to low--> need to adjust to -1,53mm

2. Calibration=  -1,53mm was far to low --> need to adjust to -1.375mm

3. Calibration= -1,375mm just fine

Startet a print and -1,375 just fine.

Seems like the whole Minda is a design fault. I think this is not acceptable, needs to be fixed. 

Why do so few users have this problem? Should actually affect everyone.

This post was modified 8 months ago by Simon
Posted : 23/03/2020 11:02 am
C Spucker liked
bobc
 bobc
(@bobc)
Reputable Member

In my experience, the first layer Z height is higher if printing from cold, and lower (ie. more squishing)  if printing from hot.

Personally, I would pay more for a temperature compensated probe. Consistent Z height would streamline the printing experience a lot. The PEI sheet is more tolerant, but I find with TXT sheet the first layer height is quite critical.

As a workaround, adding Gcode to park the head in a set position, and preheat for X minutes might help.

Posted : 23/03/2020 12:58 pm
Simon
(@simon-10)
Active Member

Just startet a print.

This time I preheatet the Minda (WTF). But I have to adjust Z-Offset from -1,375 to -1,574mm....So its not THE solution to preheat a sensor...

Posted : 23/03/2020 7:04 pm
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