A Newbie Review of the Prusa Mini

Details of the shutoff are here:

https://sites.google.com/view/smokedetectorpowershutoff/home

Filament Drying

I am using a filament drier as found at:

https://letsprint3d.net/how-to-make-a-filament-dryer/

After leaving filament out for more than about a day, quality started to decrease, with more stringing and blobs. Drying reduced those to acceptable levels. Drying is really critical for good prints. I made two sets of the tall curved pieces so I can dry two spools at once.

Cleaning The Nozzle, No Silicone Sock Available

I occasionally clean the crud off the outside of the extruder nozzle and use the supplied needle to clean the orifice. This has worked well and I haven't had to resort to cold pulls more than a couple times. I only had to remove the long Bowden tube once to get a broken piece of filament. After adding the end G-code described below, I haven't had to do either. I do wish there was a silicone sock available that fit it. Standard ones don't fit well.

I also heat the extruder up to about 270 degrees and extrude filament to clear out any old filament that may be stuck. Cold pulls are done with a max temp of 270, then cooling down to about 130-140 and pulling.

Filament Loading and Unloading

Using the park positioning modification described below with the extruder head far to the left so the Bowden tube is not as bent, I am loading and unloading filament a couple times a day with no issues.

Filament Grinding and Feed

The extruder idler adjustment is fairly sensitive. I had to loosen it some from what was shipped so it did not grind the filament. After adjustment it treats the filament pretty well.

Filament Selection and Diameter

This printer seems pretty sensitive to filament diameter. I've only used the +/- .02 or +/- .03 mm tolerance filament and had no issues. So far, I've had good luck with Prusament PETG, Prusament ASA and AzureFilm PETG. The AzureFilm PETG does take a lower temperature, but produces good results. It has less stringing than the Prusament, but may be a little weaker. I tried Sunlu PETG and it proved to be pretty weak and brittle. It suggests using quite a low temperture for PETG, so I'm not sure what it really is. From reading forums, filament that is +/- .05 mm tolerance may not work as well.

Sticking to and Unsticking from the Bed

For PETG I am having good results with the smooth PEI bed using Windex to clean it before each print and a combination of a plastic flat bladed razor and a Wilton cookie spatula to remove parts from the bed. For very thin parts I first apply some Elmers Washable School Glue stick, which is purple when wet. Then I spray on some Windex and use a Kimwipe to smush them all together evenly all over the PEI sheet. Then the parts come off easily. The ratio of glue stick to Windex determines the stickiness. If the print comes off the bed when you don't want it to, use less glue.

For ASA I am doing the same, except I wait until the bed is cooled completely to remove the parts. I clean it scrupulously with Isopropyl alcohol or even Acetone before printing. To keep tall things on the bed, I use a brim and a full height skirt draft shield you can select in the slicer.

Print Temperatures

I used the temperature tower described below to get the best printhead temperature. I am using the default bed temperatures from Prusa Slicer. For PETG, my enclosure is open so the enclosure is at room temperature. For ASA, the enclosure is closed, with a slow exhaust fan, just enough to remove the fumes but keep the enclosure temperature up so warping is minimal.

Firmware 4.2.1 And Prusa Slicer 2.2.0

I'm currently on firmware 4.2.1 and Prusa Slicer 2.2.0. It is working fairly well. I set up a static IP mapping on my router to assign it the same IP address all the time. The web interface is minimal but usable.

There are some issues that I have been able to work around by making changes in Prusa Slicer 2.2.0.

Z Azis Height Adjustment

The M.I.N.D.A. sensor is temperature sensitive and not compensated for temperature. I added the gcode I found at github.com/prusa3d/Prusa-Firmware-Buddy/issues/309 to the Start G-Code in Prusa Slicer:

;added for better Z axis control

G28 ; home all

G1 X100 Y100 F4000G1 Z1.5 F50 ; park position

;end of added section

M104 S170 ; set extruder temp for bed leveling

M140 S[first_layer_bed_temperature] ; set bed temp

M109 R170 ; wait for bed leveling temp

M190 S[first_layer_bed_temperature] ; wait for bed temp

;added

G4 S80 ; wait 80 secs when heated up

;end of added section

With this code, the leveling is consistent.

Parking After Printing, Loading and Unloading

The default position that the extruder parks after printing is a few CM above where the print finished, and close to the main support. There are two issues with this. It gets in the way when removing the print, and it leaves the Bowden tube bent when loading and unloading filament, causing binding and breakage of the filament. To fix this, I moved the park position to much higher, far left and the bed far forward. I made this change to the end G-Code in Prusa Slicer:

{if layer_z < max_print_height}G1 Z{z_offset+min(layer_z+140, max_print_height)}{endif} F720 ; Move print head up

G1 X0 Y180 F4200 ; park print head

The print is easy to remove and loading and unloading filament has been no problem. Filament that has been through the extruder gears once does have some damage. When I load the filament again, I cut off the damaged part. It does waste about 16 in of filament, but it doesn't jam.

Temperature Calibration

For each filament, I printed a temperature tower. I modified the G-Code found at projects.ttlexceeded.com/3dprinting_prusaslicer_gcode.html#other-custom-g-code that is run on layer changes:

;[layer_z]

{if layer_z <= 11.5}M104 S260

{elsif layer_z <= 21.5}M104 S255

{elsif layer_z <= 31.5}M104 S250

{elsif layer_z <= 41.5}M104 S245

{elsif layer_z <= 51.5}M104 S240

{elsif layer_z <= 61.5}M104 S235

{elsif layer_z <= 71.5}M104 S230

{elsif layer_z <= 81.5}M104 S225

{elsif layer_z <= 91.5}M104 S220

{elsif layer_z <= 101.5}M104 S215

{elsif layer_z <= 111.5}M104 S210

{endif}

It is modified to match the temperature tower where each section is 10mm high, with a 1.5 mm base. I selected the temperature that looked best to me for each filament.

The OpenScad code to generate the tower is modified from https://www.thingiverse.com/thing:2615842 to have sections start at 10mm intervals:

Sample files are linked at the end of this section.

I then adjusted the extrusion multiplier in Prusa Slicer by using the procedure here:

https://help.prusa3d.com/cs/article/extrusion-multiplier-calibration_5908

Slicer settings

I modified the slice settings for somewhat lower stringing by increasing the retraction and decreasing the Z lift a little. Making too big a change reduced stringing more, but resulted in poorer quality surfaces.

I used the method described here to print multiple copies of a test shape and vary retraction parameters:

http://projects.ttlexceeded.com/3dprinting_techniques_calibrating_retraction.html#retraction-calibration

An example is linked below with varying retraction settings to reduce stringing.

Conclusion

With the above adjustments, I have pretty consistently good results. I just start it up in the morning and print stuff for 16 hours. I don't print when someone is not awake, even though I did set it up with the smoke detector shutoff.