PETG with PLA interface supports: reasonable results with default settings on MK3S+ and MMU3.  

PETG with PLA interface supports: reasonable results with default settings on MK3S+ and MMU3.

I recently ordered the MMU3 kit for my MK3S+, mainly to be able to print more convenient supports for functional PETG objects.

Before buying I had read a lot of user reports, both here and on Reddit, and would describe the general tone as discouraging. Those posts that did report success tended to suggest that extensive configuration and tweaking was essential, that using forked firmware may be necessary, and to expect to enable purge volumes bordering on the absurd. My initial experience has been very different and quite positive, so I wanted to share it, simply as another anecdotal data point that I had not seen represented elsewhere.

I printed a series of simple objects designed to test bridging, sheer overhangs and a gradual slope from the base. The materials used were:

• Prusament PETG Galaxy Black
• “PLA Silver - Filament made for Prusa Research” (the roll that came bundled with the MK3S+)

Both filaments are approximately four years old and have suffered long periods of exposure to extreme humidity, but had spent about six hours each in a filament drier before printing.

I used the textured steel sheet, recently cleaned with concentrated dish soap then isopropyl alcohol. The nozzle is an E3D standard brass 0.4mm, coated with Slice Engineering Plastic Repellent Paint (to be honest I haven't noticed that the paint makes much difference). The hotend PTFE tubing was also recently replaced, as recommended when installing the MMU3.

Using Prusaslicer 2.9.1-alpha1 I selected the "0.20mm SOLUBLE INTERFACE @MK3" print profile and changed only the following settings:

• Print Settings > Support material > Top interface layers: 2
• Printers > Single extruder MM setup > Purging volume > 280mm³

Both filaments used their default profiles (Prusament PETG @MMU and Prusament PLA @MMU) and thus had nozzle temperatures of 230°C and 205°C respectively.

Initial results were quite adequate. Bottom layers in contact with interface material were visually uneven with a few noticeable gaps on the surface layer, but were very smooth to the touch and with dimensional accuracy to within 0.05mm. There were no blobs or other protrusions that would interfere with mechanical movement. Most surprisingly, the objects had no noticeable reduction in strength at interface layers. I applied significant force to the "legs" and did not observe any delamination, cracks, creaking, or abnormal flexing.

PLA laid down quite cleanly on top of PETG. Some deviation could be observed, but it was neatly flattened by the next pass. Printing the "slope" test object involved laying PLA directly onto the textured steel sheet at 85°C, but this progressed without any abnormality.

Having made no special configuration for the wipe tower, both materials were used interchangeably for the tower's border, and it became quite messy as prints progressed. However, trailing or lifted strands were mostly flattened by subsequent passes, or otherwise pushed out to the side. No significant amount of material got stuck to the nozzle or drawn into the main object. Keeping the wipe tower a safe distance from the target object seems beneficial, and selecting the main material for the wipe tower perimeter seems like it would be prudent for taller objects.

In one instance, interface material stuck a little stubbornly to the target object and I opted to separate it with a scalpel, simply because I was unable to grasp the tiny thread of filament with tweezers. With the tool it separated very easily, requiring almost no effort.

In conclusion, to those wishing to try this combination of materials I would encourage attempting it with some simple test objects like these to see how your printer behaves. It may be that requirements and results vary drastically between individual printers, and that I have simply been lucky here, but there is at least a chance that adequate results can be achieved without extreme purge volumes or arduous configuration (but keep your filament dry and your bed clean!).