Supports can be hard to remove and leave imperfect surfaces above them. Depending on the shape of the object, there is also the risk of ruining the print in the process. Some geometry is not even possible to print as a single piece using FFF printing.
With the MMU2S it’s possible to use water-soluble materials, like PVA+ and BVOH, to print the supports or just the 'interface' between the print and the support structure. However, printing solubles can be demanding and requires some considerations along the way. It is a process more suitable for advanced users.
The first consideration is that the main material and the soluble support material must have similar melting/printing temperatures. PVA+ and BVOH are similar in many ways, but BVOH is absolutely the better (but more expensive) option. It is stiffer and will behave much better than PVA+ when printed. This is why it is better to be printed with PETG.
For PETG you can practically only use BVOH. PLA can be printed with both PVA+ and BVOH.
There is a specific configuration for the soluble support materials in PrusaSlicer. Once you have selected the Original Prusa MMU2S printer profile, you will find some new options in the 'Print Settings' drop-down menu. For 0.15 mm and 0.2 mm layer height, you now have the additional SOLUBLE FULL and SOLUBLE INTERFACE (purple squares).
This option will use the soluble material for the entire support structure (left picture). This is recommended where you have complex geometry that requires a lot of internal supports only. PVA+ and BVOH adhere poorly to the Smooth PEI or Textured steel sheet but will adhere well when printed directly onto PLA and PETG. If you are printing something with supports on the build plate, we recommend the SOLUBLE INTERFACE preset.
This preset applies the soluble material only in the more densely printed layers between the actual support structure and the model (right picture). This allows you to prints the support structure with whatever color is loaded and only uses the soluble material where it is needed. This will save soluble material, which is more expensive than PLA and PETG. If you have a print that requires a lot of support directly on the steel sheet, this is the option preset for you.
|SOLUBLE FULL - Green filament is BVOH
|SOLUBLE INTERFACE - Green filament is BVOH
PrusaSlicer or the MMU2S will not detect where you loaded the soluble filament. PrusaSlicer will default to assign the support material or interface material to Filament number 5. You can change this to any other filament by entering the number of which the filament is loaded (purple squares).
'Support material/raft/skirt extruder: 0' will use whatever tool is currently loaded to the extruder.
By default, 70 mm³ is used for both loading and unloading. When using water-soluble supports we suggest using at least 100-120 mm³. If you find any residue in your print, increase the amount. PVA+/BVOH may require at least 120 mm³ when unloaded.
To access the settings for purging volumes, press the Purging volumes... button (green arrow).
It is essential that you have performed a very precise IR sensor calibration. With the properties of PVA+ and BVOH, their diameter can easily deform with excessive pressure from the idler.
Again, due to their soft properties, the filament tips will inevitably string, so it’s important to keep an eye on them and be ready to intervene and cut them while the main filament is being printed.
The difference in Max Volumetric speed: PLA has a max volumetric speed of 15 mm³/s, while PVA+, and BVOH have 4 mm³/s. This increases the total print time.
When the print is complete and before removing the supports, immerse it in warm water (maximum 45 °C) and leave it there for a few hours to overnight. After this, the supports should come off pretty easily but may require some use of tools, depending on your print's geometry. It may help to remove as much of the supports as possible while still dry.
Once the printed object is free from the supports, immerse it in hot water again for 10-20 minutes and then brush it to remove the residual debris of PVA+/BVOH. Using a small tough toothbrush can be an ideal tool for this task.
Unless you are dissolving several kilos of PVA/BVOH per day, you can pour the resulting BVOH/PVA-solution down the sewer.
As you might have expected, both BVOH and PVA are very hygroscopic (easily absorbs water) and should be kept as dry as possible. PVA+ or BVOH should not be exposed to humidity. It must be placed right back in its zip-lock bag right after its usage, and always with one or more bags of silica. Otherwise, it will deteriorate in a matter of a few months and go soft at a much lower temperature, starting trouble already at the loading process.
When deteriorated like this it will quickly cause loading/unloading issues and your hotend to clog. The best approach is to print and store these materials in a Drybox.