Following on from my previous post on the MMU2S, I decided to test out just how multi-material it actually is. I did this by
printing some PLA & PETG samples, and testing their layer adhesion
strength. The reason for this somewhat strange combo is that both PLA &
PETG stick reasonably well to each other when being printed, however once the
part cools the two plastics are easy to separate. Making them an ideal combo
to print 0mm interface supports
The Setup
Filament used for tests:
- PLA → 3Dfillies PLA Light Gray printed at 200°C, unloaded/dribbled at 180°C, and loaded/purged at 235°C
- PETG → 3Dfillies PETG Transp printed at 225°C, unloaded/dribbled at 205°C, and loaded/purged at 235°C
Believe it or not but the stock PrusaSlicer (as of v2.2.0) does not have the
capability to smoothly transition between different material temperatures. For
example, say you want to print PLA at 200°C and PETG at 225°C. With the stock
slicer when you change from:
- PLA → PETG, the PLA will be unloaded at 225°C, leading to stringy tips
- PETG → PLA, the old PETG will be purged at 200°C, making it impossible to purge old material
To get around this I recommend using PrusaSlicer 2.2.0 DRIBBLING by antimix, as it allows you to control the load/unload temperature
UPDATE: Kayl from the Prusa forum has released PrusaSlicer 2.3.0 DRIBBLING
Below is how the 10 samples were configured in PrusaSlicer, as well as what
they looked like when printed:
Finally, to test the layer adhesion strength I made up a simple three-point bending flexural test jig:
The Results
Before we get to the data, I need to first raise a few points:
- For the single material layer strength (Single Mode) I manually purged ~2500mm³ of PLA/PETG before starting the print by feeding ~1m of filament though the hotend. This will be considered as the maximum possible layer adhesion strength for each material
- The 500mm³ Purge has two data points, one for a purge temperature of 235°C and the other for 260°C. As you would expect purging old PETG at 260°C (loading PLA) helps with PLA layer strength as it's easier for the old PETG to exit the hotend. Interestingly purging old PLA at 260°C (loading PETG) has the opposite effect, I suspect this is due to old PLA carbonising in the hotend
- To get the break weight of each sample I recorded the scales display at 1080p 60fps and then played the video back at half speed on my PC
- The largest purge volume I did when using the MMU2S is 1250mm³. I decided to stop things here and extrapolate the data I had with an exponential relationship
- Lastly, you might say that PETG is quite a "sticky" material and that it's difficult to fully purge it from the hotend. Turns out that same is true for PLA (and I suspect other materials), as in both cases you need crazy purge volumes (>2500mm³) to fully clear the hotend
With that out of the way, here is all the data I collected:
Outcome & Tips
Basically there is no easy way out when printing different materials on
the MMU2S.
You need to purge at least 2500mm³ if you want to have any chance of
removing the old material
(same is true for both PLA & PETG). Turns out the same holds true in
the injection moulding world, "General-purpose nozzles have a dead spot in the nose and take
about 50 shots to purge clean"
To give you an idea, here is how a 2500mm³ block looks like in
PrusaSlicer:
With all that said, if you do decide to use the MMU2S for multi-material
prints then I highly recommend getting an E3D v6 Pro Sock. As earlier on I found that swapping between PLA/PETG creates little
beads which if left for long enough merge and grow in size, eventually
making a big blob on the nozzle that knocks anything in it's path.
Swapping the normal silicone sock to the Pro solved this for me:
So looks like I won't be having 0mm interface supports with my Half-Life 2
AR2 project D:
