Print ABS with PLA support interface, AMS retract the PLA to prevent softening

Bambu Lab H2 Series Bambu Lab H2D
1

I want to print an ABS part that needs a perfectly smooth supported surface. So far I have only done this with PETG and it achieved perfect results.
I only rarely have this print configuration, so I dont want to buy extra support for ABS material.
PLA is only needed on a single layer.

On the H2D there are a few ways to do this, but every one seems to have its issues.

  1. one hotend: PLA, other hotend: ABS, problem: chamber temperature too high for the PLA, it would soften in the extruder
  2. one hotend connected to the AMS with both ABS and PLA, that way the PLA would only be in the chamber when it is needed and should not soften during that short time. problems: more waste, high flushing volume needed, flushing ABS with PLA is difficult
  3. possible solution: feed ABS from the external spool holder and PLA through the AMS, have the AMS retract the PLA all the way back out of the chamber when it is not needed like it would do for a filament change, except there is no filament change done by the AMS

could this be achieved through custom filament change gcode? what are the AMS retract and feed gcodes?

2

Support for ABS (HIPS) is very cheap and the proper material for this use case. You asking for trouble with PLA, and I believe ABS and PLA bond to each other, so its not a proper support material.

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3

Today I had the exact problem you describe at No.1 when trying to print a part with PETG and ABS. I had to dismantle the filament guard and cutters to get the clog out of the extruder. Not a big deal, Bambu’s instructions were great and it only took 15min, but obviously the part didnt print.

I like your No.2 idea. It might work, except the chamber etc. will still be really hot from the ABS. If the settings are lower, will it turn on the fans and wait to cool down?

Re. Your possible solution, I would do it the other way around. You want the PLA as cool as possible going in and the AMS 2 retains some internal heat. So I would put the PLA externally and the ABS in the AMS.

Curious to hear if you succeed with this. As of today Im not mixing high and low temp filaments again anytime soon.

4

toms3d.org (Thomas Sanladerer): Try these filament combinations for Multi-Material and Supports! (also timestamp 4:48 in the youtube video):

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PLA should not bond to ABS

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5

for the possible solution the PLA has to go in the AMS, on the externeal spool holder you don´t have the motor to pull it back

6

The only solution in my opinion, is to keep the chamber temperature below 40 degrees for your ABS.
With 60-65C your PLA will stick to your nozzle in the cool zone and a clog is inevitable.

Just choose a proper support filament for ABS as @maxim3D already mentioned.
I am almost sure you can use it in the future again.

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