PETG (print)/PLA (support) layer adhesion issue

[michaelc]

Problem

I’m printing a PETG model using PLA support interfaces. I think I have set the correct settings for this (top interface 0mm, etc.) - it prints mostly fine except for one specific layer where the new PETG layer has zero adhesion to the previous layer.

Here are two attempts. The red/red is where the PETG should be adhering to itself (green/green is my second attempt). The blue is the PLA support interface layer.

This is a close up of the gray PETG layer that didn’t stick to the one below it. The black is obviously again the PLA support interface layer.

Could it be some kind of PLA expansion/layer height thing pushing the PETG layer too high up? Are there any other settings I can try? Is there anything about printing model layers before support layers, or similar?

Printer/filament

Bambu A1 Combo. eSUN PLA (black in the photo) and eSUN PETG (gray).

Other info

I am using the following filament gcode to fix the PETG squish/first layer problems I’ve had with the Bambu A1, could it be interfering? I specified to only do it on the first layer, so my assumption is it wouldn’t interfere?

{if curr_bed_type=="Textured PEI Plate"}
    {if filament_type[initial_tool]=="PETG"}
        {if layer_num == 0}
            G1 Z0.065 ;move z up 0.065
            G92 Z0 ;set new z as the zero point
        {endif}
    {endif}
{endif}

[michaelc]

Ok, from this post I see it might be a flush volume issue: Support Filament --> PETG for PLA and PLA for PETG and more

I am trying some new flush volume values now and printing a test.

[lion7718]

Can you a post a link to the model…??
I understand what your trying to do, but I don’t understand why.
It seems like you can just glue those 2 pieces together.

[RocketSled]

You are going to have a significant challenge solving this problem.

The issue is purge. If you have even a tiny bit of the other filament still in the extruder, it will contaminate the layer that’s printed after a filament switch. If it’s printing a support, it doesn’t matter a huge amount if the layer sticks hard to itself. But if it’s switched from PLA to PETG and there’s any PLA remaining, the PETG layer will not stick to the previous layer.

You need massive amounts of purge when switching back to PETG. I gave up doing PETG prints with PLA supports because getting it to print and be strong wasted huge amounts of filament for the purge.

[michaelc]

Thank you @RocketSled! I didn’t end up reprinting (based on your advice that it would be hard to get right). I did manage to glue the two separate pieces together to get the final model I wanted.

Other people, please see RocketSled’s reply, and the link I posted in the second comment.

[RocketSled]

That’s what I do. Split the part and flip the top over on to the build plate.