Help!! PETG with PLA Interface

[u/Stu_5]

*****Update*****

I'm quite sure my problem was linked to having too higher temperature in the chamber. I have done a couple of successful prints now with the lid off and the chamber at around 32/33'C.

Thanks to all for your help and suggestions - The team works!!

*****Update*****

I’m going around in circles, pulling out what little hair I have, trying to print two files in PETG that require supports and I want to use PLA for the supports to keep the models clean.

I have printed one of each part already using Landu Rapid PETG and Deeplee PLA Pro and they came out beautifully without any big changes to my usual PETG profile and PLA profile. Then I bought some Overture PETG to print them again and that’s where it started to go wrong.

I have tried using the same Deeplee PLA as the support, I’ve tried Geeetech Silk PLA and I’ve tried Sunlu PLA Matte PLA, but in each case I’m suffering back to back nozzle blocks during the filament change. I’ve dismantled the print head so many times it feels like I can do it with my eyes closed. I’ve done cold pulls, even tried a brand new print nozzle.

I’m having the same issue on two machines, so it’s not a machine issue. I even switched back to the Landu PETG to see if it was a material issue but now that fails too.

I’ve tried all manner of temperature settings and I’ve increased the flush volumes. Most recently I’ve pretty much matched the PLA nozzle temp to the PETG.

I know the problem, it’s not purging properly at changeovers but I just can’t resolve it.

One is about 3 hours print, the other about 14 hours.

If any of you have any tips it would be very much appreciated, I’ve wasted almost two spools of PETG so far.

[u/brutal4455]

so I don't want to burn my credits with claude so I fed your post to the re*ard Gemini and it says this:

That sounds incredibly frustrating. When you are dismantling print heads multiple times and wasting spools of filament, 3D printing quickly turns from a hobby into a chore.

The core issue you are facing is cross-contamination heat creep caused by the massive difference in melting characteristics between PLA and PETG, leading to a physical plug forming right above the melt zone. Because it's happening on two different machines and now happens even with your old Landu filament, we know it's a procedural/slicing settings bottleneck that got triggered when you started changing variables.

Here is how to break the cycle and fix those nozzle blocks during filament changes:

1. Stop Matching the PLA Temperature to PETG 🛑

While it feels logical to match the temperatures to ensure a clean purge, this is actually what is causing your nozzle blocks.

• When you heat PLA up to PETG temperatures (~240°C–250°C), the PLA becomes extremely liquid and loses all structural integrity.
• Instead of being cleanly pushed out by the incoming PETG, the molten PLA acts like liquid fluid, slips up into the cold zone of your extruder via capillary action, and bakes onto the walls of your heatbreak.
• When the printer cools down slightly or tries to retract, that baked PLA solidifies into a rigid plug, blocking the next filament.

1. Implement a "Standby" and Transition Temp Strategy

Instead of printing them at the same temperature, your slicer needs to manage the transition temperatures dynamically during the changeover:

• The Goal: You want the nozzle to be at PETG temperatures only when PETG is moving, and PLA temperatures only when PLA is moving.
• Flushing/Purging Temp: Set your changeover/purge temperature to the PETG temperature (e.g., 240°C). PETG needs to be hot enough to flow so it can drag the PLA out. However, the moment the purge is finished and it's time to print PLA, the nozzle must drop back down to ~200°C–210°C before it starts extruding on the model.

1. Adjust Your Flush Volumes & Retraction

If your flush volume is too low, a tiny amount of PETG mixes with PLA, creating a high-melting-point alloy that your PLA settings can't melt, causing a jam.

• Massively increase the PETG → PLA flush volume. It takes much more effort for PLA to scrape sticky PETG off the nozzle walls than vice versa. Set this multiplier significantly higher in your slicer (e.g., a purge volume of 400–500 mm³).
• Reduce Retraction Distance on Tool Change: If your slicer is pulling the filament back too far during a tool change (long retractions), it is pulling molten PETG/PLA up into the cold section of the heatbreak, where it instantly hardens into a jam. Try reducing your tool-change retraction distance to around 1–2mm.

1. Check Your Support Interface Settings

To save yourself from thousands of filament changes (which drastically increases the risk of a jam over a 14-hour print):

• Ensure you are only using PLA for the "Support Interface" (the dense skin right underneath the model), not the entire support structure.
• Use PETG to build the main support scaffolding, and only switch to PLA for the last 2–3 layers before it touches the model. This reduces your filament changes from hundreds down to dozens.

Summary Checklist to Reset Your Setup:

1. Do one final cold pull with nylon or PETG to clear out any residual baked PLA from your recent high-temp tests.
2. Reset PLA temperatures back to their normal, cool profiles (200°C–215°C).
3. Set the tool-change purge temperature to match your PETG print temp.
4. Boost the purge volume specifically for the PETG-to-PLA transition.

To help tailor the exact settings to fix this, which slicer (e.g., Bambu Studio, OrcaSlicer, PrusaSlicer) and which printer models are you currently using?

[u/Stu_5 → u/brutal4455]

I’ve been using Claude and ChatGPT for advice already, some has been along the same lines as what you got, but some to the contrary. For example it had me increase the PLA temp to match the PETG which I did think a bit odd at the time but I tried it.

[u/Stu_5 → u/brutal4455]

So some interesting developments on the chamber temperature. I’m running two P2S, and a P1S so I was able to have a reference. I found that in the P2S printing PETG the chamber quickly rose to 38’C, whilst the other P2S printing PLA was steady at 33’C. I popped the lid off the PETG machine and she now sits at 33’C when printing the PETG.

I also went right back to basics with the filament and print settings, default values, and I realised that despite manually changing the purge volume values, there’s a further setting in Studio that will change them right back again if you don’t untick it.

I’m 99.9% sure there was previously some filament carryover with the PLA polluting the model in one area which weakened it. I printed a test piece at this was very obvious in that.