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Weak layer adherence with dual extrusion

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I am printing a model with PETG (with CTE setting, 245 degrees) + PVA as support.

The layers where there is only PETG are fine. However the PETG layers where there is a mix og PETG+PVA is quite weak. As a result, the model splits like this:


For the record, I used PVA as support in the open area in the middle.

The layer that was split seems quite poor:


What can be causing this? I have a theory that the filament is retracted too much when it changes materials.

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My first thought relates to the fact that:

A) Higher glass temp materials (like ABS, PETG) have trouble more trouble bonding - you really have to melt the layer below a bit as you add the layer above. This is normally solved by less fan (or heated chamber). The symptom is weak layers.


B)My UM3 fans run at 100% speed when you set the fan power to about 8%. So you need fans around 2 or 3% on the UM3 to get the cooling cut in half.

So my first thought is that maybe it's not properly lowering the fan speed when it switches to PLA on the same layer that had some PVA. You could check that in the gcode. PVA doesn't really need any fan anyway.

My second thought is the same as yours - maybe some kind of underextrusion? Maybe add a prime tower. I'm not sure what I'm seeing where the red arrow points - I can't tell if there is any underextrusion or if it's just bad layer adhesion.

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Thanks gr5. I checked the fan speed - the profile setting is at 50% on both extruders. I will try to disable the fan.

It looks like under extrusion of PETG in the mixed layers. When looking on the model up close, there are miniature gaps between the layers. Oddly enough, there is no sign of that if I print with only PETG. I tried to print the same model with only PETG. It is super solid.

I am printing with a priming tower. I will try to increase the size of it and see if it makes a difference.

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50% is BS. It's the exact same fan speed as 20% and as 100%. You have to get below 10% before fan slows down. Experiment in TUNE menu while printer is printing.

Anyway it sounds like underextrusion - not layer adhesion issue. So I would concentrate on the tower option. Put it in the rear center - it tends to fall over in the default location (right rear) because the glass usually warps down in the rear two corners.

Also view all the "line width" settings. Make sure they are all resonable for your nozzle size e.g. .35mm to .4mm for a .4mm nozzle. Cura has so many strange settings for infill where it prints it at double speed for example or thinner line width which is all unwise in my opinion.

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