About half the time recently when printing with PLA, I see that holes or loops have a line of filament running approximately through the middle which seems to be one of the inner perimeters having detached and contracted, but is still attached on both sides. Is it a temperature thing, an extrusion thing? I can’t find a pattern. Bed adhesion is great. Bambu H2C, mostly printing with Bambu Basic PLA.
I’d considered raising the temperature a little to get the perimeter to stick better to its neighbor, but I wasn’t sure if hotter printing would worsen the thermal contraction, which is what I was originally suspecting was happening. Nothing ever pulls off from an outer perimeter, it’s only inner perimeters of empty loops or holes.
Isn’t that obvious?
Try making a circle of a certain size on a flat, empty table, by dragging a string. The circle will become smaller, than the target size because of the part that is already laid down will be dragged inwards.
Now drag the string around a gluestick, it can’t be smaller than the gluestick’s circle because the gluestick is in the way.
Your nozzle won’t travel anywhere outside of your model’s outer perimeter because it has no reason to (unless your g-code is super borked, see my comment about your slicer above) but it will be dancing around within the space between the outer perimeter and center of your model many hundreds of times. Any extrusions pulled off on the outer perimeter would stay somewhere within the model.
Not quite sure what your concern is with the top fill pattern. It is a load-bearing part, so it kind of needs to be the way it is to retain the part coming out the other side. As far as the extruder exiting the perimeter of the model, I would remind you of the possibility of printing more than one model on a build plate. Although, I hadn’t yet considered trying to print only one to see if it still happens, as a troubleshooting approach. I’ll try that later, to rule out the possibility that it’s being mechanically pulled off by an extruder transport move.
Unless you’ve deliberately reversed your walls/infill printing order, the default is to print walls first. Your print head and nozzle won’t have any reason to leave the perimeter of the model even if you’re printing multiple examples of the part until the entire layer is complete on one of them. It will move to the next part in the array only after finishing the infill, which is well after your problem may have occurred on either the inner or outer perimeters on any particular layer.
I’m not sure what you’re on about with top fill. I didn’t say anything about your fill pattern or percentage.