Why does a raft adhere to the bed while skirt does not?

Question

I'm experimenting with my new Ender 3 V2 using PETG filament on the textured side of the included glass bed without any additional adhesives.

I'm been having trouble with adhesion until I found out that enabling raft on my prints helps solve the problem. However, I can't help to wonder whether skirts work any different than rafts.

In my test prints, all settings are identical except for the raft vs. skirt setting (using Cura as my slicer). However, the outer line of a raft adheres perfectly to the glass, while the outer line of a skirt fails to stick completely and moves around when the nozzle goes "around the corner".

Is there any difference between the outer border of a raft and a skirt as produced by Cura?

EDIT: I've managed to upload a video comparison of raft and skirt in the same project.

Answer 1

The raft base (initial layer) is usually printed with very wide lines. Cura's default is double the nozzle diameter, so 0.8 mm with standard 0.4 mm nozzle. This is ridiculously high flow, especially with the default 0.3 mm thickness, which is why the raft base lines are (and must be, unless you have a seriously overpowered hotend) printed so slowly. So, each line has significantly increased (double) surface contact, more pressure against the bed, more forgiveness if the bed clearance is too high (since it will still smash against the bed, just spreading out less than the whole 0.8 mm width), comes out slower (giving it better opportunity to adhere), and does not involve any curves (where the nozzle may "pull" just-extruded material that's not yet adhering).

You may be able to get a lot of the same benefit with skirts (or brims) by increasing the "Skirt/Brim Line Width" setting and slowing down its speed.