I have a problem when i want to print an empty shell. I get a vertical line on it. How can i resolve this?

The seam happens because of the pause between layer change. Lower temp decreases the ooze during the pause. Increasing speed decreases the pause. Increasing retraction on layer changes helps.
Printing the “spiral vase” method would eliminate seams entirely but I don’t know if your software handles that or if it’s something you even want to mess with.

-Jesse