Why is the spring strengthened in the middle?
It doesn’t seem to affect the spring’s buckling characteristics.
My speculation is that it’s to reduce spring noise. That strengthened region at the middle is where the spring will buckle outwards most, resting against the barely visible side rails on the inside of the case. Instead of just one wobbly contact point it now has three rigid ones as a “skate” to reduce the stick-slip noise when opening and retracting the tip. Is this right?
(The pen is a Mitsubishi Uni-Ball Power Tank, pretty much my favorite model.)
My guess too. A longer spring would have more deflection in the middle. This design could reduce that and increase the spring force.
how would it deflect when confined in a plastic cylinder?
It wouldn’t much, the spring wouldn’t be the exact diameter of the chamber. I could see a Japanese or German pen company over-engineering the spring this way because the cad stress analyzer shows a weakness there
no, you can clearly see from the image posted the spring is snug against the interior wall of the cylinder it’s enclosed in.
Your confusing an external rib for the internal wall. I can see at least a .5mm gap