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.)
This prevents spring distortion by effectively creating two shorter spring segments, probably because the pen designer wanted a longer barrel and the mechanical engineer was told to “figure it out that’s why we pay you” and the truth is I have no idea it just sounds plausible enough.
My guess too. A longer spring would have more deflection in the middle. This design could reduce that and increase the spring force.