Conical Face VS Flat Face plunger design:
Conical–faced designs extend the useful range of a solenoid to provide higher forces for strokes typically over 1.5mm. The pole surface area is greater and the distance between the tapered cone face is approximately one half of the gap between the faces (for 30° angles), providing the effect of a closer air gap.
While some of the force component is lost because the force vector is not parallel with the plunger motion, the shorter gap and higher flux density combine to provide more output force for longer strokes.
For shorter strokes, the magnetic flux density increases and causes the iron to saturate rapidly as the poles move closer, thus reducing the efficiency of the conical-faced design. At this point, the flat-faced plunger is more efficient.
The main advantage of the flat-faced pole over the conical is that the full component of force is useable because the force vector is parallel with the pole motion.