Non axi-symmetric nature of cans.
Body structural stiffness.
Change of curvature.
Body and lid flange design changes around the periphery.
The curling is generally uneven around the lid.
The process (machine) influences the final result.

 

Cam & Lever mechanism.
The most widely used mechanism nowadays.
Plunge method (rapid attack).
Software to detect trajectory errors.

 

Progressive attack / classic method	Plunge method / fast attack
Each operation is done in several turns.	Each operation is done in just one turn.
1st Operation 5-7 turns	1st Operation 1 turn
2nd Operation 2-3 turns	2nd Operation 1 turn
Slower machines	Improved performance
More free-forming process	Material better controlled

 

Curling.

Solves discontinuity between curled and uncurled lid zones.

Deep curling most useful for some difficult cans.