The FEM codes for sheet metal forming may be classified accordingly to their type of integration: implicit or explicit
Explicit methods solve for equilibrium at time t by direct time integration. The explicit procedure is conditionally stable since iterative procedures are not implemented to reach equilibrium and also ∆t is limited by natural time.
Implicit codes solve for equilibrium at every time step (t + ∆t). Depending on the procedure chosen, each iteration requires the formation and solution of the linear system of equations.