The properties of a LPAPI-matrix derived from an extended Direct-SIMPLE scheme are demonstrated. It is shown that such LPAPI-matrix for a 3D strong P V coupling problem is difficult to numerically solve by nature, though the Direct-SIMPLE algorithm is theoretically efficient and computationally available for a 2D STP-based simulation. Convergent computation can not be achieved for an entire 3D solidification process using SOR, Jacobi and ICCG iterative methods to solve the 3D P V coupling. This appeals some special solution techniques to the LPAPI-matrix for an efficient 3D STP-based simulation with an extended Direct-SIMPLE algorithm.