In order to define boundary conditions for contact detection, a distinction must be established between regions of the free surface where contact has not yet been detected and regions where contact has occurred. Before contact, dynamic and kinematic conditions (Equation 15–1 and Equation 15–2) dictate the displacement of the free surface. A normal force can be applied on an internal free surface to take the inflation into account. Once contact occurs, free surface displacement becomes impossible in the direction normal to the mold surface, and the velocity of the fluid becomes identical to the velocity of the mold. In the tangential direction, a "slipping" law can be defined. At the same time, the force vector (normal and tangential components) is no longer prescribed: an essential condition on the velocity replaces the force condition that was previously prescribed.