# POLYFLOW - About the force driven motion (e.g. for mould in contact application, blow moulding, etc.)

 How can one define a 'moving rigid' wall boundary condition whose motion is driven by a force? (as may possibly appear e.g. in blow moulding/thermoforming or squeeze flow, where a force is applied on the contact mould)POLYFLOW does not offer a specific boundary condition for 'moving solid walls' driven by a force, such as the head of a piston in a pump, a valve opening or closing in a pipeline or the mobile central part of a die in a parison extrusion process. It is however quite possible to define such a 'moving boundary' condition in POLYDATA via the 'Global force imposed' boundary condition. The technique just requires the definition of a Lagrangian remeshing (it means that the setup must contain at least one free surface).To illustrate the technique, let's take a simple squeeze flow as illustrated in the figure squeeze_force.gif attached below.Boundary Set 1 (BS1) is the axis of symmetry, BS2 corresponds to the moving rigid wall, BS3 is a free surface and BS4 represents the lower plate of the device. This plate does not move nor deform.On BS2, we select a 'Global force imposed' boundary condition that forces the velocity field to be identical on all points of the boundary, making this boundary set to be 'rigid'. The quantity to enter actually corresponds to the force (in Newtons) applied on the upper plate.The free surface on BS3 allows defining a Lagrangian remeshing method. This Lagrangian remeshing will make the 'rigid wall' (BS2) to actually move.LIMITATIONS: trials were performed with a flat boundary setNOTE: in versino 13.0, it is possible to define a force-driven mould motion with POLYDATA.