Modelling of the isothermal replication of surface microstructures in polymer melts

The forming of micro surface structures on polymer materials is well established in polymer-processing operations. Numerical flow calculations were performed using the Lagrangian Integral Method where the fluid was described by a MSF constitutive model. The numerical modelling of the flow was performed on two length scales, at a macro level describing the steady flow (relative to the flow front) between the mould plates and at a micro level describing the transient free surface flow into the surface structure.

The information from the macro level was passed to the micro level as an applied local boundary condition. This allows an investigation of the effect of the rheological properties of the polymer melt on the ability of the material to fill small structures in a mould surface. Series of isothermal compression moulding experiments were performed with a polycarbonate (PC) and a polystyrene (PS) melt.

A round PC or PS plate with a thickness of 3 mm and a diameter of 59 mm was placed between and at the centre of two squared steel plate. The bottom plate is equipped with a microstructured nickel insert, positioned 46 mm from the middle of the steel plate. The insert contained 10 parallel, rectangularly-shaped channels with a depth of 9.4 micrometers a width of 22 micrometers and a distance between the channels of 18 micrometers.

The steel plates were pressed together, applying a constant pressure on the top. This allowed the polymer melt to flow outwards. The channels were positioned parallel to the incoming molten plastic flow. Just before the flow-front of the melt reached the end of the inserts the polymer was frozen. The replicated PC and PS micro-structures were examined using a confocal laser scanning microscope.

Uniaxial elongational viscosity and linear viscoelasticity were used to characterize the flow behaviour of the PC melt. All the replicated PC structures matched the numerical calculations within the experimental scattering.