The effects of polymer melt rheology on the replication of surface microstructures in isothermal moulding

Series of isothermal compression moulding experiments were performed with a polycarbonate and a polystyrene melt in a hot press. The bottom plate in the hot press was equipped with a microstructured nickel insert. The insert contained 10 parallel, rectangularly shaped microchannels with a depth of 9.4 micrometer, a width of 22 micrometer and a distance between the channels of 18 micrometer. The channels were positioned parallel to the incoming molten plastic flow. The polymer melt was frozen just before the flow-front of the melt reached the end of the inserts. The partly replicated microstructures were examined using a confocal laser scanning microscope. With increasing Deborah number, defined as De = G'/G'', there is a considerable decrease in the (non-dimensional) length the flow front has to move in order to fill the microchannels. Numerical flow calculations were performed using the Lagrangian Integral Method where the fluid is described by a molecular stress function (MSF) constitutive model. The numerical modelling of the flow was performed on two length scales, at a macro-level describing the flow between the mould plates and at a micro-level describing the flow into the structure. The information from the macro-level was passed to the micro-level as an applied local boundary condition.