1 Department of Management Engineering, Technical University of Denmark
Thermoplastic injection moulding is a widely used industrial process that involves surface generation by replication. The surface topography of injection moulded plastic parts can be important for aesthetical or technical reasons. With the emergence of microengineering and nanotechnology additional importance of surface topography follows. In general the replication is not perfect and the topography of the plastic part differs from the inverse topography of the mould cavity. It is desirable to be able to control the degree of replication perfection or replication quality. This requires an understanding of the physical mechanisms of replication. Such understanding can lead to improved process design and facilitate in-line process quality control with respect to surface properties. The purpose of the project is to identify critical factors that affect topography replication quality and to obtain an understanding of the mechanisms controlling topography replication. Surface micro topography replication in injection moulding depends on the main elements of Process conditions Plastic material Mould topography In this work, the process conditions is the main factor considered, but the impact of plastic material, the topography itself, and other factors were also investigated. The experimental work is based on a multi-purpose experimental injection mould with a collection of test surface inserts manufactured by EDM (electrical discharge machining). Experimental production took place with an injection moulding machine in a clean room environment. The mould and the injection moulding machine were fitted with transducers for subsequent process analysis. A total of 13 different plastic material grades were applied. Topographical characterisation was performed with an optical laser focus detection instrument. Replication is conceived to proceed at two different replication modes, viscous or solid, depending on the temperature of the material in the surface region. The viscous or early replication mechanism dictates that the replication happens while the plastic material is predominantly in a viscous state and takes place very early after the melt front passes the mould topography. The solid or late replication mechanism takes on the form of solid deformation of the surface layer and occurs relatively late when a high pressure has been built up in the cavity. Experimental data supported the hypothesis that replication of coarse topography is primarily controlled by the viscous or early replication mechanism, while fine topography is mainly controlled by the solid or late replication mechanism.