Impact of orientation, position and surrounding structures on injection moulding replication accuracy of micro-ridges on polymer surfaces

In this research, directional optical properties were achieved with a functional micro‐structured surface[1] composed of a close array of ridges (Figure 1), defined by their width and angle with respect to the main surface. The ridges were 1.2 mm long, 0.2 mm wide and with a 10˚ slope angle, for a depth of about 0.03 mm. Each matrix was defined by 10x10 pixels and each pixel was composed of 6 ridges. This study investigated the effect of the ridge orientation with respect to the direction of the flow front and the effect of the surrounding pixels, i.e. isolated pixels vs pixels surrounded by the full geometry, on the replication. To investigate this, 4 inserts were fabricated (Figure 2) by a precision five axis milling machine; one insert presented the full structure of 100 pixels, whilst only 9 pixels from selected locations were machined on the remaining 3 inserts. The pixels were selected to maintain the same topography of the original surface at 3 different distances from the gate. The inserts have then been replicated using acrylonitrile butadiene styrene (ABS) by means of injection molding. Inserts and replicates were characterized measuring values of ridge height and ridge inclination angle.