Ultrasonic micro-injection moulding: characterisation of interfacial friction by varying feedstock shape and high-speed thermal imaging for microneedle feature replication

This study explores the interfacial friction in ultrasonic micro-injection moulding by using different polymer feedstock shapes, characterisation of micromoulding melts through thermal imaging and assessing microneedle feature replication. Industry standard polypropylene pellets and discs with different thicknesses were used for varying the amount of interfacial friction during sonication. High-speed thermal imaging and tooling containing sapphire windows were used to visualise the melt characteristics. Moulded products were characterised using laser-scanning confocal microscopy to quantify microneedle replication. The study demonstrates that (i) the interfacial area for the different feedstock shapes affects the heating in ultrasonic micro-injection moulding significantly, (ii) disc-shaped feedstocks result in initially higher flow front velocities and exhibit dominance of viscoelastic heating over interfacial friction and (iii) industrial pellet feedstocks provide a good combination interfacial friction and viscoelastic heating and more viscosity reduction in overall leading to better microreplication efficiency. The results presented could have a significant impact on the process development of ultrasonic micro-injection moulding where process repeatability can be improved by controlling the interfacial friction. The research provides an essential contribution to the development of this process, where interfacial frictional heating can be tailored specifically for miniature functional components, offering improved precision and reduced energy use when compared with conventional methods.