The plastics industry is a fast evolving sector, with customers requiring higher volumes of products, complex shapes, miniaturisation and added functionality, in an environment friendly manner at competitive cost. Ultrasonic welding is one of the processes used to assembly injected moulded components to fulfil the above requirements, in various industries: automotive, medical, electrical and electronic, communications, appliances, consumer products, toys, textile, packaging, etc. This economic process is fast, clean, efficient, repeatable, and can significantly increase production and lower assembly cost. Offering flexibility and versatility, high reliability, consistency, long life, and low-cost makes ultrasonic welding the preferred method of assembly. The ultrasonic created bonds are desired to be strong and integral, and retaining the properties of the pristine material. Microinjection moulding makes possible the production of fine details with high precision and accuracy, requirements extended to the assembly process. The bonding process needs to maintain the crystallinity, the optical and mechanical properties of the original component – attributes highly influenced by the heat generated from high-frequency mechanical motion. This study will show how the quality of welds varies due to the mechanical motion produced by a 20 kHz ultrasonic welding system, along with applied force that creates frictional heat at the plastic components joint area. The paper includes the manufacturing (microinjection moulding), the physico-chemical (DSC, confocal microscopy, SEM, microCT) and mechanical (tensile testing) characterisation of polystyrene components. Parameters such as: energy director shape, ultrasound amplitude and energy, weld time, hold time and pressure will be discussed in the assessment of welds resulting from this high-frequency mechanical motion process.