Characterization and analysis of weld lines on micro-injection moulded parts using atomic force microscopy (AFM)

In recent years plastic moulding techniques, such as injection moulding, have been developed to fulfil the needs of micro-components fabrication. Micro-injection moulding (SLIM) is the process which enables the mass production of polymer micro-systems such as micro-mechanical parts, micro-fluidic systems, micro-optics, micro-components for medical devices, etc. A key factor in micro-polymer based systems is the possibility of designing and therefore replicating very complex geometries, allowing an increasing number of applications and micro-products. Complex geometries bring challenges during the filling stage of the process. Many features cause melt front separation: through holes, presence of pins and cores, changes of thickness, multi-gating systems, existence of micro-inserts, two component injection moulding, etc. When two or more melt flows join each other after separation, a layer between the two original flows will generate and a weld line is formed on the surface of the micro-moulded part. This phenomenon has to be avoided or at least reduced, since in the weld line area the mechanical properties are poorer than in the bulk part, creating strength problems on the final part. Although weld lines are unavoidable, the micro-injection moulding process can indeed be optimized in order to on one hand obtain a good filling of the cavity and on the other hand decrease the size of the weld lines. In this paper, an investigation devoted to the determination of the influence of typical injection moulding parameters on the weld lines' dimensions is presented, using an atomic force microscope (AFM). Depth and width of weld lines were chosen as parameters to be optimized.