Micro-assembly injection molding

Assembly injection molding is a well-established process for the manufacturing of assemblies using similar (compatible) or dissimilar (incompatible) materials to create fixed or movable joints. Usually, a first component is injection-molded and subsequently overmolded with a second shot. Within the joining of two polymers the quality of the resulting interface is dominantly affected by the thermal history at the contact area between the two components. With the scale down to micro assemblies, the manufacturing of material bonds becomes more challenging. In this study the mechanical properties of assembly injection molded tensile rods with a diameter of 0.6 mm were investigated and compared to weld lines and the reference material properties varying the process conditions. Polyoxymethylene (POM) was chosen for this study because of its fast crystallization kinetics and morphology visualization possibilities. The mechanical properties (Young’s modulus, elongation at break and tensile strength) were linked to the processing parameters (volumetric flow rate, mold temperature and melt temperature) using a design of experiments (DoE). Compared to the reference and weld line samples always showing a ductile fracture, the assembly injection molded specimen showed a brittle failure with different properties and no adhesion for some of the molding set-ups.