Injection moulding of PA12: influence of processing history on the thermal and mechanical behaviour

The presented PhD thesis will summarise the work conducted on the influence of different processing histories, due to either process settings or mould geometry, on the thermal and mechanical behaviour of injection moulded Polyamide 12 (PA12), currently being used extensively in the sporting goods industry. The research project lies within the overall scope of improving existing knowledge of process parameter influences on semi-crystalline polymeric materials, in combination with more advanced testing methods to increase the quality of final part property predictions, based on simplified test specimen data. Furthermore, an initial proof of concept on in-plant recycled material, followed by a final application verification study utilising components from the sporting goods industry will highlight PA12 as a possible material candidate for the so-called circular economy. The influence of processing parameters was addressed with a focus on the applied melt temperatures during the injection moulding process, by testing simplified test samples, originating from test plates produced at four different temperature settings. These results directly influenced the subsequent reprocessing investigations, also varying the temperature settings. A newly developed mould system (Advanced Mould Concept - AMC), which enables the creation of certain melt flow phenomena during dynamic flow (e.g. weld-lines and/or localised flow hesitation) by either adding or removing certain mould features, was utilised to study the influence of different mould geometries. It was found that melt temperatures above 300°C lead to increased tensile ductility in PA12, correlating well with impact test results on more complex geometries from recycled material, where a decreased ductility for components produced at lower melt temperatures led to several premature failures and a change in failure mode (ductile to brittle transition). With respect to melt flow modifications by different mould geometries, flow hesitation shows a significant influence on the weld-line strength of a sample, increasing the observed tensile toughness. Besides the petroleum-based PA12, PA610 was also used as material for sample production, representing a partly bio-based and therefore perceived a more sustainable PA12 alternative. The resulting mechanical properties were analysed by quasi-static tensile and instrumented impact tests, while material characterisation was conducted utilising Differential Scanning Calorimetry (DSC) and Gel Permeation Chromatography (GPC).