Polypropylene for material extrusion: Evidence that flow-enhanced crystallization restricts welding

We present a comprehensive investigation into the effect that both the crystallization kinetics and the molecular weight of polypropylene feedstock have on the welding properties in material extrusion 3D-printing (MatEx). As sufficient welding at the interfaces between printed layers may be restricted by the onset of crystallization, there is a delicate balance between polymer interdiffusion and the development of spherulites within the weld region. We monitor these processes during a cooling cycle using a well-established theoretical model that requires full characterisation of the rheology and crystallization. The model shows that the largest effect of increasing molecular weight is a reduction in interdiffusion; this mechanism leads to a reduction in the weld strength as demonstrated by mechanical testing. For the highest molecular weight feedstock, we find that the fracture pattern changes from plastic deformation to brittle mode. Furthermore, we observe that the spherulites within the weld are about an order of magnitude smaller than in other samples. These observations cannot be described by the quiescent crystallization kinetics alone. Our findings suggest that it is a flow-enhanced crystallization mechanism that accelerates nucleation leading to smaller spherulites developing in the weld earlier in the cooling process. This significantly restricts interdiffusion at the welds.