Matthieu Fischer

The properties of plastic components show a particularly strong dependence on the processing history, compared to other material categories. Interactions with p The properties of plastic components show a particularly strong dependence on the processing history, compared to other material categories. Interactions with p

The complex multiphase morphology of thermoplastic elastomers based on styrene-block copolymers (TPSs) affects their flow behavior significantly and in a way which may not be considered by commonly used characterization and evaluation procedures. To evaluate the relevance of non-Newtonian flow phenomena for the validity of rheometric data in processing, three commercially available TPSs with comparable hardness of about 60 Shore A but with different application fields were selected and characterized using parallel plate and high-pressure capillary rheometry. The validity of the rheometric data is assessed by modeling the flow in a high-pressure capillary rheometer by a computational fluid dynamics (CFD) simulation. The results were discussed in conjunction with close-up images of samples taken after the measurement. The materials show clearly different rheological behaviors but depend on the respective shear and geometrical conditions. In particular, for the material with the lowest viscosity, doubling the capillary diameter resulted in a disproportionate increase of the pressure loss by up to one third. Only the capillary flow of this material could not be reproduced by the CFD simulation. The results indicate that conventionally determined rheometric data of TPSs are of limited use in evaluating process flows for various material grades.

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.

The interactive effects between additives and weld lines, which are frequent injection-moulding defects, were studied in high-density polyethylene (HDPE) and compared to weld-line-free reference samples. These materials were formulated around a D- and I-optimal experimental design, based on a quadratic Scheffé polynomial model, with up to 60 wt% calcium carbonate, masterbatched carbon black and a stabiliser package. Where reasonable and appropriate, the behaviours of the systems were modelled using statistical techniques, for a better understanding of the underlying trends. The characterisations were performed through the use of conventional tensile testing, digital image correlation (DIC) and scanning electron microscopy (SEM). A range of complex interactive effects were found during conventional tensile testing, with DIC used to better understand and explain these effects. SEM is used to better understand the failure mechanics of some of these systems through fractography, particularly regarding particle effects. A measure is introduced to quantify the deviation of the pre-yield deformation curve from the ideal elastic case. Novel analysis of DIC results is proposed, through the use of combined time-series plots and measures quantifying the extent and localisation of peak deformation. Through this, it could be found that strong shifts in the deformation mechanisms occur as a function of formulation and the presence/absence of weld lines. Primarily, changes are noted in the onset of continuous inter- and intralamellar slip and cavitation/fibrillation, seen through the onset of localised deformation and stress-whitening.