Compounding is an important step in processing base polymers and is used to incorporate various additives into a polymer. For this purpose, different screw elements are used for dispersive and distributive mixing on a co-rotating twin-screw extruder. Optimising the screw configuration requires precise knowledge of the screw elements’ mixing properties, which have not been thoroughly investigated. This study analyses the mixing behaviour of individual screw elements regarding dispersive and distributive mixing using 3D CFD flow simulations with subsequent particle tracking. For distributive mixing, the particle distribution behind the screw elements in the XY plane is analysed and the mixing index MQ, which relates the standard deviation and the mean value of the triangular areas between the particles, is calculated. For dispersive mixing, the maximum shear stress on the particle path and the integral of the shear stress over the residence time of each individual particle are determined. The results show that screw element geometry and rotation speed have a significant influence on dispersive and distributive mixing. In addition, better dispersive mixing is achievable with highly viscous materials. These findings enable the optimisation of the mixing zone of a co-rotating twin-screw extruder for the efficient mixing of mineral fillers.

Due to their valuable properties (low weight, and good thermal and mechanical properties), glass fiber reinforced thermoplastics are becoming increasingly important. Fiber-reinforced thermoplastics are mainly manufactured by injection molding and extrusion, whereby the extrusion compounding process is primarily used to produce fiber-filled granulates. Reproducible production of high-quality components requires a granulate in which the fiber length is even and high. However, the extrusion process leads to the fact that fiber breakages can occur during processing. To enable a significant quality enhancement, experimentally validated modeling is required. In this study, short glass fiber reinforced thermoplastics (polypropylene) were produced on two different twin-screw extruders. Therefore, the machine-specific process behavior is of major interest regarding its influence. First, the fiber length change after processing was determined by experimental investigations and then simulated with the SIGMA simulation software. By comparing the simulation and experimental tests, important insights could be gained and the effects on fiber lengths could be determined in advance. The resulting fiber lengths and distributions were different, not only for different screw configurations (SC), but also for the same screw configurations on different twin-screw extruders. This may have been due to manufacturer-specific tolerances.

Many theoretical analyses of extrusion ignore the effect of the flight clearance when predicting the pumping capability of a screw. This might be reasonable for conventional extruder screws with “normal” clearances but leads to errors when more advanced screw designs are considered. We present new leakage-flow models that allow the effect of the flight clearance to be included in the analysis of melt-conveying zones. Rather than directly correcting the drag and pressure flows, we derived regression models to predict locally the shear-thinning flow through the flight clearance. Using a hybrid modeling approach that includes analytical, numerical, and data-based modeling techniques enabled us to construct fast and accurate regressions for calculating flow rate and dissipation rate in the leakage gap. Using the novel regression models in combination with network theory, the new approximations consider the effect of the flight clearance in the predictions of pumping capability, power consumption and temperature development without modifying the equations for the down-channel flow. Unlike other approaches, our method is not limited to any specific screw designs or processing conditions.

During the cooling process of the molten material, residual stresses appear because the reduced volume of the cooled material cannot fully fill the space formerly occupied by the molten material. The morphology in and around the weld is formed by different factors depending on the material and process parameters. Different morphological structures relate to different mechanical properties. The process parameters and the welding results including morphology and residual stress are linked together. In this article, residual stresses and the mechanical properties of a hot-plate-welded polypropylene specimen with 0.1 wt.-% content of carbon black are investigated in relation to the morphology. Different measurement positions and joining displacements of parts to be joined result in different residual stress states and morphological structures. The higher the joining displacement, the higher the residual stress. Investigations of the morphology show a relation between the size of the alpha spherulites and the joining displacement. Diffractions patterns of wide-angle X-ray scattering (WAXS) are not able to resolve the beta phase of the specimen.

Additive manufacturing processes, like the Fused Deposition Modeling (FDM) process, do not need product-specific tools and create parts directly from the CAD data. In the FDM process, the semi-finished product, a wire of a thermoplastic polymer, is melted and forced through a nozzle. The continuous positioning of this nozzle allows the polymer to weld together strand by strand and layer by layer to produce a component. Because no mold is used in the FDM process, no holding pressure can be generated as in injection molding processes, in which the holding pressure is used to minimize the shrinkage and warpage of the part. In the FDM process, the part is generated in an ambient pressure environment. Each strand cools down and shrinks separately. This causes residual stresses in the part that can lead to major warpage and a complete stoppage of the process. This is the main reason why the material selection in the FDM process is restricted in comparison to conventional polymer processing technologies. In this paper, the warpage of different polymers is quantified as a criterion for evaluating the processability of polymers in the FDM process. Due to the process principle, the part properties in the FDM process are mainly influenced by the machine quality and the data processing, so that it is difficult to test a material for FDM independently of the machine and the data processing. Considering these influences, a custom-built specimen is created to test and quantify the warpage of different types of blended and reinforced polyamide 6. Considering the experimentally investigated warpage, the materials can be evaluated and the warpage can be related to the shrinkage investigated in pvT measurements. This procedure allows the machine- and process-independent rating of the processability in terms of warpage for different materials. Alongside other criteria, this is a necessary step to develop new materials with good processability in the FDM process.