Advanced Thermoplastic Composites and Manufacturing Processes

Overmolding is a technology in which a thermoplastic composite laminate is thermoformed and subsequently injection overmolded. This near-net-shape manufacturing process is well suited for automated large series production of complex 3D structures with excellent structural performance and a high level of function integration. There is a need for process design tools, which are currently lacking. For this purpose, the bond strength between the overmolded composite laminate and the injected polymer resin was studied by means of process modeling and mechanical testing. Starting from De Gennes' classical reptation theory for reptation and healing of amorphous polymers, an alternative approach was developed to describe the strength development for semi-crystalline materials. A rudimentary description of the degree of melting was implemented to predict the bond strength as a function of the thermo-mechanical history at the interface during forming and subsequent resin injection for PA6 and PEEK, both semi-crystalline matrix materials. Dedicated coupon geometries were developed, manufactured and tested to evaluate the bond strength under tensile and shear loading conditions. Both a single step and a dual step process were evaluated, leading to distinctly different process mechanisms and resulting material structure and mechanical performance of the parts. Suggestions are presented for optimum process conditions and improved design features to further mature this technology.

Continuous fiber reinforced composites were made using in-situ polymerization of a new liquid thermoplastic resin. Depending on the temperature profile in the die and the initiators used, it was shown that PREDICI software enables the simulation of the radical polymerization reaction. By using an appropriate sizing for the fibers, the properties of the pultruded part are comparable to standard thermoset resins. Moreover, the specific thermoplastic behavior of the matrix provides the unique advantage of being post-formable or shapeable as desired while maintaining mechanical performances.

Absorption-based microcomputed tomography (μCT) scans using polychromatic X-ray sources are frequently used to analyse the microstructure of polymer-matrix composites. Assuming the proportionality between linear X-ray attenuation coefficients and gray levels in the reconstructed μCT images, segmentation techniques can be used to conduct qualitative and quantitative analyses. Nevertheless, the reliability of such analyses is limited in partially consolidated composites formed of commingled yarns due to low contrast induced by beam hardening and partial volume effect. This paper aims at investigating the possibility of using low-contrast μCT images to analyse fiber bed deformation and impregnation mechanism by staged consolidation of a weft-knitted fabric of commingled thermoplastic/glass yarns. The experimental work is focused on the effect of the compaction ratio of the consolidation cycle. Absorption-based μCT analyses are conducted for representative samples using a voxel size of 10 μm and a constant energy-level of the X-rays source. Preliminary qualitative analyses of the raw reconstructed images indicate that beam hardening is the most significant in the case of the non-consolidated sample and that partial volume effect makes it difficult to visually distinguish between dry and impregnated zones inside the yarns at a compaction ratio of 63%. Thus, two image-quality descriptors, viz. contrast and signal-to-noise ratio are evaluated based on the K-means clustering method to follow their variation for four consolidation levels. The corresponding volume fractions of glass fibers, thermoplastic matrix, and voids are compared with the results of a second weight-controlled thresholding method. Considering that the weight-controlled thresholding method is less sensitive to beam hardening, it is confirmed that obtaining reliable segmentation results based on the K-means method requires a contrast around 0.5 and signal-to-noise ratio lower than 3. The results from the comparison with the classical consolidation scenario of commingled yarns confirm the potential of using μCT images of different qualities to characterize partially consolidated weft-knitted fabrics.