Rajasundar Chandran
There is a growing interest in exploring novel material combinations and processing technology to manufacture complex 3D shaped polymer and composite structures. In the field of prosthetics and orthotics, this need is significant with additional challenges of providing patient specific customization to these devices in a cost-effective manner. Thermoplastic based materials(TP) including thermoplastic elastomers (TPE) and their composite counterparts have the potential to address these applications. The objectives of the research are to develop composite preforms that can conform to complex 3D structures, understand the processing phenomena in particular the fusion bonding mechanisms of different thermoplastic materials forming hard and soft interfaces and finally develop processing strategies that are suitable for the manufacture of complex 3D structures with these multimaterials. A novel TPE based composite preform consisting of continuous glass fibres was first manufactured by a continuous melt impregnation technique, extremely well suited for the studied TPE material. This process resulted in superior fibre wetting and impregnation characteristics for the composite preform. Furthermore, TPE reinforced by glass fibres provided preforms which can conform easily to 3D shapes owing to the flexible nature of the matrix. Then isothermal integration of polypropylene based glass fibre (iPPGF) composites with the TPE materials was studied and the bond strengths of bilayer specimens under different processing conditions were determined. The bonding temperature and pressure were tuned to promote the best fusion bonding conditions which was verified from peel tests and microscopic characterization of the interfaces. Further improvements in bond strength and fusion bonding time was achieved by non-isothermal fusion bonding techniques namely overmolding and 3D printing. The processing windows were then obtained for these two processes under a wide range of fusion bonding conditions and for various TPE and TP based materials. The presence of a continuous plasticized iPP phase in the TPE was determined to play an important role in the bonding through the establishment of intimate contact and interdiffusion mechanisms during the molding process. The influence of the interface temperature and bonding pressure was shown to govern the wetting and intimate contact of the interfaces and to compensate shrinkage respectively. The established processing conditions permitted tailoring of the bond strengths for TPE-iPP and TPE-iPPGF interfaces. These results can be directly applied for high volume manufacture of complex 3D shaped composite parts. The effects of the TPE composition (fillers, reinforcements, TP content, surface roughnessâŠ) and of bonding temperature, pressure and time, were thus better understood under the non-isothermal bonding conditions encountered in the additive processes. The results of the FDM printing can be combined with other low pressure processing techniques such as thermoforming to manufacture hybrid multimaterials 3D structures. The research is applied in developing a hybrid processing technology which proposes the combination of thermoplastic-based multimaterials and different processing techniques that are suitable for the manufacture of the next generation of composite applications in particular for customized affordable prosthetic limbs.
EPFL2017
