Thermal issues in thermoplastic composites manufacturing
Abstract: Heat transfer in thermoplastic composites duringmanufacturing is treated both numerically and experimentally inthis thesis. The related issues crystallization kinetics,residual stresses, and geometrical changes are alsoinvestigated. Three different manufacturing methods arestudied:thermoplastic pultrusion, double-belt presslamination, and compression moulding. In the experimental workglass fibre reinforced polypropylene (PP), polyamide 12 (PA12),and polyethylene terephthalate (PET) are used.First, an experimental parametric study on thermoplasticpultrusion is carried out. The effect of changing variousprocess variables on mechanical properties, surface finish, andfibre distribution is investigated. The results show that thedesired effect on the different evaluated composite propertiesrequires contradictory process conditions and therefore it isnecessary to decide which properties are the mostimportant.Heat transfer in thermoplastic composites duringmanufacturing is modelled in one dimension, numericallyimplemented using finite difference methods, and in threedimensions using finite element methods. Temperature dependentthermal conductivity and specific heat, as well as non-infinitethermal contact conductance at interfaces are included in themodels and the heat generation from crystallization is takeninto account. The temperature predictions are compared withexperimental data and the agreement is generally good.Moreover, it is found that the use of relevant thermal contactconductances at interfaces is important for accuratepredictions, especially when multi-layered items arestudied.The non-isothermal crystallization in thermoplasticcomposites during manufacturing is predicted using amacro-kinetic model, which is on differential form and thuswell suited for coupling together with the heat transfer model.Despite being mathematically uncomplicated, the modeladequately represents the most essential aspects of the processand agrees well with experimental data.Using the above described predictions as basis, residualstresses and geometrical changes in compression moulded glassmat reinforced plastics (GMT) are investigated using twodifferent material models, one transversely isotropic linearelastic and the other isotropic linear viscoelastic.Predictions are compared with some experimental data concerninggeometrical changes and the agreement is good. The limitationwith the elastic model is that only geometrical changes can bestudied. Further, as the GMT material is consideredtransversely isotropic, the results from the isotropicviscoelastic model are qualitative.Keywords:thermoplastic, composite, manufacturing,characterisation, pultrusion, double-belt press, compressionmoulding, modelling, heat transfer, crystallization, residualstresses.
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