Screening method for the onset of bonding of molten polyamide resin layers to continuous fiber reinforced laminate sheets

Thermoplastic polymer composites (TPC) with multiple fabric layers of continuous fiber reinforcement are laminate sheets designed to be thermally stamped and overmolded leading to low cycle times and thus high volume composite parts. Injection over-molding is one way to combine the high processability of thermoplastic melts with the less processable TPC sheets, for complex part making. To simulate the adhesion development during processes like over-molding, a non-isothermal screening method for bonding molten polyamide (PA) to a solid TPC surface has been developed and applied to evaluate differences in the threshold temperatures for good bond formation as a function of TPC matrix composition. Pure PA66 and aromatic high temperature nylons (PPA) in addition to miscible PPA blends comprising different ratios of PA66, were used as TPC matrices, and retain most of the very high melting point of 100% PPA. We have devised experiments to address two critical aspects: The first is to develop a screening test that probes very short timescales relevant to practical part making. The second is to investigate various miscible semi-crystalline blend ratios as matrices for the TPC where both blend species are crystalline. This allows us to explore a practical improvement of a lower required bonding temperature without substantially decreasing the high melting point of the PPA. DSC was also used to correlate the various blend melting transitions with the temperature required for the onset of bonding. The interfaces are shown to develop high strength partly because of substantial surface melting of certain blend components of the initially solid TPC, and this quickly allows interpenetration and inter-diffusion across the interface, which then crystallizes and strengthens during cooling.

Optimization of hybrid thermoplastic composite structures using surrogate models and genetic algorithms

Jan-Anders Månson, Martyn Douglas Wakeman

Using an integrated processing methodology, an injection or compression moulded thermoplastic structure can be locally reinforced, for example, with textile structures or uni-directional fibre tows. Netshape components can be produced with cycle times typical for injection or compression moulding but with higher mechanical properties. As the inclusion of local reinforcements adds complexity to the design process, both in the terms of amounts and location, mathematical optimization techniques in conjunction with finite element simulations can be a useful tool to design weight and cost effective structures. This work evaluates approximation models used in conjunction with genetic algorithms by using a generic but industry relevant beam structure as an example problem. The underlying finite element model is based on layered shell elements with up to four different materials through the thickness and with nine design variables controlling the material thicknesses in different areas. Material non-linear behaviour is included for the over-moulding polymer and for the compressive response of one specific fabric material. The accuracy of four different approximation methods was assessed, polynomial models with and without term selection, radial basis functions and Kriging. Rather than solely examine the mean approximation error, a method is proposed of how to treat single large approximation errors. It is shown that a Multi-Island Genetic algorithm can be successfully used to optimize the structural weight under 64 constraints with continuous and mixed continuous-discrete design variables.

2007

Phase separation at the fiber-matrix interface in composites based on a thermoplastic/polyester blend

Xavier Bulliard, Jan-Anders Månson, Véronique Michaud

A study of the microstructure developing at the surface of glass fibers in a poly(vinyl acetate) (PVAc)/polyester blend is presented. Three different experimental methods are used: a technique based on the Wilhelmy method to measure the wettability of the fibers before curing, and both optical microscopy and atomic force microscopy in the pulsed-force mode to characterize potential phases splitting at the fibermatrix interface after curing. It was found that, depending on the curing conditions and the concentration in PVAc, the surface treatment of the fiber could have a significant influence on the microstructure. For a concentration in PVAc lower than 5 wt% and a curing temperature of 80°C, extreme cases, such as the development of layers of one of the phases at the surface or the formation of lenses of one phase, were observed. In other cases, in particular for elevated temperatures and higher concentrations in PVAc, the fibers did not exert a significant influence on the morphology. It was also found that in such a reactive system, surface tension considerations alone are insufficient to explain the configuration of the phases at the surface of the fibers.

2005

Procede non isothermique de fabrication de parties composites creuses

Jan-Anders Månson, Pascal Vuilliomenet, Martyn Douglas Wakeman

A process for making hollow composite structures or vessels which includes the steps of: A) heating a mixture of thermoplastic matrix and reinforcing fibres wrapped over a rigid or semi-rigid thermoplastic liner or bladder above the melting point of the thermoplastic composite matrix outside of a moulding tool; B) transferring the heated assembly to a mould that is maintained below the melting temperature of the thermoplastic matrix of the composite; C) closure of the mould and application of internal fluid pressure to the liner or bladder to apply pressure to the thermoplastic matrix and reinforcing fibres; D) optionally the use of a special coupling system for rapid connection of the internal pressure; E) cooling of the liner or bladder and thermoplastic matrix and reinforcing fibre assembly in contact with the cold or warm mould while consolidation of the assembly occurs; F) opening of the mould and removal of the finished assembly. Suitable thermoplastic materials for the liner/bladder and thermoplastic composite matrix material include: polypropylene, polyamide, polyethylene, cross-linked polyethylene, polybutylene terephthalate, polyethylene terephthalate, polyoxymethylene, polyphenylene sulfide and polyetheretherketone.

2005