Composites Part A: Applied Science and Manufacturing
Commingled yarn composites for rapid processing of complex shapes
Introduction
Among the possible routes for manufacturing fibre reinforced thermoplastic composites, consolidation of preforms based on commingled yarns has emerged as a cost-effective method for the production of complex-shaped parts. As the polymer flow distance for impregnation is reduced when the matrix is already commingled with the fibres, the time for impregnation or the need for high-applied pressure are limited, leading to a reduction in manufacturing cost. Moreover, compared to other intermediate product forms such as pre-impregnated tows or powder-impregnated fibre bundles, the flexible commingled yarns can easily be converted into a highly drapeable textile fabric, reducing the likelihood of wrinkles during the forming of complex shapes. To fully benefit from these advantages, the process parameters governing consolidation must be identified and optimised. Consolidation of commingled yarns has been studied and modelled by several authors [1], [2], [3], [4], [5], [6] and has recently been reviewed by Svensson et al. [7]. In all cases, the consolidation quality was characterised by the amount of residual porosity in the part. In general, the polymer fibres melt rapidly and form liquid pools surrounding the reinforcing fibres. The problem thus reduces to that of infiltration of dry fibre bundles surrounded by molten polymer pools. As reinforcing fibres and polymer fibres are in general not distributed homogeneously within a yarn, commingling quality becomes one of the most critical parameters.
Friedrich et al. [1], [2], [3], [4] modelled the fibre bed impregnation mechanism by describing matrix flowing in one direction, orthogonal to the fibre axis. They assumed that part of the yarn was impregnated quickly by well-distributed polymer fibres or during melting of the polymer, and also defined a commingling parameter corresponding to an adjusted thickness of the dry bundle. Van West et al. [5] proposed a more elaborate finite element model for impregnation of fibre bundles of elliptic cross-section. They considered that the fibres may move in the bundle during flow of the resin, assuming a linear distribution of fibre velocities along the radius of the bundle, but did not account for the resulting volume fraction change. They also assumed that the matrix formed a continuous network at some point. An increase in void gas pressure followed as impregnation further proceeded, leading to a value of residual porosity. Cain et al. [6] adapted the model developed by Van West for the case of non-Newtonian fluids. Limitations encountered in all these models include the assimilation of the superficial velocity reported in Darcy's law to the matrix velocity and the consideration of a Carman–Kozeny approach for the fibrous bed permeability, which adds another parameter to be determined by fitting with experimental measurements of permeability or void content. In addition, these models were all compared to experimental data obtained from compression of flat plates.
The present authors recently proposed a detailed analysis of the laws governing fibre bundle impregnation, and developed a new model for consolidation of commingled fibre yarns [8]. The analysis relied on a geometrical description of the yarn structure, taking into account a distribution of cylindrical dry fibre bundles of different sizes at the onset of impregnation, as schematically described in Fig. 1. Each population is therefore impregnated concomitantly, with radial flow orthogonal to the fibre axis, but with different kinetics. The overall void volume fraction includes the contribution of the complete distribution. The model also included a possible gas pressure increase due to closing of the void within a bundle at some point during impregnation. It was demonstrated, for unidirectional commingled yarn laminates made from Polyamide 12 (PA12) and carbon fibres (CF), that the model accurately predicts the influence of the processing variables (i.e. time, pressure and temperature) on the part quality (in terms of void content) after consolidation, if appropriate selection of a critical radius related to closing of pores is made. Nevertheless, model solutions for limiting cases, considering that pores are closed from the onset of infiltration or, on the other hand, remain open all along, provided adequate brackets for an estimation of laminate void contents with no fitting parameter.
In this paper, we propose to investigate the applicability of the model previously validated for a flat geometry, a unidirectional yarn configuration and isothermal consolidation conditions to other processing techniques and yarn assembly types, always keeping the same type of individual yarn. Three case studies are proposed: (i) manufacturing of a tube by bladder inflation moulding of braided yarns, (ii) automated yarn placement and pre-consolidation, followed by polymer over-injection for the manufacture of selectively reinforced composites, and (iii) rapid non-isothermal compression moulding of a stack of unidirectional commingled yarn fabrics, pre-consolidated or not.
Section snippets
Materials
Commingled yarns composed of stretch-broken carbon fibres and staple PA12 fibres were used in this study. Microscopic observations of yarn cross-sections revealed a distribution of fibre bundle sizes within the yarn, as commingling was not perfectly homogeneous. As a first approach for the consolidation analysis, it was assumed that a representative commingled yarn was composed of only two populations of reinforcing fibre bundles, referred to as small and large bundles, respectively, containing
Theory
For the three processing routes presented in the previous chapter, the architecture of the yarn assembly and the process conditions, in terms of applied pressure, time of pressure application, mould temperature and cooling rate are rather different. However, the individual yarns are identical for the three cases and the physical mechanisms are also similar. Indeed, in the case of Bladder Inflation Moulding, the material is heated up, pressure is then applied by the membrane, impregnation takes
Bladder inflation moulding
Fig. 6 shows a micrograph of a part of a laminate cross-section, after 2 min consolidation at 210°C under 2 bar, and a close-up of an incompletely consolidated yarn region. It is observed that pores still remain in the form of large uninfiltrated areas located within reinforcing fibre bundles. Pores can thus be approximated as cylindrical tubes along the yarn length. This confirms the model assumption that impregnation occurs radially in the various fibre bundles comprised in a yarn. The two
Conclusions
The consolidation model, previously developed and validated for unidirectional CF/PA12 commingled yarn fabrics processed in a flat matched-die mould, was applied to other fabric architectures and to processing techniques capable of producing complex-shaped parts. Bladder inflation moulding enabled hollow composite parts to be manufactured from tubular commingled yarn braids. Another process, referred to as integrated processing, was used to manufacture polymeric parts selectively reinforced by
Acknowledgements
The authors would like to thank the Swiss Priority Program on Materials Research, with Ems-Chemie AG and Sulzer Innotec AG as industrial partners, for the financial support of this research. Dr V. Michaud acknowledges support from the Fonds National de la Recherche Scientifique under contract 20-52625.97. Schappe Techniques is also gratefully acknowledged for supplying the materials used in this study. The collaboration of H.Q. Tran and N. Weibel was greatly appreciated.
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