Compression moulding fibre orientation

Hojo, H., Kim, E. G., Yaguchi, H. and Onodera, T. (1988), Simulation of compression moulding with matrix-fibre separation and fiber orientation for long fibre-reinforced thermoplastics . International Polymer Processing, 3,54-61. 

Fig. 7.9 Cross-sectional SEM micrographs (a, b) neat ABS, and (c) ABS/10 wt% carbon fibre 3D printed parts and (d) compression-moulded ABS/10 wt% carbon fibre (Reprinted from Composites Science and Technology, Vol. 105, Halil L. Tekinalp, Vlastimil Kune, Gregorio M. Velez-Garcia, Chad E. Duty, Lonnie J. Love, Amit K. Naskar, Craig A. Blue, Soydan Ozcan, Highly oriented carb

It is the combination of fibre length, orientation and ability to compress the fibre/resin mixture whilst moulding which determine the level of property achievable. Only some materials can be used with some processes and this is illustrated in Table 11.3. 

Mechanical properties, particularly tensile strengths and stiffness, depend upon the degree of orientation achieved. This is limited to some extent by the fabrication method and type of article produced, as shown schematically in Fig. 8.15. Thus, a compression-moulded unoriented LCP has mechanical properties similar to that of a conventional isotropic polymer. On injection moulding, tensile bars of MCLCPs generally show superior mechanical moduli to that of conventional glass-fibre-reinforced isotropic thermoplastic (Fig. 8.16),... 

Most tests will be made on standard test pieces which may be pieces cut from a component or a sheet, or they may have been moulded separately from the same material. Where test pieces or sheet are produced for the trials it is important that they are produced in as near as possible the same way as the product and that the processing conditions are recorded. Different results can be expected from compression and injection moulding or from extrusion (where a choice is possible). Directional properties can result from the conditions of flowing and cooling in a mould. For example, in a study at ERA, the creep strain of unfilled HDPE, either individually moulded or cut from square plaques, varies by up to a factor of two depending on the orientation of flow [40]. This difference becomes even more marked with short fibre reinforcement. 

The process uses crystallizable polymers, of which the most important in PET. The first step is to injection mould (hence the name) a parison, or preform as it is more usually termed here. The preform is closed at the bottom and is considerably shorter and thicker than the final bottle. It is rapidly cooled (quenched) by using chilled water to cool the injection mould and this ensures that it is in its amorphous condition, i.e. no crystalline structure. Next it is reheated with infra-red elements to above its Tg, about 90-100 C for PET and enters the bottle mould and the mould is closed. The blow pin enters and pushes the soft preform downwards almost simultaneously the blow occurs, compressed air blowing the material outwards. The result is biaxial orientation - downwards from the movement of the blow pin, outwards from the action of the expanding air. The orientation induces crystallization, but in the form of lamellar crystals rather than spherulitic ones. This type of crystallization is strain-induced, and is characteristic of synthetic fibres and film, e.g. Melinex. It gives a transparent product with enhanced physical properties, both important for bottling carbonated drinks. The alternative name for the process is the stretch-blow process. Its main feature as a process is the control of the crystallinity of the polymer at its different stages. 

These polymers can be moulded by injection or compression and can be cast as films fi om organic solvents. Their isotropic tensile moduli are well beyond the range of known isotropic polymers. They have outstanding mechanical properties, considerably better than those of the best polymeric materials, and can even be compared to structural metals [66] (see Mechanical properties). The rigid PPP backbone is presented as the microscopic equivalent of the fibre in a fibre-polymer composite, such as oriented carbon fibres embedded in a suitable polymer matrix the pendant groups play the role of the suitable polymer matrix. Applications in the design and construction of military and commercial aircraft, sports and industrial equipment and automobile components are proposed. 

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