Injection moulding fibre orientation

These long fibres give better product performance although injection moulding machine modifications may be necessary to prevent fibre damage and reduce undesirable fibre orientation effects in the mould. 

The effects of processing will be illustrated by considering injection moulding of a semicrystalline polymer. The molten plastic is injected into the mould under high pressure and temperature. The edges of the mould retard flow and cool more rapidly, leading to a boundary layer of high shear, which in semicrystalline polymers leads to orientation of the polymer chains and of short fibre reinforcements parallel to the direction of flow. At the centre the structure is less oriented. Where two separate flow streams meet, there is a... 

In summary, therefore, processing, and in particular injection moulding, can introduce limited chemical degradation, local polymer orientation, orientation of short fibre reinforcements, internal stresses, warpage, shrinkage and defects such as weld lines and voids. 

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. 

With mixed aramid fibre/particulate flake compositions in polycarbonate, however, the previously mentioned processing operations significantly reduced the levels of fibre and flake orientation relative to corresponding observations with single particle compounds [167]. For example, in sheet extrusion and injection moulding, the flakes oriented perpendicular to the fibres losing their biaxial orientation parallel to the surrounding machine surfaces. 

PLA is a polymer that may not be well suited to injection moulding. Its rate of crystallisation is too slow to allow cycle times typical of those for commodity thermoplastics such as polystyrene. Stress induced crystallisation that can enhance PLA crystallisation is better suited to processes such as fibre spinning or biaxial orientation of film. 

Short fibre polymer composites are being increasingly used as engineering materials because they provide mechanical properties superior to neat polymers and can be processed easily by the same fabrication methods, e.g. injection moulding. The mechanical properties of these materials are dependent on a complex combination of several internal variables, such as type of matrix, fibre-matrix interface, fibre content, fibre dimensions, fibre orientation, and external... 

Control of fibre orientation with short fibres, such as those sometimes used in injection moulding or reaction injection moulding (RIM) is also important. The flow in the mould may be constrained in two directions such as width and thickness, giving linear flow only in one direction. Or, the flow can instead be radial, that is, constrained in just one direction, for example the thickness, in which case the usual shear forces due to the mould walls will operate, but there will be an additional extensional force, caused by the expanding flow front. This results in fibre orientation being normal to the flow direction. The orientation angle is usually very small at the surface, but it can be almost 90° at the mid-plane [7]. 

When the material contains short glass fibres, or other reinforcements, their orientation will also be determined the flow pattern, figure 83 shows a section through an injection-moulded part made fiom glass-reinforced polypropylene near the surface, the fibres are oriented preferentially in the flow direction, whilst in the central region they are aligned in the direction transverse to flow. A full e q>lanatk>n of these effects is beyond the scope of this book. 

Sections ((a) parallel to the flow direction and (b) normal to the flow direction) through a glass-reinforced polyp>ropylene injection moulding, showing the short fibres near the surface oriented parallel to ttie flow direction, whilst those in the central region tend to be transverse to flow (after M. W. Darlington). 

The following techniques are useful for analysis and charaterization for the sizing on the fibres SEM, IR and 6PC of extracts, IGC and DMA (T ) for the fibre-matrix bond micro bond, micro failure with SEM [2], confocal laser scanning microscopy [3] DMA, flexural strength, ILSS and impact of the uni-directional composites (fixed fibre length, orientation and volume fraction distributions). After this detailed analysis, the fibres are tested in their application, which is an injection moulded compound for thermoplasts, where the microstructure (fibre length, fibre orientation and fibre-fibre distance distributions) as well as the fibre- matrix adhesion determine the mechanical properties. 

Vincent and co-workers [151] studied the influence of flow on the fibre orientation and mechanical properties in the injection moulding and extrusion of fibre reinforced thermoplastics. 

Tool design and manufacture is a very expensive business, and before a design is committed to it prototypes will usually be made and tested. They are made by machining from solid material of the same type as that proposed for the injection-moulded product, if available. Some care has to be exercised in results of tests on prototypes made in this way, because orientation of reinforcing fibres, or of polymer molecular chains in machined gears may be inconsistent with those in mouldings. 

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. 

The improvement in the initial modulus of as-spun PP fibres which is provided by the LCPs is illustrated in Table I. This observation accords with those of Baird and coworkers (12,13), who noted improvements in the moduli of injection-moulded samples of PP where LCP was incorporated. The tensile strengths are, however, still low, so that fibre drawing remains essential, to develop an oriented molecular structure for the PP matrix. 

M. Akay and D. Barkley, Fibre orientation and mechanical behaviour in reinforced thermoplastic injection mouldings. Journal of Materials Science, 26, 2731-2742 (1991). 

H.-C. Ludwig, G. Fischer and H. Becker, A quantitative comparison of morphology and fibre orientation in push-pull processed and conventional injection-moulded parts, Composites Science and Technology, 53, 235-239 (1995). 

The mechanical properties of the polymer-based composites are dependent on the short fibres orientation, and, on its turn, this one is influenced by the characteristics of melt flow, injection conditions, and mould geometry [1254, 1256]. 

The fibres orientation with respect to the stress direction essentially influences the fracture mechanism, as it was proved for a composite with semicrystalline matrix, PET reinforced with short fibres, having the structural characteristics presented in Figure 3.496 [1267], It can be observed that different fibre orientation normally exists across the thickness of injection moulded plaques of short fiber reinforced thermoplastics. 

Surface finish can be a problem with composites, although there are many well-known examples of painted composite surfaces which are judged to be of Class A surface finish. However, the most consistently under-appreciated problem with fibre-reinforced composites is anisotropy, i.e. the directional dependence of mechanical and dimensional properties. It arises from the orientation of the fibres, and consequently is most pronounced when a component has been shaped by a high speed melt flow process injection moulding is the prime example. It is best to assume that anisotropy is always present in a composite, unless isotropy has been designed into the material either by the use of a random glass mat, or by a deliberate layering process in which the orientation in different layers is balanced out. 

A and B through fibre orientation in injection moulding C because samples cut in direction of continuous fibre reinforcement. 

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