Injection moulding flow mechanisms

Cast material is stated to have a number average molecular weight of about 10. Whilst the Tg is about 104°C the molecular entanglements are so extensive that the material is incapable of flow below its decomposition temperature (approx. 170°C). There is thus a reasonably wide rubbery range and it is in this phase that such material is normally shaped. For injection moulding and extrusion much lower molecular weight materials are employed. Such polymers have a reasonable melt viscosity but marginally lower heat distortion temperatures and mechanical properties. 

In this paper, all the blends and composites are designated by the type of matrix (G for the neat nylon, D for the 8 wt % rubber-modified nylon and N for the 20 wt % rubber-modified nylon), the concentration of fibres and the type of fibre/matrix interface (A or B). As an example, a material designed DlOB is a ternary blend made of DZ matrix and 10 wt% of type B fibres. After drying the specimens for 24 hours at 100°C, they were stored in plastic bags inside a desiccator. In comparison with freshly injection moulded samples, the moisture content in the specimens ready for mechanical testing is about 2 wt%. All the mechanical tests were conducted in an environmental chamber in controlled conditions a temperature of 20°C under a continuous argon flow. 

In summarizing, it can be concluded that the microhardness of elongational flow injection moulded PE is influenced by a local double mechanical contribution (a) a plastic deformation of crystal lamellae under the indenter, and (b) an elastic recovery of shish-fibrils parallel to the injection direction after load removal. Further, the Shish-crystals are preferentially formed when high orientation occurs, i.e. at zones near the centre of the mould and at an optimum processing temperature Tp around 145-150 °C. Below this temperature overall orientation decreases due to a wall-sliding mechanism of the mbber-like molten polymer. 

The anisotropy of the mechanical properties, caused by unidirectional orientation, is clearly demonstrated by Fig. 1. It shows a microtome section of the wall of an injection-moulded beaker perpendicular to the direction of flow. Bending causes numerous cracks to be formed in the oriented surface layers but they stop abruptly at the unoriented spherulitic core, because this material does not split so easily. Samples taken parallel to the direction of flow do not show these premature cracks on bending, on the contrary, the samples are less brittle than unoriented material. 

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. 

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. 

Attention is focused on improving the flow of the material in order to reduce cycle times for injection moulding complex and thin-walled components. GE Plastics has found for example that the viscosity of PEI can be reduced by 50% with the addition of a small amount of PPO polymer. The mechanical properties of PEI remain constant with the addition of the PPO. 

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