Anisotropy injection moulding

Fig. 25. Influence of temperature in injection moulder on anisotropy behaviour at the surface of injection moulded PE ( ) high molecular grade PE (M 450 000) (O) typical-moulding grade (M -100000) 18>...

Figure 7.2 schematically shows the geometry of an injection-moulded polymer bar in which z is the injection direction. Indentations were made on the yz plane. In all cases an indentation anisotropy arises because the microhardness is maximum when the indentation diagonal is parallel to the injection direction (Hj) and minimum when the diagonal is normal to it (Hy). The large value corresponds to... 

Figure 7.5. Correlation between indentation anisotropy AH and birefringence An for injection-moulded oriented PE. AH and An were measured at the centre of the PE bar along the z axis (Fig. 7.2). (After Rueda et al, 1989.)...

Is the mean length of the glass fibre reinforcement in injection-moulded polypropylene (fig. 4.28a) sufficient enough to give optimal stress transfer to the fibres Explain how the stiffness anisotropy in such a moulding arises. What new mechanisms of energy absorption arise when polystyrene is converted into a low density closed cell foam, and how can the compressive yield stress be controlled over a range of values ... 

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. 

Polymer liquid memoiy plays an important part in determining orientation, anisotropy and warping in injection moulding. In extrusion, the... 

Anisotropy and fibre distribution in the case of short GF reinforced items cause the biggest disadvantage in injection moulding, as regards the final mechanical performance, however, the economy of process with short GF is much more promising than use of continuous fibres. 

Figure 3.447. Tensile strength of tested LCPs for both test directions. Visible is the high anisotropy between testing (a) parallel (T) and (b) perpendicular (L) to the mould-filling direction. ( ) Separated skin layer measured, separated core layer measured, ( ) injection+moulded material measured and dl injection-moulded material calculated [1263].

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. 

The characteristic feature of liquid crystal polymers is that the stiff, rod-like molecules become highly ordered in the melt when subjected to a process such as injection moulding. This morphology is retained on cooling in the mould. The highly ordered aggregates impart a wood-like structure to the solid, which may be described as self-reinforcing. Compared to other moulded materials, liquid crystal polymers exhibit considerable anisotropy. 

Compared with injection moulding, there is a big reduction in the filling pressnres reqnired. There is also less orientation of the material, which can improve properties and rednce anisotropy. This can be of special importance in moulding thin-walled components where dimensional stability is important, and with transparent mouldings where good optical properties are required. 

Measurements at the surface of the injected bars give values of about H 60 MPa, which are close to the values obtained for linear isotropic PE with a density p = 0.96 g cm . The low orientation at the surface of the mouldings explains the low indentation anisotropy (AH = 10%) shown in Fig. 7.3. 

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