Directionally reinforced moulding

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... 

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 application of this technique to talc-reinforced polypropylene has shown that the microstructure of platelets and the resulting physical properties of the moulded composites are markedly affected [171)]. With two live feeds located at either end of the mould cavity, the talc platelets exhibited strong talc platelet alignment throughout the thickness of the moulding in the direction of the ap-... 

Uniaxial tensile tests have been performed for four-ply and one-ply composites based on glass plain-weave reinforcement — Table 2.3. The composites are produced by resin infusion moulding using Dow Derakane 8084 Epoxy-Vinyl Ester resin. The weave architecmre is slightiy unbalanced exhibiting different crimp in warp and weft directions. The lay-up of four-ply composite has been balanced. 

As a result, when they are used to reinforce plastics, significant increases in modulus (with talc and mica) and strength (with mica onfy) can be obtained at Uttle or no increase in cost There is also a further advantage over fibres when platelets are aligned parallel to each other (as they are in the surface layers of an injection moulding) they provide reinforcement in all directions in their plane, not merely in one direction as with uniaxially aligned fibres. 

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). 

Epoxy prepreg also may be compression moulded. This may have glass or carbon fibre as the reinforcement in uni directional or bi-directional geometry. 

New glass fibre-reinforcement products for u.se in fibre-directed preform processes are another focus of development, designed specifically for liquid composite moulding applications such as resin transfer moulding with thermoset polyesters. 

---