Compression moulding variation

The injection-compression process is an important variation in which rubber is injected into a partly closed mould under low pressure used for moulding-on of rubber soles to footwear uppers. Rubber is injected into a gap between the shoe upper and the sole plate, which is held 3-4 mm wider than it would be in its final position. After injection at the first mould station of a multistation machine the sole plate is closed in the manner of compression moulding at the second mould station and rubber is pressed along the sole and over the toe cap. The importance of the process is that it becomes possible to mould without using normal high injection pressures which tear and distort canvas, leather or synthetic uppers. 

Fig.36. Variation in electrical conductivity (o) with molecular weight for polyethylene composites filled with 4% by volume carbon black, demonstrating the effects of orientation (I), degradation (II) and flow-induced segregation of carbon black aggregates (III). ( ) injection moulded (O) compression moulded (unoriented) [181]...

Figure 4.22. Variation of hardness H with tensile Yt (solid symbols) and compressive Yc yield stresses (open symbols). O,, compression-moulded samples A, A, annealed samples at atmospheric pressure , , CEPE samples. (From Flores et al, 2000.)...

Transfer moulding is a variation of compression moulding, where by a rubber preform is prepared, heated in the transfer chamber and injected via a plunger into a mould refer to Figures 6.24(a) and (b). 

A patent (19) assigned to Phillips Petroleum Co. covers the use of di-, tri-, or tetra-thiols which react with sulfur in the presence of basic catalysts such as amines. Typical materials are prepared by heating, at 135 °C, a mixture of up to 20 parts by weight of a poly thiol containing 0.1 parts tributylamine with 100 parts of sulfur. The resultant mixture was compression-moulded at 24,000 psi and 100 °C. Typical results for tensile strength and Shore D hardness are given in Table I. No mention is made of the possible variation in these properties with storage time. 

This is a marriage of two processes pultrusion and compression moulding. A pultrusion in-feed system is employed for management of the reinforcement and to apply the resin mix but the conventional pultrusion die is replaced with one which will open and close. This allows variations in the cross section and linearity of the component which are not possible with pultrusion. 

Local thickness variations can be incorporated without incurring moulding problems (cf injection and compression moulding). 

In order to minimise as far as possible the influence of processing variables, studies have been carried out using tensile creep tests on carefully prepared compression moulded specimens. It must be realised that, vv ith injection moulded articles, the creep properties will also be subject to variation with the amount and direction of residual flow orientation, while, with crystalline polymers such as the polyolefins, the creep effects will also be influenced by variations in density caused by a combination of flow orientation, compressive packing, and cooling effects. Stresses will generally be complex and will often involve compressive and flexural components. However, articles should normally be designed to limit the strains occurring to quite low levels, where a reasonable correlation can be expected between tensile, compressive, and flexural creep data. 

Compression resin transfer moulding (CRTM), schematically described in Fig. 7.2, is another variation of RTM, in which the rigid upper mould is partially closed, maintaining a given cavity thickness greater than the final part thickness. The desired quantity of resin is then injected. Finally the mould is closed to its final position, compacting the preform to its final thickness, and establishing a compression-driven flow which pushes the resin into the... 

During blowing of a bottle, the inner surface of a preform is cooled by the expansion of the compressed air used for inflation, the outer surface is cooled by contact with the cold mould whilst the internal thermal energy is raised by strain heating. In addition, the contour shape of the container and the complex blowing process itself produces considerable variation in wall thickness and direction of stretching at different positions in the container. The... 

It can be seen that injection-compression is a variation of the injection moulding process. Other variations on the injeetion moulding process such as this will be discussed in Chapter 10. 

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