Moulding processes high-pressure

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... 

EniChem through Snamprogetti EniChem process, high pressure, autoclave or tubular reactor. LDPE (density 918-935 kg/m ), MFI = 0.1-400 g/10 min. EVA copolymer (3-40% VA content) for film, injection moulding, profiles, sheets, cable sheetings, crosslinking, and foaming. 

At the present time, doped ICPs are not normally capable of being processed like normal thermoplastics. Processes usually Involve high-pressure moulding of finely powdered polymers under vacuum or an inert gas. However, modification of some ICPs with, for example, alkyl or alkoxy side groups may produce soluble, and thus more tractable, polymers. 

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. 

In some cases the raw material is already in a fluid state with thermosets as a resin for casting or impregnating, with thermoplasts, as an exception, sometimes as a monomer which is able to polymerise in the mould. In these cases the forming processes are relatively simple, since they can be carried out on low-viscous fluids which do not require high pressures to be transported. 

Notwithstanding this fact, high pressures are required to transport the fluid mass through the transport channels or into the mould of a processing machine with an acceptable speed. Pressure levels of 50 to 150 MPa (500 to 1500 bar) are normal, so that the machines have to be very robust. 

Recent work has included tests that examine the effects of shear rate and cure simultaneously (which will also be discussed below). The models derived from these tests are recombined to provide an overall chemoviscosity model to be used in processing applications. The effect of pressure on chemoviscosity has not been studied extensively however, system pressure may be relevant to high-pressure injection-moulding and transfer-moulding processes. 

Thermoset injection moulding is a high-pressure process typically used for DMC or BMC polyester materials, and is shown in Figure 6.18. 

A vacuum is then drawn inside the cover membrane (to remove volatiles and porosity) while the pressure and temperature inside the autoclave are separately controlled (to provide even control of pressure across the surface, and thermal control of cure). Optimization of the application of pressure and vacuum will prevent the formation of dry laminates (which typically occurs when pressure is applied too early and low-viscosity resin is forced out) and porous laminates (when pressure is applied too late with high-viscosity cured resins). In general, consistent mouldings of high quality can be produced, but the process is slow and capital-intensive. 

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