Direct injection moulding

One alternative approach to the two-stage steam moulding process is that in which impregnated beads are fed directly to an injection moulding machine or extruder so that expansion and consolidation occur simultaneously. This approach has been used to produce expanded polystyrene sheet and paper by a tubular process reminiscent of that used with polyethylene. Bubble nucleating... 

The foam effect is achieved by the dispersion of inert gas throughout the molten resin directly before moulding. Introduction of the gas is usually carried out by pre-blending the resin with a chemical blowing agent which releases gas when heated, or by direct injection of the gas (usually nitrogen). When the compressed gas/resin mixture is rapidly injected into the mould cavity, the gas expands explosively and forces the material into all parts of the mould. An internal cellular structure is thus formed within a solid skin. 

The results of the above section show that the significant nonuniformity of the distribution of the filler particles in the thickness of sample is observed during injection moulding of the filled polymers. This nonuniformity must affect the electrical properties of CCM owing to the strong dependence of the CCM conductivity on the filler concentration. Although there are no direct comparisons of the concentration profiles and conductivity in the publications, there is data on the distribution of conductivity over the cross-section of the moulded samples. 

NMR microscopy is appropriate to study the flow behavior of complex materials, the flow in complex geometric structures and processes such as extrusion, injection moulding, flow in nozzles, pipes, etc., because the velocity vectors can be directly... 

Thermoplastic polymer macromolecules usually tend to become oriented (molecular chain axis aligns along the extrusion direction) upon extrusion or injection moulding. This can have implications on the mechanical and physical properties of the polymer. By orienting the sample with respect to the coordinate system of the instrument and analysing the sample with polarised Raman (or infrared) light, we are able to get information on the preferred orientation of the polymer chains (see, for example, Chapter 8). Many polymers may also exist in either an amorphous or crystalline form (degree of crystallinity usually below 50%, which is a consequence of their thermal and stress history), see, for example, Chapter 7. 

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. 

PP-structural foam mouldings were produced on an injection moulding machine in a pre-pressurised mould cavity by the classical low-pressure process and an alternative low-pressure process. Melt temperature, injection direction and sprae diameter were varied. Cross-sections cut from the middle of the small cylinder in longitudenal orientation were investigated by site-resolved X-ray scattering. Morphological properties were investigated. 4 refs. 

Fig. 30. Design principles of direct compounding injection moulding machine for processing highly filled polymer compositions...

In marked contrast to conventional injection moulding, where orientation effects normally depress conductivity, in this investigation the injection-moulded composite material yielded not only a lower percolation threshold than compression moulded samples, but in the injection direction, also gave conductivity values two to three orders of magnitude higher than the latter. 

Fig. 35. In-mould magnetization injection moulding technology for filled thermoplastics (a) constructional features of mould (b) magnetic flux generation in the radial direction [177]...

Three types of moulding processes are available - compression, transfer and injection moulding. In compression moulding the rubber blank is placed directly into the cavity of the mould where it is heated by conduction which causes rubber flow by application of pressure. Transfer moulding uses prewarmed rubber which is heated during transfer and forced through small orifices into the mould cavities in a three-part mould. In the injection moulding process, the rubber compound is pushed under pressure from an injection head where it has been heated and plasticized into a closed heated mould where cure is completed. 

Table 4. Properties of thin injection-moulded samples measured along the direction of flow... 

Injection moulding is by far the most frequently applied technique for manufacturing end-products directly from a thermoplastic raw material. In this process articles are formed by injecting the molten polymer into a cooled mould. Also on rubbers and thermosets this method is applied, though at a smaller scale in these cases the mould is heated rather than cooled in order to promote the vulcanisation or curing reaction. 

In this case the biopolymers are directly plasticized by thermomechanical means, and transformed through the classical forming technologies of the plastic industry injection-moulding, extrusion and thermoforming. These agromaterials keep a natural aspect, they are sensitive to the atmospheric conditions like wood, but they have no shape restrictions (Figure 5.20). 

The metathesis polymerisation of dicyclopentadiene, an inexpensive monomer (commercially available cyclopentadiene dimer produced by a Diels-Alder addition reaction containing ca 95 % endo and ca 5 % exo form), leads to a polymer that may be transformed into a technically useful elastomer [144-146, 179] and thermosetting resin [180,181]. The polymerisation has characteristics that make it readily adaptable to the reaction injection moulding ( rim ) process [182], The main feature of this process comes from the fact that the polymerisation is carried out directly in the mould of the desired final product. The active metathesis catalyst is formed when two separate reactants, a precatalyst (tungsten-based) component and an activator (aluminium-based) component, are combined. Monomer streams containing one respective component are mixed directly just before entering the mould, and the polymerisation into a partly crosslinked material takes place directly in this mould (Figure 6.5) [147,168,183-186],... 

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