Flow in an injection mould

The flow process in an injection mould is complicated by the fact that the mould cavity walls are below the freezing point of the polymer melt. In these circumstances the technologist is generally more concerned with the ability to fill the cavity rather than with the magnitude of the melt viscosity. In one analysis made of the injection moulding situation, Barrie showed that it was possible to calculate a mouldability index (p.) for a melt which was a function of the flow parameters K and the thermal diffusivity and the relevant processing temperatures (melt temperature and mould temperature) but which was independent of the geometry of the cavity and the flow pattern within the cavity. 

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Some typical data for this mouldability index are given in Figure 8.8. One limitation of these data is that they do not explicitly show whether or not a mould will fill in an injection moulding operation. This will clearly depend on the thickness of the moulding, the flow distances required and operational parameters such as melt and mould temperatures. One very crude estimate that is widely used is the flow path ratio, the ratio of flow distance to section thickness. The assumption is that if this is greater than the ratio (distance from gate to furthest point from gate)/section thickness, then the mould will fill. Whilst... 

In extensional flows, the velocity increases (fibre melt spinning) or decreases (radial flow from the sprue in an injection mould) along the streamlines, but there is no velocity gradient in the perpendicular direction. Figure 5.5 shows fibre melt spinning where the velocity increases with distance x from the spinneret, as the result of a tensile stress along the fibre. The tensile strain rate 6x is defined by... 

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. 

Plastic closures can be made by injection moulding, where the mould has two parts - a core which has the inside features of the closure and a cavity which has the features of the outside of the closure. Molten plastic is injected into the mould, which is water-cooled. The plastic solidifies, and the mould opens and the completed closure is ejected. If the closure is a two-piece design (i.e. fitted with a loose or flowed in-liner), then this is added later. Another technology for moulding plastic closures for the beverage market is compression moulding. A hot pellet of PP is positioned in a mould, similar to the cavity of an injection mould. As the mould closes, the pellet is squashed and defoimed into the shape... 

In this method a mandrel is placed in the mould of an injection moulding machine, the mould heated, and thermoplastic forced in to flow round the mandrel and form a tube with a closed end mandrel and plastic then are removed and passed while still hot to the blowing mould, where air introduced through the mandrel blows the material to the shape required. 

The characteristics of organic based heat stabilisers used in PVC are outlined and a comparison is made of the performance of an organic based stabiliser one-pack with a traditional lead stabiliser one-pack in the injection moulding of an unplasticised PVC pipe compound. Spiral flow and colour development during injection moulding are discussed and the results of customer trials on a large... 

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. 

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. 

Moreover, there were signs of defects on the plastic inside the metal tube, at a point below the subcritical cracks. They comprised a series of flutes (Fig. 10.26) and a curious wavy line on the inner surface (Fig. 10.27). This combination of features observed on the plastic insert indicated material degradation, because this part should have had a smooth blemish-free surface. Being an injection moulding, the surface had been in contact with the steel core. If the polymer had flowed, then the viscosity must have dropped to allow the material to flow when the product was removed from the tool, showing that the molecular weight was too low. 

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