In-mould operation

Polymers have some obvious advantages for wheels. The wheel can be moulded in one operation, replacing a metal wheel which must be assembled from parts. It requires no further finishing, plating or painting. And its naturally low coefficient of friction means that, when loads are low, the axle may run on the polymer itself. 

Through the development of such techniques as injection moulding it is possible to make highly complex parts in one operation without the need for assembly work or the generation of more than a notional amount of scrap material. 

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... 

When rods are required they are placed in wooden trays in a formolising bath. If the requirement is for a disc or blank such as used by the button trade the extrudate is cut up by an automatic guillotine and the blanks are immersed in the formalin solution. For manufacture of sheets the rods are placed in moulds and pressed into sheets before formolising. Many attractive patterns may be made by pressing different coloured rods into grooves set on the bias to the rods, thus forming new multi-coloured rods. This operation may be repeated a number of times in order to produce complex patterns. 

Compression moulding is one of the most common methods used to produce articles from thermosetting plastics. The process can also be used for thermoplastics but this is less conunon - the most familiar example is the production of LP records. The moulding operation as used for thermosets is illustrated in Fig. 4.62. A pre-weighed charge of partially polymerised thermoset is placed in the lower half of a heated mould and the upper half is then forced down. This causes the material to be squeezed out to take the shape of the mould. The application of the heat and pressure accelerates the polymerisation of the... 

To facilitate and accelerate folding and crystallization of polymer chains, internal plasticizers are often added to PET to serve as crystallation promoters. Such additives are usually based on poly(ether ester)s. These plasticizers are liquids that are typically added at levels of 2-4 wt%. They reduce cycle time in injection moulding operations by increasing the rate of crystalline formation. They also plasticize the resin and act as processing aids by virtue of their lubricating action in the melt. On a molecular level, these plasticizers reduce the intermolecular... 

The possibility of bulk colouring and in-mould decoration, which can simplify or avoid the finishing operations. 

In-mould decoration (IMD) with films reduces finishing operations. If the process and its operating conditions are suitable, the demoulded parts are finished. 

Personal 8-h TWA measurements taken in 1978 and 1979 in companies where acrylonitrile-butadiene-styrene moulding operations were conducted showed levels of < 0.05-1.9 mg/nr (Burroughs, 1979 Belanger Elesh, 1980 Ruhe Jannerfeldt, 1980). In a polybutadiene rubber warehouse, levels of 0.003 ppm [0.007 mg/m ] were found in area samples area and personal samples taken in tyre plants found 0.007-0.05 ppm [0.016-0.11 mg/m ] (Rubber Manufacturers Association, 1984). In a tyre and tube manufacturing plant in the United States in 1975, a cutter man/Banbmy operator was reported to have been exposed to butadiene at 2.1 ppm [4.6 mg/m ] (personal 6-h sample) (Ropert, 1976). 

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