Mould operation

For many applications (e.g. body pressings), it is inconvenient to use continuous fibres. It is a remarkable feature of these materials that chopped fibre composites (convenient for moulding operations) are nearly as strong as those with continuous fibres, provided the fibre length exceeds a critical value. 

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... 

Post moulding operations Finishing - painting. Paint chips off easily Finishing - not required -painting not required. Compounded colour retention permanent. 

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... 

During a rotational moulding operation an aluminium mould with a uniform thickness of 3 mm is put into an oven at 300°C. If the initial temperature of the mould is 23°C, estimate the time taken for it to reach 250°C. The natural convection heat transfer coefficient is 28.4 J/m s. 

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... 

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). 

Moulding defects up to 5% are allowed in the moulding operation. The common moulding defects and their causes and remedies are given in the following table 14.2. 

Saeki and Kaneda (1988) examined the apparent mean viscosity, minimum viscosity and apparent gel time of an epoxy-resin moulding compound during the transfer-moulding operation by using an instrumented transfer mould. 

Lubricants may be added either as an additive or at the fabrication stage as a processing aid. In general lubricants prevent adhesion with metal parts. Lubricants may be solids, such as waxes, stearates, i.e. fatty acid esters, fatty acid amides, or liquids such as liquid paraffins. Lubricants may also improve the flow and thereby lower the temperature of the moulding operation. For example, a silicone additive polydimethyl siloxane may be used in LDPE and HOPE films to improve flow. Stearamides are also used. 

Mould layout becomes more critical in the case of multicavity mouldings where it is essential that each mould operates under similar conditions. Although it may occasionally be necessary to blank off a mould or moulds (e.g. damaged mould inserts), there is always a possibility that this may upset the delicate balance between mouldings. 

Although the above are the major moulding processes for thermosets, injection moulding can now be used for the selected thermoset moulding operations. The major use of thermosets still remains with screw closures. These are made on either a multicavity platen tool (up to 96 cavities) or a rotary type press which resembles a rotary tableting machine. 

As compounder above if compounding occurs, e.g. colourants added immediately prior to the moulding operation. 

The level of sinkage/shrinkage is also influenced by other factors such as fillers, pigments, melt flow index, density and cooling rate. It should be noted that materials which are oriented, either purposely or accidentally (e.g. in a moulding, operation), will shrink significantly if they are heated above the temperature used for the orientation process. Amorphous materials show less shrinkage than crystalline polymers. 

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