Mould 32-cavity

During firing the wax burns out of the ceramic mould to leave a perfectly shaped mould cavity. 

A silver replica of a holly leaf is to be made by investment casting. (A natural leaf is coated with ceramic slurry which is then dried and fired. During firing the leaf burns away, leaving a mould cavity.) The thickness of the leaf is 0.4 mm. Calculate the liquid head needed to force the molten silver into the mould cavity. It can be assumed that molten silver does not wet the mould walls. 

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. 

Internal stresses occur because when the melt is sheared as it enters the mould cavity the molecules tend to be distorted from the favoured coiled state. If such molecules are allowed to freeze before they can re-coil ( relax ) then they will set up a stress in the mass of the polymer as they attempt to regain the coiled form. Stressed mouldings will be more brittle than unstressed mouldings and are liable to crack and craze, particularly in media such as white spirit. They also show a characteristic pattern when viewed through crossed Polaroids. It is because compression mouldings exhibit less frozen-in stresses that they are preferred for comparative testing. 

In the low-pressure systems a shot of material is injected into the mould which, if it did not expand, would give a short shot. However, the expanding gas causes the polymer to fill the mould cavity. One important form of the low-pressure process is the Union Carbide process in which the polymer is fed to and melted in an extruder. It is blended with nitrogen which is fed directly into the extruder. The extruder then feeds the polymer melt into an accumulator which holds it under pressure (14-35 MPa) to prevent premature expansion until a predetermined shot builds up. When this has been obtained a valve opens and the accumulator plunger rams the melt into the mould. At this point the mould is only partially filled but the pressurised gas within the melt allows it to expand. 

In order to overcome such disadvantages the injection-compression process has been developed. A conventional compression press is coupled to a screw preplasticising unit which can deliver preheated and softened material direct to a compression mould cavity. 

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 earliest injection moulding machines were of the plunger type as illustrated in Fig. 4.30 and there are still many of these machines in use today. A predetermined quantity of moulding material drops from the feed hopper into the barrel. The plunger then conveys the material along the barrel where it is heated by conduction from the external heaters. The material is thus plasticised under pressure so that it may be forced through the nozzle into the mould cavity. In order to split up the mass of material in the barrel and improve the heat transfer, a torpedo is fitted in the barrel as shown. 

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. 

The first stage of the cycle is the flow of molten polymer into the mould cavity through a standard feed system. Before this flow of polymer is complete, the injection of a predetermined quantity of gas into the melt begins through a special nozzle located within the cavity or feed system as shown in Fig. 4.45. The timing, pressure and speed of the gas injection is critical. 

A practical difficulty which arises during injection moulding of reinforced plastics is the increased wear of the moulding machine and mould due to the abrasive nature of the fibres. However, if hardened tool steels are used in the manufacture of screws, barrels and mould cavities then the problem may be negligible. 

Polyethylene is injected into a mould at a temperature of 170°C and a pressure of 100 MN/m. If the mould cavity has the form of a long channel with a rectangular cross-section 6 mm X 1 mm deep, estimate the length of the flow path after 1 second. The flow may be assumed to be isothermal and over the range of shear rates experienced (10 -10 s ) the material may be considered to be a power law fluid. 

In transfer moulding, the channels in the mould which transfer mbber compound to the actual moulding cavities. 

Many rubber compounds have a tendency to stick in the mould cavity after vulcanisation and require some type of mould release agent. The substances used are surface-active materials such as detergents, soaps, wetting agents, silicone emulsions, aqueous dispersions of talc, mica and fatty acids, applied by spray or brush. Alternatively, dry types based on polytetrafluoroethylene or polyethylene, usually carried in a solvent, can be aerosol applied. An alternative is the addition of an incompatible material to the rubber compound which will bleed to the rubber surface during vulcanisation. 

These products are applied to the mould cavities and do not cause product faults and generally do not cause bond problems. 

Injection moulding machines can be in a horizontal or vertical mode, with or without tiebars, or with a C frame structure for easy mould fitting and access. Machines are also available which can deal with two colour/compound injection into the same mould cavity. 

Machines differ in their mode of heating, plasticisation of the mix and delivery of the mix to the mould cavity. This may be by simple ram (plunger or piston) or by screw. 

This process uses the plasticising and heat advantages of the injection unit to impart good flow properties to the rubber mix. It also offers the advantages of the flexibility of the transfer layout without the sprue and runners of the balanced runner system required by injection moulding. The space used by runners in other systems can be profitably used by more mould cavities. 

With compression moulding, to ensure dimensional consistency, it is necessary to allow the excess material to move away from the edge of the cavity so that the lands between the cavities can contact with minimum thickness of rubber (flash) between them. Spew grooves and channels are provided of sufficient dimensions to accommodate this excess, and also to allow the escape of air from the mould cavity. In some cases, where the shape is complex, it may be necessary to provide extra venting to allow air to escape from a blind area, where it is likely to be trapped. 

In moulds for complex product shapes it can be advantageous to provide vacuum extraction to the cavities so that air and evolved gases can be removed from the mould cavity, before and during vulcanisation to ensure complete cavity fdling, and a product without air (gas) blemishes. 

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