Mould clamping

Note that if Pq is substituted back into the expression for P then the expression used earlier (Chapter 4) to calculate the mould clamping force is obtained. That is... 

Though its production, e.g. in an injection moulding machine, requires special tricks, the injection pressures and the mould clamping forces required are much lower than with a solid product. 

The two main parameters for machine size are, the maximum shot size that can be injected in a single forward movement of the screw, and the maximum mould clamping force / damp- The shot size is usually quoted in terms of the mass of polystyrene that can be injected. The clamping force, typically in the range from 100 kN to 100 MN, restricts the maximum projected area A of a moulding onto the mould parting plane (Fig. 5.24). A typical average melt pressure p in the mould is 20 MPa, and the condition... 

In a ram- or plunger-type machine, the extruder feeds the material to an accumulator/head device. The plunger pushes the material rapidly through the head-die assembly once the desired volume has accumulated. The transfer of material does not need to occur via a blowing station in the mould clamp mechanism. The part is blown, cooled and removed from the mould, and the next parison is only extruded after the part has been removed. 

Pneumatic and hydraulic drives provide independent control, but a greater amount of space is needed to install and cool the cylinder. In manifold nozzles the cylinder is usually set into the mould clamping plate, and in some designs it is made as part of the plate. The hydraulic cylinder temperature may not exceed 80-100 °C, so at high processing temperatures the clamping plate must be cooled. There are also systems with an extra plate accommodating a complete drive system, e.g., Mold-Masters systems. 

The telescopic connector is rendered leakproof by a fit with a very small clearance (0.005 mm, lapped aperture) and by differentiation of thermal expansion. The bushings are heated by band heaters, whereas the connectors are heated by cartridge heaters. The basic nozzle in this system is one with a heated torpedo. The nozzle cartridge heater may easily be replaced through a hole in the mould clamping plate. 

Through an insert which interacts with the nozzle, the pressure on the nozzle is transferred directly to the mould clamping plate. The heaters are set and embedded on the tube between the pivot pins and insulated by a jacket that reflects thermal radiation. Because of the low weight of the tube, there is a lowering of energy consumption to l/6th of the energy necessary for a plate manifold in its traditional version (according to the manufacturer). 

The contact area of the pressure discs with the mould clamping plate has been rednced (by recessing) to decrease heat losses, and very large snrface pressures are transferred through them (see Chapter 4.2.1.5). 

The mould clamping plate is subject to large surface pressures originating from the manifold pressure discs, and there is also a risk of seizure of the plate surface by the discs during thermal expansion of the manifold. It is therefore recommended that the clamping plate be made of toughened steel, or that hardened inserts be located in it (see Figure 4.61). 

Moving the central nozzle away from the mould axis causes an asymmetrical mould structure, asymmetrical loading of the ejection system and an asymmetrical stress distribution in the injection machine columns. The effective mould clamping force is diminished. 

The next section will consider issues relating to the mould clamping force requirements. 

It is found in most cases, that flash (see Chapter 9 for a description) is not caused by inadequate mould clamping force or locking pressme, but by an insufficient rigidity of the mould in the areas concerned. 

Flowability Very good, as LSR systems have low viscosity Therefore, the injection pressure and mould clamping forces required are also very low... 

In order to remove the air blister even at faster initial injection speeds, the mould clamping force, which had previously been set very high (too high) was reduced until satisfactory ruiming was achieved again. This example shows that a mould clamping force which is not too high, and which is satisfactorily set,... 

Moreover, the equipment is looked after better the lower the mould clamping force, the lower the wear. 

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