Injection moulding clamping force

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... 

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. 

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,... 

Wall sections in foam moulding are thicker than in solid material. Longer cycle times can therefore be expected due to both the wall thickness and the low thermal conductivity of the cellular material. In contrast, however, the injection pressures in foam moulding are low when compared with conventional injection moulding. This means that less clamping force is needed per unit area of moulding and mould costs are less because lower strength mould materials may be used. 

Since the reactants have a low viscosity, the injection pressures are relatively low in the RIM process. Thus, comparing a conventional injection moulding machine with a RIM machine having the same clamp force, the RIM machine could produce a moulding with a much greater projected area (typically about 10 times greater). Therefore the RIM process is particularly suitable for large... 

The container shown at the top of p. 341 is injection moulded using a gate at point A. If the injection pressure at the nozzle is 140 MN/m and the pressure loss coefficient, m, is 0.5, estimate (i) the flow ratio and (ii) the clamping force needed. 

It may be seen from the above analysis and that in Chapter 4 for the calculation of clamping force on an injection moulding machine the Mean Effective pressure (MEP) across the cavity may be obtained from... 

Fig. 5.27 shows the variation of MEP with flow ratio (R/H) for spreading flow in discs of different depths. The material is polypropylene and the constant injection rate is 3.4 x 10 m /s. This is a high injection rate but has been chosen because the clamp forces predicted by this diagram are representative of those occurring in real moulding situations (even though it is based... 

After the mould has closed, but before final clamping, a measured quantity of rubber is injected into the transfer port. The rubber forces the transfer port open until it contains the required volume. With a top injection system the mould is forced downwards and the transfer piston remains stationary. With a bottom injection unit the mould is stationary and the transfer piston is raised. In both cases the effective volume of the port is increased by the filling action of the rubber. 

This process—injection compression moulding—offers other advantages besides lower clamping forces compression of the hot melt gives excellent surface finish and lower pressure at injection reduces orientation in the melt, so improving dimensional stability. The method is used currently to make compact discs of high quality from polycarbonate. 

Ah electrical fitting, injection-moulded in PES, will operate at 150 C. It will be bolted to a steel support, and the total clamping force must not fall below 10 kN after 10 seconds. Washers with inside and outside diameters of 10 mm and 20 mm, respectivety, will be used. The maximum allowable strain for PES at 15(PC is 0.7%. Calculate the number of bolts needed, using data given in Hgures 8.13 and 8.14. 

The extruder feeds one of two presses from an overhead gantry. Presses are of the hydraulic upstroking type with a clamping capacity of 150 tonnes - a compact injection-moulding press producing the same-sized mouldings would need a locking force of approximately 1500 tonnes. Press platen area is 2.15 x 1.5 m (7 x 5 ft). 

Elco Plastics Ltd of High Wycombe, Bucks, mould the TV screen surround on a Battenfeld BM 2 x 3200/500 S-C co-injection machine. That is, a 500 ton (5 MN) clamp force and two plasticizing cylinders each with 3200 cm swept volume capacity. The reciprocating screws in the plasticizing cylinders were 110 mm diameter with a L D ratio (length to diameter) of 15.5 1. 

In conventional injection moulding, a polymer melt is prepared in a heated barrel, using a compression-type extruder screw the melt is injected under high rates of shear into a cooled mould, which is clamped closed by a (high) force, sufficient to resist the hydraulic pressure of the injecting melt. 

Energy savings resulting from the lower clamping force required from the injection moulding machine... 

The projected area of a mould includes cavities and feed systems. The injection pressure is a function of part thickness, melt temperature, and mould temperature. It can vary within the mould. The three-zone reciprocating screw should achieve good plasticisation of the material. The estimation of the clamping force needed to keep the mould shut is a complex function of the projected area and the injection pressure. A rule of thumb for the clamping force required for PE is 0.2-0.5 tons/cm of projected area. A large safety factor of 25-50% is advisable. Computer simulation of flow will provide a more accurate estimation of the injection pressure and mould opening force. 

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