Injection moulding gating

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... 

Injection moulding and extrusion may be carried out at temperatures in the range of 300-380°C. The polymer has a high melt viscosity and melt fracture occurs at a lower shear rate (about 10 s ) than with low-density polyethylene (about 10 s ) or nylon 66 (about 10 s ). Extruders should thus be designed to operate at low shear rates whilst large runners and gates are employed in injection moulds. 

One of the major innovations in recent years is the use of pulsed pressure through the gates to introduce and control the orientation of the structure (or fillers) in injection moulded products. A special manifold is attached to the machine nozzle as illustrated in Fig. 4.46. This diagram relates to the double live feed of melt although up to four pistons, capable of applying oscillating pressure may be used. 

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. 

Polythene at 170°C is used to injection mould a disc with a diameter of 120 mm and thickness 3 mm. A sprue gate is used to feed the material into the centre of the disc. If the... 

Polymer melts cool slowly in contact with air, so it is possible for a weld to form when two sections of melt are pressed together, as in the base of a blow-moulded container. Welds occur in injection mouldings when the melt from neighbouring gates meet (Fig. 5.27), or when the melt stream parts to flow past a hole in the product, and then recombines. Extruded products... 

Figure 6.8 An edge gated polyethylene injection moulding, before and after melting, in a bath of silicone oil.

A 508 mm diameter cycle wheel, having five spokes, is injection moulded using a toughened nylon. It is gated at the hub and is 4 mm thick. Comment on the maximum flow lengths and whether the positions of the weld lines may cause problems. 

The Lost Foam process (Figure 2.36) starts with the 3D-CAD design and production of a precision moulded foam pattern, made from expanded polystyrene (EPS) or PMMA, produced by automated injection moulding machines. These patterns can be made from one piece or by assembling a number of parts by adhesives. Here usually, a number of patterns, dependent on size, are fixed to a runner-down gating system, made from the same material as the pattern, thereby generating a cluster. 

The investigations were done on isotactic polypropylene (iPP), grade HD 120 MO from Borealis. Both injection and compression moulded plates were used. The melt tempjerature in injection moulding was 245 °C, the mould temperature was 40 °C. The mould has a film gate with a width of the half of plates thickness. 

O90ni Onishi, P., Hashemi, S. Effect of fibre concentration and strain rate on mechanical properties of single-gated and double-gated injection-moulded short glass fibre-reinforced polypropylene copolymer composites. J. Mater. Sci. 44 (2009) 3445-3456. 

Eght types of gate used in injection moulding (after ICI pic). 

A plate of polystyrene is to be injection-moulded. When the cavity is sealed freezing of the gate, the melt is at a temperature and pressure of 120 C and 20 MPa, respective. Assuming uniform cooling, predict the linear mould shrinkage of the plate when it is removed from the mould at 20°C Use the following data for polystyrene. 

Any thick blodc of pofymer, quench-cooled from the melt, usually contains residual thermal stress which is tensile in the interior of the block and compressive at the surface (see Figure 7.18). Injection moulded components, however, especially near the gate, are sometimes found to contain residual stress which is compressive in the interior and tensile at the surface. 

A rectangular box measuring 20 X12 cm and 10 cm deep is injection-moulded from a gate at the centre of the base. Assuming that at any given time the speed of the flow front is the same everywhere in the mould, determine the positions of the weld lines. 

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