Runner diameter

Example 4.6 The mould shown in Fig. 4.35 produces four cup shaped ABS mouldings. The depth of the cups is 60 mm, the diameter at the is 90 mm and the wall thickness is 1.0 mm. The distance from the sprue to the cavity is 40 mm and the runner diameter is 6 mm. Calculate the clamp force necessary on the moulding machine and estimate how the clamp force would change if the mould was designed so as to feed the cups through a pin gate in the centre of the base (as illustrated in Fig. 4.38). The clamp pressure data in Fig. 4.42 should be used and the taper on the side of the cups may be ignored. 

An injection moulding is in the form of a flat sheet 100 mm square and 4 nun thick. The melt temperature is 230°C, the mould temperature is 30°C and the plastic may be ejected from the mould at a centre-line temperature of %°C. If the runner design criterion is that it should be ejectable at the same instant as the moulding, eshmate the required runner diameter. The thermal diffusivity of the melt is 1 x 10 m /s. 

To minimize pressure and heat losses, the runners should be round and as short as possible. Another preferred choice is trapezoid runners that are easy to machine and provide the same amount of resistance to flow as round runners. They should blend smoothly into the gates, without abrupt cross-sectional changes. The runner s length should be reduced and its diameter increased when the thickness of the molded part increases. A runner diameter of at least 6 mm is required for a 10 mm thick part. Thicker parts require runners that are 50%-100% larger in diameter than the thickness of the part. As in injection molding, pressure drop and material loss are de-... 

A molten polymer at 295 C is injection-molded into a circular runner (diameter of 0.05 m). The runner wall temperature is kept constant at 90°C. Estimate the solidification profile for the flowing polymer and a temperature distribution across the runner diameter. Polymer properties are ... 

Reposition or reposition to balance the flow to a thick section in the gate or reducing the runner diameter may resolve the problem. Surface texture can be added to part design and steel wall cavities also solve the problem by masking the flow marks. Flow chart 5.2.16 shows the schematic ways to solve the problem of flow marks, folds, and back fills. High injection speed will improve the flow length and will also reduce the visibility of flow marks. 

Runner layouts should be designed to deliver the plastic melt at the same time and at the same temperature, pressure, and velocity to each cavity of a multicavity mold. Such a layout is known as a balanced runner (Fig. 7.15). A balanced runner will usually consume more material than an unbalanced type, but this disadvantage is outweighed by the improvement in the uniformity and quality of the moldings. Balance in a multi-cavity mold with dissimilar cavities (known as a family mold) can be achieved by careful variation of runner diameter in order to produce equal pressure drops in each flow path. Such balancing can only be achieved efficiently by the use of computer flow simu-... 

Runner diameters range fi om 4mm to 7.5mm (0.157 to 0.295in) a typical diameter is 6mm (0.236in). Diameters above 7.5mm (0.295in) tend only to increase the cycle time and do not significantly assist in the filling of the mold. Runner lengths should be kept as short as possible and suit-ably-sized, ejectors positioned at runner junctions to ensure that the feed system is removed from the mold without difficulty. 

S.R.Cleveland, J.P. Latchaw. Runner Diameter and Length Effects on Molded-in Stresses of Injection Molded Parts. SPE Technical Papers ANTEC 2004, P834... 

Table 5 Runner diameters for short primary runner...

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