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The choice of runner section is also influenced by the question whether positive ejection of the runner system is possible. Consider, for instance, the case of a two-plate mod in which a circular runner has been machined from both parting surface. In this case, as the mod opens, the runner is pulled from its channel in one mod half and it is then ejected from the other mod half either directly, by ejector pins, or by relying on its attachment to the moldings by the gates (Figure 4.5).
For multi-plate molds, however, positive ejection of the runner system is not practicable. Here the basic trapezoidal-type runner is always specified, the runner channel being machined into the injection half from which it is pulled as the mod opens. In this way the runner is free to fall under gravity between mod plates. If a circular runner had been pecified, however, the runner system could well adhere to its channel and make its removal difficult (Figure 4.6).
Summing up the points concerning cross-sectional shape, we can say that for simple two-plate molds which have a flat parting surface the fully round runner or hexagonal runner is to be prefaced, the increased mod cost being relatively small. For molds which have complex parting surface, where it would be difficult to match accurately the semicircular channels of the round runner or, for multi-plate molds, the trapezoidal or modified trapezoidal section should be used.
2.Runner size
When deciding the size of the runner the designer must consider the following factors: (i) the wall section and volume of the molding (ii) the distance of the impression from the main runner or sprue, (iii) runner cooling considerations, (iv) the range of mouldmaker's cutters available and (v) the plastics material to be used.
(i) The cross-sectional area of the runner must be sufficient to permit the freezes and for packing pressure to be applied for shrinkage compensation if required, Because of this, runners below 2 mm (3/32 in) diameter are seldom used and even this diameter is normally limited to branch runners under 25mm (1 in) in length.
(ii) The further the plastic melt has to travel alone the runner the greater is the resistance to flow. Hence the distance the impression is from the sprue has a direct bearing on the choice of cross-sectional size of the runner. For example, whereas a 5mm (3/16 in)
(iii) The cross-sectional area of the runner should not be such that it controls the injection cycle, although this is sometimes unavoidable for very light moldings The larger he cross-sec ~tion area of the runner the greater is the bulk of material it contains and the longer the period it takes to cool sufficiently to enable the mod to be opened and the moldings and runner ejected. For this reason it is undesirable to make the runner larger than 10 mm (] in) diameter for most materials. However, the rigid PVCs and the acrylics are exceptions due to their high viscosity and diameters up to 13 mm (1/2 in) are used.
(iv) The size chosen for the runner should be in a range consistent with the mouldmakers's not having to carry in stock a multitude of different! sizes of cutters. In practice the following are the more common sizes: 2-13 mm in I mm steps in the metric range and ~-? in With ~ in steps in the imperial unit range. The following empirical formula is suggested as a guide of the size of the runner or branch runner for moldings weighing up to 200g(I g (7 oz), and with wall sections less than 3 mm (0.125 in). For the rigid PVCs and the acrylics, increase the calculated diameter by 25%.
The formula is used in conjunction with the notes given previously.
(i) The runner should not be below 2 mm (3/32 in) diameter, nor above 10 mm (3/8 in) diameter (or 13 mm (1/2 in) diameter where applicable).
(ii) The calculated size should be increased to the next suitable cutter size
Figure 4.7 shows a plot of diameter versus length of runner for various weights of molding, adopting the metric system of dimensioning. Figure 4.8 shows a corresponding plot using the Imperial dimensioning system. For example, a 120 g (4 oz) molding in polyethylene being fed by a 50 mm (2 in) long runner will require a diameter of 7 mm (5/16 in).