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Theoretically the cross-sectional area of the main runner should be equal to, or in excess of, the combined cross-sectional areas of the branch runners that it is feeding. This relationship is, however, ignored when the maximum suggested diameter is reached
The main objection to the fully round runner is that this runner is formed from two semicircular channels machined one in each of the mod plates. It is essential that these channels are accurately matched to prevent an undesirable and inefficient runner system being developed. A similar argument applies to the hexagonal runner system. The fact that these channels must be accurately matched means that the mod cost for a mod containing round or hexagonal runner will be greater than for one containing trapezoidal runners.
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 specified, however, the runner system could well adhere to its channel and make its removal difficult
3.Runner layout
The layout of the runner system will depend upon the following factors: (i) the number of impressions, (ii) the shape of the components, (iii) the type of mod (i.e., two-plate or multi-plate mold), (iv) the type of gate. There are two main considerations when designing a runner layout.
The runner length should always be kept to a minimum to reduce pressure losses, and the runner system should be balanced.
(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.
(Runner balancing means that the distance the plastic material travels from the sprue 1o the gate should be the same for each molding This system ensures that all the impressions will fill uniformly and without interruption providing the gate lands and the gate areas are identical, Figure 4.9 shows example~ of molds all based on the balanced runner principle.
It is not always practicable, however, to have a balanced runner system and this particularly applies to molds which incorporate a large number of differently shaped impressions (Figure 4.10). In these cases balanced filling of the impression can be achieved ~y varying the gate dimensions. That is by balanced gating (Section 4.3.2).
Single-Impression MOLDS
Single-impression molds are usually fed by a direct sprue feed into the impression (Figure 4.14) and hence no runner system is required. However, it may be desirable ;o edge gate (for example when sprue marks must not appear on the main surface) in which case a short runner as shown in Figure 4.9a may be used. But note that by gating a single impression in this way the impression itself must be offset, This is undesirable, particularly with a large impression, as the injection pressure will exert an unbalanced force which will tend to open the mod one side and may result in flashed moldings
Two-Impression MOLDS