【正文】 ovide a maximum crosssectional area from the standpoint of pressure transfer and minimum crosssectional area to periphery will, therefore, give a direct indication of the efficiency of the runner design。 whereas the ratios exhibited by the semicircular and rectangular types make their use generally undesirable. Unfortunately, the square runner is not very satisfactory either, but for another reason: it is difficult to eject. In practice, because of this, an angle of 10176。s cutters available and (v) the plastics material to be used. (i) The crosssectional area of the runner must be sufficient to permit the freezes and for packing pressure to be applied for shrinkage pensation 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 crosssectional size of the runner. For example, whereas a 5mm (3/16 in) (iii) The crosssectional 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 crosssec ~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 mouldmakers39。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 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 TwoImpression MOLDS The various alternatives for feeding two impressions are shown in Figure 4,9b. c and d. The simplest case (b) is where the runner takes the shortest path between the two impressions. Unfortunately, it is not always possible to adopt this short runner. This is because, as shown in the following discussion, the most desirable position for the gate may not be on the centerline of the mold If we consider Figure , which schematically shows the plan of a mod for two rectangular blocks, it is seen that solely from the viewpoint of mod layout it is desirable to have tine impressions positioned as shown with short runners to the sides of the impressions, thus enabling the size of the mod to be kept to a minimum. However, there are other constructions, such as that of correct gating, and it may be desirable to gate at one end of the impression To achieve these end gates it is necessary to alter the design of the runner layout, so that either a Tshaped runner extends beyond the impressions and is then connected to the gates by short branch runners (Figure ), or the runner, in the form of an S, sweeps round 1o the gales (d), without the necessity for branch runners. In general, providing that the impressions are approximately the same size and shape, no difficulty should be experienced in designing balanced runner systems for twoimpression molds THREEIMPRESSION MOLDS Figure illustrate a balanced runner system for three similar impressions. In this case the impressions are placed on a pitch circle diameter 120176。 this design allows the runners to be kept to a minimum length. When, however, the impressions are of different shapes and sizes the layout may be as shown in Figure . The large impression is shown being fed directly from the sprue via a short runner, w