【正文】 tched 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 twoplate 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 ). For multiplate molds, however, positive ejection of the runner system is not practicable. Here the basic trapezoidaltype 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 layout The layout of the runner system will depend upon the following factors: (i) the number of impressions, (ii) the shape of the ponents, (iii) the type of mod (., twoplate or multiplate 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 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. (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 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 ). In these cases balanced filling of the impression can be achieved ~y varying the gate dimensions. That is by balanced gating (Section ). SingleImpression MOLDS Singleimpression molds are usually fed by a direct sprue feed into the impression (Figure ) and hence no runner system is required. However, it may be desirable 。is incorporated on the runner well, thus modifying the square to the trapezoidal section. The volume of the trapezoidal runner is approximately 25% greater than that of a round runner with corresponding dimensions (W=D, Figure ). To reduce this difference and still maintain corresponding dimensions, a modified trapezoidal form has been developed (Figure ) in which the volume is only 14% greater (approximately) than its round counterpart. The hexagonal runner is basically a double trapezoidal runner, where the crosssectional area of this runner type is about 82% of that of the corresponding round runner. Naturally if similar crosssectional areas are required, then the value for D(Figure ) must be increased accord the hexagonal runner pared with matching the two halves of a round runner. This point applies particularly to runners which are less 3mm (1/8 in) in width. As the plastic melt progresses through the runner and mod system the melt adjacent to the cold mod surface will rapidly decrease in temperature and solidify. The material which follows will pass through the center of this solidified material and, because of the low thermal conductivity th