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【正文】 at most thermoplastics posses, the solidified material acts as an insulation and maintains the temperature of the central melt flow region. Ideally, the gate should therefore be positioned in line with the center of the runner to receive the material from the central flow stream. This condition may be achieved with the fully round runner (Figure ), and also with the hexagonal runner The basic trapezoidal designs (Figure and c) are not as satisfactory in this respect since the gate cannot normally be positioned in line with the central flow stream. 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 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 pecified, however, the runner system could well adhere to its channel and make its removal difficult (Figure ). Summing up the points concerning crosssectional shape, we can say that for simple twoplate 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 plex parting surface, where it would be difficult to match accurately the semicircular channels of the round runner or, for multiplate molds, the trapezoidal or modified trapezoidal section should be used. 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 mouldmaker39。 the runner section are giver in Figure . As can be seen, the various types of standpoint。in line of draw39。 The Injection Mold The design of Runner basic The runner is a channel machined into the mod plate to connect the sprue with the entrance(gate) to the impression. In the basic twoplate mod the runner is positioned on the surface while on the more plex designs the runner may be positioned below the parting surface The wall of the runner channel must be smooth to prevent any restriction to flow. Also, as the runner has to be removed with the molding, there must be no machine marks left, which would tend to retain the runner in the mod plate. To ensure that these points are met, it39。s desirable for the mod designer to specify that the runner(channel) is polished 39。. There are some other considerations for the designer to bear in mind: (i) the shape of the cross section of the runner, (ii) the size of the runner. Runner crosssection shape The crosssectional shape of the runner used in a mod is usually one of four forms (Figure ): fully round (a), trapezoidal (b), modified trapezoidal (c) and hexagonal (d). The reason why these particular forms are used in preference to others are outlined below. The criterion of efficient runner design is that the runner should pr
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