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1、The Injection Mold-The design of Runner1.The basicThe runner is a channel machined into the mod plate to connect the sprue with the entrance(gate) to the impression. In the basic two-plate mod the runner is positioned on the surface while on the more complex designs the runner may be positioned belo
2、w the parting surfaceThe 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, its desirab
3、le for the mod designer to specify that the runner(channel) is polished in line of draw.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 cross-section shape The cross-sectional shape of the runn
4、er used in a mod is usually one of four forms (Figure 4.2): 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 provi
5、de a maximum cross-sectional area from the standpoint of pressure transfer and minimum cross-sectional area to periphery will, therefore, give a direct indication of the efficiency of the runner design; the runner section are giver in Figure 4.3. As can be seen, the various types of standpoint; wher
6、eas 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 10is incorporated on the runner well,
7、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 4.2). To reduce this difference and still maintain corresponding dimensions, a modified trapezoidal form has
8、been developed (Figure 4.2c) in which the volume is only 14% greater (approximately) than its round counterpart.The hexagonal runner is basically a double trapezoidal runner, where the cross-sectional area of this runner type is about 82% of that of the corresponding round runner. Naturally if simil
9、ar cross-sectional areas are required, then the value for D(Figure 4.2c) must be increased accord the hexagonal runner compared 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 r
10、unner 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 that most thermoplastics posses, the solidified material
11、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 4.4a),
12、and also with the hexagonal runner The basic trapezoidal designs (Figure 4.2b 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 semicirc
13、ular 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
14、 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-pl
15、ate 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 th
16、e 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
