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The 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 below the parting surface7510
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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, it's desirable 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 runner 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 provide 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; 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 10°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 4.2). To reduce this difference and still maintain corresponding dimensions, a modified trapezoidal form has 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 similar 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 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 that 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 4.4a), 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 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.