1、 The Injection Mold -The design of Runner 1.The 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 two-plate mod the runner is positioned on the surface while on the more complex designs the runner may be positioned
2、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, its de
3、sirable 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 t
4、he 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 shou
5、ld 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 standpoi
6、nt； 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 10is incorporated on the runn
7、er 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
8、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. Naturall
9、y 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 th
10、rough 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 solidifie
11、d 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 (Fig
12、ure 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 t
13、wo 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 accurat
14、ely 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
15、 of a two-plate 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 mo
16、ldings by the 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
17、 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 4.6). Summing up the points concerning cross-sectional shape, we can say that for simple two-plate mold
18、s 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 complex parting surface, where it would be difficult to match accurately the semicircular channels of the round runner or, for multi-p
19、late molds, the trapezoidal or modified trapezoidal section should be used. 2.Runner 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) run
20、ner cooling considerations, (iv) the range of mouldmakers cutters available and (v) the plastics material to be used. (i) The cross-sectional area of the runner must be sufficient to permit the freezes and for packing pressure to be applied for shrinkage compensation if required, Because of this, ru
21、nners 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 ha
22、s a direct bearing on the choice of cross-sectional size of the runner. For example, whereas a 5mm (3/16 in) (iii) The cross-sectional 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 cross-sec ti
23、on 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.
24、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 mouldmakerss not having to carry in stock a multitude of different! sizes of cutters. In pract
25、ice the following are the more common sizes: 2-13 mm in I mm steps in the metric range and -? in With in steps in the imperial unit range. The following empirical formula is suggested as a guide of the size of the runner or branch runner for moldings weighing up to 200g(I g (7 oz), and with wall sec
26、tions less than 3 mm (0.125 in). For the rigid PVCs and the acrylics, increase the calculated diameter by 25%. The formula is used in conjunction with the notes given previously. (i) The runner should not be below 2 mm (3/32 in) diameter, nor above 10 mm (3/8 in) diameter (or 13 mm (1/2 in) diameter
27、 where applicable). (ii) The calculated size should be increased to the next suitable cutter size Figure 4.7 shows a plot of diameter versus length of runner for various weights of molding, adopting the metric system of dimensioning. Figure 4.8 shows a corresponding plot using the Imperial dimensioning system. For example, a 120 g (4 oz) molding in polyethylene being fed by a 50 mm (2 in) long runner will require a diameter of 7 mm (5/16 in).
