注塑模具设计-毕业论文外文翻译.doc

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 below 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, 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.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-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 moldings 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 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 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 complex parting surface, where it would be difficult to match accurately the semicircular channels of the round runner or, for multi-plate molds, the trapezoidal or modified trapezoidal section should be used.2.Runner sizeWhen 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 mouldmaker's 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, 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.(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 has 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 tion 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. 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 mouldmakers's not having to carry in stock a multitude of different! sizes of cutters. In practice 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 sections 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 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). Theoretically the cross-sectional area of the main runner should be equal to, or in excess of, the combined cross-sectional areas of the branch runners that it is feeding. This relationship is, however, ignored when the maximum suggested diameter is reached 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 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 moldings 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 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 3.Runner layoutThe layout of the runner system will depend upon the following factors: (i) the number of impressions, (ii) the shape of the components, (iii) the type of mod (i.e., two-plate or multi-plate 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 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, 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 4.9 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 4.10). In these cases balanced filling of the impression can be achieved y varying the gate dimensions. That is by balanced gating (Section 4.3.2). Single-Impression MOLDS Single-impression molds are usually fed by a direct sprue feed into the impression (Figure 4.14) and hence no runner system is required. However, it may be desirable ;o edge gate (for example when sprue marks must not appear on the main surface) in which case a short runner as shown in Figure 4.9a may be used. But note that by gating a single impression in this way the impression itself must be offset, This is undesirable, particularly with a large impression, as the injection pressure will exert an unbalanced force which will tend to open the mod one side and may result in flashed moldingsTwo-Impression MOLDS The various alternatives for feeding two impressions are shown in Figure 4,9b. c and d. The simplest case (b) is where the runner takes the shortest path between the two impressions. Unfortunately, it is not always possible to adopt this short runner. This is because, as shown in the following discussion, the most desirable position for the gate may not be on the centerline of the mold If we consider Figure 4.9b, which schematically shows the plan of a mod for two rectangular blocks, it is seen that solely from the viewpoint of mod layout it is desirable to have tine impressions positioned as shown with short runners to the sides of the impressions, thus enabling the size of the mod to be kept to a minimum. However, there are other constructions, such as that of correct gating, and it may be desirable to gate at one end of the impression To achieve these end gates it is necessary to alter the design of the runner layout, so that either a T-shaped runner extends beyond the impressions and is then connected to the gates by short branch runners (Figure 4.9c), or the runner, in the form of an S, sweeps round 1o the gales (d), without the necessity for branch runners. In general, providing that the impressions are approximately the same size and shape, no difficulty should be experienced in designing balanced runner systems for two-impression molds THREE-IMPRESSION MOLDS Figure 4.9e illustrate a balanced runner system for three similar impressions. In this case the impressions are placed on a pitch circle diameter 120° apart; this design allows the runners to be kept to a minimum length. When, however, the impressions are of different shapes and sizes the layout may be as shown in Figure 4.10a. The large impression is shown being fed directly from the sprue via a short runner, while the smaller components are fed via a branch and main runner system. Balance in the feed system is ultimately attained by adjustment of gate size OTHER MULTI-IMPRESSION MOLDS For four or more impression molds the design of the runner layout is simply an extension of the previous discussion. For example, balanced runner systems for molds containing four, five, six and eight impressions are shown in Figure 4.9. From the runner balancing standpoint it is simpler by far to situate the impressions on a pitch circle diameter and feed each impression directly from the sprue via a runner rather than to incorporate a main and branch runner system. As the number of impressions increase, however,the pitch circle diameter design becomes progressively more impracticable as the runner length, which is a function of the pitch circle diameter, is also increased. This results in large-diameter runners being required which progressively length the moulding cycle and more scrap (although re