注塑模具设计-毕业论文外文翻译.doc

《注塑模具设计-毕业论文外文翻译.doc》由会员分享，可在线阅读，更多相关《注塑模具设计-毕业论文外文翻译.doc（9页珍藏版）》请在淘文阁 - 分享文档赚钱的网站上搜索。

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 f

17、ree 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

18、 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,

19、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 co

20、nsiderations, (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, runners below 2

21、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 bear

22、ing 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 r

23、unner 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 rig

24、id 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 practice the following

25、 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

26、 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)

27、. (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 ex

28、ample, 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.

29、 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 preve

30、nt 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 trapezoid

31、al 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 runn

32、er 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 th

33、e 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

34、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

35、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 appli

36、ed 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 gat

37、e 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, howev

38、er, 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). S

39、ingle-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

40、 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

41、 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

42、 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 desirabl

43、e 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 achiev

44、e 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 necessi

45、ty 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 impr

46、essions. 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 d

47、irectly 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 a

48、n 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 di

49、rectly 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