全英文注塑资料注塑机结构-动作-成型工艺..docx
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1、The question of testing mould1. Introduction Injection moulding without rejects is the ideal moulders try to attain. This article describes two auxiliary devices that could increase the repeatability of an injection moulding machine. Once the optimum parameters are set, the physical dimensions, weig
2、ht and other physical properties of the part will stay almost constant. The two devices do clamping force measurement/control and cavity pressure switchover to holding pressure. For injection moulding of high precision optical parts, or parts with a high added value like appliance cases, the payoff
3、of reduced rejects is high. Figure 1 shows the part weight distribution of quality moulding and suboptimal moulding. The nominal weight is 60 g, allowable deviation is +0.1 g and -0.05 g. The white cubes denote parts within the tolerance band. Figure 1. Quality injection moulding and otherwise 2. Cl
4、amping force measurement/control The clamping mechanism of injection moulding machines falls mainly into two categories: toggle and direct hydraulic. The former is more widely used; the latter has the property of automatic clamping force regulation. As a result, direct hydraulic clamp machine do not
5、 need clamping force measurement/control. 2.1 Toggle clamp A 5-point double toggle clamp is shown in Figure 2. After amplification by the toggle mechanism, the clamping cylinder, attached to the tail platen, extends, pushing the moving platen to lock the mould halves together. We will investigate in
6、 various ways how the clamping force is generated. Figure 2. The 5-point double toggles 2.2 Clamping force At its simplest, the rated clamping force Fo is calculated according to the following formula.Fo = Ps * A * M - (1)wherePs = system pressure,A = clamping cylinder cross sectional area,M = mecha
7、nical advantage of the toggles. In most machines, M has a value of between 22 and 30. It is a function of the toggle dimensions and the stiffness of the toggles and tiebars.2.3 Rated clamping force and adequate clamping force The clamping force found in the specification table of an injection mouldi
8、ng machine is the rated clamping force Fo. By considering the various design parameters, the machine designer calculates it using formula (1).In using an injection moulding machine, it is best to use the minimal but adequate clamping force F. An adequate clamping force holds the mould halves togethe
9、r against the cavity pressure during the injection phase.An excessive clamping force distorts the mould and the mould cavity unnecessarily, affecting the precision of the moulded part. Furthermore, a high clamping force compresses the toggles and the mould, and stretches the tiebars, reducing the fa
10、tigue lives of the toggle pins, the mould and the tiebars.2.4 Clamping force problem and solution The problem with toggle clamped injection moulding machines is with only the hydraulic pressure meter available there is no way to set an accurate clamping force when the mould is installed and to maint
11、ain it constant during injection. As the mould heats up, it expands, increasing the clamping force. The solution is to attach a device to measure the clamping force and to control the clamping force to within a tolerance as mould temperature changes.For the engineers, the following sections details
12、how clamping force is generated. They also relate to the second device: switchover to holding pressure by cavity pressure measurement.2.5 Mould height adjustment mechanism Since not all moulds have the same mould height, a toggle clamped injection moulding machine has a mould height adjustment mecha
13、nism for that purpose. Basically, the tail platen is moved forward or backward so that with the toggles almost fully extended (q 0) the mould halves just touch each other. At this time, the clamping force is zero. See Figure 3a.To generate maximum clamping force (clamping force 0) and to self lock,
14、the toggles are fully extended (q = 0). This is done by extending the clamping stroke further and through the toggles, moves the moving platen forward by am, which is the amount by which the mould is compressed. At the same time, the tiebars, attached between the stationary and tail platens, are elo
15、ngated by at. See Figure 3b. Self locking means even when the hydraulic pressure in the clamping cylinder is relieved, the clamping force is maintained. This could only be achieved when the toggles are fully extended.Figure 3. Generation of clamping force 2.6 Revisiting clamping force Assume the mou
16、ld and the tiebars are in the elastic region at the rated clamping force. Their respective compression and elongation could be analysed using Figure 4.In Figure 4a, at the adequate clamping force F, the mould is compressed by am and the tiebars elongated by at. Since the tiebars are long and thin, t
17、hey are more flexible than the mould. Hence, the tiebars line is shallower. Technically speaking, Kt = tan at Km = tan am. As an example, with clamping force measured in tonnes and elongation/compression measured in microns (1 micron = 0.001 mm), an injection moulding machine with 60 mm diameter tie
18、bars has ato, a 300mm thick, 170 mm square steel mould has am = 64o. To facilitate the following analysis, the mould compression line is moved right to intersect the tiebars elongation line at F. See Figure 4b.Figure 4. Clamping force analysis When the melt is injected into the mould cavity, the cla
19、mping force is increased to F1. See Equation (2). In practice, this increase in clamping force could be observed by a clamping force measuring device. See Figure 5.Figure 5. The effect of cavity pressure on clamping force Figure 6 shows the free-body diagram of the mould halves with cavity pressure
20、introduced. Each mould half is balanced by the force equilibriumF1 = Fc + Fr - (2)whereFc = cavity pressure force,Fr = residual clamping force on the mould.Figure 6. Free body diagram of mould halves The cavity pressure force Fc offsets part of the clamping force F1, leaving only F1 - Fc to compress
21、 the mould. As a result, the mould compression is reduced from am to am. The difference is taken up by the tiebars elongating more from at to at, increasing the clamping force to F1. See Figure 7.The mould opening force Fc due to cavity pressure is seen between the tiebars line and the mould line. T
22、his is the graphical way of showing equation (2).Figure 7. Adding cavity pressure From Figures 3 and 7, one can see that the distance between the moving platen and the stationary platen is increased (by am - am) during injection. In practice, this could be measured by a dial gauge between the platen
23、s. In the extreme case when the cavity pressure is so high that the residual clamping force is reduced to zero, the mould opens and flashing occurs. At this point, the mould compression is zero, and cavity pressure force Fc = F2, the clamping force when flashing occurs. See the dashed line in Figure
24、 7.As an example, take F = 75 tonnes, the rated clamping force of Tat Ming ME75 III. For a 300 mm thick, 170 mm square steel mould, am = 0.037 mm. With such a mould mounted, the ME75 III toggle clamp will open at F2 = 81 tonnes, 6 tonnes above its rating. Everything else equal, an injection moulding
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