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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, they are more flexible than the mould. Hence, the tiebars line is shallower. Technic
