The claming units main function is to open and close the mold along with ejecting the parts. The two most common types of mold clamps are the are the direct hydraulic and the toggle clamps. Toggle clamps are actuated by hydraulic cylinders. These clamps utilize mechanical linkages to generate higher forces than a direct connection from a hydraulic cylinder of the same size.
A 5-point double toggle clamp is shown in Figure 1. After amplification by the toggle mechanism, the clamping cylinder, attached to the tail platen, extends, pushing the moving platen to lock the mold halves together. We will investigate in various ways how the clamping force is generated.
Figure 1. 5 point Toggle Clamp
Clamping force problem and solution
The problem with toggle clamped Injection Molding machines is with only a hydraulic pressure meter available, there is no way to set an accurate clamping force, when the mold is installed and to expected to maintain a constant pressure during injection. As the moud 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 tolerance as mold temperature changes.
For the engineers, the following sections details how clamping force is generated. They also relate to the second device: switch over to holding pressure by cavity pressure measurement.
At its simplest, the rated clamping force Fo is calculated according to the following formula.
Fo = Ps * A * M ------------------------------------------- (1)
Ps = system pressure,
A = clamping cylinder cross sectional area,
M = mechanical 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 tie bars.
The clamping force found in the specification table of an Injection Molding 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 Molding machine, it is best to use the minimal but adequate clamping force F. An adequate clamping force holds the mold halves together against the cavity pressure during the injection phase.
An excessive clamping force distorts the mold and the mold cavity unnecessarily, affecting the precision of the molded part. Furthermore, a high clamping force compresses the toggles and the mold, and stretches the tie bars, reducing the fatigue lives of the toggle pins, the mold and the tie bars.
Mold height adjustments
Since not all molds have the same mold height, a toggle clamped Injection Molding machine has a mold height adjustment mechanism for that purpose. Basically, the tail platen is moved forward or backward so that with the toggles almost fully extended (q > 0) the mold 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, the toggles are fully extended (q = 0). This is done by extending the clamping stroke further and through the toggles, moving the platen forward by am, which is the amount by which the mold is compressed. At the same time, the tie bars, attached between the stationary and tail platens, are elongated by at. see Figure 3b. Self locking means that even when the hydraulic pressure in the clamping cylinder is relieved, the clamping force is maintained. This can only be achieved when the toggles are fully extended.
Figure 3. Toggle Clamp generating force
Revisiting clamping force
Assume the mold and the tie bars are in the elastic region at the rated clamping force. Their respective compression and elongation could be analyzed using Figure 4.
In Figure 4a, at the adequate clamping force F, the mold is compressed by am and the tie bars elongated by at. Since the tie bars are long and thin, they are more flexible than the mold. Hence, the tie bars line is shallower. Technically speaking, Kt = tan at < Km = tan am. for example, with clamping force measured in tons and elongation/compression measured in microns (1 micron = 0.001 mm), an Injection Molding machine with 60 mm diameter tie bars has at = 9.2o, a 300mm thick, 170 mm square steel mold has am = 64o. To facilitate the following analysis, the mold compression line is moved right to intersect the tie bars elongation line at F. See Figure 4b.
Clamp Force Analysis
When the melt is injected into the mold cavity, the clamping 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 mold halves with cavity pressure introduced. Each mold half is balanced by the force equilibrium or:
F1 = Fc + Fr
Fc = cavity pressure force,
Fr = residual clamping force on the mold.
Figure 6. Body diagram of mold halves
The cavity pressure force Fc offsets part of the clamping force F1, leaving only F1 - Fc to compress the mold. As a result, the mold compression is reduced from am to am'. The difference is taken up by the tie bars elongating more from at to at', increasing the clamping force to F1.(see Figure 7).
The mold opening force Fc due to cavity pressure is seen between the tie bars line and the mold line. This 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 platens. In the extreme case when the cavity pressure is so high that the residual clamping force is reduced to zero, the mold opens and flashing occurs. At this point, the mold compression is zero, and cavity pressure force Fc = F2, is the clamping force when flashing occurs. See the dashed line in Figure 7.
As an example, take F = 75 ton. For a 300 mm thick, 170 mm square steel mold, am = 0.037 mm. With such a mold mounted, the toggle clamp will open at F2 = 81 tons, 6 tons above its rating if everything else equal, an Injection Molding machine with 50 mm diameter tie bars will open at F2 =78 tons, three tons above its rating.
As the mold heats up, it expands. The clamping force is increased as the mechanical interference is increased by the amount of the mold expansion. This is shown in Figure 8 in which the mold line is moved further right by the expansion, intersecting the tie bars line at a higher clamping force F". In this diagram, the tie bars are elongated more (at" - at ) and generated the additional clamping force F" - F.
To restore the clamping force, a mold height adjustment is made to restore the mold (now hot) compression to am before the next shot is injected. Such adjustment is clamping force control.
As an example, a 300 mm thick steel mold heated up by 10oC expands by 0.045 mm. On the ME75 III Injection Molding machine, the increase in clamping force is 7.3 tons, which is almost 10% of the rated clamping force.
Figure 8. mold expansion increases clamping force
The clamping force measuring device
Since fast response is not needed, a strain gauge-based device is sufficient to measure and control clamping force. The simplest means is a strain gauge attached to the tail or stationary platen, which deflects under the clamping force.
Alternatively, a strain gauge is attached to a tie bar which extends as the mold is locked. An assumption is made that the tie bars are evenly stretched which may not be true if the mold faces are not parallel, the mold cavity is not symmetrical or the tie bars are not balanced out of the factory.
The strain gauge output is amplified and digitally displayed. The display is calibrated to read in tons. Such a device is sufficient to help the operator set up an adequate clamping force initially (during mold height adjustment). When, for example, a 5% deviation from the initial clamping force is detected (after the mold is closed but before injection), the operator could do a mold height adjustment to restore the clamping force to its original value.
Alternatively, the computer in the Injection Molding machine could set up the clamping force during the initial mold height adjustment, and to restore the clamping force by another mold height adjustment when a prescribed deviation is detected.