06.2021| 7 min
By. András Szűcs PhD, Károly BelinaPhD

In the first part of the article, the environment of the experiment was described and the measuring results were recorded. In this part contains the analysis of the results.

Measurement results

Figure 4 shows cavity pressure curves measured in one cycle. 2 sensors per cavity provide the signal, so in altogether 4 curves displayed belong together in pairs. The sensors were calibrated so the experienced differences of the pressure peaks can arise from the differences of the mould geometry. The previously mentioned four phases can be well separated.
 

Figure 4. Cavity pressure curve in function of time

By zooming in the rising section of the curve, the filling and compression phase becomes visible (figure 5). The melt reaches the sensors located near the gate in approximately 85ms, although measurable pressure appears only at the end of flow path, after 100ms. The measured pressure difference between the sensors (dp1 and dp2) in the filling phase is almost constant in a short section and proportionate to the material viscosity, the passed time between rising of two signals is proportionate to the injection speed (shear speed).

Figure 5. Cavity pressure curve in function of time

Table 2 summarises the Δp and injection pressure values
 

Table 2. The Δp and injection pressure in function of injection speed

The curves seen in figure 6 are similar to the flow curve [3]. The explanation for this is that the shear stress is determined by the pressure drop and flow geometry, while the shear speed comes from the flow volume and flow geometry. By increasing the injection speed, the measured pressure difference between the sensors is changing non-linearly, so the material viscosity is decreasing which can be measured on-line with the system. The effect of the material temperature on the flowability is unambiguous. Lower pressure was required to fill the mould cavity with higher temperature plastic. The registered injection pressure shows similar tendency, which has an entirely different explanation. The limit of the injection pressure set on the machine is 1500 bar. The measured pressure in the injection phase is significantly lower than this, which means that this parameter does not play a role in the forming, the real injection pressure can freely build up. The difference can be explained more by the reaction time of the machine, which changes between 0,01 and 0,1s according to the machine type. By increasing the speed of the screw piston, the inertia of the screw is increasing, i.e. it will stop later. The switchover was 10mm at all the settings, but the screw stopped later each time. Figure 7 shows the pressure curve of the 1. sensor in function of injection speed. (To make the comparison of curves easier, in the following we use the measured results of the 1. sensor.).

Figure 6. Δp values in function of injection speed

Figure 7. Cavity pressure in function of injection speed

At exact same parameters the injection speed has a significant effect on the cavity pressure. The specimens made at the speed of 120 and 140 mm/s have flash, which can be explained by the high cavity pressure. In case of flash it must considered whether it was formed in the filling or the holding phase. In this case the impulse-like pressure surge slightly opens the mould, and the low viscosity melt forms flash. In cases like this, flash can be stopped unambiguously by decreasing the injection speed, the holding pressure does not affect it.

Changing the switching point brought similar results. At basic setting the switchover from injection pressure to holding pressure happens at the 10 mm position. If the switchover earlier, i.e. at 11 mm happened, it only slightly reduced the holding pressure. This means that the compression happened in the holding phase. If the switchover takes place later, then the injection speed shows the same phenomenon as before. Since the upper limit of the injection pressure was 1500 bar, the injection moulding machine did not limit the real, built up injection pressure. Due the late switchover flash was formed on the part (figure 8.).

Figure 8. The effect of the switching point on the injection pressure

To analyse the process more precisely, worth to examine the filling and compression phase a little more (figure 9.). The pressure curves measured in the filling phase are the same, the lines overlap. The switchover of 11 and 10 mm bends slightly due the relatively early switchover. The curves are running together even in the compression phase, while the injection speed made the curve run-ups steeper or flatter. After switchover we use 300 bar holding pressure at all setting. The decrease of cavity pressure peak over time can be explained by the backflow of the material. There was a 200…250 bar cavity pressure until the sealing. This difference can be explained by the compressibility of the polymer, the not-Newtonian flow, and the structural changes in the flow cross-section during the cooling [4].

Figure 9. Effect of the switchover on cavity pressure

The holding time is usually determined by the mass measurement of the part. With cavity pressure measurement system, the effective holding time can be determined faster and more accurately (figure 10). Since all the parameters are constant, the curves run together until the end of the holding time. When the holding is shorter than sealing it can result in a sudden break, which also the result of the free material flow. If the holding time is longer than the sealing time, it is not shown in the curve, because the material parts frozen in the impediment do not allow either inflow or outflow. After sealing – in the cooling phase – the steepness of the curves is approximately constant, which is analysed in the cooling test.

Figure 10. The effect holding time on cavity pressure

The testing of holding pressure brought very interesting results (figure 11). The cavity pressure is slightly lower than the set value. The reason for this was explained previously. When the holding pressure is lower than the cavity pressure, then backflow formed in the impediment and the pressure decreases. But when it is higher, then the compression is formed by holding pressure, reaching the pressure peak becomes slower, and it stays on the same value until the sealing.

At either of the settings are the specimen with flash, although the cavity pressure was significantly higher than during the change of injection speed. The reason of this is that the shell of the part is continuously cooling during the filling. When the temperature of shell layer decrease under the crystallisation temperature, then a solid shell surrounds the melt. Accordingly, the melted material is not able to break through the shell, flow into the dividing plane, form flash. Consequently at the production of thick-walled products the holding pressure value should be profiled in order to reduce the sink marks, recommended to increase it under time.

Figure 11. Effect of the holding pressure on cavity pressure

Figure 12 shows the effect of material temperature on cavity pressure. It is immediately apparent that by increasing the temperature of the plastic, the cavity pressure is increasing, and the sealing also happens more and more later. Above 230°C the 5 seconds long holding counts as short, the reason for this the higher temperature melt seal later.

Figure 12. Effect of material temperature on cavity pressure

The function in figure 13 are first derivative in time of the figure 12 curves. It contains many interesting points in terms of process analysisIt shows well the influence of the material temperature on moulding process. In the compression phase can be seen the first and the biggest peak in a positive direction, which a negative peak follows, this is proportionate to the outflow rate of the material. In the holding phase a negative steepness can be experienced, the pressure slightly decreases. The next negative peak results in a more significant change in function of temperature. Continuous transition is shown between the switchover and cooling, when the sealing happens, if the holding time is short, then there is a negative peak again. In cooling phase all the steepness is constant, since after the sealing the cooling speed does not change substantially.

Figure 13. First derivative in time of the cavity pressure curves at different material temperatures

The mould temperature influenced the curves similarly to the material temperature but did not increase the pressure peak in the same amount (figure 14). By increasing the mould temperature, the sealing time shifts to later again. At 53°C the 5s holding time is not enough, the material flows out of the cavity even before the sealing happens. In the holding phase the pressure decreases less at a higher mould temperature. This can be explained by the slower cooling and the slower decrease of specific volume.

Figure 14. The effect of mould temperature on cacity pressure

By evaluating the derivative of figure 14, the effective holding time and cooling speed can be verified (figure 15). The higher mould temperature results in slower cooling, which is shown well on the diagram. The analysis of the derived function is used in the algorithm of the Cavity Eye control and interfering process, which is under development.

Figure 15. First derivative in time of the cavity pressure curves at different mould temperatures

Summary

As result of the last years research, we developed a cavity pressure measuring system, which includes the development of the measuring cells, hardware components, and software. The instrument was created according to the industrial requirements, but it is suitable for laboratory measurements too. In our article we introduced the possibilities of the pressure measurement and the conditions of the injection moulding process. The measurement results demonstrate well how each technological parameter effects the four phases of injection moulding. The analysis and control of the injection moulding process with the curves and the first time derivate of the curves ensures further automatization possibilities. The use of the system can make production launch easier and faster. In case of the manufacture of faulty products it helps to determine the cause of the problem, and to sort and separate the scrap due the instability of the technology.

Bibliography
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[3] Szűcs, A.: Rheological and thermal analysis of the filling stage of injection moulding, eXPRESS Polymer Letters, 6/8, 672–679 (2012).
[4] Pantani, R.; Coccorullo, I.; Speranza, V.; Titomanlio, G.: Morphology evolution during injection molding: Effect of packing pressure, Polymer, 48, 2778–2790, (2007)