06. 2021 | 4 min
By  András Szűcs PhD, Károly Belina PhD

Introduction

The quality requirements for the injection moulding products are increasing, enough to think of the automotive or telecommunication products, which mechanical properties, dimensional accuracy and appearance developed greatly in the last years. Thanks to the appearance of more accurate injection moulding machines, moulds, and more special polymers on the market. One of the corner stone of the continuous production of the right quality product is the correctly set injection moulding technology. Of course, the machine setting means not only the parameters of the injection cycle, but the correct preparation and drying of the raw material, the tempering of the mould and other settings. It makes difficult the reproducibility of production when we work with multi cavity mould due the different filling of the cavities. To test the reproducibility and stability of the technology we can use the monitoring tools of the injection moulding machine. This method is more or less effective, but it has its limits since we do not measure the parameters where the forming happens so confounding factors limit the reliability of the monitoring. The cavity pressure measurement is the only technique that gives a closed feedback for system operation. Switchover based on cavity pressure increases the system stability, product separation based on the pressure curves – by setting correct tolerances - ensures scrap-free production, storing the measured values provides quality control and traceability. Unfortunately, these systems are not widespread in the industrial environment, therefore at the GAMF Faculty of Kecskemét College we developed a new cavity pressure measurement system, which opens new possibilities for the users. In our article we show the potential of the measurement system with the help mould of thick-walled product from a practical point of view.

Experiment

For the tests we used the Tipelin R-959A (TVK Nyrt.) type of polypropylene, which is recommended for injection moulding, and has low viscosity, and its flow index is 45g/10 min measured on 230°C and at a load of 2,16 kg.

The experiments were performed on an ARBURG Allrounder 470A injection moulding machine with monitoring system. During the testing/production process the electronic data carrier continuously store the measured values (injection pressure, injection time etc.), so they can be quite easily and quickly analysed later.

For the measurements we used the mould of an altered standard specimen. The inlet system of the mould can be equipped with inserts, so products can be manufactured by side, film, unilateral and bilateral gate. The thickness of the cavity is 4 mm. In both cavity the ejector pins are located at the beginning and at the end of the flow path, through them indirect measurement can be performed. Under the ejector pins we installed our self-developed π-cell pressure sensors. The measuring range of the sensors is 15 kN, which is really necessary since the reaction force is approximately 7850 N in case of a 10 mm diameter ejector pin and 1000 bar cavity pressure. Thanks to the minimized signal-to-noise ratio the measured values are shown in rather big resolution, so even a few N load can be measured accurately, and the characteristic is linear in the entire range. The sensors are not sensitive to temperature change. The signals were processed by “Cavity Eye” measuring system (Figure 1) and software which were developed especially for industrial environment, but suitable for laboratory measurements too. The standard version of the instrument handles 8 channels – sensors – and can be expanded indefinitely. It contains many automated functions, for example electronic access, storage, and compression of all the measuring results, statistical evaluation, automated identification of the mould etc.

The analysis of injection moulding process
The cavity pressure and hydraulic pressure curves are more or less connected. The injection pressure clearly influences the cavity pressure, but the moulding happens in the mould cavity, so we can only get correct information about the production process if the pressure is examined in the mould cavity. A general cavity pressure and hydraulic pressure curve is shown on Figure 2. The curve can be divided into several distinct sections, which are

–filling,
–packing,
–holding pressure,
–cooling[1].

Figure 2. Cavity pressure and injection pressure in function of time

In the filling phase the material flows freely in the cavity. The pressure required to maintain the flow is relatively small, and it primarily depends on the flowability of the material, speed of the melt front and cross-section of the flow. There are fundamental differences between the thin- and thick-walled products. The filling of thin-walled products with long flow path is more complicated, there is less time available, and in the filling bigger pressure formed in the mould cavity while the cooling time also significantly shorter.

In the compression phase the mould cavity is entirely filling up with polymer melt. The material does not flow, so when it is slightly compressed, the cavity pressure is built up in 0,01…0,1 second. Compression basically means that we squeeze several precent more material into the mould cavity than it could contain on atmospheric pressure. (The compressibility of the material can be determined by p-v-T measurement.) After the compression there is a switchover to holding pressure, this means the real built-up injection pressure is reduced momentarily or within a specified “ramp” time to the holding pressure value. The “ramp” can be always set on a modern injection moulding machine, which greatly perpetuates the process, since the PID control electronics is not able to handle steadily momentarily changes. The switchover is one of the most sensitive parameters of the injection moulding process.

During the decrease of the material temperature, the specific volume is decreasing, this is compensated by the injected material in the holding phase. During holding phase decreasing, increasing, and possibly pulsing pressure profiles can be used. Leaving the holding pressure or setting it incorrectly can result in sink marks on the surface of the product or vacuum void inside the product. The holding time is useful until the sealing i.e., when the material temperature in the cross-section of the impediment is decreased under the yield or crystallisation temperature then it solidifies and prevents the flow of material.

In cooling phase, the material continuous to cool, the pressure in the mould cavity decreases, but due the sealing the material does not flow. Therefore, the pressure decrease is approximately proportional to the decrease of the specific volume, i.e. the shrinkage of the product. The negative steepness in the cooling phase is proportional to the cooling speed.

One of the most important machine parameters is the switchover. In industrial environment the most common to set the switchover according to the path, which means that the switchover from speed control to the pressure controlled holding phase happens at a given screw path. In case of modern machines this position can be held with 0,001-0,01 mm accuracy. The fault of system is that it does not take into consideration the external effects on the filling process. In case of the failure or slight wear of the non-return valve, the path of switchover will be constant, although the amount of the injected material will fluctuate in each cycle. The least common method is the switchover according to time. It has the benefit to be able to measure the time with extreme accuracy, therefore it can be used for control. The most effective method in term of stability is to measure the cavity pressure. The pressure sensor is located near the gate or far away from it. If it is located near the gate then the entire filling process is well traceable, if it is at the end of the flow path then the filling of the product can be checked. We get the most information if there are sensors both at the beginning and end of the flow path. This is especially important at thin-walled products. Since the goal is to produce the same product quality in every cycle, the largest stability can be reached by switchover based on cavity pressure, which means a closed feedback [3]. The figure 3 shows the product manufacturing process.

Figure 3. Flow chart of product manufacturing

Measurement results

For the experimental production a stable technology was set (table 1.), compared to this the following parameters were changed:

–    injection speed(vfröccs),
–    material temperature(Ta),
–    mould temperature(Tsz),
–    switching point(sát),
–    holding pressure(pu),
–    holding time (tu).

 

Table 1. Technological parameters