Reduction of energy consumption in injection molding of polypropylene parts through the optimization of mold thermal control

Giovanni Lucchetta, Davide Masato, Marco Sorgato, Department of Industrial Engineering,

University of Padua, Italy

Abstract

The environmental impact of the injection molding

process is mostly due to electricity consumption. This is particularly significant for packaging applications, which are the largest application sector for the plastics industry. In this work, electricity consumption measurements of the process were performed, considering a large packaging plant. In particular, the energy consumption related to mold thermal control were analyzed and minimized through a representative case study. The effects of alternative cooling channels configurations and different process parameters were experimentally investigated, considering also their influence on the quality of the molded parts. The results indicated that the common industrial practice for mold thermal control is highly inefficient. The implementation of electricity consumption measurements allowed the optimization of molds thermal control leading to considerable economic savings.

Introduction

Energy savings and emissions reduction related to manufacturing are crucial issues, especially considering large-scale processes, as injection molding [1]. Indeed, the higher the process diffusion the more marked the reduction of its environmental impact due to a slight increase in its overall efficiency [2].

The assessment of the environmental impact of the plastic industry started more than 40 years ago [3], when manufacturing enterprises began implementing energy consumption evaluation in order to improve both their economic benefit and environmental performance [4]. Researchers have been aiming at improving injection molding environmental performance by providing the designers with solutions to optimize electricity consumption [5,6].

The energetic impact of the injection molding process is particularly important for the packaging industry, which constitutes about the 40% of the demand for plastics, thus having a large impact on the plastic manufacturing economy [7]. Injection molding plants for packaging applications commonly function on 24 hours shifts for 7 days a week, thus being particularly intense in terms of electrical energy demand, because of the high-energy absorption related to the functioning of main injection molding machines units (i.e. injection, clamping and cooling units) [8].

Moreover, the energy efficiency of packaging industrial plants is usually not properly controlled because of its common configuration in which all the injection molding machines work with a centralized mold temperature control system [9]. Traditional water chillers are used for the thermal control of several molds. Consequently, each mold temperature is controlled by the temperature set in the water chiller, which is the same for all the injection molding machines. This configuration is economic, since dedicated temperature controllers are not needed, but it can be negative in terms of quality of the molded parts and overall energy consumption of the plant.

The reduction of the energy consumption in injection molding is usually pursued by following two different strategies: improvement of machinery (i.e. hardware and auxiliary equipment) or optimization of the processing parameters [8]. However, most of the studies reported in the literature focused their analysis on the former, without considering the potential improvement offered by process optimization [10].

Moreover, industrial common practice is usually inspired by general guidelines, such as maximize the water flow rate and connect mold cooling circuits in parallel to minimize the increase of water temperature. Exalting these guidelines without measuring their energy impact in any specific case can lead to significant inefficiencies. In order to support this consideration with experimental evidence, in this work the impact of mold thermal control on the injection molding energy consumption was investigated. The analysis of the efficiency of the mold cooling system adopted in a mono-material packaging plant was studied by measuring the electrical consumption. Different configurations of cooling channels were analyzed considering the electrical absorption and the quality of the molded parts. The effect of process parameters was then analyzed considering the energy consumption of the injection molding machine, the chiller and the water circulation pumps.

Materials and Methods

The analysis of the contribution of mold thermal control to the energy consumption of the injection molding process was performed considering a mono-material plant for manufacturing aerosol sprayers. The plant runs continuously 24 hours a day, 7 days a week, to produce more than 350 million products per year, thus being particularly energy consuming.

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Product and Mold Design In particular, the investigation focused on the injection molding of a single component: the body of a trigger sprayer (Fig. 1).

Figure 1. Molded product.

The part is made of a polypropylene random copolymer (INEOS, PP 200CA). The material is ideal for the manufacturing of thin-wall injection molded parts. Table 1 reports the main properties of the polymer.

Table 1. Main property of the INEOS PP 200CA.

Property Units Test Method Value Density g/cm3 ISO 1183 0.93 MFR 230°C – 2.16 kg

g/10 min ISO 1133 40

Crystallization Temperature

°C ISO 6721 119

Measuring Equipment

The flow rate of the water circulating in the mold

cooling channels was measured using a portable ultrasound flow meter (Ultraflux, Uf 801 P), which allowed the non-intrusive monitoring of the coolant flow rate without stopping the process. Two thermocouples were used to monitor the inlet and the outlet water temperatures.

The electrical power consumption absorbed by the injection molding machine was monitored using a power analyzer, directly connected to the machine electrical panel.

Experimental

Optimization of cooling configuration

The mold was excluded from the centralized water

distribution system and connected to a dedicated chiller. Both series and parallel connection configurations for the

cooling channels were tested. As shown in Fig. 2 (a), when the coolant inlets are connected in parallel, three different cooling circuits independently work within the mold, while with the connection in series all the channels are connect in one cooling circuit (Fig. 2(b)).

(a) (b)

Figure 2. Cooling connections configurations: (a) parallel, (b) series.

For both connection configurations, the effect of a

gradual reduction of the coolant flow rate was investigated considering its influence on molded parts quality, in order to determine the minimum allowable flow rate value. The temperature of the cooling water was monitored and related to the different flow rate values.

Process parameter optimization

After determining the best connection configuration

for the mold cooling circuits, the dependence of energy consumption on coolant flow rate was investigated in combination with the main injection molding parameters, following a Design of Experiments (DoE) approach. A two-level, full factorial plan was employed (Table 2) to investigate four factors: coolant flow rate, Q, temperature of the hot runner, Thr, temperature of the nozzle, Tm, and packing pressure, Ph.

The overall power absorbed by the injection molding machine, the water chiller and the water circulation pump was selected as the response variable for the analysis. For each run three acquisitions were performed using the power analyzer.

Table 2. Factors and levels for the designed factorial plan.

Factor Low level (-) High level (+) Q [m3/h] 6.0 7.4 Thr [°C] 210 250 Tm [°C] 225 290 Ph [bar] 40 60

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Results and Discussions

Cooling configuration The parallel configuration was tested decreasing the

flow rate from 9.4 m3/h to 8.0 m3/h with intermediate steps of 9.1 and 8.5 m3/h. For each flow rate value, the chiller power absorption and the quality of the molded parts were monitored. Moreover, the inlet and the outlet temperature were measured and their difference, ΔT, was calculated to evaluate the efficiency of the cooling system. The results for the parallel configuration are reported in Table 3. Table 3. Thermal efficiency and power absorption for the

parallel configuration of the cooling circuit. Flow Rate

[m3/h] ΔT

[°C] Power absorption

[kW] 9.4 1.0 10.9 9.1 1.1 11.6 8.5 1.2 11.9 8.0 1.4 13.0

Fig. 3 shows that the power absorption increases

decreasing the flow rate. Moreover, for this configuration, with a flow rate of 8.0 m3/h, the quality of the molded parts was not acceptable. In fact, as shown in Fig. 4 (a), at a flow rate of 8.0 m3/h, the thermal gradient at the injection location was not sufficient for the correct thermal break of the gate, causing an aesthetic defect.

This is due to the fact that at a flow rate of 8.0 m3/h or lower, the heat conduction at the mold surface was not sufficient for a fast solidification of the polymer. As reported in Table 3, the difference between the outlet and the inlet temperature (ΔT) slightly increases decreasing the flow rate but its value is critical, since a typical allowable increase in the coolant temperature is 1 °C [11].

Figure 3. Power absorption as a function of flow rate for the parallel configuration of the cooling circuit.

(a) (b)

Figure 4. Defects of the molded parts for (a) flow rate of 8.0 m3/h for the parallel configuration and (b) flow rate of 4.0 m3/h for the series configuration

The series configuration was analyzed starting with a

flow rate of 7.4 m3/h and decreasing it down to 4.0 m3/h, with intermediate steps of 6.4, 5.5, 5.0 and 4.5 m3/h. The results of the conducted experiments are reported in Table 4 and summarized in Fig. 5. It is interesting to observe the trend of the power absorption for this configuration. Referring to Fig. 5, at high values of flow rate, the power absorption reaches its maximum value (9.9 kW/h). Decreasing the flow rate, the power absorption decreases until reaching a minimum value in correspondence of 5.0 m3/h and then it increased. The minimum flow rate value for this configuration was 4.0 m3/h, below which the quality of the molded parts was not more acceptable due to the appearing of the aesthetic defect shown in Fig. 4 (b).

Comparing the two cooling configurations, the series was observed to allow an overall lower power absorption and higher quality of the molded parts. As reported in Table 4, the difference between the outlet and the inlet temperature (ΔT) slightly increases decreasing the flow rate even tough its value is critical. In this specific case, the industrial common practice of maximizing the water flow rate and connecting the mold cooling circuits in parallel, to minimize the increase of water temperature, would be satisfactory in terms of part quality but not optimal from the point of view of energy consumption.

Table 4. Thermal efficiency and power absorption for the

series configuration of the cooling circuit. Flow Rate

[m3/h] ΔT

[°C] Power absorption

[kW] 7.4 1.1 9.9 6.4 1.3 9.7 5.5 1.4 9.0 5.0 1.5 8.7 4.5 1.8 9.4 4.0 2.1 9.8

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Figure 5. Power absorption as a function of flow rate for the series configuration of the cooling circuit.

Process optimization

The optimization of process parameters was performed considering the series configuration. The results of the ANOVA for the optimization plan (Table 5) indicated that all the factors except the packing pressure significantly affect the energy consumption during the injection molding process, even though the influence of coolant flow rate is much lower compared to the others. In particular, increasing the flow rate from 6.0 to 7.4 m3/h, the power consumption increases by 2% (Fig. 6).

The hot runner temperature is the factor that most strongly affects the power absorption during the process. In fact, increasing Thr from 210 to 250 °C, the power absorption increases by 5%, while increasing Tm from 225 to 290 °C, the absorption increases by 3.5%.

A further optimization analysis was conducted by varying only the coolant flow rate in order to minimize the total energy consumption (overall electricity absorbed by the injection molding machine, the water chiller and the pump). The injection molding process parameters were fixed at the following constant values:

• Hot runner temperature: 210°C • Nozzle temperature: 225°C • Packing pressure: 40 bar.

In particular, the hot runner and nozzle temperature were set to the lowest suitable values. As in the previous tests, the parallel cooling circuit configuration was used.

In this second analysis, the coolant flow rate was varied starting from 5 m3/h down to 2 m3/h. Also in this case, the response variables were the power absorption and the quality of the molded parts.

Table 5. ANOVA table for the optimization DoE pla

Process parameter Unit p-value Coolant flow rate (Q) m3/h 0.058 Hot runner temperature (Thr) °C 0.000 Nozzle temperature (Tm) °C 0.000 Packing pressure (Ph) bar 0.127

The quality of the molded parts resulted acceptable

even at the lowest value of the set flow rate, due to the fact that the melt temperature was much lower compared to previous experiments.

Table 6 reports the results of the power absorption as a function of coolant flow rate. It is interesting to notice that the power absorption reaches a minimum value at a flow rate of 2.50 m3/h. Table 6. ANOVA table for the optimization DoE plan

Flow rate [m3/h] Power absorption [kW] 5.50 24.8 5.00 24.5 4.5 24.3 4.0 24.1 3.5 24.1 2.5 23.7 2.0 23.9

Figure 6. Main effect plots for the analyzed factors.

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The analyzed injection molding is actually operated setting a hot runner temperature of 240°C and a nozzle temperature of 250°C. Considering that the actual energy consumption is 26.53 kW/h, and given a price for the power supply of 0.148 c€/kWh, the estimated saving per year for just the analyzed part is around 3000 €. The implementation of this approach to the rest of the production could potentially yield savings a hundred times higher.

Conclusions

In this work, electricity consumption measurements of the injection molding process were performed, considering a large packaging plant. In particular, the energy consumption related to mold thermal control were analyzed and minimized through a representative case study. The effects of series and parallel connection configurations for the cooling channels were experimentally investigated, considering also their influence on the quality of the molded parts. The results indicated that the series allows an overall lower power absorption and higher quality of the molded parts. The difference between the outlet and the inlet temperature slightly increases decreasing the flow rate but its value is critical, since a typical allowable increase in the coolant temperature is 1 °C [11]. Therefore, a complete evaluation of part quality needs to be performed. In this specific case, the industrial common practice of maximizing the water flow rate and connecting the mold cooling circuits in parallel, to minimize the increase of water temperature, would be satisfactory in terms of part quality but not optimal from the point of view of energy consumption.

Moreover, by setting the hot runner and nozzle temperatures to the lowest suitable values, the coolant flow rate could be significantly reduced without compromising the quality of the molded parts and reaching a minimum value of the power absorption of 2.50 m3/h.

The implementation of electricity consumption measurements allowed the optimization of molds thermal control leading to a saving of approximately 3000 € per year, for just the analyzed part The implementation of this approach to the rest of the production could potentially yield savings a hundred times higher.

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