Mold cooling in thermoplastics injection molding: Effectiveness and energy efficiency

Abstract

Energy use by thermoplastics injection molding machines is estimated to result in global CO2 emissions in the order of 80 million metric tons annually. Shortening the molding cycle time is a key factor in improving energy efficiency and since cooling occupies a major part of the cycle, effective design and operation of cooling systems is essential. While guidelines exist, there is a lack of quantitative generic information to complement these. To provide this, a parametric study of mold tool cooling is carried out using numerical simulations, examining coolant channel layout, coolant flowrate and temperature, and tooling thermal properties. Briefly, some findings for representative cases include:Within recommended guidelines for coolant channel layout (channel diameter, pitch and distance from the cavity) cooling time for the worst case was found to be 70% longer than for the best.Reduction of coolant temperature by 5 °C (35 °C to 30 °C) allows reduction of coolant flowrate by a factor of more than two while keeping the cooling time unchanged.Use of an aluminum tooling alloy reduces cooling time, as compared with tool steel, by about 30% (15 s–10 s in an example) across a range of coolant flowrates and temperatures.If the maximum plastic temperature variation on ejection is to be no more than 5 °C, coolant channel pitch should be less that 50 mm when the channels are 10 mm from the cavity, and 80 mm when at 20 mm.A coolant heat transfer coefficient of 5,000 W/m2K is recommended. This corresponds to a Reynolds number of 10,000 in a coolant channel of 10 mm diameter.The effectiveness of higher heat transfer coefficients is limited by the thermal resistance of the tool and rapidly increasing pumping costs.Cooling times can be collapsed onto a single line when plotted against an overall thermal resistance that takes into account the coolant channel layout, tooling thermal conductivity, and coolant heat transfer coefficient.A widely promoted formula for cooling time is found to be inadequate and an improved formula incorporating this overall thermal resistance provides better estimates.The need for careful balancing of opposing effects to optimize energy use in cooling is emphasized. The present results will assist with this in the early-stage design, with the aim of shortening cycle time to better amortize base loads. Furthermore, insights gained will be valuable in providing better estimates of cooling time for predictions of productivity, energy use and environmental impacts.