Plastics Technology

JUL 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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The Energy Density vs. ΔT/in. relationship is an important step forward in pursuit of a science-based approach to cooling-circuit design. Here's why. An Engineering Approach to Mold-Cooling Circuit Design In a November 2015 feature in Plastics Technology ("Improve the Cooling Performance of Your Molds"), Burger & Brown Engineering discussed techniques for determining the cooling-capacity requirement for an injection mold. We demonstrated how to calculate the required cooling energy for a part based on shot size, cycle time, material properties, pro- cessing temperature, and safe ejection temperature. We also dem- onstrated a method of calculating the required coolant flow rate based on an estimated change in coolant temperature (ΔT) as it flows through the mold. That article ended as follows: "The foundation of designing cooling circuits is a good ΔT estimate." We also offered the sugges- tion, "Processors can conduct internal studies of cooling-circuit ΔT to develop a sound basis for future designs." We felt then that this conclusion left a lot to be desired and begged for a sound science- and engineering-based approach to predicting ΔT. Since then we have been busy thinking about and studying the ΔT issue. We have spoken before about our "mold simulators" that pump electrical energy into steel mold-shaped components. While we pump heat into our "molds" we also pump water through cooling circuits just like in a real mold. One simulator is a 12-in. square mold base with drilled cooling passages. The other is 1.5-in. diam. core that can be cooled using a baffle or a bubbler tube. We measure the coolant flow rate, coolant temperature, and steel tempera- ture to within a tenth of a degree. All the sensors are connected to a data-acquisition system that feeds the data to a laptop. The measurements are converted into thousands of values in an Excel spreadsheet. Over the past few years we have spent hundreds of hours conducting experiments, changing variables, and studying the data. We are now prepared to tell a story about coolant ΔT. Graphical presentation of data sometimes reveals a novel way of thinking about relationships between variables. Our studies accurately measured the heat input to the mold, simulating molten polymer being injected. This energy flow is represented by the letter Q (BTU/ By Philip M. Burger Burger & Brown Engineering QUESTIONS ABOUT HEATING & COOLING? Visit the Heating and Cooling Zone. Energy Density also influences mold temperature and is useful in predicting the temperature. This shows an early attempt at understanding the relationship between heat input and ΔT/in. We plotted data from our round core and square mold plate with similar heat inputs but very different cooling-circuit geometry. The much shorter core cooling circuit produces a much higher ΔT/in. value. FIG 1 Coolant ΔT/in. vs. Heat Input (Coolant 75 F @ 1 GPM) 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0 Coolant ΔT/in. of Flow, F Heat Input, BTU/hr Round Core Square Mold Plate 0 500 1000 1500 2000 2500 3000 3500 44 JULY 2018 Plastics Technology PTonline.com Tips and Techniques

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