Plastics Technology

JUL 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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broader range of mold geometries. We have examined cooling data from molds running in our own molding shop and find good agreement between shop-floor data and our Energy Density research. Here is a step-by-step procedure explaining how an engineer or designer would use the Energy Density method to design a cooling circuit with a desired (or required) heat-removal capacity for the thick-wall cap shown in Fig. 8. 1. Calculate the amount of heat that must be removed by the cavity side of a mold to cool a single-cavity HDPE cap. Its weight is 19.4 g (0.043 lb.) running on a 12-sec cycle. The heat capacity: C = 0.60 BTU/lb.-°F. The latent heat value: H L = 119 BTU/lb. The processing (melt) temperature is 450 F and the safe ejection temperature is 150 F. The required change in temperature, ΔT = 450-150 = 300 °F. BTU/shot = W × ((C × ΔT) + HL) = 0.043 × ((0.60 × 300) + 119) = 12.86 BTU/shot. BTU/hr = BTU/shot × shots/hr. Shots/hr = 3600 sec/hr ÷ 12 sec = 300 shots/hr. The letter Q is used to represent energy flow, BTU/hr. Q = 12.86 BTU/shot × 300 shots/hr = 3857 BTU/hr. We will assume that 45% of the cooling for this part is accom- plished by the cavity with a single cross-drilled cooling circuit as shown in Fig. 9. Our cooling circuit must be able to remove 0.45 x 3857 BTU/hr = 1736 BTU/hr. 2. Calculate cooling-circuit surface area based on a circuit of 0.339-in. diam. × 21.1 in. long. A = π × d × L = 3.1416 × 0.339 in. × 21.1 in. = 22.47 in.² 3. Calculate Energy Density, DE = Q/A, BTU/hr./in.² Energy Density = 1736 BTU/hr/22.47 in.² = 77.26 BTU/hr/in.² 4. Estimate ΔT/in. using the calculated energy density and the 1 GPM line on Fig. 4. The value is about 0.19 °F/in. ABOUT THE AUTHOR: Phil Burger, P.E., founded Burger & Brown Engineering Inc. in 1978 and served as president until 2005. Burger & Brown manufactures engineered products related to mold cooling and in-mold sensing and holds 10 patents for its products. Burger currently works part-time for the firm and has most recently developed an educa- tional program called Scientific Cooling that was launched in October 2013. Contact: (816) 878-6675;; 5. Calculate total coolant ΔT: ΔT = ΔT/in. × L = 0.19 × 21.1 = 4.01 °F 6. Calculate required coolant flow rate using the expression: • GPM = Q/(500.4 × ΔT). • GPM = 1736 BTU/hr ÷ (500.4 × 4.01 °F) = 0.87 GPM. • Check to be sure that flow will be turbulent considering the circuit diameter and coolant temperature. 7. Note that the Energy Density value calculated in Step 3 is also useful for estimating mold temperature based on Figs. 5-7. Having studied and understood this example you might have some questions. We anticipated a couple: • We used a 1 GPM line on the graph to calcu- late a required cooling flow rate of just 0.87 GPM. What gives? The heat input comes from the molten plastic or resistance heaters in the case of our lab simulation. Not all the heat goes to the coolant. Some heat trans- fers to the mold and the surrounding environment, depending on mold temperature. The ΔT/in. value is influenced only by the heat that makes its way into the cooling circuit. • If you wanted to design your cooling circuit to have lower Energy Density or ΔT/in. values, what can you change? Make the cooling circuit longer or larger in diameter, or add additional circuits—in other words, create more cooling-circuit area. You can also pump more water through the circuit; but as we have discussed many times, more flow yields diminishing returns after the transition to turbulent flow. In the case of our example circuit and the choice of 1 GPM as the flow rate, we are already at a Reynolds number of nearly double the minimum for turbulent flow. More flow wouldn't accomplish much. FIG 8 Using this thick-wall cap as a reference, we created a step-by-step procedure for how an engineer or designer would use the Energy Density method to design a cooling circuit with a specific heat-removal capacity. FIG 9 Our calculations assumed that 45% of the cooling for this part is accomplished by the cavity with a single cross-drilled cooling circuit, as shown here. @plastechmag 47 Plastics Technology M O L D C O O L I N G

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