Plastics Technology

JUL 2018

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hr). We measured the total increase in temperature (ΔT) of the coolant as it passed through the cooling circuits. We divided the ΔT by total length of the cooling circuit to create a value of ΔT/in. We under- stood that there was a relationship between heat input and ΔT/in. Figure 1 shows an early attempt at understanding the relationship. 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. Clearly cooling-circuit length alone does not fully describe the power or capacity of the circuit. We knew that all the heat being removed by the coolant must pass through the walls of the cooling circuit and into the coolant flowing through the circuit. Thus, the true expression of heat transferred into the coolant has to involve the area, not just the length, of the cooling circuit. It must also involve energy flow, Q (BTU/hr) along with circuit area, A (in. 2 ). We named this value Energy Density and it is described as the energy flow divided by cooling-circuit area: Energy Density = Q/A in BTU/hr/in.2 Our next step was to graph the relationship between Energy Density and coolant ΔT/in. We plotted Energy Density on the horizontal axis and coolant ΔT/in. on the vertical axis of a typical X-Y graph. As in Fig. 1, we included data from our round core and square mold plate as separate plots on the same graph. This time the plots from two very different types of cooling circuits showed a remarkably linear relationship. The Energy Density with the round core was much higher than the square mold, but the two trials showed a clear linear relationship between Energy Density and ΔT/in. Figure 2 shows this interesting finding. At this point we felt we were onto something meaningful and useful. Figure 3 shows a further refinement of this data, using a trend line that provides a user-friendly tool for estimating ΔT/in. based on the easily calculated Energy Density value. For example, an Energy Density value of 80 BTU/hr/in.² would produce a ΔT/ in. value of about 0.20 °F/in. Calculating the Energy Density value is as simple as deter- mining the BTU/hr that must be removed to cool your part and dividing that value by the area of the cooling circuit. Let's say you need to remove 1500 BTU/hr to cool the part and the cooling circuit has an area of 20.6 in.² (7/16 in. diam. × 15 in. long). The Energy Density value would be: FIG 2 Coolant ΔT/in. vs. Energy Density (Coolant 75 F @ 1 GPM) FIG 3 Coolant ΔT/in. vs. Energy Density (Coolant 75 F @ 1 GPM) FIG 4 Energy Density vs. Coolant ΔT/in. (Consolidated Round Core Baffle and DME Square 0.44 in. Diameter 75 F Coolant) In graphing the relationship between Energy Density and coolant ΔT/in. of flow, we plotted Energy Density on the horizontal axis and coolant ΔT/in. on the vertical axis of a typical X-Y graph. This time the plots from two very different types of cooling circuits showed a remarkably linear relationship. The Energy Density with the round core was much higher than the square mold, but the two trials showed a clear linear relationship between Energy Density and ΔT/in. 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0 BTU/hr/in.² Cooling Surface BTU/hr/in.² Cooling Surface Coolant ΔT/in., F Coolant ΔT/in., F 0 50 100 150 200 250 300 0 50 100 150 200 250 300 0 50 100 150 200 250 300 Here we used a trend line that provides a user-friendly tool for estimating ΔT/in. based on the easily calculated Energy Density value. For example, an Energy Density value of 80 BTU/hr/in.² would produce a ΔT/in. value of about 0.20 °F/in. 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0 1.80 1.60 1.40 1.20 1.00 0.80 0.60 0.40 0.20 0 Expanded studies used new data from our core simulator and standard mold base. The studies were conducted at four different heat inputs and four different coolant flow rates. These results confirm our belief that the Energy Density vs. ΔT/in. relationship is an important step forward in the pursuit of a science-based approach to cooling-circuit design. Linear (Consolidated Trendline 1.39 GPM) Linear (Consolidated Trendline 1.00 GPM) Linear (Consolidated Trendline 0.71 GPM) Linear (Consolidated Trendline 0.39 GPM) Energy Density, BTU/hr/in. 2 Coolant ΔT/in., F Round Core Linear (Square Mold Plate) Linear (Data Consolidation) @plastechmag 45 Plastics Technology M O L D C O O L I N G

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