Plastics Technology

JUN 2013

Plastics Technology - Dedicated to improving Plastics Processing.

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tips and techniques Measuring the Magic of turbulent-Flow Mold cooling engineers have long understood the benefts of turbulent fow as it applies to heat transfer, and for decades molding experts have talked about the importance of achieving turbulent fow in moldcooling circuits. But what is it about turbulence that makes it so important? What can be measured and demonstrated to have some tangible value to molders? As manufacturers of cooling-related products in this industry, we have a natural curiosity about these questions. Several months ago we began a quest to fnd some answers. We elected to do our research with a test mold equipped with electric cartridge heaters. We chose this method because it is much easier and less costly to vary an electric heat source to simulate injection molding than to use an actual press shooting hundreds of pounds of resin. As any experienced molder knows, it can take hours for the mold temperature to stabilize after a process change. Our test mold setup provided a convenient and accurate way to control the variables and replicate the thermal responses of the big hunk of steel in a molding press that we call a mold. Thus, it is a much more user-friendly research tool. The test mold (see Fig. 1) is of a size and shape that resembles a small tool that might run in a 100-150 ton press. Two 750W heater cartridges powered by a variable AC voltage source can be cranked up to about 5100 BTU/hr—roughly the equivalent of molding 15.4 lb/hr of nylon 6 at 570 F melt temperature. The test mold has two 7/16-in. ID cross-drilled, U-shaped cooling loops of different lengths. We can use either or both cooling circuits and they can be plumbed in parallel or series. The inside walls of these circuits were clean and smooth. A temperature sensor was placed strategically between the heaters and the cooling loops to study the temperature response of the steel to variations in cooling conditions for a given heat input. Coolant fow and temperature were monitored and recorded to a suitable level of accuracy. The temperature change (ΔT) of the coolant moving through the mold becomes small as fow increases so those temperature measurements must be very accurate and repeatable. Water inlet and outlet temperature were measured with a matched pair of laboratory-grade RTD sensors and readers that produce results consistent within 0.1° C. Flow measurements were made with our NIST-traceable fow calibration system, more accurate than necessary, but convenient for us to use. To date, we have conducted experiments with a single cooling 22 june 2013 Plastics technology FIG. 1 The Test Mold Cooling Circuit 1 inlet outlet Cooling Circuit 2 inlet outlet steel temperaure measured here midway between heaters and cooling circuits electric heater cartridges The test mold used in Burger & Brown's research into the benefts of turbulent-fow mold cooling is of a size and shape that that might run in a 100-150 ton press. FIG. 2 Calculating Temperature Change Of Coolant Through the Mold Btu/gal vs. Flow Rate .44-in. iD cooling circuit Water @ 66F 160 heat Removed, Btu/gal By Philip M. Burger, Burger & Brown Engineering 140 t-Flow transition 120 Power 100 80 60 40 20 0 0 0.25 0.5 0.75 1 Flow Rate, gPM 1.25 1.5 Data collected in numerous trials allowed Burger & Brown to calculate accurately the temperature change (ΔT) of the coolant as it passes through the mold. Knowing ΔT permitted calculation of the BTUs removed by each gallon of water. BTU/gal was plotted against fow rate. This chart shows clearly that BTU/gal decreases as fow rate increases, even as the fow transitions to turbulent (red vertical line).

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