Plastics Technology

JUN 2013

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mold cooling FIG. 3 the Value Curve steel temp vs. Flow 130 t abl 110 uns 115 mp 120 Predicted t-Flow l te e steel temp, F 125 s te e circuit and with two circuits plumbed both in parallel and in series. The coolant is local tap water with treatments to control pH and biological agents. Trial runs were conducted at a fxed power input while coolant fow was varied from substantially below to well above the turbulent range. After each change in fow rate, suffcient time was allowed for the steel temperature to stabilize, sometimes taking as long as 3 hr. Coolant inlet and outlet temperatures, fow rate, and steel temperature were all recorded throughout the fow range. The data collected in our numerous trials allowed us to calculate accurately the temperature change (ΔT) of the coolant as it passes through the mold. Knowing ΔT we could calculate the BTUs removed by each gallon of water. We plotted BTU/gal against fow rate (see Fig. 2). This chart shows clearly that BTU/ gal decreases as fow-rate increases, even as the fow transitions to turbulent. We were surprised by this result, but the math shows us that we should have expected it. Heat transfer is based simply on the increase in water temperature: BTU/gal = ΔT x (1 BTU/lb-°F) x 8.3 lb/gal The faster the water goes through the mold, the less the temperature increases. This result leaves us still wondering: Where is the "magic" of turbulence? We believe our data sheds some light, but we had to think in a different way. But before we get to that, let's back up and think about what molding cooling is really about. What is it that we really need to accomplish? The real objective is to control the cooling rate and temperature of the parts so they can be ejected at the earliest possible time, while maintaining the desired properties and dimensions. Today's process monitoring and control technology is highly advanced and sophisticated, but to imagine directly controlling part temperature in the mold is still science fction. So we do the next best thing—we try to control the temperature of the mold, thereby indirectly controlling part cooling. Now let's think of the mold as a very strange kind of heat exchanger. Molten plastic supplies heat to the mold and the coolant takes the heat away. However, there are some key differences between our mold/heat exchanger and a conventional heat exchanger. One is that we are trying to control the temperature of the body of our heat exchanger (the mold). Two, our heat exchanger is a large mass of metal with a very slow temperature response. Think of it as a big thermal fywheel. Any "normal" heat exchanger has certain performance characteristics related to inlet and outlet conditions for both fuids. The heat-transfer performance of a mold is revealed by plotting steel temperature (measured at a strategic point) vs. coolant fow rate (Fig. 3). It is true that there are endless sizes of molds with endless different heat inputs and cooling schemes, and each one will have its own unique response to the thermal inputs and cooling system. For each one it would be possible to create a graph of mold temperature (measured at a meaningful point) vs. coolant fow rate— a heat-exchanger performance curve. We believe this curve shows us something interesting about turbulence and effcient mold cooling. For that reason we decided to call it the "Value Curve." Here is what we believe the Value Curve tells us: The vertical 105 100 95 0.00 stable & effcient Wasted cap ac 0.50 1.00 cooling Flow Rate, gPM ity 1.50 Heat-transfer performance of a mold is revealed by plotting steel temperature (measured at a strategic point) vs. coolant fow rate. This has been dubbed the Value Curve. At fow values well below turbulence, steel temperature is less stable, changing signifcantly with small changes in fow rate. But after the transition to turbulent fow the curve fattens out and steel temperature changes little with increasing fow—in other words, it becomes more stable. axis represents steel temperature and the horizontal axis represents water fow rate. The distinctive L-shaped curve is defned by low fow and a nearly vertical leg at the left and by higher fow toward the right as the curve becomes more horizontal. Note that the transition to a more horizontal shape occurs around the predicted turbulent fow rate. The steep curve shows a large change in steel temperature resulting from a very small change in fow rate. The more horizontal portion of the curve shows a range where steel temperature changes only slightly as fow increases substantially. You could say that at fow values well below turbulence, steel temperature is less stable, changing signifcantly with small changes in fow rate. In contrast, after the transition to turbulent fow the curve fattens out and steel temperature changes little with increasing fow—in other words, it becomes more stable. Increasing fow to well above turbulence yields little additional beneft and is costly in terms of fow capacity. Thus, the Value Curve suggests that operation with coolant fow slightly above the turbulent rate offers stable and economical operation and that substantially higher coolant fow might be considered wasteful. about the author Phil Burger founded Burger & Brown engineering in 1978 and served as president until succeeded by Mark a. Brown in 2005. Based in grandview, Mo., 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 frm developing an educational program called scientifc cooling that will be offered in the Fall of 2013. contact: (816) 878-6675; email: trumpetman44@hotmail.com; web: burgereng.com Plastics technology june 2013 23

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