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Mold Design 22 MoldMaking Technology —— APRIL 2017 By Rocky Huber Get More Coolant Flow Through Smaller Bubblers This method for calculating the size of nonround water passages is designed to increase coolant flow, and reduce pressure loss and cycle time. M ost mold designers are familiar with design- ing a parallel arrangement of water lines, whether when designing water manifolds or circuits to feed bubblers, such as in Figure 1. It's a pretty straightforward process: The area of the drilled sublines is calculated, and then a feeding line size is chosen that has at least as much area as the total area of the lines being fed. However, what if the channels to be fed are not round? Some examples would be a cooling chan- nel around a round cavity or a bubbler residing in a drilled hole. Most moldmakers and mold design- ers have been taught that the cooling channel area should be equal to or smaller than the area of the hole that feeds it, regardless of the shape. This works for round channels, but problems arise when trying to design non-round passages, including annular ones. Here, I will present an alternative method for calculating proper non-round channel sizes to optimize coolant flow with minimal pressure losses. Hydraulic Diameter While reading fluid mechanics books for writing software, I stumbled upon the equivalent hydraulic diameter method for determining water channel sizes. Those in the heating, ventilation and air conditioning trades have been using this calculation for years to size ventilation channels and duct work, but I have never seen it used in plastics or tooling. This method is only used on nonround shapes to calculate the size that an equivalent round passage would be, based on a ratio of the area of a nonround passage's cross section to that pas- sage's perimeter. When designing nonround passages, the hydraulic diameter is also substituted for diameter "d" in equations used to deter- mine laminar or turbulent flow and pressure losses, and to Figures courtesy of DZynSource LLC. Most mold designers are familiar with a parallel arrangement of water lines to feed bubblers. FIGURE 1 define friction factor and relative roughness. Fortunately, the area and perimeter of the shapes in question usually no longer need to be calculated manually, as that information can usu- ally be extracted from today's CAD software. More elaborate methods for fluids calculations exist, but the hydraulic diameter equation can provide reasonable results accurately and quickly. The exact equation is: dh = 4 A / p (in which "dh" is the hydraulic diameter, "A" is the area section of the passage, and "p" is the perimeter of the passage). The application is always the same, regardless of whether the shape is round, square or irregular: Round. If we test this equation on a round passage (see Figure 2) in which "π" represents the ratio of a circle's circum- ference to its diameter and "D" is its actual diameter, we get: dh = 4 A / p A = π D 2 / 4 p = π D so dh = ((4 (π D 2 / 4)) / π D).