Advertising in MoldMaking Technology offers
Issue link: https://mmt.epubxp.com/i/802173
Mold Design 24 MoldMaking Technology —— APRIL 2017 When Science Works Here is an example of the application of the equiva- lent hydraulic diameter method in mold design: A customer's mold had a parallel water design feed- ing eight cores/bubblers in a row. The water en- tered at a lower level, flowed up inside the bubbler tube and down the outside, returning to the upper- level water line and out of the mold. The mold was originally built with a standard-sized, equal-area- based drilled hole and bubbler. The hole in each core was 0.093 inch (3/32), and the high flow tube had a 0.072-inch outside diam- eter with a 0.060-inch inside diameter. During the mold's first run, only 0.5 gallons per minute flowed through the row of eight cores. The pressure on the feed side was 85 psi, and the pressure on the outlet side was 5 psi. This water circuit was experiencing a 75-psi pressure loss to get a half-gallon per min- ute of water flow through the circuit. The core's drilled hole could not be enlargered any further, so the bubbler tubes were redesigned by cutting the existing 0.072-inch-diameter bubbler until it was in the larger relief diameter behind the 0.094-inch drilled hole. Next, a bubbler with an 0.059-inch outer diameter was welded inside of the 0.072-inch bubbler, which resulted in a smaller bubbler inside the 0.094-inch drilled hole. When reconnected to the water source, the new flow rate in- creased to 6 gallons per minute with only a 20-psi pressure loss. Using a smaller bubbler yielded more flow with less pressure loss. Flow Outside of a Bubbler Tube The hydraulic diameter of the area between the outside of a bubbler and the inside of a drilled hole is simply the drilled-hole diameter minus the bubbler outside diameter. Consequently, the drilled-hole diameter that provides an equivalent hydraulic diameter outside the bubbler can be cal- culated by adding the bubbler's inside and outside diameters. In one example, a supplier-published chart shows a bubbler with an outside diameter of 0.437 inch, an inside diameter of 0.307 inch and a recommended channel size of 0.531 inch. These sizes are based on what we have all been taught about "the areas being equal inside and outside the bubbler." In order to have the annular channel's hydraulic diameter be equivalent to the inside diameter of the bubbler, the hole would have to be 0.737 inch in diameter (0.531 + 0.307). This is especially important with smaller bubblers that are prone to clogging. The concept of hydraulic diameter may seem counter- intuitive, as you can get more fluid flow and less pressure drop with a smaller bubbler when the hole size is kept constant (if it was calculated by the equivalent area method). This is because the excessive pressure loss is caused by the restriction of flow outside the bubbler when using the equivalent area method to size the holes. I have actually increased water flow and reduced cycle time in existing molds by moving to a smaller bubbler when the hole size must be kept constant. Simple Solutions Some suppliers offer high-flow tubes for water bubblers. These are thin-wall tubes with an integral thread available in two dif- ferent sizes for each tube diameter. There are high-flow tubes with a larger tube for each thread and ones with a smaller tube for the same thread. If you check your bubbler and hole sizes, and discover that your mold was designed with these parts sized using the equivalent area method and that your mold has the high flow tube with the larger tube for the given thread size, simply make a new bubbler with the smaller tube version. This will immediately increase flow rate, decrease pressure drop and potentially reduce cycle time. (Keep in mind that this may not always work, however, as there are many others parameters that could be controlling cycle time.) The bottom line is that any non-round water channel has a higher-pressure-loss and Reynolds number than its area would indicate. A solution is to use the hydraulic diameter equation to design larger non-round channels. The annular channel is the area between the hole diameter "D" and the outside of the bubbler "d." The formula for hydraulic diameter is dh = (D - d). FIGURE 4 FOR MORE INFORMATION DZynSource LLC / rhuber@dzynsource.com Ivanhoe Tool & Die / rhuber@ivanhoetool.com CONTRIBUTOR Rocky Huber is an engineering manager for Ivanhoe Tool & Die Co. LLC and owner of DZynSource LLC.