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Mold Design 20 MoldMaking Technology —— MARCH 2018 By Rocky Huber This polycarbonate part with an L/T (length-to-thickness) ratio of less than 65 and a final wall thickness of 0.027 of an inch (0.68 millimeter) was successfully injection-molded using a standard-sized bubbler. Figures courtesy of DZynSource. T he plastics industry has been incorrectly sizing water- lines for years. This is most prevalent in mold designs that use bubblers to deliver the cooling medium. In the equivalent hydraulic diameter method (DH = 4A/P), "dh" is the hydraulic diameter, "A" is the area section of the passage, and "P" is the perimeter of the passage. Using this method to deter- mine the correct bubbler size to maximize coolant flow rates increases water flow, reduces clogging and decreases cycle time. However, it also can have some unexpected, negative results. Here are five reasons not to maximize coolant flow rates, especially when using bubbler tubes to cool the cores. 1. Incomplete cavity fill. When sizing bubblers using the equivalent hydraulic diameter method, the cooling is so drastically increased that thin sections of the part may freeze before the cavity filling is complete. Figure 1 shows an example of a polycarbonate part with an L/T (length-to-thickness) ratio of less than 65 and a final wall thickness of 0.027 of an inch (0.68 millimeter). The moldmaker heated the mold for this part to 160–200°F and injected the polycarbonate at pressures up to 40,000 psi (pounds per square inch). Also, the part was valve-gated at the top flange. When the moldmaker installed an optimized smaller bubbler, the part froze prematurely and could not be filled. The optimized bubbler allowed fluid to pass through the bubbler/core circuit at a rate of 5 gallons per minute. The part was injection-molded successfully using a stan- dard-sized bubbler that gave equal area inside and outside of the bubbler. The standardized bubbler also greatly reduced the flow rate to 0.7 gallons per minute. The problem was not temperature. It was heat transfer. 2. Wide variation in shrinkage rates. In parts with thick and thin sections, there will be a wider difference in shrink- age rates. The thin sections of a plastic part cool very rap- idly because of increased heat transfer that occurs when using bubblers sized with the equivalent hydraulic diameter Unintended Consequences of Maximized Cooling Here are five reasons not to maximize coolant flow rates with cores that are cooled by bubbler tubes. method. The thick sections cool faster too, but the thin sec- tions freeze so quickly that the expected shrinkage rate is not achieved. As a result, there may be very little shrinkage in the part's thinner sections (see Figure 2). 3. Excessive flow rates. Larger parts that permit more room for increased cooling channel sizes can have excessive coolant flow in the mold. It is important for moldmakers to calculate the appropriate amount of cooling when incor- porating the cooling system into the mold design. It can be expensive and wasteful to exceed the amount of cooling that is required. FIGURE 1