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24 MoldMaking Technology May 2015 Pellet 2 Part (P2P) 1 2 3 4 5 7 6 8 Image courtesy of KraussMaffei. H 2 O By Rich Oles A mold is essentially a heat exchanger—a cooling fixture for the molded part that must be engineered by the moldmaker for the lowest cost of ownership, to produce the highest quality part, within specification, and to provide the largest processing window for the process engineer. The mold's water system is essential to these tasks. Here are some considerations for designing a mold water system. Establish a process. If the water system is not designed correctly, the molder is less likely to have a consistent process on which to rely every time. The goal is to establish the correct molding process during PPAP. If a process change is required during pro- duction, stop. Find the root cause and fix it before changing anything else. Keeping the original process setup sheet for the life of the program should be the standard. It is common to target a mold-build budget to receive the purchase order. Unfortunately, this means features will be cut from the build plan and the right water system can be com- promised. It is a challenge to communicate the value of the correct water system (and other performance features) through the molder's procurement team to the end customer. But it is important to understand how investing a little more on mold engineering will pay back dividends over the life of the pro- gram. For example, water cooling is one mold feature whose impact is often minimized, but it actually has the potential to improve quality and cycle time for the least amount of expense and time. Understand the fundamentals. Engineering a water system capable of moving more energy than the resin can give up during the cooling phase will bring no return on investment. So it is important to first know the resin's rate of thermal dif- fusivity. Go further and educate yourself on industry resin standards related to mold steel temperatures, barrel heat profiles and drying, and pay close attention to water-line position and size related to part geometry, steel conditions and cycle-time targets. Also consider the targeted change in temperature, or delta T, of the water as it goes to and from the process. A target delta T of 1°F or less is recommended. Designing a Better Heat Exchanger In a three-part sub-series, we review considerations for mold water cooling systems. Achieving this requires a well-engineered water system, along with an overall process concept review that considers target- ed cycle time, periphial hardware available and water quality. In addition, you will want to engineer the maximum total surface area of water channel walls, because more surface area means more potential heat transfer into the water when This article reviews the mold water system and the steps for designing a better heat exchanger.