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20 MoldMaking Technology April 2015 Pellet 2 Part (P2P) 1 2 3 4 5 7 6 8 Image courtesy of KraussMaffei. H 2 O By Rich Oles In injection molding, energy management—thermal energy, not electricity—requires understanding how energy is applied and removed as a plastic pellet travels through the screw and hot runner system (in this case, the heated melt delivery system). Without this fundamental knowledge, a mold can be set up for failure during a molder's first sam- ple run. To avoid this outcome, the molder first needs to understand the body being heated and its contact to the world around it (heat sinks), and then the basic components of a conven- tional heating system: the energy source (heaters), body temperature sensors (thermocouples), power and control systems, hot runner controller, and cables and adapters. The energy source is a heater that can be sol- dered, clamped or pressed into position. It heats the vessel (the barrel or hot runner manifold system) containing the plastic. Problems can result from wiring, heat-sink and mois- ture issues. Heaters can be wired in series or in parallel, but parallel is the more common method. A heat sink is an engineered contact point used to mini- mize the energy loss in a hot runner system. It occurs where the heated hot runner makes contact with the mold. Once a heat sink is present, the system cannot heat up properly, which causes the power to cycle on and off more frequently than planned. As a result, the system struggles to reach the set-point temperature, which can impact performance and cause part defects. A heat sink can also prevent a thermocouple (TC), a temperature-measuring device, from receiving the signal that a nozzle or manifold has reached the set-point temperature for the system. This can cause an overrun and ultimately degrade the resin in the melt delivery system. Another TC issue is cross wiring the TC and heater in the barrel. This could lead to a TC for one heater controlling another heater, resulting in unplanned temperature overruns and underruns. TCs require proper location and wiring. When a particular TC type is connected to a cable or an adaptor made from a dissimilar material, the temperature may be read in the Controlling the Melt Delivery System Heat Source Understanding the heating systems along the melt delivery path is critical to setting up a robust process and properly diagnosing problems. wrong location. This can be extremely difficult to identify if the operator has no prior experience with these devices. When moisture is present inside the heater and full power is applied, the moisture reaches the boiling point (expand- ing at a rate greater than the heater sheath can withstand). This can result in a break in the heating element, commonly known as heater failure or burnout. To determine the appropriate operating temperature for the injection molding system, the molder uses a watt-to- mass calculation based on the cubic units of the body being heated, heater layout (including surface area contacting the body), heater wattage, TC placement, planned heat sinks and the maximum time it takes to get the body to a specific temperature. If not calculated properly at the beginning of a project, the body being heated will never reach the correct operating temperature. Power and control systems use the TC signal to control the power output and cycle. This is critical to keeping the resin at the planned temperatures throughout the melt delivery system. Modern molding machines use a human machine This article reviews key strategies for controlling the melt delivery system (4).