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moldmakingtechnology.com 25 fills the cavity can impact the dimensions, cosmetics and functional strength of the part. However, as important as this filling of a cavity is, the molder can't actually see this part of the process. Properly positioned cavity sensors can help solve this dilemma. The most basic use of these cavity sensors is monitoring the process. The time the melt takes to reach the sensors, as well as the resulting pressure and/or temperature curves, can serve as a reference point for a solid process. Combining pressure and temperature sensors into a single cavity enables oth- erwise unmeasurable variables such as shear rate, shear stress and viscosity to be calculated, and this data can serve as reference points for troubleshooting or dialing-in a process when a mold is moved to a different molding machine. Cavity sensors also can be used for statistical process control to deter- mine the rejection of a given shot or cavity. Instead of relying on feedback from the injection molding machine to determine if a cycle was successful, the molder can look to the data related to cavity fill provided by the sensors, which is a more accurate indicator of a successful shot. Depending on the application, this can reduce or elimi- nate part inspection while still ensuring that parts meet specifications. A final application for sensor data is as a trigger for other equipment. For example, information from the cav- ity sensors can be used to transfer the injection molding machine from the injection phase to the holding phase, to adjust hot runner temperatures to achieve balance, and to signal when it is time to open or close a pin in a valve- gated hot runner system. Valve Gate Hot Runner Technology Traditional valve gate hot runners control the flow of melted plastic into the mold cavity by simply opening and closing the valve pin on the nozzle. With these systems, the user initiates a pin move based on either time, screw position or a signal from a cavity sen- sor. Once a pin move is initiated, the molder has no control over the speed or distance of the pin movement. Recent technological advances now enable that control over the speed and stroke of the valve pin. Controlling the valve pin movement. In producing large, sequentially filled parts, pressure builds up behind the delayed valve pins. The abrupt opening of these delayed pins can cause sudden accelerations in melt-front velocity near the gates and stagnation away from the gates, often resulting in cosmetic defects on the parts. Slowing the initial opening speed of the valve pins on the delayed gates can reduce the