'Buy Metric Locating Rings At DME Today. Metric locating ring are locating rings that are measured in size by the metric system. A Locating Ring (Register Ring) is used to locate the mold on the stationary plate (injection side) assuring that the nozzle on the molding machine will interface with the sprue bushing allowing plastic to flow into the mold. There are many standard sizes. The most common is a 3.990 in. locating ring