What You Need to Know About Locating Rings: Part 1

The outside diameter of an American standard locating ring is typically 3.990 in. Smaller ones or rings mounted in a counter-bore that is larger than 3.991 in. can also cause misalignment.

There are several different types of locating rings. Almost all of them are mounted in a 7/32-in.-deep counter-bore in the top or injection clamp plate. The basic or “standard” type of locating ring can properly align a mold to a platen, but it is not user-friendly, especially on large molds. It only extends beyond the face of the clamp plate by 5/16 in. The “LN” version, which is specifically for the old Lester and Newbury machines, extends only 7/32 in. If you factor in the 0.100-in. chamfer on the front face of the rings, there’s not a lot of “land” or bearing surface holding the ring in the platen-alignment hole.

length required to hold the ring in the platen. In this case, you’ll need a taller locating ring designed specifically for an insulator sheet. Unfortunately, very few suppliers offer this type of ring with a tapered lead-in to help guide the mold into the platen orifice.

Another type of locating ring is the “clamp” type. The inside diameter of this type of ring is stepped down to 13/4 in. to retain the sprue bushing and prevent it from “blowing back.” Bushings are usually bolted to the mold base with one or two 1/4-20 socket-head cap screws through the bushing’s shoulder. The clamp-type locating ring eliminates the need for this extra machining step, and there are no exposed screws on the face of the bushing to fill up with plastic.

However, you will need to add a dowel pin, key, or other alignment method when the bushing needs to be oriented due to the runner or part geometry. If the sprue bushing’s orientation is critical, it is best to key the bushing to the mold base—not the locating ring. Most locating rings are retained with socket-head cap screws. The screws have clearance around their head and body, which allows the locating ring (and sprue bushing) to rotate as much as one or more degrees.

The best type of locating ring to use is the flanged or “platen-backed” type. It can provide three additional advantages that the other types of rings cannot. First, since the screws are mounted outboard of the 3.990 in. diam., it’s almost impossible to fill up the heads of the mounting bolts or the threads of the jackscrew holes with plastic. Second, they provide a little more support to the center of the mold because the flange butts up against the platen. Third, you can add shim to the underside of the flange to compensate for a worn or “dished” platen. Depending on how much shim is used, a flanged locating ring can actually preload the center of the mold to help prevent parting-line flash. How important is that! If your platen is dished and you try to process around this flash, your process window becomes less robust. If you do not process around this flash, your parting-line will eventually hob.

Most locating rings have an inside diam. of 2 to 23/8 in. If the sprue bushing is recessed in the mold, you will not be able to seat the machine nozzle tip against the bushing without ripping the