Locating ring in injection molding

A locating ring is the round steel ring on the fixed half of an injection mold that fits into the center bore of the machine's stationary platen. It centers the mold on the platen, so that the mold's sprue bushing lines up with the machine nozzle. The locating ring locates the mold but does not hold it: that is the job of the mold clamps.

Where the locating ring sits

• On the mold: the ring is screwed to the outside of the mold's fixed clamping plate, around the sprue bushing. On many molds it also holds the sprue bushing in place.
• In the platen: the center bore of the stationary platen, highlighted in the drawing, receives the ring. Some machines have a replaceable centering ring or insert in this bore, so molds with different ring sizes can be used.
• On the moving side: some molds have a second locating ring on the ejector half that fits the center bore of the moving platen. On most machines that bore is used by the central ejector rod.

How the locating ring works

The ring's outside diameter is a close sliding fit in the platen bore. When the mold is hung in the machine, the ring is guided into the bore and the fixed half is brought flat against the platen face before the clamps are tightened. The bore is concentric with the nozzle, so the sprue bushing ends up on the nozzle axis.

This matters on every shot. The nozzle tip presses against the seat of the sprue bushing, and the two only seal if they meet squarely and on center. An off-center mold leads to:

• melt leaking between the nozzle and the sprue bushing and building up as a lump of plastic around the nozzle;
• a sprue that sticks in the bushing, because the nozzle orifice does not line up with the sprue opening;
• damage to the nozzle tip and the sprue bushing seat.

The nozzle guide explains how the nozzle radius and orifice have to match the sprue bushing.

Sizes and fit

There is no single locating ring size. The machine's data sheet gives the diameter of the stationary platen's center bore, and the mold's ring must match it with a small clearance, so it slides in without force. Mold component suppliers make locating rings in standard metric and inch diameters, and the size is usually given on the mold drawing. When a mold moves to another machine, check the ring diameter first: a ring that is too large will not go in, and one that is too small leaves the mold off center.

The mold clamping plate / locating ring we supply is made of 45# steel, black oxide or zinc-plated, and is listed for Haitian MA and MA II series machines.

Signs of a problem

• The mold will not go into the bore: wrong size, burrs on the ring or in the bore, or plastic and dirt in the bore.
• Plastic leaking at the nozzle, or a sticking sprue, after a mold change.
• A bent, dented or burred ring, often from a mold that was dropped or swung into the platen.
• Loose ring screws.
• A worn or damaged platen bore, or an injection unit that is no longer on center with the platen.

Checking nozzle alignment

1. Follow your setting procedure and the machine's safety instructions: the nozzle and sprue bushing are hot.
2. With the mold clamped, bring the nozzle forward slowly at low force against a piece of thin paper held on the sprue bushing seat.
3. Look at the print the nozzle leaves. It should be a full, even ring around the center of the seat.
4. If the print is off center and the locating ring and bore are correct, check the alignment of the injection unit and its carriage on the linear guide, as the manual describes.

Replacing a locating ring

1. Take the mold out of the machine, or reach the ring with the mold safely supported.
2. Remove the ring screws and take off the ring. Check whether it also retains the sprue bushing, and keep the bushing in place.
3. Clean the seat and inspect the sprue bushing.
4. Check the new ring's outside diameter against the platen bore, fit it, and tighten the screws evenly.

What to send for a replacement quote

Send the ring's outside diameter, thickness and inside diameter; the screw size and bolt circle; the diameter of the platen's center bore from the machine data; and the machine brand and model. A photo of the old ring helps. Request a quote, and see the platens guide for the other platen features.