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Last month's column reviewed the various standard types of locating rings available today. This month I will get into the details of their physical characteristics and precautions to avoid platen damage. Mold-component suppliers specify the steel type and hardness values of their ejector pins, leader pins, bushings, etc. They also specify the dimensions of these components with tolerances down to four decimal places. But, for some reason, many suppliers neglect to specify these same important attributes of their locating rings. Let's first look at the importance of the size and dimensional tolerances. Most stationary platens of an injection molding machine have an alignment hole with an inside diameter of 4 in. Many machine manufacturers tolerance that dimension to +0.002-0.000 in. This diameter is often larger with older machines that are damaged due to wear, abuse, or accidental attempts to lift the mold out of the press while the locating ring is still engaged. Nothing gets a supervisor's heart racing like watching a novice setup person accidentally lift a molding machine off the floor. Locating Rings and Platen Damage Corrosion is also an issue with both the locating ring and the machine platen. Most locating rings are made of mild carbon steel. Platens are made of cast iron. They are both subjected to the corrosive outgasses from various molding materials, espe- cially when purging, and they rust. Over time, their dimensions can be altered. The outside diameter of a standard locating ring on a mold is, or is supposed to be, 3.990 in. It is nearly impossible to perfectly center the locating ring in the alignment hole in the platen with a crane, A-frame, or forklift. Therefore, depending on how much tension is on the hoist, the locating ring is going to abut the top or bottom edge of the hole in the platen. As a result, the mold is verti- cally misaligned by at least the 0.005 in. clearance between the ring and the platen. A locating ring may be 3.990 in. at room temperature, but how many molds run at 72 F? The coefficient of thermal expansion for carbon steel is 6.7 × 10 -6 in./in.-°F. Therefore, the formula for the amount a standard locating ring expands or contracts due to temperature is: 3.990 × (Locating Ring Temperature – Ambient Temperature) × 6.7 × 10 -6 Once the molding process has stabilized, the locating ring is going to be about the same temperature as the object it's been mounted to—typically the top clamp plate. For cold-runner molds, locating-ring expansion is not an issue. In fact, any thermal expan- sion will help align the locating ring in the platen by reducing the amount of clearance. But if you have a mold with a hot sprue Nothing gets a supervisor's heart racing like watching a novice setup person accidentally lift a molding machine off the floor. Selecting the right locating ring and putting it to good use takes careful consideration. Get more insights on tooling from our expert authors: short.ptonline.com/toolingKH Learn more at PTonline.com KNOW HOW TOOLING By Jim Fattori PART 2 Locating- Ring Temperature, F Ambient Temperature, F Diameter at Ambient Temperature, in. Coefficient of Thermal Expansion, in./in.-°F Resulting Locating- Ring Diameter, in. Glycol Chiller 45 72 3.990 0.0000067 3.989 Tower 72 3.990 Mold TCU 125 3.991 Oil Heater 225 3.994 Hot Runner 450 4.000 Dimensional Change of Locating Rings at Various Temperatures 36 APRIL 2017 Plastics Technology PTonline.com K now How TOOLING