Ring Valves

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Ring valves

Ring valves use a ring instead of a ball to provide shut-off (figure 8). The ring is located at the base of the valve and fits around the body of the valve. The diameter of the ring is slightly larger than the diameter of the valve body, allowing it to move slightly forward or backwards. When the ring is pushed forward, melt can move into the valve from the screw. When it is pushed back, it closes the opening. A shorter ring stroke (or the distance from the forward to the back position) usually provides greater shut-off consistency and shot control, but may also restrict the flow of the melt.

Figure 8
General purpose ring valves usually have deep, smooth flute areas which give them good free flow characteristics. A relatively short ring stroke provides fast and positive shut off. On average, this type of valve provides an approximate 2:1 compression ratio and will process about 90% of all materials.

As the melt moves forward in the screw (figure 9), it produces pressure which forces the ring forward, opening the valve. This allows the plastic to flow into the valve (through the space between the body of the valve and the ring) and then out of the valve (through openings in the flutes) into the melt chamber. At the end of the cycle, as the screw injects the plastic into the mold (figure 10), the backward pressure of the plastic forces the ring back against the valve seat to shut off the opening to the screw.

Figure 9
As the melt moves forward in the screw, it produces pressure which forces the ring forward, opening the valve. This allows the plastic to flow into the valve (through the space between the body of the valve and the ring) and then out of the valve (through openings in the flutes) into the melt chamber.

Figure 10
At the end of the cycle, as the screw injects the plastic into the mold, the backward pressure of the plastic forces the ring back against the valve seat to shut off the opening to the screw.

Advantages of a Ring valve
As the melt flows into a ring valve, it moves in a relatively straight flow line, first into the space between the body of the valve and the ring and then into the flute area where it is discharged into the melt chamber. This relatively smooth flow line produces less compression and friction than that produced by a ball check valve, making it a better choice for materials that are heat or shear sensitive.

The smooth flow lines of a ring-valve usually cause few material hang-ups within the valve, meaning it is usually more self cleaning in process than a ball check valve. It is also easier to clean once removed from the machine.

Because the ring section of a ring valve ‘free floats’, and does not spin along with the screw, a ring valve is less likely than a ball check valve to produce gauling against the sides of the barrel.

Disadvantages of a Ring valve
In most non return valves, it is the backward pressure of the melt against either the ring or the ball that closes off the valve. Because the seating area of a ring valve is larger, flatter and not as well defined as that of a ball check valve, the closure of a ring valve is often not quite as quick or as positive as a ball check valve.

A ring valve is often more susceptible to wear than a ball check valve. In a ring valve, the body of the valve spins with the screw while the ring free floats. This creates metal to metal, or what is known as adhesive wear in the area at the base of the flute where the ring rides against the flute. Approximately 90% of ring valve wear occurs in this area. Abrasive wear, or wear from abrasive materials such as glass fillers etc., accounts for the remaining wear.

There are however, many different styles of ring valve to choose from, and by choosing wisely, a ring valve can usually be found that will provide satisfactory shut-off and shot control for almost any type of material.

3, 4, 5-piece valves - Free flow valves