Choosing a Non-Return Valve

Types of Non-Return Valves: Ring Valve Variations

There are many different types of ring valves available, each with different characteristics, advantages and disadvantages. Choosing the correct valve for the job will depend mostly on the type of materials being processed, but there are other factors to consider as well, such as how often color changes will be made, how important it is to not interrupt production etc.

General Purpose
Free-Flow Valves
Castle Valves
Carbide Tips

General purpose, 3-piece, 4-piece, 5-piece valves

General purpose ring valves usually have moderately deep flute areas and open flow paths. Compression ratios are higher than a free-flow, but shut off is usually more positive. This type of valve design is usually an effective compromise for processing most of today’s engineering grade resins. A high quality, general purpose valve is often capable of providing satisfactory performance in over 90% of all material applications.

Things to look for in a general purpose ring valve:

Smooth, unencumbered flow surfaces - this makes color changes easier and creates less chance for shear damage
Low compression ratios - 2:1 is an excellent number to shoot for and will allow most shear sensitive materials to be processed successfully
Short ring stroke - the shorter the ring stroke, usually the quicker the shut off and the better the shot control will be

Figure 10.
Many molders prefer a 4 or 5 piece valve such as this because the areas of highest wear (usually the flute piece, the ring or the seat) can be replaced without having to replace the whole valve.

Should I choose a 3- piece, 4-piece or 5-piece Valve?

A 3-piece ring-valve design incorporates a body, a ring and a seat. The body of the valve (which includes the flute area) screws into the machine screw and turns as it turns. The seat fits tightly around the body of the valve and provides a shut off point between the ring and the screw. The ring floats loosely around the body of the valve. Because most of the wear occurs between the flute area (which is turning rapidly along with the screw) and the ring (which is not turning), some valves are constructed in 4 or 5 pieces (figure 11) which allows only those pieces which wear to be replaced. However, because the entire valve must be removed from the screw to replace the worn pieces, changing these pieces can often be both costly and time consuming.

Free-Flow Valves

EMI's 100% Free Flow valve is an excellent choice when running shear sensative or fiber filled materials.

Free flow valves have very low compression ratios. A 1:1 compression ratio (which would be 100% free flow) means that the material will experience no compression as it flows through the valve (a 2:1 compression ratio would exert twice the compression etc.). This means that as the material leaves the screw and enters into the valve, the area it is entering into is equal to the area it is leaving, with no restrictions or areas of compression. In the design of a free flow valve it is important that there are no reductions in the aggregate cross sectional areas of the valve’s flow paths.

Advantages of a free flow valve
A free flow valve is an excellent choice for shear sensitive materials, materials with a relatively high viscosity or fiber filled materials. Because there are fewer places for material to become trapped, it is also an excellent choice when material changes need to be made often.

Disadvantages of a free flow valve
While the free flow characteristics of this type of valve allow the melt to flow easily forward, it also allows it to flow easily backwards. Because a free flow ring valve has a relatively longer ring stroke than a general purpose ring valve, it takes longer for shut-off to occur. In some applications, especially when the shot size is less than 30% of the machine’s shot capacity, this can reduce it’s overall shut-off consistency.

Castle Valves

The ring in a castle valve has cut outs (like the top of a castle turret) that fit into the spaces made by the indentations of the flutes. Although the ring moves forward with the melt and back to shut-off, it is always interlocked with the flute. This causes it to spin with the flute, eliminating the wear against the ring and the flute. While this can minimize wear within the valve, the spinning ring can cause wear to the inside of the barrel. Repairing this type of damage can often be much more costly than replacing the valve.

Carbide Wear-Resistant Valves

Since the area of highest wear in a ring valve is usually between the ring and the base of the flutes, some manufactures will imbed extremely durable, carbide inserts at the base of each flute (figure 12). As the valve spins against the ring, the inserts protect the flute area. This is an excellent choice when abrasive materials are often run or for any operation that is looking for a highly effective extended wear valve. Also available are carbide-faced valves. These valves have a thin carbide coating applied to the valve’s wear surfaces (between the base of the flute and the ring, or between the seat and the ring) to protect these areas from wear.

Next: Types of Non-Return Valves: Spring and Smear Valves