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Choosing a Non-Return Valve
Types of Non-Return Valves: Ball Check Valves
Figure 3
This exploded view of a front discharge ball check valve shows the insert assembly (which includes a ball, pin, and shell) removed from the body of the valve. |
A ball check valve uses a smooth, round ball within the valve to provide shut-off figure 3). As the plastic moves to the end of the screw, it pushes the ball away from the
opening allowing the plastic to flow into the valve, around the ball and then out into the melt chamber (figure 4). At the end of the cycle, as the screw injects the plastic
into the mold, the backward pressure of the plastic forces the ball back into the valve opening. This blocks the opening and prevents the plastic from flowing back to the screw
(figure 5). Different types of ball checks include hi-flow valves, front discharge valves or side discharge valves (figure 6).
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Figure 4.
During recovery phase of the cycle, the plastic (shown in green) forces the ball up against the retaining pin, permitting material to flow around the ball and into the
chamber ahead of the screw. |
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Figure 5.
During the recovery phase, the plastic moves to the end of the screw, it pushes the ball away from the opening allowing the plastic to flow into the valve, around the ball and into the melt chamber. |
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Advantages of a Ball Check Valve
A ball check valve incorporates a small, very well-defined area for the ball to seat against. This allows the valve to shut-off quickly and completely, providing
accurate and consistent shot control. It is an excellent choice for processing high volume, smooth flowing, basic plastics that are not easily damaged by heat or friction such as
polypropylene, polyethylene or polystyrene. For large barrel diameter machines, an even greater range of plastics can be processed successfully. In fact, many molders consider a ball
check, the best choice for large barrel diameter machines.
Cost wise, a removable insert ball check valve provides the added advantage of being able
to replace the ball insert assembly (which is where most wear occurs)
without having to replace the entire valve (figure
3). This can be
a real time saver over other types of valves because the body of a
ball check valve does not have to be removed to replace the ball insert
assembly.
A removable insert ball check valve also allows different size balls to be interchanged
for varying compressions and recovery rates of different materials.
A side discharge ball check valve channels the melt out through holes at the base of the cone. As the melt flows out, it flushes away any material trapped between the end
cone of the valve and the end cap and nozzle. This helps to self clean the assembly and make color changes easier. |
Figure 6.
The ball shut off in a side discharge valve is usually fast, accurate
and resistant to wear. A disadvantage is that the plastic must flow
through two 90 degree angles to exit the valve, which can be damaging
to some shear sensitive materials |
Disadvantages of a ball check valve
As plastic is forced through the sharp-angled flow paths of a ball check valve, compression and friction occurs. This creates additional heat within the valve, which may damage
shear sensitive materials, such as engineered plastics (figure 6). This type of valve may also not be the best choice for materials with fiber fillers because the fibers may bend
or break as they are forced through the sharp angles.
A ball check valve can also produce metal to metal friction as the relatively large area of the valve body rotates against the barrel of the machine. In applications
that run high screw RPM’s this could cause gauling to the inside of the barrel.
When using a front discharge ball check valve, the many surfaces within the valve, as well as the build up of plastic in the cushion area (which is the area
between the sides of the valve where it begins to taper and the tapered end of the barrel’s end cap), can often make color or material changes difficult.
A side discharge
ball check valve (figure
6 and 7) can provide self-cleaning of materials in this area,
but the added direction changes of the melt flow inside the valve
can create additional
heat producing compression and friction.
Hi-flow ball check valves
To extend the range of materials that can be processed without shear damage,
hi-flow ball check valves are available - some with compression ratios as low
as 1.5:1 (which is significantly less than many general purpose ring valves).
Because of their enlarged flow paths, these valves will process a wide range
of materials without shear damage while still providing superior shot control.
Hi-flow ball checks are also often an excellent choice for many high speed
molding operations.
Next: Types of Non-Return Valves: Ring Valves
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