It's no secret that one of the common, everyday obstacles to achieving the desired part-to-part consistency is the behavior of the typical existing non-return valves - ring and ball-type check valves. As any molder can testify, these screw tips do not always close immediately at the start of injection - or even close at all on some shots.
Ring and ball valves are closed by the action of melt flow through the valve. That is, when forward movement of the screw is initiated to start injection, some backflow of melt through the valve occurs, causing a pressure drop on the upstream side of the sliding or floating member. This causes the valve to close, assuming that the seats are aligned and clear of foreign matter and that valve wear does not cause leakage through sealing surfaces. Screw pull back is frequently used to provide ample opportunity for the valve to close. However, the R. Dray Repeater valve, closes without either any melt flow through the valve or screw movement.
How it Works
Unlike traditional ring and ball check valves, the Repeater closes without melt flow through the valve body or screw movement. As seen in Figure 1 & 2 this valve has a central piston with a larger surface area on the downstream end than on the upstream end. This piston moves freely under the influence of differential pressure on the two ends.
If the pressure on the larger (downstream) end (P1) multiplied by the piston area (A1) is different from the pressure on the smaller (upstream) end (P2) times the piston area (A2), then there will be a net force tending to move the piston in one direction or the other.
P2 x A2 > P1 x A1
After screw rotation ceases and backpressure is shut off, pressure will decay gradually on both ends of the piston. Since A1 > A2, when P1 = P2 then:
P1 x A1 > P2 x A2
Consequently, the valve will close.
With the Repeater, it is possible to "capture" the full, exact amount of melt metered by the screw. This is done by preclosing the valve prior to injection. All that is needed is to maintain hydraulic backpressure on the screw for one or two seconds after screw rotation ends. That will genearate a higher pressure on the downstream end of the valve, immediately forcing it closed.
Some degree of screw pullback prior to injection may be used with the Repeater without causing the valve to reopen, provided that the pullback stroke does not exceed the overstroke of the valve piston. Even if the valve is not preclosed prior to inject, it should be apparent that the valve will close very quickly upon initiating the forward stroke of injection. Since the length of the piston travel necessary to close the valve is very small, the time reuqired to close the valve, and thus the amount of backflow leakage will be far less than with other valves.
Preclosing the valve via pressure differential ensures shot-to-shot repeatability. Unlike traditional valves that close as a function of backflow leakage, the Repeater consistently provides accurate shots. Even when pullback is used the Repeater will close much faster and more consistently than traditional rear seat closing ring valves and ball valves. The sliding closure and short distance-to-close ensures fast, positive closure.
The repeater's closure-by-pressure feature is unique, as backflow is no longer an inherent and necessary aspect of valve closure. With preclosing the Repeater prior to injection, backflow can be virtually eliminated. Also, wear is no longer a threat to consistent valve closing. The only moving part is the internal piston, and its sliding surfaces are not contacted by plastic flow. In other valves, sealing surfaces are constantly in contact with polymer flow. With abrasive filled materials, erosion of those surfaces does affect valve performance. Also, sealing areas in ring and ball valves are constantly exposed to any contaminants in the melt stream that may hinder effective valve seating.