Nonreturn check ring

As plastic is pushed forward (overcomingbackpressure) the screw is forced to retract. There is now a cylinder of plastic in front of the screw tip and nonreturn valve. Upon injection, the screw moves forward seating the nonreturn valve or check ring, which acts like a plimger, piston, or ram to push the plastic out the nozzle tip into the sprue hushing of the mold. 

Figure 5.60 Nonreturn valves (a) sliding ring/ (b) ball check valve/ and (c) smear tip. ...

The complexity of injection molding equipment is illustrated with the choice of nonreturn valves which prevent flow of melt back to the screw. There are two basic types ball check, and ring check, and their variations. Valve selection extends further, to include general-purpose three-piece, four-piece, and five-piece valves free-flow valves castle valves smear valves and spring (poppet) valves [12]. 

Because the mold is usually maintained at temperatures below the melting point of the resin, corrosion on the mold surface is less than in the molding machine. Nonreturn ball check valves and ring check valves are used the latter is preferred for PFA. A streamlined flow must pass through the valve, preventing areas of stagnant flow or holdup and localized degradation. 

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