Plastics Technology

OCT 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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One of the most important of the hundreds of details involved in the injection molding process is the non-return valve, or check ring. There are various versions: three- piece, four-piece, full-flow, ball-check, etc. In more recent years, sliding- and locking-ring types have been added to the list. Which one is right for you? They all have their issues, and the focus here is on the issues between sliding- vs. locking- ring types. The terms "non-return valve" and "check ring" will be used inter- changeably, for both terms are common in the injection molding community. First, what is the purpose of the check ring and how does it work? The purpose of the non-return valve is to ensure molten plastic flows in only one direction. It is a one-way valve. It is supposed to act like a plunger seal in a syringe. Its function is two-fold: During screw rotation, plastic melt is pumped through the non-return valve by the metering flights of the screw. Molten plastic is forced through the non- return valve to develop the appropriate shot size. The pressure required to do this is determined by the backpressure set on the machine. Backpressures are typically 500 to 750 psi (34–52 bar) plastic pressure for non-filled resins. For glass-filled or resins sensitive to degradation, lower backpressures are recommended. Once the shot size is established and the mold is closed, nozzle-contact force is developed and injection of the plastic Sliding vs. Locking Ring: Which Non-Return Valve Is Right for You? begins. As the screw starts moving forward, the non-return valve seat mates with the ring to make a seal to prevent molten plastic from flowing back over the flights of the screw. The pressure on the check ring and seat can be 20,000 to 45,000 psi (1380–3102 bar) as plastic is injected into a thin-walled part. Two seals must be made— one between the ring and the seat of the valve and another between the ring and the barrel wall. The seal between the ring and the barrel wall is accomplished by having a very close fit or tolerance between the ring and the wall. The seal between the ring and its seat depends on perfect mating between the ring and seat. The molder does not want any plastic to backflow. Sounds simple enough, but under high pressures and normal molding conditions there can be some major problems. How well the check ring functions to prevent backflow determines the consistency of shot size and cushion. The locking-ring style appears to dominate the market, as most believe it makes a make a better seal and leaks less. But is this really so? Get more insights on Injection Molding from our expert authors: Learn more at KNOW HOW INJECTION MOLDING With a sliding-ring check valve, the ring does not rotate with the screw during plastication or screw recovery. It moves backward during screw rotation and forward during injection. FIG 2 Locking-Type Check Valve In locking-ring check valves, the ring rotates with the screw during plastication and moves horizontally during injection. FIG 1 Sliding-Ring Check Valve Locking Tooth 28 OCTOBER 2018 Plastics Technology K now How By John Bozzelli INJECTION MOLDING

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