Radical redesign of injection check valve solves repeatability problems.

Radical redesign of injection check valve solves repeatability problems

Reportedly eliminating one of the most common causes of inconsistency in injection molding is a unique new approach to the design of the check, or nonreturn, valve at the end of the screw. Unlike today's ball or sliding-ring valves, the new "Repeater" valve requires no motion of the screw or melt to actuate it; instead, it works by pressure differential alone, so its action is virtually instantaneous and allegedly fail-safe. The new valve comes from R. Dray Manufacturing, Inc. of Hamilton, Texas, a screw designer and manufacturer. According to company president Bob Dray, the new valve works more like a ball than a ring valve, but with the ball replaced by a dual-diameter piston, and the melt path around the outside of the valve, rather than through its center. The piston has a larger diameter on the downstream than upstream end; that means that it requires higher pressure upstream to move the piston to open the valve. That happens the moment backpressure is applied by screw rotation; and the instant that backpressure drops, the valve closes (it travels a very short distance). Dray says this makes the valve action extremely repeatable shot-to-shot. This has been demonstrated in lab testing with three major injection machine manufacturers and in 24-hour-a-day production of housewares at a custom molder on three 300- and 700-ton machines. At least nine different parts weighing up to 747 g were molded with at most about 1 g variation. The new valve is also said to be relatively unaffected by wear.