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.
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|Title Annotation:||injection molding|
|Date:||Dec 1, 1991|
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