Innovative check valve solves repeatability & wear problems.
The first major rethinking in decades of the humble injection molding nonreturn or check valve looks like it can eliminate one of the most important and persistent sources of shot-size variability. So says inventor Robert Dray, president of R. Dray Manufacturing Inc. of Hamilton, Texas, a well-known screw designer and manufacturer. Dray has set up a new company, U.S. Valves, Inc. in Evansville, Ind., to manufacture and market the new valve, dubbed the Repeater.
First published details on the Repeater valve appeared last month (see PT, Dec. '91, p. 9). Here are more details, as well as the first look at the valve's novel principle of operation.
NO MORE INCONSISTENT
Today's ball- and ring-type check valves all depend on some degree of backflow to actuate valve closing at the start of injection. This undesired backflow produces a pressure drop, causing a force on the moving member (ball or ring) that moves it to the closed position. The trouble is that the amount of this backflow while the valve is closing is inconsistent. And this inconsistency of melt "leakage" back over the screw increases with wear on the valve, especially with sliding-ring types.
According to consultant Jack Stroup of Cuyahoga Falls, Ohio (formerly an executive at Van Dorn Plastic Machinery), who has been working with Dray to develop the new valve, typical nonreturn valves require excess pullback beyond that which is needed to prevent drool into an open mold. That is done to provide sufficient flow over the valve at the start of injection to ensure the valve closes. But even this is not sufficient to avoid inconsistent closing. Stroup adds that designing ring valves with a greater contact area to reduce wear results in even poorer consistency of operation.
Enter the Repeater, which is actuated solely by a pressure differential on either side of a piston, not by melt flow. As shown in the accompanying schematics, the valve has a center piston that slides back and forth, alternately blocking and unblocking a number of melt passages from the center of the valve body to the outside of the nozzle cone. The piston is designed with a larger surface area on the downstream end than on the upstream end. This means that an equal melt pressure on both ends will result in a net force (pressure x area) driving the piston backward and closing off flow through the valve.
The valve opens to permit flow only when there is a sufficiently higher melt pressure on the smaller (upstream) end of the piston. This happens when pressure is applied by screw rotation. The movement of the piston is very small, and the degree of valve opening is also small - just enough so that the pressure drop caused by flow through the valve equals the pressure differential between both ends of the piston.
At the start of injection, the valve will close quickly and completely as soon as the screw begins to move forward, producing higher force on the forward end of the piston. This requires no flow of melt and only slight screw movement.
Unique to this valve is the ability to preclose it before the actual start of injection. This is done by maintaining hydraulic backpressure on the melt pool ahead of the screw for a short time after screw rotation ceases (normally they are cut off simultaneously). In such a circumstance, higher force immediately develops on the forward end of the piston, driving it closed with no movement of the melt or screw. Stroup expects that preclosing will further enhance the shot repeatability afforded by the valve, but this remains to be proven.
The degree of over-travel of the piston in closing is reportedly sufficient to keep it closed during initial screw pullback. Likewise, any slight erosion of the rear end of the piston due to wear would not compromise valve action, Stroup says. Because the piston slides over the melt-discharge channel at right angles, there is no possibility of incomplete closing due to build-up of foreign matter on the seat, as can happen with other valves.
The key point about the Repeater valve, he emphasizes, is not that it doesn't leak at all - it does - but that it leaks only a small amount and very consistently. Some slight leakage does occur through the clearance between the valve body and the barrel wall, but that leakage is strictly a function of pressure, not mechanical action, and should be the same from shot to shot.
INITIAL RESULTS PROMISING
The Repeater valve has been tested extensively in the labs of three major injection machine manufacturers, and three valves have been in 24-hour-a-day production at what Dray describes as the number-two housewares molder in the country. The valves were simply substituted for existing ring and ball valves on one 300-ton and two 700-ton HPM machines. Nine different parts, weighing up to 747 grams were molded with at most about 1 g variation from shot to shot, Dray reports. Detailed production results with the new valve will be presented in a further article in April.
Patents on the new valve have been applied for. Repeater valves cost about the same as a four-piece ring valve. U.S. Valves may be reached toll-free at (800) 944-6662.
PHOTO : Screw rotation produces higher force on the rear end of the piston than on the front end, forcing the valve to open.
PHOTO : The valve closes the moment the screw starts its forward travel - with essentially no flow of melt through the valve. Unlike other valves, this one can be preclosed before injection simply by maintaining backpressure a few moments after screw rotation ceases.
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|Author:||Naitove, Matthew H.|
|Date:||Jan 1, 1992|
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