The check ring and cooling the barrel:Will this design fly?

Some in the industry argue that the most important part of the injection molding machine is the check ring. Its integrity and ability to maintain tolerances and injection pressure are critical. Paul Allen, president of Logic Corp., has developed a check ring designed to increase shot-to-shot reliability.

Allen contends that the screw is not always centered. While the screw is rotating and retracting, material is forced around the flights and check ring, keeping them centered. When it stops, forces holding the screw and check ring centered relax, allowing the weight of the screw to force material out from under the flights until one or more touch the inside of the barrel. When injection begins, the screw is a column confined in the barrel. Allen calculates that the force on a 2-inch screw at 20,000 psi in a barrel with a clearance of .006 inch is about 9 psi at the tip of the screw—a force that’s difficult to overcome.

The check ring, thanks to its lighter weight and larger surface area between it and the barrel, floats on a thin film of material. When injection forward begins, the off-center screw touches only one side of the check ring first.

If the check ring remains centered in this situation, sealoff is incomplete, pressure does not increase, and a short shot can occur. Sealoff can occur only if the screw recenters itself, or if the check ring tilts off-center to match the angle of the screw. When this happens, says Allen, the check ring is pushed into the side of the barrel, creating metal-to-metal contact (Figure 1). The result is wear that creates an out-of-spec gap between the check ring and barrel.

A Solution
Allen’s design is a check ring that encourages self-alignment (Figure 2). It’s used in conjunction with front zone barrel cooling, first described in IMM a few months ago (June 1999, pp. 76-77). By cooling the front zone, a skin of material develops between the barrel wall and the check ring, inhibiting backflow and centering the check ring. In one test Allen conducted with a nylon, he ran successful cycles with a rear zone temperature of 600F, a midzone temperature of 625F, and a front zone temperature of 325F (as low as 295F at times).

The check ring Allen designed takes advantage of this skin of cool material. The check ring has on it several narrow fingers that receive the seat and center the screw before sealoff starts. This ball-and-socket system allows the check ring to remain parallel to the interior of the barrel, even if the screw is not.

Space between each finger allows material to be pushed out of the way as the seat and check ring mate. Because the surface area of the check ring is so much greater than that of the screw flights, the material tends to hold the check ring centered while the screw is guided into alignment. Also, the cooler material between the check ring and the barrel will help hold the check ring on-center without twisting it in the barrel.

The goal of the design, says Allen, is to get the plastic itself to shield the screw, barrel, and check ring from wear. Chemical attack is reduced thanks to the lower temperature, and wear is reduced because the viscosity of the material around the check ring prevents metal-to-metal contact.