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According to Paul Maguire, president of color and additive feeding systems supplier Riverdale Global, Aston, Pa., there has been an inherent limitation to injection molding with color or additive metering at the machine throat—until now, that is. Maguire points out that typically about 25% of the material enters the screw during actual injection; the other 75% enters during screw recovery. The problem is that color/additive metering systems—whether for liquids or solid pellet masterbatch—remain inactive during the injection phase and add color only during screw recovery. This presents a problem in situations where the shot size
is close to the barrel capacity (which is generally considered a desirable condition). Adding a premixer for resin and color/additive above the feed throat is one solution, but the need to clean the premixer adds time and trouble to material or color changeovers.

Maguire’s answer, which is expected to become available later this year—and will be exhibited at next May’s NPE show in Orlando, Fla.—is what he terms “100% injection coloring.” Patent-pending modifications to the controls of Riverdale metering systems (and those of sister company Maguire Products) allows them to pick up two machine signals—screw recovery and injection—and actively feed liquid or solid color during both phases, with the metering system running about three times faster during the injection phase. This reportedly ensures that color or additive is being metered at all times that resin is entering the screw. By ensuring more uniform coloration throughout the shot—without need for a premixer—molders can use less colorant in translucent parts without risk of under-coloring.