INJECTION MOLDING: Medium-Pressure Machine For Large Solid or Foam Parts

Large-shot, 800-ton press straddles the gap between standard injection and structural-foam machines.

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Wilmington Machinery, Wilmington, N.C., has developed an 800-ton, two-platen press with two-stage (screw/plunger) injection and 44-lb shot capacity that offers flexibility for either solid or structural-foam molding. It offers twice the tonnage of a typical foam press and around twice the injection pressure, as well. At 10,000 psi injection capability, Wilmington calls this a “medium-pressure” machine. It also has an injection rate of 20 lb/sec, about twice the usual for structural foam.

It can utilize standard hot-runner molds for solid foam molding (perhaps with a little chemical blowing agent to prevent sinks); or it can use an external nozzle manifold for structural foam with direct gas injection. Hot runners offer faster changeover and setup for short runs of solid parts; while an external nozzle manifold permits easier cleaning when running 100% recycled resins that may be prone to contamination that can clog hot-runner nozzles.

The new MP800 press has bimetallic barrels for both screw and plunger, electric drive on the extruder, servohydraulic drive on the plunger and clamp, and Allen-Bradley controls. Clamp speed is 2000 in./min. The clamp design is said to be unusual in that both the mechanical locking and high-pressure cylinders are mounted on the moving platen rather than on the fixed platen.