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A mixing head for a reaction injection molding machine, which incorporates several beneficial features, comprises a body having a bore, which defines a mixing chamber, and a plurality of nozzle orifices opening into the mixing chamber for conducting reactive polymeric components thereto. A plunger i

A mixing head for a reaction injection molding machine, which incorporates several beneficial features, comprises a body having a bore, which defines a mixing chamber, and a plurality of nozzle orifices opening into the mixing chamber for conducting reactive polymeric components thereto. A plunger is mounted for reciprocal movement in the mixing chamber to control flow of the reactive components through the nozzle orifices to form a homogeneous polymer mix, e.g. a polyurethane or polyisocyanurate mix, in the chamber, and serves also to eject the resulting mix from the chamber. The plunger is controlled by an actuator piston and is linked thereto by a universal coupling that minimizes plunger binding in the mixing chamber due to misalignment between the plunger and actuator. The plunger is also sheathed in a sleeve of antifrictional material that permits maintaining a very tight piston fit without excessive friction, and thus resists build-up of reactive material which is a common cause of plunger "freezing" or excess wear. Flow of polymeric components through individual nozzle orifices is regulated by a restrictor valve for each component which includes a packing gland-retainer assembly as well as a position locking arrangement. Accordingly, the restrictor valve may be adjusted without leakage while the mixing head is in operation or while the polymeric composition is under pressure.

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