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An injection molding apparatus includes an injection piston that is slidable within a nozzle having a movable valve gate pin. The injection piston is movable from a retracted position to an extended position in order to force melt towards a mold cavity. A valve is located at a forward end of the pis

An injection molding apparatus includes an injection piston that is slidable within a nozzle having a movable valve gate pin. The injection piston is movable from a retracted position to an extended position in order to force melt towards a mold cavity. A valve is located at a forward end of the piston to selectively block communication between a recess, which is provided in an outer wall of the piston adjacent the valve, and a melt chamber of the nozzle. Movement of the injection piston from the retracted position to the extended position causes the valve to close so that the predetermined volume of melt located below the valve is forced into the mold cavity, when the valve gate pin opens the mold gate.

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1. A hot runner injection molding apparatus comprising:a manifold having a manifold channel for receiving a melt stream of moldable material under pressure, said manifold channel having an outlet for delivering the melt stream to a nozzle channel of a hot runner nozzle, said nozzle being positioned

1. A hot runner injection molding apparatus comprising:a manifold having a manifold channel for receiving a melt stream of moldable material under pressure, said manifold channel having an outlet for delivering the melt stream to a nozzle channel of a hot runner nozzle, said nozzle being positioned within a nozzle bore of a mold cavity plate and being spaced from said mold cavity plate by an insulative air space; a mold cavity for receiving said melt stream from said nozzle, said nozzle channel directly communicating with said mold cavity through a mold gate; a gating mechanism driven by an actuation means for selectively opening and closing said mold gate; an injection piston extending through said nozzle channel of said nozzle and being slidable therethrough, an outer wall of said injection piston abutting an inner wall of said nozzle channel, said injection piston being movable from a retracted position to an extended position to force melt into said mold cavity; a valve located at a forward end of said injection piston, said valve being selectively movable to block communication between a recess, which is provided in said outer wall of said injection piston upstream of said valve, and a melt chamber of said nozzle channel that is proximate to said mold gate, said valve being open to allow melt to flow from said manifold channel into said recess and into said melt chamber of said nozzle channel when said injection piston is in said retracted position; wherein movement of said injection piston towards said extended position forces melt located in said melt chamber of said nozzle channel to flow through said mold gate into said mold cavity when said gating mechanism is in an open position. 2. An injection molding apparatus as claimed in claim 1, wherein a known volume of melt is located in said melt chamber of said nozzle channel when the gating mechanism is in a closed position.3. An injection molding apparatus as claimed in claim 2, wherein said gating mechanism is a valve pin driven by a piston.4. An injection molding apparatus as claimed in claim 2 wherein movement of said injection piston from said retracted position to said extended position causes said valve to close.5. An injection molding apparatus as claimed in claim 2, wherein movement of said injection piston from said extended position to said retracted position causes said valve to open.6. An injection molding apparatus as claimed in claim 1, wherein movement of said injection piston is controlled by a controller that receives information from a pressure sensor that senses the pressure of at least one of said mold cavity, said nozzle channel and said manifold channel.7. An injection molding apparatus as claimed in claim 6, wherein said controller further receives information from a temperature sensor that senses the temperature of at least one of said mold cavity, said nozzle channel and said manifold channel.8. A hot runner injection molding apparatus comprising:a manifold having a manifold channel for receiving a melt stream of moldable material under pressure; a hot runner nozzle positioned within a nozzle bore in a mold cavity plate having a nozzle channel for receiving the melt stream from the manifold channel; a mold cavity having a mold gate that receives the melt stream from the nozzle channel; a gating mechanism driven by an actuation mechanism for selectively closing and opening the mold gate; and an injection piston extending through the nozzle channel of the nozzle and being slidable therethrough, wherein the injection piston includes, a piston body having an upstream portion with a first diameter and a downstream portion with a recess formed in an outer wall of the piston body for receiving the melt stream from the manifold channel, the injection piston being movable from a retracted position to an extended position, and a valve located at a forward end of the piston body, the valve being selectively openable to allow communication between the recess of the piston body and a downstream portion of the nozzle channel, wherein movement of the injection piston towards the extended position allows the upstream portion of the piston body to stop the melt stream from the manifold channel from entering the recess of the downstream portion of the piston body and forces melt located in the downstream portion of the nozzle channel to flow into the mold cavity. 9. The injection molding apparatus of claim 8 further comprising:a heater thermally coupled to the nozzle. 10. The injection molding apparatus of claim 8, wherein the gating mechanism is a valve pin that extends within the nozzle channel.11. The injection molding apparatus of claim 10, wherein the valve pin is receivable within a central bore of the injection piston and is slidable therethrough to open and close the mold gate.12. The injection molding apparatus of claim 8, wherein the downstream portion of the nozzle channel is a melt chamber of a known volume located proximate to the mold gate.13. The injection molding apparatus of claim 8, wherein the downstream portion of the piston body has a second diameter that is less than the first diameter of the upstream portion of the piston body such that the recess in the downstream portion is annular.14. A valve-gated nozzle for a hot runner injection molding apparatus for providing a melt to a mold cavity via a mold gate and being positionable within a nozzle bore of a mold cavity plate, the nozzle comprising:a nozzle body having a nozzle melt channel; a valve pin selectively movable by an actuator within the nozzle melt channel to open and close the mold gate; and an injection piston slidingly receivable within the nozzle melt channel, the injection piston including, an injection piston body having a recess in an outer wall thereof for fluidly connecting a melt source to the nozzle melt channel, and a valve located on a forward end of the injection piston body downstream of the recess, the valve being selectively closable to block fluid communication between the recess in the outer wall of the injection piston body and a downstream portion of the nozzle melt channel proximate the mold gate, wherein when the injection piston is in a retracted position the valve is open to allow melt to flow from the recess past the valve into the nozzle melt channel and when the injection piston is moved toward an extended position the valve is closed and melt in the nozzle melt channel is forced into the mold cavity when the valve pin is unseated from the mold gate. 15. The injection molding apparatus of claim 14 further comprising:an injection piston body portion that blocks communication between the melt source and the nozzle melt channel. 16. The injection molding apparatus of claim 14, wherein the valve pin is slidably receivable within a central bore of the injection piston to open and close the mold gate.17. The injection molding apparatus of claim 14, wherein a downstream portion of the nozzle channel is a melt chamber of a known volume.18. The injection molding apparatus of claim 14, wherein an upstream portion of the piston body has a first diameter and a downstream portion of the piston body has a second diameter that is less than the first diameter such that the recess in the piston body is annular.19. A hot runner injection molding apparatus comprising:a manifold having a manifold channel for receiving a melt stream of moldable material under pressure; a hot runner nozzle positioned within a nozzle bore of a mold cavity plate and having a nozzle channel in fluid communication with the manifold channel for receiving the melt stream; a mold cavity for receiving the melt stream from the nozzle channel, the nozzle channel communicating with the mold cavity through a mold gate; a gating mechanism for selectively closing the mold gate including a movable valve pin coaxial with the mold gate and seatable therein; a melt chamber located in the nozzle channel proximate to the mold gate, the melt chamber having a known volume; an injection piston positioned coaxial with the movable valve pin and slidably extending within the nozzle channel between an extended position and a retracted position; and a valve located at a forward end of the injection piston upstream of the melt chamber, the valve being selectively movable to control melt flow from the manifold channel into the melt chamber upon extension and retraction of the injection piston, wherein the known volume of melt is injected into the mold cavity when the injection piston is slid into the extended position and the valve pin is unseated from the mold gate. 20. The injection molding apparatus of claim 19, wherein the valve includes a first valve member fixedly attached to the injection piston and a second valve member movably attached to the injection piston.21. The injection molding apparatus of claim 20, wherein the first valve member and the second valve member include cutouts that allow the melt to flow through the valve when the first and second valve members are spaced one from another in an open position.