Injection Molded Preform and Manufacture Thereof

A method of injection moulding a thermoplastic preform for blow moulding to form a container, the method comprising the steps of:

• a. providing an injection mould defining a mould cavity for moulding a preform comprising a lower closed base portion, a hollow body portion, a hollow transition portion between the lower closed base portion and the hollow body portion, and an upper open end portion adjacent to an upper part of the hollow body portion, the injection mould comprising a first mould part defining the outer surface of the mould cavity and a second mould part in the form of a movable core defining the inner surface of the mould cavity, wherein the core has a free forward end, the free forward end of the core having a central region, which central region extends over at least 50% of an external radius of the free forward end and has a convex external curvature, and a first transition region including a first radially outwardly tapering region extending away from the central region to connect to an elongate body region of the core, the first tapering region being inclined to a longitudinal axis of the mould cavity, wherein the first mould part comprises an end opposite to the free forward end of the core, the end comprising a gate surrounding the longitudinal axis of the core and the mould cavity and a first peripheral surface surrounding the gate, the first peripheral surface being concave, and the first mould part further comprises a second transition region including a second radially outwardly tapering region extending away from the first peripheral surface to connect to an elongate body region of the first mould part, the second tapering region being inclined to the longitudinal axis of the mould cavity,
• b. disposing the movable core in a preliminary forward position,
• c. injecting molten thermoplastic material into the mould cavity thorough the gate opposite to the free forward end of the core,
• d. moving the core rearwardly during the injection, the first mould part and the free forward end of the core forming therebetween, during at least a portion of the injection, an annular substantially frusto-conical funnel connecting a first cavity region adjacent to the gate with a sidewall cavity region, the molten thermoplastic material flowing through the funnel into the sidewall cavity region;
• and
• e. moving the core forwardly after the thermoplastic material has been injected into the mould cavity to dispose the core in a final moulding forward position in which the mould cavity defines the final shape and dimensions of the thermoplastic preform, wherein in the final moulding forward position the first peripheral surface, and a second peripheral surface of the central region of the core which is opposite to the first peripheral surface, define a first, downwardly and radially inwardly tapering cavity portion therebetween which increases in width from a radially outer end of the first tapering cavity portion adjacent to a cavity portion for the hollow transition portion to a radially inner end of the first tapering cavity portion adjacent to the gate and a middle part of the central region which is opposite the gate, the first tapering cavity portion and a gate portion surrounded by the first tapering cavity portion comprising a central part of the preform base portion, and the transition portion comprises a second, upwardly and radially outwardly tapering cavity portion extending away from the central part to connect to the hollow body portion, the second tapering cavity portion increasing in width from the central part to the hollow body portion, wherein the forward movement of the core causes injected thermoplastic material to be flowed back, by pressure applied by the core, through the gate from the first tapering cavity portion.