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 having an annular wall, 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, the first mould part having an end, including a gate, for forming the base portion, a frustoconical portion for forming the transition portion and an elongate portion for forming the body portion, wherein the frustoconical portions of the first mould part is inclined at an angle of from 18 to 24 degrees to a longitudinal axis of the first mould part,
• b. disposing the movable core in a preliminary forward position,
• c. injecting molten thermoplastic material into the mould cavity thorough the gate opposite to a free forward end of the core, the free forward end of the core having a central part, which extends over at least 50% of an external radius of the free forward end and is either substantially flat or has a shallow convex or concave external curvature, and a transition part including a radially outwardly tapering part extending away from the central part to connect to an elongate body part of the core, the tapering part being frustoconical, wherein the frustoconical tapering part of the core is inclined at an angle of from 12 to less than 18 degrees to the longitudinal axis 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 frustoconical funnel of progressively decreasing thickness 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, wherein the funnel is formed between the tapering part of the core and the frustoconical portion of the first mould part;
• and
• e. moving the core forwardly after at least a proportion of 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 the tapering part of the core and the frustoconical portion of the first mould part define in the preform the transition portion which comprises an upwardly and radially outwardly tapering portion extending away from the central portion to connect to the hollow body portion, the tapering portion being frustoconical and increasing in thickness from the central portion to the hollow body portion.