Process for the manufacture of oriented hollow plastic articles

In a process for manufacturing oriented hollow plastic articles by pre-blowing, in a preform mold, a portion of a cylindrical parison into a preform having a cylindrical shape over one part of its length and a sealed end defining the base of the preform, thermally conditioning the preform to bring it to a temperture at which the plastic will undergo molecular orientation during a subsequent blowing operation, and then placing the conditioned preform in a final blow-mold and effecting such subsequent blowing operation by applying excess pressure internally of the preform to cause it to be shaped by stretching into its final form, the improvement wherein said step of pre-blowing comprises:

• sealing the end of the parison, in the preform mold, along a straight line to define the base;
• and applying excess pressure internally of the sealed parison in the preform mold to produce the preform in a manner such that the base is substantially equal in length to one-half the perimeter of the cylindrical part and to create an intermediate length part which is interposed between the cylindrical part and the base and has a cross section which varies progressively from the cylindrical shape of the cylindrical part to the straight-line shape of the base while maintaining a substantially constant perimeter.