| 摘要 |
Parisons (1), formed by extruding plastics material through an annular gap partially defined by a mandrel (4) and blocks (17), have portion(s) of constant wall thickness controlled by longitudinal movement of the mandrel and portion(s) of increased wall thickness controlled by transverse movement of the movable blocks. Thickness is controlled by storing signals indicative of the gap width in dependence on desired thickness values of the moulded articles in a control unit (24), arranged to control the positions of the mandrel and the blocks. A first parison is extruded whilst making marks (12) at predetermined positions on at least one portion of constant wall thickness and on at least one portion of increased wall thickness. The parison is then blow moulded into a test article (15) whose wall thickness is measured at the marks and compared with desired values. The values stored in the control unit are adjusted to eliminate or reduce the difference between the measured and desired thickness values. Subsequent articles are then moulded from further parisons after monitoring the parison length and comparing it with a desired length and adjusting all the values stored in the control unit such that the length of the next parison is substantially equal to the desired length. <IMAGE> |
