Stretch blow molding

Stretch blow molding produces a part with biaxial molecular alignment. In the process a preform, or parison, elongated mechanically in the mold and than expanded radially in a blowing process. A desirable resulting molecular orientation yields a material with increased strength. This means that products that are strength-based designs can be produced using less material than if they were to be produced using simpler blow molding techniques.

A goal in stretch blow molding is a designed work material developed by producing desirable molecular orientation. In order to produce and retain desired structure and specified properties the stretching and blowing processes need to be carried out at temperature lower than in other blow molding processes and the allowable temperature range will be smaller and so more difficult to control. A temperature conditioning station in-line is required, or a re-heating operation needed for preforms allowed to cool before use or for purchased preforms. This increased the difficulty of process design and operation and material specification since polymer properties depend on temperature history, e.g., on temperature, time at temperature and number of temperature cycles.

Important polymer properties to be considered:

  • Tensile strenth and yield above Tg
  • Effect of orientation on gas permeability through the polymer