Stretch blow molding is a two-stage process similar to injection blow molding. Firstly, a test-tube like preform is made using plastic injection moulding or a similar process. The neck of the preforms is fully finished but the diameter and length of the body portion are much smaller than the final product. The preform then undergos a stretch-and-blow process.
A small piece of thermoplastic material is preformed to a test-tube shape and installed near the tip of the blow stem. The setup is placed in the closed chamber of a divided mold.
Compressed air is injected into the preform via the blow stem while the blow stem is pushing forward to stretch the preform further into the mold.
The blowing and stretching process continues. The blow stem stops near the far end of the mold.
Compressed air continues to blow in until the preform reaches the wall of the mold.
The plastic piece is removed from the mold after it cools and hardens.
The single most important product made by stretch blow molding is the 2-liter PET bottle for carbonated soft drinks introduced in 1978 and updated to one-piece bottle in early nineties. The PET bottles are virtually unbreakable, lightweight, transparent. They and have various barrier properties. The traditional glass containers cannot compete with PET bottles and are almost extinct from the field of carbonated soft drinks.