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Stretch Blow Molding
Stretch blow molding is a two-stage process similar to that of injection blow molding. First, a test-tube like preform is made using injection molding 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.
1. 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.
2. 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.
3. The blowing and stretching process continues. The blow stem stops near the far end of the mold.
4. Compressed air continues to blow in until the preform reaches the wall of the mold.
5. 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 (good) barrier properties. The traditional glass containers cannot compete with PET bottles and are almost extinct from the field of carbonated soft drinks.

Pros and Cons of Stretch Blow Molding
Pros
  • Very fast production speed
  • Very repeatable and stable product quality
Cons
  • Very high setup costs which can only be justified by extremely high production volumes (in the millions).
  • Will take longer to setup to produce the first product.
  • Less flexible in adjusting for different product gram weights.
  • Limited to certain plastic materials, such as PET (Polyethylene Terephthalate).
  • Limited to hollow parts.
  • Limited to smaller containers.
  • Limitedd to simple shapes with no handles.

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