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Plastics Processing: Blow Molding

Blow molding produces hollow thermoplastic products such as bottles, jugs, cans, food containers, balls, toys, and automobile fuel tanks.

Extrusion Blow Molding
Extrusion blow molding accounts for 75% of all blow molded products. The basic machine used is called the bow-and-drop unit. It extrudes the plastic into a hollow tube shape (parison) between two mold halves, which clamp shut, holding and cutting the parison. The parison is then blown against the cooled mold walls, where it is held with 80 to 120 p.s.i. air pressure until the resin sets. The mold opens, dropping the part which is trimmed of "flash" (excess plastic) and made ready for use. 80% of the machine's cycle time is spent cooling the parison.

Injection Blow Molding
Injection blow molding occurs in two steps. The resin is injection-molded into a preform, where it is temperature conditioned and shaped around a core pin. The core pin then transfers the parison (hollow tube) to the blow mold where air inflates it to its final shape.

Injection blow molding offers the following advantages:

  • Production of completely finished bottles; no secondary operations are required
  • Scrap-free production to close-tolerance, and positive weight control (+/- 0.1 g) bottles
  • Bottles oriented at the ejection station, allowing for automated filling and labeling
  • Improvements in bilateral molecular orientation, clarity, and strength
  • Minimal operator supervision requirements

Multilayer Blow Molding
This high-growth form of blow molding is expected to capture 27% of the 175-billion-unit market for U.S. food packaging. It produces multilayer containers through coextrusion. Two to seven extruders feed individual components into a coextrusion head where a multi-layer hollow parison is formed. After that the parison (hollow tube) is blow molded in the conventional way. The results are multilayered containers which offer aroma and taste protection, hot-fill capability, and increased shelf life for foodstuffs and permeation barriers for solvents and gasoline. The containers are brilliant, printable, scuff-resistant, lightweight, squeezable, shatterproof, and are easily recycled and / or reused making them great for food, cosmetics and pharmaceuticals.