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Polycarbonate Plastics Blow Molding | Process & Materials | Cove

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Blow molding: Efficient production of hollow items

The blow molding process is used to efficiently produce hollow items, such as water bottles and light globes. Covestro offers specialty grades of Makrolon® polycarbonate specifically designed for blow molding.

Blow Molding
Five-Gallon water bottles made of Makrolon®

Two types of blow molding are extrusion blow molding and injection blow molding.

During extrusion blow molding, mold halves pinch the end of a hanging extruded tube – called a parison – until it seals. Air pressure applied into the tube expands the tube and forces it against the walls of the hollow mold. The blown shape cools as a thin-walled hollow shape. A secondary step removes the vestige at the pinch-off area.

Injection blow molding substitutes a molded shape in place of the extruded parison. Air pressure applied from inside the still-soft molded shape expands the shape into the form of the hollow mold. Injection stretch blow molding is a variation of injection blow molding.

Regardless of process selected, annealing and relaxation of the material is essential to achieving desired part quality

When evaluating the suitability of blow molding your item, it’s important to consider the following:

  • Wall thickness may vary throughout the part and may change with processing.
  • Blow molding cannot produce features that project from the surface, such as ribs and bosses.
  • Part geometry determines mold and equipment costs.

Covestro offers a special grade of Makrolon® WB1239 polycarbonate for the production of returnable water bottles. This material enables high productivity and cost efficiency when blow molded.

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