韩慧珍:High Quality Low Cost Blow Molding

High Quality Low Cost Blow Molding

Blow molding is a primary method for forming hollow plastic objects such as soda bottles. The process includes clamping the ends of a softened polymer tube which can be either extruded or reheated, inflating the polymer against the mold walls with a blow pin, and then cooling the product via conduction or evaporation of volatile fluids in the container.

There are three general types of blow molding:

• Extrusion blow molding

  Extrusion blow molding is usually used to make items of weight greater than 12 oz. such as containers for food, laundry, or waste.

  This process normally uses commodity materials such as PVC, PS, PP, LDPE, and HDPE.

  Important factors one should consider for extrusion blow molding include the following:

  • Polymer viscosity at high & low shear rates
  • Melt strength (important for uniform wall thickness, no holes)
  • Strain recovery (MW & Distribution)
  • Crystallization rate (slow rate desired)
  • Thermal properties (thermal diffusitivity, thermal conductivity, specific heat, etc.)

• Injection blow molding

  Injection blow molding is used to achieve very accurate wall thickness, high-quality neck finish, and to process polymers that cannot be extruded. Usual applications include pharmaceutical bottles, cosmetic packaging, and single serving liquor bottles that weigh less than 12 oz.

  These factors are critical to this process:

  • Shear & temperature dependent viscosity
  • Temperature-dependent tensile strength on the pin
  • Tensile elongation during inflation
  • Crystallization kinetics on the core pin
  • Crystallization kinetics during blowing and cooling
  • Stretch blow molding Stretch blow molding is generally only used for difficult to blow crystalline and crystallizable polymers such as polypropylene and polyethylene terephthalate.

  Important polymer properties to be considered:

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

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