Stress & strain analysis is a highly important quality inspection method which determines the stress and strain in PET materials which are subjected to forces applied. The test is conducted on a polariscope-strain viewer. This process holds huge importance for quality PET bottle production.

How PET preforms acquire stress & strain defects?

While manufacturing a PET bottle, the very initial stage from which a PET material undergoes is the ‘injection stretch blow molding’ process.

This process is divided into four stages:

  1.      Injection 
Molten polymer flows into the injection cavity through the hot runner block and produces the preform. After a particular set of time, the injection mold and the preform held in a neck carrier is rotated to 90° angle.
  2.      Stretching
Once conditioned to the correct temperature, the preform is ready for stretching. When the preform is within the blow mold area, the mold close, a stretch rod is introduced to elongate the preform longitudinally.
  3.      Blowing

Post the stretching, using two levels of air pressure, the preform is blown circumferentially.

  4.      Discharge 
After a particular set of time, when the mold is cooled off, it opens and the preform is removed via drop chutes or robotics.


For inquiry on testing instruments for PET preform manufacturing unit, kindly click here.

During this process of stretch blow molding, the preform may incorporate various kind of stress distributions or strains on itself. This defect can disturb the required thickness distribution and properties of the final bottle. As the thickness distribution is one of the most important criteria in the production of bottles, the strain can be considered as a major quality hampering factor for PET bottle manufacturers.

Stress & Strain analysis to detect PET preform defects

Stress analysis in PET preform works by observing the macroscopic view of the mechanical characteristic of materials. It is assumed that the stresses which are observed are related to strain (deformation) of the material. This deformation can be detected using the photoelastic method on a polariscope strain viewer.

The photoelastic process describes the changes in the optical properties of a preform having mechanical deformation. It determines the strain in PET preform by providing a picture of stress distributions around discontinuities in the preform.

A proficient polariscope can detect minute stress or strain occurred in the product and can help the manufacturer in testing the quality and transparency of PET preforms prior to final production.

To know more about the technical specification of polariscope, kindly follow this link.