PET bottles have a beneficial property of self-leveling. Due to this characteristic, when one section of a preform begins to stretch, it induces strain-hardening, making it tougher to stretch as a result. This eventually forces adjacent parts to stretch, and in this stretching and pausing, the preform develops into a bottle that may show as little as 0.001 in. the difference in wall thickness around the circumference of a round bottle. But many bottles show much greater differences than that. Hence, it is important to conduct an accurate testing for uneven wall thickness in a stretch-blow molded bottle.

Following are the various issues that can skew wall thickness, in order of relevance:

  • Off-Center Gate 

It is the most common reason for wall-thickness issues. When high-pressure air inflates the pre-inflated preform bubble into a bottle, it is important that the vestige of the injection molding process must be centered to the blow mold and kept there firmly. Otherwise, wall thickness will go where the gate goes and no amount of self-leveling can avoid this from happening.

  • Uneven Heating or Cooling 

It often confuses processors when the gate is in the center of a round bottle but the walls show differences up to 0.004 in. or above. It indicates that one section of the preform is cooler than the other. The warmer side stretches more and so gets thinner. In single-stage processes, this is very common, as mentioned above, but can also occur in two-stage stretch-blow.

  • Small Stretch Ratios 

For the self-leveling effect mentioned above, preforms must be stretched in both the vertical and hoop direction. Minimum ratios are 2:1 in the vertical and 4: 1 in the hoop plane. But there are design limitations, especially for small bottles below 12 oz that may prevent designers from implementing large enough stretch ratios. As a result of this, the material cannot fully stretch out the cooler section and they stay thicker.

Know more about the accurate testing of wall thickness and improve production line in PET bottles. Have a word with our experts today!

Testing uneven wall thickness with high accuracy

MagnaMike-8600 is a highly precise and accurate instrument that is used in many industries for determination of the thickness of various products through non-destructive testing. It is a compact product offered by Olympus Corporation that is used for testing the non-ferrous materials such as plastic bottles, glass bottles, water tanks, aluminum, PET etc. The instrument works on the principle of Hall Effect.

It is a very light weighted and handy device that works on a magnetic principle for measuring the thickness of non-ferrous materials with consistency and repeatability.

For more details on MagnaMike-8600 or to get a quote for the same, kindly contact us by filling up a quick form.