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What causes uneven wall thickness PET stretch blow molding

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What causes uneven wall thickness PET stretch blow molding

09/10/2018

What causes uneven wall thickness PET stretch blow molding

One of the advantages of PET stretch blow molding machine is its self-leveling functionality. In this case, as the preform stretches, it also induces the strain-hardening; this usually makes it tougher to stretch. The PET bottles produced under this process are generally great, but some small differences might come through. Without proper procedures, some of it might be with a difference of 0.001 inches. For uniform bottles, you must ensure that some conditions are correctly maintained. Here are some of the challenges that contribute to wall thickness skewing:

Off-center gate

This is one of the most common problems that contribute to the wall-thickness difference. The vestige in injection molding process should always be centered and held firmly as the air with high pressure inflates the preform bubble to form a bottle. Otherwise if not held firmly, the thickness will be in line with the gate and nothing will prevent it from happening. The common causes of wall thickness in this case are:

  1. a)    In standard cases, the stretch rod should have a distance with blow mold bottom of about 0.04 inches that is less than the thickness of the preform gate wall. This ensures the preform does not slip when blown with high pressure. The gaps usually become small when the preform is made thinner.
  2. b)    Pre-blow pressure is applied too early or too high. Before the preform is blown off the center by pre-blow pressure, the stretch rod should connect with it. Therefore, when connected, the pre-blow pressure will not be so high. The pressure required in this case depends on the temperature and preform thickness.
  3. c)    When pressure is applied too early
  4. d)    When blow mold has not been machined properly. When preform has some bending when entering the blow mold

Uneven cooling and heating

Sometimes the walls of the PET bottle show the difference of about 0.004 inches and more. This implies that the one side of the preform can be cooler than the other. You will note that the warmer side will stretch more than the cooler side. This problem is usually common in the single-stage process.

Small stretch ratios

PET stretch blow molding machine usually has a self-leveling effect. This implies that the preform has to be stretched in a hoop and vertical direction. The minimum ratio in the hope direction is normally 4:1 while the vertical should be 2:1. In some cases, the preform can have the right weight and neck finish but has not been designed for specific application. This will lead to improper ratios in some parts of the bottle.

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