A thin base in a blow-molded PET bottle is a common quality defect, often leading to reduced top-load strength, poor pressure resistance, and potential leakage. The root causes are typically related to the preform design, process parameters, and mold condition.

Here are the key reasons, categorized for clarity:

1. Preform (Parison) Related Causes :

A.Incorrect Preform Base Temperature: The preform base is often the coldest part of the preform as it is not directly heated by the heater bands. If it is too cold, the material will not stretch properly, leading to inadequate distribution and a thick, but weak, “pancake” base. Conversely, if it is too hot (e.g., from excessive infrared radiation or convection from the oven), the material becomes too fluid and stretches excessively, resulting in a thin spot.

B. Suboptimal Preform Design: The base geometry and wall thickness of the preform are critical. If the preform’s base is too thin, it has insufficient material to stretch into the final bottle base. An incorrect design of the “feed-web” (the supporting ring that holds the preform in the injection mold) can lead to uneven material distribution in the base area from the start.

2. Process Parameter Related Causes:

A. Insufficient Stretch Rod Delay Time: The stretch rod begins moving forward to stretch the preform axially. If the delay time between the start of pre-blow pressure and the stretch rod movement is too short, the preform is not properly inflated before being pushed. This can cause the rod to “poke” through the soft material, creating a thin spot or even a hole in the base.

B. Excessive Stretch Rod Speed or Force: An overly aggressive stretch rod can physically puncture or over-stretch the preform material before it has uniformly expanded against the mold, thinning the base.

C. Incorrect Blow Pressure Profile:

a. Low Pre-Blow Pressure: Pre-blow pressure is used to initially inflate the preform into a bubble. If this pressure is too low, it fails to properly form and support the material in the base before the stretch rod completes its travel and the high-pressure blow occurs. This can lead to uneven stretching and thinning.

b. Timing of High-Blow Pressure: If the switch from pre-blow to high-blow pressure is too late, the material has already cooled and cannot stretch properly, potentially leaving some areas thick and others thin due to uneven flow.

3. Mold (Tooling) Related Causes

A. Incorrect Mold Base Cooling: The bottle base is often the most difficult area to cool efficiently. If the cooling channels in the mold base are poorly designed, clogged, or the coolant flow rate/temperature is incorrect, the hot PET can “stick” to the mold surface. As the material shrinks away during cooling, this sticking can cause localized thinning.

B. Contaminated or Damaged Mold Surface: Dirt, oil, or damage (scratches, pits) in the mold cavity, especially in the base area, can hinder the smooth release and even stretching of the material, leading to uneven wall thickness.

C. Misaligned Mold Parts: Improper alignment between the mold halves and the base plug can create non-uniform gaps, causing asymmetric material flow and resulting in thin areas on one side of the base.

4. Material Related Causes Material Quality: Variations in the PET resin (e.g., IV – Intrinsic Viscosity – too low, contamination, or inconsistent regrind content) can lead to uneven stretch behavior, making the material more prone to thinning in certain areas.

5. Summary of Corrective Actions

To troubleshoot and resolve base thinning, consider the following actions:

A.Optimize Oven Heating Profile: Ensure the preform base is at the correct temperature. Shield it from excessive heat if necessary.

B. Adjust Process Parameters:

1. Increase the stretch rod delay time.
2. Reduce the stretch rod speed/force.
3. Optimize the pre-blow pressure and timing to properly form the bubble before the stretch rod finishes its stroke.

C. Check Mold Condition: Verify that cooling channels are clear and functional. Inspect for and clean any contamination or damage on the mold surface. Ensure all mold parts are correctly aligned.

D. Review Preform Design: Consult with your preform supplier to ensure the base design and wall thickness are optimized for your bottle and machine. By systematically investigating these areas, the cause of base thinning can be identified and corrected.

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