Utilizing PET Bottles for Hot-Filled Beverages: Practical Insights and Solutions

PET bottles are extensively used today, especially with the rise of juice and tea drinks. Their safety and cost-effectiveness make them a preferred choice for mainstream beverage products.

 

Requirements for Hot Filling Process in PET Bottles:

 

Enhanced heat resistance is crucial to withstand hot liquids, preventing softening and volume shrinkage (1%-1.5% at 85℃-90℃). PET bottles must endure negative pressure post-liquid cooling to prevent deformation.

 

Blowing Methods for Hot-Filled PET Bottles:

 

One-step method: Direct blowing of crystallized preforms with a high-temperature mold offers high output but limited heat resistance.

 

Two-step method: Utilizes two sets of molds for stretching, heat treatment, and shaping, resulting in superior heat resistance but lower output.

 

The process of blowing PET bottles involves several precise steps to ensure quality and consistency:

 

1. Preform Sorting and Heating:

 

Preforms are sorted and sent to the preform heating furnace.
As the preform heats, the rotating furnace evenly warms the bottle mouth, while a fan ensures uniform heating of the inner and outer walls.

 

2. Transfer to Blowing Station:

 

The heated preform is moved to the bottle blowing station using a blank conveying manipulator.

 

3. Pre-Blowing and Stretching:

 

Once in the blowing mold, pre-blowing air expands the preform in a circular motion.
High-pressure air then enters the mold cavity, further stretching the preform until its wall aligns with the mold.

 

4. Stress Elimination and Crystallinity Enhancement:

 

High-pressure gas remains in the mold to relieve internal stress and improve the plastic's crystallinity.

 

5. Cooling and Shaping:

 

Exhaust initiates, and high-pressure cooling gas shapes and cools the bottle wall.
Low-pressure air aids demolding from the bottom mold, preventing issues like protrusions or stuck bottles.

 

6. Bottle Removal and Conveyance:

 

Once complete, the bottle is removed from the mold by the conveying manipulator and transferred to the bottle conveying line. Quality control is crucial in the production of heat-resistant PET bottles. Here are some factors affecting their quality and solutions to common issues:

 

Factors Affecting Quality:

 

1. Preform Quality: Ensure the intrinsic viscosity, color purity, and storage time of preforms.

 

2. Heating Process: Maintain uniform heating in the oven to prevent deformities.

 

3. Pre-blowing: Control pre-blowing pressure, position, and airflow for proper shaping.

 

4. Mold Temperature: Maintain optimal mold temperature to prevent deformations and increase plastic crystallinity.

 

5. Environmental Conditions: Control room temperature and humidity for better production outcomes.

 

Common Quality Issues and Solutions:

 

1. Bottleneck Skew: Unblock oil passages and stretch rod holes, and replace damaged seals.

 

2. Center Point Deviation: Adjust pre-blowing pressure, flow, and position, and ensure rod alignment.

 

3. Bottom Deformation: Regulate mold oil temperature and valve function, and control preform temperature.

 

4. Folds at the Bottom: Adjust pre-blowing pressure, flow, and timing for proper shaping.

 

5. Stiff Neck: Increase neck heating, adjust pre-blowing pressure and flow, and ensure proper furnace positioning.

 

6. Poor Parting Line Forming: Replace damaged seals and adjust mold gaps.

 

7. Wall Deformation: Extend cooling blow time, regulate mold temperature, and ensure proper material distribution.

 

8. Large Shrinkage: Increase mold temperature, preform setting temperature, and clear oil passages.

 

9. Diameter Variation: Adjust cooling blow time and ensure even material distribution in the blowing process.

 

Common Issues with PET Bottles in Hot Filling Lines and How to Solve Them

 

When it comes to using PET bottles in hot filling lines, several common problems can arise, impacting performance and quality. Let's delve into these issues and explore effective solutions.

 

1. Storage and Transportation Concerns:

 

The hygroscopic nature of PET makes it susceptible to moisture absorption from the air, which can compromise its performance, particularly its heat resistance. Proper storage and transportation practices are essential to mitigate this issue:

 

• Consider the size and type of PET bottles and adhere to recommended storage timeframes.

 

• Embrace lightweight bottle designs and connected production processes to minimize storage duration.

 

• Maintain optimal storage conditions, including controlling room temperature and relative humidity levels, to preserve technical indicators.

 

• Adjust blowing processes and parameters based on factors such as material type, storage conditions, and customer specifications.

 

• Ensure PET raw materials remain dry to prevent hydrolysis reactions and degradation.

 

• Exercise caution with drying procedures, as excessive drying can lead to PET degradation and compromise bottle quality.

 

2. Poor Heat Resistance:

 

• Achieving adequate heat resistance in PET bottles requires meticulous attention to mold design and temperature control:

 

• Employ specialized mold designs capable of withstanding negative pressure to prevent bottle deformation.

 

• Increase mold temperature using high-temperature oil to eliminate internal stress and enhance crystallinity.

 

• Optimize preform and bottle designs to ensure uniform wall thickness distribution and prevent distortion.

 

• Fine-tune injection and stretch-blow molding parameters to prevent residual stress release and subsequent bottle deformation.

 

• Utilize temperature adjustment technologies for blowing molds to enhance crystallinity while maintaining transparency.

 

• Implement cycle blowing techniques to control post-demolding bottle deformation effectively.

 

3. Capacity Fluctuations:

 

• PET bottles exhibit a shrinkage rate that can lead to capacity fluctuations, influenced by various factors:

 

• Mold size and shape significantly impact bottle capacity, with thinner bottles and higher ambient temperatures exacerbating shrinkage.

 

• Production processes, including blowing pressure and mold temperature, play crucial roles in determining bottle form and capacity.

 

• Natural shrinkage over time necessitates adjustable mold designs to maintain consistent bottle dimensions.

 

• Different filling methods exert varying degrees of influence on volume control, with self-weight filling posing the greatest challenge.

 

• Adjusting molds and production processes and optimizing storage conditions are key strategies for mitigating capacity fluctuations and ensuring product consistency.