Preheating
The preform is heated and softened by infrared high-temperature lamps, targeting the body section. To maintain the shape of the bottle mouth, the neck area of the preform does not require heating and is instead cooled using a cooling device.
Blow Molding
This stage involves placing the preheated preform into a pre-designed mold and applying high-pressure air to inflate and stretch it into the desired bottle shape. Fully automatic blow molding machines integrate both steps using robotic arms, eliminating the need for manual transfer of preheated preforms into the mold. This significantly increases production speed.
Blow Molding Process Flow
The blow molding process involves biaxial stretching, during which PET molecular chains align in both directions, enhancing the mechanical properties of the bottle wall. This improves tensile, tensile strength, and impact resistance while ensuring excellent gas barrier performance. However, excessive stretching should be avoided. The stretch blow ratio must be controlled: radial stretch should not exceed 3.5–4.2 times, and axial stretch should not exceed 2.8–3.1 times. The wall thickness of the preform should not exceed 4.3 mm.
Blow molding occurs between the glass transition temperature and crystallization temperature, typically controlled within 90–110°C. Within this range, PET exhibits high elasticity. Rapid blow molding and cooling result in the formation of transparent bottles.
The blow molding process involves three rapid actions: stretching, pre-blowing, and final blowing. These steps must be well-coordinated, as the first two determine material distribution and overall bottle quality. Key parameters to optimize include:
Stretching start timing and speed
Pre-blowing start and end times
Pre-blowing air pressure and flow rate
Temperature distribution across the preform
Temperature gradient between the inner and outer walls of the preform
During rapid blow molding and cooling, induced stress forms within the bottle wall. For carbonated beverage bottles, this stress helps resist internal pressure. However, for hot-fill bottles, stress must be fully released above the glass transition temperature.
Classification of PET Blown Bottles
PET blown bottles can be divided into two categories:
Pressure-resistant bottles: Used for carbonated beverages.
Non-pressure bottles: Used for water, tea, edible oil, etc.
Tea beverage bottles are often modified PET bottles blended with polyethylene naphthalate (PEN) or composite bottles made of PET and thermoplastic polyarylate. These are classified as hot-fill bottles, capable of withstanding temperatures above 85°C. Water bottles, on the other hand, are cold-fill bottles with no specific heat resistance requirements. The molding processes for hot-fill and cold-fill bottles are similar.
Equipment
Currently, leading international manufacturers of fully automatic PET blow molding machines include France’s SIDEL and Germany’s KRONES. Domestic manufacturers, such as China’s Quan Guan, also produce such equipment. Although manufacturers differ, the working principles of these machines are similar, generally comprising five main systems:
Preform feeding system
Heating system
Blow molding system
Control system
Auxiliary equipment
Blow Molding Process
The blow molding process of PET bottles is influenced by several critical factors, including preform quality, heating, pre-blowing, mold design, and production environment.
2.1 Preform
Preforms are injection-molded from PET pellets. Requirements include:
Limited use of recycled material (below 5%)
Recycled material should not be reused more than twice
Molecular weight and viscosity must not be too low (MW: 31,000–50,000; IV: 0.78–0.85 cm³/g)
Compliance with national food safety laws: recycled materials are prohibited for food and pharmaceutical packaging
Preforms must be stored for over 24 hours before use. Reheated preforms that remain unused must be stored for an additional 48 hours before reuse. The shelf life of preforms should not exceed six months.
The quality of preforms largely depends on the PET material used. Materials that are easy to blow and shape are preferred. Experiments show that preforms made from imported PET materials are easier to blow than those made from domestic materials, even with the same viscosity. Preforms from the same batch but produced on different dates may require different blow molding parameters. Ideal preforms are clean, transparent, free of impurities and discoloration, with appropriate injection point length and minimal surrounding haze.
2.2 Heating
Preforms are heated using an oven with manually set and automatically adjusted temperatures. Far-infrared lamps radiate heat, while a bottom fan ensures uniform temperature distribution. Preforms rotate as they move through the oven to ensure even heating.
The lamps are typically arranged in a “zone” pattern, with more lamps at the ends and fewer in the middle. Heating is controlled by the number of active lamps, overall temperature settings, oven power, and segment-specific heating ratios. Lamp activation must be coordinated with pre-blowing adjustments.
Proper adjustment of oven height and cooling plates is critical. Incorrect settings can lead to defects such as expanded bottle mouths or stiff necks.
2.3 Pre-Blowing
Pre-blowing is a critical step in the two-stage blow molding process. It involves injecting air while the stretch rod descends, giving the preform its initial shape. Key parameters include pre-blowing position, pressure, and air flow rate.
The shape of the pre-blown preform determines the ease of blow molding and the quality of the final bottle. The ideal shape is spindle-like, while abnormal shapes (e.g., dumbbell or handle-like) may result from uneven heating, insufficient pre-blowing pressure, or inadequate air flow. Pre-blown preform size depends on pre-blowing pressure and position. Consistency in pre-blown preform size and shape across the machine is essential. Deviations require adjustments in heating or pre-blowing parameters.
Pre-blowing pressure varies based on bottle specifications and machine capacity. Generally, larger bottles require lower pre-blowing pressure, while higher production rates require higher pressure.
2.4 Auxiliary Equipment and Mold
Auxiliary equipment primarily includes devices for maintaining constant mold temperature. Consistent mold temperature is crucial for product stability. Typically, the bottle body requires higher temperatures, while the base requires lower temperatures. For cold-fill bottles, base temperature should be controlled at 5–8°C to optimize molecular orientation. Hot-fill bottles require significantly higher base temperatures.
2.5 Environment
The production environment significantly impacts process stability. Constant temperature and low humidity conditions are ideal for PET bottle blow molding.
Additional Requirements
Pressure-resistant bottles must meet both stress testing and pressure resistance testing requirements. Stress testing ensures molecular integrity to prevent cracking or leakage when the bottle base comes into contact with lubricants (alkaline) during filling. Pressure testing ensures the bottle can withstand internal gas pressure without bursting. The thickness of the center point must be controlled within a specific range:
Thinner center points improve stress testing results but reduce pressure resistance.
Thicker center points improve pressure resistance but may worsen stress testing results.
Stress testing results are also influenced by material distribution around the center point, which requires practical experience to optimize.
