The use of PET in beverage packaging has significantly accelerated the development of the beverage packaging sector. In turn, the growth of this industry has provided expanded opportunities for the application of PET. Strict control over PET preform injection and blow molding processes is essential to ensure both the appearance and cost-effectiveness of PET bottles.

Properties of PET

PET is a condensation product of ethylene glycol and terephthalic acid, forming a saturated thermoplastic polymer. PET molecules exhibit linear and semi-crystalline structures.

The simplest production process involves the reaction of terephthalic acid with ethylene glycol to form monomers (esterification), followed by polycondensation into long-chain PET polymers. The degree of polymerization varies with temperature and pressure.

Like many plastics, PET undergoes three phase transitions during processing: glassy state, high-elastic state, and viscous-flow state. These transitions involve three key temperatures: glass transition temperature (Tg), crystallization temperature (Tc), and melting temperature (Tf).

The transition from an amorphous glassy state to a rubbery state is known as the glass transition, indicating the initiation of long-chain segment movement. External heating increases molecular (chain segment) freedom, allowing molecules frozen in the glassy state to move. The glass transition depends on PET’s morphology. When the intrinsic viscosity (IV) is high, crystallization is more pronounced, molecular chain freedom is restricted, and Tg is higher.

As temperature increases, localized spherulites form, leading to molecular rearrangement due to intermolecular forces—i.e., crystallization. For PET, the maximum crystallinity is approximately 55%, limited by the slow rearrangement of aromatic rings, which hinders crystal formation.

If T < Tc, PET’s viscosity prevents chain segments from moving into an ordered arrangement (inhibiting crystallization). If T > Tc, thermal effects hinder the formation of amorphous regions (promoting crystallization).

The melting point (Tf) is the temperature at which all crystals disintegrate.

Hydrolysis of PET

Solid PET is highly hygroscopic and readily absorbs moisture from the air. During storage, PET absorbs moisture until saturated with ambient conditions, with saturation levels reaching up to 0.6% by weight.

Typically, PET shipped by suppliers has a moisture content below 0.1% by weight. To achieve optimal product performance, it is necessary to reduce moisture content to 0.004%, ideally below 30 ppm before melting.

Even trace amounts of moisture in the resin can trigger a series of reactions:

At temperatures above PET’s melting point (approximately 250°C), water rapidly causes polymer degradation (chain scission due to hydrolysis), reducing molecular weight, apparent viscosity, and related physical properties. In fact, hydrolysis begins at lower temperatures (e.g., 150°C) but at a slower rate, accelerating with increasing temperature. Under drying and molding conditions, the decrease in IV should not exceed 0.02 dl/g. Excessive viscosity reduction increases crystallization speed, adversely affecting preform transparency and reducing the mechanical properties of bottles, including load-bearing and impact strength.

Thermal Degradation of PET

Temperature’s effect on drying PET is complex, influencing not only the diffusion rate of moisture but also chemical processes during drying, ultimately affecting resin properties. Considering potential hydrolysis and thermal processes is crucial. As mentioned, hydrolysis accelerates above 150°C with decreasing IV. Since thermal transition processes are faster than diffusion processes, prematurely raising the temperature during drying is detrimental.

Similarly, even if most moisture is removed, excessively high temperatures (e.g., above 180°C) cause thermal degradation and thermo-oxidation (in air drying systems), leading to polymer chain scission and the release of byproducts, resulting in reduced physical properties.

Byproducts include acetaldehyde (AA). Changes in physical properties manifest in preforms as hazy crystallization, decreased IV, and yellowing.

Molding of PET Preforms

During preform molding, optimal conditions involve rapid, uniform, and complete melting at the lowest possible temperature and shortest time, minimizing IV reduction and AA generation while maximizing transparency. Related process conditions include:

1. Temperature
   Molding temperature refers to the temperatures of the barrel and hot runner. Only 30% of the heat during molding comes from external heating; 70% is generated from internal friction heat. Thus, in addition to appropriate heating, shear heat must be utilized effectively.

2. Injection and Holding Pressure
   Injection overcomes resistance in the flow channel to fill the mold with molten material. For preforms, a three-stage speed and pressure profile, sequentially decreasing, is ideal.

   • Too slow injection speed results in insufficient shear, cooling before filling, causing incomplete filling or short shots.

   • Too fast injection speed prevents adequate cavity venting, leading to incomplete filling, shrinkage, and high AA levels.
     Holding pressure has two key functions: preventing backflow and ensuring cooling under pressure (improving cooling efficiency).

   • Too high holding pressure causes overfilling, mold expansion, high internal stress, and potential crystallization.

   • Too low holding pressure results in shrinkage, preform deformation (insufficient cooling), and gate issues such as pinholes and bubbles due to reduced gate cooling rate. Holding time must also be appropriate; too short time causes pinholes and stringing.

3. Pressure Release
   Pressure release reduces pressure in the hot runner to prevent gate blockage and needle valve malfunction. Excessive release can cause shrinkage, stringing, and pinholes.

4. Back Pressure
   Back pressure is the kneading force applied to the melt by the hydraulic system via the screw during rotation driven by the oil motor. Its functions include enhancing PET plasticization and eliminating bubbles. Initially set to zero during startup, it should be gradually increased until preforms are free of bubbles or defects. Excessively high back pressure causes strong shear effects, leading to molding defects, gate blockage, and thermal degradation.

5. Cushion Zone
   The cushion zone is the residual material in front of the screw after each injection. Too little causes molding defects; too much leads to PET decomposition. Adjust gradually from low to high until preforms are clear and free of crystallization.

6. Cooling
   PET is inherently opaque; preform transparency relies on cooling. Inadequate cooling reduces the cooling rate, causing shrinkage, preform deformation, and extended cycle times. To avoid this, ensure proper water treatment, regular cleaning of water channels, checking water flow and pressure, and cleaning cores and cavities.

Blow Molding Process

Blow molding is a biaxial stretching process where PET chains undergo bidirectional extension, orientation, and alignment, enhancing the mechanical properties of the bottle wall, improving tensile, tensile strength, and impact strength, and ensuring excellent gas barrier properties. Although stretching improves strength, overstretching must be avoided. Control the stretch blow ratio: radial ratio should not exceed 3.5–4.2, and axial ratio should not exceed 2.8–3.1. Preform wall thickness should not exceed 4.5 mm.

Blow molding occurs between the glass transition temperature and crystallization temperature, typically controlled between 90–120°C. In this range, PET exhibits high elasticity. Rapid blow molding and cooling result in transparent bottles. In one-step machines, this temperature is determined by the cooling time during injection molding (e.g., Aoki blow molding machines), so coordination between injection and blow molding stations is crucial.

The blow molding process involves three rapid actions: stretching, pre-blowing, and final blowing. These actions must be well-coordinated, as the first two steps determine material distribution and blow molding quality. Adjustments include: stretching start timing and speed, pre-blowing start and end timing, pre-blowing air pressure, and pre-blowing air flow. If possible, control the overall temperature distribution of the preform and the temperature gradient between its inner and outer walls.

During rapid blow molding and cooling, induced stress forms within the bottle wall. For carbonated beverage bottles, this stress resists internal pressure, which is beneficial. However, for hot-fill bottles, ensure stress is fully released above the glass transition temperature.

Common Issues and Solutions

1. Thick Top, Thin Bottom: Delay pre-blowing time, reduce pre-blowing pressure, or decrease air flow.

2. Thick Bottom, Thin Top: Opposite of the above.

3. Wrinkles Below the Neck: Pre-blowing too late, pre-blowing pressure too low, or inadequate cooling in this area.

4. White Bottom: Preform too cold; overstretching; pre-blowing too early or pressure too high.

5. Magnifying Glass Effect at Bottom: Excess material at bottom; pre-blowing too late or pressure too low.

6. Wrinkles Inside Bottom: Bottom temperature too high (poor cooling at gate); pre-blowing too late, pressure too low, or flow too small.

7. Overall Haze (Opaque): Insufficient cooling.

8. Local Whitening: Overstretching; local temperature too low; pre-blowing too early; or contact with stretch rod.

9. Eccentric Bottom: Related to preform temperature, stretching, pre-blowing, or high-pressure blowing. Solutions: lower preform temperature; increase stretching speed; check gap between stretch rod and bottom mold; delay pre-blowing and reduce pressure; delay high-pressure blowing; check preform eccentricity.

Conclusion

1. PET bottles are widely used in daily consumer goods. Due to their lightweight, excellent preservation properties, and enhanced heat and pressure resistance, PET bottles have become the mainstream packaging for beverages. Many beverages requiring high-temperature sterilization, such as flavored water, juice, and sports drinks, are increasingly using PET bottles.

2. With low environmental impact and energy consumption, PET bottles are gradually replacing traditional packaging materials in today’s environmentally conscious world. Their heat and pressure resistance advantages are leading to the replacement of PVC bottles, aluminum cans, tin cans, and glass bottles, making PET the packaging material with the greatest growth potential.

3. PET processing is highly demanding, from mold manufacturing to machinery. While entry into the field is accessible, mastering it is challenging.