Precision Control: An In-Depth Analysis of Five Core Process Parameters Determining PET Preform Quality

Injection molding of PET preforms is a process highly sensitive to physical parameters. Even slight deviations can result in defects such as whitening, silver streaks, uneven wall thickness, or excessive acetaldehyde (AA) levels. The following are five core process parameters that must be strictly controlled in engineering practice.

1. Drying Process: The Art of Dew Point and Temperature

PET is a hygroscopic resin; if drying is incomplete, hydrolysis reduces the molecular weight, directly compromising the bottle's strength.

Core Logic: Drying temperature should be set between 170°C and 175°C, with a drying time of no less than 4–6 hours.

Critical Metrics: Moisture content of raw material entering the hopper must be below 0.002% (20 ppm). Dew point readings should remain consistently below -40°C.

2. Injection Speed: Multi-Stage Pressure Control

Due to the high viscosity and shear sensitivity of PET melt, the injection process typically employs a "slow-fast-slow" strategy.

Process Settings: Low speed is used initially upon entering the gate to prevent air bubbles; medium-to-high speed is used to fill the mold cavity; and low speed is switched to for holding pressure before the cavity is fully filled to prevent flash.

Parameter Recommendation: Injection pressure is generally set between 100 and 140 MPa, depending on the preform wall thickness.

3. Cooling Time: The Bottleneck of the Production Cycle

In PET preform injection molding, cooling time often accounts for 80% of the total cycle.

Engineering Details: Mold cooling water temperature should be controlled between 8°C and 12°C.

Key Points: Insufficient cooling may cause secondary crystallization in the preform core, leading to whitening; excessive cooling increases energy consumption and prolongs the cycle.

4. Back Pressure Control: Ensuring Plasticization Quality

Back pressure determines the melt quality and degassing efficiency during screw retraction.

Parameter Logic: Excessively low back pressure can result in trapped air or unmelted particles in the melt; excessively high back pressure causes shear overheating, increasing AA levels. Recommended value: Typically set between 0.5 and 1.5 MPa, requiring fine-tuning based on screw speed.

5. Holding Pressure and Switchover Point: Preventing Sink Marks and Stress

The purpose of the holding phase is to compensate for volumetric shrinkage as the PET cools.

Core strategy: Holding pressure is typically set at 50%–70% of the injection pressure.

Switchover logic: Switch to the holding phase when the injection position reaches over 95% to prevent over-packing the mold cavity and generating internal stress; this is crucial for ensuring uniformity during the subsequent blow molding process.

Summary: The essence of PET preform production lies in the precise management of heat. Only by codifying these critical parameters into Standard Operating Procedures (SOPs) can a consistently high yield rate be maintained in large-scale production.