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Injection Molding of Preforms

Injection Molding of Preforms

The subsequent pivotal phase in PET container fabrication involves the thermoplastic injection molding of parisons. These tubular precursors, geometrically resembling laboratory test tubes with integrated neck finishes, constitute the essential substrate for ultimate container formation.

Thermal Conditioning Process

The transformation initiates with precise thermal regulation of PET resin granules within the feeding apparatus:

  1. Thermal Profiling:
    Granular feedstock undergoes staged heating (260-280°C) through multiple barrel zones, achieving optimal viscoelastic properties while preventing thermal degradation. The enthalpy-controlled transition ensures complete crystalline structure annihilation.
  2. Rheological Monitoring:
    Melt flow index (MFI) is continuously tracked using capillary rheometry, with target viscosity ranges of 200-400 Pa·s (at shear rates of 1000 s⁻¹). Deviations beyond ±5% trigger automatic process adjustments to maintain morphological consistency.

Molding System Architecture

The injection molding apparatus comprises three functionally integrated subsystems:

  1. Injection Module:
    • Reciprocating screw mechanism with L/D ratio ≥20:1
    • Dynamic pressure control (80-120 MPa injection pressure)
    • Melt cushion precision (±0.3mm)
  2. Tooling System:
    • P20 tool steel molds with conformal cooling channels
    • Cavity pressure sensors (50-100 MPa monitoring range)
    • Micro-textured surfaces (Ra 0.05-0.2μm) for controlled release
  3. Solidification Control:
    • Differential cooling rates (5-15°C/s) through pulsed coolant flow
    • In-mold crystallization monitoring via dielectric sensors
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Post-Molding Operations

The demolding sequence incorporates:

  1. Automated Ejection:
    • Servo-driven ejector pins with <0.01mm positional repeatability
    • Vision-guided robotic handling (cycle time <3s)
  2. Quality Assurance Protocol:
    • Dimensional verification (CMM with ±5μm tolerance)
    • Optical inspection for flow marks/sink marks
    • Intrinsic viscosity measurement (0.72-0.84 dl/g range)

This meticulously controlled manufacturing sequence ensures parisons possess the requisite molecular orientation and structural integrity for subsequent stretch blow molding operations, where axial stretch ratios of 2.5-3.0:1 will be achieved.