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How to use an electric injection molding machine to solve PET preform thickness errors?

2026/01/02 By le zhan

PET preform injection molding(1)

Even minute deviations in PET preform wall thickness can lead to unstable stretch blow molding performance, weak areas in the bottle body, weight discrepancies, and high scrap rates. In high-speed production, tolerances are typically only a few millimeters, and the margin for human error is tiny. Therefore, many manufacturers are now turning to electric injection molding machine to stabilize production, reduce variability, and improve yield. Electric injection molding provides the precise motion control, rapid data sampling, and deterministic feedback loop required for PET preform forming. When combined with integrated injection molding processes, these electric injection molding machines can transform variable manual operations into a highly repeatable, stable injection molding system.

PET Preform Thickness Errors: Root Causes and Early Warning Signs

Before adjusting any injection molding machine, a reliable diagnostic should be performed. PET preform thickness errors rarely occur out of thin air; they result from the interaction of multiple factors, including materials, machines, molds, and processes. Common root causes include:

1. Unstable Melt Temperature: PET is heat-sensitive; small temperature changes can alter its viscosity and flowability. Hot and cold spots in the barrel or nozzle can cause thickness variations.

2. Inconsistent Injection Speed/Pressure: Variations in speed or changeover time can lead to uneven filling and holding pressure.

3. Incorrect Speed/Pressure Switching: Switching too early or too late can alter the final part’s quality and Shore thickness.

4. Screw or Barrel Wear: Worn screw blades reduce melt uniformity, causing pressure and injection weight drift.

PET Preform Thickness Errors

Why do electric injection molding machine improve the injection stability of PET preforms?

Electric platforms offer several advantages that significantly reduce thickness deviations during PET preform molding:

High-Resolution Servo Control: Electric servo drives enable millisecond-level position and speed control. This ensures highly repeatable injection profiles—crucial for thin-walled PET preforms, as filling dynamics determine the final thickness distribution.

Fast, Deterministic Response: Compared to many hydraulic systems, electric injection molding machines can reach and stop at target speeds with significantly less deviation. This minimizes deviations in speed/pressure switching and holding pressure profiles.

Integrated Digital Control and High-Speed Data: Electric injection molding machines typically employ a high-speed bus architecture, enabling real-time coordination among injection, screw reset, barrel temperature controllers, and peripheral devices. (For example, Topstar’s electric injection molding machines achieve high-speed data transmission and collaborative control between control units. Digital control allows for precise monitoring and control of various parameters during the injection process, thereby improving production efficiency and stability.)

Energy Efficiency and Lower Operating Temperatures: Electric drives generate less heat than hydraulic systems, helping to maintain a stable thermal environment around the machine and mold—crucial for sensitive materials like PET.

These characteristics provide processors with practical tools for stable filling, precise control of the process window, and the minimization of preform thickness errors.

Why do electric injection molding machine improve the injection stability of PET preforms

How to Adjust Injection, Holding Pressure, and Switching for Thickness Control?

After diagnosing potential causes and confirming the injection molding machine’s operating status, make targeted parameter adjustments. You can use the following structured adjustment path to reduce thickness deviations.

1. Stabilize Melt and Barrel Temperatures

  • The goal is to keep the melt temperature stable throughout the cycles, with fluctuations within ±2°C. Use appropriately positioned thermocouples and verify through melt flow tests.
  • Use uniform barrel area control; confirm nozzle tip temperature stability.
  • If possible, use active nozzle heating with feedback.

2. Optimize the injection speed profile.

  • Start with a conservative filling speed to reduce shear force and flow front variations, then gradually increase to the minimum speed required for balanced filling, avoiding flow stalls.
  • Implement multi-stage, or “ramp,” speeds: increase the initial filling speed, then control the speed to slow near the gate for better material distribution.

3. Precisely set V/P switching.

  • Use a chamber pressure sensor or a weight-based switching method. Chamber pressure V/P is most reliable: the switching point should maintain the same relative pressure rise point across all cycles.
  • In electric injection molding machines, switching operations can employ smaller timing jitters. Set the switching operation before the gate freezes, but late enough to ensure adequate sealing.

4. Control Back Pressure and Screw Return

  • Moderate back pressure improves melt uniformity, but excessive back pressure increases shear stress and heat, requiring adjustments to achieve a uniform melt density.
  • Maintain consistent screw return stroke (±0.5 mm) and control return speed to avoid inconsistent injection volumes.

5. Minimize Cycle Variation

  • Utilize motor position control to ensure injection/platen movement proceeds as expected.
  • Lock formulation parameters to prevent operator adjustments without formal change control.
How to Adjust Injection, Holding Pressure, and Switching for Thickness Control

Mold and Cooling Design: Balancing Thermal Fields to Reduce Thickness Deviation

No matter how precise the injection molding machine is, a poorly designed mold will affect the final result. The thickness of PET preforms largely depends on the mold’s thermal conductivity during filling and packaging.

In design, we need to ensure that cooling channels are symmetrically arranged around each cavity. Flow rates should be balanced, and flow simulations can guide the selection of manifold dimensions and channel spacing. Additionally, ensuring short flow paths to vents/gates minimizes dwell time and flow-front instability, thereby preventing sudden cross-sectional changes, such as localized shearing and thinning. Simultaneously, insulation around the gate is crucial, as the gate area often becomes a grouting point due to uneven grout thickness. Controlling gate geometry and implementing localized cooling prevent overheating. Furthermore, conformal cooling can be used when necessary; for complex geometries, conformal channels or baffles can maintain a uniform temperature on the mold surface.

Stable PET Preform Production Achieved Through Integrated Electric Injection Molding Machine

Stable and repeatable PET preform thickness is achievable thanks to intelligent diagnostics, precise calibration, superior molds, and a suitable machine platform. Electric injection molding machines provide the exact motion, high-speed data transmission, and deterministic control required for modern PET preform manufacturing. Topstar’s electric injection molding machine utilize high-speed data transmission, cross-unit collaborative control, and an integrated injection molding process concept—providing a precise, real-time adjustment tool to minimize thickness errors and maximize PET preform yield.

 

 

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