5 Common Problems in PET Preform Moulding and How to Solve Them Effectively
In the highly competitive beverage and packaging industries, producing flawless PET preforms is essential for maintaining production efficiency and reducing cost-per-piece. However, even with advanced injection systems, technical challenges can arise during the injection moulding process.
As a leading global PET preform mold manufacturer, DEBODI Mould has over a decade of expertise in designing high-precision, multi-cavity hot runner molds. In this technical guide, our engineering team breaks down the 5 most common PET preform moulding defects and provides actionable engineering solutions to optimize your production line.
1. Acetaldehyde (AA) Level is Too High
The Issue: High AA levels impart an unpleasant chemical taste to water and beverages, which can ruin brand reputation. AA is caused by thermal degradation of the PET material during the melting process.
- Lower the barrel temperature settings, particularly at the rear zones.
- Reduce back pressure and screw rotation speed to minimize shear heat.
- Ensure your mold features an optimized hot runner balanced manifold—like the systems engineered by DEBODI Mould—to avoid material stagnation and overheating.
2. Preform Flash (Excess Material at Parting Lines)
The Issue: Flash occurs when molten plastic escapes from the mold cavity, typically appearing at the neck finish or parting lines. This leads to wasted material and poor post-blowing results.
- Increase the injection molding machine clamping force to ensure tight closure.
- Reduce injection pressure or injection speed in the final stage.
- Check for mold wear. High-precision tooling from DEBODI utilizes premium steel (like S136) and strict tolerances to completely prevent parting-line mismatch.
3. Long Gate / Stringing (Drooling at the Injection Point)
The Issue: Material stays attached to the preform bottom after injection, creating a long tail (stringing) or causing hot runner drooling. This blocks downstream blow molding equipment.
- Optimize the hot runner nozzle tip temperature—slightly lower it to ensure clean gate freeze-off.
- Increase the cooling time or adjust the valve gate pin timing if utilizing a valve-gated hot runner mold.
- DEBODI's advanced pneumatic valve-gate systems guarantee clean cut-offs without manual trimming.
4. Moisture & Haze (Cloudy Preforms)
The Issue: The preform appears white or cloudy instead of crystal clear. This is typically caused by insufficient raw material drying or poor resin crystallization control.
- Ensure PET resin is dried thoroughly in a desiccant dryer (moisture level must be under 0.005%).
- Check your mold's cooling system. Inefficient cooling promotes crystallization (whiteness). DEBODI molds optimize internal water cooling channels to ensure lightning-fast, uniform cooling across all cavities.
5. Wall Thickness Eccentricity (Core Shift)
The Issue: Uneven wall thickness across the preform body causes thin spots that burst during the stretch blow moulding process.
- Verify the alignment between the injection machine nozzle and the mold gate.
- Inspect clamping and alignment wear components within the tooling.
- Partner with a manufacturer utilizing high-precision machinery. DEBODI employs dual-taper positioning and self-locking design structures to prevent any core shift under high pressure.
Quick Troubleshooting Summary Table
| PET Preform Defect | Primary Cause | Key Engineering Solution |
|---|---|---|
| High AA Levels | Thermal degradation / Shear heat | Reduce barrel temperatures & back pressure |
| Preform Flash | Insufficient clamping / Tooling wear | Increase clamp force; Use S136 high-precision molds |
| Long Gate / Stringing | Improper nozzle tip temperature | Optimize hot runner temperature; Use valve gates |
| Cloudy / Haze Defect | Poor drying / Slow cooling rate | Ensure moisture < 0.005%; Optimize water channels |
| Eccentricity / Core Shift | Misalignment under injection pressure | Utilize double-taper self-locking mold structures |
