PET Bottle Base Whitening and Bursting? Check These 4 Key Parameters

During the PET container blow molding process, the bottle base is the region with the highest stress concentration and the most significant variations in material stretch ratios. In workshop operations, base "whitening" (crazing) or "high-pressure bursting" are classic process anomalies: the former is typically associated with "stress whitening" caused by low stretch temperatures or over-stretching; the latter stems mostly from uneven wall thickness distribution, localized overheating annealing, or stress concentration.

When base distribution defects occur frequently on a blow molding line, there is no need to blindly alter whole-machine settings. Systematically checking the following 4 key process parameters will help locate and resolve the issue.

I. Base Pre-Heating Temperature and Heating Zone Distribution

The molecular orientation of the PET bottle base relies on a precise temperature window. Excessively high or low temperatures directly impact base appearance and physical strength:

• Whitening Issues (Low Temperature): When the preform base heating temperature falls below the glass transition temperature of the polymer, the PET material cannot flow smoothly under stretch force. Macromolecular chains are forcibly displaced, creating micro-cracks that manifest as "stress whitening." During troubleshooting, moderately increase lamp power in the lower heating zones or fine-tune oven ventilation.

• Bursting Issues (High Temperature): If the preform base is overheated, the material undergoes localized crystallization or even thermal degradation, losing its essential toughness. During subsequent high-pressure blowing or filling tests, the overheated zone fails to withstand internal pressure and suffers brittle bursting. At this stage, lower the heating proportion of the base zone or strengthen cooling airflow at the preform base.

II. Pre-Blowing Pressure and High-Pressure Switchover Timing

The function of pre-blowing is to inflate the preform uniformly into a preliminary shape while the stretch rod descends. The interplay between pressure and timing directly dictates the extension pattern of the base material:

• Premature Pre-Blowing or Excessive Pressure: The preform is over-inflated before the stretch rod reaches the bottom, forcing material prematurely against the mold sidewalls while leaving excessive residual resin at the base. When high-pressure air switches in, stress concentration occurs at the base transition zone due to abrupt wall thickness changes, resulting in bursting.

• Delayed Pre-Blowing or Insufficient Pressure: The stretch rod reaches the bottom without adequate pre-blowing air, causing the rod to stretch the base material too thin or even puncture it. This creates an extremely thin base area prone to rupture during the high-pressure stage, or causes base center whitening due to instantaneous over-stretching.

III. Stretch Rod Speed and Bottom Clearance Stroke

The stretch rod controls the axial orientation of the PET material. The operational rhythm of the stretch rod and the bottom clearance (the gap between the stretch rod tip and the top surface of the bottom mold) are critical to base quality:

• Excessive Clearance: The stretch rod fails to deliver the preform base material to its intended position, resulting in excessive resin accumulation at the base center that cools insufficiently, causing crystalline whitening or reduced internal pressure resistance.

• Insufficient Clearance or Mechanical Collision: The stretch rod descends too deep, directly striking the inner wall of the bottle base and causing mechanical damage or micro-cracks. High-pressure blowing expands these cracks instantly, causing bursting. Check stretch cylinder stroke settings and mechanical stop alignment.

IV. Bottom Mold Cooling Circuit and Water Temperature Control

In the cooling and shaping phase after blowing, the bottom plug absorbs residual heat from the bottle base. Because the base is usually the thickest section of the container, it demands high-efficiency heat dissipation:

• Clogged Cooling Channels or High Water Temperature: If the bottom mold cooling channels scale up, suffer insufficient flow, or run at elevated water temperatures, the base material cannot solidify fully before demolding. Upon exiting the mold, residual heat triggers secondary thermal crystallization, leading to whitening or causing the base to expand and burst due to residual internal air pressure.

• Mold Water Accumulation and Temperature Variances: Ensure smooth water circulation and stable temperatures in the bottom mold to prevent uneven material cooling caused by localized temperature differentials.

Conclusion Troubleshooting PET blow molding base defects should follow a logical sequence: "Check temperature first, check timing second, and verify mechanicals and cooling last." By systematically fine-tuning heating zones, pre-blowing sequence, stretch stroke, and bottom mold cooling, operators can rapidly suppress base whitening and bursting rates, ensuring efficient and stable workshop performance.