Strategies to Enhance Effective Cooling in PET Preform Molding for Slash Cycles

According to an article by Ottmar Brandau in Plastics Technology, effective cooling is influenced by ten key factors: injection time, holding time, cooling time, robot take-out or free-fall, demolding temperature, machine drying cycle time, water temperature, water pressure, coolant composition and tool configuration.

Among the ten factors, the injection time, holding time, cooling time, and ejection temperature are typically the most salient, as they directly influence the quality of the molded part and the efficiency of the production process.

1.    Injection time
This refers to the duration during which molten polymer is injected into the mold, which impacts the rate of filling and the quality of the part. An injection time that is too brief may lead to underfilling, while an excessively long time can result in overfilling and shrinkage-related defects. Resin suppliers recommend an injection rate of 8 to 12 grams per second per cavity for most preforms, but thicker preforms can be filled faster, with a 525-gram preform having a wall thickness of 8.5 mm filling in just 27 seconds for a speed of 19.4 grams per second per cavity.

2.    Hold time
The holding time is essential for maintaining pressure after injection to ensure uniform density and part integrity. A deficiency in holding time can result in voids or uneven distribution of material, while an extended holding time might induce overcooling and potential stress concentrations within the part.
It is important for ensuring proper filling of preforms as they transition from melt to solid density, and its duration should be adjusted based on preform thickness and cooling rates, with a transition point set between 6% and 8% of the total stroke for thicker preforms.

3.    Cooling time
This is the duration required for the molded part to reach the ejection temperature, which directly affects the cycle time. An insufficient cooling time may lead to premature removal of the part, potentially causing warpage or internal stresses. Conversely, an extended cooling time increases the cycle time and reduces productivity. To achieve the desired demolding temperature, it should be set at a minimum of 1.5 seconds, extending up to 20 seconds for thick preforms, to achieve the desired demolding temperature.

4.    Demolding Temperature 
The ejection temperature is crucial for facilitating the removal of the part from the mold without causing warpage or internal stresses. An ejection temperature that is too high can lead to part deformation, while one that is too low might cause the part to stick to the mold. It should be set at 45°C (113°F) to minimize visual flaws. The impact of these flaws on the final blown bottles depends on the stretch ratio, with high stretch ratios allowing for higher demolding temperatures without compromising aesthetics.

While other factors such as the method of part removal (robotics or gravity drop), machine drying cycle time, water temperature, water pressure, coolant composition, and tool configuration also play significant roles, they are often optimized by adjusting the previously mentioned key parameters.