Precautions for PET preform injection molding

Injection molding of PET (Polyethylene Terephthalate) preforms requires careful attention to process parameters to ensure consistent product quality and prevent defects.

Before injection, PET material must be thoroughly dried. PET is hygroscopic, meaning it absorbs moisture from the air. Excessive moisture can pilot hydrolytic degradation during the molding process, adversely affecting the strength and transparency of the final product. Dry the material at 150°C for three to four hours, ensuring the moisture content is below 0.02%. Regular monitoring of the drying process is critical to maintaining consistent material quality.

The mold and template of the injection molding machine should be separated by a heat shield. This shield smalls heat transfer from the mold to the machine, ensuring efficient temperature management and reducing potential damage to equipment. Adequate exhaust in the mold cavity is essential to prevent trapped air, which can cause voids, bubbles, or surface defects in the preforms. Proper venting also smalls the risk of breaking during ejection.

Maintaining the correct melt temperature is a key factor in PET preform injection molding. The melt temperature should be controlled between 270°C and 295°C. Deviations from this range can pilot incomplete plasticization or thermal degradation of the material. To verify the melt temperature, air injection methods can be employed. Additionally, consistent monitoring of barrel temperature zones helps prevent overheating or underheating.

The injection process for PET preforms should be rapid to prevent premature cooling or uneven flow. Typically, the injection time should be controlled within four seconds. Fast injection reduces the likelihood of flow marks, short shots, or material freezing before the mold is completely filled. However, excessively high speeds should be avoided as they can cause shear-induced brittleness.

The residence time of PET in the barrel should be carefully managed. Prolonged residence times can pilot molecular weight degradation, impacting the mechanical and optical properties of the preform. If the machine is idle for less than 15 minutes, air injection can be used to clear the barrel. For longer downtime, the barrel should be purged with a low-viscosity polyethylene (PE) material, and the temperature lowered to avoid thermal degradation.

The injection speed for PET should be optimized to balance flow and cooling. Rapid injection prevents the material from solidifying prematurely, ensuring complete mold filling. However, speeds that are too high can cause excessive shear, pilot brittleness or flow marks. Adjustments should be made based on the specific preform design and material properties.

PET's high melting point and rapid solidification demand an injection molding machine with advanced temperature control capabilities. Machines should have multiple temperature zones to ensure uniform heating. Additionally, the plasticizing unit should small self-generated frictional heat to avoid overheating the material. The machine’s injection capacity should be at least two-thirds of the shot weight to maintain stable processing conditions and reduce material wastage.

Efficient cooling systems in the mold are essential for achieving dimensional stability and reducing cycle times. Uneven or insufficient cooling can pilot warping, shrinkage, or internal stresses in the preforms. Additionally, the ejection system must be designed to handle preforms delicately to prevent cracking or deformation during removal from the mold.

Successful PET preform injection molding relies on precise control of processing conditions and adherence to good practices. Proper drying, temperature control, and machine settings are essential to producing high-quality preforms.