Requirements for PET Preform Molds and Injection Molding Processes

　　PET preform molds are a rather specialized type of plastic mold, requiring careful attention to design, manufacturing, and machine‑setting procedures. Today, Funeng will walk you through the properties of PET material and highlight key considerations.

　　1. Injection Molding Machine Selection

　　Due to its short thermal stability after the melting point and its relatively high melting temperature, PET requires an injection system with multiple temperature zones and minimal self‑frictional heat generation during plasticization. Furthermore, the actual weight of the molded part (including sprue material) must not be less than two-thirds of the machine’s rated shot volume.

　　2. Plastic Processing

　　Because PET macromolecules contain ester groups, they exhibit a certain degree of hydrophilicity. Pellets are relatively sensitive to moisture at elevated temperatures; when the moisture content exceeds a critical threshold, the molecular weight of PET decreases during processing, resulting in discolored and brittle products.

　　Therefore, prior to processing, the material must be dried at 150°C for at least 4 hours; alternatively, it can be dried at 170°C for 3–4 hours. The air‑blowing method can be used to verify that the material is completely dry. The proportion of recycled PET preform resin should generally not exceed 25%, and the recycled material must be thoroughly dried.

　　3. Mold and Gate Design

　　PET preforms are typically molded using hot-runner molds. It is advisable to install an insulating plate between the mold and the injection molding machine platen, with a thickness of approximately 12 mm, ensuring that the plate can withstand high pressures. Adequate venting is essential to prevent localized overheating or cracking; however, the vent depth should generally not exceed 0.03 mm, as deeper vents can easily lead to flash.

　　4. Melt Temperature

　　It can be measured using the air‑melt method, with a range of 270–295°C; for the reinforced grade GF‑PET, the temperature can be set to 290–315°C, among others.

　　5. Injection speed

　　Typically, the injection speed should be fast to prevent premature solidification during molding. However, excessively high speeds can lead to elevated shear rates, making the material prone to degradation. The injection phase is usually completed within 4 seconds.

　　6. Back Pressure

　　The lower, the better, to avoid wear. Typically, it should not exceed 100 bar; in most cases, it is not necessary to use it.

　　7. Residence Time

　　Do not use excessively long dwell times to prevent molecular weight degradation, and avoid temperatures above 300°C whenever possible. If the shutdown is less than 15 minutes, perform only a dry purge; if it exceeds 15 minutes, clean with viscous PE and reduce the barrel temperature to the PE processing range until startup.

　　8. Precautions

　　(1) Recycled material should not be too large; otherwise, bridging may occur at the feed point, adversely affecting plasticization.

　　(2) If mold temperature is poorly controlled or material temperature is improperly regulated, “white haze” and opacity are likely to occur; conversely, when mold temperature is low yet uniform, cooling is rapid, and crystallization is minimal, the molded part will be transparent.