What should be taken into consideration during PET preform injection molding?

　　PET, chemically known as polyethylene terephthalate, is also referred to as polyester. Currently, the most widely used grade among customers is GF‑PET, primarily for blow molding preforms. PET exhibits favorable rheological behavior in the molten state, with pressure having a more pronounced effect on viscosity than temperature; therefore, adjustments to melt flow are mainly achieved by controlling pressure.

　　1. 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 the permissible limit, the molecular weight of PET decreases during processing, resulting in discolored and brittle finished products. Therefore, it is essential to dry the material prior to processing—at a temperature of 150°C for at least 4 hours, typically 170°C for 3–4 hours. The air‑blowing test can be used to verify whether the material has been thoroughly dried. The proportion of recycled material should generally not exceed 25%, and any recycled material must be completely dried.

　　2. Selection of Injection Molding Machines

　　Because PET has a short stable window after its melting point and a relatively high melting temperature, it requires an injection system with multiple temperature zones and minimal self‑generated heat during plasticization. Additionally, the actual weight of the finished part—including sprue material—must not be less than two-thirds of the machine’s rated shot volume. In response to these requirements, Funeng has recently developed dedicated PET plasticization systems for medium‑ and small‑tonnage machines. Clamping force should be selected at a level exceeding 6,300 t/m².

　　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; this plate must be capable of withstanding high pressure. Adequate venting is essential to prevent localized overheating or cracking, but the vent depth should generally not exceed 0.03 mm, as deeper vents can easily lead to flash.

　　4. Hot-melt temperature

　　It can be measured using the air‑assisted injection molding method. The processing temperature ranges from 270 to 295°C; for the reinforced grade GF‑PET, it can be set between 290 and 315°C, among other options.

　　5. Injection speed

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

　　6. Back Pressure

　　The lower, the better—this helps prevent wear. Typically, it should not exceed 100 bar. In most cases, it is not necessary to use it.

　　7. Duration of Stay

　　Do not allow excessively long dwell times to prevent molecular weight reduction. Avoid temperatures above 300°C whenever possible. If the shutdown is less than 15 minutes, perform an empty‑shot purge; if it exceeds 15 minutes, flush the system with a PE‑grade resin and reduce the barrel temperature to the PE processing range until the next startup.

　　8. Precautions

　　⑴ The recycled material should not be too large; otherwise, bridging is likely to occur at the feed port, adversely affecting plasticization.

　　⑵ If mold temperature is not properly controlled or melt temperature is improperly managed, “white haze” may occur, resulting in a lack of transparency. A lower and more uniform mold temperature, combined with rapid cooling and reduced crystallization, yields a transparent finished product.