Analysis of the PET Plastic Injection Molding Process
Release date:
2021-06-26 14:16
Author:
Editor
PET plastic injection molding is one of the key processes in plastics manufacturing. For PET bottles, most packaging manufacturers opt for blow molding; however, injection molding offers distinct processing advantages. When performing PET injection molding, technicians must take numerous factors into account, as outlined below.
Introduction to PET Plastic: Chemically, PET is known as polyethylene terephthalate, also referred to as polyester. Currently, most customers use GF‑PET, which is primarily used for bottle preforms.
PET exhibits favorable rheological behavior in the molten state, with pressure having a greater influence on viscosity than temperature. Consequently, pressure is primarily used to modulate the melt’s flow characteristics.
Physical and chemical properties of PET: The glass transition temperature of PET is approximately 165°C, and its crystallization temperature range is 120–220°C. PET exhibits strong hygroscopicity at elevated temperatures. For glass-fiber-reinforced PET materials, bending deformation can easily occur under high-temperature conditions.
“Crystallinity can be enhanced by adding crystallization promoters. Transparent PET‑processed parts exhibit gloss and a higher heat distortion temperature. Special additives such as mica can be incorporated into PET to minimize warpage. Additionally, using unfilled PET resins at elevated mold temperatures can also yield transparent products.”
Injection molding process: Injection molding is primarily used to enhance the molding of PET. A screw-type injection molding machine is typically employed. The screw should generally undergo hardening treatment to prevent wear over prolonged use. The nozzle length of the injection molding machine should be as short as possible, with a diameter maintained at approximately 3 mm.
The melting point of enhanced PET reaches as high as 260°C. To prevent nozzle clogging, a higher‑power heater should be installed. Additionally, the nozzle orifice is machined into an inverted conical shape, as shown in Figure 1, which facilitates easy removal of molten material from the runner and the nozzle.
Injection Molding Machine: Injection molding is primarily used to enhance the molding of PET. PET can generally only be processed using screw-type injection molding machines.
It uses a set screw with a non‑turning collar at the head, featuring a hard surface and excellent wear resistance. The length-to-diameter ratio is not L/D = (15–20):1; the compression ratio is approximately 3:1.
Materials with an excessively high L/D ratio remain in the barrel for too long, leading to overheating and degradation that can compromise product performance. If the compression ratio is too low, shear‑induced heat generation is insufficient, resulting in poor plasticization and inferior part quality. Conversely, excessive glass fiber breakage can reduce mechanical properties. When processing glass‑fiber‑reinforced PET, severe wear occurs on the barrel inner surface; therefore, the barrel should be made of wear‑resistant material or lined with such material.
The nozzle should be short, its inner wall polished, and its orifice as large as possible. It is recommended to use a hydraulic‑actuated valve nozzle. The nozzle must be equipped with insulation and temperature‑control measures to prevent freezing and clogging; however, the nozzle temperature should not be too high, as this can lead to drooling. Before commencing molding, the barrel must be purged with low‑pressure polypropylene material.
Main injection molding conditions for PET:
1. Barrel temperature. PET has a relatively narrow processing temperature range, and the temperature level directly affects the part’s performance. If the temperature is too low, plasticization is inadequate, leading to defects such as sink marks and material shortages; conversely, excessive temperature can cause flash, nozzle drooling, darkening of color, reduced mechanical strength, and even degradation. Typically, the barrel temperature is maintained at 240–280°C; for glass-fiber-reinforced PET, the barrel temperature is 250–290°C, with a maximum of 300°C. The nozzle temperature is generally 10–20°C lower than the barrel temperature.
2. Mold temperature. Mold temperature directly affects the cooling rate and degree of crystallinity of the melt. Different degrees of crystallinity correspond to different properties of the molded part. Typically, mold temperature is maintained within the range of 100–140°C. For molding thin-walled parts, a lower temperature is preferred; for thick-walled parts, a higher temperature may be used.
3. Injection pressure. PET melt exhibits good flowability and is easy to mold. Medium injection pressures, typically 80–140 MPa, are generally used. For glass-fiber-reinforced PET, the injection pressure ranges from 90 to 150 MPa. The selection of injection pressure should take into account the viscosity, type, and amount of PET.
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