Application of PET materials

PET plastic injection molding process is one of the very important technical processes in plastic processing.

For PET plastic bottles, in most cases plastic packaging factories will use blow molding to produce and process them, but the injection molding process has unique processing advantages. For PET injection molding, technicians need to consider many factors.The chemical name of PET is polyethylene terephthalate, also known as polyester. Currently, GF-PET is used most among customers, mainly for preform making.

PET has good rheology in the molten state, and pressure has a greater impact on viscosity than temperature. Therefore, pressure is mainly used to change the fluidity of the melt.The glass transition temperature of PET is around 165°C, and the crystallization temperature of the material ranges from 120 to 220°C.

PET is highly hygroscopic at high temperatures. For glass fiber reinforced PET materials, bending deformation is very easy to occur at high temperatures. The degree of crystallization of the material can be increased by adding crystallization enhancers.Transparent products processed with PET have high gloss and heat distortion temperature. Special additives such as mica can be added to PET to minimize bending deformation.If lower mold temperatures are used, transparent products can also be obtained using unfilled PET materials.

The injection molding method is mainly used to enhance the molding of PET, usually using a screw injection machine.Screws generally need to be hardened to avoid wear after long-term use. The length of the injection machine nozzle hole should be as short as possible, and its diameter should be controlled at about 3mm.

The melting point of reinforced PET is as high as 260°C. To prevent nozzle clogging, a more powerful heater should be installed. In addition, the tip of the nozzle hole is preferably processed into a reverse cone type so that the molten material in the flow channel and nozzle can be easily cut off.

Injection molding is mainly used to enhance the molding of PET. PET can generally only be formed using a screw injection molding machine.

It is best to use a mutant screw with a non-return ring on the top. Its surface is hard and wear-resistant. The length-to-diameter ratio is not L/D=(15~20):1 and the compression ratio is about 3:1. If the L/D is too large, the material will stay in the barrel for too long, and excessive heating will easily cause degradation and affect the performance of the product.

If the compression ratio is too small, the shear heat will be less, which will result in poor plasticization and poor product performance. On the contrary, more glass fibers will break and the mechanical properties will decrease. When processing glass fiber reinforced PET, the inner wall of the barrel is severely worn, and the barrel is made of wear-resistant materials or lined with wear-resistant materials.

The nozzle should be short, the inner wall should be polished, and the hole diameter should be as large as possible. It is better to use hydraulic brake valve type nozzle. The nozzle must have insulation and temperature control measures to ensure that the nozzle will not freeze and block. However, the temperature of the nozzle should not be too high, otherwise it will cause salivation. Before starting molding, low-pressure PP material must be used and the barrel must be cleaned.

1. Barrel temperature

The molding temperature range of PET is narrow, and the temperature will directly affect the performance of the product.

If the temperature is too low, the plasticization will not be good, causing defects such as dents and missing materials in the plastic parts; conversely, if the temperature is too high, it will cause overflow, drooling of the nozzle, darker color, reduced mechanical strength, and even degradation.

Usually, the barrel temperature is controlled at 240~280℃. The barrel temperature of glass fiber reinforced PET is 250~290℃ and shall not exceed 300℃. The nozzle temperature is generally 10~20℃ lower than the barrel temperature.

2. Mold temperature

Mold temperature directly affects the cooling rate and crystallinity of the melt. Different crystallinity will result in different properties of the plastic parts.

Usually the mold temperature is controlled at 100~140℃. When forming thin-walled plastic parts, a smaller value can be taken; when forming thick-walled plastic parts, a larger value can be taken.

3. Injection pressure

PET melt has good fluidity and is easy to form. It usually uses medium pressure, with a pressure of 80~140MPa. The injection pressure of glass fiber reinforced PET is 90~150MPa.

The injection pressure should be determined by considering the viscosity of PET, the type and quantity of fillers, the location and size of the gate, the shape and size of the plastic part, the mold temperature, and the type of injection molding machine.