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2021

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Analysis of PET plastic injection molding process


PET plastic injection molding process is one of the important processes in plastic processing. For PET plastic bottles, in most cases, plastic packaging plants will use blow molding to produce and process, but injection molding technology has special processing advantages. For PET injection molding, technicians need to consider many factors, as follows.

Introduction to PET plastics: PET is chemically known as polyethylene terephthalate, also known as polyester. Currently, most customers use GF-PET, mainly for bottle preforms.

PET has good rheological properties in the molten state, and the effect of pressure on viscosity is greater than that of temperature. Therefore, pressure is mainly used to change the fluidity of the melt.

Physical and chemical properties of PET: The glass transition temperature of PET is about 165 ℃, and the crystallization temperature of the material ranges from 120 to 220 ℃. PET has strong moisture absorption at high temperatures. Glass fiber reinforced PET materials are prone to bending deformation at high temperatures.

"The crystallinity of materials can be improved by adding crystallization promoters. Transparent products processed with PET have a gloss and thermal deformation temperature. Special additives such as mica can be added to PET to minimize bending deformation. If the lower mold uses temperature, using non filled PET materials can also obtain transparent products.".

Injection molding process: Injection molding is mainly used for the molding of reinforced PET. Screw injection machines are commonly used. Screws generally require hardening treatment to avoid wear after long-term use. The length of the spray hole of the injection machine 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 ℃. To prevent nozzle clogging, higher power heaters should be installed. In addition, the nozzle hole is processed into an inverted cone as shown in Figure 1, which can easily cut off the flow path and the molten material in the nozzle.

Injection molding machine: Injection molding is mainly used for the molding of reinforced PET. PET can generally only be molded using screw injection molding machines.

Use raised head screws with non inverted rings on the top, with high surface hardness and wear resistance. The aspect ratio is not L/D=(15-20): 1, and the compression ratio is about 3:1.

Material with excessive L/D stays in the barrel for too long, which can easily cause degradation and affect product performance due to overheating. Compression ratio is too small, shear heat generation is less, plasticization is poor, and product performance is poor. On the contrary, more glass fibers will break and their 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 polished, and the aperture should be as large as possible. The use of hydraulic brake valve nozzles is recommended. The nozzle should have thermal 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, the barrel must be cleaned using low pressure PP material.

Main injection conditions for PET:

1. Barrel temperature. The molding temperature range of PET is relatively narrow, and the temperature directly affects the performance of the product. Low temperature, poor plasticization, resulting in defects such as dents and lack of material in plastic parts; Conversely, if the temperature is too high, it can cause edge overflow, nozzle drooling, darkened color, reduced mechanical strength, and even degradation. Generally, the barrel temperature is controlled at 240~280 ℃, and the barrel temperature of glass fiber reinforced PET is 250~290 ℃, not exceeding 300 ℃. The nozzle temperature is generally 10-20 ℃ lower than the barrel temperature.

2. Mold temperature. The mold temperature directly affects the cooling rate and crystallinity of the melt. Different crystallinity means different plastic properties. Generally, the mold temperature is controlled at 100~140 ℃. When forming thin-walled plastic parts, small values may be taken; When molding thick wall plastic parts, it can be a large value.

3. Injection pressure. PET melt has good fluidity and is easy to form. Generally, medium pressure is used, with a pressure of 80~140MPa. The injection pressure of glass fiber reinforced PET is 90~150MPa. The determination of injection pressure should consider the viscosity, type, and quantity of PET