Analysis Of PET Heat Shrinkage Tubing

- Apr 16, 2019-

The molecular structure of PET has rigid benzene ring and polar ester group. PET is a typical crystalline polymer with high glass transition temperature and melting point, which is characterized by high rigidity and poor toughness. It is easy to hydrolyze under high temperature and humidity, and its density is 1.38-1.41.

* performance

1. Mechanical properties

The mechanical properties are related to crystallinity and orientation, and are anisotropic. The mechanical properties of glass fibers are improved when glass fibers are added.

2. thermal properties

PET has obvious melting point, 256 265 and glass transition temperature 81 with low melting viscosity, good fluidity and decomposition temperature of 300.

3. water absorbency

PET has a certain water absorption, it must be dried before processing, and the air tightness of PET is better.

4. Optical properties

PET has good transmittance and high transparency, which is related to crystallinity and decreases the crystallinity.

5. Chemical resistance

Chemical resistance is better at room temperature. With the increase of temperature, chemical resistance decreases and hydrolysis is easy at high temperature and humidity.

6. weatherability

Long-term exposure to the outdoor environment leads to a decrease in mechanical properties and an increase in weatherability after glass fibers are added.

* processing

1. PET is water-absorbent. The raw materials containing water will hydrolyze under high temperature and humidity, which will lead to discoloration and brittleness of the products. Therefore, it must be fully dried before processing. The drying temperature is 140-170 C and the drying time is 4-6H. For the products with high transparency, it is necessary to fully dry them. Otherwise, the rate of defective products will increase, mainly bubbles, and the best way is dehumidification drying.

2. Heating shrinkage temperature

Because of the obvious melting point and easy decomposition of PET, its processing temperature is narrower. The general barrel temperature is 240-290 C. When the temperature is higher than 295 C, it will decompose and turn black. The processing and forming of PET should be strictly controlled.

3. Mould temperature

PET is a crystalline polymer with slow crystallization speed. If we want to get high crystallization products, the mould temperature will be higher, generally 100-130 C, and a longer cooling time. If we want to get products with high transparency, the mould temperature will be lower, generally 20-30 C, so that it can be cooled rapidly to below the glass transition temperature, showing an amorphous structure. Crystallization will reduce the transparency.

4. Injection speed, pressure and back pressure

Because of the low viscosity, good fluidity and easy filling of PET in melting state, it can adopt faster injection speed and lower injection pressure, and the back pressure can not be too high, the back pressure is too high, the shear is too large, the melt temperature rises greatly, and the melt is easy to decompose. At the same time, salivation and wire drawing will occur, but the products will have bubbles because of the bad exhaust gas when the back pressure is too low.

5. hot runner

Hot runner, especially bottle embryo, can be used in PET injection processing to improve work efficiency. Hot runner is very important for temperature control. It will salivate and draw when the temperature is too high. If the temperature is too low, the products will have defects such as fusion lines and low transparency.

6. post processing

The crystallization and orientation temperature of PET is high, and it will not relax after orientation. If the product has the requirement of dimension accuracy, it should be processed generally.

* Special screw for PET

According to the characteristics of PET, the special screw double-wall heat shrinkable pipe for PET has good effect in the actual production of PET bottle embryo, and has the advantages of stable quality, high yield and high production efficiency.

The characteristics of screw;

1. In the same type of machine, large diameter screw is used, and the length-diameter ratio is large.

2. Compression ratio is small, with auxiliary threads in homogenization section.

3. The feeding section is longer and the groove is deeper.

Advantage:

1. Using large diameter screw, the feeding section is longer, the screw groove is deeper, the conveying capacity is improved, the amount of melt glue is large, the speed of melt glue is fast, and the injection volume is large and the production efficiency is high for the same size machine.

2. Using twin threads, large length-diameter ratio, optimum design of screw parameters, strengthening the ability of plasticizing, homogenizing and exhausting, reducing defects such as white spots, bubbles and crazes, ensuring product quality, improving product yield and production efficiency.