Pet contains polyethylene terephthalate (PET), which is the most important variety of thermoplastic polyester. It is also called pet or PEIT in English, commonly known as polyester resin. It is a condensation polymer of terephthalic acid and ethylene glycol, together with PBT, collectively known as thermoplastic polyester, or saturated polyester.
Pet has the following advantages:
1. It has good mechanical properties, impact strength is 3 ~ 5 times of other films, and has good folding resistance.
2. Resistant to oil, fat, enoic acid, dilute alkali and most solvents.
3. The heat resistance of pure pet is not high, the hot deformation temperature is only about 85 ℃, but it is greatly improved after strengthening treatment. The mechanical properties of Glass Fiber Reinforced PET are similar to those of PC, PA and other engineering plastics, and the thermal deformation temperature can reach 225 ℃; pet has good thermal aging resistance, embrittlement temperature is - 70 ℃, and it still has certain toughness at - 30 ℃; pet is not easy to burn, and the flame is yellow, and the combustion oil drops.
4. The permeability of gas and water vapor is low, which has excellent gas blocking, water, oil and odor performance.
5. It has high transparency, can block ultraviolet rays, and has good luster.
6. It is non-toxic, tasteless, hygienic and safe, and can be directly used in food packaging.
Pet is an opalescent or light yellow highly crystalline polymer with smooth and glossy surface. It has good creep resistance, fatigue resistance, wear resistance and dimensional stability, small wear and high hardness. It has the largest toughness in thermoplastic plastics: good electrical insulation performance, little influence by temperature, but poor corona resistance. It is non-toxic, weather resistant, chemical resistant, stable, low water absorption, weak acid and organic solvent resistance, but not resistant to hot water immersion and alkali.
PET resin has high glass transition temperature, slow crystallization rate, long molding cycle, large molding shrinkage, poor dimensional stability, brittle molding and low heat resistance.
Through the improvement of nucleating agent, crystallizer and glass fiber, pet has the following characteristics besides PBT.
1. The hot deformation temperature and long-term service temperature are the highest among the general engineering thermoplastics
2. Because of its high heat resistance, the Reinforced PET can be immersed in 250 ℃ solder bath for 10s without deformation or discoloration. It is especially suitable for the preparation of electronic and electrical parts for tin soldering
3. The bending strength is 200MPa, the elastic modulus is 4000mpa, the creep and fatigue resistance are also very good, the surface hardness is high, and the mechanical properties are similar to those of thermosetting plastics.
4. Because the price of ethylene glycol used in PET production is almost half as much as that of butanediol used in PBT production, PET resin and Reinforced PET are the lowest price among engineering plastics and have high cost performance ratio.
Process conditions of pet injection molding
1. The hygroscopicity of pet is slightly higher than that of PBT, and the granular material is more sensitive to water at high temperature. When the moisture content exceeds the standard, hydrolysis will occur during molding process, which greatly reduces the quality. Therefore, the raw materials must be dried before molding.
2. The crystallinity of pet is smaller than that of PBT, and decreases with the increase of molecular weight. Therefore, the molding cycle of pet is longer and can not be as rapid as PBT.
3. The shrinkage of PBT is about 1.8%, and that of glass fiber reinforced pet is from 0.2% to 1.0%. The shrinkage of plastic parts with high mold temperature is large, while that of low mold temperature plastic parts is small.
4. Pet products often have residual internal stress, so post-treatment is often needed after molding.
5. The addition of heat to the pet should not be sufficient when the amount of heat added to the molding does not exceed 20%.
6. The rheological property of pet in the melt state is non Newtonian, and the temperature has little effect on the melt viscosity. The pressure can be adjusted to increase the shear stress or shear rate.
7. Pet has an obvious melting point, and its processing temperature range is 270 ~ 290 ℃, and thermal degradation occurs when the temperature exceeds 300 ℃.
8. The holding time of pet molding should not be too long, otherwise the internal stress will be increased, generally 20 ~ 50s
9. Because of the rapid curing of PET melt, high-speed injection should be used to prevent the melt from filling the mold cavity as soon as possible.
10. Like PBT, pet is very sensitive to sharp corners and notches, where stress concentration is easy to occur, which greatly reduces the bearing capacity, and is prone to fracture under stress or impact. Therefore, arc transition should be adopted as far as possible for these places.
11. Pet mold runner should reduce the melt pressure drop as much as possible, avoid using high melt temperature, short and thick runner is better, the ratio of surface area to cross-sectional area should be small, and circular runner should be used more.
12. On the premise of avoiding the injection of melt, the pet gate should be set at the thickest part of the plastic part, which can avoid the shortcomings of high flow resistance and fast cooling, and ensure that it fills the mold cavity.
13. The orientation of pet gate should be facing the cavity wall or thick core, which can change the flow direction, reduce the flow rate, make the melt fill the cavity evenly and avoid surface defects.
14. Pet is pseudoplastic in melting.