The PET plastic sheet has good performance and meets the requirements of the national hygiene index of molded products and the international environmental protection requirements. PET sheet is relatively complicated in the production process. It is produced in a series of compression polymerization processes using high-precision machinery. Generally, the plastic is a translucent substance, but the transparency can be changed in real time according to its specific actual requirements. Moreover, the ductility and plasticity are good, and the combustion will not produce polluting gas, and the harm to the environment is small.
Production technology of PET plastic sheet
1. PET plastic sheet
Like other plastics, the properties of PET sheet are closely related to molecular weight. Molecular weight is determined by intrinsic viscosity. The higher the intrinsic viscosity, the better the physicochemical properties, but the poor fluidity makes it difficult to form. The lower the intrinsic viscosity, the worse the physicochemical properties and impact strength. Therefore, the intrinsic viscosity of the PET sheet should be 0.8dl/g-0.9dl/g.
2. Production process of PET plastic sheet
The main production equipment of PET sheet includes crystallization tower, drying tower, extruder, die head, three-roll calender and coiler. The production process is: raw material crystallization-drying-extrusion plasticization-extrusion molding-calendering-setting-rolling products.
The PET chips are heated and crystallized in a crystallization tower to make the molecules align, and then the glass transition temperature of the chips is raised to prevent sticking and clogging of the hopper during the drying process. Crystallization is usually an essential step. Crystallization takes 30-90 minutes at temperatures below 149°C.
At high temperatures, water hydrolyzes and degrades pet, causing its characteristic adhesion to decrease. Its physical properties, especially impact strength, decrease with decreasing molecular weight. Therefore, before melting and extrusion, pet should be dried to reduce the moisture content, which should be less than 0.005%. Due to the hygroscopicity of the PET material, when the water penetrates deep into the surface of the slice, molecular bonds will be formed, and another part of the water will penetrate deep into the slice, making drying difficult. Therefore, ordinary hot air cannot be used. The hot air dew point is required to be lower than -40C, and the hot air enters the drying hopper through a closed circuit for continuous drying.
After crystallization and drying, pet is transformed into a high polymer with a clear melting point. The polymer molding temperature is high and the temperature control range is narrow. A polyester-specific barrier screw is used to separate unmelted particles from the melt, which helps maintain a longer shear process and increases the output of the extruder. Features a flexible lip die with a streamlined throttle rod. The die head is tapered. Streamlined runners and scratch-free die lips indicate that the finish should be good. The mold heater has a drain and cleaning function.
- Cooling and forming
After the melt exits the head, it directly enters the three-roll calender for calendering and cooling. The distance between the three-roll calender and the machine head is generally kept at about 8cm, because if the distance is too large, the sheet is prone to sag and wrinkle, resulting in poor sheet finish. In addition, due to the long distance, slow heat dissipation and cooling, the crystals turn white, which is not conducive to rolling. The three-roll calendering unit consists of upper, middle and lower rolls. The axis of the intermediate drum is fixed. During the cooling calendering process, the roll surface temperature is 40°c-50c. The axis of the up and down rollers can move up and down.
The advantages of PET plastic sheet
- It has excellent transparency and gloss, and does not transmit ultraviolet rays.
- It has good bending resistance, good performance and high impact strength, which is several times that of ordinary materials.
- Good drug resistance, not easy to react with other chemicals.
- Good surface decoration, can be printed without surface treatment, easy to metallize.
- It has good barrier properties to water, liquid, edible oil and gas, and is not easy to penetrate.
- High molecular conductive polymer coating, low surface resistivity, stable performance, not affected by ambient temperature.
- Odorless and non-toxic, it can be directly used in food packaging.
Application of PET plastic sheet
PET sheet has the characteristics of good product transparency, small specific gravity, good surface gloss, bright color, crystallization point, less water ripple, toughness and impact resistance, chemical resistance and easy processing and forming. The material has the characteristics of good ductility, strong isolation, medical use and impermeability, and is also suitable for other fields.
- In terms of plastic films: such as food, medicine, and sterile packaging films; as well as packaging films for precision machinery, electronic product packaging, and large machinery.
- Bottled packaging: such as drinking bottled water, mineral water, beverage bottles, daily oil, medicinal bottles, daily chemical products, etc.
- Precision instruments for household appliances: such as electronic connecting wires, integrated circuits, various switch boxes, unpacking boxes, TV accessories, etc.
- Chemical machinery and equipment: mechanical parts, chains, frames, etc.
Reasons for the brittleness of PET plastic sheets
A large part of the brittleness of PET sheets is caused by internal stress. There are many reasons for product brittleness, including:
(1) There are dead corners or obstacles in the barrel, which can easily promote the degradation of the molten material.
(2) The plasticizing capacity of the machine is too small, and the plasticizing of the plastic in the barrel is insufficient. The plasticizing capacity of the machine is too large, the plastic is heated and sheared in the barrel for too long, the plastic is easy to age, and the product becomes brittle.
(3) The ejector is inclined or unbalanced, the top trunk section is small or the distribution is unreasonable.
(1) If the gate is too small, adjust the gate size or increase the auxiliary gate.
(2) If the shunt channel is too small or improperly configured, try to reasonably balance or increase the size of the shunt channel.
(3) Abnormal injection cycle caused by poor mold structure.
(1) The temperature of the cylinder and nozzle is too low, adjust. If the material degrades easily, the temperature of the cylinder and nozzle should be increased.
(2) Reduce the back pressure and speed of the screw pre-forming, make the material slightly loose, and reduce the degradation of plastic due to shear overheating.
(3) The mold temperature is too high, and it is difficult to demold. The mold temperature is too low, the plastic cools prematurely, the welding is not good, and it is easy to crack. Especially high melting point plastics such as PET sheets.
(4) The cavity core should have an appropriate draft slope. When the core is difficult to demould, the cavity temperature should be increased and the cooling time should be shortened. When it is difficult to remove the cavity, the cavity temperature should be lowered and the cooling time should be extended.
(5) Minimize the use of metal inserts, such as polystyrene plastics with large brittle heat and cold specific volumes, let alone insert injection molding.
Raw material level
(1) Other impurities or inappropriate or excessive solvents or other additives are mixed into the raw materials.
(2) PET sheet, heated under humid conditions, will undergo catalytic cracking reaction with water vapor, resulting in large strain of parts.
(3) Too many plastic recycling times or too high recycled material content, or too long heating time in the barrel will promote the brittleness of the parts.
(4) The quality of the plastic itself is not good, such as large molecular weight distribution, too many components with uneven structure such as rigid molecular chains; or pollution caused by other plastics, bad additives, dust impurities, etc.
Product design level
(1) Products with sharp corners, gaps or parts with large thickness differences are prone to stress cracking.
(2) The product design is too thin or too hollow.
DESU is an excellent plastic sheet and coil manufacturer in China, the main products are PP plastic sheet, PS/HIPS plastic sheet, PET/RPET/PETG/GAG plastic sheet, EVOH high barrier composite plastic sheet, etc. We have a number of complete high-quality plastic sheet production lines and strict and high-standard testing departments to ensure stable product quality and worry-free quality.
Our PET plastic sheets are made of high quality 100% virgin raw materials, we can also customize the production of conductive anti-static PET sheets, coating PET sheets, printing PET sheets, etc. according to your needs. If you are interested in our products, you are welcome to contact us at any time!