What is pet plastic sheet?
PET plastic sheet is a new type of plastic product developed in recent years. This is a thermoplastic ring-wrapped plastic product. It is odorless and tasteless after burning, and does not produce toxic gas. High transparency, no water ripple, no crystal point, no direction, no whiteness, strong oil resistance and chemical resistance, good toughness and rigidity, good impact resistance (low temperature impact resistance is also good, 20% higher than PVC about).
It has a good barrier, high transparency, high impact resistance, good cold resistance, and is not easy to be broken; high strength, not afraid of friction, and excellent material properties. The special PET plastic sheet also has its unique properties: low temperature heat sealing, High temperature resistance, anti-mirror electricity.
Advantages of PET plastic sheet:
1. Bending resistance, good performance, high impact strength, several times that of ordinary materials.
2. Not easy to react with other chemicals.
3. Water, liquid, edible oil and gas are easy to block and not easy to penetrate.
4. Transparent color, anti-ultraviolet penetration.
5. No smell, non-toxic, can be directly used in food packaging.
The product has good transparency, small specific gravity, good surface gloss, bright color, less crystal points and water ripples, and has the characteristics of impact resistance, strong chemical resistance, and easy processing and molding.
What is the production process of PET plastic sheet?
The main production equipment of PET plastic sheet is: Crystallization tower, Drying tower, Extruder, machine head, Three-roll calender, Coiler.
The production process is: Raw material crystallization– Drying–extrusion plasticization– Extrusion molding– Calendering and setting–Winding products.
Step one is crystallization.
The PET plastic sheet is heated and crystallized in the crystallization tower, so that the molecules are arranged neatly, and then the glass transition temperature of the slice is increased. The purpose is to prevent adhesion in the drying process and block the hopper. Crystallization is usually an essential step and takes between 30 minutes and 90 minutes at temperatures below 149 ° C.
Step two is drying.
Under high temperature conditions, water will cause hydrolysis degradation of PET, resulting in the decline of its characteristic adhesion, its physical properties, especially impact strength, will decrease with the reduction of molecular weight. Therefore, PET must be dried before melting and extrusion to reduce the moisture content, which should be less than 0.005%. Desiccant dryers are used for drying. Due to the hygroscopic nature of PET materials, molecular bonds form when water penetrates the slice surface. Another part of the moisture will be deep in the slice, making drying difficult. Therefore, ordinary hot air should not be used.
Step three is extrusion molding.
PET is transformed into high polymer with obvious melting point after crystallization and drying. The high polymer molding temperature is high and the temperature control range is narrow. The barrier screw specially used for polyester is used to separate the unmelted particles from the melt, which helps to maintain a longer shearing process and improve the output of PET foam production line. A streamlined runner and no scratch die lip indicate a good finish, the nose heater has a drainage and cleaning function, and the gap between the head die lip should ensure good uniformity.
The uniform die lip gap of the mold directly affects the lateral thickness deviation of the sheet and the flatness of calendering. When extruding into PET sheets, the front of the fuselage, the middle of the fuselage, the rear of the fuselage, the screen changer, and the fuselage head The corresponding temperatures are 240°C-260C, 265°C-275°C and 260C-265°C, respectively, 260C-265C, 255°C-265C.
Step four is cooling and shaping.
After the melt comes out of the head, it directly enters the three-roll calendering machine for calendering and cooling. The distance between the three-roll calendering machine and the head is generally kept at about 8cm. If the distance is too large, the sheet is easy to sag and wrinkle, resulting in poor flatness of the sheet. In addition, due to the large distance, heat dissipation and cooling are too slow, resulting in white crystals, which are not conducive to calendering.
The three-roll calendering unit is composed of the upper, middle and lower three rollers. The upper and lower rollers can move up and down by moving the shaft up and down to adjust the roll gap. The temperature of the upper and lower rollers is 30℃-60℃ and 52℃-68℃ respectively. The three rollers should ensure the same speed, so as to offset the expansion of the material when it leaves the mold, weaken the internal stress of the plate and make the wrinkles disappear. PET crystallizes in the range of 100°C to 250°C, and at a faster rate of 140°C to 180°C, so it can be crystallized in a few seconds.
PET plastic sheets need to pass through this crystallization temperature zone quickly and strictly control the temperature of the three rollers. The aim is to counteract the expansion of the material as it leaves the die, weakening the internal stress of the sheet and making wrinkles disappear. PET crystallizes in the range of 100°C to 250°C, and at a faster rate of 140°C to 180°C, so it can be crystallized in a few seconds.
Step five is winding products
PET plastic sheet roll by roll for packaging.