Introduction of Biaxial Stretching Process of PET Materials
Taking BOPET film as an example, the main equipment and processes are briefly described as follows:
① The raw materials used in batching and mixing ordinary BOPET film are mainly master batch chips and glossy chips. Masterbatch chips refer to PET chips containing additives, such as silicon dioxide, calcium carbonate, barium sulfate, kaolin, etc. The corresponding masterbatch chips are selected according to the different uses of the film.
Polyester film generally uses a certain amount of silicon-containing masterbatch chips and light chips to match, its function is to increase the microscopic roughness of the film surface through the distribution of silica particles in the film, so that the gap between the films during winding is increased. Holds a very small amount of air, preventing the film from sticking. The bright chips and a certain proportion of master batch chips are mixed by a metering mixer and then enter the next process.
②Crystallization and drying For high polymers that tend to absorb moisture (such as PET, PA, PC, etc.), pre-crystallization and drying must be performed before biaxial stretching. The purpose of this is to: increase the softening point of the polymer to prevent the resin particles from sticking to each other or agglomerate during the drying and melt extrusion process; remove the moisture in the resin and prevent the polymer containing ester groups from being melted during the extrusion process. Hydrolysis occurs or bubbles are generated.
The pre-crystallization and drying equipment of PET generally adopts a packed tower with a crystallization bed, and is equipped with a dry air preparation device, including an air compressor, a molecular sieve dehumidifier, and a heater. The pre-crystallization and drying temperature is 150~170℃, the drying time is about 3.5~4h, and the moisture content of the PET chips after drying is required to be controlled at 30~50PPm.
③ Melt extrusion The PET chips after crystallization and drying treatment enter into a single screw extruder for heating, melting and plasticization. In order to ensure good plasticizing quality of PET chips and stable extrusion melt pressure, the structural design of the screw is very important.
In addition to certain requirements for length-diameter ratio, line-pressing ratio, and each functional section, the Barrier-type screw is also required. The screw of this structure is beneficial to ensure the good plasticization of the extruded material and the temperature of the material at the outlet of the extruder. Uniformity, stable discharge of extruder and good exhaust, and it is beneficial to improve extrusion capacity. If the extrusion volume is not too large, a vented twin-screw extruder is recommended.
This extruder has two exhaust ports connected with two vacuum systems, and has good exhaust and dehumidification functions, which can extract the moisture and oligomers contained in the material, so it can save A complex pre-crystallization/drying system saves investment and reduces operating costs. The extruder temperature setting is about 210~280℃ from the feeding port to the die head.
④Casting
The die head is the key mold of the cast sheet, which directly determines the shape and thickness uniformity of the cast sheet. BOPET often uses a hanger-type die. The opening of the die is initially adjusted by several push/pull differential bolts with heating coils, and is automatically measured by an online thickness gauge and fed back to the heating bolts of the die. Fine-tuning of die lip opening. The temperature of the die head is controlled at 275~280℃.
Quench roll (also known as casting roll, commonly known as cold drum). The PET melt flowing out of the die head is rapidly cooled to below its glass transition temperature on a uniformly rotating quench roll to form a glass casting sheet with uniform thickness. The purpose of quenching is to make the thick sheet form an amorphous structure and minimize its crystallization, so as to avoid adverse effects on the next drawing process. For this reason, the surface temperature of the quench roll is required to be uniform and the cooling effect is good. At the same time, the speed of the quench roll is required to be uniform and stable. In addition, cooling water at about 30°C is passed through the quenching roll to ensure that the slab is cooled to below 50°C.
Electrostatic adsorption device. The function of the electrostatic adsorption device is to make the cast sheet and the quenching roll in close contact, and prevent the quenching roll from being involved in air when the quenching roll rotates rapidly, so as to ensure the heat transfer/cooling effect. The electrostatic adsorption device is composed of wire electrodes, high-voltage generators, and electrode retracting and discharging torque motors. Its working principle is: using the thousands of volts of DC voltage generated by the high-voltage generator, the electrode wire and the quenching roll are turned into positive and negative electrodes respectively (the quenching roll is grounded). The electrostatic charges of opposite polarities of the rollers, under the effect of the attraction of opposite poles, make the cast sheet and the surface of the quenching rollers tightly adsorbed together to achieve the purpose of removing air and good heat transfer.
⑤Longitudinal stretching The thick sheet from the casting machine is heated to a high elastic state in the longitudinal stretching unit for a certain multiple of longitudinal stretching. The longitudinal stretching machine is composed of preheating roll, stretching roll, cooling roll, tension roll and rubber pressing roll, infrared heating tube, heating unit and driving device. Longitudinal stretching is usually single-point stretching, but also multi-point stretching, such as two-point or three-point stretching. The longitudinal draw ratio is generated by the speed difference between the slow drawing roller and the fast drawing roller, which is generally 3.3 to 4 times.
⑥ Lateral stretching The lateral stretching machine is composed of an oven, a chain clamp and guide rail, a static pressure box, a chain tensioner, a guide rail width adjustment device, an opening and closing clamp, a hot air circulation system, a lubrication system and an EPC. Its function is to complete the transverse stretching of the film by preheating, tentering, heat setting and cooling respectively in the transverse stretching machine. The horizontal pull ratio is 3.5 to 4 times.
⑦ Traction winding and slitting The equipment for this process consists of several traction guide rollers, cooling rollers, flattening rollers, tension rollers, tracking rollers, edge trimming devices, thickness gauges and corona treaters. The biaxially stretched film can be wound and slitted after trimming, thickness measurement and corona treatment, and it is the finished product after passing the inspection.
The longitudinal and transverse physical and mechanical properties of the sheet produced by this process are very different, and there is a large internal stress. When blister molding, it is easy to produce wrinkles or be sucked and break, and the qualified rate of the product is low.
