Analysis of biaxial stretching process of PET material

1, ingredients and mixing

The raw materials used in ordinary BOPET films are mainly masterbatch chips and polished sections. The masterbatch slice refers to a PET slice containing an additive, and the additive is silica, calcium carbonate, barium sulfate, kaolin, etc., and the corresponding masterbatch slice is selected according to the different uses of the film.

The polyester film is generally prepared by using a certain amount of silicon-containing masterbatch and light-slipping, and its function is to increase the ancestral roughness of the surface of the film by the distribution of the silica particles in the film, so that the film is formed at the time of winding. A very small amount of air is contained between them to prevent film sticking. The light slice and a certain proportion of the masterbatch slice are mixed by the metering mixer and then enter the next process.

2, crystallization and drying

For high-viscosity polymers (such as PET, PA, PC, etc.), pre-crystallization and drying must be carried out before biaxial stretching. The purpose of this is to increase the softening point of the polymer, avoiding the adhesion or agglomeration of the resin particles in the process of melt extrusion, removing the moisture in the resin, and preventing the polymer containing the ester group from being melted and extruded. Hydrolysis or bubble generation occurs.

PET pre-crystallization and drying equipment generally employs a packed column with a crystal bed, and is equipped with a dry air preparation device, including an air compressor, a molecular sieve dehumidifier, a heater, and the like. The pre-crystallization and drying temperature is 150-170 ° C, the drying time is about 3.5-4 h, and the moisture content of the dried PET slice is controlled to be 30-50 ppm.

3, melt extrusion

The crystallized and dried PET chips were placed in a single screw extruder for heat and melt plasticization. In order to ensure good plasticizing quality and stable extrusion melt pressure of PET chips, the structural design of the screw is very important.

In addition to the long diameter ratio, the pressure line ratio, and various functional sections, it is also required to be a Barrier type screw. The screw of this structure is beneficial to ensure the good plasticization of the extruded material and the temperature of the outlet material of the extruder. Uniform consistency, stable discharge of the extruder and good venting, and help to improve extrusion capacity. If the amount of extrusion is not too large, it is recommended to use a vented twin-screw extruder.

The extruder has two exhaust ports connected to two vacuum systems, which have a good function of exhausting and dehumidifying, which can remove the moisture and oligomers contained in the materials, thus eliminating the need for A sophisticated pre-crystallization/drying system saves investment and reduces operating costs. The extruder temperature setting is approximately 210-280 ° C from the feed port to the head.

4, cast

The die is the key mold for casting the slab and directly determines the shape and thickness uniformity of the slab. BOPET often uses a hanger-type die, and the die opening is firstly adjusted by a plurality of pull-type differential bolts with heating coils, and the mold is automatically measured by the thickness gauge of the online thickness gauge and fed back to the heating bolt of the die. Fine adjustment of the lip opening. The die temperature is controlled at 275-280 °C.

Quench roll (also known as cast roll, commonly known as cold drum). The PET melt flowing out of the die in a fresh fluid state is rapidly cooled to a temperature below the glass transition temperature on a uniformly rotating chill roll to form a glass slab having a uniform thickness. The purpose of quenching is to form the slab into an amorphous structure and minimize its crystallization to avoid adverse effects on the next stretching process. For this reason, it is required that the surface temperature of the quenching roller is uniform and the cooling effect is good. At the same time, the quenching roller speed is required to be uniform and stable. In addition, cooling water of about 30 ° C is passed through the quenching roller to ensure that the cast piece is cooled to 50 ° C or lower.

Electrostatic adsorption device. The function of the electrostatic adsorption device is to make the cast piece and the ram roller in close contact, and prevent the chill roll from being caught in the air when it is rotated rapidly to ensure the heat transfer/cooling effect. The electrostatic adsorption device is composed of a wire electrode, a high voltage generator, and an electrode retracting torque motor. The working principle is: using the voltage of several thousand volts generated by the high voltage generator, the electrode wire and the quenching roller become the positive electrode and the negative electrode respectively (the grounding of the quenching roller), and the cast piece is brought and quenched by the electrostatic induction in the high voltage electrostatic field. The static charge of the opposite polarity of the roller, under the action of the heteropolar attraction, the slab and the surface of the chill roll are closely adsorbed together to achieve the purpose of eliminating air and good heat transfer.

5, longitudinal stretching

The slab from the casting machine is heated in a longitudinal drawing unit to a high elastic state for a certain multiple longitudinal stretching. The longitudinal stretching machine is composed of a preheating roller, a stretching roller, a cooling roller, a tension roller and a rubber pressure roller, an infrared heating tube, a heating unit, and a driving device. Longitudinal stretching is usually a single point stretching, but also a multiple point stretching, such as two or three point stretching. The longitudinal draw ratio is produced by the difference in speed between the draw rolls and the fast pull rolls, typically 3.3-4 times.

6, horizontal stretching

The transverse stretching machine consists of an oven, a chain clamp and a guide rail, a static pressure box, a chain tensioner, a rail width adjustment device, an opening and closing clamp, a hot air circulation system, a lubrication system, and an EPC. The function is to complete the transverse stretching of the film by preheating, tentering, heat setting and cooling in the longitudinally stretched film. The cross-ratio is 3.5-4 times.

7, traction winding and slitting

The equipment of this process consists of several traction guide rollers, cooling rollers, flattening rollers, tension rollers, tracking rollers, trimming devices, thickness gauges and corona treatment machines. The biaxially stretched film can be wound and slit after trimming, thickness measurement and corona treatment, and is finished after passing the test.

The longitudinal and transverse physical and mechanical properties of the sheet produced by this process are large, and there is a large internal stress. When the plastic forming is formed, wrinkles or suction are easily generated, and the product is qualified to be low.