Introduction to the biaxial stretching process of PET material

Taking BOPET film as an example, the main equipment and processes are briefly described as follows:

1) Ingredients and mixing

The raw materials used in ordinary BOPET film are mainly masterbatch chips and bright chips. Masterbatch chips refer to PET chips containing additives. Additives include silica, 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 slices and bright slices. Its function is to increase the microscopic roughness of the film surface through the distribution of silicon dioxide particles in the film, so that the film Contains a very small amount of air to prevent the film from sticking. The bright slices and a certain proportion of the master batch slices are mixed through a metering mixer and then enter the next process.

2) Crystallization and drying

For polymers with a tendency to absorb moisture (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 to prevent the resin particles from adhering or agglomerate during the drying and melt extrusion process; to remove the moisture in the resin to prevent the polymer containing ester groups from being melted during the extrusion process. Hydrolysis occurs or bubbles are generated.

PET pre-crystallization and drying equipment generally uses a packed tower with a crystallization bed, and is equipped with dry air preparation devices, including air compressors, molecular sieve dehumidifiers, heaters, etc. The pre-crystallization and drying temperature is 150~170℃, the drying time is about 3.5~4h, and the moisture content of the dried PET chips should be controlled within 30~50PPm.

3) Melt extrusion

After the crystallization and drying process, the PET chips enter the single-screw extruder for heating, melting and plasticizing. In order to ensure good plasticization quality of PET chips and stable extrusion melt pressure, the structural design of the screw is very important.

In addition to certain requirements for the length-to-diameter ratio, pressure line ratio, and each functional section, it is also particularly required to be a Barrier type screw. This structure of the screw is helpful to ensure the good plasticization of the extruded material and the temperature of the extruder outlet material. Uniformity, stable discharge of the extruder and good exhaust, and help to improve the 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, which has good exhaust and dehumidification functions, which can pump away the moisture and oligomers contained in the material, so it can be omitted A complex pre-crystallization/drying system saves investment and reduces operating costs. The temperature setting of the extruder is about 210~280℃ from the feeding port to the die head.

4) Casting

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 clothes hanger-type die heads. The opening of the die head is initially adjusted by a number of push/pull differential bolts with heating coils, and the automatic thickness measurement of the online thickness gauge and feedback to the heating bolts of the die head are carried out. Fine adjustment of the die lip opening. The temperature of the die head is controlled at 275~280℃.

Quench rollers (also known as cast rolls, commonly known as cold drums). The PET melt in a new fluid state from the die is quickly cooled to below its glass transition temperature on a quench roll that rotates at a constant speed to form a glassy cast sheet with uniform thickness. The purpose of quenching is to form an amorphous structure of the thick sheet and minimize its crystallization, so as not to have an adverse effect on the next stretching process. For this reason, it is required that the surface temperature of the quench roll is 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 quench roll to ensure that the cast slab is cooled to below 50°C.

Electrostatic adsorption device. The function of the electrostatic adsorption device is to make the casting sheet and the sharp punching roller come into close contact to prevent the entrapment of air during the rapid rotation of the quench roller to ensure the heat transfer/cooling effect. The electrostatic adsorption device is composed of a wire electrode, a high-voltage generator and an electrode retractable torque motor. Its working principle is: use the thousands of volts DC voltage generated by the high-voltage generator to make the electrode wire and the quench roller become positive and negative respectively (the quench roller is grounded), and the cast piece is carried and quenched by electrostatic induction in this high-voltage electrostatic field. The static charge of the rollers with opposite polarities, under the action of the attraction of the opposite poles, the cast sheet and the surface of the quench roller are closely adsorbed together to achieve the purpose of air removal and good heat transfer.

5) Longitudinal stretch

The thick sheet from the casting machine is heated to a high elastic state in the longitudinal stretching unit to carry out a certain multiple of longitudinal stretching. The longitudinal stretching machine is composed of preheating roller, stretching roller, cooling roller, tension roller and rubber pressing roller, 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 produced by the speed difference between the slow draw roll and the fast draw roll, generally 3.3 to 4 times.

6) Horizontal stretch

The transverse stretching machine is composed of oven, chain clamp and guide rail, static pressure box, chain tensioner, guide rail width adjustment device, opening and closing clamp, hot air circulation system, lubrication system and EPC. Its function is to complete the transverse stretch of the film through preheating, tentering, heat setting and cooling in the transverse stretcher. The cross-ratio is 3.5~4 times.

7) Traction winding and slitting

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

The sheet material produced by this process has a large difference in physical and mechanical properties in the vertical and horizontal directions, and there is a large internal stress, which is prone to wrinkles or breakage during blister molding, and the qualified rate of the product is low.

END

From Thermoforming