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

1) ingredients and mix

ordinary BOPET film material is mainly used by masterbatch biopsy and there is light. Masterbatch slice refers to PET slice containing additives, additives are silica, calcium carbonate, barium sulfate, kaolin, etc., according to the different uses of the film to choose the corresponding masterbatch slice.

Polyester film is generally used with a certain amount of silicon-containing masterbatch slices and optical slices, its role is to increase the microscopic roughness of the surface of the film through the distribution of silica particles in the film, so that the film contains a very small amount of air between the winding film, so as to prevent the film from sticking. The light slice and a certain proportion of the masterbatch slice are mixed through the metering mixer to enter the next process.

¢Ú Crystallization and drying

For polymers with a tendency to moisture absorption (such as PET, PA, PC, etc.), pre-crystallization and drying must be carried out before bidirectional stretching. The purpose of this is: to improve the softening point of the polymer, to avoid the resin particles in the process of operation and melting extrusion adhesion or caking; Remove the water in the resin to prevent the polymer containing ester groups from hydrolyzing or forming bubbles during the melt extrusion process.

PET pre-crystallization and drying equipment generally uses a filled 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¡æ, and the drying time is about 3.5~4h. The moisture content of PET slices after drying is required to be controlled in 30~50PPm.

¢Û Molten extrusion

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

In addition to the length-diameter ratio, pressure ratio, each functional segment have certain requirements, but also special requirements are Barrier screw, the screw square of this structure is conducive to ensure the good plasticization of extruded material, extruder outlet material temperature uniformity, extruder stable discharge and good exhaust, and is conducive to improving the extrusion capacity. If the amount of extrusion is not too large, it is recommended to use the exhaust twin screw extruder.

This extruder has two exhaust ports connected with two vacuum systems, has a good pumping exhaust, dehumidification function, can remove the moisture and oligomers contained in the material, so it can save a complex pre-crystallization/drying system, save investment and reduce operating costs. Extruder temperature setting from the feeding port to the head of about 210~280¡æ.

(4) casting pieces

die head is the key to the stretch film casting piece mold, directly decided to cast slice shape and thickness uniformity. BOPET often uses a hanger type die head, die head opening through a number of heating coil do pull type differential bolt for initial adjustment, and through the online thickness gauge automatic thickness measurement, feedback to the die head heating bolt for fine tuning of die lip opening. The temperature of the die head is controlled at 275~280¡æ.

quench roll (also known as casting roll, commonly known as cold drum). The PET melt flowing out of the die head in a new flow state is quickly cooled to below its glass transition temperature on the uniformly rotating quench roll, forming a glass cast sheet with uniform thickness. The purpose of quenching is to make the thick sheet form an amorphous structure, minimize its crystallization, so as not to have an adverse effect on the next drawing process. To this end, the surface temperature of the quench roller is required to be uniform and the cooling effect is good. At the same time, the speed of the quench roller is required to be uniform and stable. In addition, the cooling water of about 30¡æ is passed in the quenching roll to ensure that the casting sheet is cooled to below 50¡æ.

Electrostatic adsorption device. The function of the electrostatic adsorption device is to make the casting sheet and the rush roller close contact to prevent the rush roller from getting into the air when it rapidly rotates to ensure the heat transfer/cooling effect. The electrostatic adsorption device is composed of a metal wire electrode, a high voltage generator and a torque motor for electrode retraction. Its working principle is: Using the thousands of volts DC voltage generated by the high voltage generator, the electrode wire and the quench roller become positive and negative respectively (quench roller grounding), the cast sheet in this high voltage electrostatic field because of electrostatic induction and the quench roller polar opposite electrostatic charge, under the action of polar attraction, the cast sheet and the quench roller surface tightly adsorbed together to achieve the purpose of eliminating air and good heat transfer.

¢Ý Longitudinal 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 the longitudinal stretching. The longitudinal stretching machine is composed of preheating roller, stretching roller, cooling roller, tension roller and rubber press roller, infrared heating tube, heating unit and driving device. The longitudinal drawing is usually a single point drawing, and there are also multi-point drawing, such as two or three point drawing. The longitudinal pull ratio is generated by the speed difference between the bread pull roller and the fast pull roller, which is generally 3.3 to 4 times.

Transverse stretching machine by oven, chain clamp and guide rail, plenum, chain tensioner, guide rail width adjustment device, open and close clamp, hot air circulation system, lubrication system and EPC. Its function is to complete the transverse stretching of the film through preheating, tensioning, heat setting and cooling in the transverse drawing machine. The transverse tensile ratio is 3.5 to 4 times.