There are 20 different companies all over the world producing PET film. PET film is characterized by good quality and low price. It has many excellent properties, such as: high mechanical strength, good optical permeability, excellent electrical insulation, good dimensional stability, high thermal stability, excellent chemical resistance, and hygiene and safety. In the initial stage, the market for PET film was limited to the areas of electrically insulating metal-plated longitudinal split wires and stamping foils.







In the 1970s, the PET film market expanded to photographic film, capacitors, and tracing paper, and then further developed into various tapes, such as audio tapes, computers and video tapes, which led to the rapid growth of PET film production. In the 1980s, with the development of the electronics and information industry,
PET film has found new application areas, such as electronic components, graphics products and other office and factory automation products. Due to the rapid expansion of the PET film market, the consumption of PET film has increased significantly. At present, the world's annual PET consumption is about 1.1 million tons.







The first step in producing PET film is to melt the PET pellets in an extruder, then form the substrate through a long and narrow die, and finally quench the substrate on a casting drum. Since the obtained substrate is non-oriented and amorphous, the physical properties are not very good. The amorphous substrate is heated and stretched in both directions in the longitudinal and transverse directions. As the stretching progresses, the polymer molecular chains are bidirectionally oriented, improving physical properties. However, this stretched film has a defect, that is, shrinks greatly at high temperatures. In order to eliminate this defect, the stretched film is thermoset under tension, which promotes crystallization and eliminates the stress caused by stretching. The key feature of PET film production and processing is biaxial stretching, which is usually not used in the production of general-purpose PE and PVC films. Biaxial stretching technology has developed rapidly with the development of the PET film industry. After that, it was used to produce PP and polyamide films and other films.



1. Overview of thin film manufacturing process
Because cast PET films are brittle, most PET films need to be biaxially stretched. The biaxial stretching process is divided into the following types (LD is the longitudinal direction; TD is the transverse direction).

(1) Synchronous biaxial stretching process of tenter frame The two processes are the scaling process and the pitch process, but these two processes are not very common due to the following reasons:
Synchronous bidirectional stretching: Tenter process (A)



Tube film technology (B)




Stepwise biaxial stretching: LD→TD stretching (C)

TD→LD stretching (D)


The structure of the stenter is complicated, so maintenance is difficult; it is not suitable for high-speed processing; it is difficult to change the stretching ratio in the ID direction. With the development of new technologies, the advantages of the existing synchronous stretching method have been expanded and its disadvantages have been avoided. Good processability (not easy to break, can produce ultra-thin film, high speed, high efficiency, stretch parameters can be changed quickly, less maintenance); synchronous relaxation (for stretched film, shrinkage is not large).







(2) Synchronous biaxial stretching process of tube film method
The tubular cast film is formed by a round die, heated and expanded by the increase of gas pressure to form a bi-oriented film. This process has the following advantages and disadvantages.

advantage:
·Low investment cost;
·High production efficiency (no scraps produced in the tenter process).

Disadvantages:
·Due to the instability of blown film processing, poor thickness uniformity and uneven film;
·Not suitable for high-speed processing;
·Not suitable for producing thick film.

The PET film produced by the tube film process is mainly limited to shrink film, such as shrink packaging of food and wires.







(3) Stepwise bidirectional stretching (LD→TD stretching)

As mentioned in the past, simultaneous biaxial stretching is not suitable for the production of PET film. Most PET films are produced by a stepwise biaxial stretching process. The two-step biaxial stretching is composed of the following three steps. Firstly, the cast substrate is stretched in one direction, then the film is stretched uniaxially in the other direction, and finally the film is thermally cured. Stepwise biaxial stretching is divided into two processes, one is LD→TD stretching (the film is first stretched in the LD direction, and then in the TD direction); the other is TD-LD stretching (the film first Stretched in the TD direction, and then stretched in the LD direction). LD stretching is usually carried out between two pairs of rotating rollers, and the stretching ratio is determined by the difference in rotation speed between the rollers. TD stretching is carried out in a moving tenter fixture, and the TD stretching ratio is determined by the ratio of the film width at the inlet and outlet of the tenter. LD→TD stretching is the main PET film processing technology in the world.

(4) Stepwise biaxial stretching (TD→LD stretching)

The process has the following advantages:

·The film stretched in the LD direction is easy to process;
·The "bow" phenomenon is low;
·Easy for high-speed processing (because the production speed is not limited by the speed of the tenter).

However, the serious problem of this process is that it is difficult to stretch a wide film in the longitudinal direction, because the wider the roll, the more likely it is to bend, and the wider nip roll usually cannot provide uniform and straight extrusion. force. At present, the main process of PET film production in the world is the stepwise biaxial stretching method (LD→TD).



Nanjing Sumino Precision Machinery Co., Ltd. is a high-tech enterprise integrating scientific research, production, and sales. It has always adhered to the business philosophy of integrity, equality, and customer benefits first. In order to better improve the quality of equipment, it has entered a more professional and standardized Production track. We have established independent research and development technologies for seven categories of equipment including plastic sheet material, casting, biaxial stretching, non-woven non-woven meltblown cloth, film laminating, recycled granulation environmental protection recycling system and coating machine manufacturing system. The center and production department provide a full set of professional technical solutions to meet the special needs of each customer. Among them, the biaxially oriented film manufacturing system includes: BOPP biaxially oriented film production system, PVC biaxially oriented shrink film production line, PET biaxially oriented film production line, BOPA biaxially oriented film production line, BOPA biaxially oriented film production line, etc.