The transverse through value of PET film is between 390 MPa and 390.5 mpa-f However, this film is not enough for long-time playback or small-size tape base film. Because these tapes require greater film hardness. The effective hardness of the film is proportional to the film thickness 3x and the film modulus. Even if the film is very thin, it should have a higher modulus. High modulus films are obtained by increasing the molecular orientation of the films. Because the longitudinal modulus is related to the motion of the tape and the transverse modulus is related to the contact of the magnetic head, it is required to have a high modulus in both directions. (the motion of the tape is usually evaluated by the elongation of the tape, the edge damage of the tape and the folding condition of the tape).

Some types of stretched films are listed below:
① Semi tensile film (longitudinal F-5 value is 130-150mpa);
② General tensile film (longitudinal F-5 value is 150 ~ 180MPa);
③ Over stretched film (longitudinal F-5 value is 180 ~ 220MPa);
④ Exceeding the tensile film (longitudinal F-5 value is 220 ~ 260mpa);

⑤ Bilateral tensile film (longitudinal F-5 value is 150MPa; transverse F-5 value is 140MPa)

With the increase of stretching, the thermal shrinkage of PET film increases, and the relationship between the change of thermal shrinkage and F-5 value is irreversible.

Precautions for pet stretched film
The basic process of producing stretched film is as follows:
(1) LD direction stretching; (2) Stretching in TD direction;
(3) Re stretching in LD direction;
(4) If necessary, stretch again in TD direction;

(5) Heat curing.

Re stretching in LD direction is quite difficult because the film is very thin and wide, which will lead to (1) both sides should be formed quite thick to prevent this width from decreasing;

(2) The deviation between preheating and stretching roll should be as small as possible;

(3) The tensile temperature shall rise as the film passes through the machine.

Since the re stretching process is usually expensive, another method is proposed to reduce the manufacturing cost. The characteristic of this new process is that the first drawing in LD direction is carried out in two steps:

(1) The cast film is first stretched in the LD direction at 120 ~ 150 ℃, and the stretching ratio is controlled to make the birefringence value of the stretched film between 0.005 ~ 0.025;

(2) After that, the second drawing in LD direction is carried out between 60 ~ 100 ℃;

(3) The unidirectional tensile film obtained in this way is stretched at a tensile ratio of more than 4.3 in the TD direction;
(4) After obtaining the biaxially oriented film, it is reheated and cured.



Precautions for pet stretched film
According to such a processing process, the manufacturing cost of the stretched film is low because the re stretching process is not required. There are various methods to produce stretched films, some of which are listed below.

(1) Re synchronous biaxial stretching of Bi oriented films

The drawing ratio of the second synchronous drawing is 1.05~1.90 in LD direction and 1.0~1.9 in TD direction The drawing temperature of synchronous drawing is between (TG + 10) ℃ and (TM-40) ℃ (TG: glass transition temperature; TM. Melting temperature).

(2) Multi step LD direction stretching and re stretching are combined.

The cast film is stretched in the LD direction in multi-step form into a single oriented film with birefringence value of 0.02 ~ 0.10, and then the film is stretched in the TD direction with a stretching ratio of 2.5 ~ 45, and re stretched in the LD direction with a stretching ratio of 1.5 ~ 2.5. After LD, TD and LD drawing, the drawing ratio of the product is higher than 25

(3) Multi - step LD stretching is combined with synchronous biaxial stretching

The cast film is stretched into a single oriented film with birefringence value of 0.02 ~ 0.10 in the LD direction in multi-step form. After that, the film is stretched in TD direction with a tensile ratio higher than 2.5, and then synchronously biaxially stretched. At this time, the tensile ratio in both directions is 1.4 ~ 2.5, and the total tensile ratio of the product is more than 27.



Precautions for pet stretched film



(4) Zone stretching with zone annealing.

Zone stretching and annealing include film stretching and annealing under the tension condition of pressing point heating. The purpose of region stretching is to obtain higher molecular orientation while preventing crystallization. It is obtained by necking stretching of the film at approximately TS temperature. The purpose of zone annealing is to anneal the stretched film under the high tension between TC TM temperature to obtain the crystal with molecular chain elongation. The modulus in LD direction of the region annealed film is almost twice that of the original film, although the modulus in TD direction decreases.

Nanjing Sumino Precision Machinery Co., Ltd. has a professional sheet production line. The company's main sheet machine manufacturing production lines are: PET/PRET sheet production line, PP sheet production line, PP vacuum profile extrusion production line, PP/PC vacuum sheet production line, PC/PP corrugated board production line, PVDF photovoltaic back sheet production line, PMMA acrylic sheet production line, PE waterproof membrane production line, high-performance barrier sheet production line, optical panel production line, multi-layer co-extrusion sheet production line, etc.
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