How PET film optimizes product quality

1. Bow phenomenon

At the entrance of the stenter machine, the transverse traction film is in a straight line, while at the exit of the capping machine, the film star and the same curved shape. Bow phenomena cause differences in lateral properties. The expansion of the PE film is different between the center and both sides. In the middle, the expansion ratio of the film in any measurement direction is the same, while on both sides, the expansion ratio changes with the measurement direction. The middle part is "isotropic", while the two sides are "anisotropic". The anisotropy on both sides of the film is caused by the warp image. In severe cases, anisotropy will cause major problems.

2. The mechanism of the bow
Set a film with a straight line mark in the heating furnace of the tenter frame, then preheat, stretch, heat cure, and cool, and the process stops at this time. Stretched film with marking lines is spread out in the tenter oven from the entrance to the exit. The experimental results are shown in Figure 6.68 [23], bowing deformation occurs along the length of the tenter heating furnace. The straight line at the entrance of the stenter (the bow shape becomes 0%), first bends to the "exit side bow curve" (the bow shape is negative) at the beginning of stretching, and then changes back to a straight line at the end of stretching, and finally, After heat curing becomes "entry side bow curve" (bow shape becomes positive). There was no change in the bow curve cooling zone of the heat curing zone or at the exit of the tenter oven.

The distribution of the bow phenomenon in the heating furnace of the stenter is estimated. The mechanism of the bow is due to the necking stress in the stretching zone, which causes the film to be thermally cured. At this time, the temperature is higher and the film hardness is relatively low. The refractive ellipsoid of cast film is a "spheroid" shape that becomes "football" in the longitudinally stretched region. Then in the transverse stretching zone, the direction of the main orientation axis gradually changes to the transverse direction due to the influence of transverse stretching virtual force. The orientation of the ellipsoid and the principal axis varies along the transverse direction. In the heat curing zone, the ellipsoid shape changes at every point in the transverse direction, because the front shear stress in the middle of the tenter is 0, and the shear stress increases as the width increases. In general, isotropic films cannot be fabricated with the usual tentering process.



3. Reduce bowing phenomenon
(1) Prevent stress spread
Assuming that necking stress in the transverse stretching zone is the main cause of bowing, it was proposed that bowing could be reduced by reducing the stress spread between the stretched and heat-cured zones. One idea is to cool the stretched film before it enters the heat curing zone. A decrease in film temperature leads to an increase in hardness, which prevents stress propagation to some extent. Another idea is to release the clamp right after transverse stretching and then re-clamp the film as it enters the heat-cure zone. Still another suggestion is to have a nip roll in the stretching and heat curing zone, the role of the nip roll is to prevent stress from spreading.

(2) Multi-step thermal curing
The transversely stretched film is cooled below the glass transition temperature;
·Heat curing at T1 temperature 200~240℃;
Heat curing again at T2 temperature (T2 temperature is between T1 and T3), after heat curing
When melted, the transverse stretch ratio of the film is 1%~20%;
·The film is thermally cured again at T3 temperature 100~200℃.

(3) Changes in lateral temperature
One method is to relax the biaxially stretched film slightly, the longitudinal tension is below 10kg/m, the center of the film is heated to above 150°C, and the two sides are heated to 10°C or higher than the center temperature. Heated by infrared radiation heater. Another method is to heat the temperature of both sides located in the middle zone of the stretching and heat curing zone to between the glass transition temperature and the heat curing temperature. Another way is to create a temperature gradient from high temperatures on the sides to low temperatures in the middle.

(4) Others.
Such as: put the curled film into the heat-cured tenter heating furnace again, and correct the bow phenomenon by conveying the film in reverse. However, this method reduces production efficiency.

As stated above, many techniques have been proposed to improve bowing. However, there is not yet an effective way to solve it, especially from an industrial point of view. In fact, at present, PET films generally have more or less bowing phenomenon.



3. Thermal stability
The film is required to have two kinds of thermal stability, one is the stability below 100°C; the other is the thermal stability above 150°C. Such as: the former is required for the magnetic tape industry, while the latter is very important in general industrial applications. High temperature thermal stability is readily obtained by thermal curing in the relaxed state. Ease of heat curing in the transverse direction in the tenter oven as the clamping chains are easy to separate and combine. However, longitudinal relaxation is difficult in tenter processing. Therefore, longitudinal thermal stability is sometimes not sufficient for some demanding applications. In order to improve the longitudinal thermal stability, the PET film is thermally cured again in a heating furnace at 180~200°C under the condition of longitudinal relaxation. This "off-line" thermal curing is very effective in reducing the longitudinal thermal shrinkage. However, this method is costly due to an increased heat curing process. In order to reduce the longitudinal heat shrinkage on the film forming line, the following remedial measures can be taken.

(1) Longitudinal relaxation is obtained by reducing the distance between the clamps in the thermal curing zone of the tenter. The main feature of this method is that the tenter adopts a special clip and chain mechanism.

(2) Longitudinal relaxation can be obtained between the two hot rollers of the roller heat positioner behind the tenter heating furnace.

(3) Longitudinal relaxation can also be obtained by different speeds of the tenter clamping chain and the vacuum suction roller at the rear of the tenter heating furnace outlet. It is said that this method is very effective in improving low temperature thermal stability.

(4) The thermally cured film is thermally cured again in a moving state. Because the film is in motion, this method is easy to obtain a two-way relaxed film. The thermal stability of the film is essentially improved by relaxation heat curing, a method that aims to achieve maximum relaxation while keeping the film flat.



4. Thickness uniformity
Thickness uniformity is one of the most important film quality criteria, as it greatly affects the flatness of the film, the configuration of the rolls, and thus the suitability for various manufacturing processes.
There are two key factors that improve thickness uniformity:
① How to obtain a cast substrate with good thickness uniformity;
② How to avoid the deterioration of thickness uniformity during stretching.

5. Methods to improve thickness uniformity:
Cast substrate with good thickness uniformity
An effective method is to use a die head with an automatic thickness control system, in which the thickness is automatically adjusted by a signal displayed by a thickness gauge located at the exit of the casting roll or at the heating furnace of the tenter frame. There are four ways to adjust the thickness deviation:

①The die head gap is adjusted by the bolts of the die head, and the bolts expand or contract with the thermal energy change controlled by the thickness change signal;

②The die head bolts are controlled by piezoelectric elements;

③The die head bolt is controlled by the servo motor operating in the horizontal direction;

④ Die lip temperature is controlled by thickness gauge. Changes in temperature cause changes in the viscosity of the molten polymer and, as a result, changes in thickness. In order to improve small deviations in film thickness or surface defects, it is proposed to introduce special metals such as chromium on the inner surface of the die lip. In order to improve the electrical pressing property of the casting area, it is proposed to add additives to increase the dielectric constant of the molten polymer.



6. Thickness uniformity during stretching

① It is very effective to use electric pressing in the process of longitudinal stretching to set it at the proper position of the fixed stretching point;

② The control of the surface of the ceramic roller is extremely useful in reducing surface defects in longitudinal stretching;

③Low temperature stretching is effective for improving thickness uniformity (longitudinal stretching temperature 55~80°C, transverse stretching temperature 50~100°C)


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 interests 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 major types of equipment, including plastic sheet, casting, biaxial stretching, non-woven non-woven melt-blown fabric, laminating compound, regeneration granulation environmental protection recycling system and coating machine manufacturing system. The center and production department provide a complete 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 manufacturing system, PVC biaxially oriented shrink film production line, PET biaxially oriented synchronously oriented film production line, BOPA biaxially oriented film production line, BOPLA biaxially oriented film production line, etc.