Film stretching device

Abstract

To provide a film stretching device that can suppress unevenness in the thickness of a stretched film.SOLUTION: A film stretching device 1 according to one aspect of the present invention heats and stretches a film F composed of a thermoplastic resin while transporting it. The film stretching device 1 comprises a pair of driving rolls 11, 12 that drive the film F, a pair of tension rolls 31, 32 which stretch the film between the pair of driving rolls 11, 12, and a pair of rod- or band-shaped infrared heaters 41, 42 disposed one on each side of the film F, facing each other across the full width of the film F between the pair of tension rolls 31, 32. There is no heat source other than the infrared heaters 41, 42 between the pair of driving rolls 11, 12.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a film stretching apparatus. [Background technology]

[0002] To produce retardation films and the like, a film stretching device is used in which a web-like film made of a thermoplastic resin is heated while being transported and stretched in the transport direction by increasing the speed of a downstream drive roller. In conventional film stretching devices, the film is generally stretched using, for example, a heating furnace. It has also been proposed to install far-infrared heaters at various points along the film transport path to heat the film in stages and suppress sudden temperature changes in the film, thereby suppressing the occurrence of wrinkles during film stretching (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2023-135432 Summary of the Invention [Problem to be solved by the invention]

[0004] The specifications required for stretched films, such as thickness and retardation, are becoming more sophisticated year by year, and a technology that can more effectively suppress unevenness in stretched films is desired. Therefore, an object of the present invention is to provide a film stretching device that can suppress unevenness in stretched films. [Means for solving the problem]

[0005] As a result of studying the heating pattern (temperature profile) along the film's conveying direction, the inventor discovered that unevenness in a stretched film can be suppressed by drastically shortening the time the film remains at a high temperature, rather than gradually heating and cooling the film, and thus completed the present invention.

[0006] (1) A film stretching device according to one embodiment of the present invention is a film stretching device that heats and stretches a film made of a thermoplastic resin while transporting the film, and includes a pair of drive rolls that drive the film, a pair of tension rolls between which the film is stretched, and a pair of rod-shaped or strip-shaped infrared heaters, one on each side of the film, facing each other across the entire width of the film between the pair of tension rolls, and no heat source other than the infrared heaters between the pair of drive rolls.

[0007] (2) In the film stretching device of (1), the heater may be a rod-shaped ceramic heater.

[0008] (3) The film stretching device of (1) to (2) may further include a pair of reflectors disposed on opposite sides of the pair of infrared heaters from the film, and configured to reflect infrared rays emitted from the infrared heaters.

[0009] (4) In the film stretching device of (3), the reflector may narrow the range on the film where the illuminance of the infrared light is at least ¼ of the maximum value, compared to when the reflector is not present. [Effects of the Invention]

[0010] According to the present invention, unevenness in the stretched film can be suppressed. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a schematic diagram showing the configuration of a film stretching device according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Fig. 1 is a schematic diagram showing the configuration of a film stretching apparatus 1 according to one embodiment of the present invention. For convenience, hatching and component symbols may be omitted, and the dimensions of various components in the drawings have been adjusted for clarity.

[0013] The film stretching apparatus 1 heats and stretches a web-like film F made of a thermoplastic resin in the transport direction while continuously transporting the film. The film stretching apparatus 1 includes a pair of drive rolls (first drive roll 11 and second drive roll 12), a pair of nip rolls (first nip roll 21 and second nip roll 22), a pair of tension rolls (first tension roll 31 and second tension roll 32), a pair of infrared heaters (first infrared heater 41 and second infrared heater 42), and a pair of reflectors (first reflector 51 and second reflector 52). The film stretching apparatus 1 does not include any heat source other than the infrared heaters 41 and 42 for heating the film F between the pair of drive rolls 11 and 12.

[0014] The drive rolls 11 and 12 are driven to rotate, driving the film F by their rotation. In other words, the peripheral speeds of the first drive roll 11 and the second drive roll 12 determine the transport speed of the film F. The peripheral speed of the second drive roll 12 is faster than that of the first drive roll 11. Due to the difference in transport speed between the first drive roll 11 and the second drive roll 12, the film F is stretched between the tension rolls 31 and 32, more specifically, within the range heated by the infrared heaters 41 and 42. The peripheral speed of the upstream first drive roll 11 (the transport speed of the film F before stretching) is preferably 0.2 m / min to 2.0 m / min, more preferably 0.3 m / min to 1.5 m / min, in order to minimize the heating time of the film F. If the transport speed of the film F is too low, the heating time will be long, causing the film F to shrink in the width direction during stretching, making it more likely to develop wrinkles and thickness unevenness. If the transport speed of the film F is too high, it may be difficult to heat the film appropriately, and the film may not be stretched stably.

[0015] The nip rolls 21 and 22 press the film F against the drive rolls 11 and 12, thereby increasing the frictional force between the drive rolls 11 and 12 and the film F. This prevents the film F from slipping against the first drive roll 11 and the second drive roll 12, and matches the transport speed of the film F by the first drive roll 11 and the second drive roll 12 to the peripheral speed of the first drive roll 11 and the second drive roll 12, respectively.

[0016] The tension rolls 31 and 32 are arranged parallel to each other and rotatably between a pair of drive rolls 11 and 12, and the film F is stretched between them. In other words, no other members come into contact with the film F between the tension rolls 31 and 32. The film F is stretched in the longitudinal direction between the pair of tension rolls 31 and 32. To support the film F flatly by the tension rolls 31 and 32 while avoiding interference with other members, the wrap angle of the film F at the tension rolls 31 and 32 is preferably 30° to 120°, and more preferably 45° to 90°. Furthermore, to ensure reliable support of the film F while miniaturizing the device, the diameter of the tension rolls 31 and 32 is preferably 30 mm to 300 mm, and more preferably 50 mm to 200 mm.

[0017] To suppress waviness of the film F during stretching, it is preferable that the distance between the tension rolls 31, 32 is small, as long as there is enough space for the second infrared heater 42. The specific center-to-center distance between the tension rolls 31, 32 depends on the diameter of the tension rolls 31, 32, the width of the reflectors 51, 52, etc., but is preferably 100 mm or more and 800 mm or less, and more preferably 200 mm or more and 500 mm or less.

[0018] The infrared heaters 41, 42 are each formed in a rod or strip shape and are arranged to face each other continuously across the entire width of the film F on the front and back sides of the film F between the pair of tension rolls 31, 32. Specifically, the first infrared heater 41 is arranged above the film F, and the second infrared heater 42 is arranged below the film F between the pair of tension rolls 31, 32. The infrared heaters 41, 42 heat the film F between the pair of tension rolls 31, 32. The infrared heaters 41, 42 are preferably rod-shaped ceramic heaters so that they can selectively heat a short section of the film F in the transport direction. The first infrared heater 41 and the second infrared heater 42 are preferably arranged to face each other across the film F so that they heat the same area of ​​the film F. To achieve uniform heating across the entire width of the film F, the effective length of the infrared heaters 41, 42 is preferably greater than the width of the film F. Specifically, the effective length of the infrared heaters 41 and 42 is preferably 1.15 times or more the width of the film F, and more preferably 1.25 times or more.

[0019] By heating the film F for a short time using the rod-shaped or strip-shaped infrared heaters 41, 42 and stretching it for a short time, shrinkage of the film F in the width direction can be suppressed, and unevenness in the thickness and retardation of the film F in the width direction can be suppressed. However, if the heating time of the film F is too short, unevenness in the thickness and retardation of the film F in the length direction may occur due to changes in conditions over time, such as flapping of the film or the influence of wind. Specifically, the heating time of the film F is preferably from 3 to 20 seconds, and more preferably from 5 to 15 seconds.

[0020] The distance of the infrared heaters 41, 42 from the film F is preferably 10 mm or more and 50 mm or less, and more preferably 15 mm or more and 30 mm or less, so as to selectively heat the film F within a narrow range in the conveyance direction without overheating the film F. Furthermore, because air convection is suppressed below the film F, resulting in a higher ambient temperature, the distance of the lower second infrared heater 42 from the film F may be greater than the distance of the upper first infrared heater 41 from the film F. For the same reason, the output of the lower second infrared heater 42 may be greater than the output of the upper first infrared heater 41.

[0021] The reflectors 51 and 52 are disposed on the opposite sides of the pair of infrared heaters 41 and 42 from the film F, respectively, and reflect the infrared rays emitted from the infrared heaters 41 and 42 toward the film F, thereby facilitating heating of the film F. To shorten the heating time of the film F, the reflectors 51 and 52 preferably have a shape that narrows the range on the film F where the infrared illuminance is at least ¼ of the maximum value compared to when the reflectors 51 and 52 are not present. In other words, the illuminance distribution of the infrared rays in the film transport direction, including reflection by the reflectors 51 and 52, is preferably a distribution with a shorter base than when the reflectors 51 and 52 are not present, such as a peakier Poisson distribution, a triangular or trapezoidal distribution, or the like. Specific cross-sectional shapes of the reflectors 51 and 52 may be, for example, cylindrical, elliptical, parabolic, triangular, or trapezoidal.

[0022] In the film stretching device having the above-described configuration, the film F is locally heated by infrared heaters 41, 42, one on the front and one on the back, and stretched in a relatively short time. As a result, shrinkage of the film F in the width direction is suppressed, and a stretched film with little unevenness in thickness and retardation can be produced.

[0023] Although the present invention has been described above with reference to an embodiment, it is not limited to the above embodiment and various modifications and variations are possible. For example, the reflector is not an essential component of the film stretching apparatus according to the present invention. Furthermore, the film stretching apparatus according to the present invention may further include a guide roll for adjusting the wrap angle of the film between the drive roll and the tension roll. [Example]

[0024] The present invention will be specifically described below based on examples, but the present invention is not limited to the following examples.

[0025] Example 1 A pair of rod-shaped infrared heaters (IRI-210-75, a ceramic heater with a reflector cap, manufactured by Nippon Heater Co., Ltd., 18 mm diameter, 750 mm effective length, 1.2 kW rated output) was positioned midway between a pair of hard chrome-plated tension rolls with a diameter of 150 mm and a center-to-center distance of 270 mm, sandwiching the film between them in the conveying direction. A 500 mm wide thermoplastic transparent polyimide film was stretched using this heater. The distance between the upper infrared heater and the film was 17 mm, and the distance between the lower infrared heater and the film was 25 mm. The peripheral speed of the upstream first drive roll was 0.5 m / min, and the peripheral speed of the downstream second drive roll was 0.9 m / min (stretch ratio: 1.8x). The output of the infrared heaters was adjusted to achieve a film temperature of 180°C at the center. The output of the upper infrared heater was 21% and the output of the lower infrared heater was 20%. The temperature unevenness (difference between the maximum and minimum values) in the width direction at the position of the film directly facing the infrared heater was 10.3°C.

[0026] <Example 2> The film was stretched under the same conditions as in Example 1, except that the peripheral speed of the downstream second drive roll was 1.1 m / min and the stretching ratio was 2.2 times. The temperature unevenness in the width direction of the film was 10.3°C.

[0027] <Comparative Example 1> The film was stretched under the same conditions as in Example 1, except that an infrared heater was placed only on the upper side of the film and the output of the infrared heater was adjusted to 39% so that the temperature at the center of the film would be 180°C. The temperature unevenness in the width direction of the film was 9.4°C.

[0028] <Comparative Example 2> The film was stretched under the same conditions as in Comparative Example 1, except that the peripheral speed of the downstream second drive roll was 1.1 m / min and the stretch ratio was 2.2 times. The temperature unevenness in the width direction of the film was 9.4°C.

[0029] <Comparative Example 3> Five 125 mm square plate-type infrared heaters ("PLC-326" manufactured by Noritake Co., Ltd., rated output 0.6 kW x 5) were connected together in the width direction of the film only on the upper side of the film, with the heaters having an effective heating length of 625 mm and positioned 40 mm away from the film. The film was stretched under the same conditions as in Example 1, except that the output of the infrared heaters was adjusted so that the temperature at the center of the film was 180°C. The output of the infrared heaters at both ends was 29%, and the output of the three inner infrared heaters was 27%. The temperature unevenness in the width direction of the film was 9.9°C.

[0030] <Comparative Example 4> The distance between the infrared heater and the film was 80 mm, and the output of the infrared heaters at both ends was set to 61%, the output of the second and fourth infrared heaters to 34%, and the output of the central infrared heater to 39%, so that the temperature at the center of the film would be 180°C. The temperature unevenness in the width direction of the film was 9.9°C.

[0031] The thickness unevenness of the stretched film was measured. "Thickness unevenness" was defined as the difference between the maximum and minimum values ​​of the thickness measured at 1 mm intervals across the width of the film using a contact thickness meter "TOF-5R" manufactured by Yamabun Denki Co., Ltd. The results, along with the stretching conditions, are summarized in Table 1 below.

[0032] [Table 1]

[0033] As described above, in Examples 1 and 2, rod-shaped infrared heaters were placed on both sides of the film, shortening the time required for temperature rise and stretching, thereby reducing thickness unevenness. In Comparative Examples 1 and 2, where infrared heaters were placed only on the top side of the film, increased output was required to heat the film sufficiently, which is thought to have resulted in a wider range of heating. This is presumably why Comparative Examples 1 and 2 were able to reduce temperature unevenness in the film's width direction, but increased thickness unevenness. Furthermore, Comparative Examples 3 and 4, where heating was performed over a wide width in the film transport direction, were also unable to reduce thickness unevenness. Thus, contrary to the conventional technical wisdom that rapid temperature changes in the film are best avoided, it was confirmed that the film can be stretched evenly by stretching the film with extremely short heating times. [Explanation of symbols]

[0034] 1. Film stretching device 11,12 Drive roll 21,22 Nip roll 31,32 Tension roll 41,42 Infrared heater 51,52 Reflector

Claims

1. A film stretching apparatus that heats and stretches a film made of a thermoplastic resin while transporting the film, a pair of drive rolls for driving the film; a pair of tension rolls between which the film is tensioned; a pair of rod-shaped or strip-shaped infrared heaters, one on each side of the film, facing each other across the entire width of the film between the pair of tension rolls; Equipped with The film stretching apparatus has no heat source other than the infrared heater between the pair of drive rolls.

2. The film stretching apparatus according to claim 1 , wherein the heater is a rod-shaped ceramic heater.

3. The film stretching apparatus according to claim 1 or 2, further comprising a pair of reflectors arranged on opposite sides of the pair of infrared heaters from the film, the reflectors reflecting infrared rays emitted from the infrared heaters.

4. The film stretching apparatus according to claim 3 , wherein the reflector narrows the range on the film where the illuminance of the infrared light is equal to or greater than one-fourth of the maximum value, compared to when the reflector is not present.

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