Two-axis extension device

Abstract

To provide a biaxial orientation apparatus with low equipment cost, suppression of membrane rupture and neck-in, high yield, arbitrary and easy adjustment of MD draw ratio, and good workability.SOLUTION: An orientation zone of a biaxial orientation apparatus is divided into two zones along a MD, and of the two zones, a zone upstream of the MD is a first orientation zone 21 and a zone downstream of the MD is a second orientation zone 22. In the first orientation zone 21, an MD distance between adjacent holding parts 16 along a guide rail and a TD distance between the holding parts 16 moving along the guide rail opposite the TD increases from entrance to exit. An MD orientation ratio in the first orientation zone 21 is fixed. In the second orientation zone 22, the TD distance between the holding parts 16 moving along each of the guide rails facing the TD is constant. A mechanism capable of changing the MD distance between the holding parts 16 independently of the MD distance between the holding parts 16 in the first orientation zone 21 is provided.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a biaxial stretching apparatus (BO) for stretching an object to be processed, such as a microporous film, particularly for stretching the object to be processed in the machine direction (MD) and a direction crossing thereto (TD), and a method for producing a film. [Background technology]

[0002] Polyolefin microporous films are widely used in applications such as battery separators for lithium batteries, electrolytic capacitor diaphragms, breathable waterproof clothing, and various filtration membranes. When using polyolefin microporous films as battery separators, the pore size and strength ratio of the microporous film are deeply related to the battery's performance and safety. However, because different customers' applications (e.g., automotive batteries and mobile device batteries) and different battery manufacturing methods require different pore sizes and strength ratios, battery separator manufacturers must tailor the pore size and strength ratio to meet their requirements. However, depending on the composition and molecular weight of the resin used in the microporous film, standard BO stretching at a ratio of 1:1 cannot achieve the desired strength ratio or pore size. Therefore, additional MD stretching or MD shrinkage is required to adjust the MD stretch ratio and strength ratio. The following methods are generally known as means for adjusting the MD stretching ratio. (i) Full-range variable BO (ii) Pre-MDO (iii) Post-MDO (iv) Preheating zone MD stretching ratio variable BO (v) Late extension zone MD additional extension BO.

[0003] (i) is a method described in Patent Document 1 in which movable points are provided on the guide rail of the BO over the entire MD range, and the MD stretching ratio and TD stretching ratio are changed as desired.

[0004] As described in Patent Document 2, (ii) and (iii) are methods in which multiple rolls are provided on the MD upstream and downstream sides of the BO, respectively, and MD stretching is performed by the speed difference between the rolls.

[0005] (iv) is a method in which the MD stretching is performed in the latter half of the preheating zone of the BO by operating the guide rail in the latter half of the preheating zone, and the MD stretching ratio is adjusted.

[0006] (v) is a method in which, as described in Patent Document 3, a step of performing only MD stretching is provided in the latter half of the BO stretching zone. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2019-93570 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-343958 [Patent Document 3] Japanese Patent Application Laid-Open No. 2011-203427 Summary of the Invention [Problem to be solved by the invention]

[0008] In the method (i) described in Patent Document 1, movable points must be provided on the guide rail throughout the MD, which makes the cost much higher than that of general BO. Furthermore, the rail must be movable throughout the MD of the BO when adjusting the MD stretch ratio, resulting in poor operability. Furthermore, the guide rail bends at the movable points, which causes a sudden increase in strain rate at the many movable points provided throughout the MD, leading to membrane rupture and poor yield.

[0009] In addition, the methods (ii) and (iii) described in Patent Document 2 are not expensive, but because rolls are used for stretching, the film is subject to scratches and stains caused by the rolls, resulting in a decrease in yield. Furthermore, unlike BO, roll stretching does not grip the side edges of the film, resulting in a large neck-in amount and a decrease in yield. Furthermore, in method (vi), in addition to the preheating zone length required for raising the film temperature, a zone for stretching in the MD must be provided, which increases the furnace length and increases costs. Furthermore, to change the MD stretch ratio, it is necessary to move the guide rails not only in the preheating zone but also throughout the MD region, which reduces operability.

[0010] In addition, in the method (v) described in Patent Document 3, the MD stretching ratio is fixed and cannot be changed arbitrarily, so the hole size and strength ratio cannot be adjusted according to customer requests, resulting in poor productivity. Also, since only MD stretching is possible, it cannot accommodate cases where MD shrinkage is required according to customer requests.

[0011] In response to the above-mentioned problems, the present invention aims to provide a BO that requires low equipment costs, has a high yield because membrane rupture and neck-in can be suppressed, and has good operability because the MD stretching ratio can be freely and easily adjusted. [Means for solving the problem]

[0012] The inventors have conducted extensive research into BOs that allow adjustment of the MD stretch ratio, and have found that by providing a magnification-varying mechanism in the latter half of the stretching zone, operability is improved because only the latter half of the guide rail needs to be moved; and compared to full-area variable BOs, the magnification-varying mechanism only needs to be provided in the stretching zone, reducing the number of variable mechanisms and resulting in lower equipment costs; and, unlike full-area variable BOs, curved guide rails can be used in the former half of the stretching zone, preventing film rupture due to sudden changes in strain rate; and, unlike Pre-MDO and Post-MDO, roll stretching is not required before or after BO, preventing scratches and stains caused by rolls; and, because MD stretching is performed while the film's side edges are held by the gripping parts, the amount of necking can be reduced compared to roll stretching, resulting in a higher yield.

[0013] That is, the BO of the present invention is a BO in which an object to be processed is stretched in a conveying direction (MD) and a direction intersecting therewith (TD) in a stretching zone while being conveyed, the BO having an inlet for carrying in the object to be processed, an outlet for carrying out the object to be processed, guide rails extending in MD from the inlet to the outlet and provided in pairs with respect to TD, and a plurality of gripping parts which grip the side end portions of the object to be processed and move along the guide rails from the inlet to the outlet, the stretching zone being divided into two zones along MD, and the zone upstream in MD of the two zones is called a first stretching zone. The BO has a first stretching zone and the zone downstream in MD of the two zones is a second stretching zone. In the first stretching zone, the MD distance between the gripping parts adjacent to each other along the guide rail and the TD distance between the gripping parts moving along each of the guide rails facing the TD increase from the entrance to the exit. In the second stretching zone, the TD distance between the gripping parts moving along each of the guide rails facing the TD is constant, and the BO has a mechanism that can change the MD distance between the gripping parts independently of the MD distance between the gripping parts in the first stretching zone. [Effects of the Invention]

[0014] By using the BO of the present invention, at least one of the following effects (1) to (5) can be obtained. (1) The MD stretching ratio can be adjusted, so the pore size and strength ratio of the microporous film can be adjusted as desired. (2) The number of movable points provided on the guide rail can be reduced, thereby reducing the equipment costs. (3) Since only the guide rails in the latter half of the stretching zone need to be moved, operability is good. (4) Curved rails can be used in the first half of the stretching zone, which can prevent film breakage due to sudden changes in strain rate. (5) Since roll stretching before and after BO is not required, scratches and stains caused by rolls can be suppressed. Also, since MD stretching is performed while gripping with the gripping part, the amount of necking can be suppressed compared to MD stretching using rolls such as Pre-MDO and Post-MDO, and the yield can be increased. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a top view showing the outline of the configuration of a BO according to one embodiment of the present invention. [Figure 2] FIG. 1 is a top view schematically illustrating the configuration of a pantograph-type BO according to an embodiment of the present invention. [Figure 3] This is an explanatory diagram showing an enlarged view of the vicinity of the guide rail in a pantograph-type BO according to one embodiment of the present invention, in which the MD magnification of the second stretching zone is 1. Note that the gripping portion and the link mechanism are omitted. [Figure 4] This is an explanatory diagram showing an enlarged view of the vicinity of the guide rail in a pantograph-type BO according to one embodiment of the present invention, in which the MD magnification of the second stretching zone is greater than 1. Note that the gripping portion and the link mechanism are omitted. [Figure 5] This is an explanatory diagram showing an enlarged view of the vicinity of the guide rail in a pantograph-type BO according to one embodiment of the present invention, in which the MD magnification in the second stretching zone is less than 1. Note that the gripping portion and the link mechanism are omitted. DETAILED DESCRIPTION OF THE INVENTION

[0016] The present invention provides a biaxial stretching apparatus (BO) that stretches a material to be processed in a machine direction (MD) and a direction crossing the machine direction (TD) in a stretching zone while transporting the material, The biaxial stretching device has an inlet for carrying in the material to be processed on the upstream side and an outlet for carrying out the material to be processed on the downstream side, a pair of guide rails extending from the entrance to the exit in the MD and provided to sandwich the transport area of ​​the object to be processed from both end sides of the TD; a plurality of gripping parts provided on each of the guide rails, which grip the TD-side end of the object to be processed and move from the entrance to the exit along the guide rails, The stretching zone is divided into two zones along the MD. The zone on the upstream side of the two zones in the MD is called the first stretching zone, and the zone on the downstream side of the two zones in the MD is called the second stretching zone. The biaxial stretching apparatus (BO) is characterized in that, in the first stretching zone, the MD distance between the gripping parts adjacent to each other along the guide rail and the TD distance between the gripping parts moving along the respective guide rails facing the TD are set to increase from the entrance to the exit, and in the second stretching zone, the TD distance between the gripping parts moving along the respective guide rails facing the TD is set to be constant, and the MD distance between the gripping parts has a mechanism that can change the MD distance between the gripping parts independently of the MD distance between the gripping parts in the first stretching zone.

[0017] The BO of the present invention is a biaxial stretching device that stretches a material to be processed in a machine direction (MD) and a direction crossing the machine direction (TD) in a stretching zone while transporting the material, The biaxial stretching device includes an inlet for carrying in the object to be processed on the upstream side, an outlet for carrying out the object to be processed on the downstream side, and a pair of guide rails extending in MD from the inlet to the outlet, the guide rails being provided so as to sandwich a transport region for the object to be processed from both end sides of TD; a plurality of gripping parts provided on each of the guide rails and moving from the entrance to the exit along the guide rails while gripping the TD-side end of the object to be processed, wherein the stretching zone is divided into two zones along the MD, and the zone on the upstream side of the two zones in MD is designated as a first stretching zone, and the zone on the downstream side of the two zones in MD is designated as a second stretching zone, in the first stretching zone, an MD distance between the gripping parts adjacent to each other along the guide rails and a TD distance between the gripping parts moving along each of the guide rails facing the TD are set to increase from the entrance to the exit, It is preferable that in the second stretching zone, the TD distance between the gripping parts that move along the respective guide rails facing the TD is set to a constant value, and the BO has a mechanism that can change the MD distance between the gripping parts independently of the MD distance between the gripping parts in the first stretching zone.

[0018] The BO of the present invention can be processed, for example, by kneading a resin and a plasticizer, extruding the mixture into a sheet through a die, and solidifying it by cooling. The film is made of a resin and a plasticizer. Examples of the resin include, but are not limited to, PP and PE. In addition to the resin and plasticizer, additives such as antioxidants may also be added.

[0019] It is preferable to provide a step of immersing the film stretched by the BO of the present invention in a solvent to replace the plasticizer, and then drying the solvent to form a microporous structure in the film and obtain a microporous film.

[0020] The pore size and strength ratio of the microporous film change depending on the TD stretch ratio and MD stretch ratio of the BO of the present invention, but since the MD stretch ratio of the BO of the present invention can be adjusted as described above, the pore size and strength ratio of the microporous film can be adjusted as desired. Specific forms of the BO of the present invention are described below with reference to Figure 2. [Example]

[0021] The BO of the present invention has an entrance 11 for carrying in the object to be treated and an exit 12 for carrying out the object, and a guide rail 13 extends from the entrance 11 to the exit 12 in the MD in pair with the TD.

[0022] The guide rail 13 has a plurality of gripping portions that move along the guide rail 13 from the entrance 11 to the exit 12 while gripping the side edges of the object to be processed.

[0023] The MD stretch ratio is the ratio of the MD distance between the gripping parts 16 along the guide rail 13 at the entrance 11 to the MD distance between the gripping parts 16 along the guide rail 13 at the end of the stretching zone. For example, when the MD distance between the gripping parts 16 along the guide rail 13 at the entrance 11 is 20 mm and the MD distance between the gripping parts 16 along the guide rail 13 at the end of the stretching zone is 100 mm, the MD stretch ratio is 5 times.

[0024] The BO of the present invention has at least a stretching zone 2 in which the material to be treated is stretched in the TD and MD, and may also have a preheating zone 1 in which the material to be treated is heated to a predetermined temperature on the entrance 11 side of the stretching zone 2, and a heat setting zone 3 in which the material to be treated is heat-treated at a predetermined temperature on the exit 12 side of the stretching zone.

[0025] In the present invention, the stretching zone 2 is divided into two stretching zones along the MD, and of the two zones, the zone on the upstream side in MD is referred to as the first stretching zone 21, and the zone on the downstream side in MD is referred to as the second stretching zone 22. In the first stretching zone 21, the most upstream side in MD is the start point of the first stretching zone 21, and the most downstream side is the end point of the first stretching zone 21. Similarly, in the second stretching zone 22, the most upstream side in MD is the start point of the second stretching zone 22, and the most downstream side is the end point of the second stretching zone 22. That is, the end point of the first stretching zone 21 and the start point of the second stretching zone 22 overlap in MD.

[0026] The ratio of the MD distance between the gripping portions 16 along the guide rail 13 at the start of the first stretching zone 21 to the MD distance between the gripping portions 16 along the guide rail 13 at the end of the first stretching zone 21 is defined as a first MD stretch ratio, the ratio of the MD distance between the gripping portions 16 along the guide rail 13 at the start of the second stretching zone 22 to the MD distance between the gripping portions 16 along the guide rail 13 at the end of the second stretching zone 22 is defined as a second MD stretch ratio, and the product of the first MD stretch ratio and the second MD stretch ratio is defined as a final MD stretch ratio. The second MD stretch ratio may be a value less than 1, in which case the object to be processed will be shrunk in the MD from the start of the second stretching zone 22 to the end of the second stretching zone 22. Here, the first MD stretch ratio is preferably in the range of 2 to 10 times, and more preferably in the range of 4 to 7 times. The second MD stretching ratio is preferably in the range of 0.1 times or more and 2 times or less, more preferably in the range of 0.5 times or more and 1.5 times or less.

[0027] In the first stretching zone 21, the TD distance between the guide rails arranged in pairs at the TD and the MD distance between the plurality of gripping units 16 are configured to gradually increase from the entrance 11 to the exit 12. Mechanisms for increasing the MD distance between the plurality of gripping units 16 include, for example, a linear motor type, a pantograph type, and a motor chain drive type, with the linear motor type and the pantograph type being preferred, and the pantograph type being particularly preferred.

[0028] In the second stretching zone 22, the TD distance between the guide rails 13 arranged in pairs at the TD is constant, and a mechanism is provided that can change the MD distance between the multiple gripping units 16 independently of the MD distance between the multiple gripping units 16 in the first stretching zone 21. As with the first stretching zone 21, the mechanism may be, for example, a linear motor type, a pantograph type, or a motor chain drive type, with the linear motor type and the pantograph type being preferred, and the pantograph type being particularly preferred. It is also preferable to select the same mechanism for the first stretching zone 21 and the second stretching zone 22.

[0029] In the case of a linear motor type, the MD distance between the multiple gripping parts 16 can be changed by changing the speed between the multiple gripping parts 16 driven by the linear motor in the second stretching zone 22 from the speed in the first stretching zone 21.

[0030] A detailed configuration of the pantograph type will be described below with reference to FIG. The guide rail 13 is composed of an inner guide rail 14 and an outer guide rail 15 that are adjacent to each other in the horizontal direction. In the first stretching zone 21, the guide rail 13 composed of the inner guide rail 14 and the outer guide rail 15 is preferably a rail that is curved in the horizontal direction. That is, it is preferable that the TD distance between the paired guide rails 13 arranged in the TD gradually increases from the entrance to the exit, and that the distance between the inner guide rail 14 and the outer guide rail 15 gradually decreases. A link mechanism 17 composed of a plurality of links is arranged along the inner guide rail 14 and the outer guide rail 15, and the link mechanism 17 is provided with a plurality of gripping units 16 that grip the workpiece. As the link mechanism 17 moves from the entrance 11 to the exit 12 along the inner guide rail 14 and the outer guide rail 15, the MD distance between the gripping units 16 increases, and the workpiece is stretched in the MD. Here, when the horizontally curved rail is used in the first stretching zone 21, the first MD stretch ratio is determined by the shape of the horizontally curved rail, and therefore is a fixed value that cannot be changed.

[0031] In the second stretching zone 22, the inner guide rail 14 is straight and not curved, and the TD distance between the paired inner guide rails 14 is constant. Meanwhile, the outer guide rail 15 has two or more bending points 18 and a pivoting mechanism. The pivoting mechanism rotates the outer guide rail 15 around the bending points 18, thereby changing the TD distance between the inner guide rail 14 and the outer guide rail 15. The bending points 18 are not particularly limited as long as the outer guide rail 15 is bendable. For example, they may have joints, be made of a low-rigidity material, or have multiple slits to facilitate bending. While two or more bending points 18 can achieve the objective of the present invention, providing four or more bending points 18 allows for the selection of a rail pattern that suppresses changes in strain rate, which is preferable from the perspective of preventing membrane rupture. However, since increasing the number of bending points 18 increases equipment costs, the number of bending points 18 can be determined based on a balance between costs and the bending points. The rotation mechanism is not particularly limited as long as it rotates the outer guide rail 15 around the bending point 18, but examples include a mechanism that connects a linear motion mechanism such as a spindle or rack and pinion to the outer guide rail 15 or the bending point 18, and rotates the outer guide rail 15 by the linear motion of the linear motion mechanism.

[0032] 3 is an enlarged view of one side of the paired guide rails 13 arranged in the TD near the stretching zone 2. The gripping units 16 and the link mechanism 17 are omitted. The TD distance between the inner guide rail 14 and the outer guide rail 15 of the second stretching zone shown in FIG. 3 is constant in the MD, and when the link mechanism 17 moves along the inner guide rail 14 and the outer guide rail 15 from the start point of the second stretching zone 22 toward the end point of the second stretching zone 22, the MD distance between the gripping units 16 does not change, so the second MD stretch ratio is 1.0.

[0033] On the other hand, as shown in Figure 4, if the TD distance between the inner guide rail 14 and the outer guide rail 15 at the start point of the second stretching zone 22 is smaller than the TD distance between the inner guide rail 14 and the outer guide rail 15 at the end point of the second stretching zone 22, the link mechanism 17 moves along the inner guide rail 14 and the outer guide rail 15 from the start point of the second stretching zone 22 toward the end point of the second stretching zone 22, thereby increasing the MD distance between the gripping portions 16, and the object to be processed is stretched in the MD from the start point of the second stretching zone 22 to the end point of the second stretching zone 22, and the second MD stretch ratio becomes a value greater than 1.

[0034] Furthermore, as shown in Figure 5, if the TD distance between the inner guide rail 14 and the outer guide rail 15 at the start point of the second stretching zone 22 is greater than the TD distance between the inner guide rail 14 and the outer guide rail 15 at the end point of the second stretching zone 22, the link mechanism 17 moves along the inner guide rail 14 and the outer guide rail 15 from the start point of the second stretching zone 22 toward the end point of the second stretching zone 22, thereby reducing the MD distance between the gripping portions 16, and the object to be processed is contracted in MD from the start point of the second stretching zone 22 to the end point of the second stretching zone 22, and the second MD stretch ratio becomes a value less than 1.

[0035] 3, 4, and 5, in the second stretching zone 22, the length between the bending points 18 of the inner guide rail 14 varies depending on the second MD stretch ratio, but the inner guide rail 14 is provided with an extension / contraction mechanism, which adjusts the length between the bending points 18 according to the second MD stretch ratio. Alternatively, the inner guide rail 14 may be made of an extendable material without providing the extension / contraction mechanism.

[0036] 2, 3, 4, and 5, an adjustment mechanism 19 is provided closer to the exit 12 than the heat setting zone 3, and this mechanism keeps the TD distance between the inner guide rail 14 and the outer guide rail 15 constant relative to the MD in the heat setting zone 3. The adjustment mechanism 19 is not essential to achieve the object of the present invention, and is unnecessary if the heat setting zone 3 is not provided.

[0037] With the above-described configuration, in the second stretching zone 22, the MD distance between adjacent gripping portions 16 along the guide rail 13 can be changed independently of the MD distance between the gripping portions 16 in the first stretching zone 21, i.e., the second MD stretching ratio can be changed independently of the first MD stretching ratio.

[0038] With the above-mentioned configuration, the BO of the present invention has good operability because only the latter half of the MD of the guide rail 13 needs to be moved. Also, since a magnification-varying mechanism only needs to be provided in the stretching zone 2, the number of variable mechanisms can be reduced compared to a full-range variable BO, resulting in lower equipment costs. Unlike a full-range variable BO, a curved guide rail can be used in the former half of the stretching zone, preventing film breakage due to a sudden change in strain rate. Furthermore, unlike Pre-MDO and Post-MDO, roll stretching is not required before or after BO, preventing scratches and stains caused by rolls. Furthermore, MD stretching is performed while the film side edge is held by the gripping section, so the amount of necking can be reduced compared to roll stretching, resulting in a higher yield.

[0039] A step of measuring the physical properties of the microporous film may be provided after the BO of the present invention. In the step of measuring the physical properties, the physical properties of the microporous film may be obtained, and the physical properties may be fed back to the BO of the present invention, and a system for adjusting the second MD stretch ratio may be provided. In this way, the physical properties can be adjusted in a short time, and productivity can be improved.

[0040] Another embodiment of the present invention is a film manufacturing method including a stretching step of stretching a film, the stretching step comprising a biaxial stretching step of stretching the film in MD and TD, and an additional MD stretching step of stretching the film in MD immediately after the biaxial stretching step, wherein the additional MD stretching step is a step of adjusting the MD stretching ratio to a ratio different from the MD stretching ratio in the biaxial stretching step. The film is not particularly limited, but is suitably used as a method for manufacturing a microporous film.

[0041] In this production method, it is preferable that the MD stretching ratio in the biaxial stretching step is fixed. It is also preferable that the method further includes a measuring step of measuring the physical properties of the film, and the additional MD stretching step is a step of adjusting the MD stretching ratio by feeding back the physical properties obtained in the measuring step.

[0042] This production method allows the MD stretching ratio to be changed by additional MD stretching or MD shrinkage immediately after the biaxial stretching step, depending on the pore size and strength ratio of the microporous film desired by the customer, making it possible to easily adjust the pore size and strength ratio of the microporous film in a short time, thereby improving the productivity of the microporous film. [Explanation of symbols]

[0043] 1 Preheating Zone 2 Extension Zone 3 Heat Fixation Zone 11 Entrance 12 Exit 13 Guide rail 14 Inner guide rail 15 Outer guide rail 16 Gripping part 17 Link mechanism 18 Inflection Point 19 Adjustment mechanism 21 First Extension Zone 22 Second Stretching Zone

Claims

1. A pantograph-type biaxial stretching device in which a material to be processed is stretched in a machine direction (MD) and a direction intersecting therewith (TD) in a stretching zone while being transported, The biaxial stretching device has an inlet for carrying in the material to be processed on the upstream side and an outlet for carrying out the material to be processed on the downstream side, a pair of guide rails extending in the MD from the entrance to the exit, the guide rails being provided so as to sandwich a transport area for the object to be processed from both end sides of the TD; a plurality of gripping portions provided on each of the guide rails, the gripping portions moving from the entrance to the exit along the guide rails while gripping the TD side end of the object to be processed; Each of the pair of guide rails comprises an inner guide rail and an outer guide rail, The stretching zone is divided into two zones along the MD. The zone on the upstream side of the two zones in the MD is called the first stretching zone, and the zone on the downstream side of the two zones in the MD is called the second stretching zone. In the first stretching zone, an MD distance between adjacent gripping portions along the guide rail and a TD distance between the gripping portions moving along the guide rails opposite to each other in the TD are set to increase from the entrance to the exit, In the second stretching zone, a TD distance between the gripping units that move along the guide rails facing each other in the TD is set to a constant value, and a mechanism is provided that can change the MD distance between the gripping units independently of the MD distance between the gripping units in the first stretching zone, The mechanism comprises: In the second stretching zone, the outer guide rail has two or more bending points and a pivot mechanism; The biaxial stretching device is configured such that the distance between the inner guide rail and the outer guide rail is changed by rotating the outer guide rail around the bending point using the rotating mechanism, and by changing the distance between the inner guide rail and the outer guide rail, the MD distance between adjacent gripping portions along the guide rails can be changed independently of the MD distance between the gripping portions in the first stretching zone.

2. 2. The biaxial stretching apparatus according to claim 1, wherein the MD stretch ratio in the first stretching zone is fixed.

3. 3. The biaxial stretching apparatus according to claim 1, wherein the inner and outer guide rails in the first stretching zone are curved rails that curve away from the conveying region.

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