JPH11198157A - Method for forming film by casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve a thickness unevenness by extruding a polyamide acid composition having a specific viscosity in a die, thereby preventing an entraining of foams at the time of casting of a resin film. SOLUTION: The method for forming a resin film by casting is applied to a polyimide film. More specifically, a polyamide acid resin composition is preferably regulated to 450 poise or less, further 300 poise or less or particularly to 50 to 300 poise of a viscosity. To this end, the degree of polymerization of the composition is lowered, a solvent ratio is raised, and a liquid temperature is raised. When such a method for casting is used, an angle θ formed between a curtain 22 and a conveyor belt 24 becomes larger than a conventional angle to prevent entraining of foams. And, an elastic force of the curtain 22 is reduced. And, a change of a periodic landing point to the belt 24 is reduced, thereby preventing an occurrence of so-called tiger stripes on a resin film 26.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a casting method using a T-die method in extrusion molding, which is one of the methods for producing a plastic film. More specifically, the present invention relates to a casting method for preventing a resin solution (hereinafter, referred to as a curtain) extruded from a die from entraining bubbles in a T-die method and preventing uneven thickness.

[0002]

2. Description of the Related Art Plastics have been widely applied as excellent materials in various fields because of their properties such as elasticity, plasticity, elasticity, strength, and insulating property and ease of molding. In particular, those having a film shape are used for laminates, capacitors, wire rod coating agents, etc., depending on various thicknesses, and are applied to electric and electronic equipment parts.

Polyimide has excellent properties such as heat resistance, insulation property, solvent resistance and low temperature resistance among plastic materials, and is used as a material for electric and electronic parts. Wiring board, base film of TAB carrier tape, wire covering agent for aircraft, etc.
Examples include a base material for a magnetic recording tape, a wire coating material for a superconducting coil, and the like. In these various uses, a polyimide film suitable for each use is appropriately selected.

[0004] As electric and electronic parts become smaller and thinner, circuits become thinner, and dimensional changes in the members used cause failures such as disconnection and short-circuiting in the thinned circuit configuration. There is concern. Accordingly, plastics are required to have high dimensional stability with high accuracy in members used for electric / electronic parts, and films used for such members are also required to have high dimensional accuracy.

In the production of the plastic film, for example, as shown in FIG. 1, a resin in the form of a liquid or slurry which is mixed and kneaded in an extruder 2 is spread from the extruder 2 in the width direction. After that, the film passes through a narrow slit-shaped gap of the slit die 4 and is extruded from the die lip 6 into a film. Extruded resin composition,
It becomes the resin film 8 and is continuously mounted on the take-up machine 10.

[0006] Conventionally, a polyimide film is usually produced by continuously extruding a polyamic acid composition, which is a precursor of the polyimide film, into a smooth thin film on an endless belt by a slit die. After that, it is manufactured by a method of solidifying to the extent that it has self-supporting properties by drying and cooling while advancing imidation.

[0007]

However, in the above method for producing a film, the viscosity of the liquid or slurry resin composition is usually in the range of 500 to 2000 poise.

In the case of a thermosetting polyimide film, a polyamic acid composition, which is a precursor of polyimide, is treated with T
By the casting method using a die, through film forming, heating and drying, imidization is completed to obtain a polyimide film.In the casting process, when the imidization reaction proceeds rapidly, partial imidization of the resin film occurs. This has caused problems such as gel defects occurring in the film and gel stuck in the slit die, resulting in coating streaks. Therefore, in order to suppress the imidization reaction, the polyamic acid composition is generally cooled to 0 ° C. or lower, and the viscosity of the polyamic acid composition tends to be particularly high.

When the viscosity is in the above range, the following inconvenience may occur. That is, when such a resin composition having a relatively high viscosity is used, the resin composition has elasticity. Therefore, as shown in FIG.
The curtain 102 of the resin composition having fluidity extruded from 0 is pulled in the traveling direction as the belt speed increases. When the curtain 102 is pulled in the traveling direction, the landing angle θ between the curtain 102 and the belt 104 of the take-off machine decreases, and when the curtain 102 lands on the surface of the belt 104, the surrounding air is embraced. It will be easier.

As a result, air is sealed between the resin film 106 and the belt 104, and large and small foam-like protrusions remain on the surface of the resin film 106. This phenomenon of bubble entrapment significantly impairs the surface properties of the resin film, for example, in the process of drying the resin film, the thickness of the resin film becomes thinner, or the entrained air thermally expands to break the resin film and form a defective portion. Cause.

Further, as described above, a high-viscosity curtain has a higher elastic force than a low-viscosity curtain, and furthermore has a large adhesive force to a belt. Therefore, the curtain is pulled in the traveling direction by the movement of the belt. However, when the curtain is pulled in the traveling direction by a certain distance by the belt, a force opposite to the traveling direction acts due to the elastic force of the resin film, so that the landing point of the curtain fluctuates periodically. Due to this periodic fluctuation,
The thickness of the resin film to be manufactured fluctuates. As a result, there is a problem that periodic unevenness in thickness in the advancing direction occurs, and a stripe pattern appears in the shape of the final product film.

The present inventors have conducted intensive studies in order to prevent the entrainment of bubbles during casting of the resin film and to improve the thickness unevenness. The present inventors have conceived a method for casting a resin film, which reduces the thickness and prevents deterioration in thickness unevenness, and particularly a method for casting a polyimide film.

[0013]

That is, the gist of the method for casting a resin film according to the present invention is to extrude a polyamic acid composition having a viscosity of 450 poise or less in a die.

Another aspect of the method for casting a resin film according to the present invention is to extrude a resin composition having a viscosity of 300 poise or less in a die.

Another point of the method for casting a resin film according to the present invention is that the viscosity in the die is 50 to 50%.
Extruding a 300 poise polyamic acid composition.

Further, in the method for casting a resin film, the resin composition is a polyamic acid composition, and the resin film is a polyimide film.

Further, in the method for casting a resin film, the resin film is a thermosetting polyimide film.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, an example of an embodiment of a method for casting a resin film according to the present invention will be described.

Further, the term “curtain” in the present invention means
A resin composition having fluidity is extruded from a slit die and has a curtain-like shape existing in a gap of an air gap from when it is grounded to a belt.

Further, the term “landing angle” in the present invention refers to
The angle of incidence of the curtain with respect to the belt.

The method for casting a resin film according to the present invention can be basically applied to any resin film. For example, it can be applied to a polyester film, a polyamide film and the like. In particular, it is useful for electric and electronic devices because of its characteristics, and is preferably applied to a polyimide film that requires high precision.

The casting film forming method according to the present invention can be realized by adjusting the viscosity of the resin composition.

Specifically, the resin composition is adjusted to have a viscosity of 450
It is preferably adjusted to not more than poise, more preferably not more than 300 poise, particularly preferably 50 to 300 poise.

As a method for adjusting the viscosity of the above resin composition to the above range, generally, the degree of polymerization of the resin composition is lowered. Increase the solvent ratio in the resin composition. Increase the solvent ratio in the curing agent. Increase the liquid temperature of the resin composition. The following four methods are conceivable. The methods (1) and (2) are specifically to increase the solvent ratio, and are substantially the same. These methods can be used alone, but two or more of them may be used in combination.

In the casting film forming method according to the present invention, in the step of producing a polyimide resin, basically,
Any of the above methods may be used for viscosity adjustment,
In the above method, as the liquid temperature increases, the imidization reaction proceeds quickly, and partial gelation occurs in the T-die or piping,
This may result in gel defects in the film. Therefore, in the application to the production of polyimide films, especially thermosetting polyimide films, a method of lowering the degree of polymerization of the resin composition, and a method using a solvent, and further, a method in which these methods are combined to adjust the viscosity. It is preferably used.

As a method for lowering the degree of polymerization of the resin composition, for example, in the case of a polyimide film, it can be realized by adjusting the mixing ratio of aromatic tetracarboxylic dianhydride and aromatic diamine. .

Hereinafter, a casting method in the production of a thermosetting polyimide film will be described as an embodiment of the present invention.

After the polyimide film is continuously extruded from a slit die as a smooth thin film curtain of a polyamic acid composition as a precursor of the polyimide and lands on a conveyor belt, the resin film is dried. , Manufactured by solidification by cooling. The thickness of the resin film that can be produced by the method according to the present invention is not particularly limited, and it can be usually carried out in a range of about 5 to 200 μm.
Applies to film thickness in the range of about 10 μm to 50 μm. Films thinner than this range will result in a thinner curtain,
It is susceptible to wind and the like, and it is easy for bubbles to enter. Also,
A film having a thickness exceeding this range has a large discharge amount and a thick and heavy curtain, so that the problem of entrainment of bubbles does not occur.

The polyamic acid composition basically comprises a polymerization reaction of a tetracarboxylic dianhydride typified by pyromellitic dianhydride and a diamine typified by 4,4'-diaminodiphenyl ether in an organic polar solvent. The main component is a polyamic acid organic polar solvent in which the polyamic acid composition is uniformly dissolved in the organic polar solvent. In addition, various additives such as an imidization accelerator, a viscosity modifier, and a filler such as calcium phosphate can be added in order to form a stable polyimide film.

First, one or two kinds of diamines are diffused in a solution or slurry in an organic solvent under an atmosphere of an inert gas such as argon or nitrogen. At least one or more tetracarboxylic dianhydrides are added to this solution in a solid state, an organic solvent solution state, or a slurry state to obtain a polyamic acid solution. The reaction temperature at this time is from −20 ° C. to 50 ° C., preferably 20 ° C. or less. The reaction time is from 1 hour to 2 hours.

In this reaction, contrary to the above-mentioned addition procedure, tetracarboxylic dianhydride is first dissolved or diffused in an organic solvent, and a solution or slurry of the solid or organic solvent of the diamine is added to this solution. You may let it. Further, the reaction may be performed simultaneously, and the mixing order of the acid dianhydride component and the diamine component is not limited.

An aromatic tetracarboxylic dianhydride component,
One or more aromatic diamine components may be used, respectively.

Examples of the organic polar solvent used in the polymerization reaction of the polyamic acid include sulfoxide solvents such as diaminoethyl sulfoxide, N, N-dimethylformamide, formamide solvents, acetamide solvents, pyrrolidone solvents, and phenol solvents. Etc. can be given. These can be used in one kind or in a mixed solvent of two or more kinds, or mixed with an aromatic hydrocarbon.

The molar ratio of the aromatic tetracarboxylic dianhydride to the aromatic diamine component is, for example, 1:
About 1.005 to 1.05 is preferable, and within this range,
Either diamine or acid dianhydride may be large. Beyond this range, the finally obtained polyimide film will have a low degree of polymerization, and if it is lower than 1.005, the polymerization will proceed excessively and the viscosity of the polyamic acid solution will increase, so it is preferable. Absent.

The solid content of the polyamic acid in the solvent is 1
A range of 5% to 25% is preferred, more preferably 15%.
% To 20%.

As described above, the average molecular weight is 50.00
A polyamic acid organic polar solvent solution having a low degree of polymerization of about 0 to 130.000 can be produced.

The viscosity may be adjusted by further adding the solvent to the polyimide organic solvent solution.

As described above, the viscosity of the resin composition of the present invention exemplified by the polyamic acid composition is preferably 450 poise or less, more preferably 300 poise or less, and particularly preferably 50 to 300 poise. If the viscosity is not less than this range, the R value, which is an index of the variation in the film thickness, is remarkably increased, so-called tragima occurs in the polyimide film, and the phenomenon of embracing bubbles is apt to occur. Further, when the poise is 50 poise or less, it is difficult to stably form a film in the present invention using a casting method using a die.

The “R value” in the present invention refers to the difference between the maximum thickness and the minimum thickness of the obtained polyimide film, and the unit is μm. The R value is preferably 8% or less of the thickness.

When the casting method according to the present invention is used, as shown in FIG.
Is larger than the conventional angle, the entrapment of bubbles can be prevented, the elastic force of the curtain 22 is reduced, and the change of the periodic landing point on the conveyor belt 24 is reduced. In addition, it is possible to prevent the so-called “trazima” from being generated in the resin film 26.

The resin film of the polyamic acid composition thus formed is dehydrated and closed on a drum or an endless belt, and cured at least to a degree having self-supporting properties to produce a polyimide film. . The manufactured film is further subjected to a heat treatment or the like as necessary, and is used as a stable polyimide film.

While the casting method according to the present invention has been described above with respect to a polyimide film, particularly a thermosetting polyimide film, the present invention is not limited to this. For example, a polyester film, a polyamide film, etc. Can be applied to the method of casting a plastic film of the present invention, and the present invention can be carried out in various modified, modified, and changed modes based on the knowledge of those skilled in the art without departing from the gist of the present invention. .

Hereinafter, specific examples will be described, but the present invention is not limited to these examples.

[0044]

EXAMPLES The R value in the MD direction was measured as an index of the state of the resin films produced in the following Examples and Comparative Examples.
Specifically, the “R value in the MD direction” in the present invention is obtained by sampling the central part of the obtained polyimide film in the MD direction by 5 m, performing continuous thickness measurement using a contact type continuous thickness gauge, and measuring the maximum thickness. And the minimum thickness were read from the chart.

R value = [maximum thickness] − [minimum thickness] The unit is μm.

Example 1 As an aromatic diamine, 4,4
^' -Diaminodiphenyl ether as an aromatic tetracarboxylic dianhydride, pyromellitic dianhydride in a molar ratio of 1.02: 1, and a polyamic acid DM
An imidizing agent consisting of 300 g of acetic anhydride, 40 g of isoquinoline, and 150 g of DMF was rapidly stirred with a mixer at a weight ratio of 40% with respect to the polyamic acid DMF solution in the F solution (solid content concentration: 20%), extruded from a T-die, and extruded under a die 30
mm at a speed of 6.3 m / min on a running stainless steel endless belt. The viscosity is 2 at 0 ° C.
It was 10 poise.

This resin film was heated at 130 ° C. for 60 seconds at 300 ° C.
X 30 seconds, dried and cured at 450 ° C x 30 seconds, 25 μm
Was obtained. The center of the film was sampled 5 m in the MD direction, and the R value of the MD thickness was measured. It was 1.55 μm. When the film was examined, no entrainment of bubbles was observed.

Example 2 As an aromatic diamine, 4,4
^' -Diaminodiphenyl ether as aromatic tetracarboxylic dianhydride and pyromellitic dianhydride in a molar ratio of 1.005: 1, the polyamic acid D
An imidizing agent consisting of 300 g of acetic anhydride, 40 g of isoquinoline, and 150 g of DMF was rapidly stirred with a mixer in an MF solution (solid content: 5%) at a weight ratio of 40% with respect to the polyamic acid DMF solution, and extruded from a T-die. 30
mm at a speed of 6.3 m / min on a running stainless steel endless belt. The viscosity is 1 at 0 ° C
It was 60 poise.

The resin film was heated at 130 ° C. for 60 seconds at 300 ° C.
X 30 seconds, dried and cured at 450 ° C x 30 seconds, 25 μm
Was obtained. The center of the film was sampled 5 m in the MD direction, and the R value of the MD thickness was measured. 1.45 μm. When the film was examined, no entrainment of bubbles was observed.

Example 3 As an aromatic diamine, 4,4
^' -Diaminodiphenyl ether as aromatic tetracarboxylic dianhydride and pyromellitic dianhydride in a molar ratio of 1.005: 1, the polyamic acid D
An imidizing agent consisting of 300 g of acetic anhydride, 40 g of isoquinoline and 150 g of DMF was rapidly stirred in a MF solution (solid content: 20%) at a weight ratio of 40% with respect to the polyamic acid DMF solution by a mixer, extruded from a T-die, and extruded under a die. 3
0 mm was cast at a speed of 6.3 m / min on a running stainless steel endless belt. The viscosity was 65 poise at 0 ° C.

The resin film was heated at 130 ° C. for 60 seconds at 300 ° C.
X 30 seconds, dried and cured at 450 ° C x 30 seconds, 25 μm
Was obtained. The center of the film was sampled 5 m in the MD direction, and the R value of the MD thickness was measured. 1.45 μm. When the film was examined, no entrainment of bubbles was observed.

Example 4 As an aromatic diamine, 4,4
^' -Diaminodiphenyl ether as aromatic tetracarboxylic dianhydride and pyromellitic dianhydride in a molar ratio of 1.005: 1, the polyamic acid D
An imidizing agent consisting of 300 g of acetic anhydride, 40 g of isoquinoline, and 150 g of DMF was rapidly stirred in a MF solution (solid content: 20%) at a weight ratio of 40% with respect to the polyamic acid DMF solution by a mixer, and extruded from a T-die. Was cast on a stainless steel endless belt running at a speed of 6.3 m / min. The viscosity is 12
It was 300 poise at ° C.

The resin film was heated at 130 ° C. × 60 seconds at 300 ° C.
X 30 seconds, dried and cured at 450 ° C x 30 seconds, 25 μm
Was obtained. The center of the film was sampled 5 m in the MD direction, and the R value of the MD thickness was measured. It was 1.6 μm. When the film was examined, no entrainment of bubbles was observed.

(Example 5) An imidizing agent and a polyamic acid DMF solution having the same composition as those used in Example 4 were heated at 25 ° C.
, And was cast 30 mm below the die on an SUS endless belt running at a speed of 6.3 m / min. The viscosity was 185 poise at 16 ° C.

This resin film was heated at 130 ° C. for 60 seconds at 300 ° C.
X 30 seconds, dried and cured at 450 ° C x 30 seconds, 25 μm
Was obtained. The center of this film was sampled 5 m in the MD direction, and the R value of the MD thickness was measured to be 1.55 μm. When the film was examined, no entrainment of bubbles was observed.

Example 6 A solution of an imidizing agent and a polyamic acid DMF having the same composition as used in Example 1 was cooled to -10 ° C. with brine to make the viscosity 450 poise.
It was extruded from the die and cast 30 mm below the die onto an SUS endless belt running at a speed of 6.3 m / min.

This resin film was heated at 130 ° C. for 60 seconds at 300 ° C.
X 30 seconds, dried and cured at 450 ° C x 30 seconds, 25 μm
Was obtained. The center part of this film was sampled 5 m in the MD direction, and the R value of the MD thickness was measured to be 1.90 μm. When the film was examined, no entrainment of bubbles was observed.

Comparative Example 1 As an aromatic diamine, 4,4
Polyamide acid D obtained by polymerizing ^' -diaminodiphenyl ether as an aromatic tetracarboxylic dianhydride and pyromellitic dianhydride in a molar ratio of 1.005: 1.
An imidizing agent consisting of 300 g of acetic anhydride, 40 g of isoquinoline, and 150 g of DMF was rapidly stirred in a MF solution (solid content: 20%) at a weight ratio of 40% with respect to the polyamic acid DMF solution by a mixer, and extruded from a T-die. Was cast on a stainless steel endless belt running at a speed of 6.3 m / min. Viscosity is 0 ° C
Was 660 poise.

The resin film was heated at 130 ° C. for 60 seconds at 300 ° C.
X 30 seconds, dried and cured at 450 ° C x 30 seconds, 25 μm
Was obtained. The center of the film was sampled 5 m in the MD direction, and the R value of the MD thickness was measured. 2.2 μm. When the film was examined, no entrainment of bubbles was observed.

Comparative Example 2 As an aromatic diamine, 4,4
^' -Diaminodiphenyl ether as aromatic tetracarboxylic dianhydride and pyromellitic dianhydride in a molar ratio of 1.005: 1, the polyamic acid D
An imidizing agent consisting of 300 g of acetic anhydride, 40 g of isoquinoline, and 150 g of DMF was rapidly stirred in a MF solution (solid content: 20%) at a weight ratio of 40% with respect to the polyamic acid DMF solution by a mixer, and extruded from a T-die. Was cast on a stainless steel endless belt running at a speed of 6.3 m / min. The viscosity was 880 poise at 0 ° C.

The resin film was heated at 130 ° C. for 60 seconds at 300 ° C.
X 30 seconds, dried and cured at 450 ° C x 30 seconds, 25 μm
Was obtained. The center of the film was sampled 5 m in the MD direction, and the R value of the MD thickness was measured. It was 2.6 μm. When this film was examined, countless bubbles of about 5 mm in diameter were found at both ends of the film.

The following is a polyamic acid D having a controlled degree of polymerization.
A casting stability test was performed on an example in which the viscosity was reduced by further adding a solvent to the MF solution.

Example 7 As an aromatic diamine, 4,4
^' -Diaminodiphenyl ether as aromatic tetracarboxylic dianhydride and pyromellitic dianhydride in a molar ratio of 1.005: 1, the polyamic acid D
DMF was further added to the MF solution (solid content concentration: 20%) at a weight ratio of 60%, followed by uniform stirring. The viscosity was 300 poise at 0 ° C. This was extruded from a T-die and cast on a stainless steel endless belt running at a speed of 10 m / min. At first there was countless bubbles involved, but as the distance between the die and the belt (hereinafter referred to as the air gap) was gradually reduced, the bubbles gradually decreased, and the film was formed without bubbles in the air gap of 25 mm. We were able to. When the traveling speed is 12 m / min, the air gap is 15 mm, and when the traveling speed is 14 m / min, the air gap is 5 mm.
It was possible to cast with no bubbles involved. However, at 16 m / min, entrapment of bubbles occurred no matter how close it was.

Example 8 A polyamic acid DMF solution (solid content: 20%) was polymerized in the same manner as in Example 7.
Further, DMF was added at a weight ratio of 70% and the mixture was stirred uniformly. The viscosity was 186 poise at 0 ° C. This was extruded from a T-die and cast on a stainless steel endless belt running at a speed of 10 m / min. Air gap 35m
m, a film could be formed without involving bubbles. The belt speed is 20 mm at 12 m / min, and 10 m at 14 m / min.
At m and 16 m / min, casting was possible without air entrapment with an air gap of 5 mm.

Example 9 In the same manner as in Example 7, a polymerized polyamic acid DMF solution (solid content: 20%) was added
Further, DMF was added at 100% by weight and stirred uniformly.
The viscosity was 65 poise at 0 ° C. This was extruded from a T-die and cast on a stainless steel endless belt running at a speed of 12 m / min. Air gap is 50
mm or more, the film could be formed without involving bubbles. At a belt speed of 14 m / min, the air gap is 50 mm,
At 16 m / min, casting was possible at 30 mm without foam entrapment.

Comparative Example 3 A polymerized polyamic acid DMF solution (solid content: 20%) was prepared in the same manner as in Example 7.
Further, DMF was added at 40% by weight and uniformly stirred. The viscosity was 660 poise at 0 ° C. This was extruded from a T-die and cast on a stainless steel endless belt running at a speed of 10 m / min. Air gap 10m
m, a film could be formed without involving bubbles. At 12 m / min, it was 5 mm, and at 14 m / min, it was 3 mm. However, 16m /
In minutes, no matter how close they were, bubbles were trapped.

For Examples 7 to 9 and Comparative Example 3,
Table 1 shows the relationship between the belt speed and the air gap that can be cast without foam entrapment.

[0068]

[Table 1]

It can be seen from Examples 1 to 6 that by adjusting the viscosity, a film having no MD and a small R value of the MD thickness can be obtained without entrainment of bubbles.

In comparison, Comparative Examples 1 and 2
The R value of the MD thickness is large, and in Comparative Example 3,
Foam entrainment occurred.

Further, from Tables 1 of Examples 7 to 9 and Comparative Example 3, it can be seen that as the viscosity is adjusted to be lower, the belt speed at which casting can be performed without entrainment of bubbles increases. Therefore, by setting the viscosity to be low, the production efficiency of the film can be increased.

[0072]

As described above, in the present invention, in the step of casting and coating a sheet by die casting, the viscosity is adjusted to be low, so that even if the belt speed is increased, the curtain is pulled in the traveling direction. The incidence angle of the curtain on the belt can be kept large. Therefore, at the time when the curtain lands on the belt, it is possible to prevent breakage of the film due to entrainment of bubbles,
In addition, since the landing point is stable, it is possible to prevent the occurrence of tragima due to thickness unevenness, and it is possible to provide a stable casting film forming method with high workability.

[Brief description of the drawings]

FIG. 1 shows a conventional plastic film manufacturing process.

FIG. 2 shows a state of a curtain extruded from a die lip by a casting film forming method according to the present invention.

FIG. 3 shows a conventional state of a curtain extruded from a die lip.

[Explanation of symbols]

 2; Extruder 4: Slit die 6, 20, 100; Die lip 8, 26, 106; Resin film 10, 24, 104; Conveyor belt 22, 102; Curtain

Claims (5)

[Claims] 1. A method for casting a resin film, comprising extruding a resin composition having a viscosity of 450 poise or less in a die. 2. A method for casting a resin film, comprising extruding a resin composition having a viscosity of 300 poise or less in a die. 3. A method for casting a resin film, comprising extruding a resin composition having a viscosity of 50 to 300 poise in a die. 4. The casting method according to claim 1, wherein the resin composition is a polyamic acid composition, and the resin film is a polyimide film. Method. 5. The method for casting a resin film according to claim 1, wherein the resin film is a thermosetting polyimide film.