Patent Description:
The following first describes a general method for producing a microporous polyolefin resin sheet. <FIG> is a schematic diagram of a general apparatus for producing a microporous polyolefin resin sheet. This production apparatus illustrated in <FIG> includes a die <NUM> having a discharge port <NUM> for discharging a sheet material <NUM> containing a polyolefin resin and a diluent, a casting device <NUM> cooling and solidifying the sheet material <NUM> discharged from the discharge port <NUM> while conveying the sheet material <NUM>, and a decompression chamber <NUM> disposed upstream of the discharge port <NUM> in a sheet conveying direction, covering a space <NUM> between the sheet material <NUM> and the casting device <NUM>, and sucking air to form a decompression space. A method for producing a sheet is known in which the sheet material <NUM> is discharged from the discharge port <NUM> of the die <NUM> toward the casting device <NUM>, air in the space <NUM> is sucked from openings <NUM> of exhaust nozzles <NUM> disposed outside both ends of the sheet material <NUM> in a sheet width direction to bring the sheet material <NUM> into intimate contact with the casting device <NUM>, and the casting device <NUM> cools and solidifies the sheet material <NUM> while conveying the sheet material <NUM>. In general, a small gap is formed between a side wall 4a of the decompression chamber <NUM> covering a face of the space <NUM> perpendicular to the sheet width direction and the casting device <NUM> in order to prevent contact between the casting device <NUM> and the decompression chamber <NUM> while reducing an inflow of outside air.

By the way, as a technique to improve sheet quality, there is an apparatus for producing a resin sheet disclosed in Patent Literature <NUM>. Patent Literature <NUM> discloses an apparatus for producing a resin sheet including shielding plates near the ends of the sheet material for the purpose of reducing thickness unevenness caused by vibration of the sheet material. <FIG> is a schematic diagram of the apparatus for producing a resin sheet of Patent Literature <NUM> observed from the top in the vertical direction. As illustrated in <FIG>, in the production apparatus of Patent Literature <NUM>, the rectifying effect by shielding plates <NUM> can reduce the formation of vortexes at the ends of the sheet material <NUM>, reduce vibration of the sheet material, and reduce thickness unevenness of the resin sheet.

With an increase in the volume of production of resin sheets in recent years, there is a strong demand to increase the discharge amount of the sheet material and to extend a continuous film forming time. However, increasing the discharge amount of the sheet material and extending the continuous film forming time also increase the opportunity for low molecular weight components, diluents, and additive-derived gases in the sheet material to adhere near the discharge port of the die and liquefy (hereafter, droplets). When these droplets come into intimate contact with the casting device and the sheet material due to an airflow flowing in the decompression chamber, they cause sheet defects and sheet tears.

However, at present, no method has been developed to reduce the scattering of droplets.

Given these circumstances, the present invention provides a method and an apparatus for producing a microporous polyolefin resin sheet that can stably produce a high-quality sheet by reducing the scattering of droplets to the casting device and the sheet material due to the airflow generated in the decompression chamber.

Provided is a method for producing a microporous polyolefin resin sheet according to the present invention to solve the object. The method includes: discharging a sheet material containing a polyolefin resin and a diluent from a discharge port of a die toward a casting device; covering a space between the sheet material and the casting device by a decompression chamber disposed upstream of the discharge port in a sheet conveying direction; sucking air in the decompression chamber to make a decompression space and bringing the sheet material into intimate contact with the casting device; and cooling and solidifying the sheet material while conveying the sheet material by the casting device, the suction of air in the decompression chamber being performed from exhaust nozzles disposed outside both ends of the sheet material in a width direction such that openings of the exhaust nozzles face each other, Y1/H ≤ <NUM> and <NUM> ≤ Y2/H ≤ <NUM> when a shortest distance from the discharge port of the die to an outer peripheral face of the casting device is H, a shortest distance from an upper end of the opening to an end of the sheet material is Y1, and a shortest distance from a lower end of the opening to the end of the sheet material is Y2.

In the method for producing a microporous polyolefin resin sheet according to the present invention, it is desirable that a suction width L1, which is a horizontal distance from an end of the sheet material <NUM> to an upstream end in the sheet conveying direction at the upper end of the opening of the exhaust nozzle observed from outside in a sheet width direction is set to be L1/H ≤ <NUM>.

In the method for producing a microporous polyolefin resin sheet according to the present invention, it is desirable that a seal gap, which is a gap positioned immediately below the opening of the exhaust nozzle in a gap between a side wall of the decompression chamber and the casting device, is physically blocked by a seal material, the side wall being perpendicular to a sheet width direction.

In the method for producing a microporous polyolefin resin sheet according to the present invention, it is desirable that the seal material is an elastic body, and the seal material is pressed against the casting device to block the seal gap.

In the method for producing a microporous polyolefin resin sheet according to the present invention, it is desirable that the seal material is pressed against and fixed to the decompression chamber by a pressing member.

Provided is an apparatus for producing a microporous polyolefin resin sheet according to the present invention to solve the object. The apparatus includes: a die including a discharge port discharging a sheet material containing a polyolefin resin and a diluent; a casting device configured to cool and solidify the sheet material discharged from the discharge port while conveying the sheet material; and a decompression chamber disposed upstream of the discharge port in a sheet conveying direction, the decompression chamber being configured to cover a space between the sheet material and the casting device, and suck air to make a decompression space, in the decompression chamber, exhaust nozzles configured to suck air in the decompression chamber being disposed outside both ends of the sheet material in a width direction such that openings of the exhaust nozzles face each other, the die, the casting device, and the exhaust nozzles being disposed such that Y1/H ≤ <NUM> and <NUM> ≤ Y2/H ≤ <NUM> when a shortest distance from the discharge port of the die to an outer peripheral face of the casting device is H, a shortest distance from an upper end of the opening of the exhaust nozzle to an end of the sheet material is Y1, and a shortest distance from a lower end of the opening of the exhaust nozzle to the end of the sheet material is Y2.

In the apparatus for producing a microporous polyolefin resin sheet according to the present invention, it is desirable that the exhaust nozzle is disposed such that a suction width L1, which is a horizontal distance from an end of the sheet material <NUM> to an upstream end in the sheet conveying direction at the upper end of the opening of the exhaust nozzle observed from outside in a sheet width direction, gives L1/H ≤ <NUM>.

It is desirable that the apparatus for producing a microporous polyolefin resin sheet according to the present invention further includes a seal material configured to physically block a seal gap, which is a gap positioned immediately below the opening of the exhaust nozzle in a gap between a side wall of the decompression chamber and the casting device, the side wall being perpendicular to a sheet width direction.

In the apparatus for producing a microporous polyolefin resin sheet according to the present invention, it is desirable that the seal material is an elastic body, and the seal material is pressed against the casting device.

It is desirable that the apparatus for producing a microporous polyolefin resin sheet according to the present invention further includes a pressing member configured to press the seal material against and fix the seal material to the decompression chamber.

The method for producing and the apparatus for producing a polyolefin resin sheet of the present invention reduce the scattering of droplets due to the airflow in the decompression chamber to the casting device and the sheet material and can thereby stably produce a high-quality sheet.

The following describes the meanings of respective terms in the present invention.

The "sheet material" is a material forming a sheet. As the sheet material, a resin of a polyolefin solution prepared by mixing a polyolefin resin such as polyethylene, polypropylene, polystyrene, or polymethylpentene with a diluent and heating and melting the mixture can be used, for example. The diluent is not limited to a particular diluent so long as it is a substance that can be mixed with or dissolved in the polyolefin resin. Materials that are miscible with the polyolefin in a melt-kneaded state but solid at room temperature may be used as the diluent. Examples of such a solid diluent include stearyl alcohol, ceryl alcohol, and paraffin waxes. The diluent is preferably liquid at room temperature in order to prevent stretching instability and in consideration of later application. Examples of the liquid diluent include aliphatic, cyclic aliphatic, or aromatic hydrocarbons such as nonane, decane, decalin, paraxylene, undecane, dodecane, and liquid paraffin, mineral oil fractions with boiling points corresponding thereto, and phthalates that are liquid at room temperature such as dibutyl phthalate and dioctyl phthalate. Liquid paraffin is further preferably used in order to obtain a stable gel-like sheet. In addition, the viscosity of the liquid diluent is preferably <NUM> to <NUM> cSt at <NUM>. The ratio between the polyolefin resin and the diluent blended is preferably <NUM> to <NUM>% by mass of the polyolefin resin and <NUM> to <NUM>% by mass of the diluent with the total of the polyolefin resin and the diluent being <NUM>% by mass from the viewpoint of making the moldability of an extrudate favorable. The uniform melt-kneading step for the sheet material is not limited to particular means, and examples include calenders, various mixers, and extruders with screws.

The "casting device" refers to a device bringing the sheet material discharged from a discharge port of a die to therewith to convey the sheet material downstream while cooling and solidifying the sheet material. The form is not limited to a particular form, and examples include rolls and belts.

The "width direction" refers to a direction matching a width direction when the sheet material is formed into a sheet shape by the die.

The "sheet conveying direction" refers to a direction in which the casting device conveys the sheet. The destination of conveyance is downstream, whereas the opposite is upstream.

The "decompression space" is a space formed between the sheet material and the casting device, the space being covered by a decompression chamber to have negative pressure.

The "decompression chamber" refers to a device covering and decompressing the space between the sheet material and the casting device to bring the sheet material into intimate contact with the casting device. In general, the pressure in the decompression chamber is -<NUM>,<NUM> Pa or higher and -<NUM> Pa or lower with respect to atmospheric pressure. The pressure in the decompression chamber may be controlled in accordance with film forming conditions.

The "opening" is a portion of an exhaust nozzle sucking air from inside the decompression chamber. The shape of the opening is not limited to a particular shape, and examples include a square, a rectangle, a trapezoid, a circle, and an oval. The opening may be subjected to surface treatment such as nickel plating, chromium plating, or zinc plating. The power source for suction is not limited to a particular power source, and examples include blowers and vacuum pumps.

The "side wall" refers to a side face of the decompression chamber, the side face being perpendicular to the sheet width direction.

The following describes an apparatus for producing and a method for producing a polyolefin resin sheet of the present invention in detail with reference to the accompanying drawings. However, the present invention is not limited to the embodiments shown here. Components having the same uses and functions as those of the conventional technique may have the same symbols as those thereof.

Referring to <FIG>, the following describes the influence of each dimension of an opening <NUM> of an exhaust nozzle <NUM> included in a decompression chamber <NUM> on an airflow in the decompression chamber <NUM>. <FIG> is a schematic diagram of an aspect of the apparatus for producing a polyolefin resin sheet of the present invention observed from upstream of the sheet conveying direction. The symbols Y1 and Y2 in the drawing are shortest distances from the opening <NUM> to an end of the sheet material <NUM>, in which the shortest distance at an upper end of the opening <NUM> is Y1 and the shortest distance at a lower end of the opening <NUM> is Y2. The symbol H is a shortest distance from a discharge port <NUM> of a die <NUM> to an outer peripheral face of a casting device <NUM>. The length of H is determined by the film forming conditions, but if the opening <NUM> is small, suction is made unstable due to clogging of droplets and the like, and thus from the viewpoint of ensuring the height of the opening <NUM> from the upper end to the lower end, H is preferably <NUM> or more and more preferably <NUM> or more. The inventors of the present invention have conducted earnest studies to find that Y1/H and Y2/H, in which Y1 and Y2 are standardized by H, affect the airflow in the decompression chamber <NUM>, which by extension scatters droplets in the decompression chamber <NUM>.

Referring to <FIG> and <FIG>, the following describes pressure and airflows near the end of the sheet material <NUM>. <FIG> is a partially enlarged sectional view perpendicular to the vertical direction along the X-X line in <FIG>, the diagram illustrating the pressure near the end of the sheet material <NUM> with contour lines. <FIG> is a diagram illustrating the pressure near the end of the sheet material <NUM> with contour lines when the distance of Y1 is longer than that in the state in <FIG> (when H is the same, and Y1/H is larger). The following gives the pressure as specific numerical values in order to accurately convey the main point of the present invention, but the numerical values are only by way of example and do not limit the scope of the present invention.

First, in the state in <FIG>, it is assumed that the pressure at each place is atmospheric pressure near the end of the sheet material <NUM> downstream of the sheet conveying direction (a point A), (atmospheric pressure - <NUM>) Pa near the end of the sheet material <NUM> inside the decompression chamber <NUM> (a point B), (atmospheric pressure - <NUM>) Pa downstream of the opening <NUM> (a point C), and (atmospheric pressure - <NUM>) Pa near the end of the sheet material <NUM> closer to the exhaust nozzle <NUM> (a point D). In this case, the pressure contour lines are as illustrated in <FIG>, and air is characterized to flow from higher pressure to lower pressure due to the Navier-Stokes equation, and thus most of air flows to the point C, which is lower in pressure than the point A, that is, to the opening <NUM> of the exhaust nozzle <NUM> as in the air flow 8a.

As the distance of Y1 becomes longer and Y1/H increases from the state in <FIG>, the pressure gradient from the point A to the point C becomes gentler, and the pressure at the point D becomes higher up to (atmospheric pressure - <NUM>) Pa than that in the state in <FIG>. Then, the pressure distribution becomes as in <FIG>, and part of air flows from the point A through the point D to the point B, which is lower in pressure, that is, also into the decompression chamber <NUM> as in the airflow 8b. This airflow 8b scatters the droplets <NUM> near the discharge port <NUM> of the die <NUM> into the body of the decompression chamber <NUM>, which adhere to the casting device <NUM> and sheet material <NUM>, causing sheet defects and sheet tears.

If the shortest distance Y2 from the lower end of the opening <NUM> to the end of the sheet material <NUM> is shorter, or if the shortest distance H from the discharge port <NUM> of the die <NUM> to the outer peripheral face of the casting device <NUM> is longer, and Y2/H is too small, the airtightness in the decompression chamber <NUM> increases, the pressure inside the decompression chamber <NUM> becomes lower than that near the end of the sheet material <NUM> closer to the exhaust nozzle <NUM>, and, as in the case of <FIG>, the airflow 8b, which flows from downstream of the sheet conveying direction through the end of the sheet material <NUM> to the inside of the decompression chamber <NUM>, is generated in the decompression chamber <NUM>.

On the other hand, if the shortest distance Y2 from the lower end of the opening <NUM> to the end of the sheet material <NUM> is longer, or if the shortest distance H from the discharge port <NUM> of the die <NUM> to the outer peripheral face of the casting device <NUM> is shorter, and Y2/H is too large, vortexes occur near the end of the sheet material <NUM>, and turbulent pressure and airflows scatter the droplets <NUM>.

The inventors of the present invention have further conducted experiments and theoretical calculations to find that by setting Y1/H ≤ <NUM> and <NUM> ≤ Y2/H ≤ <NUM>, the pressure at the point B is stably higher than that at the point D as illustrated in <FIG>, and it is possible to eliminate the airflow flowing into the decompression chamber <NUM>, prevent the scattering of the droplets <NUM> into the body of the decompression chamber <NUM>, and reduce the adherence of the droplets <NUM> to the casting device <NUM> and the sheet material <NUM>. More preferably, Y1/H ≤ <NUM>.

To achieve Y1/H ≤ <NUM> and <NUM> ≤ Y2/H ≤ <NUM>, the decompression chamber <NUM> may be designed in consideration of the discharge amount of the sheet material <NUM> from the die <NUM>, the shortest distance H from the discharge port <NUM> of the die <NUM> to the outer peripheral face of the casting device <NUM>, a suction amount from the exhaust nozzle <NUM>, and the like, but it is preferable to make side walls 4a of the decompression chamber <NUM> movable in the width direction of the sheet material <NUM> to adjust Y1 and Y2, the side walls 4a being perpendicular to the sheet width direction.

Refer now to <FIG> is a schematic diagram of an aspect of the apparatus for producing a polyolefin resin sheet of the present invention observed from the sheet width direction. In reality, the sheet material <NUM> is blocked by the decompression chamber <NUM> and cannot be seen, but <FIG> illustrates the sheet material <NUM> with the assumption that it can be observed through the decompression chamber <NUM> for the purpose of understanding the present invention. A suction width L1 in the drawing is a horizontal distance from the end of the sheet material <NUM> to an upstream end in the sheet conveying direction at the upper end of the opening <NUM> of the exhaust nozzle <NUM> observed from outside in the sheet width direction. In the method for producing a polyolefin resin sheet of the present invention, it is preferable to set L1/H ≤ <NUM>. Even if L1/H is larger than <NUM>, the effect of the present invention can be obtained by setting Y1, Y2, and H to be in the above ranges, but by setting L1/H to <NUM> or less, the amount of air sucked from inside the body of the decompression chamber <NUM> reduces, further, the pressure inside the decompression chamber <NUM> can be increased, and the airflow from downstream of the sheet conveying direction through the end of the sheet material <NUM> to the inside of the decompression chamber <NUM> can be reduced. In the apparatus for producing a microporous polyolefin resin sheet of the present invention illustrated in <FIG> and <FIG>, only the exhaust nozzles <NUM> make the space <NUM> in the body of the decompression chamber <NUM> a decompressed space without performing decompression from the body of the decompression chamber <NUM> by a vacuum pump or the like, but the inside of the body of the decompression chamber <NUM> may also be decompressed by a vacuum pump. For adjustment of Y1 and Y2, it is preferable to decompress the inside of the body of the decompression chamber <NUM> only by the exhaust nozzles <NUM> without decompressing it by a vacuum pump or the like.

Refer again to <FIG>. A seal gap <NUM> in the drawing is a gap positioned immediately below the opening <NUM> of the exhaust nozzle <NUM> in a gap between the side wall 4a of the decompression chamber <NUM> and the casting device <NUM>, the side wall 4a being perpendicular to the sheet width direction. As illustrated in <FIG>, in the method for producing a polyolefin resin sheet of the present invention, the seal gap <NUM> is preferably blocked by a seal material <NUM>. Although the effect of the present invention can be obtained even without this seal material <NUM>, providing the seal material <NUM> can eliminate an airflow flowing in through the seal gap <NUM> to disrupt the pressure balance near the end of the sheet material <NUM> and reduce the scattering of the droplets <NUM> into the decompression chamber. The material of the seal material <NUM> is not limited to a particular material, and examples include resin, rubber, and ceramic.

Further, in the method for producing a polyolefin resin sheet of the present invention, the seal material <NUM> is preferably an elastic body, and the seal gap <NUM> is preferably blocked by pressing the seal material <NUM> against the casting device <NUM>. By pressing the seal material <NUM> as an elastic body against the casting device <NUM>, due to its compressive load, the seal material <NUM>, while becoming deformed, can also prevent a minute gap with respect to the casting device <NUM>. More preferably, the seal material <NUM> is a non-conductive, porous resin or rubber that does not impart conductivity to a product even if wear particles are mixed into the product. The hardness of the seal material <NUM> is preferably Shore E10 to <NUM>, which can impart sealability at low loads. More preferably, the hardness is Shore E25 to <NUM>, with which distortion due to friction with the casting device <NUM> is small.

The method for fixing the seal material <NUM> is not limited to a particular method, and examples include a method of fastening it directly to the side wall 4a with a plurality of bolts. However, the seal material <NUM> pressed against the casting device <NUM> may become distorted or misaligned due to friction with the casting device <NUM> and fail to exhibit its sealing function, and thus care must be taken in the method of fixing. If the seal material <NUM> is directly bolted, the fixation may be lax in a narrow gap between the exhaust nozzle <NUM> and the casting device <NUM> and the like, and the seal material <NUM> may become distorted or misaligned. Thus, in the method for producing a polyolefin resin sheet of the present invention, the seal material <NUM> is preferably pressed against and fixed to the decompression chamber <NUM> by a pressing member <NUM>. <FIG> are diagrams illustrating the seal material <NUM> and the pressing member <NUM> disposed between the casting device <NUM> and the decompression chamber <NUM> of the apparatus for producing a microporous polyolefin resin sheet in <FIG>. As illustrated in <FIG>, the seal material <NUM> is preferably fixed with bolts <NUM> via the pressing member <NUM>. Even in the narrow gap between the exhaust nozzle <NUM> and the casting device <NUM>, in which the bolt <NUM> cannot be disposed, like that illustrated in <FIG>, the seal material <NUM> is fixed through the entire face being in contact with the pressing member <NUM>, and thus distortion and misalignment are reduced, and the seal gap <NUM> can be stably blocked by the seal material <NUM>. The shape of the pressing member <NUM> is not limited to a particular shape, and it is preferably an L-shaped plate also covering the top of the seal material <NUM> like that illustrated in <FIG>, but it may be a flat plate. Further, the frictional force of the contact surface with respect to the seal material <NUM> of the pressing member <NUM> may be increased in order to prevent the seal material <NUM> from becoming misaligned. Examples include providing protruding parts, increasing surface roughness, and applying an adhesive.

The following shows examples of polyolefin resin sheet production using the method for producing a polyolefin resin sheet of the present invention.

The following describes a result of actually producing a polyolefin resin sheet using the method for producing a polyolefin resin sheet and evaluating defects caused by droplet scattering. Specific sheet production conditions and a specific method for evaluating defects in the present embodiment are as follows.

An extruder was used to extrude the sheet material at a flow rate of <NUM>/h, which was passed through a gear pump and a filter and was then fed to a die. The apparatus temperature up to the die is <NUM>.

The sheet material was discharged from a <NUM> wide, <NUM> gap discharge port and was formed into a sheet shape. The amount of LP volatilized was measured from the discharged sheet to be about <NUM>/h.

The sheet was brought into intimate contact with a sheet forming roll rotating at a speed of <NUM>/minute at a temperature of <NUM> and was cooled and solidified. The discharge port of the die was installed at a position directly above the center of the sheet forming roll, with the shortest distance H from the discharge port of the die to the outer peripheral face of the sheet forming roll being <NUM>.

The decompression chamber is installed upstream of the discharge port of the die in the conveying direction and is shaped to cover the space between the sheet material discharged from the discharge port and the casting device. The wall faces at both ends of the decompression chamber are disposed with exhaust nozzles facing each other, and the opening of the exhaust nozzle is rectangular with the horizontal direction as the long side, with the long side being <NUM> and the short side being <NUM>. With the die installed under the conditions for film forming, the distance from the discharge port of the die to the upper end of the opening was <NUM> and the distance from the lower end of the opening to the sheet forming roll was <NUM>. In this state, the inside of the decompression chamber was evacuated at -<NUM> Pa from atmospheric pressure by a blower connected to the openings. The decompression chamber includes a mechanism to adjust the positions of the wall faces at both ends and was adjusted such that with the sheet material being discharged from the discharge port of the die, Y1 was <NUM> (Y1/H = <NUM>), Y2 was <NUM> (Y2/H = <NUM>), and L1 was <NUM> (L1/H = <NUM>). The decompression chamber was disposed such that the seal gap was <NUM>.

An inspection device MaxEye. Impact (Futech Inc. ) was used to count the number of defects due to droplet scattering on the sheet (<NUM> wide and <NUM> long) collected under the above conditions. The number of defects was five.

Sheets were collected and inspected for the number of defects in the same manner as in Example <NUM> except that the respective dimensions were changed as listed in Table <NUM>.

A sheet was collected and inspected for the number of defects in the same manner as in Example <NUM> except that a seal material (polytetrafluoroethylene, hardness Shore D55) was fixed to the decompression chamber with bolts to physically block the seal gap.

A sheet was collected and inspected for the number of defects in the same manner as in Example <NUM> except that the seal material was silicone sponge rubber as an elastic body (hardness Shore E30), and the seal material was pressed against the casting device and fixed to the decompression chamber with bolts to block the seal gap.

A sheet was collected and inspected for the number of defects in the same manner as in Example <NUM> except that the seal material was pressed against and fixed to the decompression chamber by a pressing member (SUS304, a <NUM> thick flat plate).

Tables <NUM> and <NUM> summarize the production conditions and the number of defects having occurred in the sheets for Examples <NUM> to <NUM> and Comparative Examples <NUM> to <NUM>, respectively.

Referring to <FIG>, the following describes the relation between Y1/H and Y2/H in each of the examples and the comparative examples. <FIG> is a graph showing the position of Y1/H and Y2/H for each of the examples and the comparative examples, with Y1/H on the vertical axis and Y2/H on the horizontal axis. "O" is Examples <NUM> to <NUM>, "X" is Comparative Examples <NUM> to <NUM>, and the area within the dotted line is a range satisfying the conditions "Y1/H ≤ <NUM>" and "<NUM> ≤ Y2/H ≤ <NUM>. " As illustrated in <FIG> and listed in Table <NUM>, in Examples <NUM> to <NUM>, which satisfied the conditions "Y1/H ≤ <NUM>" and "<NUM> ≤ Y2/H ≤ <NUM>," the pressure in the decompression chamber was higher than that at the end of the sheet material, and the airflow flowing into the decompression chamber was reduced, and thus the number of defects was small, which was four and five.

On the other hand, as illustrated in <FIG> and listed in Table <NUM>, in Comparative Examples <NUM> to <NUM>, which did not satisfy the condition "Y1/H ≤ <NUM>" or "<NUM> ≤ Y2/H ≤ <NUM>," the number of defects was large, which was <NUM> to <NUM>.

Further, in Example <NUM>, which satisfied the condition "L1/H ≤ <NUM>," L1 was small, thereby further reducing the airflow flowing into the decompression chamber, and thus the number of defects was three, which was less than that of Examples <NUM> to <NUM>.

In Example <NUM>, in which the seal gap was blocked by the seal material, the airflow flowing in through the gap to disrupt the pressure balance near the end of the sheet material was eliminated, and thus the number of defects was two, which was less than that of Examples <NUM> to <NUM>.

In Example <NUM>, in which the seal material was the elastic body, and the seal material was pressed against the casting device to block the seal gap, the minute gap with respect to the casting device was also able to be prevented, and thus the number of defects was one, which was less than that of Example <NUM>.

In Example <NUM>, in which the seal material was pressed against and fixed to the decompression chamber by the pressing member, the distortion and misalignment of the seal material were reduced, and thus the number of defects was zero, which was less than that of Example <NUM>.

Claim 1:
A method for producing a microporous polyolefin resin sheet, the method comprising:
discharging a sheet material (<NUM>) containing a polyolefin resin and a diluent from a discharge port (<NUM>) of a die (<NUM>) toward a casting device (<NUM>);
covering a space between the sheet material and the casting device by a decompression chamber (<NUM>) disposed upstream of the discharge port (<NUM>) in a sheet conveying direction;
sucking air in the decompression chamber (<NUM>) to make a decompression space and bringing the sheet material into intimate contact with the casting device; and
cooling and solidifying the sheet material while conveying the sheet material by the casting device,
the suction of air in the decompression chamber (<NUM>) being performed from exhaust nozzles (<NUM>) disposed outside both ends of the sheet material in a width direction such that openings (<NUM>) of the exhaust nozzles face each other, the method being characterized in that
Y1/H ≤ <NUM> and <NUM> ≤ Y2/H ≤ <NUM> when a shortest distance from the discharge port (<NUM>) of the die (<NUM>) to an outer peripheral face of the casting device (<NUM>) is H, a shortest distance from an upper end of the opening (<NUM>) to an end of the sheet material is Y1, and a shortest distance from a lower end of the opening (<NUM>) to the end of the sheet material (<NUM>) is Y2.