JPS58212918A - Formation of blown film and cooling ring for forming the same - Google Patents

Abstract

PURPOSE:To form stably blown film excellent in strength, transparency and lustre by cooling a cylindrical molten resin film extruded from a die slit in a specified condition. CONSTITUTION:A cylindrical molten resin film (a) is sprayed with a large quantity of gaseous cooling medium from multiple preliminarily cooling slits 10, 11 to be cooled, expanded and stretched by the compressed gas sealed in the resin film (a) to the intended diameter. Then, the film is quenched by cooling medium spouted from the normal cooling slit 12 provided near the frost line (c) and a little toward the die 4. By the preliminary cooling, the cylindrical molten resin film is cooled in a range between 60 deg.C preferably 40 deg.C higher than the crystallization initiating temperature and the crystallization initiating temperature or a little higher.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention particularly relates to a method for forming a blown film with excellent film formation stability and transparency, and a cooling ring used in carrying out this forming method.

High-low density polyethylene, polyzolopylene, poly-1-butene, propylene-ethylene copolymer, ethylene-α-
A method for stably forming blown films with excellent strength, transparency, and gloss using polyolefin resins such as olefin copolymers and ethylene-vinyl acetate copolymers alone or in mixtures as raw materials, especially with low melt tension. , provides a means that can exhibit the most remarkable effect when forming a blown film using an ethylene-α-olefin copolymer as a raw material, which has a fast crystallization rate and tends to grow large spherulites. .

Ethylene-α-olefin co-polymerized toha, α-olefin which is the molecular skeleton of 03 to C8 is 1 to 20 vt%1,
Preferably 3 to 15 wt, and 99 to 80 w of ethylene
t%, preferably 97 to 85 wt%, and is a linear low-density ethylene copolymer consisting of ethylene and a molecular skeleton of 03 to C8 by ionic reaction using a catalyst combining a transition metal compound and an organometallic compound. III produced by copolymerizing certain α-olefins under relatively low pressure
Ili and its density is 0.910 to 0.94017
cm3, preferably 0.916-0.9351/l*3
, MFRO, is in the range of 1 to 20, preferably 0.1 to 5, and is a method for converting ethylene by a radical reaction under high pressure using the generally known -C oxygen radical as an initiator. The branched low-density polyethylene resin produced by this process has different performance in molecular structure, melting characteristics, crystallization characteristics, and solid state physical properties.

Hereinafter, the ethylene-α-olefin copolymer will be referred to as L-, L-
Branched low density polyethylene resin is referred to as D P E.
- Referred to as LDPB.

Comparing L-LJ) PK and H, LDPFi in terms of productivity and quality of blown film, as mentioned above, L-LDPIli has lower melt tension, faster crystallization rate, and larger spherulites than H-LDPi. It has the characteristic that it is easy to generate. For that purpose, L-LDPFi
f (+- When forming a blown film using the entire cooling ring for conventional H-LDPB as a raw material resin, the stability of forming a cylindrical molten resin film is poor and satisfactory production volume and quality cannot be obtained. There are deficiencies.

For example, when molding L-LDPK as a raw material using the -1 slit type cooling ring 1 for H-LDPB shown in Fig. 1, in a low production volume range, the cooling air inlet of the cooling ring 1 is Blown from 2, 2', slits 3, 3
The stability of film formation is ensured by balancing the cooling air volume (m pressure) blown out from the die 4, the tension of the cylindrical molten resin film (a) extruded from the die 4, and the bubble internal pressure. , in a high production range, it is necessary to increase the cooling air ti blown out from the slits 3, 3';
Due to the dynamic pressure, the cylindrical d-melt resin film (a) is deformed as shown by the imaginary line (b), and the stability of film formation is significantly deteriorated.

Such a #L elephant uses H-LDPB; as the raw material side and the first
This can also be seen when forming a blown film using the cooling ring 1 shown in the figure, but especially when L-LDPE is used as a raw material, the bath melt tension is small, so it is easily affected by an increase in the cooling air volume, so the cooling air volume is We had no choice but to restrict and reduce production.

This means that with L-LDPI, the spherulites grow large due to insufficient cooling, making it impossible to produce a blown film with the excellent optical and mechanical properties of L-LDPI (!). Therefore, in view of the above-mentioned points, as a method to improve production volume and quality, we performed preliminary cooling to increase the melt tension, and then rapidly cooled the cylindrical molten resin film in a heavy tank where it crystallized. A two-stage cooling ring as shown in FIG. 2 is used.

Each of the two-stage cooling rings shown in FIG.
are arranged at a predetermined distance above and below, and these cooling rings 1, 5
In combination, it functions as a cooling ring.

The cooling ring body], 5 each has a plurality of cooling air inlets 2, 2', 6, 6' and slits 3.3', 7.7' for blowing out the cooling air.

The cylindrical molten resin film (a) extruded from the gouey 4 is pre-cooled by the cooling air blown from the cooling rings 1) 3 and 3', increasing the melt tension and imparting molding stability.

Next, the cylindrical molten resin film (a) is cooled and solidified by cooling air blown out from the slit 7, 7' just before the frost line 8 where it crystallizes, and this method improves production stability and quality. The amount can be increased.

However, even when a two-stage cooling ring as shown in FIG. 2 is used, sufficient results cannot be obtained in terms of production volume, film formation stability, and quality when L-LDPFi is used as a raw material.

In other words, there is a proportional relationship between the differential volume and the cooling air volume, and when the cooling air volume is increased, it is affected by dynamic pressure and the film formation stability decreases.
On the other hand, if the cooling air f is reduced, the problem of sagging due to the weight of the cylindrical molten resin film occurs, which naturally limits the production amount.

Under these circumstances, in order to reduce the influence of dynamic pressure and improve the cooling effect as much as possible, there is a method of blowing out cooling air obliquely upward or parallel to the direction of travel of the cylindrical molten resin film, or a cooling ring body 1. There is a method to dissipate heat by increasing the vertical distance between the cylindrical molten resin film and the slit, but since there is little leakage, the film heat transfer coefficient □ is small, so the cooling effect is poor, and the distance between the cylindrical molten resin film and the slit increases even a little. Cooling becomes ineffective, large spherulites form in the resin, and optical and mechanical properties deteriorate.

Furthermore, it is necessary to replace the slits in the upper cooling ring 5 for each film size, it is difficult to adjust the wind noise between the upper cooling ring 5 and the lower cooling ring 1, and the operability is poor. There are problems such as the relatively long time it takes for the film to be formed (product production) and the loss rate increases. However, the problem was that it was not possible to express the quality of the product, and therefore it was not possible to improve the product value.゛The present invention provides the most effective molding method when applied to molding a blown film using TJ-LDPK as a raw material, and also provides a cooling ring used for implementing this molding method. The first invention provides excellent film-forming stability and quality by multi-stage blowing of air and rapid cooling at a temperature immediately before the start of crystallization. While increasing the melt tension by cooling with a gaseous cooling medium blown out from multi-stage pre-cooling slits in the cooling ring,
Inflation characterized by expanding the cylindrical molten resin film to a predetermined diameter and then rapidly cooling the cylindrical molten resin film by a gaseous cooling medium blown out from a main cooling slit at a position immediately before crystallization of the cylindrical molten resin film starts. The second invention is a method for forming a film, in which a multi-stage pre-cooling slit is formed by blowing a gaseous cooling medium onto the upstream side around a forming passage for a cylindrical molten resin film, and the final cooling step is formed. This cooling ring for forming a blown film is characterized in that a main cooling slit is provided at a predetermined path 11 [i-] on the downstream side of the stage pre-cooling slit.

An embodiment of the present invention will be explained with reference to FIG.

The cooling ring (A) shown in FIG. 3 has a molding passage 9 in the center that passes through the die 4 of the extruder.
A plurality of pre-cooling slits and main cooling slits are provided around the molding passage 9 from upstream to downstream in the molding path through which the cylindrical molten resin film (a) completely extruded from the molding passage 9 passes. At least two slits for pre-cooling,
Preferably, two or three of them are provided in multiple stages so that a multi-current gaseous cooling medium can be sprayed onto the cylindrical molten resin film. Specifically, the first pre-cooling slit 10 and the second pre-cooling slit 11 are provided on the upstream side, and the main cooling slit 12 is provided on the upper end of the second pre-cooling slit 11 on the downstream side. .

The cylindrical molten resin film is cooled by the plurality of pre-cooling slits, and the cylindrical molten resin film is expanded and stretched to a predetermined diameter by the pressurized gas sealed within the cylindrical molten resin film.

The second preliminary cooling slit 11, which is the final stage, and the main cooling slit 12 are arranged at a predetermined distance above and below, and the main cooling slit 12 is located near the position just before the crystallization of the cylindrical molten resin film starts. Install. The position immediately before the start of crystallization can be easily found by observing the cylindrical molten resin film (a). Generally, this crystallization temperature is called the frost line, and in the present invention, the vicinity just before the crystallization start temperature refers to the frost line (approximately 10 cm from the center to the die 4).

Preferably, the position is within 5(1).

The amount of gaseous cooling medium, particularly preferably air, blown from the main cooling slits 12 corresponds to the gaseous cooling medium blown from multiple stages of pre-cooling slits.
, 5 to 5 times, preferably 1 to 3 times as many children.

In this way, in the present invention, by providing a divisor of slits for precooling i (), a large amount of gaseous cooling medium can be sprayed onto the cylindrical bath melting side fat film (a) in multiple stages. Since the solidification of the cylindrical molten resin film can be accelerated and the degree of self-sealing of the cylindrical molten resin film can be reduced, the film can be formed in a stable state.

The plurality of pre-cooling slits may be simply connected in multiple stages, but by installing an expansion chamber 13 between each pre-cooling slit, the air blown from the first pre-cooling slit 10 can be expanded. By flowing the extruded cylindrical M-molten resin film in a half line along the outer surface of the extruded cylindrical M-molten resin film (A), the pressure can be reduced by the Venturi seedlings produced, and the cylindrical bath-molten resin can be pre-stretched. The membrane (a) can be further stabilized. The cylindrical molten resin film (a) pre-expanded in this way is further heated through a second pre-cooling slit) 11
The cylindrical molten resin film is rapidly cooled to a temperature close to and higher than the crystallization start temperature of the core resin, and further expanded and fixed to a predetermined size and diameter by the gaseous medium sealed in the cylindrical molten resin film.

The expansion chamber 13 to be pre-inflated can be depressurized by a vacuum pump or the like via a pressure release pipe 4, if necessary.

In this case, as shown in FIG. 3, the multi-stage pre-cooling slit portion is a pre-cooling ring portion 15 in which the first pre-cooling slit 10 and the second pre-cooling slit 11 are integrally formed. ) The main cooling R section 16 forming the cooling ring 12 and the preliminary cooling ring section 15 are connected and integrated by an intermediate cylinder section 17 serving as a passage for a gaseous cooling medium to form a cooling ring (A). Also, Surin for preliminary cooling) 10
The gaseous cooling medium air blown out from , 11 cools the cylindrical molten resin film (a), and then is warmed and discharged through the discharge pipe 18.
, 18, and a resistance wall 19 and a perforated plate 20 are disposed inside the preliminary cooling ring 16 to uniformly disperse the gaseous cooling medium. .

In this way, the cylindrical molten resin film is precooled by the multiple stages of precooling slits, and during this time, the cylindrical molten resin film is expanded in a predetermined groove by the internal gas pressure, and the temperature is higher than the crystallization initiation temperature. Cooled to a temperature in the range of 60 °C above the onset temperature, preferably above the crystallization onset temperature and within 4 () °C above the crystallization onset temperature. Another embodiment of the present invention will now be described with reference to FIG.

The cooling ring (B) shown in FIG. 4 has a molding passage 21 that penetrates vertically in the center, and the cylindrical molten resin film (A) completely extruded from the die 4 of the extruder passes through this molding passage 21. A first molding path near the upstream side around this molding path 21.
A pre-cooling slit 22, an expansion chamber 23, a second pre-cooling slit 24 are provided, and a main cooling slit 25 is provided at the upper end on the downstream side. They are placed at a predetermined distance,
A rectifying cylinder portion is provided between the second preliminary cooling slit 24 and the main cooling slit 25.

26 is provided.

There is a cooling air inlet 28 in the bottom wall 27 of the cooling ring (Ell).
are provided, a peripheral wall portion 29, a cylindrical inner wall portion 30, a clay wall portion: 31,
Defined by the bottom wall portion 27, the upper forming wall portion 32 of the first pre-cooling slit 22, the expansion chamber forming wall portion 33, the lower shape hV of the second pre-cooling slit 24, the wall W534, the upper forming wall portion 35, etc. The annular cooling air introduction chamber 36 and the cooling air blowing port 28 are connected to each other so that cooling air is blown into the cooling air introducing chamber 36.

In addition, one end is connected to the front expansion chamber 23 and the other end is connected to the peripheral wall portion 2.
9. Insert the outlet pipe 37 that opens outside into the cooling air introduction chamber 36.
A resistance wall 315 is provided hanging from the base end side of the upper forming wall portion 35 of the second pre-cooling slit 24, and a first pre-cooling slit 2 is provided inside the resistance wall 36α.
2. A cooling air passage chamber 38 communicating with the second pre-cooling slit 24 is formed.

Further, one end is fully opened 39 inside the front wall-shaped inner wall portion 30,
A discharge pipe 40 is provided with the other end opening outside the peripheral wall 29, and the opening 39 is communicated with a space 41 defined between the cylindrical inner wall 3o and the rectifying cylinder 26, The rectifying cylinder part 26 has a large number of through holes 42, 42'e in its peripheral wall, and has guide pieces 43, 43, . . . protruding from its upper outer periphery.

The first pre-cooling slit 22 is connected to the axis of the forming passage 21 (
The second pre-cooling slit 24 has the shape of an outlet parallel to the axis (α) of the forming passage 21, and the main cooling slit has the shape of an outlet facing diagonally upward with respect to the axis (α) of the forming passage 21. (α), the diameter of the second pre-cooling slit 24 is larger than the diameter of the first pre-cooling slit 22, and the main cooling slit 25 The diameter is larger than the diameter of the second pre-biased cooling slit 24.

Note that the outlet pipes 37 and 37 communicating with the expansion chamber 23 may be closed and connected to an attached vacuum source (not shown) by appropriate means, and the main cooling slit 25 is oriented with respect to the axis (α). It may also be oriented slightly upward.

In the cooling ring (B) configured as described above, an extruder (
A cylindrical molten resin film (not shown) extruded from a die 4 of
A) is pre-cooled by the cooling air blown out from the first pre-cooling slit 22 to increase the bath melt tension.

Next, the cylindrical molten resin film (a) expands in the first stage due to the depressurizing action in the expansion chamber 23, that is, the Venturi effect generated by the cooling air blown out from the second pre-cooling slit 12, increasing its surface area and cooling. be promoted.

;-ζ is blown out from the second pre-cooling slit 24, is cooled by the entire cooling air, expands in the second stage, reaches the pre-cooling slit 25, and has a predetermined diameter.

The main cooling slit 25 is formed by the forming passage 21.7) axis (α
), and the setting position was set near the temperature immediately before crystallization of the cylindrical bath melt resin film (a), so the main cooling slit was moved to the 25 position. The portion of the cylindrical molten resin film (a) is in the temperature range immediately before crystallization, and the portion in the temperature range immediately before crystallization is rapidly cooled by the cooling air blown out from the main cooling slit 25.

The cylindrical molten resin H is blown out from the first pre-cooling river slit 22 and the second pre-cooling slit 24 and rising! Most of the cooling air acting on k(a) flows into the space through the through holes 42, 42, . It flows into the section 41 and is discharged to the outside of the ring body through the discharge pipe 40, during which time it prevents the cooling air from becoming a turbulent flow and performs a rectifying action.

The guide pieces 43, 43 guide and guide the discharge flow.

In this case, the blowing direction of the cooling air blown out from the slit 25 for the main cooling gradient 1 is perpendicular or approximately perpendicular to the molding passage 21, or in a slightly diagonally upward direction, in other words, the cylindrical bath melt resin film (a) Since the cooling air is blown out perpendicularly or slightly diagonally upward to the outer periphery and has a rapid cooling effect, the cooling air blown out from the first pre-cooling slit 22 and the second pre-cooling slit 24 and rising rises into the main cooling slit 25. In this way, the rectifying cylinder part 26 prevents the occurrence of turbulence, and the cylindrical molten resin film (a) is sufficiently cooled in a temperature range where it does not crystallize. Since the bath melt tension rises and becomes stable, it is possible to blow out multi-image cooling air from the main cooling slit 25 at right angles to the traveling direction of the cylindrical bath melt resin film (a), and the temperature at which crystallization starts is lowered. Until below.

Therefore, the height of the cylindrical molten resin film can be substantially reduced, and the film can be formed extremely stably, so the height from the die slit to the main cooling slit can be reduced. 11 diameter (d) of die 1.5-6
It can be doubled, preferably 2 to 4 times.

In the cooling ring tB), by providing the straightening cylinder m26, the gaseous cooling medium blown from the preliminary cooling slit between the final stage preliminary cooling slit and the main cooling slit is directed outside the cooling ring (B). The flow of the gaseous cooling medium can be adjusted by discharging the gaseous cooling medium. Incidentally, instead of the straightening cylinder part, a current plate may be used which is parallel to the cylindrical bath melting resin film by l'.

In addition, it is desirable that the straightening cylinder part or the straightening plate have a large number of through holes M in order to discharge the gaseous cooling medium to the outside of the system, and furthermore, the gaseous cooling medium should be formed in the upper part of the straightening cylinder part or the straightening plate. It is effective to provide a protruding guide piece to facilitate discharge outside the system.

In this way, the cylindrical molten resin film (c) cooled in the trench to almost the temperature just before crystallization starts is rapidly cooled to the crystallization start temperature by the main cooling 4II slit installed at a predetermined height downstream. . The angle at which the main cooling slit 25 sprays the cylindrical molten resin film (A) shall be set within a range that does not make the resin film unstable. The L-LDPFi molded as described above is preferably sprayed in a slightly diagonal upward direction, and particularly preferably in a substantially perpendicular direction.
The blown film has excellent optical properties and mechanical properties, and such blown film can be efficiently produced with high productivity.

Next, Examples and Comparative Examples in which blown films were formed according to the present invention will be listed.0 [Example-1] Ethyleno-butene-1 coclastic body (MFR: 1.0, density: 0, 920, Q value; 3.5, butene 1 content; 9.
An intrusion film was formed using the zwt under the following conditions.

Molding machine extruder diameter: 50mm+ (Mitsubishi Heavy Industries, Ltd. table)
Screw: iJ/D18 Compression ratio 3.0 Model Metering type motor with fluted barrier (2D): 15KW Three-phase commutator motor annular die Diameter 7 toe lip gap 3.5m Model Spiral cooling Ring: Sanen Yuka Co., Ltd. (cooling ring of the present invention shown in Fig. 3), main cooling slit inner diameter 18 () ~ φ Cooling ring blower: ■ Takagi Iron Works required AAA wind pressure 335
11111IA (1) Kt 19 m 3 / min Taking machine 22 roll height 5,000 units Nitsu roll height 800 simple winding machine: Sebbin winding machine Molding conditions Molding temperature Cylinder 1: 180℃ Cylinder 2
: 200°C Die: 200"C Screw rotation speed 5Q rpm Extrusion amount 20Kf/HK Take-up speed 26WV/m1n Film @ 235 gu (blow ratio 20) Film thickness 0.03 Cooling slit height 250■ (/ In this example, in order to understand the molding range, conditions such as extrusion filter by screw rotation speed, film thickness (blow ratio), film thickness by take-up speed, and cooling air volume were changed in order to understand the molding range. The results are shown in [Table-1].

Table-1 [Example-2] (Ethylene-butene 1 copolymer (MFRIJI), density 0, 920, Q value 3.5, content of butene 1 9.2 Wt
*) was molded into a blown film under the following conditions.

Extruder diameter: 505 mφ (manufactured by Mitsubishi Heavy Industries, Ltd.)
Screw: L/D1g Compression ratio 3.0 Model Metering type motor with fluted barrier (2D): 15KW Three-phase commutator annular die roller Diameter 75- Lip gap 3.5 4 Model Spiral cooling Ring: Manufactured by Mitsubishi Yuka Co., Ltd. (cooling ring of the present invention shown in Fig. 4), main cooling slit inner diameter 180, φ Cooling ring blower: @Takagi Iron Works Kulum A wind pressure 335
Cabinet Aq Air volume 19 m”/min Take-up machine 22 roll height 5.000 ■ Molding conditions Molding temperature Cylinder 1180°C Cylinder 2200°C Die 200°C Screw rotation speed 5 Orpm Extrusion @ 20 Kf/HR Take-up speed 2f3 @/min Film width , 235m (blow ratio 20) Film thickness 0.03 ms Kimoto cooling slit height 250m (from die exit)
[Comparative Example-1] A -1 slit cooling ring that blows cooling air diagonally upward in the direction of travel of the cylindrical molten resin film as shown in Fig. 1 was used, but compared to the above-mentioned [Example-2]. Since film formation was not possible under these conditions, the extrusion rate was reduced to 15 Kf/HR (screw rotation speed: 38
rpm) and changed the blower wind noise. Other conditions were the same as in [Example-2].

[Comparative Example-2] As shown in Figure 2, another -1 slit cooling ring was reinstalled at a position 7DHi upward from the lower single slit cooling ring, and a blower was added for the upper single slit cooling ring. did.

Other conditions were the same as in [Example-2].

[Comparative Example-3] A double slit cooling ring according to Japanese Patent Publication No. 49-39914 was used. Other conditions were as in [Example-2] and h.

[Example-2], [Comparative Example-1], [Comparative Example-2]
, [Comparative Example 3] and tC of the blown film are shown in Table 2).

Table 2 *1 Regarding film formation stability, ○ indicates a state in which blown film can be produced extremely stably and continuously.
Δ indicates a state in which the cylindrical molten resin film is subject to shaking and changes in the outside air; × indicates a state in which film formation is not possible under the above conditions 0 *2 HAZIn: ASTM Di003
()LO8S: ASTM D-523-72 (2
00-2110) Punching impact; ASTM D-78
1 Tensile strength: ASTM D-638-77a table-2
As seen in , the cooling rate of the present invention is fast in the crystallization start temperature region of 'L-LDP] If [Comparative Example-1]
, [Comparative Example-2], and [Comparative Example-3], the optical properties, mechanical properties, etc. are excellent, and the effect is particularly remarkable in the optical properties.

[Example 3] In order to understand the film forming range of the present invention, the extrusion amount depending on the screw rotation speed, the blowing ratio (film thickness depending on the film take-up speed, and the cooling air volume) were varied. The results are shown in [Table 3] together with [Comparative Example-4] and [Comparative Example-5].Other conditions were the same as [Example-2].

[Comparative Example-4] Using the two-stage cooling ring of [Comparative Example-2] [Example-
3] was used.

[Comparative Example-5] Using the double slit cooling ring of [Comparative Example-3] [Example-
3] was used.

In addition, in the comparative example, - heavy slit cooling ring has an extrusion amount:
Since the productivity was only 15 odors/HH and the commercial value was low, it was excluded from comparison.

Clothing-3 As seen in Table-3, in [Comparative Example-4], the cooling efficiency was low and the film formation stability was poor, so there was an eye limit in the molding speed and film formation range. On the other hand, in the method of the present invention, a large amount of cooling air can be blown out to sufficiently pre-cool the molten resin film, so the cooling efficiency is excellent and the molding range is wide.
It is possible to sufficiently form a film at a forming speed of 1 ootr/min without impairing film formation stability such as wrinkles, uneven thickness, meandering, and variations in fold diameter. In addition, [Comparative Example-5] has a wide film formation range, but
It has quality defects.

[Brief explanation of the drawing]

Fig. 1 is a sectional view of a -1 slit cooling ring, Fig. 2 is a sectional view of a two-stage cooling ring, Fig. 3 is a sectional view of the cooling ring of the present invention, and Fig. 4 is another embodiment of the cooling ring of the present invention. FIG. (A), (B)... Cooling ring of the present invention, 4... Die, 9, 21... Molding passage, 1 (1, 11, 22, 2
4... Pre-cooling slit, 12, 25... Main cooling slit, 13, 23... Expansion chamber, 26...
Rectifying cylinder part, 18, 40...Discharge pipe line, (A)...
Cylindrical molten resin film, (α)...axis of molding passage. Figure l Figure 2 Figure 3 Question

Claims (1)

[Scope of Claims] [1] Picture-shaped bath melt resin g extruded from a die slit
is cooled by a gaseous cooling medium blown out from multi-stage pre-cooling slits in the cooling ring and expanded to a predetermined diameter while melting tension, and then 1n before the start of crystallization of the cylindrical molten resin film. Monyoku melting resin Lit at a nearby location! A method for forming blown film that involves rapid cooling using a gaseous cooling medium blown out from cooling slits. [l) Multi-stage pre-cooling slits for blowing out a gaseous cooling medium are formed near the upstream side around the passage for forming the molten resin film, and a pre-cooling slit is formed at a predetermined distance downstream of the final stage pre-cooling slit. A cooling ring for blown film forming, characterized in that a main cooling slit is provided in the cooling ring.