JPH0551452B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for producing a blown film, and more particularly to an improvement in a method for cooling a molten resin tubular body extruded into a tubular shape through an annular extrusion die. [Background technology and its problems] In the production of inflation film,
In order to obtain a film with excellent transparency and to enable high-speed molding, it is necessary to rapidly cool the extruded molten resin tubular body. However, if the amount of cooling air from an air ring or the like is increased in order to improve the cooling effect, the stability of the molten resin tubular body and the bubble that is formed when this tubular body is fully expanded is impaired, so the amount of cooling air is increased. There is naturally a limit to the improvement of the cooling effect by increasing the amount. In particular, resins with low melt viscosity, such as linear low density polyethylene (LLDPE), tend to make the molten resin tubular body unstable, making it difficult to form a film with excellent transparency. By the way, a so-called double slit type air ring having double annular slits is provided on the annular extrusion die, and the molten resin tubular body is stabilized by utilizing the depressurization phenomenon caused by the cooling air flow discharged from the double annular slits. A method for increasing sex is known (Japanese Unexamined Patent Publication No. 77258/1983). However, with this method, the cooling effect itself was not sufficiently improved, so it was not possible to significantly improve the transparency etc. of the formed film. [Object of the Invention] The object of the present invention is to provide a method for producing a blown film that can improve the stability of a valve during blown film formation and improve the transparency of the formed film by improving the cooling effect. is to provide. [Means and effects for solving the problems] The present invention combines a double slit air ring and a cylindrical member, thereby achieving bubble stability through the pressure reduction phenomenon caused by the double annular slits of the double slit air ring. In addition to ensuring that
The purpose is to improve the cooling effect by allowing the cooling air discharged from the slit by the cylindrical member to stay around the molten resin tubular body for a predetermined period of time, thereby achieving the above object. Specifically, in a method for manufacturing a blown film, the process involves cooling a molten resin tubular body, which is the base end of a bubble, using a double slit air ring having double air discharge slits.
The slit of the air ring is surrounded by a cylindrical member having an inner diameter larger than the diameter of the slit forming part of the air ring and smaller than the diameter of the valve after fully inflated, and the upper tip of the cylindrical member and the inner cone of the air ring are connected to each other. The angle between the line connecting the tip and the horizontal line is set at 35 to 55 degrees. Here, if the angle is smaller than 35 degrees, not only the cooling effect will be insufficient, but also the bubble stability will be poor, whereas if the angle is larger than 55 degrees, the bubble stability will be poor. Further, the angle is preferably 40 to 50 degrees. Furthermore, the inner diameter of the cylindrical member is 0.6 to 0.6 of the bubble diameter after complete expansion.
0.9 times is better. In this case, if it is smaller than 0.6 times the bubble diameter, the bubble stability will be poor, while if it is larger than 0.9 times the bubble diameter, not only will the cooling effect be insufficient, but the bubble stability will also be lacking. Furthermore, the gap between the cylindrical member and the molten resin tubular body is preferably 100 mm or less, and is preferably 100 mm or less.
If it exceeds this, the cooling effect will be insufficient. The resin used in the present invention includes thermoplastic resins in general, and is not particularly limited.
It is effective when used with resins with low melt viscosity such as linear low-density polyethylene (LLLDPE), low-density polyethylene (LDPE), and high-density polyethylene (HDPE), especially when melt index (MI) Suitable for LLDPE and LDPE of 0.2 to 2. [Example] Hereinafter, an example of the present invention will be described with reference to the drawings. In the figure, molten resin 2 inside an annular extrusion die 1
is continuously extruded in an annular shape from the resin extrusion port 3 of the annular extrusion die 1 to form a molten resin tubular body 4, and inside this molten resin tubular body 4, air is extruded from the air discharge port 5 of the annular extrusion die 1. Compressed air is enclosed, and the molten resin tubular body 4 is expanded at a predetermined blow-up ratio to form a resin bubble 6. A frost line F appears at a predetermined position of the resin bubble 6, and at this frost line F, the bubble 6 reaches a crystallization temperature, and the properties of the resin differ significantly beyond the frost line F. ing. Further, after being cooled and solidified, the resin bubble 6 is pinched by nip rolls (not shown) to be folded to a predetermined diameter, and is further continuously wound to obtain a tubular film. A double slit air ring 10 is disposed in the upper part of the annular extrusion die 1, and this air ring 10 has an inner annular slit 12 partitioned by an inner cone 11 and an inner annular slit 12.
It has a double annular slit for discharging cooling air consisting of an outer annular slit 13 on the outer peripheral side of the cooling air. Among these slits 12 and 13, the direction of discharge of cooling air from the outer annular slit 13 is directed toward the outside in the radial direction of the molten resin tubular body 4 with respect to the resin extrusion direction. On the other hand, the inner annular resin slit 12
The blowing direction of the cooling air is not particularly limited, and may be the same direction as the resin extrusion direction, or radially inward or outward of the molten resin tubular body 4 with respect to the resin extrusion direction. However, it is preferable to have it facing slightly outward in order to improve molding stability. Furthermore, the volume of cooling air discharged from the annular slits 12 and 13 is such that the volume of cooling air blown from the outer annular slit 13 is larger than the volume of cooling air discharged from the inner annular slit 12. This is preferable in terms of improving molding stability. Furthermore, a stepped edge is formed inside the bottom of the air ring 10. A cylindrical member 20 is erected and fixed on the upper surface of the air ring 10, and the inner diameter of this cylindrical member 20 is
D _C is larger than the diameter of the portion of the air ring 10 where the outer annular slit 13 is formed, and smaller than the bubble diameter D _B after complete expansion. Specifically, the dimensions are as follows: inner diameter D _C of the cylindrical member 20 = bubble diameter D _B × (0.6 to 0.9). Here, D _C ＜0.6D _B
If D _C < 0.9D _B
If so, not only the cooling effect will be insufficient, but also the bubble stability will be poor. Also, the upper tip portion 20A of the cylindrical member 20 and the tip portion 11A of the inner cone 11 of the air ring 10.
The angle θ that the line connecting the lines makes with the horizontal line is 35~
The angle is 55 degrees, preferably 40 to 50 degrees. At this time, if the angle θ<35 degrees, not only the cooling effect will be insufficient but also the bubble stability will be poor, whereas if the angle θ<55 degrees, the bubble stability will be poor. Further, the distance S between the cylindrical member 20 and the molten resin tubular body 4 is set to be 100 mm or less. This is because if S>100 mm, the amount of cooling air remaining in the cylindrical member 20 will decrease, and the cooling effect will become insufficient. Next, the operation of this embodiment will be explained. The molten resin tubular body 4 extruded in an annular shape from the annular extrusion die 1 is formed into a double slit air ring 1.
It is immediately cooled by the cooling air discharged from the two slits 12 and 13 of 0. However, the cooling air discharged from the outer annular slit 13 is directed outward in the radial direction of the molten resin tubular body 4, and this cooling air Due to the pressure reducing effect of the flow, the molten resin tubular body 4 is sucked radially outward. Therefore, the molten resin tubular body 4 is expanded from the inside by the internal pressure of compressed air introduced from the air outlet 5, and from the outside by the cooling air flow discharged from the outer annular slit 13 in the expansion direction. The stability of the molten resin tubular body 4 which is being sucked and is in the process of expansion is greatly enhanced. At this time, the discharge amount from the outer annular slit 13 is larger than the discharge amount from the inner annular slit 12.
When the discharge direction is radially outward, the radially outward suction effect on the molten resin tubular body 4 is more stable. Note that even if the molten resin tubular body 4 is sucked radially outward by the cooling air flow discharged from the outer annular slit 13, the molten resin tubular body 4 is
Since the molten resin tubular body 4 is pressed in a direction away from the air ring 10, there is no possibility that the molten resin tubular body 4 will come into contact with the air ring 10 and become sticky. In this way, the base of the molten resin tubular body 4 is cooled to some extent, and the outer peripheral surface of the molten resin tubular body 4 that is expanding is
The resin bubble 6 is further cooled by the cooling air remaining inside the cylindrical member 20 discharged from the resin bubble 6 to a temperature close to the crystallization temperature, and a frost line F is formed in the resin bubble 6. According to this embodiment, the air ring 1
The stability of the molten resin tubular body 4 and, in turn, of the resin bubble 6 can be significantly enhanced by the suction action of the molten resin tubular body 4 radially outward by the cooling air discharged from the outer annular slit 13 of 0. Therefore, the blow-up ratio of the molten resin tubular body 4 can be increased, and the moldable fold diameter can be expanded. In this case, even when using resins with extremely low melt viscosity such as linear low-density polyethylene, blown film can be produced at high speed with excellent stability over a wide range of desired blow-up ratios. can be molded. In addition, since the cylindrical member 20 is arranged, the cooling effect can be significantly improved by the action of the cooling air retained by the cylindrical member 20, and the physical properties of the molded film, such as the transparency of the molded film, can be improved. can be significantly improved. Furthermore, since the cooling effect is high in this way, the position of the frost line F is not raised, and the height of the entire apparatus can be reduced. In addition, there is no possibility of occurrence of blocking or the like due to insufficient cooling in the Nipprol. Next, the present invention will be explained in more detail using experimental examples and comparative examples of the present invention based on specific numerical values. The experimental conditions of Experimental Examples 1 to 5 and Comparative Examples 1 to 3 are as follows. Molding machine: Inflation molding machine manufactured by Plako Co., Ltd. diameter 50 mm, die resin extrusion diameter 100 mm Molding conditions: Molding temperature 200°C, resin extrusion amount 40 kg/
hr Resin used: Linear low density polyethylene, MI1.0
g/10 minutes, density 0.92 g/cm ³ Cooling method: A double slit air ring was installed so that the tip of the inner cone was located at a height of 40 mm from the top of the extrusion die. Under the above conditions, the cylindrical member 20 Inflation molding was performed by changing the type and dimensions of the film, as well as the angle θ and blow-up ratio, and the bubble stability and haze of the molded film were examined. The results are shown in Table 1.

〔Effect of the invention〕

As described above, according to the present invention, there is an effect that the cooling effect is large and that stable bubble molding can be performed even when using a resin having a low melt viscosity.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing the main parts of an apparatus to which an embodiment of the blown film manufacturing method according to the present invention is applied, and FIGS. 2 to 4 show different types of equipment used in the embodiment, respectively. A perspective view of the cylindrical member, FIG. 5 is a sectional view showing the main part of a modified example of the device to which the above embodiment is applied, FIG. 6 is a graph measuring the maximum discharge amount with respect to MI of resin, and FIG. It is a graph measuring the maximum blow ratio with respect to MI. 1... Annular extrusion die, 4... Molten resin tubular body,
6... Resin bubble, 10... Double slit air ring, 11... Inner cone, 11A... Tip, 20... Cylindrical member, 20A... Upper tip,
D _B ...bubble diameter, D _C ...inner diameter of cylindrical member, θ...
…angle.

Claims (1)

[Scope of Claims] 1. In a method for manufacturing an blown film, which is carried out while cooling a molten resin tubular body that is the base end of a valve, using a double slit air ring having double air discharge slits, Linear low-density polyethylene, low-density polyethylene, or high-density polyethylene with a mass (MI) of 0.2 to 2 is used, and the diameter is larger than the diameter of the slit forming part of the air ring, and 0.6 to 0.9 times the bubble diameter after complete expansion. The slit of the air ring is surrounded by a cylindrical member having an inner diameter of
A method for producing an inflation film characterized by setting the temperature to ~55 degrees. 2. A method for producing an blown film according to claim 1, characterized in that the distance between the cylindrical member and the molten resin tubular body is set to 100 mm or less. .