JPH0246375B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for forming a blown film, which is taken out of an extruder into a cylindrical shape by a so-called air-cooled inflation method during the process of manufacturing a synthetic resin tubular film. The inflation method involves winding up a resin film that exits from the annular outlet of a molten resin extruder and is cooled and advances into a cylindrical shape due to internal pressure and take-up. With this manufacturing method, haze value (haze degree),
In order to obtain a homogeneous film with a good gloss value, it is considered necessary to cool the film as rapidly as possible, and various cooling methods have been used. For example, the Japanese Patent Publication No. 48-10065 ``Cooling Device for Film Manufacturing Machines'' has a cool air and cold water injection ring directed upward, and a suction ring for sucking the cold air and cold water, above the cooling air injection ring provided at the exit of the extruder as in the past. is installed to cool the film in a strip. JP-A No. 53-146764 "Inflation film forming method" also uses a similar two-stage cooling method, in which the upper injection ring is placed at a suitable distance from the lower injection ring without using water or a suction ring. The cooling air is directed upward or diagonally upward. Although the injection ring is said to be aligned with the frost line, the frost line appears at the cooled position. That is, when cooling the blown film with a fluid, the most advanced known technique was to add an upper injection ring above the lower injection ring and direct the jet upward or diagonally upward. The present inventors had also proceeded with research on cooling methods based on similar knowledge. However, as I researched the relationship between cooling blown film and product quality, especially transparency, I realized that there was something missing in the common knowledge that cooling a film quickly increases its transparency. No matter how rapidly the product is cooled immediately after exiting the extruder, it only lowers the frost line and does not affect transparency. What is affected is the time required for the film to pass through the frost line region, where the film changes from a transparent liquid phase to a translucent solid phase, that is, the time from when solid phase components begin to form in the liquid phase until the entire solid phase is completed. Therefore, further shortening this relatively short time by rapid cooling is effective in improving transparency. Therefore, it is unnecessary to cool the film over a wide area as in the past, and it is sufficient to rapidly cool the film strongly just before the solid phase is generated in the liquid phase film, and the solid phase is completed instantly. I understand what happened. Specifically, inflation and
Simply advance the film to create a frost line, then quickly cool the area just below this frost line to create a new frost line here. However, as a rapid cooling method, if a sufficient amount of cooling air is blown diagonally upward according to the common sense mentioned above,
The advancing film will oscillate, resulting in defective products unless the advancing speed is slowed down. Fortunately, the inventor had saved the suction ring for stabilizing the inflation film described in Utility Model Publication No. 53-15826, so he attempted to reversely use this suction ring as an injection ring to blow cool air directly below the frost line. When sprayed, high transparency was obtained without causing any disturbance to the film. This success was achieved by not applying the jet to a wide area of the film, but by cooling a narrow area below the frost line, which could be called a horizontal line, and applying the jet at right angles from close range. In order to promote solid phase formation of the film, it is sufficient to rapidly cool a line on the circumference of the film, and solid phase formation begins and ends instantly on this line. Jetting to other parts has nothing to do with improving transparency and only causes agitation to the film. If a jet of the same strength is applied at right angles to the film, that is, in a horizontal direction, the film will only be squeezed in that horizontal plane, but if it is applied diagonally upward, the film will maintain a perfect circle and move upwards. Because the pressure is not on the ground, the height of the pressure varies depending on the location, increasing the strain. Furthermore, errors in manufacturing accuracy of the injection slit have a large effect when the injection is applied obliquely. When the two overlap, the distortion of the film increases and the oscillation becomes stronger. In addition, we believe that the large amount of jet flow creates an upward vortex, and that the degree of cooling at the same height is different, resulting in temperature differences (strength differences), resulting in uneven elongation due to take-up, which increases agitation and leads to quality deterioration. It will be done. Now, the configuration and embodiments of the present invention obtained as a result of the above-mentioned research will be explained with reference to the drawings. FIG. 1 shows an embodiment of an inflation film forming apparatus to which the present invention is applied, in which 1 is a molten resin extruder, 2 is a die, and is equipped with an annular outlet 2a. There is an air port for internal pressure in the center of the die 2 (not shown).
A known cooling air injection ring (air ring) 3 is provided on the outer periphery of the annular outlet 2a, and as the resin exits from the annular outlet 2a, it is cooled, increases its viscosity, and moves upward in a cylindrical shape. The cylindrical inflation film F moves upward in a cylindrical shape as shown in the figure without adding the upper injection ring 10 according to the present invention, is folded into a flat shape by the guide plate 4, and is moved by a number of taking-off nip rollers 5. It passes through a guide roller and is wound up by a winder 6. Two blowers 7 send air to injection rings 3 and 10. As before, the injection ring 3 surrounding the outer periphery of the annular outlet 2a
As mentioned above, the cooling air is sent diagonally upward, and the explanation is omitted, but inside the extruder 1, for example, 150℃, 200℃
It works by rapidly cooling a material with low viscosity, such as ℃, at the same time as it is extruded, increasing its viscosity to the point where it can move upward while maintaining its cylindrical shape. Nitz Prora 5 while moving up
The material is stretched and becomes even thinner, but because it is thin, it is quickly cooled by the outside air, and even without secondary cooling,
Eventually, the freezing point of around 100°C is reached, and the film F, which had been transparent until then, changes from frost line ₀ to translucent. Now, a procedure for applying the molding method of the present invention to a blown film produced using such conventional equipment will be described. First, a cooling air injection ring 10 having an annular injection slit 11 on its inner circumference is prepared, which has a size suitable for surrounding the outer circumference of the target inflation film F in the vicinity of frost line ₀ with a small gap. This is installed above the injection ring 3 so that it can move up and down before making the inflation film F.
Of course, this is vertical movement while maintaining a horizontal posture. However, this new injection ring 10 may be divided into two parts so that it can be installed on the outer periphery of the inflation film F that is already moving upward. Injection ring 10
The center line of the annular outlet 2a is made to coincide with that of the annular outlet 2a. The vertical positioning of the injection ring 10 is performed as follows based on the principle of the invention described above. That is, the outer periphery of the transparent portion of the film slightly below the frost line ₀ already formed on the inflation film F is fixed at a position where it can be linearly cooled. Slightly lower means lower, commensurate with the cooling capacity, and not too far down, since the farther downward you go, the more the instantaneous cooling amount until solidification increases. Injection ring 10
The height of the injection ring 10 may be determined so that it can move up and down only for preliminary experiments, and the injection ring 10 for production may be one that does not move up and down. Also, the transparent portion refers to a portion where no solid phase has yet been generated in the film and thus maintains the same transparency as the high temperature portion. In areas close to the frost line where solid phase components are beginning to form due to slow cooling, the rapid solidification effect of the present invention is reduced. In the left half of Figure 2, there is a frost line formed only by the inflation film F and its lower injection ring 3.
₀ and the injection slit 11 of the new injection ring for this
Indicates the location of Please note that this is an explanatory drawing and is not drawn to scale in proportion to the actual product. The right half of FIG. 2 shows the tendency of the temperature distribution of the film F by a temperature curve T. For example, the film extrusion temperature T ₁ when exiting from the annular outlet 2a of the extruder is
At a high temperature of 150°C or 200°C, it is cooled rapidly by the lower injection ring 3 and is stretched into a thin layer as it moves upward. Then, as the solidification temperature T ₂ approaches, for example, 110° C., a solid phase component begins to form in the liquid phase of the film. The area where this solid phase content increases and becomes visible to the naked eye and soon completely solidifies is called a frost line. In this invention, the injection slit 1 of the new injection ring 10 is formed in the transparent part which is close to the frost line ₀ and which is slightly higher than the solidification start temperature at which solid phase components do not begin to appear.
1 at a nearly right angle. By injection, that part is rapidly cooled and immediately subcooled to a temperature even lower than the final solidification temperature, leaving a faint frost line there as it moves upwards, whereupon it is cooled by the temperature gradient of natural cooling and then wound up. The new injection ring 10 applies a jet of cooling air substantially perpendicular to the surface of the film F and with an intensity that causes a reversible indentation C in the film F. By applying the jet stream almost perpendicularly to the film surface, most of the jet stream comes into contact with the film surface, which works effectively for cooling, and unlike conventional methods, most of the air flow does not come into contact with the film surface, resulting in a laminar flow. It does not flow away and has high cooling efficiency. The angle between the jet stream of the upper injection ring 10 and the film F surface can be used if it is 70° to 110° with 90° as the center, preferably 80° to 100°, and of course 90° is the maximum. . The fact that the jet flow is strong enough to cause a recoverable dent C in the film F means that the jet flow will not make it impossible for the film to advance smoothly, and it will also prevent the dents that naturally occur due to spraying. , the intention is to limit the amount to a level that allows for smooth recovery through upward movement, and this is sufficient to achieve the purpose of the invention. The cooling air injection has a shape in which the injection slit 11 of the injection ring 10 allows the jet to proceed straight without dispersion, and
It is desirable that the distance between the jet slit 11 and the film surface is not so far as to cause dispersion of the jet midway.
At least the vertical opening angle of the upper and lower plates of the slit 11.
It should not be more than 10°. By narrowing down the cooling target to the horizontal linear part of the outer circumference of the film F and using the method with the highest cooling efficiency of hitting the linear part almost at right angles, complete cooling can be achieved with the minimum amount of injection. None, the objective of improving transparency was achieved without adversely affecting the progress of the inflation film F as in conventional methods. This seems to be due to the fact that the upper injection ring 10 has the function of squeezing the arbitrarily expanded cross section of the cylindrical film F into a correct circular shape, and that it does not have the above-mentioned disadvantage of the obliquely upward jet flow. In addition to improving transparency, this invention has the effect of reducing and stabilizing the oscillation of the advancing film tube. As a result, a significant effect was added in that the thickness unevenness (uneven thickness) of the film F was significantly reduced. In addition, the film F is less shaken and the cooling effect is improved, making it possible to take it off at high speed, making it possible to produce a highly transparent film at a production speed of 40 to 90 m/min, which was hitherto unimaginable. The synthetic resins that this invention mainly targets are polyolefin polymer resins, including homopolymers and copolymers of high-pressure polyethylene, medium-low-pressure polyethylene, polypropylene, polybutene-1, etc., ethylene, propylene, butene-1, etc. A mixture thereof, etc. The above-mentioned linear low-density polyethylene includes ethylene and α-olefins having 3 to 12 carbon atoms, such as propylene, butene-1, hexene-1,4-methyl-1-
At least one of pentene, octene-1, decene-1, etc. is produced by a conventionally known medium-low pressure method or high-pressure method in the presence of a Ziegler type catalyst. Further, as the medium-low pressure method, any method such as a gas phase method, a slurry method, a solution method, etc. may be used. In particular, the present invention is extremely effective for polyolefin resins made of linear low-density polyethylene and mixtures thereof with other polyolefin resins.
Transparency, gloss, and optical properties that were previously unobtainable with high-pressure low-density polyethylene (for example, 4% haze)
(Gross 110% or more) can be obtained with high productivity. Next, experimental results will be shown in which the present invention is applied and not applied to various resin inflation methods. The experimental results in the table below are based on the following conditions. Extruder cylinder diameter 50mm Diameter of extruder annular outlet 2a 150mm Exit gap 2.5mm Finished film thickness 20μ Blow (expansion) ratio 1.3 Injection slit 3.0mm Injection slit, distance between film surfaces 5.0mm Frost line ₀ and injection slit Approximately 50mm distance from

【table】

【table】

[Table] Table 1 shows the results to which this invention is applied, and (a) in Table 2
Experiments Nos. 7 to 10 show the case where the upper jet spray angle (with respect to the film surface) is eliminated to 90 degrees, and similarly No. 11 shows a conventional method in which the upper jet is not used and the take-up speed is reduced. Needless to explain, the excellence of this invention is (re)
It often appears in each term (nu), (ru), and (o). Although the present invention has been described above with reference to an illustrated embodiment, it goes without saying that the embodiments of the present invention can be varied and applied in various ways without changing the gist thereof. The terms vertical and horizontal are relative terms and do not mean absolute vertical or horizontal. The direction in which the film is taken may be upward, downward, horizontal, or diagonal. Cooling air is not limited to air. The number of upper injection rings 10 is not limited to one, and injection rings for pre-cooling and post-cooling may be added. Also, when using a particularly powerful jet, after the jet has cooled the film,
It is desirable to take measures to prevent the flow from becoming a vortex. In addition, the effects of this invention will be further enhanced by the ingenuity of field engineers who implement it. This invention has clarified the principle of improving the quality of blown film by cooling it, which had hitherto been unclear, and clarified that cooling is required within an extremely limited range from the start of solidification to the completion of solidification. Specifically, we use the frost line that occurs only by cooling the lower jet as a guide, and locally cool the lower side in a linear manner.
Because solidification starts, completes, and supercools on the spot, the conventional slow cooling and opacity caused by solidification is drastically reduced. Furthermore, as a linear cooling means, we achieved great success by applying a jet flow almost perpendicular to the film surface, breaking from the conventional wisdom of cooling with a jet flow along the film surface. That is, the present invention greatly contributes to the theoretical and practical aspects of resin film molding technology using the inflation method.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of an embodiment of the present invention, and FIG. 2 is an enlarged view of the main part and a diagram illustrating temperature distribution. F...Inflation film, ₀ ...
...Frost line, C...Film dent, 10...
...Cooling air injection ring, 11...Injection slit.

Claims (1)

[Claims] 1. The molten resin discharged from the annular outlet of the extruder is appropriately cooled by an arbitrary cooling device near the outlet,
Linear cooling that uses internal pressure and withdrawal to advance the film as an inflation film with a cylindrical frost line, and has a jet slit on the inner periphery that allows the jet to travel straight, almost perpendicular to the surface of the cylindrical film, without dispersion. The outer periphery of the film near the above-mentioned frost line is surrounded by a cooling air injection ring for use with a small gap, and the position of the injection ring is adjusted so as to be concentric with the annular outlet, so that the jet is slightly smaller than the position of the above-mentioned frost line. The outer periphery of the transparent part near the extruder, where no solid phase has yet formed in the film, is fixed at a position where it can be cooled in a linear manner, and the jet is sprayed. The cooling capacity shall be such that a recoverable linear depression is created on the circumferential surface of the applied film, and the cooling capacity shall be capable of instantaneously lowering the film at a temperature slightly higher than the solidification start temperature to a temperature slightly lower than the solidification end temperature. A method for forming an inflation film, which is characterized by: