JPH0246376B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for forming a synthetic resin tubular film, in which a tubular film is taken out from an extruder by a so-called air-cooled inflation method during the manufacturing process. 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, Japanese Patent Publication No. 48-10065 ``Cooling device for film manufacturing machine'' has an obliquely upward cool air and cold water injection ring above the cooling air injection ring provided at the exit of the extruder, as well as a suction ring that sucks the cold air and cold water. The film is cooled in a band-like manner. 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. That is, when cooling the blown film with a fluid, the most advanced known technology 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 we researched the relationship between cooling blown film and product quality, especially transparency, we discovered that
It has become clear that the common technical knowledge that rapidly cooling a film increases its transparency is lacking.
No matter how quickly you cool down the area just out of the extruder,
Transparency is not affected by simply lowering the frost line. 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, the inflation film can be advanced to form a frost line using a conventional general method, and the area immediately below this frost line can be rapidly cooled to form a new frost line. 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 blown film described in Utility Model Publication No. 53-15826, so he attempted to reversely use this suction ring as an injection ring to inject cold air perpendicularly 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 over a wide area of the film, but instead targeting 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, embodiments of the configuration of the present invention obtained as a result of the above-mentioned research will be described with reference to the drawings. FIG. 1 shows an embodiment of an inflation film forming apparatus to which the present invention is applied and whose take-off direction is upward.
Equipped with a. 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 flat by the guide plate 4, and is moved by the taking-off nip roller 5 and a large number of 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 made using such conventional equipment will be described. First, it has dimensions suitable for surrounding the outer periphery of the target inflation film F near the frost line ₀ with a small gap, and has an annular injection slit 11 on the inner periphery and a rectifying annular 12 parallel to the upper plate. A cooling air injection ring 10 is prepared. 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°. In addition, when the production speed is increased, the rising speed of the film F becomes faster and the cooling time becomes shorter.
It is necessary to increase the cooling rate. Therefore, a large amount of cooling gas is sprayed at high speed, and although the linear cooling part of the film that is uniformly pressurized all around is good, the film surface that follows the upward and downward dispersion flow is affected. You will begin to receive it. Even if the lower film surface is distorted due to the downwardly dispersing flow, it is not a big problem because it will be tightened uniformly when it advances to the linear cooling part, but the upwardly dispersing flow cannot be left alone. As a countermeasure against this upwardly dispersing flow, the above-mentioned rectifying ring plate 12 is provided along the inner periphery of the upper surface of the injection ring 10. Figure 3 is an enlarged explanatory diagram showing one example.
Annular plate 12 for dispersion flow rectification, which coaxially overlaps the upper plate of slit 11 with a gap therebetween, has an inner diameter similar to that of the upper plate, and has an annular plate portion parallel to slit 11 as the main part.
It is. In this case, legs 13 are erected at various places on the upper surface of the upper ring plate of the horizontal injection gap 11, and the ring plate 12 is placed on top of the legs 13.
is fixed parallel to the slit 11. The distance between the ring plate 12 and the upper plate of the narrow gap 11 was set at 5 to 20 mm, but the dimensions and position of the ring plate 12 were determined through experiments at a position that would give the best air flow stabilization effect. This ring plate 1
2 is a mere flat plate in the embodiment shown in FIG. 3, but as in the embodiments shown in FIGS.
The shape and dimensions, such as adding b, are left to the designer, and the point is that it should be able to prevent turbulence and vortices along the film F surface and cause distortion and agitation to the extent that the film F becomes problematic. Bye. Fig. 6 shows six examples of the cross-sectional shapes of the injection edge of the slit 11 and the inner edge of the ring plate 12 that achieved the effect, and Fig. 7 shows three examples A', B', C' where the effect was not obtained, and the conventional Two examples of blow-up types, D' and E', are shown. These main dimensions,
In other words, the narrow gap a, the minimum distance b between the upper plate of the narrow gap 11 and the ring plate 12, the inner edge height c of the ring 12, the inner radius difference d between the narrow gap 11 and the ring plate 12, etc. illustrated in FIG. This will be described later along with the experimental results. 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. 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. The above-mentioned linear low-density polyethylene includes ethylene and an α-olefin having 3 to 12 carbon atoms, such as propylene.
At least one of butene-1, hexene-1, 4methyl-1-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. It is something that will be done. 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. The present invention focuses on cooling the horizontal linear portion of the outer periphery of the film F, employs a method with the highest cooling efficiency of applying jets to the linear portion at almost right angles, and provides a rectifying ring plate 12. As a result, complete cooling can be achieved with a minimum amount of injection, and the objective of improving transparency can be achieved without adversely affecting the progress of the blown 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 highly transparent films at a production speed of 90 m/min, which was previously unimaginable. Next, the present invention was applied to the inflation method of linear low density polyethylene, high pressure low density polyethylene, and blends thereof, and the cross-sectional shapes of the inner circumference of the injection ring on the frost line side are shown in Figs. 7
Tables 1 and 2 show the results of experiments conducted with changes as shown in the figure.
Shown as a table. All experiments were conducted under the following conditions. Extruder cylinder diameter 50mm Diameter of extruder annular outlet 2a 150mm Exit gap 2.5mm Finished film thickness 20μ Film folding width 300mm Injection slit, distance between films 5.0mm Distance between frost line ₀ and injection slit Approx. 50mm Slit, Thickness of ring plate material: 5 to 8 mm Distance from die 2 to upper injection ring 10: 720 mm (Experiment numbers 1 to 4)

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[Table] Experiment numbers 1 to 11 are those of this invention, 1 to 8 obtained good results, and 9 to 11 obtained poor results. Experiment numbers 12 and 13 added for reference used the conventional frost line side injection ring, and the injection angle for 12 was 60°, and for 13
45° and a take-up speed of 60 m/min, the film is unstable and cannot be used. There is no need to explain the details, but (F), (G),
The excellence of this invention is clearly shown in (Y), (T), (R), and (S), and if the shape and dimensions of the rectifying ring plate 12 are contrary to its purpose, bad results will occur. There is. Although the present invention has been described above with reference to the illustrated embodiments, 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. There are various ways to improve the rectifying effect, such as using multiple rectifying ring plates instead of one, adding one to the bottom of the jet slit, or actively sending airflow between the ring plate and the upper plate of the slit to strengthen the rectifying effect. The effects of this invention will be further enhanced by making the cross-section more complex and having a higher rectifying effect, or by making other efforts by the designers and 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 of the line in a linear manner to cause solidification to begin, complete, and supercool on the spot. Opaqueness due to solidification has been 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. In addition, especially when the flow velocity and flow rate of the jet in the upper injection ring are increased to increase productivity, even if the cooling area is minimized, the film stability cannot be avoided. By adding a rectifying ring plate above the gap (take-up side), it was possible to suppress film oscillation and destabilization to a level that would not cause any practical problems. 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 one embodiment of this invention, Fig. 2 is an enlarged view of the main part and explanatory diagram of temperature distribution, Fig. 3 is an enlarged explanatory diagram of the main part of Fig. 1, and Figs. 4 and 5 are an explanatory diagram of the main part of this invention. Figures 6 and 7 are cross-sectional explanatory views of the inner peripheral part of the injection ring used in the experiment. F...Inflation film, ₀ ...
...Frost line, C...Film dent, 10...
... Cooling air injection ring, 11 ... Injection slit, 12 ... Rectification ring plate.

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,
An injection slit that advances as an inflation film with a cylindrical frost line using internal pressure and withdrawal, and allows the jet to travel straight without dispersion almost at right angles to the surface of the cylindrical film, and an upper plate of the slit. For linear cooling, the inner periphery includes a ring plate for rectifying diffused air, which overlaps coaxially with a gap above the upper plate, has an inner diameter similar to the above upper plate, and has a ring plate part parallel to the above gap as the main part. A cooling air injection ring surrounds the outer periphery of the film near the frost line with a small gap, and the injection ring is positioned concentrically with the annular outlet so that the jet stream is slightly below the position of the frost line in the extruder. The outer periphery of the transparent part of the film, where no solid phase has yet formed, is fixed at a position where it can be linearly cooled, and the injection is performed.The strength of the jet from the injection ring is determined by applying the jet at a right angle. The cooling capacity shall be such that a recoverable linear depression is created on the film circumference, and the cooling capacity shall be such that solidification is started and completed instantly at the linear cooling position from the temperature of the transparent part of the film receiving the jet flow. A method for forming an inflation film, which is characterized by: