JPS5913964B2 - Inflation film molding method for high-density polyethylene resin - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a method for manufacturing a thin tubular film made of high density polyethylene resin by an air-cooled inflation method, and in particular, to improve a method for manufacturing a thin tubular film of high density polyethylene resin using an air-cooled inflation method, and in particular, to improve a method for producing a thin tubular film of high density polyethylene resin by an air-cooled inflation method. The purpose is to provide a method that allows molding at high speed.

Conventionally, films formed from high-density polyethylene by the inflation method have been widely used for packaging, agriculture, etc., but they have gradually become thinner and thinner.
There is a need for high-quality films with well-balanced lateral strength, and it is also desired to provide a method for producing such films at high speed. Several methods have already been proposed to solve these industrial problems, but these methods tend to reduce production efficiency when trying to improve the quality of the product film 25, and vice versa. Attempts to increase production efficiency tend to result in a decline in the quality of the product film, and no truly improved method has yet been established. In order to produce a high-quality thin film with balanced longitudinal and lateral strength, the molten resin tube discharged from the die can be stretched in the drawing direction as well as in a direction perpendicular to the drawing direction. This method is known to be effective, and involves taking the molten resin tube at a speed higher than the extrusion speed of the resin, and expanding the discharged molten resin tube 35 by gas internal pressure. In addition, as a method for cooling and solidifying the discharged molten resin tube, a method is adopted in which a ring surrounding the molten resin tube is installed at the top of the die, and room temperature air is blown through a small hole or slit provided in the ring. There is. The discharged molten resin tube is stretched and moved in the take-off direction, while its temperature is gradually lowered by air cooling, and when it reaches a temperature suitable for being stretched in the transverse direction, it begins to expand. Then, before reaching the take-off machine, it is sufficiently naturally cooled and wound up as a solid film. In order to continuously produce high-quality thin films stretched vertically and horizontally at high speed, the resin used must have favorable physical properties, especially favorable melt viscoelasticity at the resin temperature during the cooling process. It is essential to use a suitable raw material resin and to set more precise molding conditions. Normally, when a raw material resin is specified, optimal molding conditions are determined based on the results of trial experiments, taking into consideration the performance of the desired product film. Important molding conditions include resin extrusion speed, resin temperature at the die, position of the air cooling ring, cooling air amount and air blowing direction, take-up speed, expansion ratio, frost line height, etc. . By appropriately combining and setting these molding conditions, not only the performance of the product film is determined, but also the shape of the molten resin tube discharged from the die until it is taken off. The shapes include a type in which the tube expands from just above the die surface (type 1), a type in which the tube diameter gradually decreases after being discharged from the die and expands following a constriction where the diameter reaches the minimum (type 1); It is roughly divided into an intermediate type (type 1) between the above-mentioned type 1 and type 1. In order to form a thin film at high speed and to stably produce high-strength, high-quality products without fluctuations in quality, the molding method using the above-mentioned No. H mold is most preferable, but the molding method using this No. 1 mold This method has another drawback in that the molten resin tube discharged from the die is likely to sway during the time it cools and reaches the take-off machine, making it difficult to stabilize the film quality.

To solve this problem, a method described in Japanese Unexamined Patent Publication No. 49-61252 has been proposed. However, JP-A-49-61
When forming a thin film at high speed by the method described in Publication No. 252, the cross section has already expanded into a perfect circle at the beginning of expansion due to the influence of the guide plate immediately in front of the pulling machine. This has the drawback that the film is easily folded, causing wrinkles, slack, and fluctuations in folded diameter when folded by a pulling machine. As a result of various studies aimed at improving the inflation molding method using the above-mentioned mold 1, the inventors of the present invention have found that the film obtained when molded while maintaining the shape of the mold H does not have wrinkles, slack, or folds. We found that the cause of diameter fluctuations is that the expansion does not occur in a perfectly circular cross section when the expansion starts following the constriction, and in order to prevent this constriction from occurring, we By bringing the molten resin tube into contact with a metal hollow mandrel having a diameter larger than the lip diameter of the die, the molten resin tube whose diameter has naturally contracted is expanded, and when the expansion is started by the internal gas pressure on the mandrel, the constriction is removed. It has been found that the expansion occurs in a perfectly circular cross section without any swelling, and that a high-quality thin film without wrinkles, slack, or fluctuations in fold diameter can be efficiently formed.

The object of the present invention is to provide a method for forming a blown film by causing a natural contraction in the tube diameter of a molten resin tube of high-density polyethylene continuously extruded from a die, and then expanding the tube by internal gas pressure. An object of the present invention is to provide an inflation film forming method capable of producing a thin high-density polyethylene film at high speed without wrinkles, slack, or changes in fold diameter by causing perfect circular expansion.
In the high-density polyethylene resin inflation film molding method of the present invention, high-density polyethylene resin is continuously extruded from a die into a molten tube shape, and is allowed to rise while being air-cooled from the outside to cause natural contraction of the tube diameter. In an inflation film molding method for high-density polyethylene resin, in which a tubular film that has been expanded by internal gas pressure and cooled and solidified is taken out via a guide plate, A mandrel has a hollow cylindrical part made of metal with a diameter 1.01 to 1.2 times the lip diameter and a sufficient length and a roughened surface. The molten resin tube that has undergone contraction is brought into contact with the molten resin tube before the constriction occurs, and the expansion of the molten resin tube by the gas internal pressure is started on the cylindrical surface, and the expansion ratio is 3 to 3.
It is characterized in that it is expanded at a stretching ratio of 4 to 70 in the drawing direction.

The high-density polyethylene resin used in the molding method of the present invention is
It is produced by a normal medium-low pressure polymerization method and is easily available as a commercial product.

Preferably, the melt strength is 59 or higher;
In particular, it is about 6 to 89. The above melt strength values are 1mm in diameter and 5m77 in length! Melt high density polyethylene resin at 190℃ from a nozzle with 10.0C
This value is measured under conditions of extrusion at rrL/min and withdrawal at 6.3 m/min.

Using the above high-density polyethylene resin, the high-density polyethylene resin is continuously extruded into a molten tube from a die attached to the tip of the melt extruder, and then air-cooled using an air-cooling ring so that expansion starts at an appropriate height from the outside. raise it while
When the film is drawn at a high speed through a guide plate and expanded by internal gas pressure, the shape of the mold is maintained as described above, and a thin film is formed.

The shape of this first mold is representatively shown in FIG. 1 of the accompanying drawings. Fig. 1 shows that the molten resin tube discharged from the lip 2 of the die 1 is cooled by the air cooling ring 4, and is drawn out while being stretched to form an elongated naturally contracted portion 7, which continues to the constriction 11 where the tube diameter is the smallest. Expansion begins due to the internal pressure of the gas conducted from the gas communication hole 3, indicating that an expanded portion 8 is formed. The shape of this first mold, especially the shape in which the tube diameter gradually narrows to form a constriction and then swell, is formed because the molten resin tube discharged from the die slip is drawn in at high speed before expanding. This is because hoop stress occurs in the molten resin tube due to stretching, and natural contraction of the tube diameter occurs to relieve this stress. Therefore, the region where the constriction occurs is the region where the cross-sectional shape of the molten resin pipe is unlikely to be a perfect circle and is most likely to become distorted. In the molding method of the present invention, a portion of the molten resin tube extending an appropriate length from the region where the constriction occurs is brought into contact with a specific mandrel having a diameter larger than the lip diameter of the die to expand the diameter of the mandrel. The feature is that by raising the surface in a sliding manner, the occurrence of constrictions is avoided, and the cause of defective molding is eliminated and molding is performed. The mandrel used in the molding method of the present invention is illustrated in FIG. 2 of the accompanying drawings.

In the figure, a hollow metal cylinder 6 is integrally connected to the lower end of a thin cylindrical support rod 5 which is fixed vertically to the central upper surface of the die 1 so as to be concentric with the axis of the die. The mandrel is shown fitted with the screw 10 on the support rod 5 by the screw fitting part 12 and fixed at an appropriate position in the vertical direction, and the mandrel is discharged from the lip 2 of the die and the natural contraction of the tube diameter occurs. The rising molten resin tube 7 contacts the surface of the hollow cylinder 6 of the mandrel before the constriction occurs,
The molding method of the present invention is also illustrated in which the film 8 rises while sliding on the surface and then begins to expand on the surface of the cylinder 6 due to the internal gas pressure to form the expanded film 8.
The support rod 5 is provided with a screw on its lower end circumferential surface,
This is fixed to the die 1 by fitting it into a screw provided in a recess at the center of the die. In addition, the support rod 5
is provided with an opening through hole 13 passing from its lower end to its upper end, and air for inflation passes through this hole and exits from the upper end opening. Further, the support rod 5 has a gas communication hole 1 located below the cylinder 6 fixed thereto, which communicates with the through hole 13 and opens on the surface of the support rod 5.
4 is provided, and a portion of the air flowing into the through hole 13 exits from this hole to prevent the molten resin pipe 7 from deforming due to external pressure. Further, the lower end corner of the cylinder 6 is rounded, and the molten resin pipe 7 in contact with the lower end corner is rounded.
The structure allows for smooth contact and sliding. The mandrel used in the present invention is fixed to a die so that the molten resin tube that comes into contact with it does not shake, and the lower part of the mandrel is sufficiently thin so that the molten resin tube, which has naturally contracted in diameter, does not come into contact with it. In addition, the cylindrical portion of the mandrel is arranged perpendicular to the die surface so as to be concentric with the axis of the die slip so as to avoid tilting of the molten resin tube.

The cylindrical part of the mandrel is for expanding the diameter of the molten resin tube whose diameter has naturally contracted and raising it while making contact with it, so that expansion starts on the surface of the cylindrical part.
It requires a diameter larger than the die lip diameter and sufficient length. A tightening force is generated on the molten resin pipe due to the contact diameter expansion, but while the molten resin pipe contacts and slides upward on the surface of the cylindrical part, its cross section is formed into a perfect circle, and
The tightening force acts uniformly around the molten resin tube, and when expansion begins on the surface of the cylindrical part, the uniformly maintained tightening force also overcomes the flattening force exerted by the guide plate, and the cross section Causes a perfect circular expansion. However, if the diameter of the cylindrical part becomes too large, the tightening force that acts on it will also become excessive, which often causes the molten resin tube to break at the point where the molten resin tube starts to expand, or causes resistance to pulling it out. is necessary. Of course, if the diameter of the cylindrical portion is too small, the tightening force will be insufficient and a sufficient effect will not be exhibited. Thus, the diameter of the cylindrical portion is 1 relative to the lip diameter of the die.
．． A diameter of 0.01 to 1.2 times is appropriate, especially a diameter of 1.01 to 1.0
．． 10 times is preferable. The cylindrical portion is filled with molten resin so that the molten resin tube has a perfectly circular cross section and uniform tightening force while the molten resin tube slides in contact with the surface of the cylindrical portion and rises. Sufficient length must be provided downward from the beginning of the tube's expansion. However, if the length is too long, the contact area of the molten resin tube increases, which increases the resistance to take-up, and also reduces the strength of the product film. Thus, the length of the cylindrical portion is preferably 1.5 to 3 times the lip diameter of the die. The height from the die surface where the cylindrical part should be placed is:
The length from the point on the surface of the cylindrical part where expansion begins to the top of the cylinder should be about 0.1 to 0.8 times the diameter of the die slip, which is the vicinity where constriction occurs when the mandrel is not used. Preferably, a cylindrical part is provided. The cylindrical portion of the mandrel used in the present invention is a hollow metal body whose surface is roughened.

As mentioned above, the cylindrical part is preferably made of metal, which is required to have non-deformability, durability, and wear resistance, and is easy to manufacture, since the molten resin tube having a tightening force slides in contact with the cylindrical part. Furthermore, by making it metal, the temperature balance between the molten resin and the cylindrical part that are in contact with each other can be easily achieved. However, if the heat capacity is large, it will be easy to absorb heat from the molten resin pipe, so in order to prevent this, it is preferable that the cylindrical part is a hollow body. Furthermore, by making the surface uniformly rough, it is possible to absorb heat from the molten resin pipe. It is possible to prevent the tube from sticking and rapid heat absorption, and to maintain a better temperature balance between the molten resin tube and the cylindrical portion that are in contact with each other, while allowing the molten resin tube to smoothly slide into contact with the surface of the cylindrical portion. can. The surface of the cylindrical portion can be easily roughened, for example, by cutting grooves into the surface of the cylindrical portion so as to form uniformly pointed threads. The molding method of the present invention involves bringing a molten resin tube extruded from a die, whose diameter has naturally contracted, into contact with the surface of the cylindrical portion of the mandrel before constriction occurs, and sliding the molten resin tube on the surface of the cylindrical portion. Then, the internal gas pressure starts the expansion of the molten resin tube on the cylindrical surface to an expansion ratio of 3-! The film is expanded to -5 and taken off at a draw direction draw ratio of 4 to 70.

Normally, the resin temperature at the die part is 190 to 230°C, and the molten resin tube extruded from the die is cooled by cold air from the air cooling ring, and when it is cooled to the expansion start temperature, expansion begins. As described above, according to the present invention, the initiation of expansion occurs on the surface of the cylindrical portion of the mandrel, the cross section of which is perfectly circular. To obtain a film with well-balanced strength in the longitudinal and transverse directions, the expansion ratio should be 3 to 5 and the take-up direction should be 4 to 5.
The take-up speed and the internal gas pressure for expansion are adjusted so that the temperature becomes 70. Normally, according to the above-mentioned molding method, the height from the die surface to the expansion start part is 3 to 3 times the lip diameter of the die.
The length is equivalent to 10 times as long. After being expanded to the above-mentioned expansion ratio, the molten resin tube cools down further, and eventually a frost line appears. Next, the expanded tubular film, which has been completely cooled and solidified, is picked up by a pulling machine while being sandwiched between guide plates and folded. The above expansion ratio is expressed by the ratio of the diameter of the tube after expansion to the diameter of the die slip, and the drawing ratio in the drawing direction is expressed by the ratio of the drawing line speed to the extrusion line speed of the molten resin. It is an important factor for setting the strength balance of the film. The present invention
After the diameter of the molten resin tube naturally shrinks as described above, the diameter is expanded using the cylindrical part of the mandrel to a diameter larger than the diameter of the die slip, and the molten resin tube is expanded while maintaining a uniform tightening force. By stretching the film, a film with good strength balance can be formed without wrinkles, slack, or changes in fold diameter. According to the molding method of the present invention, the folded diameter is 500 m1.
A thin tubular film with a width of L or more and a thickness of about 5 to 15 μm can be easily formed at a high bowing speed of 40 to 100 m/min or more, and the vertical and horizontal strength are balanced, In addition, it has excellent tear strength, impact strength, etc., has little variation in fold diameter, and can produce smooth, high-quality films with no wrinkles, slack, etc. at high speeds with good quality stability.

Examples in which films were manufactured by the method of the present invention will be shown in more detail below, but the technical scope of the present invention is not limited thereto.

EXAMPLE A die having a lip diameter of 100 m, TIL, and lip clearance of 1.0 was attached to an extruder.

Separately, the diameter is 16m7 so that it is concentric with the center axis of the die and perpendicular to it! A round steel bar with a length of L1 of 8001 m was attached to a die. This round steel bar has an open through hole at its axial center extending from the lower end to the upper end, and further extends 10 mm from the lower end.
0m77! There is also a hole in the rod surface that communicates with the through hole. Furthermore, this round steel bar is provided with a threaded groove around its circumference. Separately, diameter 102m77! Length 200m7
By fitting the threaded fitting part provided at the lower end of the steel hollow cylinder of fL with the thread groove on the surface of the round steel bar, the distance from the die surface to the lower end of the cylinder is 5007 mm.
F! It was attached to the part marked m. The cylinder is grooved so that a uniformly pointed thread is formed on its surface. In this way, after preparing an apparatus in which a mandrel was attached to a die, high-density polyethylene with a melt index of 0.07 was extruded at a die part resin temperature of 210°C and an extrusion speed of 44°C.
kg/hour, and after being brought into contact with the mandrel, it was withdrawn at a speed of 60 m/minute, and expanded at an expansion ratio of 4.0 while being cooled by an air cooling ring, resulting in a mass of 6701L7 from the die surface! Expansion occurred on the cylindrical surface at L.

The expanded cooling tubular film was continuously wound up via a guide plate, and there was almost no shaking of the resin tube during operation, allowing continuous operation for a long period of time. There was no slack in the film, and no wrinkles were observed in the product film. The thickness of the product film was 10 μm, and almost no thickness unevenness or variation in fold diameter was observed. This product film has a tensile strength of 524kg/CfL and 461k in the longitudinal and transverse directions.
g/d, and the Elmendorf longitudinal tear strength and transverse tear strength are 9.4 kg/CrlL and 25.71<g/C, respectively.
It was m. Moreover, the impact strength was 7.2 kg·α when measured by ASTM D-1709 method. As a comparative example, the operation was carried out in exactly the same manner as in the above example except that the cylinder was removed. Although the resin tube did not shake much, wrinkles were still observed in the product film and fluctuations in the fold diameter were observed. It was done.

[Brief explanation of the drawing]

FIG. 1 is a vertical cross-section of a main part showing a molding method in which a molten resin tube discharged from a die is expanded after forming a constriction when molding is performed by a conventional method.

Claims (1)

[Claims] 1 High-density polyethylene resin is continuously extruded into a molten tubular shape from a die, raised while cooling with air from the outside, and after natural contraction of the tube diameter occurs, it is expanded by gas internal pressure, and the cooled and solidified tubular film is passed through a guide plate. In the method of forming a high-density polyethylene resin blown film by drawing the resin through a blown film, the film is erected and fixed on the central upper surface of the die, and the lip diameter of the die is 1.01 to 1.
On the surface of the cylindrical part of the mandrel, which has a hollow cylindrical part made of metal that has twice the diameter and sufficient length and has a roughened surface,
The molten resin tube whose diameter has naturally contracted is brought into contact with the molten resin tube before constriction occurs, and the molten resin tube starts to expand on the surface of the cylindrical part due to the gas internal pressure, and is expanded to an expansion ratio of 3 to 5 and then taken off. A method for forming a blown film of high-density polyethylene resin, characterized in that the film is drawn at a directional stretching ratio of 4 to 70.