JPH0425859B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical field to be utilized] The present invention relates to a method for producing a synthetic resin film by an inflation method, and mainly relates to the production of an HMW-HDPE film. [Prior art] In general, in this type of film forming method, when the height of the frost line is considered constant, there is a method in which the bubble diameter is rapidly expanded to a large extent from the annular die outlet, and a method in which the bubble diameter is expanded rapidly up to the vicinity of the frost line. Films formed using the former method are said to have a large degree of longitudinal orientation, while films formed using the latter method are said to have less of this tendency. In recent years, there has been a tendency to adopt the latter method with a high blow ratio in order to reduce the tendency for vertical orientation, but the height at which the bubbles start expanding (net height) is lower than the annular discharge port. Increase the height to about 4 to 6 times the diameter, and blow at a blow ratio of 2.5 to 5.
Some manufacturing methods have been implemented in which the discharge rate of double bubbles is approximately 1.0 to 2.0 kg/h per 1 cm of circumferential length of the annular outlet, and the gap of the annular outlet of the die is 1 to 2 mm. It's getting old. In such a known method, the pull-off speed and the longitudinal and transverse tear strength ratio (Tear Stnength Ratio
The relationship between TD/MD) and Dart Impact Strength tends to be as shown by the gram solid line in Figure 1.
At around 100 m/min, the dart impact strength value is high and the longitudinal/width tear strength ratio is low, but at other take-up speeds, these two values regarding the properties of the film deteriorate rapidly. However, in reality, the pick-up speed is 100m/
It is difficult to achieve a high speed of 1 min because the bubble will vibrate due to an increase in the amount of cooling air. Next, some methods have been published to provide a stabilizer inside the bubble, but in the above-mentioned known method and device, if a stabilizer is used, the curves of the longitudinal and transverse tear strength ratio and dart impact strength will be reduced. As shown by the dotted line in the graph of FIG. 1, the curve becomes gentle, but these properties deteriorate as a whole. Therefore, although the use of a stabilizer contributes to the stabilization of the bubble, it has been considered to have a rather negative effect on the strength of the film. On the other hand, as shown in the graph in Figure 2, the correlation between net height, dart impact strength, and vertical/horizontal tear strength ratio generally shows a tendency to improve as the net height increases, but the data shown in the figure shows a tendency to improve as the net height increases. This is when the diameter of the annular discharge port is small and the blow ratio is small, and it was not known at all what kind of tendency it would be when the diameter was large, the blow ratio was high, and the neck height was high. [Problem] However, in recent years, film molding manufacturers have been requesting even higher output mass production, but if the die gap of the annular outlet of the die is kept as is and high-speed take-off is performed, the orientation becomes increasingly large. At the same time, the flow velocity of the molten resin flowing through the die increases, the heat generation of the molten resin increases, the resin temperature rises, bubbles become unstable, melt fracture occurs, and it becomes difficult to form high-quality films. Become. On the other hand, if the die gap is simply widened, the bubbles immediately after being discharged have high fluidity and become considerably heavy as the thickness increases, so that the molten bubbles immediately after the die exit cause a so-called sagging phenomenon. Therefore, it is necessary to rapidly cool the bubble up to the net height by bringing it into contact with a large amount of cooling air, but increasing the wind speed causes vibrations to the bubble, causing cooling spots, which can lead to rough skin on the bubble surface and breakage. Even if the bubble can be cooled to a level that can prevent irregular widths, uneven wall thickness, etc., and the phenomenon of bubble sitting, the inside of the wall is still quite hot (170 to 190 degrees Celsius) when expanding. temperature is high, the effect of biaxial orientation is not exhibited, and the vertical orientation after expansion remains strongly in the film after molding, resulting in a film that is easily torn in the vertical direction, with markedly different tear strengths in the vertical and horizontal directions. It was thought that high discharge molding with a discharge rate exceeding 2.0 kg/h per 1 cm in the circumferential direction of the annular discharge port was difficult or impossible. [Purpose] This invention further improves the above-mentioned discharge rate limit, enables the discharge rate per 1 cm in the circumferential direction of the die in the annular discharge direction to reach 4 kg/h at once, and furthermore,
The vertical and horizontal tear strength is well-balanced, and the folding diameter and
This is a method for forming high-quality films with uniform wall thickness dimensions. [Means for Solving the Problems] The present invention guides the bubble discharged from the annular discharge port of the inflation molding die downstream without changing its diameter, and creates a bubble having an outer diameter approximately equal to this diameter. a lower stable body having a cylindrical outer circumferential surface with a diameter of approximately 0.5 to 1 times the diameter of the annular outlet than the position of the annular outlet; and a cylindrical outer periphery having approximately the same diameter as the lower stable body. The bubbles have a surface and are placed at a height of 2 to 5 times the diameter of the annular outlet from the position of the annular outlet, and the diameter of the bubble passing through the upper stabilizer is almost increased. The bubbles are gradually cooled by cooling air that extends in the axial direction without causing any damage and blows downstream from around the annular discharge port along the bubble surface, so that the bubbles that have passed the stabilizer are cooled in the axial direction. At the downstream end position of the stretching section, the bubble temperature is cooled until it approaches the orientation residual upper limit temperature, and the height (neck height) above the annular discharge port is adjusted until the bubble starts expanding in the radial direction. This is a high discharge rate film forming method characterized by making the diameter 7 to 12 times the diameter of the discharge port, and then expanding the bubbles all at once to a desired expansion ratio while the bubble temperature is within the orientation residual temperature. This solved the problem. Next, the method of the present invention will be specifically explained based on FIG. 3. First, the equipment for carrying out this method is as follows:
A stabilizer 13 is attached to a mandrel 11 that stands perpendicularly to the axis of the die 10 (usually also serves as an air supply pipe for adjusting the bubble internal pressure and cooling the inner surface), as shown in FIG. In this case, the stabilizer 13 is further divided into a lower stabilizer 13a and an upper stabilizer 13b, and the diameter of the lower stabilizer 13a is approximately equal to or smaller than the diameter D of the annular discharge port 12.
The lower stabilizer 13a is provided at a distance from the annular discharge port 12 by 1/2 of the diameter D or equivalent to this diameter. The upper stable body 13b has a circular outer circumferential surface 1 whose outer diameter is approximately equal to the diameter D and whose length is 2 to 3 times the dimension D.
4, even if the diameter is the same over the entire length,
Inclination angle 2~ where the diameter on the lower end side is slightly smaller than the D dimension
It may be tapered at 5 degrees. The lower end of this upper stabilizer 13b is 2 to 3 times the dimension D,
It is fixed to the mandrel 11 at a distance from the annular discharge port 12. Of course, the upper stabilizer 13b is not limited to the above-mentioned shape, and may have the same shape as the lower stabilizer 13a. The upper stabilizer 13b is connected to the annular discharge port 1.
2 at a distance of 2 to 5 times the diameter. These stabilizers 13 are made of aluminum,
The outer peripheral surface is carved with fine circumferential grooves or spiral grooves, and non-woven fabric, paper, etc. are pasted thereon. [Embodiment] First, the die gap of the annular discharge port 12 is 1.5 to 3 mm.
The circumferential direction 1 of this annular discharge port 12 is
The bubbles 15 that have just been discharged from the annular discharge port 12 are discharged at a discharge rate of 2 to 4 kg/h per cm, and the blow ratio (BUR) is set to 4 to 7. The bubble 15 is brought into contact with the upper stabilizer 13b at the above position, and the bubble 15 is pulled in the downstream direction at a take-up speed that is optimal for the above conditions, and the internal pressure is also adjusted to adjust the air volume blown out from the air ring device 16, thereby increasing the net height H. The D dimension should be 7 to 12,
Shape. [Effect] In this way, the thick, high-temperature bubble 15 discharged from the annular discharge port 12 is pulled slightly in the axial direction, and the temperature of its outer peripheral surface is lowered by cooling air from the outside. , stable body 1
By contacting the stabilizer 13, the unstable bubble is prevented from lateral shaking of its axis and vibration in the radial direction of the bubble, and the tensile force due to the pull-up is applied to the bubble 15 below the stabilizer 13. At the same time, the weight of the bubble less than this is supported to some extent by the frictional force between the stabilizer 13 and the stabilizer 13. Next, the bubble 15 passing through the two upper and lower stabilizers 13a and 13b has a thickness equal to that of the annular discharge port 12.
is reduced to 1/2 to 1/3 of the gap dimension of , 13b is further stretched in the axial direction, and near the upper end of the neck height H is 1/5 to 1/15 of the die gap dimension.
However, since the temperature is higher than the temperature at which molecular orientation remains, even by stretching the bubbles 15 in the axial direction, no axial orientation remains in the formed film. Since the bubbles expand all at once in the temperature range where molecular orientation remains, they are stretched in both the vertical and horizontal directions in this section, and the formed film has little difference in tear strength in the vertical and horizontal directions. . In addition, in the above-described method, even when molding is performed at a high discharge rate, the bubble 15 is formed in the stable body 1 including the stabilizers 13a and 13b in a portion close to the fluid state.
3, no excessive tensile force is applied to the bubble in this part, and the vibration of the bubble is suppressed by a small amount of cooling air and the two stabilizers 13a, 13.
Due to the synergistic effect of contact with b, it is possible to produce a film with excellent fold stability and uniform thickness. If the stabilizers 13a, 13b are too close to the annular outlet 12 than the above-mentioned range, the bubbles 15 after passing through the stabilizer 13 will become unstable, and if they are too far apart, the bubbles before reaching the annular outlet 12 will be unstable. For these distances and outer diameter dimensions, optimal values are selected within the above range depending on the purpose. In order to lower the height of the net height H below the above range, it is necessary to increase the volume of cooling air, which inevitably increases the wind speed, causing the bubbles 15 to vibrate and causing the temperature inside the bubbles 15 to become oriented. The residual degree does not decrease to near the upper limit, the temperature during expansion is too high, and the orientation residual due to subsequent axial tension remains strong, making it impossible to obtain a good quality film. In addition, increasing the net height H not only unnecessarily increases the size of the device itself, but also causes the bubble to become unstable, or the resin crystals in the film that make up the bubble to grow, resulting in a decrease in quality or This could lead to unevenness. Furthermore, if the outer diameter of the stabilizers 13a, 13b is too large or the stabilizers are too long, the inner surface of the bubble 15 may be damaged or the frictional resistance may be too large.
5 may break, and if it is too thin or too short, no stabilizing effect can be expected. [Effects of the Embodiment] In a method in which the discharge amount per 1 cm in the circumferential direction of the annular discharge port 12 is approximately 1.5 to 4 kg/h, productivity is significantly improved. In a method in which the blow ratio is set to 4 to 7, a film with particularly small difference in longitudinal and lateral tear strength can be obtained. In the method in which the die gap is set to 1.5 to 3 mm, the discharge speed is not very high, and the resin flow rate in the die is not excessively high, so that deterioration in melting does not occur. Next, data obtained through experiments using the method of the present invention are shown below.

【table】

[Table] An experiment was conducted under the above conditions using high-density polyethylene with an average molecular weight of 150,000 to 200,000, and the following results were obtained.

【table】 [Brief explanation of drawings]

The figures relate to this invention. Figure 1 is a graph showing the relationship between take-up speed, vertical and horizontal tear strength ratio, and dart impact strength. Figure 2 is a graph showing the relationship between net height, dart impact strength, and vertical and horizontal tear strength. The graph showing the relationship between ratios and FIG. 3 are schematic diagrams of an apparatus for carrying out the present invention. In the figure, 10...die, 12...annular discharge port, 1
3a, 13b... Stable body, 15... Bubble, D...
...Diameter of annular discharge port, A...Frost line, H
... Netsk height.

Claims (1)

[Scope of Claims] 1. A bubble discharged from an annular discharge port of a die for forming an inflation film is guided downstream without changing its diameter, and a cylindrical outer circumferential surface having an outer diameter approximately equal to this diameter is formed. and a lower stable body disposed at a height of about 0.5 to 1 times the diameter of the annular outlet from the position of the annular outlet, and a cylindrical outer peripheral surface having approximately the same diameter as the lower stabilizer. The bubbles passing through the upper stable body are successively passed through an upper stabilizer placed at a height of 2 to 5 times the diameter of the annular discharge outlet from the outlet position, and the bubbles that have passed through the upper stabilizer are axially The bubbles are gradually cooled by cooling air blowing downstream from around the annular discharge port along the bubble surface to form a section extending in the axial direction of the bubble past the stabilizer. At the downstream end position, the height (net height) above the annular outlet until the bubble starts to expand in the radial direction after cooling the bubble until it reaches the orientation residual upper limit temperature is defined as the diameter of the annular outlet. 7
1. A high discharge rate film forming method characterized by expanding the bubble to a desired expansion ratio at once to a desired expansion ratio while the temperature of the bubble is within the orientation residual temperature. 2 In the above method, the circumferential direction 1 of the annular discharge port
2. The high output film forming method according to claim 1, wherein the output per cm is about 15 to 4 kg/h. 3. Claim 1, characterized in that the blow ratio in the method is set to 4 to 7.
High output film forming method. 4. The high discharge rate film forming method according to claim 1, characterized in that the die cap of the annular discharge port of the die for discharging the bubbles is used with a diameter of 1.5 to 3 mm.