JPH0375027B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to the improvement of electrostatic pinning technology used in producing sheets or films (including sheets thicker than films may also be referred to as films) from thermoplastic resin. The present invention relates to a film forming technique capable of maintaining a stable electrostatic pinning state and capable of obtaining a film whose thickness is kept at a high level of uniformity. [Prior Art] It is well known to form a sheet by melting a thermoplastic resin, extruding it into a belt (sheet) from a slit die, and cooling and solidifying it on the surface of a rotating cooling roll. It is possible to make the sheet thickness uniform by adjusting the lip interval of a slit die, or to make the sheet thickness uniform by sandwiching and rolling an unsolidified sheet at a temperature higher than the softening point between a pair of rolls, etc. This is a means for adjusting the thickness. In applications requiring a high degree of quality and uniformity, various means are used, either alone or in combination, to ensure uniform sheet thickness. Thermoplastic resin is kneaded in a molten state so that it has a uniform heating temperature and a uniform melt viscosity, and a metering pump is used to maintain a constant flow rate per hour when extruding into a sheet. Sometimes.
Furthermore, in order to avoid local fluctuations in the amount of molten resin flowing out in the die portion, a set of adjustment bolts that expand or narrow the gap between the die lips as described above may be provided along the slit of the die, or Uniform thickness control means are known, such as arranging a large number of fine heating and cooling means along the slit of the die to adjust the local flow rate of the melt. It is also known that these local flow adjustments of the melt are associated with automatic control devices that correct differences between the measured sheet thickness and a reference value. The thickness of the sheet can be measured using optical means, such as changes in the amount of infrared rays (transmitted light amount), β-ray thickness meter, etc. ) thickness can be measured. Further, by scanning the thickness measuring means in the sheet width direction, the thickness state in the sheet width direction can be detected. In the conventional technology, the thickness in the sheet width direction is detected during the sheet manufacturing process, and by feeding back the thickness detection results, the die is melted locally so that the thickness in the sheet width direction is constant. Thickness control technology that adjusts body flow has been put into practical use. By the way, even if the molten resin is extruded to a uniform thickness along the width direction of the die and the extrusion amount is completely constant, a sheet of molten resin will be deposited on the surface of the cooling roll to a uniform thickness. Without solidification, a sheet of uniform thickness cannot be obtained. The technology for cooling and solidifying a molten resin sheet requires that the cooling roll rotates at a uniform speed with almost no fluctuation even momentarily, and that the molten resin sheet contacts the cooling roll at a predetermined position without fluctuation. It is essential that the touch-down condition be stable. The latter technique, a film forming technique using electrostatic pinning, in which a molten resin sheet is statically brought into close contact with a cooling roll, is effective (Japanese Patent Publication No. 37-6142). In this electrostatic pinning, if the resin sheet and the cooling roll do not come into perfect contact with each other, air will be drawn in, resulting in uneven adhesion on the sheet surface. A condition for good electrostatic pinning is a stable connection in which the air accompanying the resin sheet as it travels is removed at the position where it contacts the cooling roll. From another point of view, it is necessary for the resin sheet to have a constant state of contact with the cooling roll and a spatially constant position of contact with the cooling roll surface in order to form a sheet with a uniform thickness. [Technical Problems to be Solved] In the electrostatic pinning method, in order to obtain a good and stable film casting state and a resin sheet with a uniform thickness, it is necessary to use a live part exposed electrode, such as a needle-like electrode or a wire-like electrode, It is necessary to place it in an appropriate spatial position. For example, the distance between the electrode and the resin sheet must be appropriate in terms of the relative position of the electrode and the touch-down line (also called landing line) of the molten resin sheet to the surface of the cooling roll. If the electrode and the resin sheet are too close together, the molten sheet receives excessive static electricity and vibrates, changing the touch-down line and causing uneven thickness of the resin sheet in the flow direction. This is because the vibration causes unevenness in the amount of deposit over time, which is observed as pulsating thickness unevenness in the longitudinal direction of the resin sheet. Furthermore, when the electrode is moved closer to the die than the touch-down line, the electric potential gradient becomes smaller as the electrode moves further away, and the electrostatic charge applied to the molten resin sheet decreases, increasing the adhesion of the resin sheet to the cooling roll. becomes weaker. If the entrained air cannot be completely removed when the resin sheet is brought into close contact with the cooling roll, the air will be drawn into the sheet or removed, and the entrained air will be scattered as fine bubbles on the sheet surface, making it impossible to make it come into close contact with the cooling roll. Traces of these air bubbles remain on the surface of the broken resin sheet. A resin sheet in which such air bubbles are scattered has "undulation"-like thickness unevenness. As described above, the electrodes need to be positioned appropriately depending on the thermoplastic resin used to form the sheet. In conventional technology, adjustment of the electrode position relies on the operation of a skilled worker based on the measured value of the sheet thickness during manufacturing, and in fact, the thickness is determined based on the state of the final product. Fine adjustments were made to the position of the electrodes based on inspection results such as the presence or absence of spots and traces of microbubbles. However, this method requires a high degree of skill on the part of the operator, and as the molding speed increases, the range of optimal positions for the electrode narrows, so if adjustment takes time, there will be a lot of loss. Conventional problems have been pointed out, such as the use of opaque sheets and other materials that make it difficult for workers to make judgments and require automatic adjustment technology. [Object of the Invention] In order to solve the above-mentioned problems, the present invention aims to provide a method that can quickly and quantitatively adjust the position of an electrode to an optimal position that provides a good sheet forming condition. . [Configuration of the Invention] In order to achieve this object, the method for manufacturing a thermoplastic resin sheet of the present invention has the following configuration. In the present invention, when extruding a molten thermoplastic resin in the form of a sheet onto the surface of an electrically grounded cooling roll, the resin sheet is extruded by applying a DC high voltage to an exposed live electrode placed above the resin sheet. In the method of manufacturing a thermoplastic resin sheet that is electrostatically brought into close contact with the cooling roll and cooled, a change in the thickness of the resin sheet in the machine direction (longitudinal direction) at a predetermined position in the width direction of the resin sheet is detected by a detection means. The detected thickness change is separated into thickness change components in a plurality of frequency ranges by an analysis means, and the electrode and the resin are separated at positions corresponding to the predetermined position in the sheet width direction according to the thickness change components in each frequency range. Adjust each thickness change component to the minimum by changing the distance to the sheet, and then detect the thickness change at another predetermined position in the width direction of the resin sheet in the same way as the first position example, and use multiple frequencies. By repeating the process of separating the thickness change component in the region and adjusting the distance between the electrode and the resin sheet at the position corresponding to the different predetermined position according to the thickness change component, the thickness change in the entire width of the resin sheet is calculated. This is a method for producing a thermoplastic resin sheet characterized by reducing the amount of . The present invention will be explained. The present invention can be applied to forming sheets, films, and webs from general thermoplastic resins. Electrostatic pinning can increase production speed and produce sheets through uniform cooling and solidification, making it suitable for molding (film forming) where quality and productivity are requirements. Typical examples of thermoplastic resins include linear saturated aromatic polyesters such as polyethylene terephthalate, polyethylene naphthalene dicarboxylate, and polyhexamethylene naphthalene dicarboxylate; other examples include polyether, polyamide, polyolefin, etc. Naturally, thermoplastic resins can also be used. In the present invention, known means can be applied as a technique for melting the resin, extruding it through a die, and casting it on the surface of the cooling roll. That is, the resin is melt-extruded at a temperature above its melting point and cast onto a cooling roll whose surface is cooled to about room temperature. The cooling roll usually consists of a surface made of a material such as metal or ceramic, and a cavity for storing (circulating) a coolant, and is electrically grounded or connected to a power source to form a counter electrode. To explain this with reference to Figure 1 of the drawings, the molten resin is extruded from the die 1 onto the sheet 2, and in the space until it reaches the cooling roll 3, it is placed in a position close to the resin sheet but not in contact with it. An electrostatic charge is applied to the sheet by the placed light bulb 4. This light bulb uses a needle-like electrode that has a large number of needles densely arranged at approximately equal intervals across the width of a resin sheet, or a wire such as a piece of piano wire that is stretched along the width of the sheet. Electrodes are appropriate. In either case, the surface of the electrode placed near the sheet is exposed and is not covered with an insulator. This light bulb is connected to a power source, and during operation, an electrostatic charge is deposited on the surface of the sheet from the electrodes. In the present invention, as described later, the electrode 4 is arranged in the horizontal direction 5.
and a means for adjusting in the vertical direction 6,
The distance between the electrode and the resin sheet is maintained within an appropriate range. Note that the electrodes are connected to a high voltage static electricity generation source 7. In the present invention, a detection means 9 is provided for detecting the thickness of the sheet 8 at a position away from the cooling roll, usually before and after the stretching process, and as described later, detects the thickness of the sheet and changes in the thickness. is analyzed by the analysis means 10. The sheet is subjected to biaxial stretching and heat treatment via a stretching machine 11, a stenter 12, etc., and then wound up by a winding means 13 according to a conventional method. The present invention mainly consists of these electrostatic pinning means, a sheet thickness detection means, and a means for adjusting the electrode position based on the sheet thickness variation obtained by the detection means. . and,
In addition to the above-mentioned electrostatic pinning means, automatic position control of the electrodes is performed by detecting changes in the thickness of the sheet during operation during the sheet manufacturing process. In the present invention, the thickness measuring device 9 disclosed in Japanese Unexamined Patent Application Publication No. 70904/1984 can be used as the sheet thickness detection means 9 incorporated during the manufacturing process.
This device is an infrared thickness measuring device that utilizes the infrared absorption characteristics of thermoplastic resin. When the resin is polyethylene terephthalate, it uses infrared rays with a wavelength of 5.8 μm, and compares the thickness with a standard thickness (control) using the double beam method. However, the sheet thickness at a predetermined position can be measured with high precision, and a highly sensitive element made of indium antimony is used as the sensor. Such a thickness measuring device can be installed in any space from the position where the sheet leaves the cooling roll to the position where the sheet is rolled up, and it is possible to measure the change in thickness of the sheet while it is running. The thickness of the sheet can be measured at any position in the width direction. However, in the present invention, since the electrode positions are adjusted by means 5 and 6 so as to make the sheet thickness uniform, the sheet thickness can be adjusted at effective positions when adjusting the electrode positions. It is desirable to measure the pattern. Therefore, in the width direction of the sheet, changes in the thickness of the sheet over time can be controlled at positions near both ends of the sheet and at positions where there is a means for adjusting the distance between the electrode wire and the molten resin sheet, such as at the center of the sheet. Preferably, it is measured. The thickness of the sheet changes over time, in other words in the longitudinal direction, as shown by curve P in FIG.
The change in thickness is caused by a variable long period "undulation",
It is observed as a composite of small changes with short periods. That is, in Fig. 2, the frequency is 0.5
~2Hz waviness (fluctuation component; R ₁ , curve Q)
The combined fluctuation ₍ component;
R ₀ , curve P). In the present invention, the variation in sheet thickness detected by the thickness detection means 9 is detected in the above frequency range 0.5 to 2.
Hz is separated into 10 to 30 Hz, each variation component is calculated by the analysis means 10, and the optimum distance between the electrode and the resin sheet is set. FIG. 3 schematically shows the relationship between the position of the sheet and the wire electrode and the fluctuation component. If the distance between the electrode at the position corresponding to the predetermined position (the position of the electrode that causes the most change in the thickness of the part of the sheet whose thickness is being observed) and the resin sheet is moved so as to reduce the thickness variation of the waviness component, The magnitude of the waviness (the difference between the maximum value and the minimum value) as shown in the Q curve in FIG. 3 changes. It turns out that there is an optimal value for position adjustment. This adjustment can be carried out by trial and error, but if the manufacturing conditions are the same, it is possible to start production with the optimum value based on the actual product as the initial value and gradually optimize it. After repeating trial and error several times, you will come to know the following. That is, if the distance between the electrode and the molten sheet is too short, a strong charge will be applied to the molten sheet, but as a result of the sheet oscillating at this time, the sheet thickness tends to become uneven (see A in Fig. 3). region). Furthermore, if the electrode is too far away from the molten sheet, the electrostatic pinning effect will be weakened, the casting state will become unstable, and the sheet thickness will tend to become uneven (region C in FIG. 3). These conditions are shown in the Q curve as the fluctuation component in the 0.5-2 Hz frequency range and the distance between the electrode and the sheet in FIG. Next, it is necessary to determine the optimal wire electrode position for the fluctuation component in the frequency range of about 10 to 30 Hz. In this case as well, there is an appropriate distance between the electrode and the molten resin sheet that minimizes thickness unevenness. It must be noted that this striped component has an optimal distance only within a very narrow range. (See S curve in Figure 3). In the present invention, the position of the electrode wire corresponding to a predetermined position in the sheet width direction is adjusted based on the change in the thickness unevenness of the "wavy component", and furthermore, the change in the thickness unevenness of the striped component is also minimized. It is controlled by
Moreover, the conditions for determining the position of the electrode wire for optimizing the change in the waviness component and the adjustment of the change in striped thickness are performed completely independently. In the prior art, it is hardly known that adjustment of sheet thickness is performed as position control of electrode wires. Of course, it is presumed that they tried to obtain a high-quality sheet by determining the position empirically, but as in the present invention, by dividing the thickness unevenness into specific frequency regions,
By optimizing each component, the optimal position of the electrode wire (region B in FIG. 3) can be determined in a short time. Furthermore, the accuracy is higher than when processing is performed based on actual thickness variation data. The present invention measures the sheet thickness and independently adjusts the electrode position based on the analysis (calculation) result of separating the thickness fluctuation into long-period region fluctuation.
The optimum position is determined by this method, and this adjustment means and analysis means can be connected to perform automatic adjustment. [Example] Thickness (unstretched state) using the apparatus shown in Fig. 1
200 μm polyethylene terephthalate (intrinsic viscosity
A sheet of 0.60) was formed. For casting, the circumferential speed of the cooling roll was set at 37.5 m/min, and an infrared thickness measuring device (Japanese Patent Application Laid-Open No. 1983-1998) was used as a thickness detection means.
70904) was used. Measurements were taken at three locations, one at the center and at both ends of the sheet, three times for 3 seconds each, and the average value was used to repeat the adjustment in order. By adjusting the position of the wire electrode for the fourth time, a good sheet thickness was obtained. Achieved uniformity of quality.

[Table] Adjustments are made by sequentially measuring the thickness variation at the edge, center, and other edges of the sheet, then adjusting the wire electrode according to the waviness component, and then adjusting the variation in the striped component. , the first coarse adjustment at that position is completed. Repeat this adjustment at another position,
The first rough adjustment has been completed at three locations. When this process was repeated three times and the thickness unevenness average value of the 200 μm sheet was controlled to within 1.5 μm, the thickness unevenness of the 200 μm sheet was controlled to within 1.5 μm. The position of the wire electrode was connected to the wire support and the step motor, and the position of the wire electrode could be changed based on the analysis results, making it possible to adjust the position automatically and in a short time. . If a conversion table (calculation table) is prepared for each production condition, data with good reproducibility can be obtained between the analysis results and the motor drive amount. As explained above, the present invention aims to reduce sheet thickness unevenness by adjusting the electrode position in electrostatic pinning. The sheet thickness can be detected at positions other than the unstretched sheet, such as after one-stage stretching or after completion of biaxial stretching, and the electrode distance can be optimized based on the analysis result of the thickness variation. Therefore, the installation location of the thickness detection means can be arbitrarily selected. Furthermore,
In this example, approximately 1 Hz (0.5 to 2 Hz) and approximately 20 Hz (10 to 30 Hz) are used as analysis means to adjust the electrode position.
However, the frequency domain may be selected as appropriate based on the analysis of the periodicity of the thickness variation. Furthermore, in this embodiment, the electrode position is adjusted for each variation component by dividing into two parts, but it is also possible to separate the thickness variation component into three or more parts and adjust the electrode position based on each variation component. be. Since this example illustrates a wire electrode, the thickness of the sheet was measured at three points (the center and both ends).
Although the position of the wire was only adjusted at three locations, a method of adjusting the position of each individual needle electrode for a large number of needle electrodes is also within the scope of the technical idea of the present invention. [Effects of the Invention] According to the present invention, a resin sheet having an extremely uniform thickness can be obtained. And this uniformity of thickness is
This can be achieved by adjusting the distance between the electrode and the melted sheet in electrostatic pinning, and the production efficiency is extremely high in that the adjustment of the electrode position can be optimized automatically and in a short time.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of an apparatus implementing the invention. FIG. 2 is a curve showing the change in thickness of the thermoplastic resin sheet. Further, FIG. 3 is a curve schematically showing the state of change in thickness when the positions of the sheet and the electrode are changed. In the drawings, 1 is a die, 2 is a resin sheet, 3 is a cooling roll, 4 is an electrode, 5 and 6 are electrode position adjustment means, 9 is a sheet thickness detection means, 10 is a thickness variation analysis means, and 11 is a A primary stretching machine, 12 a secondary stretching machine, and 13 a winding means.

Claims (1)

[Scope of Claims] 1. When extruding a molten thermoplastic resin in the form of a sheet onto the surface of an electrically grounded cooling roll, a high DC voltage is applied to an exposed live electrode placed above the resin sheet. In a method for manufacturing a thermoplastic resin sheet in which the resin sheet is electrostatically brought into close contact with the cooling roll and cooled, the method includes a means for detecting a thickness change in the flow direction (longitudinal direction) of the resin sheet at a predetermined position in the width direction of the resin sheet. The detected thickness change is separated into thickness change components in multiple frequency regions by an analysis means, and the electrode and the resin sheet are separated at corresponding positions in the sheet width direction according to the thickness change components in each frequency region. Adjust the thickness change component to the minimum by changing the distance from A method for manufacturing a thermoplastic resin sheet, characterized in that a change in thickness over the entire width of the resin sheet is reduced by repeatedly adjusting the distance between an electrode at a position corresponding to a predetermined position and the resin sheet. 2 Detected thickness change in flow direction at 0.5 to 2 Hz
Separate the thickness change component in the frequency range of A method for producing a thermoplastic resin sheet according to claim 1, which comprises adjusting the distance. 3. The method for producing a thermoplastic resin sheet according to claim 1 or 2, wherein the thermoplastic resin is polyalkylene terephthalate. 4. The method for producing a thermoplastic resin sheet according to claim 1, wherein the live part exposed electrode is wire-shaped. 5. The method for producing a thermoplastic sheet according to claim 1, wherein the live portion exposed electrode comprises a large number of needle-like electrodes.