JPH0375026B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD This invention relates to an improved technique for an electrostatic pinning device (electrostatic adhesion device) used to obtain a film or sheet of flat and uniform thickness from thermoplastic resins such as polypropylene and polyethylene terephthalate. Conventional technology and its problems In film forming technology in which a thermoplastic resin sheet is cooled and solidified on the surface of a cooling roll, static electricity is applied to the melting sheet to electrostatically bring the melting sheet and the grounded cooling roll into close contact. A method for eliminating air that tends to be caught between the melting sheet and the cooling roll is known (see Japanese Patent Publication No. 37-6142). With this technique, the fused sheet is brought into close contact with the surface of the cooling roll, so the cooling effect is enhanced, a sheet of uniform thickness is obtained, and the production rate is maintained at a high level. When forming a thermoplastic resin film by electrostatic adhesion, it is important that the surface of the cooling roll be highly flat and smooth. Efficient cooling means that the surface condition of the cooling roll becomes the surface structure of the sheet. If the cooling roll has unevenness, the unevenness is transferred to the surface of the sheet to form protrusions or depressions. Therefore, unless the surface of the chill roll is carefully controlled to be flat and smooth, a good, flat and smooth sheet cannot be obtained. However, with the electrostatic adhesion technique, scars are often formed on the surface of the cooling roll. This is because abnormal discharge occurs from the electrode (usually a wire electrode or needle electrode) toward the cooling roll, resulting in scars on the roll surface due to thermal energy during the discharge. This abnormal discharge occurs when there is dielectric breakdown in the resin sheet due to a pinhole in the sheet, or when the electrode and the molten sheet become abnormally close to each other, resulting in an excessive potential gradient. This phenomenon occurs when dielectric breakdown occurs due to a heat sink, but it is difficult to predict in any case. Once the above-mentioned abnormal discharge occurs, scratches are created on the surface of the cooling roll, so the cooling roll must be replaced and the scratches repaired. As described above, electrostatic pinning technology is extremely effective when producing thermoplastic resin sheets under normal conditions, but once scratches occur on the surface of the cooling roll, it can lead to machine (production) stoppages. There is a need for fundamental measures to prevent abnormal discharge from occurring between the electrode and the cooling roll. OBJECT OF THE INVENTION The purpose of the present invention is to provide a new technique to solve this problem. In particular, since the occurrence of abnormal discharge is difficult to predict and impossible to avoid, it is possible to reduce the discharge energy during abnormal discharge as much as possible to weaken the scars that occur on the surface of the cooling roll. This paper presents an electrostatic adhesion device for stable film formation. Specific Solution In order to achieve the object of the present invention, the inventors discovered that in a DC high voltage power supply, by reducing the capacity of the rectifying smoothing capacitor, it is possible to suppress the energy during discharge, and the present invention was achieved. . That is, the present invention is an apparatus for electrostatically bringing a molten thermoplastic resin sheet into close contact with the surface of a cooling roll to cool and solidify it, which comprises: a cooling roll that is electrically grounded;
Consisting of an electrode that applies static electricity to the sheet, and a DC high voltage generator electrically connected to the electrode,
A static electricity adhesion device, characterized in that the DC high voltage generator is provided with a high voltage rectifying smoothing capacitor with a capacity of 5000 picofarad or less, and is provided with a function of quickly detecting a current change and shutting off the power supply in the event of an abnormal discharge. It is. The present invention will be explained. The first feature of the present invention is that a rectifying smoothing capacitor having a capacitance of 5000 picofarad or less, preferably 500 picofarad or less is applied to the DC high voltage generating power source. When an abnormal discharge occurs directly from the exposed electrode of the live part to the surface of the cooling roll, the size of the discharge mark formed on the surface of the cooling roll is the amount of discharge energy (i.e., the capacity of the rectifying smoothing capacitor of the DC high voltage power supply or the storage capacity). The capacitance of the rectifying smoothing capacitor should be set at 5000 picofarads, preferably 500 picofurads.
In other words, it was set to a value smaller than PicoFuratudo. If the discharge energy is small, the scars on the surface of the cooling roll due to abnormal discharge will naturally be small. However, if it becomes significantly smaller, the amount of static electricity applied to the resin sheet during normal operation will naturally decrease, so there is a lower limit. The distance between the electrode and the resin sheet, and the stability of that distance (does the position of the molten resin sheet hardly change or is it constantly shaking?)
The lower limit capacitor capacity also changes depending on the Unless it is well-regulated and produced at extremely high speeds, a lower capacity of about 100 picofurads is the lower limit. The second feature of the present invention is to quickly cut off the power supply in the event of an abnormal discharge, thereby suppressing continuous discharge from the electrodes. For this purpose, it is equipped with means for detecting current changes and a mechanism for shutting off the power in the event of abnormal discharge. The present invention will be explained with reference to the drawings. FIG. 1 is a schematic diagram of film forming equipment equipped with the electrostatic adhesion device of the present invention. In the figure, a thermoplastic resin is extruded into a sheet from a die 1, and this thermoplastic resin sheet 2 is cooled and solidified on the surface of a cooling roll 3. At this time, since high-voltage static electricity is applied to the sheet 2 from the electrode 4, the sheet 2 is brought into close electrostatic contact with the cooling roll which is electrically grounded. The electrode 4 is electrically connected to a DC high voltage power supply 5. FIG. 2 is a detailed diagram of the DC voltage power supply device. Based on the commercial power supply 6 (for example, a 220 volt or 440 volt AC power source), a number of
This is a device that generates a voltage of about KV to 30KV.
This device is equipped with a discharge detection circuit 7, a power circuit breaker 8 for cutting off the power in the event of abnormal discharge, a high voltage adjustment current circuit 9, a high voltage smoothing capacitor 10, and an output protection resistor 11. In the present invention, in particular, the capacitance of the smoothing capacitor 10 is set to 5000 picofarad or less, abnormal discharge is checked by the discharge detection circuit 7, and the power supply circuit breaker 8 is activated in the case of abnormal discharge. In the device of the present invention, a commercial power source is modulated and boosted to a high voltage by a high voltage adjustment current circuit, and converted into a pulsating DC voltage. The pulsating component is smoothed by a high-voltage smoothing capacitor and delivered to the DC high-voltage output terminal via an output protection resistor. This DC high voltage output terminal is electrically connected to the wire electrode shown in FIG. 1, and can apply an electrostatic charge to the thermoplastic resin sheet extruded from the die in a molten state. In the apparatus shown in FIGS. 1 and 2, discharge marks on the cooling roll when abnormal discharge occurs are detected by changing the capacity of the smoothing capacitor 10 and by changing the capacity of the power supply circuit breaker 8.
The operation time was varied and observed. Discharge marks on the surface of the cooling roll can be quantitatively evaluated using a metallurgical microscope (size of scratches on the surface) and a surface roughness meter (depth of scratches on the surface). Here, we used the following qualitative evaluation. 〇: The discharge marks are slight and the marks are not transferred to the molten sheet. ×: Discharge traces transferred to the molten sheet were generated. The observation results are shown in Table 1.

[Table] The situation in Table 1 can also be illustrated by the graph in Figure 3. The relationship between the relative magnitude of discharge energy and its duration is shown in Figure 3, which is for the experimental examples A, B, and C above.
It is clear that in the case of C where the discharge is significant, discharge marks that cause transfer occur on the cooling roll. Therefore, it can be seen that a DC high voltage power supply device having a characteristic such as characteristic A in FIG. 3 that minimizes discharge energy is preferable. The present invention can be applied to melt film formation of various thermoplastic resins. For example, polyethylene terephthalate,
Polybutylene terephthalate, polyethylene
Polyesters such as 2,6-naphthalene dicarboxylate, polyolefins, polyamides,
Suitable for casting polyvinyl, polyimide, etc. sheets. Effects of the Invention According to the present invention, the thermal energy at the time of abnormal discharge can be suppressed to a small level, so it is possible to produce sheets without leaving discharge marks on the surface of the cooling roll. Production stoppages due to roll replacement can be avoided.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram (side view) showing melt film formation of thermoplastic resin using the apparatus of the present invention, and Fig. 2 is a block diagram showing the configuration of the DC high voltage power supply device which is the main part of the apparatus of the present invention. FIG. 3 is a graph showing the relationship between discharge energy and abnormal discharge occurrence time in Examples and Comparative Examples of the present invention. The reference numbers in the drawings are as follows. 1...cap, 2...thermoplastic resin sheet, 3...cooling roll,
4... Wire-shaped electrode, 5... DC high voltage power supply device,
6...Commercial power supply, 7...Discharge detection circuit, 8...Power circuit breaker, 9...High voltage adjustment current circuit, 10...High voltage smoothing capacitor, 11...Output protection resistor, 12...DC high voltage output terminal.

Claims (1)

[Scope of Claims] 1. A device for electrostatically bringing a molten thermoplastic resin sheet into close contact with the surface of a cooling roll and cooling and solidifying it, which comprises an electrically grounded cooling roll and an electrode for applying static electricity to the sheet. , consisting of a DC high voltage generator electrically connected to the electrode, the DC high voltage generator is provided with a high voltage rectifying smoothing capacitor with a capacity of 5000 picofarad or less, and the current changes quickly in the event of abnormal discharge. A static electricity contact device characterized by having a function to detect and cut off power. 2. The electrostatic adhesion device according to claim 1, wherein the rectifying smoothing capacitor provided in the DC high voltage generator has a capacity of 500 picofarad or less.