NL8502354A - DEVICE FOR ELECTROSTATIC FASTENING OF AN EXTRUSED POLYMER FILM. - Google Patents

Description

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Device for electrostatically pinning an extruded polymer film.

The invention relates to a device 5 for electrostatically pinning an extruded polymer film on an electrically grounded casting surface, which device comprises a wire electrode which extends transversely over the casting surface and is connected to a high voltage source, the wire electrode is arranged at a distance from the extruded film near or before the area of the first contact between the film and the casting surface and outside the film edges is electrically insulated by dielectric insulators movable along the wire electrode as corresponding to the edge of the extruded movie.

Such a device is known from the

Dutch patent application 70.06768. Although this patent application describes a slidable insulating sleeve 25 for adjusting the effective width of the electrode wire, it is not known from this application to change the location of the sleeve during the operation of the pin device. A mechanism for moving and adjusting the insulating sleeve is not described, so that the adjustment is done by hand. However, this is only possible when the pin wire is not energized, since the pin wire operates with a potential of, for example, 25 kV and a current of 200 microamperes, and the distance between the electrode wire and the film is only about 10 mm.

However, the molten extruded film will constrict between the extrusion die and the casting drum, and the extent to which this takes place may vary during film making, for example, by changing film thickness during production. It has been found that sparking can occur from the uninsulated portions of the electrode wire, which are exposed above the casting drum when the edges of the film move together (the film narrows).

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The object of the invention is to provide a device in which the insulators of the fixed pin electrode are placed during extrusion, so that fluctuations in the film width are compensated and the feature of this device is that a device for measuring the linear speed of the film and a drive for moving the insulators are provided, the signal obtained from the measuring device serving to actuate the drive for the insulators.

The invention is based on the finding that the transverse fluctuations of the film edges are proportional to changes in the linear velocity of the film. Thus, the displacement of the insulators can be controlled by continuing to monitor the speed of the film. This preferably takes place by coupling a potentiometer of 15 meters to the cooling device, for example to the drive for the casting drum, so that a reference voltage is obtained, corresponding to the film speed. A second potentiometer can be coupled to the drive for the carriers of the insulating sleeves, so that a reference voltage is obtained corresponding to the distance over which the insulating carriers have moved along their guides. When comparing the two reference voltages, an electrical signal is obtained which controls electric motors coupled to the insulator carriers and thus controls the displacement of the insulators themselves. When the reference voltages are in equilibrium, the insulators are properly positioned relative to the film edges and do not need to be moved further until edge fluctuations occur again.

Since volatiles released from the molten film can condense on the relatively cool surface of the wire electrode and interfere with electrostatic discharge and also corrode the wire, it is desirable to prevent or obstruct this condensation. This can take place by heating the wire electrode to a temperature above the condensation temperature, for example electrically or by means of a hot gas flow against the wire electrode. An advantage of a hot gas flow is that the gas flow also keeps the volatiles away from the wire electrode. The hot gas flow can also be directed between the wire electrodes and the film to prevent volatiles from reaching the wire electrode. Another possibility is to run a piece over clean wire / casting surface or continuously run a wire over the casting surface and around a disc on either side of the casting surface and then through a cleaning bath or between 10 scrapers or brushes to remove any condensate that may have deposited.

The film can then be subjected to known film-forming operations, such as drawing in one or more directions to molecularly orient the film, thermal curing and coating.

The casting surface can be of any suitable shape, such as a rotating drum with a highly polished surface, from which the film can be easily pulled off and which can be cooled by passing a coolant, or a continuously moving metal belt.

The dielectric insulators may advantageously be in the form of tubular members through which the wire electrode passes and which approximately correspond to the outer size of the wire electrodes, but are loose enough to allow the insulators to move along the wire electrodes. Any suitable dielectric material can be used to make the insulators, although a fluorocarbon polymer, such as polytetrafluoroethylene, is preferably used because of its low coefficient of friction and also because of its extraordinary dielectric properties. Polytetrafluoroethylene is commercially available under the trade name Fluon.

The dielectric insulators may be mounted on supports that can be moved along the guides enough to accommodate variations in film width. The carriers 35 can be provided with a rack, which is mounted in a driven tooth. - 4 - grips the wheel, which is usually driven by an electric motor.

The device according to the invention can be used for the rapid cooling of all polymeric materials, which can be formed into a flat film and must be rapidly cooled, for example polycarbonates, polyamides such as polyhexamethylene adipamide and polycaprolactam, polyimides, poly-sulfones, polymers and copolymers , of (X olefins such as ethylene, propylene, butene and 4-methylpentene-1, polymers and copolymers of vinyl monomers such as vinyl chloride, linear polyesters and copolyesters such as polyethylene terephthalate, polyethylene 2,6-naphthalate and polyethylene 1,5-naphthalate and polyester ethers such as polyethylene-1,2-biphenoxyethane-4,4'-dicarboxylate The invention is particularly suitable for the manufacture of polyethylene-15 terephthalate film.

The invention will now be elucidated with reference to the drawing, in which

Figure 1 is a perspective view of a web of molten film cast on the surface of a revolving drum,

Figure 2 is a top view of an embodiment of the invention;

Figure 3 is a top view of another embodiment of the invention.

In Figure 1, the casting of a web of molten polyethylene terephthalate 1 film onto the surface of a rotary polishing steel casting drum which is cooled to a temperature below 80 ° C by passing a coolant through the inlet 3 and the outlet 4 is shown. the shaft around which the 30 drum revolves. The cooled film 6 is removed from the drum surface 2 via a roller (not visible through the drum in figure 1) and is guided to a known molecular orientation and thermoforming device (not sheared).

A pin wire electrode 8 is in a tensioned condition close to the film surface and close to the point where 8502354-5 the web of sprayed film touches the surface of the casting drum. The wire electrode 8 is made of a nickel-chromium alloy and has a diameter of about 0.18 mm, a specific resistance 3 of 72 microohm / cm and a measured resistance of 30-35 Ohms, depending on the working temperature of the electrode.

The pin wire electrode 8 is connected to a high voltage generator (not shown in Figure 1), which transmits a voltage of 4.5 kV and a current of 720 microamperes to the electrode. Electrostatic charges are transferred to the molten film 1 through the wire electrode 8, drawing the film onto the surface 2 of the casting drum, which is held at the potential of the earth.

The film web 1 is narrower than the surface 2 of the casting drum, so that bare edge strips 9, 15 and 10 of the casting drum surface are exposed under the wire. For this purpose, the ends of the wire 8 above the exposed edge strips 9 and 10 are insulated with tubular insulators 12 and 13 of poly-tetrafluoroethylene, which just overlap the edges of the film web 1 and which can be placed along the wire electrode 8, so that fluctuations in the film edges are collected.

Figure 2 shows a mechanism that can be used to move the insulators 12 and 13 of Figure 1. For the sake of simplicity, the frame supporting the components of the device is omitted from Figure 2. The common parts of Figures 1 and 2 are indicated with the same reference numerals. The wire 8 is kept under slight tension between two fixed points 15 and 16, the last point being connected to the high voltage generator (not shown).

Tubular insulators 12 and 13 are tightly clamped in clamps 18 and 19, which are made of a dielectric material, such as reinforced urea or phenol formaldehyde resin, and are mounted on carriers 20 and 21. These carriers 20 and 21 are each in a fixed (not shown) guide parallel to the wire electrode 8 along which they can move freely.

Carriers 20 and 21 are each provided with a toothed rack 22 and Ö5Ö2 3 d 4 - 6 - 23, which engage in a gear 24 and 25, respectively. The gears 24 and 25 are driven by electric motors (not shown). The operation of the electric motors is controlled by an electric relay circuit, so that the carriers 20 and 21 run along their guides, the tubular insulators 12 and 13 move a distance corresponding to the displacement of the film edges and in the direction of the displacement of the film edges, whereby the ends of the insulators continue to slightly overlap the film edges and prevents arcing between the wire-electrode 10 and the exposed edge strips 9 and 10 of the casting drum.

According to the invention, the movement of the insulators 12 and 13 is controlled by comparisons of the output signals from potentiometers coupled to the drive for the casting drum 2 and the drive for the carriers 20 and 21. The potentiometer, which is connected to the drive of the casting drum 2 via gears, produces a comparative voltage corresponding to the film's linear velocity. The other potentiometer is connected to the drive for the toothed wheels 24 and 25 via gears 20, thereby obtaining a comparative voltage corresponding to the degree to which the wheels 24 and 25 are rotated to move the carriers 20 and 21 and thus the insulators 12 and 13. A signal obtained by comparing the 25 reference voltages of the potentiometers is used to control the operation of the electric motors driving the toothed wheels 24 and 25 thereby forcing the insulators 12 and 13 to follow the fluctuations of the film edges. When the reference voltages are unbalanced, the comparative signal forces the drive motors to adjust the position of insulators 12 and 13 until the comparative voltages are rebalanced. The insulators 12 and 13 are in the correct position relative to the film edges, when the equilibrium state is reached and no further movement of the insulators 12 and 13 is required, 8502 3 5 4 - 7 - until further fluctuations in the film edges performance.

The device shown in Figure 3 uses a wire electrode 8, which is pulled from a supply of clean wire wound on a spool 28, guided 5 across the casting drum 2 and onto a take-up spool 29, which is driven by an electric motor 30 is powered, wound up. In this device, wire contamination due to condensation of volatile material on a static wire electrode is avoided. The displacement and the voltage of the wire electrode 10 is controlled by a torque limiter 31. The wire electrode is energized by pulling the wire over a terminal 32 of the high voltage generator located adjacent to the take-up reel 29.

In order to position the wire electrode 8 accurately at a distance from the web of molten film 1 and the surface of the casting drum 2, it is passed over guide rollers 33 and 34 which are of a dielectric material such as a urea or phenol formaldehyde resin. as well as the tubular insulators 12 and 13 through which it is pulled.

The clamps 18 and 19, the carriers 20 and 21 and their racks 22 and 23 and the toothed wheels 24 and 25 are essentially the same as the corresponding parts of Figure 2 and they act similarly to the tubular insulators 25 12 and 13 as to move too far in response to fluctuations in the film edges, so that it is ensured that the insulators 12 and 13 are positioned such that sparking between the wire electrode 8 and the exposed edge strips 9 and 10 of the casting drum 2 is prevented.

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Claims (2)

1. Device for electrostatically pinning an extruded polymer film on an electrically grounded casting surface, said device comprising a wire electrode extending transversely across the casting surface and connected to a high voltage source, the wire electrode being spaced from the extruded film near or in front of the region of the first contact between the film and the casting surface and outside the film edges is electrically insulated by dielectric insulators movable along the wire electrode as corresponding to the edge of the extruded film, characterized in that a device for measuring the linear velocity of the film (1) and a drive (20-25) for displacing the insulators (12, 13) are provided, with the measuring device signal 15 obtained serves to activate the drive of the insulators (12, 13). Device according to claim 1, characterized in that the measuring device consists of a potentiometer coupled to the casting surface drive (2), which generates a comparative voltage corresponding to the linear speed of the film (1) and that a second potentiometer is coupled to the drive (20-25) for sliding the insulators (12, 13), the drive (20-25) being actuated by comparing the voltages supplied by the potentiometers. 25 7 | f »§ ii *. -s / · <* ·