EP0252954B1 - Electrostatic device for powder spraying with triboelectric powder charging - Google Patents

Electrostatic device for powder spraying with triboelectric powder charging Download PDF

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Publication number
EP0252954B1
EP0252954B1 EP87900639A EP87900639A EP0252954B1 EP 0252954 B1 EP0252954 B1 EP 0252954B1 EP 87900639 A EP87900639 A EP 87900639A EP 87900639 A EP87900639 A EP 87900639A EP 0252954 B1 EP0252954 B1 EP 0252954B1
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EP
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Prior art keywords
powder
powder pipe
pipe
particles
electrodes
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EP87900639A
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German (de)
French (fr)
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EP0252954A1 (en
Inventor
Gerhard Friedrich VÖHRINGER
Kurt Pfeifer
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Esb Elektrostatische Sprueh- und Beschichtungsanlagen Gf Vohringer GmbH
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Esb Elektrostatische Sprueh- und Beschichtungsanlagen Gf Vohringer GmbH
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Priority to AT87900639T priority Critical patent/ATE43517T1/en
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
    • B05B—SPRAYING APPARATUS; ATOMISING APPARATUS; NOZZLES
    • B05B5/00—Electrostatic spraying apparatus; Spraying apparatus with means for charging the spray electrically; Apparatus for spraying liquids or other fluent materials by other electric means
    • B05B5/025—Discharge apparatus, e.g. electrostatic spray guns
    • B05B5/047—Discharge apparatus, e.g. electrostatic spray guns using tribo-charging

Definitions

  • the invention relates to an electrostatic powder spraying device with triboelectric powder charging by means of a powder tube enclosing a conveying channel for the spray powder and at least one displacement body arranged in the flow channel to increase the flow rate, the powder tube being earthed on its outside and having a triboelectric material on at least on its inside the frictional electrical voltage series is arranged at a distance from the material of the powder particles to be sprayed.
  • Such a device is known from DE-B-2 203 351. This device does not require an electric field between the nozzle and the workpiece.
  • a comparable powder spraying device is known from DE-A-2 938 806, according to which the powder is charged in a tubular nozzle body, the flow channel of which is narrowed by a guide body in order to increase the speed and thereby generate a flow vortex.
  • the outside of the nozzle is grounded or connected to high voltage in order to separate the opposite charges of particles in the event of contact on the wall of the nozzle lined with triboelectric material or on the displacement body.
  • This is intended to improve the charging of the particles, but a separate electric field is used between the nozzle and the workpiece to guide the charged particles to the material.
  • the electrostatic field extends to the tips, edges and other protrusions of the workpiece, neutral zones form on the workpiece.
  • the coating powder can therefore only penetrate into corners and cracks of the workpiece to an insufficient extent. The coating therefore becomes uneven, which is a major reason why surface damage such as rust stains or the like occurs early on the coated workpiece.
  • the invention is based on the electrostatic powder spraying device defined at the outset and pursues the task of designing this device in the simplest possible manner in such a way that the precipitation of the coating powder is evened out on workpieces of complicated design and the precipitation efficiency is increased.
  • a reloading device for the triboelectrically charged powder particles with the same polarity adopted in the frictional charging is used to achieve this object.
  • the charging voltage of the powder particles can be increased to such an extent that these particles find their way to the workpiece without an electrostatic guide field, which is set up externally between the spraying device and the workpiece, and largely independently of other directional forces such as gas pressure flow forces and the like.
  • the powder particles can penetrate to the bottom of even complicated recesses on the workpiece, even the coating over the entire surface and improve protection against damage or destruction from the outside.
  • the recharging should primarily be carried out by electrodes fed from an electrical high-voltage line, such as radially inwardly projecting wire electrodes. However, it may be appropriate to carry out a further recharge shortly before the powder emerges from the nozzle, which under certain circumstances can again be done triboelectrically.
  • multiply provided displacement bodies which can be provided in the flow channel, but also in an upstream delivery channel, finally just before the nozzle emerges.
  • the supporting part of the device shown in FIGS. 1 and 2 is a powder tube (1), to which a flow conveying gas with a large number of powder particles uniformly distributed therein is fed from the feed end (2) according to arrow (J), which according to FIG. 1 are shown as positive charge carriers (4) and leave the powder tube (1) at its outlet end (5) again, in order to be guided to a workpiece which is at ground potential, in particular by means of electrostatic charge forces.
  • This discharge body (7) is connected by a line (8) to an earth connection (9).
  • the discharge body (7) expediently consists of highly conductive metal such as copper or brass
  • the powder tube (1) is expediently formed by an insulator, in particular an insulating plastic.
  • Polytetrafluoroethylene (PTFE) is preferred.
  • PTFE polytetrafluoroethylene
  • a particular advantage of this material is the big difference between its electricity constants and those of the most commonly used coating powders, especially epoxy resin.
  • a recharging device (12) which is arranged close to the outlet end (5), is used, for example, with two recharging electrodes (13) designed as wire electrodes, which project approximately radially into the conveying channel delimited by the inner surface (11) and through one high-resistance protective resistor (15) are connected to a high-voltage supply (17).
  • the friction charge is reinforced by two displacement bodies (21) and (22), which, like the powder tube (1), are made of PTFE and bear against the inside (11) of the powder tube with slight pressure.
  • Both displacement bodies each have a core (23) with a square cross section, which carries guide ribs (24) arranged in a cross shape on the outside, which run parallel to the pipe axis (16) with the same circumferential division and are provided with wedge-shaped bevels (25) at the ends of the displacement bodies.
  • the edges (28) of the guide ribs (24) can also be sharpened in a cutting shape on the inflow side (arrow 3).
  • Both displacement bodies (21, 22) are rotated by 45 ° to one another as shown in FIG. 2.
  • Each displacement body reduces the free flow cross-section in the flow channel (14) to less than half the available cross-sectional area.
  • the powder grains can not flow linearly, but are deflected in the circumferential direction and thereby swirled, which brings additional contacts with the boundary surfaces of the displacement body and the powder tube (1). This also takes place at about twice the speed, which significantly increases the effect of friction charging.
  • an axial hollow of the core (23) of the displacement body (21) is a cylindrical metal rod (26) which is connected to the respective displacement body , the powder tube (1) and its grounding body (7) is connected by a radially guided pin (27).
  • This pin serves both to ground the displacement body and to secure it in position. It also serves to form a counter electrode, which includes the grounding body (7) and the metal rod (26).
  • the coating powder which is distributed as evenly as possible in its conveying gas, is blown into the powder tube (1) in the direction of arrow (3) at the feed end (2). Due to the contacts with the inner surface (11) and the displacement bodies (21) and (22), more and more charge particles release their negative charge carrier (10) to the powder tube (1) and the metal body (26) after a division process.
  • the powder is positively charged triboelectrically.
  • This charge is amplified towards the outlet end (5) by the reloading device (12).
  • An electrostatic field is formed between the recharge electrodes (13) and the counterelectrode formed by the parts (7, 26, 27), which causes a charge equalization with the negative charge carriers (10) in accordance with the field lines (18) and at the same time that brings already positively charged charge carriers (4) to a higher charge potential.
  • the endeavor of the positively charged charge carriers (4) to balance the negative charge carriers (10) leads to the entire electrostatic field moving into the flow channel (10). This prevents the field from emerging from the nozzle opening (31) and thus the formation of electrically neutral zones (Faraday cages) on the workpiece. Due to the improved charging of the coating powder, higher powder throughputs can be achieved.
  • the operating voltage on the high-voltage supply (17) can be adapted accordingly to the individual operating conditions, for example to the materials used for the powder tube (1). of the coating powder and also the type and number of displacement bodies used be turned off. It can also be adjusted automatically according to operating data, such as the distance to the workpiece, the percentage of powder deposited, and also according to the current intensity.
  • the powder tube (1) is installed in the shaft (40) of a spray gun (50) within a shaft tube (58) in the manner known from DE-A-2 559 472, which is rotatably fixed between a head piece (52) and a foot piece (47) is held on the front of the gun housing (51).
  • a tubular nozzle bushing (62) is held inside by means of a nozzle sleeve (69), in which the flow channel (14) is widened from a rear cylinder section (30) to an annular nozzle opening (31).
  • An inner collar (32) of the head piece (52) fits smoothly between the powder tube and spray nozzle into the wall of the flow channel (14).
  • a nozzle rod (39) is held in this by means of axially extending radial webs (65) and carries an impact body (67) which is adjustable in length.
  • the recharge electrodes (13) are located at a distance from the nozzle opening (31) in the rear end of the nozzle bushing (62). They are connected in the manner known from the aforementioned DE-A-2 559 472 via a high-resistance protective resistor (75) to the high-voltage feed (17), which is led to the outside in the form of a high-voltage cable at the lower end of the pistol grip (33).
  • a feed channel (34) is provided in the gun handle (33) which, like the powder tube (1) itself, is connected to a bend-like shaped body (35) in the gun housing (51).
  • an injector nozzle (37) opens into the rear extension of the powder tube (1), to which compressed air is fed through a hose (38).
  • the wall of the conveying channel (34) generally consists of the same insulating material as the parts (1, 21 and 22), so that the powder is already charged with friction. Therefore, a displacement body (19) is also attached there, which has essentially the same effects as the displacement bodies (21 and 22).
  • an impact body (671) with its central bore (72) sits on the front end of a tubular nozzle rod (661) which is led out of the back of the gun housing (51) and connected to a compressed gas supply line ( 73) is connected.
  • This nozzle rod can be moved in the direction of the double arrow (74) and the distance of the impact body (671) from the nozzle opening (31) can thus be changed.
  • the compressed air from the bore (72) on the back (78) of the impact body is also radially deflected by a nozzle device (79) and takes the flow impinging on the front (77) at the ring edge (80) in order to prevent powder particles from adhering to the back of the impact body.
  • the nozzle bushing (621) is smooth cylindrical and shortened compared to the previously described embodiment. It is held in the head piece (52) by a nozzle sleeve (691) overlapping this head piece, on which in turn a nozzle cap (70) is seated.
  • the interior (71) formed between the aforementioned parts widens from the flow channel (14) via a first frustoconical surface (41) formed on the end faces of the nozzle bushing (621) and the nozzle sleeve to a cylindrical central space (42) and narrows down to the annular nozzle opening (311) along a truncated cone surface (43).
  • the guide body (44) is provided on its circumferential side with helically arranged or formed guide elements (45), which can be designed as grooves or as ribs, so that in each case helical narrow channels are formed through which the powder flow is passed, the Powder is additionally brought into connection with the guide body (44) and the wall of the nozzle cap (70).
  • the guide body like the nozzle cap (70), is expediently made of triboelectric material such as PTFE in order to open up a further possibility of reloading.
  • a tapered guide insert (46) attached to the end of the guide body (44) serves to introduce it into the screw channels.
  • the powder voltage is initially charged exclusively by friction contacts and then the powder voltage is increased by a recharging device (12) attached from the outside
  • a third charging process by triboelectric recharging is provided here.
  • the state of charge achieved by external charging can hardly be increased significantly.
  • it is ensured that the charge state is maintained until it emerges from the nozzle opening (311) during the process of swirling the emerging flow. That without an outside
  • the applied electrostatic guide field which is supplied to the workpiece by electrostatic forces, can therefore penetrate the workpiece into the electrically neutral zones of the workpiece much more easily and thereby improve the exposure efficiency.
  • the guide body (44) has the task of converting the high linear flow velocity into a rotating powder outlet. This process results in a better powder distribution at a reduced exit speed, which additionally leads to an increase in the coating efficiency.

Abstract

A stream of carrier gas with powder particles is introduced at the feeding end (2) of a powder pipe (1) in the direction of arrow (3) for coating workpieces. The material of the coating powder, of the powder pipe (1) and of displacement bodies (21, 22) built in the latter have different dielectric constants. At a certain distance from the outlet end (5) is mounted a charging device (12) provided with boosting electrodes (15) projecting inwardly and connected through a protective resistance (15) to a high-voltage supply (17). A rear counter-electrode (7, 26, 27) comprises an earthed conductive body (7). The displacement bodies (21, 22) accelerate the flow of powder particles. The latter come repeatedly in contact with the inner surface (11) of the powder pipe (1) and the displacement bodies (21, 22), communicating negatively charged particles (10) to them, after charge separation, whereas the positively charged particles (4) remain attached to the powder particles. The concentration of negatively (10) and positively (4) charged particles at opposite ends (2, 5) of the pipe is increased through the electrostatic field acting between the boosting electrodes (13) and the counter-electrode (7, 26, 27). The coating powder is thus so highly charged that an outer electrostatic field between a spraying electrode and the workpiece can be dispensed with.

Description

Die Erfindung betrifft eine elektrostatische Pulversprühhvorrichtung mit triboelektrischer Pulveraufladung mittels eines einen Förderkanal für das Sprühpulver umschließenden Pulverrohres und wenigstens eines zum Erhöhen der Durchströmungsgeschwindigkeit im Strömungskanal angeordnet en Verdrängungskörpers, wobei das Pulverrohr auf seiner Außenseite geerdet ist und wenigstens auf seiner Innenseite einen triboelektrischen Werkstoff aufweist der in der reibungselektrischen Spannungsreihe mit Abstand vom Werkstoff der zu versprühenden Pulverpartikel eingeordnet ist.The invention relates to an electrostatic powder spraying device with triboelectric powder charging by means of a powder tube enclosing a conveying channel for the spray powder and at least one displacement body arranged in the flow channel to increase the flow rate, the powder tube being earthed on its outside and having a triboelectric material on at least on its inside the frictional electrical voltage series is arranged at a distance from the material of the powder particles to be sprayed.

Eine derartige Vorrichtung ist aus DE-B-2 203 351 bekannt. Bei dieser Vorrichtung wird kein elektrisches Feld zwischen Düse aun Werkstück benötigt.Such a device is known from DE-B-2 203 351. This device does not require an electric field between the nozzle and the workpiece.

Eine vergleichbare Pulversprühvorrichtung ist bekannt durch die DE-A-2 938 806, wonach die Aufladung des Pulvers in einem rohrförmigen Düsenkörper bewerkstelligt wird, dessen Strömungskanal durch einen Leitkörper verengt ist, um die Geschwindigkeit zu erhöhen und dabei einen Strömungswirbel zu erzeugen. Zudem ist die Außenseite der Düse geerdet oder an Hochspannung gelegt, um die entgegengesetzten Ladungen von Teilchen bei einem Kontakt an der mit triboelektrischem Werkstoff ausgekleideten Wandung der Düse beziehungsweise am Verdrändungskörper zu trennen. Auf diese Weise soll die Aufladung der Teilchen verbessert werden, aber zur Führung der aufgeladenen Teilchen zum Werkstoff wird ein gesondertes elektrisches Feld zwischen der Düse und dem Werkstück eingesetzt. Da jedoch das elektrostatische Feld zu Spitzen, Kanten und anderen Vorsprüngen des Werkstückes verläuft, bilden sich am Werkstück neutrale Zonen. Das Beschichtungspulver kann daher nur in unzureichendem Umfang in Ecken und Ritzen des Werkstückes eindringen. Die Beschichtung wird daher ungleichmäßig, was ein wesentlicher Grund dafür ist, daß sich frühzeitig Oberflächenschäden wie Rostflecken oder dergleichen am beschichteten Werkstück einstellen.A comparable powder spraying device is known from DE-A-2 938 806, according to which the powder is charged in a tubular nozzle body, the flow channel of which is narrowed by a guide body in order to increase the speed and thereby generate a flow vortex. In addition, the outside of the nozzle is grounded or connected to high voltage in order to separate the opposite charges of particles in the event of contact on the wall of the nozzle lined with triboelectric material or on the displacement body. This is intended to improve the charging of the particles, but a separate electric field is used between the nozzle and the workpiece to guide the charged particles to the material. However, since the electrostatic field extends to the tips, edges and other protrusions of the workpiece, neutral zones form on the workpiece. The coating powder can therefore only penetrate into corners and cracks of the workpiece to an insufficient extent. The coating therefore becomes uneven, which is a major reason why surface damage such as rust stains or the like occurs early on the coated workpiece.

Die Erfindung geht aus von der eingangs definierten elektrostatischen Pulversprühvorrichtung und verfolgt die Aufgabe, diese Vorrichtung auf möglichst einfache Weise so zu gestalten, daß der Niederschlag des Beschichtungspulvers auch an kompliziert gestalteten Werkstücken vergleichmäßigt und der Niederschlagswirkungsgrad erhöht wird.The invention is based on the electrostatic powder spraying device defined at the outset and pursues the task of designing this device in the simplest possible manner in such a way that the precipitation of the coating powder is evened out on workpieces of complicated design and the precipitation efficiency is increased.

Zur Lösung dieser Aufgabe dient erfindungsgemäß eine Nachladevorrichtung für die triboelektrisch aufgeladenen Pulverpartikel mit der gleichen bei der Reibungsaufladung übernommenen Polarität.According to the invention, a reloading device for the triboelectrically charged powder particles with the same polarity adopted in the frictional charging is used to achieve this object.

Auf diese Weise laßt sich die Ladespannung der Pulverpartikel soweit erhöhen, daß diese Teilchen ihren Weg zum Werkstück ohne ein außen zwischen Sprühvorrichtung und Werkstück errichtetes elektrostatisches Führungsfeld und weitgehend unabhängig von anderen Richtkräften wie Gasdruck-Strömungskräften und dergleichen finden. Dadurch können die Pulverpartikel bis auf den Grund auch komplizierter Vertiefungen am Werkstück eindringen, die Beschichtung auf der ganzen Oberfläche vergleichmäßigen und den Schutz gegen Beschädigung oder Zerstörung von außen verbessern.In this way, the charging voltage of the powder particles can be increased to such an extent that these particles find their way to the workpiece without an electrostatic guide field, which is set up externally between the spraying device and the workpiece, and largely independently of other directional forces such as gas pressure flow forces and the like. As a result, the powder particles can penetrate to the bottom of even complicated recesses on the workpiece, even the coating over the entire surface and improve protection against damage or destruction from the outside.

Es ist daher wesentlich, daß die Reibungsaufladung durch Wahl der geeigneten triboelektrischen Stoffe und des Sprühpulvers sowie durch die Führung des Pulverstromes zu den gerätfesten Wandungen möglichst intensiv bzw. mit der notwendigen Strömungsgeschwindigkeit betrieben und dafür Sorge getragen wird, daß die sich geräteseitig bildende Aufladung abgeleitet wird.It is therefore essential that the frictional charge is operated as intensively as possible or with the necessary flow rate by selecting the appropriate triboelectric substances and the spray powder and by guiding the powder flow to the device-fixed walls, and care is taken to ensure that the charge which forms on the device side is discharged .

Die Nachladung sollte vornehmlich durch aus einer elektrischen Hochspannungsleitung gespeiste Elektroden wie radial nach innen ragende Drahtelektroden betrieben werden. Es kann jedoch angebracht sein, kurz vor Austritt des Pulvers aus der Düse eine weitere Nachladung vorzunehmen, die unter Umständen wieder triboelektrisch erfolgen kann.The recharging should primarily be carried out by electrodes fed from an electrical high-voltage line, such as radially inwardly projecting wire electrodes. However, it may be appropriate to carry out a further recharge shortly before the powder emerges from the nozzle, which under certain circumstances can again be done triboelectrically.

Besondere Bedeutung kommt ferner der Ausbildung und Anordnung von mehrfach vorgesehenen Verdrängungskörpern zu, die im Strömungskanal, aber auch in einem vorgeschalteten Förderkanal, schließlich kurz vor Düsenaustritt, vorgesehen sein können.Also of particular importance is the design and arrangement of multiply provided displacement bodies, which can be provided in the flow channel, but also in an upstream delivery channel, finally just before the nozzle emerges.

Weitere Merkmale und Vorteile der Erfindung sind in den Unteransprüchen festgehalten und in der folgenden Beschreibung vonFurther features and advantages of the invention are set out in the subclaims and in the following description of

Ausführungsbeispielen der Erfindung näher erläutert. Es zeigen

  • Fig. 1 einen Teil-Längsschnitt durch eine im übrigen schematisch dargestellte elektrostatische Pulversprühvorrichtung gemäß der Erfindung,
  • Fig. 2 einen Querschnitt nach der Linie II -II in Fig.1,
  • Fig. 3 eine teilweise geschnittene Seitenansicht einer erfindungsgemäß ausgebildeten Pulversprühpistole,
  • Fig. 4 eine erste Abwandlung dieser Pistole teilweise im Längsschnitt und
  • Fig. 5 eine Weiterbildung der Ausführung Fig. 4.
Embodiments of the invention explained in more detail. Show it
  • 1 shows a partial longitudinal section through an electrostatic powder spraying device according to the invention, which is otherwise shown schematically,
  • 2 shows a cross section along the line II -II in Fig.1,
  • 3 shows a partially sectioned side view of a powder spray gun designed according to the invention,
  • Fig. 4 shows a first modification of this gun partially in longitudinal section and
  • 5 shows a development of the embodiment of FIG. 4.

Tragender Teil der in den Fig. 1 und 2 dargestellten Vorrichtung ist ein Pulverrohr (1), dem vom Zuführende (2) her gemäß Pfeil (J) ein Strom-Fördergas mit einer Vielzahl in diesem gleichmäßig verteilter Pulverpartikel zugeführt wird, die gemäß Fig. 1 als positive Ladungsträger (4) dargestellt sind und das Pulverrohr (1) an dessen Auslaßende (5) wieder verlassen, um insbesondere mittels elektrostatischer Ladungskräfte zu einem Werkstück geführt zu werden, das an Erdpotential liegt.The supporting part of the device shown in FIGS. 1 and 2 is a powder tube (1), to which a flow conveying gas with a large number of powder particles uniformly distributed therein is fed from the feed end (2) according to arrow (J), which according to FIG. 1 are shown as positive charge carriers (4) and leave the powder tube (1) at its outlet end (5) again, in order to be guided to a workpiece which is at ground potential, in particular by means of electrostatic charge forces.

In die Außenfläche (6) des Pulverrohres ist als Teil einer Gegenelektrode ein geerdeter Ableitungskörper (7) in Form einer zylindrischen Buchse eingelassen, die mit der Außenfläche (6) des Pulverrohres bündig abschließt. Dieser Ableitungskörper (7) steht durch eine Leitung (8) mit einem Erdanschluß (9) in Verbindung.A grounded lead body (7) in the form of a cylindrical bushing, which is flush with the outer surface (6) of the powder tube, is embedded in the outer surface (6) of the powder tube as part of a counter electrode. This discharge body (7) is connected by a line (8) to an earth connection (9).

Während der Ableitungskörper (7) zweckmäßigerweise aus gut leitendem Metall wie Kupfer oder Messing besteht, wird das Pulverrohr (1) zweckmäßigerweise durch einen Isolator, insbesondere einen isolierenden Kunststoff gebildet. Bevorzugt wird Polytetrafluoräthylen (PTFE). Ein besonderer Vorteil dieses Werkstoffes liegt in dem großen Unterschied zwischen seiner Elektrizitätskonstanten und denen der meistverwendeten Beschichtungspulver, insbesondere Epoxidharz. Die einzelnen Pulverpartikel kommen beim Durchlauf durch den vom Pulverrohr (1) gebildeten Strömungskanal (14) mehrmals mit dessen Innenfläche (11) in Berührung, wobei an den zunächst neutralen Teilchen eine Ladungsteilung eintritt. Die negativen Ladungsträger (10) haften an der Innenfläche (11) des Pulverrohres, während die positiven Ladungsträger (4) mit den Pulverteilchen weiterwandern.While the discharge body (7) expediently consists of highly conductive metal such as copper or brass, the powder tube (1) is expediently formed by an insulator, in particular an insulating plastic. Polytetrafluoroethylene (PTFE) is preferred. A particular advantage of this material is the big difference between its electricity constants and those of the most commonly used coating powders, especially epoxy resin. When passing through the flow channel (14) formed by the powder tube (1), the individual powder particles come into contact with its inner surface (11) several times, a charge sharing occurring at the initially neutral particles. The negative charge carriers (10) adhere to the inner surface (11) of the powder tube, while the positive charge carriers (4) move on with the powder particles.

Zur Verstärkung des positiven Ladungszustandes der Pulverpartikel dient eine hier dicht an Auslaßende (5) angeordnete Nachladevorrichtung (12) mit zum Beispiel zwei als Drahtelektroden ausgebildeten Nachladeelektroden (13), die etwa radial in den durch die Innenfläche (11) begrenzten Förderkanal vorragen und durch einen hochohmigen Schutzwiderstand (15) an eine Hochspannungszuführung (17) angeschlossen sind.To boost the positive charge state of the powder particles, a recharging device (12), which is arranged close to the outlet end (5), is used, for example, with two recharging electrodes (13) designed as wire electrodes, which project approximately radially into the conveying channel delimited by the inner surface (11) and through one high-resistance protective resistor (15) are connected to a high-voltage supply (17).

Verstärkt wird die Reibungsaufladung durch zwei Verdrängungskörper (21) und (22), die ebenso wie das Pulverrohr (1) aus PTFE bestehen und mit leichtem Druck an der Innenseite (11) des Pulverrohres anliegen.The friction charge is reinforced by two displacement bodies (21) and (22), which, like the powder tube (1), are made of PTFE and bear against the inside (11) of the powder tube with slight pressure.

Beide Verdrängungskörper haben jeweils einen im Querschnitt quadratischen Kern (23), der außen kreuzförmig angeordnete Leitrippen (24) trägt, die unter gleicher Umfangsteilung parallel zur Rohrachse (16) verlaufen und zu den Enden der Verdrängungskörper mit keilförmigen Schrägen (25) versehen sind. Die Ränder (28) der Leitrippen (24) können auch auf der Zuströmseite (Pfeil 3) schneidenförmig angeschärft sein.Both displacement bodies each have a core (23) with a square cross section, which carries guide ribs (24) arranged in a cross shape on the outside, which run parallel to the pipe axis (16) with the same circumferential division and are provided with wedge-shaped bevels (25) at the ends of the displacement bodies. The edges (28) of the guide ribs (24) can also be sharpened in a cutting shape on the inflow side (arrow 3).

Beide Verdrängungskörper (21, 22) sind gemäß der Darstellung in Fig. 2 um 45° zueinander verdreht. Jeder Verdrängungskörper mindert den freiem Strömungsquerschnitt im Strömungskanal (14) auf weniger als die Hälfte der verfügbaren Querschnittsfläche. Zudem können die Pulverkörner nicht linear durchströmen, sondern werden in Umfangsrichtung abgelenkt und dadurch verwirbelt, was zusätzliche Kontakte mit den Begrenzungsflächen der Verdrängungskörper und des Pulverrohres (1) bringt. Dies erfolgt zudem bei etwa verdoppelter Geschwindigkeit, was die Wirkung der Reibungsaufladung erheblich steigert.Both displacement bodies (21, 22) are rotated by 45 ° to one another as shown in FIG. 2. Each displacement body reduces the free flow cross-section in the flow channel (14) to less than half the available cross-sectional area. In addition, the powder grains can not flow linearly, but are deflected in the circumferential direction and thereby swirled, which brings additional contacts with the boundary surfaces of the displacement body and the powder tube (1). This also takes place at about twice the speed, which significantly increases the effect of friction charging.

Um dabei die Aufladewirkung an den Begrenzungsflächen der Verdrängungskörper etwa gleich zu halten wie an der Innenfläche (11) des Pulverrohres, sitzt in einer axialen Höhlung des Kernes (23) des Verdrängungskörpers (21) ein zylindrischer Metallstab (26), der mit dem jeweiligen Verdrängungskörper, dem Pulverrohr (1) und dessen Erdungskörper (7) durch einen radial hindurchgeführten Stift (27) verbunden ist. Dieser Stift dient dabei sowohl der Erdung des Verdrängungskörpers als auch dessen Lagesicherung. Er dient ferner zur Bildung einer Gegenelektrode, welche den Erdungskörper (7) und den Metallstab (26) einschließt.In order to keep the charging effect on the boundary surfaces of the displacement bodies approximately the same as on the inner surface (11) of the powder tube, an axial hollow of the core (23) of the displacement body (21) is a cylindrical metal rod (26) which is connected to the respective displacement body , the powder tube (1) and its grounding body (7) is connected by a radially guided pin (27). This pin serves both to ground the displacement body and to secure it in position. It also serves to form a counter electrode, which includes the grounding body (7) and the metal rod (26).

Im Betrieb wird das in seinem Fördergas möglichst gleichmäßig verteilte Beschichtungspulver in Richtung des Pfeiles (3) am Zuführende (2) in das Pulverrohr (1) eingeblasen. Durch die Kontakte mit der Innenfläche (11) und den Verdrängungskörpern (21) und (22) geben immer mehr Ladungsteilchen nach einem Teilungsvorgang ihren negativen Ladungsträger (10) an das Pulverrohr (1) und den Metallkörper (26) ab. Das Pulver wird dadurch triboelektrisch positiv aufgeladen.During operation, the coating powder, which is distributed as evenly as possible in its conveying gas, is blown into the powder tube (1) in the direction of arrow (3) at the feed end (2). Due to the contacts with the inner surface (11) and the displacement bodies (21) and (22), more and more charge particles release their negative charge carrier (10) to the powder tube (1) and the metal body (26) after a division process. The powder is positively charged triboelectrically.

Diese Aufladung wird zum Auslaßende (5) hin durch die Nachladevorrichtung (12) verstärkt. Dabei bildet sich zwischen den Nachladeelektroden (13) und der durch die Teile (7, 26, 27) gebildeten Gegenelektrode ein elektrostatisches Feld aus, das entsprechend den eingezeichneten Feldlinien (18) einen Ladungsausgleich mit den negativen Ladungsträgern (10) bewirkt und gleichzeitig noch die bereits positiv geladenen Ladungsträger (4) auf ein höheres Ladungspotential bringt. Das Bestreben der positiv geladenen Ladungsträger (4) die negativen Ladungsträger (10) auszugleichen führt dazu, daß sich das gesamte elektrostatische Feld in den Strömungskanal (10) hineinbewegt. Dadurch wird der Austritt des Feldes aus der Düsenöffnung (31) und damit die Bildung elektrisch neutraler Zonen (Faradaysche Käfige) am Werkstück verhindert. Durch die verbesserte Aufladung des Beschichtungspulvers können höhere Pulverdurchsätze gefahren werden. Da sich das künstliche Hochspannungsfeld entgegen der Zuführrichtung des Pulvers weit in den Innenraum (14) des Pulverrohres (1) hinein erstreckt, läßt sich das bisher verwendete außenliegende Hochspannungsfeld zwischen Elektrode und Sprühteil vermeiden. Da sich auch am Werkstück keine neutralen Zonen ausbilden können, wird eine vollständige Beschichtung auch im Bereich tiefliegender Ecken und Innenkanten ermöglichtThis charge is amplified towards the outlet end (5) by the reloading device (12). An electrostatic field is formed between the recharge electrodes (13) and the counterelectrode formed by the parts (7, 26, 27), which causes a charge equalization with the negative charge carriers (10) in accordance with the field lines (18) and at the same time that brings already positively charged charge carriers (4) to a higher charge potential. The endeavor of the positively charged charge carriers (4) to balance the negative charge carriers (10) leads to the entire electrostatic field moving into the flow channel (10). This prevents the field from emerging from the nozzle opening (31) and thus the formation of electrically neutral zones (Faraday cages) on the workpiece. Due to the improved charging of the coating powder, higher powder throughputs can be achieved. Since the artificial high-voltage field extends far into the interior (14) of the powder tube (1) against the feed direction of the powder, the previously used external high-voltage field between the electrode and the spray part can be avoided. Since no neutral zones can form on the workpiece either, complete coating is also possible in the area of deep corners and inner edges

Die an der Hochspannungszuführung (17) liegende Betriebsspannung kann den individuellen Betriebsverhältnissen entsprechend angepaßt, beispielsweise also auf die verwendeten Werkstoffe des Pulverrohres (1). des Beschichtungspulvers und auch die Art und Anzahl der eingesetzten Verdrängungskörper abgestellt sein. Sie läßt sich ebenso nach Betriebsdaten, etwa dem Abstand zum Werkstück, dem Prozentsatz des niedergeschlagenen Pulvers auch nach der Stromstärke selbsttätig einregeln.The operating voltage on the high-voltage supply (17) can be adapted accordingly to the individual operating conditions, for example to the materials used for the powder tube (1). of the coating powder and also the type and number of displacement bodies used be turned off. It can also be adjusted automatically according to operating data, such as the distance to the workpiece, the percentage of powder deposited, and also according to the current intensity.

Nach Fig. 3 ist das Pulverrohr (1) in der durch die DE-A-2 559 472 bekannten Weise in den Schaft (40) einer Sprühpistole (50) innerhalb eines Schaftrohres (58) eingebaut, das drehfest zwischen einem Kopfstück (52) und einem Fußstück (47) an der Vorderseite des Pistolengehäuses (51) gehalten ist.According to FIG. 3, the powder tube (1) is installed in the shaft (40) of a spray gun (50) within a shaft tube (58) in the manner known from DE-A-2 559 472, which is rotatably fixed between a head piece (52) and a foot piece (47) is held on the front of the gun housing (51).

An der Vorderseite des Kopfstückes (52) ist innen mittels einer Düsenhülse (69) eine rohrförmige Düsenbuchse (62) gehalten, in welcher der Strömungskanal (14) von einem rückseitigen Zylinderabschnitt (30) zu einer ringförmigen Düsenöffnung (31) erweitert ist. Dabei fügt sich ein Innenbund (32) des Kopfstückes (52) glattflächig zwischen Pulverrohr und Sprühdüse in die Wandung des Strömungskanals (14) ein. Im vorderen Ende der Düsenbuchse (62) ist in dieser durch sich axial erstreckende Radialstege (65) ein Düsenstab (39) gehalten, der längseinstellbar einen Prallkörper (67) trägt.On the front of the head piece (52), a tubular nozzle bushing (62) is held inside by means of a nozzle sleeve (69), in which the flow channel (14) is widened from a rear cylinder section (30) to an annular nozzle opening (31). An inner collar (32) of the head piece (52) fits smoothly between the powder tube and spray nozzle into the wall of the flow channel (14). In the front end of the nozzle bushing (62), a nozzle rod (39) is held in this by means of axially extending radial webs (65) and carries an impact body (67) which is adjustable in length.

Die Nachladeelektroden (13) sitzen hier mit Abstand von der Düsenöffnung (31) im rückseitigen Ende der Düsenbuchse (62). Sie sind in der durch die zuvor erwähnte DE-A-2 559 472 bekannten Weise über einen hochohmigen Schutzwiderstand (75) an die Hochspannungszuführung (17) angeschlossen, die in Form eines Hochspannungskabels am unteren Ende des Pistolengriffes (33) nach außen geführt ist.The recharge electrodes (13) are located at a distance from the nozzle opening (31) in the rear end of the nozzle bushing (62). They are connected in the manner known from the aforementioned DE-A-2 559 472 via a high-resistance protective resistor (75) to the high-voltage feed (17), which is led to the outside in the form of a high-voltage cable at the lower end of the pistol grip (33).

Parallel zur Führung dieses Hochspannungskabels ist im Pistolengriff (33) ein Förderkanal (34) vorgesehen, der ebenso wie das Pulverrohr (1) selbst an einen krümmerartigen Formkörper (35) im Pistolengehäuse (51) angeschlossen ist. In den Umlenkbereich (36) mündet dabei in der rückseitigen Verlängerung des Pulverrohres (1) eine Injektordüse (37), der Druckluft durch einen Schlauch (38) zugeführt wird.Parallel to the routing of this high-voltage cable, a feed channel (34) is provided in the gun handle (33) which, like the powder tube (1) itself, is connected to a bend-like shaped body (35) in the gun housing (51). In the deflection area (36), an injector nozzle (37) opens into the rear extension of the powder tube (1), to which compressed air is fed through a hose (38).

Dadurch entsteht im Umlenkbereich (36) ein Unterdruck, durch den das Pulver im Förderkanal (34) hochgesaugt und im Druckstrahl der Düse (37) mit der Förderluft nochmals durchgemischt wird.This creates a negative pressure in the deflection area (36), through which the powder is sucked up in the conveying channel (34) and mixed again with the conveying air in the pressure jet of the nozzle (37).

Die Wandung des Förderkanals (34) besteht in der Regel aus dem gleichen IsolierWerkstoff wie die Teile (1, 21 und 22), so daß hier schon die Reibungsaufladung des Pulverseinsetzt. Daher ist auch dort ein Verdrängungskörper (19) angebracht, der im wesentlichen die gleichen Wirkungen entfaltet wie die Verdrängungskörper (21 und 22).The wall of the conveying channel (34) generally consists of the same insulating material as the parts (1, 21 and 22), so that the powder is already charged with friction. Therefore, a displacement body (19) is also attached there, which has essentially the same effects as the displacement bodies (21 and 22).

Abweichend von der vorbeschriebenen Ausführung sitzt nach Fig. 4 ein Prallkörper (671) mit seiner zentrischen Bohrung (72) auf dem vorderen Ende einer rohrförmigen Düsenstange (661), die nach hinten aus dem Pistolengehäuse (51) herausgeführt und an eine Druckgas-Zuführleitung (73) angeschlossen ist. Diese Düsenstange kann in Richtung des Doppelpfeiles (74) verschoben und damit der Abstand des Prallkörpers (671) von der Düsenöffnung (31) verändert werden. Während die weitgehend axial ausgestoßenen Pulverpartikel an der Vorderseite (77) des Prallkörpers auftreffen und schräg radial nach außen abgelenkt werden, wird die Druckluft aus der Bohrung (72) an der Rückseite (78) des Prallkörpers durch eine Düseneinrichtung (79) ebenfalls radial abgelenkt und nimmt an der Ringkante (80) die auf die Vorderseite (77) auftreffende Strömung mit, um ein Anhaften von Pulverpartikeln auf der Rückseite des Prallkörpers zu verhindern.4, an impact body (671) with its central bore (72) sits on the front end of a tubular nozzle rod (661) which is led out of the back of the gun housing (51) and connected to a compressed gas supply line ( 73) is connected. This nozzle rod can be moved in the direction of the double arrow (74) and the distance of the impact body (671) from the nozzle opening (31) can thus be changed. While the largely axially ejected powder particles hit the front (77) of the impact body and are deflected diagonally radially outwards, the compressed air from the bore (72) on the back (78) of the impact body is also radially deflected by a nozzle device (79) and takes the flow impinging on the front (77) at the ring edge (80) in order to prevent powder particles from adhering to the back of the impact body.

Nach Fig. 5 ist die Düsenbuchse (621) glatt zylindrisch und gegenüber der vorbeschriebenen Ausführung verkürzt. Sie wird im Kopfstück (52) durch eine dieses Kopfstück übergreifende Düsenhülse (691) gehalten, auf der wiederum eine Düsenkappe (70) sitzt.5, the nozzle bushing (621) is smooth cylindrical and shortened compared to the previously described embodiment. It is held in the head piece (52) by a nozzle sleeve (691) overlapping this head piece, on which in turn a nozzle cap (70) is seated.

Der zwischen den vorerwähnten Teilen gebildete Innenraum (71) erweitert sich vom Strömungskanal (14) über eine erste an den Stirnflächen der Düsenbuchse (621) und der Düsenhülse angeformte Kegelstumpffläche (41) zu einem zylindrischen Mittelraum (42) und verengt sich bis zur ringförmigen Düsenöffnung (311) entlang einer Kegelstumpffläche (43). In dem zylindrischen Mittelraum (42) des Innenraumes (71) sitzt undrehbar ein Führungskörper (44), der den Düsenstab (662) mit dem Prallkörper (67) trägt.The interior (71) formed between the aforementioned parts widens from the flow channel (14) via a first frustoconical surface (41) formed on the end faces of the nozzle bushing (621) and the nozzle sleeve to a cylindrical central space (42) and narrows down to the annular nozzle opening (311) along a truncated cone surface (43). A guide body (44), which carries the nozzle rod (662) with the impact body (67), is non-rotatably seated in the cylindrical middle space (42) of the interior (71).

Der Führungskörper (44) ist an seiner Umfangsseite mit schraubenförmig angeordneten bzw. ausgebildeten Leitelementen (45) versehen, die als Nuten oder als Rippen ausgebildet sein können, so daß in jedem Fall schraubenförmige enge Kanäle entstehen, durch welche die Pulverströmung hindurchgeführt wird, wobei das Pulver zusätzlich mit dem Führungskörper (44) und der Wandung der Düsenkappe (70) in Verbindung gebracht wird. Der Führungskörper wird zweckmäßigerweise ebenso wie die Düsenkappe (70) aus triboelektrischem Werkstoff wie PTFE hergestellt, um eine weitere Nachlademöglichkeit zu eröffnen. Zum Einleiten in die Schraubenkanäle dient dabei ein stirnseitig am Führungskörper (44) angebrachter kegelartiger Leiteinsatz (46).The guide body (44) is provided on its circumferential side with helically arranged or formed guide elements (45), which can be designed as grooves or as ribs, so that in each case helical narrow channels are formed through which the powder flow is passed, the Powder is additionally brought into connection with the guide body (44) and the wall of the nozzle cap (70). The guide body, like the nozzle cap (70), is expediently made of triboelectric material such as PTFE in order to open up a further possibility of reloading. A tapered guide insert (46) attached to the end of the guide body (44) serves to introduce it into the screw channels.

Während bei den Ausführungen der Fig. 1 bis 4 zunächst ausschließlich durch Reibungskontakte aufgeladen und dann durch eine von außen angesetzte Nachladevorrichtung (12) die Pulverspannung gesteigert wird, ist hier ein dritter Aufladevorgang durch triboelektrische Nachladung vorgesehen. Auf diese Weise kann zwar der durch die Fremdaufladung erzielte Ladezustand kaum noch wesentlich erhöht werden. Es wird aber sichergestellt, daß der Ladungszustand bis zum Austritt aus der Düsenöffnung (311) während des Vorganges der Verwirbelung der austretenden Strömung erhalten bleibt. Das so ohne ein von außen angelegtes elektrostatisches Leitfeld durch elektrostatische Kräfte zum Werkstück geführte Beschichtungspulver kann daher wesentlich leichter in elektrisch neutrale Zonen des Werkstückes eindringen und dadurch den Beaufschlagungs-Wirkungsgrad verbessern. Neben den elektrischen Vorteilen hat der Führungskörper (44) die Aufgabe, die hohe lineare Strömungsgeschwindigkeit in einen rotierenden Pulveraustritt umzuwandeln. Durch diesen Vorgang wird eine bessere Pulververteilung bei reduzierter Austrittsgeschwindigkeit erreicht, was zusätzlich zur Erhöhung des Beschichtungswirkungsgrades führt.While in the embodiments of FIGS. 1 to 4, the powder voltage is initially charged exclusively by friction contacts and then the powder voltage is increased by a recharging device (12) attached from the outside, a third charging process by triboelectric recharging is provided here. In this way, the state of charge achieved by external charging can hardly be increased significantly. However, it is ensured that the charge state is maintained until it emerges from the nozzle opening (311) during the process of swirling the emerging flow. That without an outside The applied electrostatic guide field, which is supplied to the workpiece by electrostatic forces, can therefore penetrate the workpiece into the electrically neutral zones of the workpiece much more easily and thereby improve the exposure efficiency. In addition to the electrical advantages, the guide body (44) has the task of converting the high linear flow velocity into a rotating powder outlet. This process results in a better powder distribution at a reduced exit speed, which additionally leads to an increase in the coating efficiency.

Liegen die Werkstoffe in der reibungselektrischen Reihe entgegengesetzt zum obigen Beispiel, daß also die Ladungsträger am Pulverrohr positiv und an den Pulverteilchen negativ sind, so muß auch an den Nachladeelektroden (13) eine negative Spannung angelegt werden.If the materials in the frictional electrical series are opposite to the example above, that is to say that the charge carriers on the powder tube are positive and negative on the powder particles, then a negative voltage must also be applied to the recharging electrodes (13).

Claims (22)

1. Electrostatic device for powder spraying with triboelectric powder charging by means of a powder pipe (1), which surrounds a feed duct (14) for the spray powder, and by means of at least one displacement element (21, 22), which is disposed in the flow duct to increase the flow rate, the powder pipe (1) being earthed on its outer surface (6) and having, at least on its inner surface (11), a triboelectric material which is in electrostatic series at a distance from the material of the powder particles to be sprayed, characterised by a subsequent charging device (12) for the triboelectrically charged powder particles, having the same polarity which is acquired during frictional charging.
2. Device according to claim 1, characterised in that the triboelectric material is formed by an insulating plastics material, are especially polytetrafluoroethylene.
3. Device according to claim 1 or 2, characterised in that the displacement element (21, 22) is provided, at least externally, with preferably the same triboelectric material as the powder pipe (1) has internally.
4. Device according to claim 3, characterised in that powder pipe (1) and displacement element (21, 22) are each formed predominantly integrally from triboelectric material.
5. Device according to one of claims 1 to 4, characterised by a supply means (37, 38) for supplying acceleration air, which supply means serves to increase the flow rate further.
6. Device according to claim 1, characterised in that the earth terminal of the powder pipe has an earthed shunt element (7) formed from a material which is a good electrical conductor, said shunt element being mounted on the outer surface of the powder pipe and being more especially tubular.
7. Device according to one of claims 1 to 6, characterised in that the subsequent charging device (12) has boosting electrodes (13) disposed within the powder pipe (1).
8. Device according to claim 7, characterised in that the boosting electrodes (13) are formed by wire electrodes which, more especially, protrude radially inwardly from the wall of the powder pipe (1
9. Device according to claim 7 or 8, characterised in that the boosting electrodes (13) are mounted at a distance from the outlet end (5) of the powder pipe (1), and they have associated therewith a counter-electrode (7, 26, 27), which is, more especially, earthed.
10. Device according to claim 7, 8 or 9, characterised in that the boosting electrodes (13) are connected to a high-voltage supply (17) via a high-ohmic protective resistance (15).
11. Device according to one of claims 1 to 10, characterised in that at least one displacement element (21, 22) has a plurality of guide ribs (24) which are inclined relative to one another and are axially disposed in the powder pipe (1).
12. Device according to claim 11, characterised in that a plurality of displacement elements (21, 22), which are disposed one behind the other in the direction of flow, are disposed so as to be rotatable about the axis (16) of the powder pipe (1 relative to one another.
13. Device according to claim 11 or 12, characterised in that at least one displacement element (21, 22) tapers towards at least one end, the guide ribs (24) extending more especially in a wedge-shaped manner.
14. Device according to one of claims 1 to 13, characterised in that at least one displacement element (21, 22) is retained in a non-rotatable manner on the inner surface (11) of the powder pipe (1) in conductive contact.
15. Device according to claim 6 and possibly one of claims 11 to 14, characterised by a metallic connection (27) between an earthed shunt element of the powder pipe (1) and a connector (26), which is preferably metallic, of a displacement element (21, 22), said metallic connection serving more especially to prevent rotation.
16. Device according to one of claims 1 to 15, having a baffle member (671), which is disposed upstream of the outlet end (5) of the powder pipe (1) and is situated on a rod which extends through the powder pipe (1), characterised in that the rod is in the form of a hollow rod (661) and has an air supply for supplying air to the baffle member (671).
17. Device according to one of claims 7 to 10, characterised in that the powder pipe (1) has a widened portion (30, 31) between the boosting electrodes (13) and the outlet end (5).
18. Device according to claim 17, characterised in that a guide member (44) is inserted in the widened portion of the flow duct (14, 31, 32) and is provided with guide faces (45) directed towards the circumferential direction.
19. Device according to claim 18, characterised in that the outlet end (5) of the powder pipe (1) narrows again (43) from the widened portion (71, 42).
20. Device according to claim 18, characterised in that the guide member (44) is made of tetrafluoroethylene.
21. Device according to claim 20, characterised in that the guide member (44) is in the form of a holder for the baffle member.
22. Device according to claim 20, characterised in that the guide member (44) is installed to supply air to the end face of the baffle member (67).
EP87900639A 1986-01-14 1987-01-09 Electrostatic device for powder spraying with triboelectric powder charging Expired EP0252954B1 (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AT87900639T ATE43517T1 (en) 1986-01-14 1987-01-09 ELECTROSTATIC POWDER SPRAYING DEVICE WITH TRIBOELECTRIC POWDER CHARGING.

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
DE19863600808 DE3600808A1 (en) 1986-01-14 1986-01-14 ELECTROSTATIC POWDER SPRAYING DEVICE WITH TRIBOELECTRIC POWDER CHARGING
DE3600808 1986-01-14

Publications (2)

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EP0252954A1 EP0252954A1 (en) 1988-01-20
EP0252954B1 true EP0252954B1 (en) 1989-05-31

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Country Status (5)

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US (1) US4798340A (en)
EP (1) EP0252954B1 (en)
JP (1) JPH0673646B2 (en)
DE (2) DE3600808A1 (en)
WO (1) WO1987004088A1 (en)

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Also Published As

Publication number Publication date
US4798340A (en) 1989-01-17
DE3600808A1 (en) 1987-07-16
WO1987004088A1 (en) 1987-07-16
DE3760194D1 (en) 1989-07-06
JPS63502408A (en) 1988-09-14
EP0252954A1 (en) 1988-01-20
JPH0673646B2 (en) 1994-09-21

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