Apparatus for stripping a boundary air layer from a traveling web

A web to be coated travels over a deflecting roll and underneath a slot nozzle from which a curtain of a coating material falls. A suction box open toward the roll and web has upstream and downstream ends and extends full axial length of the roll. An upstream seal on the upstream side of the box is juxtaposed with the web, extends the full axial length of the roll, and has upstream and downstream edges extending transverse to the web-travel direction. A similar downstream seal is provided on the downstream side of the box juxtaposed with the web. The downstream-seal downstream edge is spaced upstream in the direction from where the curtain falls on the web. Air is withdrawn from the box between the seals to suctionally strip a boundary layer of air from the web between the seals.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US national phase of PCT application PCT/EP02/03775, filed 5 Apr. 2002, published 17 Oct. 2002 as WO 02/081102, and claiming the priority of German patent application 10117667.8 itself filed 9 Apr. 2001.

FIELD OF THE INVENTION

The invention relates to an apparatus for suctionally stripping the boundary air layer off a web deflected over a roll as well as to a system for coating a traveling web that is provide with such a stripping apparatus.

BACKGROUND OF THE INVENTION

A traveling web, such as of paper or cardboard, plastic or metal foil, is coated by a so-called curtain coater which applies the coating material (glue dispersion, ink, etc.) as a curtain dropping from a slot nozzle onto the web. In order for the curtain coater to form a uniform coating it is essential that the curtain dropping from the slot nozzle be stable, not move, and not form bubbles on the web. Thus the boundary air layer that forms on the web at high web-travel speeds of more than 400 m/min creates problems. The boundary air layer adhering to the web creates irregular disturbances in the curtain and makes bubbles that create irregularities in the coating thickness.

A curtain coater in EP 0,489,978 for creating photographic materials has upstream of the slot nozzle a shield-like element (“air shield”) which serves for stripping off the boundary air layer. The element is positioned near a deflecting roll for the web and extends in an arc over a predetermined angular portion of the deflecting roll. The element is constructed such that its upstream edge and downstream edge form zones in which the wind resistance is greater than in the zone between the two end regions. In the zone between the zones of greater wind resistance there is a chamber that is connected to a suction line to reduce its pressure. The spacing between the seal-like elements at the upstream and downstream edges and the deflecting roll is between 0.5 mm and 2 mm. The spacing between the downstream edge of the shield-like element and the curtain is between 5 mm and 30 mm.

OBJECT OF THE INVENTION

It is an object of the invention to provide an apparatus that makes it possible to reduce the boundary air layer on a wide (2 m and more) web at high web-travel speed (1000 m/min and more) such that a curtain coater provided downstream applies the coating material uniformly.

SUMMARY OF THE INVENTION

This object is attained in that a suction box is provided at the deflecting roll and has the following features:

The suction box extends a full axial length of the deflecting roll, is open toward the deflecting roll, and is connected to the intake of a fan;

It has upstream and downstream seals that form respective gaps with the web and extend the full axial length of the deflecting roll, at least the downstream edge of the upstream seal and the upstream edge of the downstream seal being positioned where the web is deflected around the deflecting roll;

The suction box has at each axial end face of the deflecting roll an end seal that is parallel to and extremely closely spaced from the respective end face;

The downstream seal forms with the web a downstream gap whose height and width measured in the web travel direction are generally constant over the entire length of the suction box, this width being more than 10 mm, preferably more than 30 mm and in particular 50 mm–70 mm, and the gap having a dimension selected depending on the web travel speed such that the product of the web-travel speed (m/min)×gap width (in mm) is less than 1500.

According to the invention instead of the “air shield” there is a suction box that suctionally strips off the boundary air layer. It has been determined that the effectiveness is highly dependent on the shape and orientation of the downstream seals. They are set in accordance with the web-travel speed such that the boundary air layer on the web and at the ends of the deflecting roll is largely eliminated downstream of the seal. the curtain of coating material of a curtain coater provided in this region is thus not disturbed.

SPECIFIC DESCRIPTION

The coating system shown in the drawing serves for applying a dispersion-type adhesive (e.g. an acrylate) in an aqueous solution to a siliconized paper web1. As a result of its advantageous features the system can also serve for applying other dispersions to paper, plastic foils, or thin strips or to coat paper or cardboard webs with ink.

The coating system has a slot nozzle2that is suspended above the path of the web1in a frame. The slot nozzle2is connected to a supply of coating material and has on its lower side an outlet slot from which the coating material passes as a free-falling curtain3that lands on the web surface. The width of the slot nozzle is greater than the width of the web1being coated. Upstream of the slot nozzle2the web1passes around a deflecting roll4whose axial length extends over the entire working width of the coating system, that is the maximum width of a web1that can be coated.

At both axial ends of the coating system underneath the slot nozzle2are edge troughs5that limit the curtain3to the desired coating width. The edge troughs are both hung from the frame of the nozzle2so that they can move in or against the web-travel direction with the nozzle2. The edge troughs5catch the coating material at the curtain ends and conduct it to a collection pan6. The collection pan6is provided immediately upstream of the deflecting roll4.

When the coating process is started the slot nozzle2and the parts carried on it are moved above the collection pan6. Then at first a stable curtain3is produced that drops into the collection pan6. Subsequently the slot nozzle2is moved into its working position above the deflecting roll4(FIG. 1) or somewhat downstream therefrom (FIG. 3).

The embodiment ofFIG. 1serves to apply a coating with a width smaller than the web width. Thus the coated web1has coating-free edges. In the embodiment according toFIG. 3the web1is coated over its entire width. The curtain3is therefore wider than the web1. The slot nozzle2in its working position is downstream of the apex line of the deflecting roll4so that a pan7can be positioned underneath the curtain3to catch the coating material falling past the web edges. The web1is fed horizontally to the region underneath the slot nozzle2and then is deflected downward over a second deflecting roll8.

In order to suctionally strip the air boundary layer off the web1and the edges of the deflecting roll4there is a vacuum apparatus spaced upstream in the web-travel direction from the slot nozzle2as shown in large scale inFIGS. 2 and 4.

A suction box8in the deflection region of the deflecting roll4is open toward the deflecting roll4, to the web1, and is connected via a suction line9in its rear wall to the intake of a fan. The suction box8is long enough to extend over the entire axial length of the deflecting roll4. It has on is its upstream side an upstream seal10and on its downstream side a downstream seal11that each extend over the full axial length of the deflecting roll4and form respective gaps12and13with the web1. At least the downstream edge of the upstream seal12and the upstream edge of the downstream seal13are positioned in the region where the web1is deflected by the deflecting roll4. At each end of the deflecting roll4the suction box8has an end seal14that extends externally parallel at a very close spacing with the end face. For sufficient sealing at the end faces the seal elements14have lengths measured radially of the deflecting roll or at least 10 mm, preferably more than 15 mm, at the ends of the deflecting roll4.

The downstream seal11is so constructed and arranged that it forms with the web1the downstream gap13whose width measured parallel to the web travel direction and height are generally constant over the entire width of the suction box, that is over the entire axial length of the deflecting roll4. The width of the downstream gap13is more than 10 mm, preferably more than 30 mm and in particular 50 mm to 70 mm. The height of the downstream gap13is set according to the web travel speed such that the value of the product of web speed (in m/min)×gap height (in mm) is smaller than 1500. With a web travel speed of 1000 m/min the gap height is thus less than 1.5 mm.

The distance along the roll surface between the downstream edge of the upstream seal10and the upstream edge of the downstream seal11is more than 10 mm, preferably 50 mm–500 mm. In this region the boundary air layer is suctioned off the web1by the subatmospheric pressure in the suction box8. It has been seen that the vacuuming is improved when the upstream seal10and the downstream seal11are constructed such that they have sharp edges at their respective upstream and downstream edges. At least the downstream seal11is thus sharp at its upstream and downstream edges, and preferably the upstream seal10has at least at its downstream edge a similar sharp edge.

For uniform distribution of the subatmospheric pressure in the suction box8, it has at least one partition wall15formed with a uniform array of holes and extending the full width of the suction box8. Preferably the partition15is formed by a perforated plate that subdivides the suction box8into a rear compartment having the suction line9and a front compartment that is open toward the web1. The partition15also serves to rigidify the structure of the suction box8.

In order to better strip the air boundary layer the suction box8is provided with a corona-charge electrode16that is connected to a high-voltage source. Such a corona-charge electrode and its use in separating gaseous laminar boundary layers are described in detail in EP 0,837,824. The deflecting roll4, which is grounded and formed of a conductive material, typically metal, forms a counter electrode on the opposite web side. The charge electrode16extends over the full width of the suction box8and has a row of parallel points17directed at the web1. The applied high voltage causes the points17of the charge electrode16to emit electrons or positrons that create turbulence in the boundary air layer and thus cause it to spin off. The points17are spaced as closely as possible from the web, with a spacing from the web1of 0.1 mm–10 mm, preferably 1 mm–5 mm. The spacing transverse to the web between adjacent points17is about 5 mm.

In the coating systems shown inFIGS. 1 and 3the suction box8is positioned such that the distance from the downstream edge of the downstream seal11to the curtain3produced by the slot nozzle2is 10 mm–200 mm, preferably 20 mm–70 mm. Such a spacing not only ensures that no new boundary air layer can form that would disturb the curtain3, but also prevents disturbances of the curtain3by sucking in air through the downstream gap13. In order that in the coating system according toFIG. 3the small distance from the downstream seal11to the curtain3can be maintained, the suction box8used there and inFIG. 4is extended at its downstream end. The downstream end of the suction box8with the downstream seal extends substantially past the apex line of the deflecting roll4.

Advantageously the upper wall of the suction box8is constructed such that it only projects slightly above the periphery of the deflecting roll4. This makes it possible to mount the edge troughs5above the suction box and directly above the web path so that they can be inclined downward opposite the web-travel direction so that the caught coating material runs into the pan6. Preferably the suction box8can be pivoted away from the deflecting roll4so that a new web can be loaded into the coating system. In addition in this manner the front side open toward the web1is made accessible for servicing.