EP0906789B1 - Process and apparatus for curtain coating of a travelling support - Google Patents

Description

The present invention relates to a method and a device according to the preamble of claim 1 or 3. This can be any coating material.

In all casting processes, but especially in the curtain casting process, the air, which is guided against the pouring point by friction from the non-cast carrier, creates problems. The pour point denotes the place where the liquid to be poured comes into contact with the carrier for the first time. In many coating processes, including curtain casting, this location is designed as a line and is referred to as a dynamic wetting line. The area near the carrier in which the air moves due to friction is called the boundary layer.

The following problems associated with the air boundary layer are known for curtain casting:

Watering means replacing the air on the carrier with a liquid. If this does not succeed, which can be the case especially at higher casting speeds, air is drawn between the carrier and the liquid film, and a coherent coating is then no longer possible.

Even if the air is not drawn in between the carrier and the liquid film, it strikes the back of the curtain with considerable force when viewed in the direction of movement of the carrier, especially at high pouring speeds. This leads to disturbances, especially in the area of the dynamic wetting line as diffuse non-uniformities in the coated film.

The air that is led through the carrier against the front of the curtain has to be exhausted somehow. To the side, upwards, backwards, but not in the direction of the carrier movement. This evacuation of the air is particularly difficult for wide carriers, which leads to the curtain being inflated like a balloon in the direction of the carrier movement. This causes on the one hand a deformation of the curtain, which can result in a reduction in the quality of the cast film, and on the other hand a deformation of the dynamic wetting line. This makes the curtain's casting behavior non-uniform across the casting direction, which in turn can lead to a loss of quality in the coated film, e.g. in the form of air pockets in the curtain edge zones.

Various methods and devices have been developed to eliminate or at least reduce the adverse effects described above. In the method and the device according to the preamble of claims 1 and 3, according to US-A-5 624 715, the boundary layer is blocked on the one hand by a body which is arranged in front of the curtain concentrically in the direction of movement of the carrier and at a short distance from the casting roll and on the other hand, the air flowing through the gap between body and wearer is drawn off through a slit nozzle and air is supplied to this location in a controlled manner.

EP-B-0 489 978 also uses a shield arranged concentrically around the casting roll in order to mechanically block the boundary layer. In addition, the air flowing through the concentric gap is drawn off in the middle of the body.

The main effect of both methods described above is based on the mechanical barrier of the air boundary layer. Although additional air is drawn off in both devices, a considerable amount of air still hits the back of the curtain, so that the quality losses described above can still occur.

In both devices, the suction device in particular is not designed efficiently due to its slot-like design. In EP-B-0 489 978 there is also a relatively large chamber between the suction slot and the concentric gap. The air is strongly swirled in this chamber so that the residual air impinges on the curtain with a non-uniform force in the direction transverse to the carrier movement and can therefore cause losses in quality. The suction device arranged in the middle of the body also causes air to be drawn from the space between the body and the curtain into the concentric gap against the carrier movement. This suction creates an additional air flow in front of the curtain and especially in the vicinity of the dynamic wetting line, which is non-uniform across the carrier movement and can therefore cause a loss of quality on the coated film.

Based on this prior art, it is an object of the present invention to provide a method and a device which produce an improved uniformity of the coated film. This object is achieved with the method and the device according to the independent claims. Further improvements and advantages result from the dependent claims.

Figur 1

zeigt im Schnitt einen Teil der erfindungsgemässen Vorrichtung,

Figur 2

zeigt das Geschwindigkeitsprofil im Spalt,

Figur 3

zeigt den Ort der Nullgeschwindigkeit der Spaltströmung, und

Figur 4

zeigt die Volumenströme für die Spaltströmung.

The invention is explained below with reference to a drawing of an embodiment.

Figure 1

shows in section a part of the device according to the invention,

Figure 2

shows the speed profile in the gap,

Figure 3

shows the location of the zero velocity of the gap flow, and

Figure 4

shows the volume flows for the gap flow.

The following description refers to an observation in the direction of the carrier movement, which is indicated by an arrow U in FIG. 1. The device according to the invention contains a body 1, which is arranged in front of the curtain 2 concentrically around the casting roll 3, and thereby forms a uniform gap 4 between the carrier surface 5 and the body 1 of 0.1-1.0 mm, preferably 0.3-0.6 mm, height H. A first cavity 6 is arranged in the first half of the body and extends over the entire width of the carrier surface and opens into a suction channel 7. A suction device known per se is connected to the suction channel.

On the carrier side, a first layer 8 of porous material is arranged in the first cavity 6, through which the air is drawn off. The cavity remaining behind the porous layer helps to ensure that the air is sucked off efficiently and uniformly over the entire length L ₂ and width of the porous layer. The length L ₂ is, for example, 15 mm.

In the second section of the body there is a second cavity 9, which likewise has a layer 10 made of porous material on the carrier side and to an air supply duct 11 is connected, whereby air is not drawn off through this second porous layer, but is fed in uniformly. The air feed, which is uniform across the carrier movement and has a length L ₄ of, for example, 10 mm, is also brought about by the cavity located behind the porous layer. The air supply duct is connected to an air supply device, not shown, so that the pressure and thus the volume flow of the air fed in can be set precisely.

The lengths L ₁ and L _{3 of} the body before and after the first porous layer 8 are dimensioned such that a fully developed flow occurs in the concentric gap 4. The minimum lengths for L ₁ and L ₃ depend primarily on the carrier speed and the gap height and are approximately 3 to 15 mm at a speed of 5 m / s and a gap height of 0.5 mm.

The air fed in causes a regulated air flow to develop in the gap between the first porous layer and the end of the gap against the curtain, which is characterized by a parabolic velocity profile such that the air in the upper part of the gap along the body from the curtain against the first porous layer and flows in the lower part of the gap along the carrier web in the direction of the curtain, see FIG. 2.

The layer thickness of the porous layers depends, among other things, on the porosity and the specific surface of the porous medium. The layer thickness must be dimensioned such that the pressure drop in the air flow through the porous layer is substantially greater, ie, for example by a factor of 100, than the pressure drop in the air flow transverse to the carrier movement in the channel behind the porous layer. at a porosity of 0.4 and a beam width of 1 m results in a layer thickness of approx. 8 mm.

u(y) =

Geschwindigkeit der Luft im Spalt

p₁ =

Umgebungsdruck hinter dem Vorhang

p₀ =

Unterdruck bei der Saugvorrichtung

Δp =

Druckdifferenz = (p₁-p₀)

µ =

Viskosität der Luft

L =

Länge des Spaltes

H =

Höhe des Spaltes

U =

Geschwindigkeit des Trägers

y =

Koordinate senkrecht zur Trägeroberfläche

The velocity profile in the concentric gap between the porous layers, along L ₃ , is characterized by the point P, which is located at a distance d from the carrier surface. The air speed at point P is zero, ie point P separates the forward flow in the direction of the carrier movement from the backward flow. The velocity profile u (y) for such a flow is calculated according to the Hagen-Poiseuille formula

u (y) =

Air speed in the gap

p ₁ =

Ambient pressure behind the curtain

p ₀ =

Vacuum in the suction device

Δp =

Pressure difference = (p ₁ -p ₀ )

µ =

Air viscosity

L =

Length of the gap

H =

Height of the gap

U =

Carrier speed

y =

Coordinate perpendicular to the surface of the beam

The distance d is calculated according to $$\mathit{\text{d}}\text{= -}\frac{\mathit{\text{a}}}{\text{2}}\text{-}\sqrt{{\left(\frac{\mathit{\text{a}}}{\text{2}}\right)}^{\text{2}}\text{-}\mathit{\text{b}}}$$ in which and $$\mathit{\text{b}}\text{=}\frac{\text{2µ}\mathit{\text{LU}}}{\text{Δ}\mathit{\text{p}}}$$ The distance d normalized with the gap height H is shown in FIG. 3 as a function of the pressure difference Δp and the location of the zero speed is thus obtained.

By integrating the speed profile over the gap height H, you get the air volume flow per gap width Q ₁ , which strikes the curtain, and the air volume flow per gap width Q ₂ , which has to be extracted:

The two volume flows are shown in FIG. 4 as a function of the pressure difference Δp.

For a given gap height, the amount of air hitting the curtain and the amount of air that has to be drawn off can be regulated as desired by setting the negative pressure p ₀ . In particular, it can be set as small as desired, thus minimizing the quality-reducing influences on the curtain. It is important that the amount of air that has to be extracted is not excessively extracted from the space between the curtain and body and thus causes disruptive air flows in front of the curtain, but that the amount of air is fed in through the porous layer at the outlet of the gap in the correct amount , in such a way that there are no disturbing air currents, both in front of the curtain and in the gap. Only the amount of air Q ₁ flowing through the gap in the direction of the moving beam and onto the curtain is allowed strikes from the space between the curtain and the body.

Claims (6)

1. Method for curtain coating a moving support with a coating material, where the air carried along by the moving support is extracted in front of the curtain, as seen in the running direction of the support, in a gap formed in parallel or concentrically to the support, and air is continuously supplied in a controlled manner, characterised in that by means of a body (1) comprising, in the running direction (U) of the support, a first cavity (6) provided with a following air suction channel (7) and, before the end of the body, a second cavity (9) with an adjacent air supply channel (11), both of these cavities (6, 9) extending across the entire width of the support and being provided on their sides facing the support with a layer (8, 10) of a porous material, the air supply is adjusted in function of the extracted air in such a manner that a parabolic velocity profile develops which comprises a point P at a distance d from the support where the air velocity is equal to zero.
2. Method according to claim 1, characterised in that the velocity profile v(y) is calculated according to the Hagen-Poiseuille formula where

   u (y) =   air velocity in the gap

   p₁ =   ambient pressure behind the curtain

   p₀ =   vacuum at the suction device

   Δ_p =   pressure difference = (p₁-p₀)

   µ =   air viscosity

   L =   gap length

   H =   gap height

   U =   support speed

   y =   coordinate perpendicular to the support surface.
3. A device for carrying out the method according to claim 1, comprising a coating roll (3) on which a support (5) is guided, and a suction (7) and air supply device (11) which in the direction of motion of the support are disposed in front of the curtain (2) and are provided on a body (1) forming a gap (4) and extending in parallel or concentrically to the support, characterised in that the body (1), seen in the running direction (U) of the support, comprises a first cavity (6) provided with a following air suction channel (7) and, before the end of the body, a second cavity (9) connected to an air supply channel (11), both of these cavities (6, 9) extending across the entire width of the support and being provided on their support sides with a layer (8, 10) of a porous material in such a manner that a uniform, concentric or parallel gap (4) is formed between the support (5) and the body (1).
4. Device according to claim 3, characterised in that the first cavity (6) is provided on the side facing the support with a layer (8) of a porous material.
5. Device according to claim 3 or 4, characterised in that the height (H) of the gap is comprised between 0.1 and 1.0 mm, more particularly between 0.3 and 0.6 mm.
6. Device according to any one of claims 3 to 5, characterised in that the minimum lengths (L₁) between the beginning of the body and the beginning of the first cavity (6) and (L₃) between the end of the first cavity and the beginning of the second cavity (9) are comprised between 3 and 15 mm.