CA1228476A

CA1228476A - Coater

Info

Publication number: CA1228476A
Application number: CA000453800A
Authority: CA
Inventors: Dan Eklund
Original assignee: Wartsila Oy AB
Current assignee: Wartsila Oy AB
Priority date: 1983-05-12
Filing date: 1984-05-08
Publication date: 1987-10-27
Also published as: BR8402361A; IT8420867A0; GB8412114D0; FI68770C; DE3416722A1; FR2545736B1; GB2141950A; JPS59213465A; SE459323B; SE8402368L; NO841914L; FR2545736A1; SE8402368D0; FI831672L; IT1173976B; FI831672A0; GB2141950B; FI68770B

Abstract

ABSTRACT

Apparatus for an one-sided or two-sided coating of a vertically travelling web comprising a coating applicator, in which said web is in contact with a coating substance held at atmospheric pressure, and a flexible blade co-operating with a back-up member for levelling a coating substance layer attached at the web. According to the invention, said applicator comprises a flow regulator which forms between it and the web a clearance. This clearance is mainly of even height or is inclined in the travel direction of the web. This height is smaller than a thickness of an eventual coating substance attached to a web without a co-operation with a regulator and the web. The applicator is further provided with means for arranging a gravity controlled over-flow and a backstream substance flux. The backstream flux is governed by the coating substance supply to the applicator, the viscosity of coating substance, the speed of the web travel and a slot of said applicator so that only a small amount of the coating substance flows downwardly through this slot during normal operation of said apparatus. Said slot forms a web entrance at the bottom of the applicator.

Description

ARRANGEMENT FOR EBB COATING

FIELD OF THE INVENTION

The invention relates to an apparatus for one-sided or two-Dodd coating of a basically vertically traveling web, for instance a paper web, by providing nonpressurized supply of coating substance onto the web and evening the coating substance sticking thereto by pulling the web through a coating nip formed by a flexible coaxing blade and a back-up member.

BACKGROUND OF THE INVENTION

In web coating arrangements, in which the web is pulled vertically through a nip formed by a blade or the like and a back-up member, the spreading of coating substance onto the web usually takes part prior the nip by means of a spreading lo apparatus. In a known arrangement the spreading apparatus comprises two rolls forming a nip through which the web is pulled. The coating substance is supplied onto at least one of these rolls the applicator roll, wherefrom it is transferred to and spread onto the web. At high web speeds, one difficulty is due to web slip in the spreading nip because the applicator roll moves slower than the web. This causes uneven coating substance transfer to the web. Another known coaler construction comprises spreading nozzles in I

lieu of applicator rolls. However, with this arran6eme~t it is difficult to spread the coating substance uniformly on the web. A common feature of both these known constructions it that, in the spreading phase, relatively great amounts of surplus coating substance are removed from the web and from the spreading unit. This excess amount has to be pumped back to the spreading point, and this forms an extra, us-economical step of the process. Further, another recycling circuit has to be provided for risque in the coating substance scraped off by the evening blade or blades. This complicates the construction end increases the manufacturing costs.

In coating units provided with a non-pressure coating applicator, that is, a coating substance applicator under atmospheric pressure, the substance attached to traveling web is leveled by a flexible blade. In such coating units, however, a phenomenon called barring causes problems in the form of coating layer variations in the longitudinal direction of the web, especially at high coating speeds (Giver 500 main Exactly why barring occurs has not been completely clarified. An improved embodiment of the invention is based on the assumption that one reason for the occurrence of barring is due to non-uniform thickness of the coaling layer on the web before the smoothing nip. This result in movements in the smoothing blade, and an uneven coating is produced because of these movements.

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US-Patent 3192895 discloses a coaler in which a coating chamber it provided by a metering bar, which regulates the flow of coating substance between said bar and the horizontally moved web. The drawback of this arrangement is that the gravity acts against coating substance between the bar and the flexible blade. The bar only forms an outlet slot adjustment but does not contribute to the level quality prior the flexible blade.

In an extrusion applicator, in which the coating substance lo is applied on the web under pressure, the situation is different. In applicators of this type, a fairly long smoothing gap is used in connection with the applicator for equalizing the coating substance pressure. The invention is not related to such coating methods.

OBJECT AND SUMMARY OF THE INVENTION

The object of the invention is to provide a web coating en-rangement, in which the spreading of the coating substance, also at high web speeds, is carried out in a reliable and economical way and with a homogeneous and even result. An arrangement according to the invention has low manufacturing costs and the amount of recycled coating substance is minimized. This results in an improved coating economy. Also the coating substance scraped off from the web by the coating nip blades, is collected for reuse- without any pumping action.

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According to the invention there is provided apparatus for an one-sided or two-sided coating of a vertically traveling web comprising a coating applicator, in which said web is arranged in contact with a coating substance held at atmospheric pressure, and a flexible blade co-operating with a back-up member for leveling a coating substance layer attached at said web, said applicator comprising a flow regulator which is arranged to form a clearance between said regulator and said web, said clearance being mainly of even height or being inclined in the travel direction of said web, said height being smaller than a thickness of an eventual coating substance attached to said web without a co-operation with said regulator and said web, said applicator further being provided with means for arranging a gravity controlled over-flow and a back stream substance flux, said back-stream flux being governed by the coating substance supply to said applicator, the viscosity of said coat-in substance, the speed of said web travel and a slot of said applicator so that only a small amount of said coating substance flows downwardly through said slot during normal operation of said apparatus, said slot forming a web entrance at the bottom of said applicator Thus, in the invention, the web is moved past a vertical coating applicator in which the web is in direct contact with the coating substance under atmospheric pressure.

pa Said applicator comprises a clearance formed between the web and a flow regulator, which clearance is of even height or arranged inclined in the web travel direction.
This height is adjusted smaller than a thickness of an eventual substance layer formed without said regulator.
At the bottom of the applicator there is a slot forming a web entrance, through which the web comes in contact with the coating substance. The coating substance supply, the width of the bottom slot, the viscosity of the substance and the speed of the web are so adapted, that they effect-lively govern gravity controlled back stream flux of coating substance through said slot. Hereby only a small amount of the coating substance flows downwardly through the slot during normal operation of the apparatus. The applicator is further provided with gravity controlled over-flow means.

The web is in direct contact with the coating substance in the applicator. The moving web then carries away coating substance, which contributes to a firm adhere once of coating substance all over the web surface.
The excess coating substance scraped off by the coating blade is, even at high web speeds collected for imp mediate reuse without any pumping means. Since only a small amount of coating substance passes through the bottom slot downwards, a column of coating sub-stance is formed in the coating chamber This I

provides a large and uniform contact between the web and the coating substance. The construction is economical as only a smell amount of coating substance flows out of the contact zone. The viscosity of the substance being optimized to the web speed, no substance flows out of the zone. Hereby no pumping means are necessary for substance recycling.

The coating substance level can easily be adjusted by means of a preferably adjustable overflow edge or duct as known per so. The applicator may have vertical adjustable side walls, the upper edge of which form the overflow edge. It should be noted that the coating applicator may have a length in the traverse direction of the web of 6...8 m in a full scale coaler. This means that also rather large end openings will nevertheless allow the applicator to remain substantially filled with coating substance.

The object of the invention is to reduce the risk of barring. This is possible by said flow regulator arranged crosswise to said web and at a distance therefrom The regulator forms between itself and said web a clearance extending in the travel direction of the web. In a preferred embodiment of the invention, this clearance converges in said travel direction. This arrangement considerably decreases the thickness variations of the coating substance layer adhering to the web before the smoothing blade, as well as other disturbances, and this substantially improves the quality and uniformity of the coating. The regulator can be positioned at the coating substance level in the applicator. This is, however not always absolutely necessary. The rear edge of the flow apply actor should be sharp to eliminate disturbing effects on the coating.

301ution resembling the improved embodiment of the invention Lo shown in several prior publications, like in Patent Specifications US 3,179,536, item 28, TJS 4,063,531, item 67 and item aye, FIX 55375 and SE 402 222. These known.
arrangements differs from said improved embodiment in that lo the flow regulating duct is not converging (US 3,179,536, FIX 55375, SE 402 222 and US 4,063,531, item aye) and in that there is no contact between the element in question and the coating layer on the web (US 4,063,531, item 67).

The height of the flow regulating clearance or duct is dependent on the dry solids content and the viscosity of the substance. If the coating substance contain much liquid and has a low viscosity, the clearance height should be reduced.
In practice, the average height of the flow regulating duct should be 0.5...5 mm.

I The length of the flow regulating duct in the web travel direction can be selected relatively freely. It has been found that the length is not of great importance. The length should be at least the same as the lowest duct height. A

recommendable duct length is 1...200 mm, preferably 10... 100 mm.

The flow regulating duct is converting in the longitudinal direction of the web. A suitable angle on convergence is

2...20, preferably 4...10. Hydrodynamics forces having 2 favorable effect on the uniformity of the coating layer occur due to the convergence of the duct. A converging duct should be of a suitable length, preferably at least 4 mm.

To ensure that the flow regulator operates correctly, it is important that the coating substance level if. the coating applicator remains basically constant. The surface-level regulation must be very accurate, if the distance between the web and the slow regulator is small. A sufficiently accurate surface level regulation it usually obtained by providing the coating applicator with an overflow system that automatically keeps the surface level at a desired height. On the other hand the economy of the over-flow system is back-up by the gravity operation.

A suitable back-up member for the coating nip in one-sided web coating can be formed as a plate extending vertically upward slightly above the coating nip and downwards to or past the bottom level of the collection tank or trough. The width of the plate should correspond to or exceed the web width. By this means the web is simply and firmly supported at its one side and the nip back-up member as well as the slot limiting member are formed by one single member. The adjustment of the clearance or the slot relative to the back-up member is easy to carry out for these stationary positioned elements. Another back-up member is a rotating roll.

In an arrangement according to the invention, the members located at opposite sides of the web may be fully or partly identical. For two-sided coating corresponding members can be arranged at both sides of the web, preferably at the same vertical level. These applicators may be supplied with the tame kind or with different kinds of coating substance. In the latter case, the slot widths and liquid level positions at both side may he considerably different.

In order to obtain an uniform coating result, the arrangement can be provided with a load member applying an adjustable pressure on the flexible coating blade, as known per so.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described more in detail with reference to the attached drawing, in which - Figure 1 is a schematic sectioned view of a first embodiment of the invention, in which a one-sided web I coating i 9 shown, - Figure 2 is a sectional side view of another embodiment in an apparatus according to the invention.

DESCRIPTION OF THE PREFERRED Embodiment erring descriptively to the drawing, reference numeral 1 indicates a moving web, which is pulled through a coating nip formed between a flexible blade 2 and a back-up member 5 I (Figures 1 and 2). The running direction of the web it indicated by an arrow. A narrow slot 5 is formed below said nip. Slot 5 is formed between a slot limiting member associated with the backup member 19 and the edge 11 of an adjustable slot width regulator 10. The slot width is lo infinitely variable up to a maximum of preferably 5 cm.
Back-up member 19 and the limiting member associated therewith may be separate or combined to a single member 7 which may be a roll, a flexible or a rigid plate or a fixed web guide member. A coating chamber is formed above slot 5.
This chamber is limited by the web 1, the flexible blade 2, a back wall 8 and side walls (not shown). The side walls may pass beyond the side edges of the web as obvious to a man skilled in art. In this case, it might be suitable to use, at that portion of the web, a properly arranged backup member (not shown). It is in most cases more convenient to slightly reduce the distance between the side walls so that their edges seal against the web. Thereby, a narrow section at each web edge is left uncoated. The upper portion 8b of the coating chamber is during operation of the apparatus more or less filled with coating substance due to the action of blade 2, which scrapes off surplus coating substance at lets it flow back to the lower portion of the coating 1 0 '.
chamber.

A purity plate 9 carries slot width regulator lo, which is movably suspended so as to allow adjustment of the width of 910t 5. Width regulator 10 may be provided with threaded members I so arranged, that turning of the members causes adjustment of the width of slot 5. The slot width regulator could alto have the form of a rotatable eccentrically ~ournalled bar extending crosswise to the web.

Coating substance 16 is supplied to the chamber through a lo supply duct 13. A collecting tank 12, open against web l, is arranged below slot 5. The small amounts of coating substance flown through slot 5 and collected in tank 12 are recycled to the chamber through an outlet duct 14, which via pumping means (not shown) it connected to supply duct 13.
Chamber is provided with means for regulating the level of coating substance contained therein. For example, coating substance may flow over the upper edge of the side walls into tank 12. The edges are preferably vertically adjustable.

The embodiment of Figure 1 includes a roll 19, which at the tame time forms the back-up member for flexible blade 2 and the limiting member for slot 5. Roll 19 is rotated in the running direction of web 1. There is an overflow wall 23 at the back of the coating applicator, keeping the level 24 of I the coating substance 16 at a constant height. At this I

height, the coating applicator has a flow regulator 26, forming a slightly converging duct 18 at the web. Flow regulator 26 reduces the amount 27 of coating substance following web 1 upwards into the upper portion 8b of the coating chamber. Layer 27 is substantially uniform in icons, which has a favorable influence on the function of coating blade 2 and thus helps to avoid barring. The distance between flow regulator 26 and the web and preferably also the angular position of the flow regulator relative to the web are adjustable.

Coating substance is pumped to applicator 8 through pipes 13, and coating substance flown over the overflow wall 23 is led to a recycling system through a pipe 33. The converging duct 18 between flow regulator 26 and the web is more clearly shown in Figure 2, which shows another back-up member 19. The angle of convergence d is about 10. The inlet-side duct height is a and the outlet-side duct height is b. The dimension a is in most cases 1.5...7 mm, preferably 2...4 my The duct length c should be greater than b and at least 4 mm, preferably 10... 100 mm.

Due to the convergence of duct 18, there will be a hydrodynamics force P in the duct. Expressed as force per width-unit in the Crosby direction of the flow regulator this force can be calculated from the known formula:

, p = gnu r in _ 2m-2 1 Tandy my , where 12 ~22 n - the viscosity of the coating substance - the speed of 'he web 1 d _ the angle of convergence of the dllct m - a/b (ratio of the duct height at the inlet end and at the outlet end) The hydraulic force P has an evening effect on the coating sub.c~ance layer 27, which improves the effect of the flow regulator 26.

The embodiments shown in Figure 1 and 2 are based on one-lo sided web coating. Basically symmetric set-ups relative to web 1 can be applied in two-sided web coating. Such embodiments are shown, for instance in British Patent Application 2085327. It is, however, not necessary to use a symmetrical coating arrangement. If, for instance, different coating substances with different viscosity are supplied to coating chambers at opposite sides of the web, the slot regulating edges 11 may be at different distances from web 1.

Collecting tank 12 is suspended at coating chamber wall 8.
Tank 12 can also be formed in one piece with wall 8 by pro-voiding the latter with a long, downwards extending portion bent at its lower edge towards web 1 to form a trough. The trough or tank 12 may extend outside the side edges of web 1 .

I

The back-up member of the coating nip and the limiting member of slot 5 may be combined into a single, mainly rigid web guiding plate.

The invention is not limited to the embodiments shown, but several modifications thereof are feasible within the scope ox the claims

Claims

Claims:

1. Apparatus for a one-sided or two-sided coating of a vertically travelling web comprising a coating applicator, in which said web is arranged in contact with a coating substance held at atmospheric pressure, and a flexible blade co-operating with a back-up member for levelling a coating substance layer attached at said web, said applicator comprising a flow regulator which is arranged to form a clearance between said regulator and said web, said clearance being mainly of even height or being in-clined in the travel direction of said web, said height being smaller than a thickness of an eventual coating substance attached to said web without a co-operation with said regulator and said web, said applicator fur-ther being provided with means for arranging a gravity controlled over-flow and a backstream substance flux, said backstream flux being governed by the coating sub-stance supply to said applicator, the viscosity of said coating substance, the speed of said web travel and a slot of said applicator so that only a small amount of said coating substance flows downwardly through said slot during normal operation of said apparatus, said slot forming a web entrance at the bottom of said applicator.

2. The apparatus of claim 1, wherein said clearance height inclination is arranged to converge in the travel direction of said web.

3. The apparatus of claim 1, wherein the height position of said flow regulator is at the level of said coating substance in said coating applicator.

4. The apparatus of any of claims 1 to 3, wherein the average height of said clearance between said flow regu-lator and said web is 0.5...5 mm.

5. The apparatus of claim 1, wherein the length of said clearance between said flow regulator and said web, in the travel direction of said web, is 1...200 mm, preferably 10...100 mm.

6. The apparatus of claim 1, wherein said flow regulator has a sharp edge forming the extreme end limit at the out-flow clearance end between said flow regulator and said web.

7. The apparatus of claim 5 or 6, wherein the length of said clearance between said flow regulator and said web, in the travel direction of said web, is at least as great as the smallest height of said clearance.

8. The apparatus of any of claims 1 to 3, wherein said clearance includes a length between said flow regulator and said web, in said travel direction, said length being at least 4 mm.

9. The apparatus of any of claims 1 to 3, wherein said clearance height includes an angle of the convergence between said flow regulator and said web of magnitude 2°...20°, preferably 4°...10°.

10. The apparatus of any of claims 1 to 3, wherein the level of said coating material is regulated by an over-flow system determining a constant level of the coating substance in said coating applicator.