CA1196780A - Coating apparatus - Google Patents

Abstract

TITLE
COATING APPARATUS
ABSTRACT OF THE INVENTION
A slide coating apparatus wherein the angle between the slide surface and a horizontal datum plane lies within a range from 35° to 50° and wherein the takeoff angle defined between a tangent to the coating roll and the slide surface lies within a range from 85° to 100°.

Description

6~

TITLE
. ~ _ COATING APPARATUS

BACKGROUNU OF THE INVENT~ON
This invention relates to slide coating .
apparatus and in particular to a slide coating apparatus wherein the slide surface angle and ta~eofr angle lie within predetermined limits.
A conventional multi-slide coating apparatus, as disclosed in United States Patent

2,761,~17 (Russell et al.), per~orms coating operations by metering individual layers ~f a coating fLuid from a supply, through a trough disposed insid~
the hopper and then through a narrow distribution slot. A coating fluid is distributed by the slot as a layer uniformly across a downwardly inclined slide sur~ace. Such layer o~ coating ~luid ~lows by gravity down the slide sur~ace and meets with the adjacent underlayers o~ coating ~luid which have been similarly metered and distributed through narrow slot~. The cambined coating ~luid layers then flow down the slide sur~ace and ~orm a ooating bead. The web to b~ coated is carried by a backup roll and is moved across the bead. The ~luid layers in the bead impinge upon the moving web which picks up the multilayer coating ~luids from the slide surface.
A cascade c~ater is a slide coating apparatus in which the impingement point between the multilayer coatin~ ~luids and the web is located above the horizontal center line of the backup roll supporting the web to be coated. Exemplary o~ such a device is that shown in United States Patent

3,289,632 (Barstow). British Patent Speci~ication 1,~68,144 discusses the desirability o~ having the slide sur~ace o~ a cascade coater de~ine an angle of D-1933 35 less than 30 with resp~ct to a predetermined hori~ontal datum.

~ 78~

It has been observed that defects ir- coating qu~lity are related to the magnitude o~ a takeof~
angle de~ined between the slide surface an~ a tan~
~recte~ a~ the point o~ impingement of the coating fluid onto the bac~up roll. In an ef~ort to overco~e such defects United States ~atent 4,2~3,443 ~Choinski) discloses a cascade coa~er having a slide sur~ace with a lip disposed at the lowerlnost end thereof which lip changes the direction of flow o~
the ~luid layers to a generally upwardly direction aligned with the upward direction o~ the web movement.
In a slide coating apparatus one or more fluids to be coated on a moving web are accelerated toward the moving web by gravity. Due to the inclination o~ the slide surface with respect to a horizontal d tum the gravitational force imposed on the coating ~luid is resolv~ble into two components, one parallel to the slide sur~ace and the second perpendicular thereto. The momentum o~ the coating fluid as it impinges upon the moving web is related to the component of the gravitational force parallel to the slid~ sur~ace. It is desirable that the momentum of the coating fluid be as high as possible in order to overcome the forces which oppose the ~5 deposi~ion o~ the coating ~luid onto the web. These opposition ~orces include forces due to the sur~ace t~nsion o~ the coating fluid, high shear ~orces due to the velocity o~ the web, and ~orces due to air drawn along the sur~ace o~ the web under the bead o~
coating fluid.
The perpendicular component of the gravit~tional force acting on the coating ~luid as it moves down the slide sur~ace tends to even the distribution of the fluid across the transver~e dimension of the slide, an effect known in the art as ~678C~

"leveling". In addition, the component o~ force perpendicular to the slide sur~ace tends to maintain the transverse dimension o~ the coating fluid an~
prevents a shrinkage or "neck in~' of tne tran~verse dimension o~ the coating ~luid at a given point. In order to maximize the leveling effect i~ is desirable that the component of the accelerating force perpendicular to the slide surface be as large as possible. However; increasing the perpendicular component has the necessary consequence o~ decreasing the pa~allel component thus decreasing the momentum, and vice versa.
Accordingly~ it would be desirable to provide a slide surface coating apparatus wherein the component o~ accelerating ~orce perpendicular to the slide surface and the component parallel thereto are both optimized so that the maximum impingement momentum is achieved while still maintaining an adequate leveling ~orce.
SUMMARY OF THE INVENTION
This invention relates ~o a slide sur~ace coating apparatus along which a coating ~luid ~lows by gravity toward an impin~ement point on a moving web carried by a coating roll 7 the slide surface defining an angle with respect to a horizontal datum plane, wherein the angle between the slide sur~ace and the datum plane lies in a rang2 ~rom 35 to 50 and the impingement point of the coating ~luid with the web is disposed above the horizontal center Line of the coating roll such that a tangent to the coating roll at that point defines a takeoff angle with respect to the slide sur~ace which takeoff angLe lies in the range ~rom 85 to 100 In the preferred embodiment the slide surface angle is 40 and the takeo~f angle is 90.

BRIEF OESCRIPTION OF THE DRAWINGS
The invention will ~e more ~ully understood ~rom the ~ollowing detailed description thereof taken in connection with the accompanying drawings ~hich ~orm a part o~ this a~plication and in whicll:
Figure 1 is a highly stylized schematic representation of a side elevational view o~ a slide ooating apparatus in accordance with the instant inventian; and Figures 2 and 3 are graphical representations illustrating the relationship between a ~orce (vacuum~ required to destabilize the coating ~luid bead as a ~unction o~ various coating speeds (web velocity) at various slide angles and at various takeo~ angles, respectively.
DETAIL~D DESCRIPTION OF THE INV~NTION
Throughout the ~ollowin~ detailed description similar re~erence numerals refer to similar elements in all ~igures o~ the drawings.
Re~erring to Figure 1 shown is a highly stylized schematic side elevational view o~ a slide coating apparatus generally indicated by reference oharacter 10 in aocordance with the teachings of the instant invention. The slide coating apparatus includes a coatlng head 12 ~ormed o~ a plurality cf distribution headers 14A and 14B joined together by a su.itable biasing arrangement 16. It is to be understood that any predetermined number o~
distribution headers 14 may be disposed within the 3~ coating head 12 and remain within the contemplation o~ the instant invention.
Each distribution header includes a metering slot 18 disposed in ~luid communication with a resexvoir 20 o~ coating ~luid~ Coating ~luid ~rom each o~ the reservoirs 20 is metered into the ~67t3~

distribution slots 18 by a pump 22. As the coating ~luid wells ~rom the mouth o~ each distribution slot 18 it is accelerated by gravity along an inclin~
slide sur~ace 24 toward a web 26 carried ~y a backup - 5 roll 28. Each o~ the slide surt`aces 24A and 248 are parallel to each other. In practice, the plan~
containing the slide surface 24B lies parallel to and slightly above the plane containing the slide sur~ace 24A. The lower edge l~E o~ the distributing head 12 lies within a predetermined close gap ~0 ~rom the surface of the web 26. SuLtable edge guides (not shcwn) may be provided along the lateral edges of the surface 24 if desired.
As the coating ~luid bridges the gap 3~ a 15 bead 32 o~ coating ~luid is ~nrmed. A vacuum box 34 is disposed upstream of the bead 320 The vacuum-box 34 withdraws air ~rom that region beneath the bead 32 thus creating a low pressure region. The pressure di~erential between the region beneath the bead 32 20 and above the coating ~luid generates a force acting in the direction 36 tending to push the bead against the web 26~ The point 38 at which the bead 32 contacts the web 26 is known as the impingement point. The impingement point 3~ lies near to the 25 intersection o~ a line extending ~rom the inclined sur~ace 24A and the sur~ace of the roll 28. Of course, the exact location o~ the impingement point 3~ is dependent upon the vacuum in the vacuum box 34, the speed o~ the web 2S and the ~low o~ the fluids 3~ down the slide sur~ace.
In accordance with this invention each slide sur~ace 24 defines a predetermined angle Theta with ~espect to a horizontal datum plane P 7 which angle Theta lies in a rang~ ~rom 35~ to 50. In addition, 35 the slide sur~aces 24A and 24B de~ine a second 78~

predetermined ~ngle Phi with respect to a reference line T erected as a ~angent to the baokup roll ~8 at the impingement point 38. The an~le Phi lies ~ithin the range ~rom 85 to 100. Preferably the angle 5 Theta is 40 and the angle Phi. is 90.
As a coating fluid emanates from it~
distribution slots 18 onto the slide sur~ace 24 it is subjected to a 3ravitational ~orce Fg. The gravitational force Fg may be resolved into a first 10 compon~nt Fm extending parallel to the sur~ace 24 and a second component Fh extending perpendicularly' thereto. The ~orce F~ accelerates the c02ting fluid along the inclined slide surface 24 toward the impingement point 38. The magnitude of the mo~nelltull~
o~ the coating ~luid toward the impingement point 38 is functionally related to the magnitude of ~he force Fm .
The ~orce component Fh tends to distribute the coating ~luid evenly across the transverse dimension o~ the distribution head 12 and opposes any ~orce which would tend to "neck ln" the coating ~luid at any point as it ~lows down the slide sur~ace 24.
With ~e~erence to Figures 2 and 3 shown are graphical representations oP the ~orce necessary to destabiLize the bead 38 (measured as a function of the Yacuum drawn by the vacuum box 34) plotted against various coating speeds (in ~eet per minute o~
the web 26) for each o~ a predetPrmined number of angles Theta ~or a predetermined coating thickness on the web 26 (Fi~ure 2) and ~or a predetermined number o~ angles Phi (Figure 3). From Figure 2 it is observed that at any constant web speed as the angle Theta increases the vaouum ~arce required to destabilize the bead 32 increases. Not apparent ~ro~
Figure 2 is the ~act that the l~velin~ eP~ect is less 7~3~

pronounced as the angle Theta is increased. Visual observations indicated that while the vacuum needed to destabilize the bead increases with increased angle Theta, when the angle Theta exceeds 50 de~ect~
due to inadequate leveling render the coating quality unacceptable.
From Figure 3 it may be observed that optimum bead st~bility at all coating speeds is obtained when the angle Phi lies in the range from 85 to l~û, with 9~ being preferred. Thus in accordance with the invention if the slide sur~ace angle Theta lies within a range ~rom 35 to 50 and the takeo~ angle Phi lies within a range ~rom 85 to 100 the (nagnitude of the momentum ~orce Fm is maximized while simultaneously the magnitude of the leveling ~orce Fh is adequate to ~aintain the coating fluid against the sur~ace 24 and promote leveling of de~ects. In the preferred embodiment o~
the invention the tak~of~ angle Phi is 90~. Thus, the momentum of the coating fluid onto the web ~6 is a maximu~.
~ hile those skilled in the art havin~
bene~its of the teaching o~ the instant invention as hereinabove set ~orth may e~fect numerous modi~ications thereto, such modifications are to be construed as lying within thP scope of the instant invention as de~ined by the appended claims.

Claims (3)

WHAT IS CLAIMED IS:

1. In a coating apparatus having a slide surface along which a coating fluid flows by gravity toward an impingement point on a moving web carried by a coating roll, the slide surface defining an angle with respect to a horizontal datum plane, the improvement which comprises:
the angle between the slide and the datum plane lies in a range from 35° to 50°
and the coating fluid impingement point is disposed above the center line of the coating roll such that a tangent to the coating roll defines a takeoff angle with respect to the slide surface which lies within a range from 85° to 100°.

2. The coating apparatus of claim 1 wherein the slide surface angle is 40°.

3. The coating apparatus of claims 1 or 2 wherein the takeoff angle is 90°.