US 3486482 A
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Description (OCR text may contain errors)
De. 30, 1969 G. K. HUNGER 3,485,482
APPARATUS FOR COATING TRAVELING WEBS Filed Dec. 30, 1966 4 Sheets-Sheet 1 FIG. I.
FRESH COAT/N6 I NVEN T OR. Gunther K. Hunger ATTORNEY Dec. 30, 1969 G. K. HUNGER 3,486,482
APPARATUS FOR COATING. TRAVELING WEBS Filed Do. so, 1966 4 Sheets-Sheet 2 A (1) N\ r I INVENTQR Gunther K. Hunger ATTORNEY G. K. HUNGER APPARATUS FOR COATING TRAVELING wEias Dec. 30, 1969 4 Sheets-Sheet 5 Filed Dec. 30, 1966 R O T N M Gunther K. Hunger BY Y.
ATTORNEY Dec. 30, 1969 G, K] HUNGER 3,486,482
APPARATUS FOR COATING TRAVELING WEBS Filed Dec. 30, 1966 4 Sheets-Sheet 4 P O i IN VENTOR. Gunther K. Hunger wy/Mm;
A TTORNE Y United States Patent 3,486,482 APPARATUS FOR COATING TRAVELING WEBS Gunther K. Hunger, Ellicott City, Md., assignor to Westvaco Corporation, a corporation of Delaware Filed Dec. 30, 1966, Ser. No. 606,164 Int. Cl. B05c 11/02, 3/02 US. Cl. 118-603 6 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the application of coatings to traveling webs such as paper, paperboard, and the like. More particularly, the invention relates to apparatus for coating a traveling web in such manner that coating is forced into the web, as well as metered thereon, as the result of the combined use of a coating impelling nip and a pressurized pond of coating.
The coating of web material, such as paper and paperboard, may be performed in general by the use of roll coaters, air knife coaters, or blade coaters. It is known that with pond type blade coaters, such as shown in United States Patent 2,368,176 to A. R. Trist, or with inverted blade coaters, such as shown in the United States Patent 3,149,005 to M. M. Brundige, a primary problem encountered is that of scratching or streaking of the coated surface. The scratches or streaks are primarily caused by hard particles in the coating which become lodged in the nip between the doctor blade and the traveling web, and as the web travels under and past the blade, the absence of coating on the web, caused by the lodged particle, takes the form of a streak or scratch on the web. This streaking continues until either the particle becomes dislodged and passes under the blade or until the particle is abraded away.
The particles which cause the streaking may be dried adhesive or dried coating from the coating supply system or they may be foreign materials which have not been removed from the coating system. Screens normally used in the industry to screen paper coatings are not fine enough to remove all scratch producing particles, and the use of finer mesh screens has not been found to be commercially feasible.
The coating apparatus of this invention readily minimizes the ability of scratch producing particles to reach the doctor blade and cause scratching, while at the same time, it applies higher coat weights than do conventional blade coaters under equivalent coating conditions. This is accomplished by a coating apparatus which includes a backing roll on which a traveling web may be caused to travel, a coating applicator pan opposing a portion of the backing roll and sealed to the atmosphere at the incoming web end, a first adjustable coating impelling and metering device forming, preferably, a positive gap with the backing roll at the outgoing web end of the applicator pan and a positive or negative gap with the web, a second metering device spaced from the outgoing web end of the applicator pan, hydrostatic pressure means forcing coating into the applicator pan and into the traveling web, and means for draining from the applicator pan the scratch producing particles which gather in an area of low streaming velocity in the bottom of the pan.
3,486,482 Patented Dec. 30, 1969 ICC The present invention is based upon the finding that a self-cleaning of the coating takes place in the applicator pan of the present coater, such that most of the scratch producing particles never reach the second, and final, metering device. This self-cleaning phenomenon is attributable to the action of the first metering device which impels coating into the web, and to flow streams which are set up in the coating pan. Scratch producing particles are carried in the flow streams and are deposited in areas of lower streaming velocity within the applicator pan. The particles which gather in the low velocity region in the bottom of the pan may be continuously drained from the same.
For the foregoing purpose, a coating reservoir is placed above the applicator pan to provide hydrostatic pressure for supplying coating to the pan and aiding in the forcement of coating into the web. Coating in the pan is picked up by the web which moves with the rotating backing roll and passes through a first metering nip formed by the web and the coating impelling device at the outgoing end of the applicator pan. The coating impelling device may be spaced from the web to form a gap of small positive clearance between the device and the web or the coating impelling device may be adjusted to provide a negative clearance between the web and the device. In any event, an excess of coating, as compared to the desired final coat weight, is metered onto the traveling web by the coating impelling device. Spaced from the outgoing web end of the applicator pan is a second metering device which smooths the coating on the web and meters the coating to the desired final coat weight. The coating impelling and metering device preferably is in the form of a bar of rigid material and is adjustable toward and away from the web and backing roll, to vary the gap between the device and the web. The second metering device preferably is a pneumatically loaded trailing blade. Means for draining scratch producing particles which gather in an area of low streaming velocity in the applicator pan are provided, and means for raising and lowering the applicator pan are also provided.
This invention will be best understood by reference to the drawings, in which:
FIGURE 1 is a side elevation view of the coater and coating system of this invention;
FIGURE 2 is an expanded, more detailed view of the applicator pan and metering devices shown in FIG- UREl;
FIGURE 3 is a perspective view of the coater of FIGURE 1;
FIGURE 4 is an end view of the coater of FIGURE 1 showing means for altering the position of the applicator pan and final metering device.
Referring to FIGURES 1-3, the backing roll 10 may be of any suitable material but is preferably a resilient roll and is driven in the direction of the arrows. A coating applicator pan, designated generally as 12, is disposed beneath and adjacent to a portion of the backing roll 10. The applicator pan 12 is comprised of a back member 14, a secondary bottom member 16 which extends from the back member 14 in the direction of the backing roll 10, a primary bottom member 18 which is located between the secondary bottom member 16 and the backing roll 10 and extends from the back member 14 in the direction of the backing roll 10 to form a pond 13 for coating material, and side members 20 and 20',
Each side 20 and 20' of the applicator pan 12 is machined along most of its top edge to fit the contour of the backing roll 10. Seals (not shown) of suitable material, such as Tefion, are suitably located on the interior surfaces of the sides 20 and 20' and bear against the traveling web W to prevent coating from leaking between the web W and sides 20 and 20'.
As best illustrated in FIGURE 2, the ingoing web end of the applicator pan 12 is sealed by blade 22 which is housed in blade holder 24 comprising jaws 25 and 26 fastened to each other by threaded screws 27. The blade holder 24 is secured by threaded screws 28 to the top portion of the back member 14. Blade 22 extends from side 20 to side 20' and bears at its tip against backing roll 10 to seal the applicator pan 12. The outgoing web end of the applicator pan 12 is sealed by a coating impelling and metering device 29 which is preferably a rigid bar mounted by screws 30 on a flexible leaf spring 32 which is suitably attached at one end by screws 34 to the primary bottom member 18. Adjusting screws 36 extend through the primary bottom member 18 and contact leaf spring 32. Turning of screws 36 adjusts leaf spring 32 and the bar 29 toward or away from the backing roll 10 to thereby control the amount of coating which passes with the web W to the final metering station. For most coating operations, it has been found preferable to provide a small positive clearance between the backing roll 10 and the metering bar 29 and to provide a small positive or negative clearance between the metering bar 29 and the web W. The web W and coating thereon passing between the metering bar 29 and the backing roll 10 seals the pond 13 at the outgoing web end.
The final metering device takes the form of a trailing blade 40 suitably mounted in a blade holder 42 which is attached to a T-bracket 44 mounted for rotation on cylinder 46. The bracket 44 carries a coating return channel member 48 which returns excess coating metered from the web by blade 40 to the coating return portion 50 of the applicator pan 12 formed by the secondary bottom member 16, the back member 14, and the primary bottom member 18. Trailing blade 40 is spaced from the outgoing web end of the applicator pan 12, in the direction of rotation of backing roll 10.
The size of the applicator pan may vary as the size of the backing roll is varied. It has been found that a contact angle of at least is preferred for best quality, regardless of the size of the backing roll, and contact angles of up to 180 are within the scope of this invention. The contact angle is the number of degrees of backing roll circumference which is in contact with the applicator pan at one time, i.e., that portion of the backing roll 10 between sealing blade 22 and metering bar 29.
In one embodiment of the invention, coating material from fresh coating feed line 69 and from conduit 67 is moved by pump 70 to tank 52 via conduit 71, valve 73, and conduit 72. Tank 52, which is equipped with an overflow conduit 75, is located above the applicator pan 12 to provide hydrostatic pressure. The coating material flows by gravity from tank 52, through conduit 54 to coating distributor 56 which is suitably secured to the back 14 of the applicator pan 12. Valve 58 controls the flow of coating from tank 52 to the applicator pan 12. By varying the height of tank 52 above the applicator pan 12, the hydrostatic pressure is varied, becoming less when the tank is lowered and more when it is raised. Conduits 54, 58, 67, 71, 72, and 74 are preferably flexible to allow for varying the height of tank 52 above the applicator pan 12.
In another embodiment of the invention, coating material is supplied under hydrostatic pressure to applicator pan 12 by means of the delivery pump 70. Three-way valve 73 is set to close conduit 72 and direct coating material from pump 70, through conduits 71 and 74 to conduit 54, tank 52, and distributor 56, and into applicator pan 12. That portion of conduit 54 which is above conduit 74, as well as tank 52, functions as a standpipe to maintain the coating material in the applicator pan 12 under relatively constant pressure, i.e., the standpipe buffers against any surges in the delivery of coating to pan 12 which may be attendant with the use of a pulsating pump 70. In this connection, conduit 58 may be used as 4- a standpipe, in this embodiment of the invention, without the necessity of employing a tank such as tank 52.
The back 14 of applicator pan 12 is slotted across its width as indicated by inlets 59. Coating material flows through distributor 56, through inlets 59 in the back member 14, and into the applicator pan 12. The applicator pan 12 is completely filled with coating material during operation of the coater, and the coating in the pan 12 is under hydrostatic pressure due to the gravity feed of coating from the elevated tank 52 or from its action as a standpipe, depending upon which embodiment of the invention is employed.
Maintaining the coating in pan 12 under pressure is an important feature of the present invention. By employing a hydrostatic head to pressurize the coating in pan 12, high coat weights are achieved by the pressurized coating being driven into the traveling web W. Further, the hydrostatic pressure cooperates with the coating impelling bar 29 to force coating into the web and through the nip between the impeller 29 and the web W, resulting in the production of high coat weights and a smooth and glossy coated product. It has been found that a web of paper is smoother before entering a pond of coating than it is after leaving the pond because the web swells in the pond and surface roughness increases. It is my understanding that the impelling bar 29 of the present invention smooths high points of the swollen web as well as forces coating into the low areas of the web, to thereby produce a coated product with more smoothness and gloss than heretofore produced by known blade coaters.
As the web travels around backing roll 10, excess coating is metered from the web to the desired final level by blade 40. The excess coating is directed by channel member 48 to the coating return portion 50 of the applicator pan 12, and the coating in the coating return 50 is drained through conduit 60 to screen 62. Conduits 64 are provided to drain scratch producing particles which collect in the pond 13 of applicator pan 12 during operation of the coater. Valves 66 control this draining operation and are normally set in a slightly open position in order to allow the particles which settle in the primary low velocity area A2 in pond 13 to be bled off continuously.
All return coating material from conduits 60 and 64 passes through screen 62 and the rejected material subsequently is removed from the coating system through conduit 68. Acceptable coating material passing through screen 62 is moved by pump 70 through conduits 71 and 72 to coating supply tank 52 for reuse or is delivered by pump 70 directly to the applicator pan 12.
In the operation of the device, which finds great utility in the coating of paper and paperboard webs, a web W passes partially around the backing roll 10 which is rotating at web speed. The web, which is wider than the width of the applicator pan, passes between the backing roll 10 and sealing blade 22 and is contacted by coating material in the pond 13 of applicator pan 12. The web W passes through pond 13, picking up coating material from the pond, to metering bar 29 which impels coating material into the web due to the localized higher hydrostatic pressure that exists in the nip between bar 29 and web W. As the web W passes bar 29, an excess of coating in layer-form is metered onto the web. The web W continues in its movement with'backing roll 10 to trailing blade 40 which meters a final and smooth layer of coating on the web. The web W then leaves backing roll 10 at a suitable point and passes to drying devices, not shown.
The bottom of the pond 13 has been designed to create the area A2 of lower streaming velocity. This has been accomplished by forming an angle in the bottom of the pond 13 between bottom member 18 and back member 14 as opposed to having the back and bottom members define a curvilinear configuration. In the drawings, the bottom member 18 and back member 14 are shown as forming an approximate right angle but any angle which creates an area of lower streaming velocity may be employed.
During operation of the coater, movement of the backing roll and web W creates flow streams in the pond 13 of applicator pan 12 as illustrated in FIGURE 2 at 74, 75, 7-6, 77 and 78. Model studies have shown that there are two areas in the pond 13 where there is relatively little coating movement, and that contaminating particles tend to gather in these regions. One such area is located at the center of the stream lines, indicated generally as A1, where some contaminating particles tend to gather. The other area of lower streaming velocity, A2, is located in the bottom of the pond 13, where contaminating particles also tend to gather. It has been found that the scratch level of the coater of this invention is much less than conventional trailing or inverted blade coaters, especially when scratch producing particles, as they gather in area A2 are drained from pond 13. These scratch producing particles are bled off through conduits 64 and are removed from the coating system by screen 62.
As a specific illustration of a typical run with the coater of this invention, an aqueous coating composition comprising 100 parts pigment, 16 /2 parts acrylic resin, and having a solids content of about 60%, was prepared and utilized in the coater of this invention and in a conventional inverted blade coater of the type shown in U.S. Patent 3,149,005. Blade loadings were identical in each coater and blade conditions were also the same, being a .015" blade, 45 blade angle, and blade extension. The clearance between metering bar 29 and backing roll 10 was set at .0015", and the contact angle (number of degrees of backing roll circumference between the pan sealing blade and the metering bar) was approximately 65. Both coaters were run at 400 feet per minute,
and the web material coated was 14 point paperboard. The following results were obtained:
Standard inverted Invention coater blade coater Blade load coat weight, coat weight,
(p.s.i. static) lbs/3,000 sq. ft. lbs/3,000 sq. ft.
It is seen that a higher coat weight is obtained with the coater of this invention at equal solids level of the coating and equal blade conditions. Further, at equal coat weights, the invention coater produced a more glossy product. For example, the above-noted paper coated with 4.6 pounds of coating (invention coater) had a gloss of 31 as measured according to Tappi Standard T480m-5 1, while the paper with 4.4 pounds of the same coating applied by the standard inverted blade coater had a gloss of only 25. Additional studies have shown that paper coated with the invention coater exhibits a Bekk smoothness of several hundred units more than the same paper coated similarly by a standard inverted blade coater.
In another series of runs made with a coating composition similar to the above, the scratch level of the coated paperboard was compared for the coater of this invention and a standard inverted blade coater. The same blade and metering bar conditions as above noted were utilized. In addition to the contaminants which are normally found in a paper coating composition, sand was added to the coatings for each coater in order to make a more drastic comparison between the scratch levels produced by each coater, and further to illustrate the selfcleaning effect which occurs in the coater of this invention. The total number of scratches found on the coated web, per strip 100 ft. long and 10 inches wide, were counted and the average results for four runs is shown below:
Number of Number of scratches scratches standard invention inverted Test coater blade coater No contaminant added 3. 2 7. 3 gram of sand passing 325 mesh screen added per gallon of coating- 16. 5 40 gram of sand passing a 200 mesh screen but retained on a 325 mesh 32 5 334 screen added per gallon of coating- As has been mentioned, and shown, the coating impelling and metering device 29 preferably is in the form of a pointed rigid bar and steel metering bars have been used. Other devices, such as a blade, a rod or wire wound rod rotating in a direction counter to backing roll rotation, maybe used as the impelling and metering device, but itiihas been found that the steel bar produces a coated product having the best quality and lowest scratch level.
The distance between the first metering nip and the trailing blade 40 of this invention should be designed at a minimum in order to prevent dewatering of the film of coating between the first and second metering devices. Distances up to about six inches have been utilized with no significant quality losses. A distance of about three inches is preferred.
Referring to FIGURE 4, means are illustrated for moving the applicator pan 12 and blade 40. Such means are provided at each side of the machine. Backing roll 10 has a shaft 11 which is supported through pillow block bearing by frame cap 81 mounted on frame member 82. Pivot arm 83, slotted at 8-4, is carried at one end on shaft 85 which is rotatively supported by bearing 86 secured to pivot arm bracket 87. Pivot arm 83 is secured at its other end to applicator pan 12. Pivot arm 88, slotted at 89, is carried at one end on shaft 90 which is rotatively supported by bearing 91 secured to pivot arm bracket 87. Pivot arm 88 is secured at its other end to applicator pan 12.
Hydraulic cylinder 94, which is securely mounted on frame members 82 beneath the pan 12, has a piston rod 96 which cooperates with pivot arm 83 by means of an extension shaft 98 which is housed in slot 100 ofpivot arm 83. The pan 12 may be retracted by the pivot armhydraulic cylinder arrangement for threading a traveling web around backing roll 10 or to clean the machine.
Blade cylinder 46 has a shaft 102 which is held by crank arm 104 pivotally secured to piston rod 106 of pneumatic cylinder 108. Pneumatic cylinder bracket is mounted on frame member 82 and pneumatic cylinder 108 is pivotally mounted on bracket 110- Blade cylinder bracket 112, mounted on frame member 82, rotatively carries shaft 102 by means of bearing 114. Adjustment of the air pressure in pneumatic cylinder 108 controls the degree of rotation of blade cylinder 46 to which blade 40 is attached, and controls the pressure loading on blade 40 and the final coat weight metered thereby.
Other mechanisms for moving pan 12 and blade 40 may be employed, but it is preferred that blade 40 be pneumatically loaded for best quality results.
The novel coating apparatus of this invention has been run at speeds varying from about 100 to 1800 feet per minute. The angle which blade 40 makes with the tangent to the backing roll 10 at the point of blade contact has been varied from about 20 to 70, with a preferred blade angle of from 45 to 60. Coat weights from about 3 to 10 pounds per 3000 square feet of web have been applied, and the clearance between backing roll 10 and metering bar 29 has been varied from a negative. clearance of about .010" to a positive clearance of about .003", with the preferred being a positive clearance of from .0015" to .003".
While preferred embodiments of the invention have.
been illustrated and described in detail, it is to be understood that changes may be made therein and the invention embodied in other structures. Hence, the present embodiments are to be considered as illustrative and not as restrictive, and it is intended to cover the invention in whatever form its principles may be utilized.
1. An apparatus for coating traveling webs which includes a rotating web backing roll on which a traveling web is caused to travel, a coating applicator pan opposed to a portion of said backing roll for supplying coating material under pressure to the. traveling web, and a trailing blade opposed to the backing roll at a metering station which is spaced from said applicator pan in the direction of rotation of said backing roll, said coating applicator pan comprising a back member, primary and secondary bottom members joined at one end to and extending from said back member in the direction of the backing roll, said primary bottom member disposed between the backing roll and the secondary bottom member and meeting with the back member angularly to form an area of low streaming velocity within the coating applicator pan adjacent the jointure of said back member and said primary bottom member, side members attached to said back member and said bottom members, each side member being shaped along one edge to fit the. contour of the backing roll, said primary and secondary bottom members defining an open channel to receive excess coating material which is metered from the traveling web by the trailing blade, sealing means mounted on said back member and extending longitudinally of said applicator pan and bearing against said backing roll for sealing the applicator pan to the atmosphere, a coating impelling and metering bar adjustably mounted on the primary bottom member of the applicator pan for impelling coating material into and for metering an excess of coating material onto the traveling web, said coating apparatus further including hydrostatic pressure means forcing coating material into said applicator pan, means for draining coating material from said area of low streaming velocity within said applicator pan, screening means through which drained coating material is passed to remove contaminant particles, and conduit means connecting the screening means and the hydrostatic pressure means through which the coating material which passes through the screening means is returned to said hydrostatic pressure means.
2. The apparatus of claim 1 in which the hydrostatic pressure means comprises a coating reservoir located at a variable height above said applicator pan, and conduit means connecting said coating reservoir and said applicator pan for delivering coating material under nonpulsating pressure from said coating reservoir to said applicator pan.
3. The apparatus of claim 1 in which at least about 60 degrees of the circumference of the backing roll is opposed by the applicator pan from said sealing means to said metering bar.
4. The apparatus of claim 1 in which said trailing blade forms an angle of from about 20 to about 70 degrees with the tangent to the backing roll at the. point of blade contact.
5. In an apparatus for coating a traveling web of paper,
paperboard and the like comprising a web backing roll about which a traveling web is passed, a coating applicator pan opposed to a portion of the backing roll for supplying coating under pressure to the roll backed web, sealing means at the incoming web end of the applicator pan opposed to the backing roll to seal the pan to the atmosphere at said incoming web end of the pan, pressure means forcing coating material into the applicator pan, a coating impelling bar adjustably mounted toward and away from the backing roll at the outgoing web end of the applicator pan for impelling coating material into the web, said coating impelling bar forming a positive gap with the backing roll through which the traveling web passes with a metered excess amount of coating material thereon, and a trailing blade opposed to the backing roll at a metering station which is spaced from said outgoing web end of the applicator pan in the direction of rotation of the backing roll, the improvement characterized by said applicator pan comprising a back member, primary and secondary bottom members joined at one. end to and extending from said back member in the direction of the backing roll, said primary bottom member disposed between the backing roll and the secondary bottom member and meeting with the back member angularly to form an area of low streaming velocity within the coating applicator pan adjacent the jointure of said back member and said primary bottom member, side members attached to said back member and said bottom members, each side member being shaped along one edge to fit thecontour of the backing roll, said primary and secondary bottom members defining an open channel to receive excess coating material which is metered from the traveling Web by the trailing blade, means for draining coating material from said area of low streaming velocity within said applicator pan, screening means through which drained coating material is passed to remove contaminant particles, and conduit means connecting the screening means and said pressure means through which the coating material which passes through the screening means is returned to said pressure means.
6. The apparatus of claim 5 in which the improvement is further characterized in that said primary and secondary bottom members extend from said back member upwardly in the direction of the backing roll, and said primary bottom member meets said back member substantially at a right angle to form said area of low streaming velocity.
References Cited UNITED STATES PATENTS 2,796,846 6/1957 Trist 118413 X 3,051,125 8/1962 Ahara et al. 118-603 3,079,889 3/1963 Jacobs et al. 118413 X 3,179,536 4/1965 Matinek 118-407 X 3,192,895 7/1965 Galer 118-413 X 3,354,867 11/1967 Pomper et a1 118126 X WALTER A. 'SCHEEL, Primary Examiner JOHN P. MCINTO'SH, Assistant Examiner US. Cl. X.R. 118126, 413