IL29042A - Apparatus and method for coating webs - Google Patents

Description

APPARATUS AND METHOD FOR COATING WEBS - - Abstract of the Disclosure An apparatus and method for coating moving webs consisting of a stationary tubular member add a longi tudinal fluid-metering means positioned adjacent thereto. The tubular member is provided wi th apertures for the admission and discharge of coating fluid into a cavity formed by the tubular member and the metering device. In use a moving web is drawn over the tubular member so as to completely enclose the cavity. Coating fluid is fed through the admission apertures and completely fi lls the cavi ty, thus preventing exposure of the coating fluid to the atmosphere. Return of excess coating may be controlled by adjusting the angle by which the advancing web wraps the tubular member.

Field of the Invention This invention relates to apparatus and method for coating moving webs therein fluid coating material is applied in excess ; the excess material being distributed across the web and then removed from the coated surface. It relates further to coating apparatus wherein the coating material is maintained in an enclosed environment during the coating process.

Prior Art Any coating apparatus must perform the functions of application, distribution and metering. Each of these functions presents special problems. Inventors in this field have generally devoted their attention toward the individual solution of thase problems. As a result the present art includes numerous knives , airkni ves , blades , aprons , and bars for accurately metering the surface flow of a coating fluid which has previously been applied and distributed.

Applications devices Incl ude pans , rolls , sprayers , and extruders .

Distribution is performed in many ways , often in combination with the application or metering steps. In general , machines which perform all three functions well are large, complex, and expensive combinations of wel l known components. In the usual case these machines are limited, in practical application, to a narrow class of coating materials.

In addition to the above limitations , existing apparatus are general ly unsatisfactory for handling hot melts or other fast setting coatings. This is caused either by early exposure to the air, or by inadequate means to remove excess coating materials from the application and metering area. Evaporation, coagulati on, and build-up are typical results. Also 1t 1s not uncommon for excessive aeration to lead to frothing of aqueous type coatings.

When hot melts are handled 1t is usually necessary to apply heat to numerous parts of the coating apparatus as well as to the coatttg fluid Itself.

Summary of the Invention Our invention 1s a coating method and a compact, mechanically simple, and inexpensive coating apparatus adapted to apply, distribute, and meter coating fl uid in a single combined operation without exposure to the air and with provision for removal of excess coating material from the coating area. The apparatus has as its main structure a large tubular member over which 1s drawn the uncoated web. This tubular member remains stationary during the coating operation. . Our apparatus includes also a longitudinal fluid-metering means positioned adjacent and parallel to the main tubular member; the intersection of the parallel members forming a longitud- inal fluid holding cavity. Themmain tubular member is provided with apertures. The admissi on apertures are so located as to provide a continaoas supply of coating to the fluid holding cavity. The discharge apertures are so located as to be enclosed by the v/eb in its approach to the fluid holding cavity. When the web is ·,: drawn over the tubular member it completely encloses a supply o$ coating fluid which has been forced through the admission ape -tures and into the fluid-holding cavity. Hydrostatic pressure forces excess coating fluid out through the discharge apertures. This arrangement of web and coating structure serves to Isol ate the coating fl uid from the atmosphere. Furthermore, when the web is rapidly irawn across therrelatlvely smal l coating region , the duration of the actual coating operation , or coating dwel l time, 1s much reduced over conventional coating methods.

It Is readi ly apparent to one ski lled in the art that our Invention may be used with a large variety of webs and coatings without the disadvantages which have been pointed put for prior apparatus. In particular coagulation, drying out, loss of sol vent bui ld up of excess coating material , frothing, and contamination from air borne di rt are substantially entirely eliminated* Metering can be accurately controlled with a minimum of surface tension ridges in the completed coating. No local heating 1s required for application of hot melts. Furthermore, when hot melts are applied the reduced coating dwell time faci litates application at temperatures near the fluid congeal ing point, thus improving coating hold-out. When water base coatings are applied, the reduced dwell time decreases soak-up of water Into the web, thus . eliminating the local viscosity increase which commonly causes ) ''; defects such as fi lm split marks in coatings applied at high solids content. For application of solvent base coatings , such as overprints for printed webs , the shortened coating dwell time makes It possible to apply overprints that would normally dissolve the inks. Also, Isolating the solvent base coating from the ai r reduces fire hazards.

Further advantages of the apparatus are simplicity and low cost of constructi on. Except for the metering means which , in the preferred embodiment, is a totating wire wound rod, no moving parts are used. The main structure may be fabricated from pipe and fl at metal plates.

Accordingly 1t is a principal object of my inventi on to provide a simple, compact coating apparatus for use wi th a wide variety of coatings , particularly those which are fast setting in nature.

Another object of our Invention is to provide a method of coating a moving web wherein the coating dwell time is very short In duration and wherein the coating material sis Isolated from the atmosphere before and during the coating operation.

Another object of our Invention is to provide a stationary coating apparatus capable of applying an excess of coating fluid to a moving web and removing the excess through surface apertures. Another object of our invention Is to provide a coating apparatus adapted to apply, distribute and meter coating fl uid to a moving web with no exposure of the coating material to the atmosphere unti l after completion of the metering.

Other and further objects wi ll be apparent from the description which fol lows.

Brief Description of the Drawings Fig. 1 is a diagrammatic drawi ng of a typical machine embodying our Invention. For simplici ty of i l lustration, the end plates which support the metering bar and prevent fluid seepage about the Fig. 2 is an enlarged sectional elevation drawing of the coating area for a machine using our Invention In the form shown n Fig. 1 Fig 3 is an enlarged sectional elevation drawing of the coating area for a machine using our invention is a different form.

Fig. 4 is a sectional drawing of our coating apparatus In the preferred embodiment.

Fig. 5 is a sectional drawing of an alternate configuration of our coating apparatus .

Descri ption of the Preferred Embodiments Referring to F§g. 1 , a web of material 10 is being pulled across a tubular member 11 which is supported by stand 18. The tubular member 11 may have any desired cross section, but, for manufacturing reasons , a ci rcular cross-section is most desirable. Stand 18 rigidly supports member 11 so as to prevent any tendency to rotate with the moving web. A metering means , in this case a wi re would rotating bar 12, 1s placed In a smal l groove which has been nl lled into member 11. The crest of bar 12 rises above the surface of member 11 to define a longitudinal fluid holding cavity. When web 10 is wrapped around the combi ned structure there is formed an enclosed pocket 15 whi ch is best illustrated in Figure 2. Referring stil l to Figure 1 , the numeral 13 represents any of a series of smal l apertures which have been dri lled into the surface of member 11 along a* line running in the longi tudinal direction. Apertures 13 provide a passageway for coating fluid whi ch is most desirably pumped into line 16 fi ll ing a supply cavity wi thin member 11 and thence out through apertures 13 into the pocket 15. A series of discharge apertures M running along another longitudinal line provide passages through which excess coating fluid flows into the interior of member 11 and thence out through Tine 19 to a reci rculation system which Is not Illustrated. A motor 17 rotates bar 12 In a direction either following or opposing the movement of web 10. The machine shown in F1g, 1 would ordinarily be provided with end plates to reduce seepage of coating fluid around the edges of the web, but for purposes of clarity they are omitted from the Illustration.

Figure 2 i llustrates In better detai l the coating function itself* he rotating wire wound bar Is completely wetted by coating fluid, but its rotation prevents the coating from running down the back side of member 11 * We have found that the metering function Is best performed when the wire wound bar is rotated at speeds between 20 and 40 r* .m. A particularly novel feature of this configuration is the location of the discharge apertures under the web in its approach path to the pocket 15. The web c n-trols the return of excess fluid by restricting flow to the discharge apertures. Coating fluid should be supplied to the admission apertures at a pressure great enough to maintain a full coating pocket as well as a steady excess flow. However, this pressure must not be so great as to cause seepage of fluid around the edges of the web. Pressure relief may be achieved by throttling the supply or In many other obvious ways. Adjustments In Internal fluid pressure may also be accomplished by altering the angle by which the web wraps member 11 ; particularly the wrap forward of discharge apertures 14 as designated 1n Fig. 2 by the angle . We have found that a magnitude of about 60 degrees is most ideal for This wrap assures good contact,, aids in preventing the formation of wrinkles , and seals the pocket 15. We have used this arrangement quite successfully in the coating of hot melts.

Figure 3 i llustrates the coating function for an alternate configuration. The various elements are numbered to correspond with similar elements in Fig. 2. The Illustrated apparatus differs from that of Figure 2 1n that the arrangement of the surface apertures has been changed and the fcotating metering bar has been replaced by a knife. However the basic principle of the invention is present In both configurations. It will be apparent to one skilled In the art that many other variations may be employed.

We do not Intend to limit the acope of our invention by illustrating only 2 of the many possible configurations of the numbered elements. Furthermore, the use of the top member 11 rather than the bottom or a dide Is not significant. Our Invention may be practiced In any desired vertical orientation.

The preferred structuail arrangement is shown In detail in Fig. 4, Tubular member 11 has been cut away In sectioning fashion to expose a supply cavity 20 jhith has been formed by fastening plate 22 Into member 11 with bolts 25. Coating fluid is forced Into cavity 20 through an end plate aperture which Is not i llustrated but which is represented generally by 23. The coating fluid 1s forced through aperture 13 into pocket 15 where it is distributed and metered as previously described. The excess coating Is forced down aperture 14 Into discharge cavity 21 which is also formed In member 11 by plate 22. The , excess coating continues out of cavity 21 through aperture 24 and thence into the recirculation system. For coating fluids which may be adversely affected by the drop through cavity 21 , It might be desirable to instal l baffles or to maintain cavity 21 In a completely fil led condition. A drain plug 27 is provided at the bottom of supply cavity 20. A guide roll 29 is mounted on an adjustable arm 34 to alter the wrap angle as above stated. Additional features which are Illustrated In Figure 4 are a far end plate 28 (near plate cut away) and a blade 26 for holding the web down in firm contact with the coating apparatus. Blade 26 Is required in only a relatively few types of coating operations but It Is bhown for completeness of illustration. The tubular - - member Π with Its Internal cavities and supply lines along with apertures 13 and 14 and the metering means 12 hereafter will be referred to as the coating head.

An alternate Internal structure 1s Illustrated 1n Fig. 5.

Here the admission apertures 13 are replaced by a longitudinal slot (also denoted by 13 because of Identity of function) which extends the entire length of member 11. A longitudinal weakening groove 32 also extends the entire length of member 11. A set of bolts 30 are fitted through openings 1n the wall of member 11 and threaded into plate 22. Washers 33 provided working surfaces between the wall of member 11 and the heads of bolts 30. When bolts 30 are tightened down, the wall of member 11 is deflected longitudinally about the weakening groove 32 thus closing slot 13. In this manner the flow of coating fluid into pocket 15 may be accurately adjusted. Springs 31 may be provided 1f necessary for the reverse operation.

The process which is Inherent In the invention's not specifically illustrated in any single figure but Is generally shown in all figures. It. consists of wrapping a web over a tubular structure adapted to form an enclosed cavity between the web and the said tubular structure. Coating fluid is continually forced Into the said cavity and metered by drawing or pulling the web across a metering device forming part of the back wall €f the cavity. Excess coating Is continually removed from the cavity; the entire operation being carried out in such a manner as to prevent exposure, of the coating fluid to the atmosphere.

We have successfully carried out the above process using apparatus as suggested by Fig. 1. Rolls of corrugating medium weighing 33 pounds per thousand square feet were coated at coat weights of approximately ten pounds per thousand square feet uniformly across the entire width of a 73 3/8 inch web at speeds up to 200 ft. per ml n. We used a commercial, wax having a; congealing temperature near 165°F. A 1/4 inch diameter rod wound with 0.024 Inch diameter wire was used for metering and was rotating at 95 RPM in the direction of web travel .

The melted wai was pumped in through 1/16 inch diameter holes and removed through slots 1/4 Inch wide. Wax flow to the coating head was controlled by throttling the supply valve.

All excess nsplten wax supplied to the coating head readily flowed through the return slots and was carried back into the supply tank by means of a. trough located* under the entire length of the coating head. We found, that it was necessary to continual ly circulate the hot wax through the coating head, when . the unit was not being used, to prevent the wax from. freezing 1n the lines and coating head.

Although we have described several particular embodiments of our Invention it should be understood that we do not intend to limit the scope of our claims thereby. What we cl aim as our invention is:

Claims (10)

1. - 1 - An apparatus for coating a continuously moving web comprising a stationary, tubular, web-supporting member provided at its surface with at least one fluid admission aperture and at least one fluid discharge aperture, and a fluid metering means oriented wi th its axis parallel to the axis of said web-supporting member; said web-supporting member and said fl uid metering means by the juncture of thei r surfaces defining a longitudinal fluid-holding cavity, and said apertures for fluid admission and fluid discharge each providing non exposed passage between the longitudinal fluid holding cavity and the interior of the web-supporting member.
2. - 2 - A coating apparatus accoriing to claim 1 , said discharge apertures being arranged substantially along a fi rst longi tudinal l ine and said admission apertures being arranged substantial ly along a second longi tudinal l ine, sai d second longitudina line being located on the surface of said web-supporting member between said fi rst longitudinal line and said metering means,
3. - 3 - A coating apparatus according to claim 2, the surface of said tubular member being shaped in the form of a continuously curved arch In the area between said longi tudinal lines , and the placement of said first longitudinal l ine being sufficiently remote from said metering means that a plane extending between said fi rst longitudinal line and the crest of the metering means wi l l intersect the surface of said tubular member; the line of intersection being situated between the metering means and said fi rst longitudinal line. '
4. A coating apparatus according to claim 3, said metering means comprising a rotatable bar.
5. - 5 - A coating apparatus according to claim 4 and further comprising a backing means for holding said web firmly against said rotatable barf
6. - 6 - A coating apparatus according to claim 4 and further comprising a guide rol l for adjusting the wrap angle of said web.
7. - 7 - A coating apparatus according to claim 1 said at least one fluid admission aperture being a longH!udinal slot running the full length of said tubular member and said coating apparatus further comprising means for deflecting the wall of said tubular member thereby to alter the width of said slot and adjust the rate of coating fluid movement therethrough.
8. - 8 - A process for coating a moving web comprising the steps of: (1) placing the web in contact with a stationary tubular coating head having a longitudinal cavi ty along i ts surface and a longi udinal fluid metering means at the rear wal l of the cavi ty, (2) drawing the web tightly across the coating head to create an enclosed pocket bounded on one side by the web and on the other side by said metering means and by the wall of said cavity. (4) pulling the web foraadd across the coating head whi le maintaining the enclosed pocket and thereby causing application of coating fluid to the under si de of the web and metering of the fl uid so applied, and (5) mai ntaining a fresh supply of coating fluid wi thin the pocket by ci culating fluid into the pocket at a rate in excess of the appl ication rate and continually expel ling fluid at a rate equal to the excess.
9. - 9 - A coating method according to claim 8 and further comprising the step of continually recirculating the expelled coating fluid back into said pocket. -
10. 10 - A coating method according to claim 9 , sai d coating fluid being a hot melt and siid method further comprising the step of heating the recirculated coating fluid to a temperature near but above the fluid congealing point thereby promoting rapid post-application congealing for improvement of coating holdout.