Gravure coating feed apparatus and method

<b>Field of the Invention</b>

The invention relates to a gravure coating feed apparatus and method. More particularly, the invention concerns a gravure coating feed apparatus and method which deposits a uniform curtain-like layer of coating composition to a gravure print surface without producing associated coating effects, such as flow lines and flow patterns.

<b>Background of the Invention</b>

Gravure print surfaces, for instance gravure cylinders, are a common means of supplying liquid compositions to webs. US Patent 4,373,443 describes the use of a gravure cylinder to provide ink in newspaper presses. Engraved upon the surface of the gravure cylinder are cells, which retain the liquid composition after being immersed in the reservoir. A doctor blade scrapes excess liquid composition from the surface of the gravure cylinder, such that the cylinder delivers a precise amount of liquid to a second surface upon contact. A number of distinct feed apparatus types which produce a variety of coating flow patterns are used to coat a gravure cylinder.

One common means of coating a gravure cylinder is a feed pan, similar to US Patent 3,936,549. The pan is constructed to hold a supply of the liquid composition with a means to maintain the liquid level. The gravure cylinder, while partially immersed in the reservoir, rotates to fill the cells and transfer the composition to a web. Drawbacks of the feed pan design include liquid waste due to large holdup volumes and the creation of "flow lines" in the coating. "Flow lines" occur when the gravure cylinder surface initially dips into the liquid composition and contacts a region with impurities. The impurities include air bubbles, globules of concentrated dye and binder, and pieces of partially dried foam. The feed pan design is conducive for the formation of stagnation regions where impurities may form and collect. Experiments with alternative pan designs were unsuccessful in eliminating "flow lines," particularly at high coating speeds.

An additional common means to coat the gravure cylinder is a reverse doctor-pond feed, as seen in US Patent 4,158,333. The pond feed, which may be open or enclosed, is centered at the nine o'clock position with a reverse angled doctor blade at the seven o'clock position. The reverse doctor blade scrapes the surface of the gravure cylinder as the surface exits the reservoir and the doctored cells pass through three quarters of a rotation before reaching the impression nip. The volatile solvents in the liquid composition remaining in the cells evaporate or "dry in" and create a leading edge pattern on the second cylinder or web. Additionally, foam generation on the pond surface may lead to "flow line" problems as with the feed pan method.

Another alternative apparatus to coat the gravure cylinder is an X-hopper coater, which is an extrusion device. A high degree of accuracy and precision are required to position the device relative to the gravure cylinder. The feed applies a thick (2-5 mils) coating to the cylinder at the three o'clock position. The surface remains wetted until reaching the doctor blade at the standard 10 o'clock position. The disadvantage of the design is primarily the cost associated with constructing a precise metering slot and a high pressure metering pump. The design is also inadequate for startup procedures, where the feeder coats the cylinder while the doctor blade and impression nip are disengaged. Under those conditions the excess coating solution would pool and splash at the hopper's lip.

More recent methods developed to coat the gravure cylinder for thermal donor media melts is the feedbar pond, the patent application for which is pending. It combines a knife coater with a puddle or pond feed with a continuous solution delivery means, an overflow and recycle. The knife or feedbar spacing provides a means for metering the coating of solution on the gravure cylinder, as it exits the bottom of the pond, at an three o<img file="EP0787584A1_D0001.tif" />clock position, prior to being doctored off at the 10 o<img file="EP0787584A1_D0001.tif" />clock position. The main disadvantages of this device is that at high speeds, the pond becomes turbulent and wavy, forming standing oscillating waves and eddies, which can cause<img file="EP0787584A1_D0002.tif" />flowline<img file="EP0787584A1_D0001.tif" /> defects. It also fails to uniformly wet the gravure cylinder at high speeds and viscosity<img file="EP0787584A1_D0001.tif" />s due to air entrainment at the air-cylinder-solution interface, causing <img file="EP0787584A1_D0002.tif" />flowpattern<img file="EP0787584A1_D0001.tif" /> defects.

The problems identified and solved by the present invention include an apparatus which supplies a uniform curtain-like layer of coating to a gravure cylinder at high coating speeds. More specifically, the apparatus of the invention supplies a coating solution to the gravure cylinder without coating defects associated with high coating speeds such as <img file="EP0787584A1_D0002.tif" />flowlines<img file="EP0787584A1_D0001.tif" /> and <img file="EP0787584A1_D0002.tif" />flowpatterns<img file="EP0787584A1_D0001.tif" />, and at the same time increases the onset speed of wetting failure due to air entrainment at the cylinder-liquid-air interface. Moreover, the apparatus of the invention reduces waste and costs associated with the gravure coating process.

<b>Summary of the Invention</b>

It is, therefore, an object of the invention to provide a coating feed apparatus that deposits coating composition onto a gravure surface without coating effects such as flowlines and flowpatterns.

Another object of the invention is to provide a coating feed apparatus that delivers coating composition in a curtain-like overflow layer to a gravure surface.

It is a feature of the apparatus of the invention that a reservoir for containing the coating composition includes an upwardly sloped base terminating in an outlet end configured to produce a curtain-like overflow coating layer for deposition onto the gravure surface.

It is an advantageous effect of the invention that when the coating composition is delivered from the reservoir and then deposited onto the gravure surface, coating effects, such as flowlines and flow patterns are eliminated.

Accordingly, for accomplishing these and other objects, features and advantages of the invention, there is provided, in one aspect of the invention, an apparatus for depositing a coating composition onto a gravure surface. A reservoir containing the coating composition includes opposing sidewalls, a rear wall, a base having lateral edges connecting the sidewalls and a first end portion connecting the back wall with the sidewalls. The base slopes upwardly from the rear wall to form an outlet end or weir at a second end portion opposite the first end portion. Further, the outlet end is configured to deliver a uniform overflow curtain-like layer of coating composition from the outlet end or weir onto the gravure surface. In this embodiment, moreover, means is provided for delivering the coating composition in the reservoir through the outlet end. Finally, at least one baffle is disposed in the reservoir downstream of the outlet end. According to the embodiment of the invention, the baffle includes a first portion attached to the base and a second portion directed substantially toward the rear wall of the reservoir.

In another aspect of the invention, a method is provided for depositing a coating composition onto a gravure surface which includes the steps of providing a coating composition, such as coating compositions described in US Patent Nos. 4,695,287; 4,700,207; and 4,698,651; hereby incorporated herein by reference, and a reservoir having a base sloped upwardly towards the outlet end, the outlet end being configured to form an uniform overflow curtain-like layer of coating composition. Also important to this embodiment is the step of providing means for delivering the coating composition from the reservoir to the gravure surface. Within this step, the coating composition in an uniform overflow curtain-like layer exits the outlet end of the reservoir, across the widthwise lip and finally deposits onto the gravure surface.

<b>Brief Description of the Drawings</b>

The foregoing as well as other objects, features and advantages of this invention will become more apparent from the appended Figures, wherein like reference numerals denote like elements, and wherein: <ul id="ul0001" list-style="none" compact="compact"><li>Figure 1 is a schematic view of the apparatus of the invention;</li><li>Figure 2 is a side partially enlarged sectioned view along line 2-2 of Fig. 1;</li><li>Figure 3 is a front plane view of the apparatus showing the relationship between the weir and gravure surface; and,</li><li>Figure 4 is a comparison of air entrainment speed limit of the coating feed apparatus of the invention.</li></ul>

<b>Detailed Description of the Invention</b>

Turning now to the drawings, and more particularly to Figures 1-3, the apparatus <b>10</b> for depositing coating composition (c) onto a gravure surface <b>12</b> is illustrated. According to Figure 1, the apparatus <b>10</b> comprises a reservoir <b>14</b> for containing the coating composition (c). Reservoir <b>14</b> has opposing similar sidewalls <b>16</b> (only one shown) and a rear wall <b>22</b>. In Fig. 2, base <b>24</b> connects with sidewalls <b>16</b> and with the rear wall <b>22</b>. In Figure 2, it is important to the invention that base <b>24</b> slopes upwardly from the rear wall 22 to form an outlet end or weir <b>26</b>. Moreover, the outlet end <b>26</b>, defining a coating weir, is configured to deliver a uniform overflow curtain-like layer of coating composition (c) from the reservoir <b>14</b> downwardly onto the gravure surface <b>12</b>.

Base <b>24</b> of reservoir <b>14</b>, as indicated, is sloped upwardly from the rear wall <b>22</b>, preferably by an angle (φ) up to about 20 degrees for both optimal capacity in the reservoir <b>14</b> as well as for preventing unpredictable and otherwise uncontrollable migration of the curtain-like layer <b>32</b> from a downwardly flow plane. We have found that slopes somewhat greater than 20 degrees are not as efficient for eliminating the migration tendency of the curtain-like coating layer <b>32</b>.

As shown in Figure 1, apparatus <b>10</b> further includes means, preferably a pump, <b>30</b>, for delivering the coating composition (c) from a source (not shown) through preferably a center fed supply line or pipe <b>29</b>, through an inlet end <b>31</b> of the reservoir <b>14</b>, and then into the reservoir <b>14</b> before passing through the outlet end <b>26</b>. The coating composition (c) then flows across the outlet end <b>26</b> in an uniform overflow curtain-like layer <b>32</b>, as described below, before depositing onto the gravure surface <b>12</b>.

According to Fig. 2, at least one baffle <b>34</b> is disposed in the reservoir <b>14</b> downstream of the outlet end <b>26</b>. Baffle <b>34</b>, preferably has a first portion <b>36</b> attached to the base <b>24</b> and a second portion <b>38</b> directed substantially toward the rear wall <b>22</b> of the reservoir <b>14.</b> This arrangement of the baffle <b>34</b> minimizes turbulent flow and eddy currents in the reservoir <b>14</b> when the coating composition (c) is being forced through an inlet port <b>37</b> and then into and out of the reservoir <b>14</b>. We prefer using only one baffle <b>34</b> to reduce the turbulent entrance effects in the reservoir <b>14</b>.

In Figs. 1-3, outlet end or weir <b>26</b> has a widthwise lip <b>40</b> extending vertically downwardly toward the gravure surface 12. Portions <b>41</b>,<b>42</b> of widthwise lip <b>40</b> extend beyond a respective opposing end edges <b>45</b>,<b>47</b> of the gravure surface or cylinder <b>12</b>, as shown in Fig. 3. It is important to the invention that width (w₁) of outlet end <b>26</b> is somewhat greater than the width (w₂) of the gravure cylinder <b>12</b>. Preferably widthwise lip <b>40</b> has a height (h) of about 0.50 inches (1.27 cm) measured from the outlet end <b>26</b> to end edge <b>44</b> of lip <b>40</b>. According to Fig. 2, end edge <b>44</b>, which is in close proximity to the gravure surface <b>12</b>, has an undercut portion <b>48</b> angled upwardly towards the base <b>24</b> and away from the widthwise lip <b>40</b>. This preferred outlet end <b>26</b> configuration enables the coating composition (c) exiting the reservoir <b>14</b> to overflow downwardly across the vertical widthwise lip <b>40</b> before being deposited onto the gravure surface <b>14</b>. The undercut portion <b>48</b> has a length (l) preferably not less than about 1.5 inches (3.81 cm), and has an angle β preferably about 10 degrees to about 40 degrees away from vertical widthwise lip <b>40</b> to provide a pinning point for the curtain-like coating meniscus, and to prevent migration of the curtain like coating layer <b>32</b>.

In an alternative embodiment, reservoir <b>14</b> may be equipped with a shield (not shown) which may be arranged to extend along the widthwise curtain-like layer <b>32</b>. The shield would prevent excessive evaporation of a volatile coating composition in reservoir <b>14</b>.

Moreover, reservoir <b>14</b> may be arranged on an adjustable mount <b>49</b> in apparatus <b>10</b> (shown clearly in Fig. 2) to either vertically or horizontally reposition the weir <b>26</b> relative to the gravure surface <b>12</b>. Being able to position the weir <b>26</b> enables the operator to set and control a desired curtain-like application deposit point, and height, angle and flowrate of the curtain-like coating layer exiting the lip <b>40</b> and depositing onto the gravure surface <b>12</b>. Experience indicates that end edge <b>44</b> of lip <b>40</b> and gravure surface <b>12</b> should be spatially separated by a distance of at least 2 inches (9.08 cm). Also, the coating flow angle (a), measured from the top <img file="EP0787584A1_D0002.tif" />dead<img file="EP0787584A1_D0001.tif" /> center <b>50</b> of the gravure surface <b>12</b> to the coating deposition point <b>52</b> on the gravure surface <b>12</b>, should not exceed about 55 degrees. Setting of these parameters insures that the gravure surface <b>12</b> is wetted immediately downstream of the impression nip <b>43</b> (Fig. 1).

As indicated, it is important to the invention that a uniform, overflow curtain-like layer <b>32</b> of coating composition (c) is formed before exiting the weir <b>26</b> and depositing onto the gravure surface <b>12</b>. It is our experience that the curtain-like coating layer <b>32</b> is robust to gravure coating process startup and idling conditions, especially when conditions indicate that the gravure cylinder <b>12</b> must remain wet, with the doctor blade <b>54</b> and impression cylinder <b>56</b> disengaged. Further, the use of forming and depositing a curtain-like layer of coating onto a gravure surface <b>12</b>, such as illustrated in Figs. 1-2, has heretofore not been known to solve problems associated with any flow separation, stagnation region and eddies, typically seen with any roll coating feed system at increasingly higher production coating speeds. Stagnation regions and eddy currents in the coating composition flow where impurities and bubbles form, are thought to also give rise to <img file="EP0787584A1_D0002.tif" />flow line<img file="EP0787584A1_D0001.tif" /> type defects (as discussed previously) on the coating layer deposited on the gravure surface <b>12</b>. Accordingly, it has been our observation that the curtain-like coating feed apparatus <b>10</b> of the invention unexpectedly provides a coating deposition process which virtually eliminates flow separation and stagnations regions, thereby eliminating the possibility of occurrence of <img file="EP0787584A1_D0002.tif" />flow line<img file="EP0787584A1_D0001.tif" /> defects

Furthermore, it is well known that the momentum of the curtain-like overflow layer of coating composition (c) depositing onto the gravure cylinder <b>12</b> provides gravitationally assisted deposition of the coating composition onto the gravure cylinder <b>12</b>. This phenomenon appears to help in displacing the air entrained by the gravure cylinder <b>12</b> at higher speeds by raising the critical speed at which wetting failure and hence <img file="EP0787584A1_D0002.tif" />flowpatterns<img file="EP0787584A1_D0001.tif" /> appear on the gravure surface <b>12</b>. According to Figure 4, a comparison is presented of the air entrainment speed limit of the curtain-coating apparatus of the invention and the feedbar pond system typically found in the prior art. The results suggest that by using the curtain coating apparatus <b>10</b> of the present invention, it is now possible to coat at higher coating speeds than ever before.

Additionally, we have observed, on the one hand, that the metered excess of coating composition (c) depositing on the gravure cylinder <b>12</b> by the curtain-like overflow layer is thin enough that it minimizes splashing at the doctor blade <b>54</b> at relatively high speeds. On the other hand, it is our observation that the coating deposition on the gravure surface <b>12</b> is thick enough to prevent excessive evaporation, which causes <img file="EP0787584A1_D0002.tif" />drying in<img file="EP0787584A1_D0001.tif" /> defects in the case of volatile solvents.

<b>Parts List</b>

<dl id="dl0001" compact="compact"><dt>10</dt><dd>apparatus</dd><dt>12</dt><dd>gravure surface or cylinder</dd><dt>14</dt><dd>reservoir/gravure surface</dd><dt>16</dt><dd>sidewalls of base</dd><dt>22</dt><dd>rear wall of base</dd><dt>24</dt><dd>base</dd><dt>26</dt><dd>outlet end or weir</dd><dt>29</dt><dd>pipe</dd><dt>31</dt><dd>inlet end</dd><dt>32</dt><dd>curtain-like coating layer</dd><dt>34</dt><dd>baffle</dd><dt>36</dt><dd>first portion</dd><dt>37</dt><dd>inlet portion</dd><dt>38</dt><dd>second portion</dd><dt>40</dt><dd>vertical widthwise lip</dd><dt>41</dt><dd>extended widthwise portions</dd><dt>42</dt><dd>extended widthwise portions</dd><dt>43</dt><dd>impression nip</dd><dt>44</dt><dd>end edge</dd><dt>45</dt><dd>opposing end edges</dd><dt>47</dt><dd>opposing end edges</dd><dt>48</dt><dd>undercut portion</dd><dt>49</dt><dd>adjustable mount</dd><dt>50</dt><dd>top <img file="EP0787584A1_D0002.tif" />dead<img file="EP0787584A1_D0001.tif" /> center</dd><dt>52</dt><dd>deposition point</dd><dt>54</dt><dd>doctor blade</dd><dt>56</dt><dd>impression cylinder</dd></dl>

The invention has therefore been described with reference to certain embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention.