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Title:
METHOD FOR APPLYING A COATING ONTO A MOVING WEB
Document Type and Number:
WIPO Patent Application WO/1998/043746
Kind Code:
A1
Abstract:
A method for using a coating apparatus (10) to improve productivity and reducing material waste when a continuous flow of a coating liquid onto a web (12) is desired and when the continuous flow is intentionally stopped to address a coating problem or to end, or temporarily postpone, a coating session. The method includes the use of flow control means (22) which enables the continuous flow to be quickly and cleanly started and stopped to address a coating problem or to end or temporarily postpone the coating session.

Inventors:
MILBOURN THOMAS M
LUDEMAN THOMAS J
YAPEL ROBERT A
BHAVE APARNA V
Application Number:
PCT/US1997/023193
Publication Date:
October 08, 1998
Filing Date:
December 11, 1997
Export Citation:
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Assignee:
IMATION CORP (US)
International Classes:
B05C5/02; B05C11/10; G03C1/74; G11B5/848; (IPC1-7): B05C11/10; B05C5/02; G03C1/74; G11B5/848
Foreign References:
US5360629A1994-11-01
US5536313A1996-07-16
US4729858A1988-03-08
US3973961A1976-08-10
US4050410A1977-09-27
US4938994A1990-07-03
US4739859A1988-04-26
US4831961A1989-05-23
US5360629A1994-11-01
US5541054A1996-07-30
EP0748456A11996-12-18
US5567507A1996-10-22
Attorney, Agent or Firm:
Weimer, William K. (P.O. Box 64898 Saint Paul, MN, US)
Hilleringmann, Jochen (Deichmannhaus, K´┐Żln, DE)
Download PDF:
Claims:
What is claimed is:
1. A method for using a coating apparatus (10) to improve productivity and reduce material waste when a continuous flow of at least one coating liquid onto a web (12) is desired and when the continuous flow is intentionally stopped to address a coating problem or to end or temporarily postpone a coating session, the coating apparatus including a coating die (14) for continuously applying a first coating liquid to the web (12), the coating die (14) having a first chamber containing the first coating liquid, the coating apparatus (10) also including a first reservoir (17) containing the first coating liquid and a first coating supply line (20) connecting the first reservoir (17) to the coating die (14) and allowing the first coating liquid to flow from the first reservoir (17) to the coating die (14), the coating apparatus (10) further including first flow control means (22) connected to the first coating supply line (20) and positioned between the first reservoir (17) and the coating die (14), the first flow control means (22) causing the first coating liquid to be directed to the coating die (14) or to be diverted away from the coating die (14), the first flow control means (22) further causing a controlled excessive flow to the coating die (14) approximately when flow is initially directed to the coating die (14) to cause the first coating liquid to bridge from the die (14) to the web (12), the first flow control means (22) ending the bridging to the web (12) and drawing first coating liquid back into the die (14) approximately when the flow is diverted away from the coating die (14); the method comprising the steps of: positioning the web (12) relative to the coating die (14) to form a desired first coating gap ; causing the web (12) to move relative to the coating die (14); causing the first coating liquid to exit the coating die (14) and bridge across the first coating gap to the web (12); applying the continuous flow of the first coating liquid to the web to create a length of coated web (12); and stopping the continuous flow of the first coating liquid to the web (12) when needed or desired to address a coating problem or to end or temporarily postpone the coating session, the stopping step being accomplished by actuating the first flow control means (22) such that the first coating liquid is diverted from the coating die (14) and such that the bridging of the first coating liquid to the web (12) is stopped and the first coating liquid is drawn back into the coating die (14).
2. The method of claim 1, further comprising the step of restarting the continuous flow of the first coating liquid to the web (12) after addressing the coating problem or when resuming after a temporary postponement of the coating session, the restarting step being accomplished by actuating the first flow control means (22) such that the first coating liquid is directed back to the coating die (14) and such that the controlled excessive flow flows to and through the coating die (14) to cause the first coating liquid to again bridge from the die to the web (12).
3. The method of claim 1, further comprising the steps of: moving the web (12) relative to and a sufficient distance from the coating die (14) to prevent contact between a web splice and the die (14) when a web splice is transported past the die (14); and moving the web (12) relative to the coating die such that the web (12) and die (14) again form the desired first coating gap.
4. The method of claim 1, wherein the stopping step is performed without obstructing the flow within the die (14), and wherein the first flow control means (22) comprises a first piston (26) and the stopping step comprises actuating the first flow control means (22) such that movement of the first piston (26) draws coating liquid exiting tne coating die (14) back into the coating die (14), the first piston (26) having an adjustable stroke length, the method further comprising the step of adjusting the stroke length of the first piston (26) to correspond to the properties of the first coating liquid and the conditions of coating.
5. The method of claim 1, wherein the first flow control means (22) includes a first spool valve (28) and the stopping step comprises actuating the first flow control means (22) such that the first spool valve (28) diverts the first coating liquid away from the coating die (14).
6. The method of claim 1, wherein the stopping step comprises diverting the first coating liquid back to the first reservoir (17), further comprising the step of filling the first chamber with the first coating liquid before the stopping step.
7. The method of claim 2, wherein the step of restarting the continuous flow comprises actuating the first flow control means (22) such that the first flow control means (22) pumps the controlled excessive flow of first coating liquid to and through the coating die (14).
8. The method of claim 1, the coating die (14) being capable of continuously applying a second coating liquid to the web (12) simultaneously with the first coating liquid, the coating die (14) further having a second chamber containing the second coating liquid, the coating apparatus (10) also including a second reservoir (17) containing the second coating liquid and a second coating supply line (20) connecting the second reservoir (17) to the coating die (14) and allowing the second coating liquid to flow from the second reservoir (17) to the coating die (14), the coating apparatus (10) further including second flow control means (22) connected to the second coating supply line (20) and positioned between the second reservoir (17) and the coating die (14), the second flow control means (22) causing the second coating liquid to be directed to the coating die (14) or to be diverted away from the coating die (14), the second flow control means (22) further causing a controlled excessive flow to the coating die (14) approximately when flow is initially directed to the coating die (14) to cause the second coating liquid to bridge from the die to the web (12), the second flow control means (22) ending the bridging to the web (12) and drawing second coating liquid back into the die (14) approximately when the flow is diverted away from the coating die (14); the method comprising the steps of: positioning the web (12) relative to the coating die (14) to form a desired second coating gap; causing the web (12) to move relative to the coating die (14) ; causing the second coating liquid to exit the coating die (14) and bridge across the second coating gap to the web (12); applying the continuous flow of the second coating liquid to the web (12) to create a significant length of coated web; and stopping the continuous flow of the second coating liquid to the web (12) when needed or desired to address a coating problem or to end or temporarily postpone the coating session, the stopping step being accomplished by actuating the second flow control means (22) such that the second coating liquid is diverted from the coating die (14) and such that the bridging of the second coating liquid to the web (12) is stopped and the second coating liquid is drawn back into the coating die (14).
9. The method of claim 8, the step of stopping the second coating liquid occurring prior to the step of stopping the first coating liquid, the step of stopping the first coating liquid occurring prior to the step of stopping the second coating liquid, the first coating liquid exiting the coating die (14) above where the second coating liquid exits the coating die (14).
10. The method of claim 12, further comprising the step of restarting the continuous flow of the second coating liquid to the web (12) after addressing the coating problem or when resuming after a temporary postponement of the coating session, the restarting step being accomplished by actuating the second flow control means (22) such that the second coating liquid is directed back to the coating die (14) and such that the controlled excessive flow flows to and through the coating die (14) to cause the second coating liquid to again bridge from the die to the web (12), the step of starting the first coating liquid occurring prior to the step of starting the second coating liquid, the step of starting the second coating liquid occurring prior to the step of starting the first coating liquid, the first coating liquid exiting the coating die (14) above where the second coating liquid exits the coating die (14), the second coating liquid exiting the coating die (14) above where the first coating liquid exits the coating die (14).
11. The method of claim 1, further comprising the step of converting the coated web into one of an imaging media and a data storage media.
Description:
METHOD FOR APPLYING A COATING ONTO A MOVING WEB TECHNICAL FIELD The present invention relates to methods for applying a coating onto a moving web and, particularly, to methods for improving the starting and stopping portions of coating processes.

BACKGROUND OF THE INVENTION Starting up a slot die coating process involves (a) transporting the web, (b) bringing the die coater close to the moving web, and (c) starting the flow of coating liquid in order to establish the coating bead. A quick and clean start is very important to achieve acceptable productivity and material usage.

A poor start involves such problems as excessively thick coating as the bead is established that can result in coating liquid coming into contact with and sticking to the idler rollers (a. k. a. coating pick-off).

When coating liquid sticks to an idler roller, it can pick-off more coating liquid from the web leading to an increased incidence of point defects. Coating pick-off can be avoided either by increased drying rates that can cause overdrying or by reduced coating speeds that reduce productivity. In some instances, undesirable saw-toothed coating occurs during start-up which can extend over many feet of the web. Inability to establish a sharp contact of the coating liquid with the web leads to increased waste before uniform coating is established and can also lead to heavy coating spots that lead to the aforementioned problems. A poor start can also lead to uneven contact lines on the die lips leading to streaks in the coated material, and unacceptable coating quality. Also, this can lead to multiple start- up attempts, which increases waste.

At the end of the coating operation, or during necessary

interruptions to the coating operation, the coating bead is broken and the die is commonly pulled back from the web. The web may be either stopped or continued depending on the nature of the interruption. It is equally important to stop the coating operation in a quick and clean fashion, i. e., the contact line between the coating liquid and the web should be broken in a clean line (without saw- toothed or concave/convex profiles) and with minimal heavy spots to avoid the aforementioned drying or productivity problems. A poor stop can also lead to coating liquid dripping down the die land, where drying of the coating liquid can lead to streaks during the subsequent start.

Establishing good starts and stops is especially important during known periodic interruptions of the coating process such as when a web splice passes through the coater head. In practice, the splice quality is such that coating over a splice is often not possible and, hence, the coating operation is interrupted and the coater die pulled back to allow passage of the splice. Minimizing waste due to splice passage (by establishing starts and stops in a quick, clean fashion) and after splice passage (by eliminating streaks) is, therefore, extremely important.

Furthermore, in situations in which multiple coating liquids are co-extruded through a slot die coater, it is desired to achieve quick and clean starts and stops with each of the coating liquids.

Various methods have been used in the past to try to achieve quick and clean starts and stops. U. S. Patent Nos. 3,973,961 and 4,050,410 to Stroszynski disclose a coating apparatus for coating photoconductor patches onto a carrier web. A main pump supplies the majority of the liquid to the die and recycle line. Excess flow is supplied to the die to obtain a transversely uniform flow of the liquid through the die to the web. Two dosing pumps, one upstream and one downstream of the die, complement the main pump by adding controlled oversupply and retraction of liquid in the die for starts and

stops, respectively, of the coating process. However, with this system, non-uniform light areas of coating occur on the front and back portions of the coated patch. Moreover, the apparatus can provide relatively sharp starts (over 2-10 millimeters) of coating on the web, but cannot coat sharp stops (requiring 30 millimeters or more to stop).

The ends of the coating are curved and cannot be made straight.

This variation leads to unacceptably high waste levels. Also, the recirculation system of this apparatus recirculates liquid through the coating die. The apparatus as described in the above patents is complex and leads to high waste levels (especially at stops).

Moreover, these patents do not recognize the need to quickly and cleanly start and stop during a continuous coating for optimum productivity and material usage, not to mention such a need when simultaneously coating multiple liquid layers.

U. S. Patent No. 4,938,994 to Choinski and a related promotional brochure entitled"Inca-2000 Patch Coater"disclose an apparatus for patch coating a plurality of incremental printed circuit boards. During operation, the coating liquid is fed through applicator lips without continuously circulating. A single patch is coated onto a single incremental board. There is no teaching to coat a plurality of patches onto a single board or substrate, and, hence, these references are not concerned with coating edge sharpness. Moreover, a positioning piston moves the die toward and away from the board to coat and to assist in breaking the coating bead. This will not work adequately at high line speeds (greater than 25 feet per minute).

Moving the die toward and away from a web to coat and assist in breaking the coating bead as described in these references does not provide for sharp front and rear edges. As a result, these references do not allow for quick and clean starts and stops. Moreover, moving the die does not permit the extrusion of coating liquid onto the web to occur while maintaining a constant distance between the coating die and the web, thereby increasing the complexity of the operation.

Furthermore, the piston taught by Choinski is inside the die which can create shock waves and cause coating defects. The piston is a flow obstacle which disrupts the coating liquid flow in the die which results in a non-uniform flow distribution across the die width which leads to non-uniform coating defects, such as streaking and banding.

Still further, the Choinski die is positioned perpendicular to the horizontal coating substrate. The die is positioned vertically with the die lips pointing downwardly toward the web surface. This can lead to two problems. First, air bubbles tend to accumulate in the die manifold leading to non-uniform coating and increased waste levels due to the increased effective compressibility of the system (dampening). With lower viscosity liquids, it is more likely for the coating liquid to dribble from the coater die lips onto the web after coating stops, therefore requiring a lip seal (which prevents or minimizes spots of heavy coating that lead to pick-off and other drying problems). Second, this can lead to streaks on the subsequent start of coating. Furthermore, as Choinski coats discrete circuit boards, there is no product beneath the die between the coatings to be ruined by the dribbling of the coating liquid. Moreover, Choinski does not teach either an apparatus or a method to rapidly start and stop multiple liquid layers during a continuous coating process.

U. S. Patent Nos. 4,739,859 and 4,831,961 to Chino et al. disclose applying a magnetic coating to a moving web. A valve helps to recirculate the coating liquid back to a reservoir through a bypass line when the coater stops. Recirculation occurs at the end of coating and ceases during the re-start of coating after the passage of a joint between two connected webs. The valve, apparently a standard pneumatic valve, starts and stops the coater and requires about 0.5-2 seconds to move. Consequently, this slow speed will lead to increasingly high waste levels with any given start and associated stop as coating line speed is increased. For example, at a line speed of 400 feet per minute, this implies that up to 26.7 feet of waste results, in

addition to the intrinsic uncoatable waste of the joint (not including further waste due to streaking or coating overthickness). Chino claims the use of a valve in the by-pass line to control the pressure in the line to be substantially equal to that of the coating liquid being applied to the web. Moreover, Chino does not describe either apparatus or method to rapidly and cleanly start and stop a coating process involving the simultaneous flow of multiple coating liquids.

A coating apparatus is noted in U. S. Patent No. 5,360,629.

This reference teaches using the coating apparatus to coat discrete, consistent, and clean-edged patches of liquid onto webs. This reference, however, does not teach, suggest, or infer using the disclosed coating apparatus for other than patch coating.

A need exists for a method which stops and restarting the flow of the coating liquid during a continuous coating process in order to improve productivity and material usage, especially for such coating processes which involve the simultaneous coating of multiple coating liquids.

SUMMARY OF THE INVENTION The present invention addresses the need for a method for using a coating apparatus to improve productivity and reducing material waste. This method is particularly useful when a continuous flow of at least one coating liquid onto a web is desired and when the continuous flow is intentionally stopped to address a coating problem or to end or temporarily postpone a coating session. The coating apparatus can include a coating die for continuously applying a first coating liquid to the web. The coating die has a first chamber containing the first coating liquid. The coating apparatus also includes a first reservoir containing the first coating liquid and a first coating supply line connecting the first reservoir to the coating die and allowing the first coating liquid to flow from the first reservoir to the coating die. The coating apparatus further includes first flow control

means connected to the first coating supply line and positioned between the first reservoir and the coating die. The first flow control means causes the first coating liquid to be directed to the coating die or to be diverted away from the coating die. The first flow control means further causes a controlled excessive flow to the coating die approximately when flow is initially directed to the coating die to cause the first coating liquid to bridge from the die to the web. The first flow control means ends the bridging to the web and draws first coating liquid back into the die approximately when the flow is diverted away from the coating die. The method includes the step of positioning the web relative to the coating die to form a desired first coating gap. Another step involves causing the web to move relative to the coating die. Another step involves causing the first coating liquid to exit the coating die and bridge across the first coating gap to the web. A further step involves applying the continuous flow of the first coating liquid to the web to create a significant length of coated web. Another step involves stopping the continuous flow of the first coating liquid to the web when needed or desired to address a coating problem or to end or temporarily postpone the coating session. The stopping step is accomplished by actuating the first flow control means such that the first coating liquid is diverted from the coating die and such that the bridging of the first coating liquid to the web is stopped and the first coating liquid is drawn back into the coating die.

Another aspect of the present invention involves the step of restarting the continuous flow of the first coating liquid to the web after addressing the coating problem or when resuming after a temporary postponement of the coating session. The restarting step is accomplished by actuating the first flow control means such that the first coating liquid is directed back to the coating die and such that the controlled excessive flow flows to and through the coating die to cause the first coating liquid to again bridge from the die to the web.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic view of the coating apparatus according to the present invention; Figure 2 is a schematic view of a flow control means within the coating apparatus shown in Figure 1; and Figure 3 is a schematic view of the flow control means in another position to divert coating liquid from a coating die.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The present invention involves a method which allows for the stopping and restarting of the flow of a coating liquid during a coating process. This ability is highly beneficial to minimize the effects of coating defects and web variations. Such adverse effects can be raw material waste, reduced productivity, and reduced production capacity.

This method is particularly effective for coating processes generally involving the continuous flow of the coating liquid onto a web (e. g., polymeric film, nonwoven, woven, or other substrate).

Specifically, such coating processes which involve the simultaneous coating of multiple coating liquids. This method can employ a coating apparatus, as shown in Figure 1. (A similar single layer coating apparatus is described in U. S. Patent No. 5,360,629 and is noted as being useful for coating patches of liquid onto a moving web. This patent is hereby incorporated by reference.) Figure 1 illustrates an embodiment of a coating apparatus 10 capable of coating two liquids (not shown) onto a moving web 12 (e. g., polymeric film, paper, nonwoven, or woven material) or other substrate. The coating apparatus generally includes an extrusion die 14 and a rotating backup roll 16 against which the web 12 is positioned.

Known extrusion dies, such as the Ultracoat and Magnacoat models made by Extrusion Dies Inc. of Chippewa Falls, Wisconsin, can

be used. Tne extrusion die can include two internal manifolds (not shown) and two associated die slots (not shown) which are adjacent the web 12 and through which the first and second coating liquids pass to form the beads which bridge to the web 12.

Each of the two coating liquids is supplied to the extrusion die 14 from a liquid reservoir 17 via a metering pump 18. The pump 18 can be a gear-type metering pump which is available from Parker Hannifin Corp., Zenith Pump Division, Sanford, NC.

Liquid supply lines 20 connect the reservoir 17, pump 18, and the extrusion die 14. A flow control means 22 communicates with the liquid supply lines 20 and is positioned between the reservoir 17 and the extrusion die 14.

As shown in Figures 2 and 3, each flow control means 22 includes a housing 24, a piston 26, a spool valve 28, and liquid seals 29. Each flow control means 22 communicates with a recirculating line 30. The recirculating line 30 further communicates with the associated liquid reservoirs 17.

The spool valve 28 can be an air-operated three-way, high speed spool valve. The spool valve either allows the coating liquid to continue on to the extrusion die via supply lines 20 as shown in Figure 2 (when coating onto the web 12 is desired), or diverts or recirculates the coating liquid back to the reservoir 17 as shown in Figure 3 (when coating onto the web 12 is not desired). The spool valve 28 does not displace the coating liquid when the spool 32 shuttles back and forth.

The piston 26 can be an air-operated piston which displaces the coating liquid without any displacement caused by the spool valve 28.

The movement of the piston 26, as shown in Figures 2 and 3, provides for a pumping effect and a drawing effect which are discussed more below. The piston can have a stroke length which can be adjusted to correspond to the properties of the coating liquid and the conditions of coating.

The nousings 24, pistons 26, and the spool valves 28 can be constructed using 316 Stainless Steel. The liquid seals can be Teflon spring seals, Model No. 220-218-0104 available from Furon Co., Los Alamitos, CA 90720. The pistons 26 and the spool valves 28 can be pneumatically actuated with air cylinders (not shown) using a pressurized nitrogen source (e. g., 300 psi) and a controller (not shown).

Coating liquid is constantly pumped from the reservoir 17 through the spool valve 28. As previously noted, the spool valve 28 is either positioned to cause the spool valve 28 to direct the coating liquid to the extrusion die 14 to coat the web 12 (Figure 2), or is positioned to cause the spool valve 28 to divert or recirculate the coating liquid back to the reservoir 17 (Figure 3).

The rapid movement of the piston 26 and the spool valve 28 and the minimal disruptive effect this movement has on the flow of the coating liquid (minimal shock/vibrational disturbances causing coating nonuniformity defects) allows the apparatus 10 to quickly start, stop, and re-start the coating of the web 12.

The piston 26 and the valve 28 are cooperatively operated.

When coating is to be started or restarted (for purposes previously noted), the valve 28 causes coating liquid to proceed to the extrusion die 14, which has been filled with coating liquid. At approximately the same time, the piston 26 moves toward the coating liquid in the liquid supply line 20 (Figure 2) to force the coating liquid toward and through the extrusion die 14 to provide a controlled excess flow of coating liquid to the extrusion die 14 and to the web 12. The quick supply of controlled excess flow causes the coating liquid to quickly bridge the gap from the die slot (not shown) to web 12 and to form a sharp edge of coating liquid on the web 12.

When the coating of the web 12 is to be stopped (for reasons previously noted), the valve 28 diverts the coating liquid back to the reservoir 17 while the piston 26 moves away from the coating liquid in

the liquid supply line 20 (Figure 3). This quickly and cleanly draws coating liquid back into the die 14 (which would have otherwise flowed out of the die slot), and provides a sharp break in the flow of the coating liquid from the extrusion die 14.

The movement of the piston 26 can precede, follow, or can operate simultaneously with the opening or closing of the valve 28.

The piston stroke can be varied to change the effective volume of coating liquid displaced. These variations can allow for quick, clean starts and stops accommodating different coating parameters such as viscosity, web speed, and coating thickness.

The time to move the spool valve 28 has been measured at about 4 milliseconds from the time the valve 28 begins to move, to the time it has finished its one-way stroke. This time period can generally be accounted for in the piston and the valve control scheme. The piston movement time is about 2 milliseconds. Thus, the apparatus 10 can stop coating in 4 milliseconds and can re-start coating in 4 milliseconds. (Faster times can be accomplished by increasing the nitrogen pressure to actuate the pneumatics, improving the piston design, or by using a hydraulic drive system, a direct electronic motor, or solenoid drive system.) As previously noted, the edge of coating liquid on the web 12 caused by a start or a stop is sharp and clean, that is, the edge is of high quality. By that, it is meant that the coating is distributed to the web such that the desired coating width, thickness, and general uniformity are instantly achieved. As a result, previously noted defects (e. g., overthickness, overdrying, pickoff) are minimized if not eliminated.

Furthermore, as previously mentioned, the ability to quickly and cleanly start and stop the flow of coating liquid from the die slot results in less drying of coating liquid on the die lips, die lands, and across to the web. As previously noted, reduced drying can result in less streaking and less cleaning. This control of the coating liquid at

the die lips also permits starts and stops, in many cases, without requiring the movement of the extrusion die 14 relative to the back-up roll 16. This results in even quicker starts and stops.

A primary aspect of the present invention is to use the apparatus for a new use, which is to improve productivity and to reduce material waste when it is desired to continuously coat or flow a liquid onto a web. As noted, this new use is a flow-interrupting and a flow-initiating (or flow control) method which specifically improves productivity and reduces waste by quickly and cleanly starting, stopping, and restarting the continuous flow when prompted to address a coating problem or to end or temporarily postpone the coating session. Such continuous coating processes include coating a photosensitive emulsion or a magnetic solution onto a polyester film to create an imaging material or a data storage media, respectively. Such coating problems which can be addressed after interrupting the coating process can involve the passage of a splice between the coating die 12 and the backup roll 16, or the presence of a foreign particle or other body on the die which is causing or may cause coating defects.

This method allows for quick, clean starts and stops at relatively high coating line speeds, for example, at 400 feet per minute. At a line speed of approximately 400 feet per minute and with total start and stop time of 10 milliseconds, the method can reduce material waste to only 0.8 inch of coated web (not including the length of the splice). In addition to reduced material usage, the method results in significant productivity improvements. Because the apparatus 10 can provide for even a shorter start and stop time and can be used with a faster line speed, even less material waste and greater productivity is achievable. With less material waste and improved productivity, this continuous coating method results in a lower cost, higher capacity process.

Because of the enhanced capability to start coating, this method

also allows the establishment (quick start) of acceptable coating at an initial coating gap which is larger than the largest gap attainable to establish the coating bead by known methods. In other words, the method allows for the initial gap set to establish the coating bead to be larger and that larger gap to be maintained during the coating process which improves coating quality by, for example, reducing streaks.

Known methods are left, instead, with two alternatives, to remain at the narrower initial gap or to increase the gap after coating is started.

Both alternatives are undesirable.

Alternatively, this method allows for the establishment (quick start) of acceptable coating at higher line speeds than known methods.

Therefore, the method addresses the need to quickly reach higher line speeds. In fact, the method may eliminate the need to adjust the line speed after coating is started.

Another important aspect of this method is accurately and independently controlling the start and stop of each coating liquid.

One embodiment of the method is to start the coating liquid (upper liquid) which exits the coating die above where another coating liquid (lower liquid) exits, just before the other coating liquid. Another embodiment is to stop the lower coating liquid before the upper coating liquid. For example, the timing delay between these starts and between these stops can be less than one second. These abilities allow for the earlier start of the coating liquid which is less prone to pick-off than the other coating liquid. These abilities also allow for a more stable coating bead start.

The actual steps involved in one embodiment of this inventive method can include the following: (a) Filling a chamber within the extrusion die with coating liquid; (b) Moving the extrusion die relative to the web to form desired coating gap; (c) Rotating the backup roll and transport the web; (d) Starting coating by actuating the spool valves and pistons to

allow flow to the extrusion die and to provide the controlled excess coating flow (burst) to the extrusion to bridge the coating liquid from the die to the web; (e) Continuing coating a continuous bead of coating liquid to the web; (f) Stopping coating as required or desired to address a coating problem or to end or temporarily postpone the session (i. e., to allow passage of a web splice, to remove any build-up on the extrusion die, or to shut down after completing the desired coating quantity) by actuating the spool valves and pistons to divert flow to the reservoirs and to draw back an amount of coating liquid from the die slot to cleanly break the bead and prevent dribble of the coating liquid onto the die lips ; (g) If the stopping step in step (f) were taken to allow passage of a splice, increasing the gap between the die and the web to reduce the risk of damage due to contact between the splice and the die; and (h) If the stopping step in step (f) were not to shut down the coater, starting coating by repeating step (d). Of the gap increasing step in step (g) were taken, return to the coating gap before starting coating).

Variations of the above-noted method are clearly contemplated and should be considered to be within the scope of the present invention. Variations of the disclosed apparatus are similarly contemplated. Rather than involving the simultaneous coating of two liquids process, the method and apparatus are useful for coating a single coating liquid or three or more coating liquids onto a web or substrate. Rather than using the noted gear-type metering pump, the coating liquid could be delivered by any other pressure feed system.

Rather than being pneumatically driven, the piston 26 or the spool valve 28 can be operated mechanically, electrically, or hydraulically.

Rather than orienting the exit positions of the first and second coating liquid such that one exit position is above the other (with an implied

horizontal orientation of the die slots), the positions could be oriented in a side-by-side orientation (with an implied vertical orientation of the die slots).

Furthermore, the flow control method can be one part of a overall process used to ultimately manufacture finished goods such as data storage media products (e. g., floppy diskettes, tape, and the like), imaging media products (e. g., photographic film, photothermographic film, medical imaging film, imagesetting film, printing plates, proofing media, ink-jet receptor media, and the like), or other end-used products. Other parts of the overall process for making such products can include known methods for drying the coated web, applying additional coating liquids to the web (and drying such additional coating liquids), slitting the coated web, winding the web or slit web, converting the web or slit web into discrete pieces or lengths, inserting discrete pieces or lengths into some form of a product carrier (diskette exterior, tape cassette, and the like), and packaging the discrete pieces in, for example, primary boxes which fit within shipper boxes.

Some of the advantages provided by the inventive method are illustrated in the following Examples Section, but the particular materials and amounts thereof recited in these examples as well as other conditions and details, should not be construed to unduly limit this invention.

EXAMPLES Example 1 (Comparative) A dual-layer photothermographic construction was simultaneously coated using a dual extrusion die (such as that shown in Figure 1). The flow-interrupting method was not used. The bottom layer was a photothermographic emulsion layer prepared substantially as described in U. S. Patent No. 5,541,054, which is hereby incorporated by reference. This bottom layer was coated at a 70-

micron wet thickness and at a solution viscosity of between 1000- 1500 centipoise. The top layer was a protective overcoat layer prepared substantially as described in the same patent. This top layer was coated at a 22-micron wet thickness and at a solution viscosity of between 1000-1500 centipoise. Uniform, streak-free coating could not be established at line speeds greater than 100 feet per minute. At 100 feet per minute, coating was stopped within approximately 100 feet of web length.

Example 2 This Example is virtually identical to Example 1 except that the flow control method was used. In using the flow control method, the top coating layer was started approximately 250 milliseconds before the bottom coating layer was started; and the bottom coating layer was stopped approximately 250 milliseconds before the top coating layer was stopped. At 125 feet per minute, uniform and streak-free coating was established within 1.0 inch of the point at which coating liquid first contacted the web. At 125 feet per minute, coating was cleanly stopped within 1.0 inch of web length.

Example 3 (Comparative) An antihilation layer was coated using a single extrusion die without using the flow-interrupting method. This coating layer contained CAB 381-20 (a cellulose acetate butyrate available from Eastman Chemicals Co., Kingsport, TN) with a squarylium dye (disclosed as dye B, Example 13, European Laid-Open Patent Application No. 0 748 456 A1, which is hereby incorporated by reference). This coating layer was at a 40-micron wet thickness and a solution viscosity of between 800-1200 centipoise. At 400 feet per minute, establishing coating was difficult and unacceptably high material waste levels occurred (greater than 100 feet). Coating was stopped within not less than 100 feet of web length.

Example 4 This Example is virtually identical to Example 3 except that the flow control method was used. At 400 feet per minute, acceptable start-up of coating was achieved within 2.0 inches of the point at which coating liquid first contacted the web. At 400 feet per minute, coating was cleanly stopped within 1.0 inch of web length.

Example 5 An ink-jet receptive layer was coated using a single extrusion die and using the flow-interrupting method. The solution was coated at a 130-micron wet thickness and at a solution viscosity of approximately 1800 centipoise. At 75 feet per minute, coating was established within 0.25 inch of the first point at which the liquid contacted the web and uniform streak-free coating was established within one inch. At 75 feet per minute, coating was cleanly stopped within 1.0 inch of web length.

The inkjet receptive layer was prepared as described in U. S.

Patent No. 5,567,507.