Coating apparatus – Projection or spray type – With hood or offtake for waste material
Reexamination Certificate
2001-11-26
2003-08-26
Crispino, Richard (Department: 1734)
Coating apparatus
Projection or spray type
With hood or offtake for waste material
C118S300000, C118S324000, C118S325000, C118SDIG004, C427S420000
Reexamination Certificate
active
06610148
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to methods and apparatus for coating objects or moving webs by curtain coating and, more particularly, to an improved curtain coating method and apparatus for the manufacture of sheet goods including films and papers.
BACKGROUND OF THE INVENTION
In curtain coating, a moving receiving surface is coated by the impingement of a free-falling curtain of liquid coating composition. The curtain may be a single coating composition or a composite of a plurality of layers of distinct coating compositions such as is described in U.S. Pat. No. 3,508,947 to Hughes. The coating receiving surface can be any surface that can be passed through a curtain. The receiving surface may be a continuous web of paper, plastic, metal, or cloth. In applications demanding high coating uniformity, such as the coating of photographic films and papers, the web must be stabilized by a precision backing roller to prevent vibrations of the tensioned web.
It is desirable to start and stop coating without the deposition of excess coating liquids on the coating receiving surface. Excess liquids necessitate additional dryer capacity and, therefore, result in additional costs. Coating composition that does not dry can result in contamination of the coating machine and cause laps in wound rolls of coated web to adhere to one another.
A method and apparatus for making starts and stops in curtain coating with minimal excess coating liquids on the receiving surface is described in U.S. Pat. No. 4,851,268 to Kozak. The startup of the curtain coating process is achieved by using a downwardly inclined catch pan device closely spaced to the backing roller and positioned to intercept the falling curtain before and during retraction of the pan. As shown in
FIG. 1
, the prior art catch pan
10
has two spaced lips on its trailing end, a primary lip
12
that retains the curtain liquid puddle formed on the device during coating startup and a secondary lip
14
. There is a pan extension area
16
between the two lips
12
,
14
that captures the curtain liquids extended from their free fall position as the primary lip
12
intercepts the curtain during its retraction. Preferably, the height of the two lips
12
,
14
and the distance between them is determined by the retraction speed of the catch pan
10
and the time it takes the curtain to fall the distance from the top of the primary lip
12
to the pan extension area
16
. The method requires passing the catch pan
10
through the curtain at high speed (50 to 200 centimeters per second) in the direction of the moving web. A disadvantage of this catch pan
10
is the risk of collision with the coating receiving surface because there is only a small gap therebetween. Another disadvantage is that it is possible for the catch pan
10
to outrun the curtain. That is, the curtain fails to contact the pan extension area
16
and secondary lip
14
because the vertical distance between the primary lip
12
and secondary lip
14
over which the curtain drops is too great.
In U.S. Pat. No. 5,017,408 to Kozak, a catch pan side wall geometry is disclosed that prevents excess liquids from being deposited on the edges of the coating receiving surface when starting or stopping. As shown in
FIGS. 2 and 3
, the sidewalls
18
of the catch pan
20
have flexible extensions
22
that make contact with the flushed edge guides
24
maintaining the curtain width. This contact removes the flushing liquid stabilizing the edges of the curtain
26
. The curtain
26
may reform slowly or not at all on the dried portion of the edge guides
24
when the catch pan
20
is retracted. The edge guides
24
can also be contaminated or damaged by contact and their function impaired. The flexible edges
22
of the catch pan
20
may curl up when coating composition dries on them such that contact with the edge guides
24
is lost and excess coating solution passes onto the receiving surface
28
through the gap thereby created.
In U.S. Pat. No. 5,885,659 to Takahashi et al., a catch pan
30
as shown in
FIG. 4
for use on a uniformly inclined upwardly running web
32
is disclosed wherein the pan
30
is retracted in the direction opposite to web motion. A receiving portion
34
of the pan
30
receives the curtain
36
before coating. A shelf portion
38
of the pan
30
receives the curtain
36
as the pan
30
is withdrawn to start coating. The shelf portion
38
and receiving portion
34
of the pan
30
are connected by an upright wall
40
of at least 8 millimeters that cuts the curtain
36
as the pan
30
is withdrawn and isolates the liquid in the receiving portion
34
of the pan
30
. The shelf portion
38
is upwardly inclined from the receiving portion
34
by at least 5 degrees from horizontal to recover the liquid intercepted by the shelf portion
38
. A disadvantage of this method is that the upright wall (level difference)
40
may have to exceed substantially 8 millimeters because of the tendency of the pooled liquids in the receiving portion
34
of the pan
30
to climb and spill over the upright wall
40
as the pan
30
retracts. The higher level difference necessitates a longer shelf
38
. Also, retracting the catch pan
30
in the direction opposite to web motion causes any excess liquids that may reach the web
32
to pass through the curtain
36
and thereby disturb it. An additional disadvantage is that coating application must be performed on an unsupported web
32
under tension. This is generally not suitable for the most demanding applications.
The prior art catch pans are directed to coating receiving surfaces that are horizontal or slightly inclined. In European Patent Specification No. 0563308 B1 to Blake and Ruschak, a curtain coating method is disclosed in which the coating receiving surface is significantly downwardly inclined. Coating speed, as well as latitude in viscosity and coating thickness can frequently be increased by this method, and there is a need for a catch pan for a downwardly inclined coating receiving surface.
Typically, prior art catch pan systems include pneumatic cylinders positioned on each side of the catch pan. The pneumatic cylinders are used to drive the catch pan into and out of a curtain intercepting position. Pneumatic cylinders allow for a pan velocity on the order of 60 to 130 centimeters per second at the point the catch pan releases the curtain. The catch pan travels a distance on the order of about 20 cm through actuation of the pneumatic cylinders. The travel length and velocity require acceleration rates on the order of 2 times gravitational acceleration. Accelerating both sides of the catch pan at the same rate in order to maintain alignment using pneumatic cylinders is problematic at best. Prior art catch systems using pneumatic cylinders must be designed to allow for some undesirable skewing of the catch pan as it is accelerated into and out of the intercepting position.
The typical configuration of curtain coating apparatus often makes it impractical to tie the sides of the catch pan together rigidly enough to maintain alignment due to weight and the required acceleration rates. Further, mechanical linking mechanisms for tying the two pneumatic cylinders and therefore, the sides of the catch pan together are prone to misalignment, backlash, deflection, and wear. Attempts have been made to control the alignment of the catch pan while driving it with pneumatic cylinders by using equal lengths of supply and exhaust tubing for each cylinder as well as using precision pressure and flow control valves. However, even using these types of measures, the misalignment of the catch pan during movement is still on the order of several millimeters. Further, the speed achieved using pneumatic cylinders is inconsistent from cycle to cycle varying by 10% or more. The alignment and speed control of these prior art pneumatically driven systems is affected by friction from mechanical components and seals, deterioration of valves and controls, as well as dirt and wear.
It should also be und
Deprez Robert J.
Ruschak Kenneth J.
Bocchetti Mark G.
Crispino Richard
Eastman Kodak Company
Lazor Michelle A
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