Coating apparatus

An extrusion type coating apparatus for applying a coating film to a support has a bent doctor edge made of cemented carbide having a very large hardness. The bent doctor edge has a very large obtuse angle and the resulting doctor edge allows the coating film to be applied smoothly to the support. Since the doctor edge is made of cemented carbide having a very high hardness, the doctor edge is not unduly worn by abrasives contained in the coating film thereby improving the wear-resistance of the apparatus.

BACKGROUND OF THE INVENTION 
This invention relates to a coating apparatus. Specifically, the present 
invention is an improved extruder for extruding a coating onto a support. 
The term "support" as used herein is intended to mean a flexible 
belt-shaped material 0.3 to 3 m in width, 45 to 10,000 m in length and 5 
to 200 mm in thickness, which is made of either a plastic film of 
polyethylene terephthalate, polyethylene-2, 6-naphthalate, cellulose 
diacetate, cellulose triacetate, cellulose acetate prpionate, polyvinyl 
chloride, polyvinylidene, polycarbonate, polyimide or polyamide; paper; 
paper on which a-polyolefin having two to ten carbons such as polyethlene, 
polypropyrene or ethylene butane copolymer is coated or laminated; or 
metal foil or aluminum, copper or tin. The support includes a belt-shaped 
material which is obtained by forming a preliminary treatment layer on the 
aforementioned belt-shaped material. 
Samples of coating solutions include a photographing photosensitive 
solution, a magnetic solution, a surface protecting solution, a charging 
preventing solution or a smoothing solution. The particular solution 
extruded onto the surface of the support depends upon the use intended. 
After the coating solution dries, the coated support is cut into pieces 
having a desired width and length. Typical products of the support are 
photographing films, print paper and magnetic tapes. 
Japanese Patent Application Laid-Open No's 138036/1975 and 90350/1978 
disclose extruders with doctor edges which can apply an extremely small 
and uniform thickness coating solution to a support which is run at a high 
speed, the speed being between 50 and 100 m/min. 
A common feature of these extruders is that the support is laid in such a 
manner as to be able to bend in the direction of its thickness between 
support guiding means such as guide rollers. Another common feature is 
that protruded surfaces of a doctor edge and a back edge, which form a 
part of the front end portion of the extruder where a slot outlet for 
extruding the coating solution is formed, confront the support in such a 
manner so as to abut against the surface of the support. In addition, the 
distance between the support and the doctor edge provided on the 
downstream side of the support is changed according to the quantity of 
supply of extrusion of the coating solution extruded through the slot when 
carrying out the coating operation. 
In the former Japanese Patent Application Laid-Out No. 13803/1975, the 
doctor edge has a long edge length, and the edge surface is a curved 
surface having a large curvature, a polygonal surface, an uneven surface, 
or a completely flat surface. The doctor edge thus formed is confronted 
with the surface of the support. Therefore, if the section of the doctor 
edge is not uniformly machined and finished also in the widthwise 
direction of the support, then the thickness of the coated film is 
unavoidably considerably non-uniform in the widthwise direction. 
Accordingly, in machining the doctor edge, high machining precision is 
required, and a material for manufacturing the doctor edge is limited. 
Recently, there has been a strong demand for a coated film more uniform in 
thickness. However, in the case of the doctor edge having the 
above-described edge surface, the ends of the doctor edge or the parts in 
the vicinity thereof which greatly affect the surface quality, thickness, 
etc. of the coated film are not precisely straight because the ends of the 
doctor edge are rounded or chamfered to remove burrs. Therefore, the 
extruder is limited in controlling the thickness of the coated film in the 
order of micrometers (.mu.m). 
Even if it were possible to machine and finish the ends of the doctor edge 
in the order of micrometers, the surface and ends of the doctor edge would 
be readily worn, because when coating, the doctor edge is often kept in 
contact for a long time with a coating solution containing a relatively 
large quantity of relatively hard and minute foreign particles such as 
ferromagnetic iron oxide powders, silver halogenide particles and abrasive 
particles. Therefore, it is necessary to correct the portions of the 
doctor edge worn by abrasion. The accuracy of the coated finish is greatly 
lowered whenever abrasion occurs. 
In the latter Japanese Patent Application Laid-Out No. 90350/1978, the 
doctor edge also has a flat surface and rounded or chamfered ends. In 
addition, a protruded surface of the back edge located on the upstream 
side of the support with respect to the slot outlet contacts the surface 
of the support. Therefore, as in the former extruder referred to above, it 
is difficult to provide a coated film uniform in thickness and 
satisfactory in surface quality. 
Furthermore, both of these prior art extruders are disadvantageous in that, 
as the quantity of the above-described abrasive particles is increased, 
defects such as longitudinal stripes are liable to be formed which degrade 
the product quality. 
In the case where the doctor edge has a flat surface, the coated surface 
can be readily improved in accuracy. However, since the doctor edge has an 
upstream edge and a downstream edge, one of these two edges pushes the 
support surface more forcefully than the other edge resulting in the 
doctoring operation on the flat surface being non-uniform. As a result, 
the tension of the support is abruptly varied (increased) at the end 
pushing more forcefully and accordingly it is difficult to form a coated 
film having a desired thickness, and the aforementioned longitudinal 
stripe defects are frequently formed. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide a coating 
apparatus in which all of the above-described difficulties accompanying 
the prior art coating apparatus are eliminated, and machining and 
maintenance of the doctor edge can be readily achieved. 
The foregoing object and other objects of the invention are achieved by the 
provision of an extrusion type coating apparatus in which a coating 
solution is continuously extruded out of an open end of a slot which is 
set close to the surface of a belt-shaped flexible support running 
continuously along a predetermined running path. The coating solution is 
applied to the surface of the flexible support while the amount of coating 
solution applied to the support is metered to a desired thickness by a 
doctor edge adjacent to the slot. The doctor edge has a doctor surface 
bent towards the support so that the doctor edge is triangular in section, 
the doctor edge being set close to the support in such a manner that the 
support is bent substantially triangular, therefore metering the amount of 
coating solution applied.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1-3 show a first embodiment of the coating apparatus of the present 
invention. The extruder 1 includes a solution supplying system 2, a pocket 
3, a slot 4, a doctor edge 5, and a back edge 6. 
The solution supplying system 2 comprises pump means (not shown) which is 
provided outside the body of the extruder 1 for continuously supplying a 
coating solution C at a predetermined flow rate. The pump means 
communicates via pipe members with the pocket 3 which extends in the body 
of the extruder 1 in the widthwise direction of a support W. 
The pocket 3 is substantially circular in cross-section as shown in FIGS. 
1-3. The pocket 3 extends in the widthwise direction of the support W with 
the sectional configuration being substantially unchanged. That is, the 
pocket 3 is a kind of solution pool. The effective length of the pocket 3 
is, in general, equal to or slightly longer than the coated width to be 
applied to the support W. 
The inside diameter of the pocket 3 is generally in a range of from 10 mm 
to 50 mm. Both open ends of the pocket 3 are closed by shield plates 7 and 
8 which are secured to both end faces of the extruder 1. 
A short pipe 9 extends from the shield plate 7. The solution supplying 
system 2 is connected to the short pipe 9 so that the pocket 3 is filled 
with the coating solution C by the pump means. The coating solution C is 
extruded from the pocket 3 through the slot 4 under a uniform solution 
pressure distribution. 
The slot 4 is provided in the body of the extruder 1 from the pocket 3 
towards the support W. The slot 4 usually has a width e between 0.03 and 2 
mm. The slot 4 is a relatively narrow flow path which extends in the 
widthwise direction of the support W as in the case of pocket 3. The 
length of the opening of the slot 4 in the widthwise direction is 
substantially equal to the coating width of the support W. 
The length of the flow path in the slot 4 extending between the pocket 3 
and support W is determined by taking into consideration various 
conditions such as the composition and properties of the coating solution 
C, and a flow rate and a pressure in supplying the coating solution C. 
That is, the length of the flow path between the pocket 3 and the support 
W is determined so that the coating solution C flows in the form of a 
layer out of the pocket 3 with the flow rate and solution pressure 
distribution being uniform in the widthwise direction of the support W. 
At the end portion of the body of the extruder 1, where the slot 4 is open, 
a step d is formed by the doctor edge 5 and the back edge 6. 
The doctor edge 5 is provided on the downstream side of the support W with 
respect to the opening of the slot 4. The doctor edge is closer, usually 
by 0.01 to 1.0 mm, to the support W than the back edge 6 which is provided 
on the upstream side of the support W, thus forming the aforementioned 
step d. 
The doctor edge 5 has edge surfaces 61, 62 which confront the support W. 
The edge surfaces 61, 62 consist of two surfaces which form a vertical 
angle .alpha. which is an obtuse angle of 165.degree. or larger, 
preferably 170.degree. to 178.degree.. One of the two surfaces, surface 
61, which is on the downstream side of the vertical angle .alpha. has a 
length l.sub.1 of 1 to 15 mm, preferably 1 to 5 mm. 
The surface 62 on the upstream side of the angle .alpha. has a length 
l.sub.2 between 0.1 and 2 mm, the length l.sub.2 preferably being between 
0.1 and 1 mm. 
The edge surfaces 61 and 62 of the doctor edge 5 are made of a cemented 
carbide material having a hardness of about 70 or larger (in Rockwell 
hardness A scale) in order to dispense with the aforementioned rounding or 
chamfering thus improving the wear-resisting characteristics of these 
surfaces 61,62. 
The back edge 6 has an edge surface 63 which is substantially parallel with 
the edge surface 62 of the doctor edge 5. The edge surface 63 of the back 
edge 6 confronts the support W with the step d as described above. The 
edge surface 63 of the back edge 6 has a length between 1 and 5 mm, 
preferably being between 1 and 3 mm. 
The doctor edge 5 has an edge 51 on the downstream side of the vertical 
angle .alpha., and an edge 53 on the upstream side of the vertical angle 
.alpha.. A top edge 52 is located between the edges 51 and 53. The edges 
51, 52 and 53 are not rounded nor chamfered; i.e., they are not machined 
at all. The edges 51 and 53 merge with sides which are extended away from 
the support W, forming an angle .beta. of about 70 to 90 degrees with the 
aforementioned two surfaces, respectively. 
In the coating apparatus thus constructed, the support W is laid over 
running and guiding means such as guide rollers (not shown) in such a 
manner that the support W is under a substantially constant tension and is 
slightly curved in the direction of its thickness. When the support W thus 
laid is caused to approach the doctor edge 5 by an extruder supporting 
mechanism (not shown) so that the support W is curved and is substantially 
parallel with the downstream surface 61 of the doctor edge 5, the solution 
supplying system 2 starts supplying the coating solution C at a desired 
flow rate. The coating solution C is extruded through the pocket 3 and the 
slot 4 at a uniform flow rate and with a uniform pressure distribution. 
The coating solution C is delivered to the opening of the slot 4 and 
partially flows onto the edge surface 63 of the back edge 6 which 
confronts the support W with a small gap corresponding to the 
aforementioned step d to form a kind of bead in the gap. The coating 
solution flows along the surface of the support W which is moving 
continuously in the direction of the arrow A in such a manner as to push 
the support W away from the edge surfaces 61, 62 of the doctor edge 5. 
As the above-described movement of the coating solution is continued, the 
support W is spaced a constant distance apart from the edge surfaces 61, 
62 of the doctor edge 5 by the coating solution which flows in the form of 
a layer over the width of the support W. 
The constant distance between the support W and the edge surfaces 61,62 of 
the doctor edge 5 depends on various conditions, such as the tension of 
the support W, the distance between the support W and the extruder 1 and 
the flow rate of the coating solution C. If, among these conditions, the 
flow rate of the coating solution C is controlled, the separation distance 
between the support W and the edge surface 61,62 of the doctor edge 5, 
i.e., the thickness of the coated film, can be determined readily and 
accurately. 
As was described before, the edge surfaces 61,62 of the doctor edge 5 
consists of the upstream and the downstream surfaces 61,62 which form the 
obtuse angle .alpha., and the entire edge surfaces 62,61 are made of the 
cemented carbide material. Therefore, the upstream and downstream surfaces 
62,61 are very straight and the surfaces merging therewith are very flat. 
Furthermore, as the length l.sub.1 of the downstream surface 61 is made 
longer than the length l.sub.2 of the upstream surface 62, the bending of 
the support W is minimized. The doctoring operation carried out between 
the edge surfaces 61, 62 of the doctor edge 5 and the support W is 
effected smoothly and the coated film is more uniform in thickness in the 
widthwise direction. Not only is extreme bending of the support W 
prevented, but a wedge effect also results facilitating a high speed 
coating operation generated around the upstream surface 62. 
FIGS. 4 and 5 show modifications of the above-described method of supplying 
the coating solution C to the pocket 3. 
In the modification shown in FIG. 4, as in the method shown in FIG. 3, the 
coating solution is supplied from one side of the extruder. However, it 
should be noted that another short pipe 10 is mounted on the other shield 
plate 8, so that a part of the coating solution C injected into the pocket 
3 through the one short pipe 9 on the one shield plate 7 is discharged 
through the short pipe 10 to thereby prevent the coating solution C from 
being retained in the pocket 3 for a long time. This is considerably 
important in applying a magnetic coating solution which is thixotropic and 
condensable. 
An extruder shown in FIG. 5 can be obtained by adding a third short pipe 11 
to the extruder shown in FIG. 4. That is, the extruder in FIG. 5 has the 
short pipe 11 in addition to short pipes 9 and 10 provided on both plates 
7, 8. The short pipe 11 communicates withe substantially central portion 
of the pocket 3. That is, the extruder has a central supply system in 
which the coating solution C is supplied through the central short pipe 
11. 
A part of the coating solution C injected into the pocket 3 is discharged 
through short pipes 9 and 10 and the remaining coating solution C is 
extruded through the slot 4 with more uniform pressure distribution 
without being retained in the pocket 3. 
The coating solution supplying methods for the coating apparatus according 
to the invention have been described with references to FIGS. 3-5, 
respectively; however, these methods may be employed in combination. The 
cylindrical pocket 3 has been described; however, it may be a rectangular 
pocket, i.e., a ship's bottom-shaped pocket. That is, any pocket may be 
employed if it is so shaped as to make the solution pressure distribution 
uniform in the widthwise direction of the support W. 
Furthermore, the coating characteristics can be maintained satisfactory 
even if the body of the extruder except for the doctor edge 5 is not made 
of a cemented carbide material, i.e., the block member forming the back 
edge 6, the slot 4 or the pocket 3. 
The above-described coating apparatus of the invention provides the 
following novel effects or merits; 
At least the doctor edge 5 of the extruder 1 is made of the cemented 
carbide material. The doctor edge 5 has the edge surfaces 61,62 of two 
surfaces forming the obtuse angle .alpha.. The downstream edge 51, the top 
edge 52 and the upstream edge 53 of the doctor edge 5 are not chamfered at 
all, and these edges and the upstream and downstream surfaces 62,61 of the 
doctor edge 5 are remarkably improved in wear resistance. This eliminates 
any loss in the coating accuracy due to repolishing. 
The upstream and downstream surfaces 62,61 forming the edge surface of the 
doctor edge 5 form the obtuse angle .alpha.. 
The extruder 1 is confronted with the surface of the support W so that the 
support W is bent substantially parallel with the downstream surface 61 of 
the doctor edge 5. Therefore, problems which result when the support W is 
extremely bent are avoided. Accordingly the thickness of the coated film 
is not varied and longitudinal strips formed by abnormal tension 
variations are also avoided. 
As described above, the doctor edge 5 is triangular in section. Therefore, 
a wedge effect results and the coating solution C is more smoothly and 
positively applied to the support W near the upstream edge of the upstream 
surface 62. Accordingly, a high speed coating operation can be carried out 
with the coating apparatus of the present invention. 
The edge surface of the back edge 6 is retracted by the step d from the 
prolongation of the upstream surface 62 of the doctor edge 5. Therefore, 
contact between the extruder and the support surface W is prevented or 
minimized. Accordingly, degradation of the coated film surface quality due 
to the unsatisfactory surface quality of the support W, such as a 
scratched surface, can be prevented. 
The coating apparatus according to the invention has been tested as 
follows. 
Materials listed in Table 1 were sufficiently mixed and dispersed in a ball 
mill, and epoxy resin (epoxy equivalent 500) of 30 parts by weight was 
added to the resultant mixture. The resultant solution was uniformly mixed 
and dispersed to obtain a magnetic coating solution. 
TABLE 1 
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.gamma.-Fe203 powder (needle 
300 parts by weight 
particles having an average 
major diameter of 0.5.mu. , 
320 Oe) 
Vinyl chloride - vinyl acetate 
30 parts by weight 
copolymer (copolymerization 
ratio 83:13, copolymerization 
degree 400) 
Electrically conductive carbon 
20 parts by weight 
Plyamide resion (Amine value 
15 parts by weight 
300) 
Lecithin 6 parts by weight 
Silicon oil 3 parts by weight 
(Dimethylpolysiloxane) 
Xylole 300 parts by weight 
Methylisobutylketone 
300 parts by weight 
N--butanol 
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The equilibrium viscosity of the magnetic coating solution thus prepared 
was measured with a "Shimazu Rheometer RM-1" manufactured by the Shimazu 
Seisakusho. It was 8 P (poise) with a shearing speed of 10 sec..sup.-1, 
and was 1 P with a shearing speed 500 sec..sup.-1. 
The coating solution was applied with the coating apparatus as shown in 
FIGS. 1-3, under the following conditions. (No chamfering was carried 
out.) 
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(1) Support 
Material--Polyethylene terephthalate film 
Thickness--20 .mu.m 
Width--300 mm 
Tension--2 kg/full width, and 4 kg/full width 
Moving Speed--50 m/min., and 100 m/min. 
(2) Extruder 
Doctor edge material--SUS-27 and cemented carbide 
Doctor edge hardness--SUS-27: (60 or less 
in Rockwell hardness A scale); and cemented 
carbide: (88 or larger in Rockwell hardness A scale) 
Doctor edge 
Obtuse angle (.alpha.)--160, 165, 170, 178, and 180 degrees 
Downstream surface length (l.sub.1)--0.5, 1, 10, 15 and 16 mm 
Upstream surface length (l.sub.2)--0.05, 0.1, 1, 2 and 3 mm 
Back edge 
Edge surface length (l.sub.3)--0.05, 1, 5 and 6 mm 
Step (d)--0.01, 0.5, 1 and 1.5 mm 
(3) Coated film thickness (after dried)--2 .mu.m and 10 .mu.m 
(4) Coating time (in total)--500 hours 
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As a result, the following could be confirmed: 
(1) The extruder made of SUS-27 was worn in about fifty hours to the extent 
that the coated film thickness was greatly changed. On the other hand, the 
extruder made of the cemented carbide was scarcely worn even after 500 
hours. 
(2) In each of the extruders made of SUS-27 unsmoothed partions like burrs 
were found at both edges, and at the top edge of the bent edge surface of 
the doctor edge. The reproduction output of the coated film formed by the 
extruder made of SUS-27 varied. 
No defects like burrs were found in any one of the extruders made of the 
cemented carbide. However, depending on the conditions set for the edges, 
some coated films were found unsatisfactory in thickness and surface 
quality. 
(a) When the angle (.alpha.) was set to 160 degrees, the contact angle of 
the extruder with the support near the top edge was increased, as a result 
of which defects such as longitudinal stripes were frequently formed. Thus 
it was determined unsatisfactory to set the angle to 160 degrees. 
When the angle was set to 180 degrees, i.e., the doctor edge was made flat, 
the contact angle at the upstream edge was increased, as a result of which 
defects such as longitudinal stripes were frequently formed, while the 
coated film thickness in the widthwise direction was nonuniform. 
(b) When the length (l.sub.1) of the downstream surface of the doctor edge 
was set to 0.5 mm, the aforementioned doctoring action was insufficient, 
as a result of which the coated film thickness was not uniform. When the 
length was set to 16 mm, the tension loss of the support increased with 
the result that the support ran unstably. Thus, it was undesirable to set 
the length of 0.5 mm or 16 mm. 
(c) When the length (l.sub.2) of the upstream surface of the doctor edge 
was set to 0.5 mm, the coating solution was excessively applied to the 
support. When the length was set to 3 mm, the coating solution was liable 
to flow in the widthwise direction from the upstream surface. Thus, it was 
not suitable to set the length to 0.05 mm or 3 mm. 
(d) The length (l.sub.3) of the edge surface of the back edge and the step 
d had variable optimum values depending on the contact angles of the 
support with the doctor edge; however, it was found that satisfactory 
results were obtained by setting the length (l.sub.3) of the edge surface 
in the range of from 1 mm to 5 mm and by setting the step d in the range 
of from 0.01 mm to 1 mm.