Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR APPLYIN& A
PHOTO POLYMER TO SURFAC~S
~=l~icat:iOn
The present invention relates to applying a photo
polymer to a surface and more particularly to a novel
method for applying a uniform, holiday-f:ree photo polymer
layer on surfaces of both smooth and/or irregular config-
uration.
The use of photo polymers in the photo fabricating
industry is extremely well-known for such valuable uses as
production of printed circuits, photo etching, photoen-
graving and chemical milling and machining. These important
processes have historically included a number of conven-
tional steps:
A surface to be etched or milled is first coated with
a light-sensitive photo polymer which, when cured, becomes
resistant to acids, alkalies or both. Hence, the first
photo polymer is commonly called a "resist". Conventionally,
a mask is placed over the resist-coated surface so as to
leave only selected portions of the resist exposed ancl
unmasked. Depending upon the type of resist used, the light
impinging upon the unmasked areas either hardens the resists
or degrades the resist so that when the resist-coated part
of ~he surface is subsequently washed in a developer, part
of the resist is removed leaving an image defined by the
mask on the surface.
The key in producing high quality work of this type is
to apply liquid resists uniformly and free from dust, dirt
and voids or pin holes, commonly referred to as holidays.
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The difficul~y of properly applying liquid resist is well-
know~ in the indu9 try and numerous techniques have been
tried. One somewhat effective prior art me~hod includes
completely immersing a surface in liquid -resist and slowly
removing the surface from the liquid to improve the uni-
formity of resist thickness. The dipping method has,
however, proved to be extremely slow and cumbersome. This
is particularly true where the surface is very large or
where placemen~ of resist on all parts of the surface is
undeslrable.
In the face o:E serious problems encountered in applying
liquid resist, at least two major suppliers of resist have
abandoned the liquid resist approach and have developed
solid film resists which are laminated to the surface.
While solid film resists are more expensive than liquid
resist, they have gained a strong foothold in the industry
because many ~eel that they can be more reliably applied
than liquid resists.
I~ would therefore be a substantial contribution to the
industry to provide a technique of applying liquid resis~ so
as to be uniform and holiday-free
The invention deEined herein provides for a ~iniform
holiday-free application of liquid resist to regular and
irregular surfaces by impregnatin~ a plastic sponge material
with a liquid resist comprising a solvent which tends to
slowly react with the fabric of the sponge. The sponge thus
impregnated has been found to uniquely apply a superior
liquid resist coating upon the surface.
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It is, therefore, a prlmary object of the present
invention to provide an improved method for applying a
liquid resist.
This and other objects and features of the present
invention will become more fully apparent from -the following
descrip~ion and appended claims taken in conjunction with
the accompanying drawing.
The drawing is a schematic flow diagram illustrating
principal steps of a preferred embodiment of the invention.
According to the presently preferred embodiment of the
invention, a desired liquid photo polymer resist 12 is
selected for application to a surface 14. Any suitably
liquid photo resist polymer such as Eastman Kodak KPR, KMR,
Dynachem GMR 5000 or the like may be used. All of the well
known photo polymer resists comprise a solvent which volat-
ilizes in the atmosphere to cure the resist.
Next, an applicator 16 is selected. Preferably, the
applicator 16 is fabricated from a cellular sponge material
pre~erably made o~ synthetic plastic. The synthetic plas-
tics are preferred because they are more easily affected bythe common solvents in conventional resists. According to
the present invention, it has been found surprisingly
effective to use a sponge applicator 16 which slowly reacts
with the solvent of the photo resist. Initially, the sponge
material is impregnated with the photo resist whereupon the
solvent in the resist is permitted to react at least with
the exterior of the applicator. Care is taken, however, to
insure that the applicator is not permitted to be exposed
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directly to the sol-vent or so long a time that the sponge
fabric disintegrates It has been found that for most
conventional plastic sponges the applicator 16 will be
effective to apply resist properly within less than one to
five minutes after impregnation with resist, ~epending upon
the particular solvent in the resist. Most sponge appLi-
cators, depending upon si~e, retain overall integrity for
several hours a~ter impregnation.
~fter the sponge applicator 16 has been impregnated
with resist, and sufficient interaction between the resist
and the applicator has taken place to permit holiday-free
application, the sponge applicator is brushed lightly across
the surface 14 in continuous strokes. After completing one
stro~e, another stroke is made with slight overlap of the
first as illustrated in the figure. Sequential application
is continued until all the desired portions of the surface
14 are covered with resist.
The solvent-softened resist-impregnated sponge appli-
cator was found surprisingly effecti~e in delivering a
uniform holiday-free resist coat. The solvent in the photo
resist causes the sponge applicator 16 to become extremely
soft so that the sponge conforms to the surface 14 and
adequately appl~es resist even to surface irregularities,
small ~its and scratches in the surface. The applicator 16
has been ~ound con~enient in applying photo resist to
surfaces ha~ing unusual configurations such as curved
suxfa~es, and inside surfaces. ~loreover, extremely large
objects which can be dipped or roller coated only with
specialized e~uipment can be easily coated with the appli-
cator 16.
In an attempt to determine why a paint brush would no~
provide the same satisfactory resul~s as obtained with
applicator 16, it was concluded that the bristles of -the
brush must (a) be made infinitely so:Et or ~b) be made to
melt or dissolve into the resist ~luid. Clearly, there
are no paint brushes with infinitely soft bristles and if
the bristles melted they would contaminate the resist. On
the other hand, it was discovered that certain plastic
sponges useful in the practice of this invention were
swelled by the solvents in the resist system but were not
lmmediately dissolved thereby. This swelling of the
plastic sponge causes the plastic sponge to become extremely
soft and hold a substantial quantity of resist thereby
providing a uniform, dust and holiday-free resist coating.
In addition, the softened plastic sponge or applicator 16
readily conforms to the various surface contours as well as
covering minor surface irregularities such as small pits,
scratches and the like.
~ fter application of the surface 14 has been complete~,
the photo resist is preferably washed from the applicator to
increase the life of the applicator. ~s mentioned above, if
the photo ~esist is allowed to remain in khe sponge for an
extended length of time, depending upon the fabric of the
sponge and the particular solvent in the resist, the sponge
will disintegrate. If the sponge is washed after each use,
it may ~e used se~eral times before replacement is required.
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The following non-limiting examples are representative
of the method.
Example 1
A plas-tic ~oam sponge was dipped into a pan of Eastman-
Kodak resist identified as KPR-3. The sponge was found to
become fully impregnated in several seconds. The impreg-
nated sponge was then lightly drawn across a printed circuit
laminate measuring 36 ~ 24 i.nches. The sponge delivered a
coat of resist four inches wide across the laminate and,
lQ after completing one stroke, another stroke was made slightly
overlapping the previously coated area. This sequence was
repea~ed until the complete laminate was covered with
resist. The slight overlap was ~ound to equalize homo~
genously so that the resist coating was substan-tially
uniform and remarkably holiday-~ree.
Example 2
An instrument panel measuring 2 inches by 3 inches to
be manufactured by the photo fabrication method was coated
with resist by dipping a sponge into Dynachem resist 315
; 20 and dragged lightly across the surface of the panel. A
uniform highly desirable coating of resist resulted.
E a ~
Cylinders having a four inch diameter were coated with
resist so that an emblem could be photoengraved onto the
exterior sur~ace. A plastic foam sponge impregnated with
Dynachem CMR 5~00 was drawn lightly across the surface to be
: photoeng~a~ed. A uniform substantially holiday-free resist
layer resulted.
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Example 4
Chemically milled screens from stainless steel were
prepared by placing a first resist coat on one side of a
stainless steel sheet as set forth in the foregoing example.
After the resist had dried, the sheet was turned over and
the other side coated in a like manner. Parts chemically
machined from this double side resist coated stainless steel
were uniform and of surprisingly high quality.
The invention may be embodied in other specific forms
without departing from its spirit or essential character-
istics. The described embodiments are to be considered in
all respects only as illustrative and not restrictive and
the scope of the invention is, therefore, indicated by the
appended claims rather than by the foregoing description.
All changes which come within the meaning and range of
equivalency of the claims are to be embraced within their
scope.
What is claimed and desired to be secured by United
States Le~ters Patent is:
