Note: Descriptions are shown in the official language in which they were submitted.
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MEq!~OD ~ND APPARATU8 FO~R GBNERAl!IN~: AN IMAG~5
ON A PRI~IT 8C:nE:EN
INV13NrI017
The present invention relates generally to khe
preparation of print screen stencils and deals more particu-
larly with a method and apparatus for generating directly on
the screen the mask of the image to be printed. The invention
more specifically relates to a method and apparatus for eject-
ing variable size or volume drops of a blocking composition
directly onto the screen to occlude apertures in the screen
mesh to produce the desired image positive.
The preparation of print screen stencils is gener-
ally well know to those skilled in the art. One method often
referred to as the ~Idirect method" of preparing print screen
stencils involves the coating of a liquid light-sensitize
emulsion directly onto the screen in order to sensitize the
screen fabric. When exposed to actinic light through a film
positive, the black portions of the positive will not allow
light to penetrate to the screen and the emulsion will remain
soft in those areas. In the areas which are exposed to light,
the emulsion will harden and become insoluble in the selected
ink system.
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1 One problem generally associated with the prior art
is that many steps are necessary to prepare the screen thus
making screen preparation somewhat time consuming and labor
intensive. One prior art method o~E preparing a print screen
typically requires coating the sur~Eace with a resist composi-
tion of some type, exposure to light using an image film posi-
tive and then, curing of the emulsion to occlude the desired
areas and washing or purging of the unoccluded area~ to permit
the printiny co~position ink to pass through to a substrate
positioned beneath the screen fabric when the screen is held
in contact with the substrate surface.
It is a general aim of the present invention to
reduce the time and labor required to generate a print screen
stencil by producing an image positive directly on the screen
mesh.
It is a further aim of the present invention to
eject drops of a blocking composition on to the screen mesh
surface in accordance with pre programmed data representative
of the desired image to directly occlude apertures in the
screen mesh to produce the image positive.
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1 ~UMM~RY OF THE INVEN~ION
In accordance with the present invention, a method
and associated apparatus are presented for generating an image
positive directly on a print screen mesh. A blocking composi~
tion which may be a polymerized material is selectively
ejected as drops ~rom a drop ejection head which scans the
surface of the screen mesh in an X-Y coordinate movement and
which drop ejection and movement is directed by a controller.
Data representative of the desired image to be generated is
input to the controller as pre-recorded digital words which
are used by the instruction set in the controller to cause the
drop ejection head to deposit or not deposit a drop of block
ing composition in an aperture of the screen mesh as the head
;~ 15 scans the surface. The result is an image positive as defined
by those apertures left unoccluded.
In another aspect of the invention, the volume of a
drop ejected by the drop ejection head is variable to control
the size of the drop deposited in the aperture with smaller
volume drops being deposited at the marginal edge areas of the
graphic resulting in the production of a well defined outline
so that a graphic printed with the stencil has a smooth and
sharply deLined outline.
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1 In a further aspect of the invention, the drop ejec-
tion head deposits larger volume drops in non-marginal edge
areas of the graphic where lass accuracy is required to speed
generation of the image positive on the screen mesh.
! In a yet further aspect of the invention, the drop
ejection head scans the surface at variable speeds moving at a
faster rate over larger occluded areas and at slower rates at
the marginal edge areas of the graphic outline to additionally
speed yeneration of the image positive on the screen mesh.
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BRIEF DE~CRIPT:i:ON OP' THE DRAWING~
Additional advantages and features of the present
invention will become readily apparent from the following
written description and drawings wherein-
Fig. 1 is a diagrammatic perspective view of imagegenarating apparatus emhodying the present invention for
directly producing an image positive stencil for use in screen
printing;
Fig. 2 illustrates an enlarged view of a portion of
a screen mesh material;
Fig. 3 is an enlarged schematic view of a portion of
the screen mesh material showing certain of the apertures
occluded using smaller drops along a marginal edge area and
largex drops in non-marginal edge areas of a portion of an
image positive stencil produced with the invention;
Fig. 4 is an enlarged schematic view of a portion of
the screen mesh material showing a backer in contact with the
surface of the screen mesh material side oppo~ite the drop
ejection head to prevent drops from passing through the aper- -
tures when shot by the drop ejection head;
Fig. 5 is a diagrammatic perspective view o~ an
alternate embodiment of the present invention wherein the
screen mesh material is held on a drum which is rotated rela-
tive to the ejection drop head as the head moves along a
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1 linear path to provide coordinate movement between the surface
and the drop ejection head;
Fig. 6 is a diagrammatic plan view of an alternate
embodiment of the present inventio;n wherein drop ejecting
heads are disposed opposite one another at opposite sides of
the screen mesh material so that a]pertures are occluded ~rom
both sides of the screen mesh.
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l DETAILED DESC~IPTIO~ OF ~NE PREFER~ED æMBODIMEN~
Turning now to the drawings and considering the
invention in further detail, Fig. 1 shows a diagrammatic
representation of exemplary apparatus for purposes o~ explana-
tion and understanding o~ the invention. The apparatus desig-
nated generally 10 has a platform 12 which includes a work
surface 14 upon which surface rests a frame 16 Eor holding the
screen mesh material 18. The desi;red graphic or image posi-
tive to be generated on the screen mesh materials is shown and
designated generally 20. Axially extending guide rods 22,24
are supported in a parallel spaced apart relationship with one
another along the marginal edge regions 26,28 respectively of
the work surface 14 of the platform 12 so that the frame 16
holding the screen mesh material 18 fits therebetween. The
guide rods 22,24 carry a gantry designated g~nerally 30 which
extends transversely between the guide rods and above the work
surface 14 of the platform 12 and the frame 16 holding the
screen mesh material 18. The gantry 30 may be slideably
coupled to the guide rods 22,24 by means of bushings 32,34,
respectively or other means well known in the art. The gantry
30 is also arranged for bi-directional movement along a recti-
linear path on the guides 22,24 in the Y-axis direction using
an appropriate drive means well known to those skilled in the
art. For purposes of explanation only and in no way as a
limitation, one such drive means uses an axially elongated
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l screw 36 which is substantially perpendicular to the direction
of motion of the gantry 30. The screw 36 is threaded throuyh
a ball nut assembly 38 attached to the gantry to move the
gantry in one direction when the screw rotates in one direc-
tion and in the opposite direction when the screw rotat~s inits opposite direction. One end 40 of the screw 36 is coupled
to a Y-drive motor 42 which is controlled to rotate in the
required direction and for a predetermined time to move the
gantry to the desired position along the work suxface.
A drop ejection head designate~ generally 44 iB
mounted on and carried by a carriage 46 transversely across
the surface of the platform and of the screen mesh in the X-
axis direction. The drop ejection head 44 moves bi-
- directionally along a rectilinear path substantially perpen-
dicular to the gantry 30. The carxiage 46 is coupled to a
drive belt 48 which is driven by an X-drive motor 50 located
at one end 52 of the gantry. The X-drive motor 50 is carried
with the gantry and when activated causes the drive belt 48 to
move in one or the other direction around a pulley 54 located
at the opposite end 56 of the gantry 30. The carriage 46
moves along guide rods 58,60 which extend in a parallel spaced
relationship with one another and between the ends 52,56 of
the gantry 3~. The rods pass through bushings 62,64 extending
transversely through the carriage. The movement of the gantry
in the Y-axis direction and the movement of the carriage and
drop ejection head in the X-axis direction produce relative
coordinate movement over the surface of the work surface to
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l locate the drop ejection head at any location along the
surface of the screen mesh material held in the frame 16 on
the work surface. Although the above discloses an illustra-
tive apparatus for purposes of explanation, it will be recog-
nized by those in the art that relative coordinate movementbetween the drop ejection head and the screen mesh material
may be obtained using other methods, for example, by moving
the work surface and frame relative to the drop ejection head
which is held stationary relativs to the work surface.
Still considering Fig. 1, a controller 66 includes a
keyboard 68 for entry of the appropriate command data to actu-
ate the apparatus and to execute an instruction set in accor-
dance with data representative of the graphic or image to be
generated on the screen mesh. The graphic information is
represented as binary coded words or data and may be pre-
programmed on a magnetic or other storage media which operates
in conjunction with the controller. As illustrated, khe
stored information is recorded on a magnetic diskette 70 for
retrieval by the controller 66 in accordance with the instruc-
tion set to cause the X and Y-drive motors to move the gantry
and the carriage carrying the drop ejection head to the speci-
fied X,Y coordinate position along the surface of the screen
mesh. Although a magnetic diskette 70 is illustrated, the
~ stored information may be recorded in semiconductor memories
such as PROMS, EPROMS, and other devices well known to those
in the art.
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1 A reservoir 72 located generally above the drop
ejection haad 44 and the work surface 14 o~ the platform 12 is
provided to receive and hold a blocking composition for
occluding the screen mesh apertures. The reservoir 72 is
coupled to the drop ejection head via a flexible conduit 74 to
permit movement of the head in the X-Y coordinate direction as
the head scans the sur~ace. Alternately, the reservoir may be
carried by the gantry or may be part o~ the drop ejection
head.
An optical scanner 76 or other suitable detector may
be mounted on the carrier 46 adjacent the drop ejection head
44 to detect and sense the approximate center of an aperture
in the screen mesh material 18 to align the drop ejection head
with the aperture. The sensing of the center of an aperture
is particularly useful to provide higher definition of the
marginal outline edge area of the graphic image since the
deposited drop is more likely to be in the opening of the
aperture rather than hitting the weave of the screen mesh
material and splattering to adjacent apertures resulting in a
roughness or fuzziness along the marginal outline edge areas
of the finished stencil.
The controller 66 is connected to the drop ejection
head 44 via an electrical conduit 78 which carries electrical
excitation signals to the head to cause the drop ejection head
to deposit or not deposit the blocking composition at a given
location in accordance with the instruction set to produce the
desired graphic or image positive stencil. The conduit 78 may
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1 also include additional electrical wires to couple signals
from the optical scanner 76 to the controller 66 when such a
scanner is utilized.
In operation, the carriage 46 carrying the drop
ejection head 44 and the gantry 30 will be located at an ini-
tial home or start position from which home position absolute
incremental movement of the drop ejection head is calculated
to move the head to the desired positions along the screen
mesh surface. The home position can be mechanically or elec-
trically sensed and detected as is known to those in the art.
A typical print screen mesh weave generally desig-
nated 80 is shown in an enlarged sectional portion in Fig. 2
and illustrates one type of weave typically associated with a
fabric material. As known in the art, the fabric as weaved
has a number of high points or spots which become abraded
after repeated usage of the stencil due to contact between the
squeegee edge as it spreads the printing composition over the
mesh surface. In order to minimize the abrading action on the
fabric material, it is desirable to encapsulate the threads
forming the screen mesh to protect them from the abrading
action normally encountered by repeated wiping of the printing
composition by the squeegee over the screen mesh surface.
In accordance with the present invention, the size
or volume of a drop of blocking composition ejected by the
drop ejection head is controllable to insure that the aperture
is fully filled and to vary the thickness of a layer of block-
ing composition deposited to ensure complete coverage of the
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l threads of the screen mesh. The surface of the occluded area
can be made smooth as a result of clepositing a heavier layer
of blocking agent so that as the printing composition ink is
squeegeed along the surface, excess ink is wiped from the sur~
face and not left to accumulate in minute pockets or cavities
on the surface which is a common occurrence in the prior art.
The ability ko deposit a variable thickness layer of blocking
composition is also important to accommodate dif~erent screen
mesh materials and characteristics. For example a wider mesh
screen requires a heavier layer o~ blocking composition to be
deposited to cover the knuckles o~ the screen mesh weave
whereas a finer mesh screen requires a thinner layer o~ block-
ing composition to cover the screen mesh weave.
The blocking composition deposited by the drop ejec-
tion head is preferably a viscous composltion which has char-
acteristic properties that cause it to adhere to the screen
mesh material in intimate contact with the screen mesh.
Preferably the blocking composition formulation has a low vis-
cosity during storage and delivery to the drop ejection head
and at the instant a drop is ejected from the head and then
hardening upon contact with the screen mesh material. It is
; ~ desirable that the blocking composition be a viscous liquid
composition to facilitate drop position control. The harden-
ing process will be dependent upon the formulation of the
blocking composition used and may include hardening by 5001-
ing, ultraviolet light, polymerization, moisture addition, and
other methods known in the art.
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1 It may also be pre~erable to match the characteris
tics o~ the screen mesh material and the blocking composition
used to ensure compatibility and optimal adherence of the
blocking composltion to the screen mesh mater:ial. In addi
tion, the screen mesh material may be heat and pressure
treated to fuse the threads of the screen mesh material to
enhance adhering of the blocking composition could improve
resistance to abrading without necessarily encapsulating the
screen mesh material.
Turning now to Fiy. 3, a portion of the screen mesh
material is shown in diagrammatic form and illustrates several
features of the present invention. Recalling ~rom above that
it is possible to regulate or adjust the size or volume of a
drop ejected from the drop ejection head and further that the
trajectory of a drop may be accurately aligned by means of an
optical scanner or other detector sensing and determining the
center of an aperture, drops represented by Dl, D2, D3, D4 and
D5 are illustrated wherein the drop of blocking composition is
sufficiently large to completely fill and occlude the respec-
tive aperture in the screen mesh material. The resultant out-
line marginal edge is seen to be fuzzy so that an image
printed using the image positive stencil will have a fuzzy or
rough outline marginal edge also. In contrast, ejecting a
smaller size or volume drop o~ blocking composition as repre-
sented by dl, d2 and d3 in the corresponding apertures o~ thescreen mesh material permits bridging in the aperture between
adjacent threads of the screen mesh and collectively pro~ides
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1 a well defined and sharp outline marginal edge. The ability
to vary the size or volume o-f a drop facilitates control of
edge acuity. Thus an image or graphic produced ~rom the sten-
cil will also tend to have a sharp and well defined outline
marginal edge.
Another ~eature of the present invention which
increases the speed at which a print screen stencil can be
produced involves varying the rate of scanning of the drop
ejection head across the surface of the screen mesh material.
Recalling that the graphic or image to be produced is repre-
sented by data information contained on a storage media, the
instructinn set in the controller calculates the location of
the peripheral marginal edges defining the graphic or image
outline and causPs a smaller size or volum~ drop to be
deposited in the corresponding apertures along the outline
marginal edge and larger size or volume dots in areas to be
occluded away from the marginal edge areas of the graphic
outline. Accordingly, since less accuracy is required in the
depositing of the blocking composition ~o occlude apertures in
the screen mesh material in those areas away from the graphic
outline marginal edge, the drop eje~tion head may be scanned
across those areas at a higher rate and thus speed the produc-
tion of the print screen stencil. For purposes of explana-
tion, area 1, represented by A1, is to be completely occluded
and therefore the drop ejection head will deposit a larger
size or volume drop in the apertures in this area at a faster
scanning speed than in areas al and a2 representative of
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l marginal edge areas of the yraphic. In other words, the drop
ejection head scans the surface of the screen mesh material at
a variable rate dependent on the accuracy with which it is
desired to locate a drop within a given aperture.
Turning now to Fig. 4, a portion of the screen mesh
material is shown in diagrammatic form wherein a planar
surface or backer 82 is located beneath or behind the screen
mesh material and in contact with the screen mesh material
surface so that when a drop of the blocking composition is
deposited from the drop ejection head, the backer forms a
barrier so that the drop is not ejected through the screen
mesh material. A backer 82 located behind the screen mesh
material permits the use of a lower viscosity blocking compo-
sition because the composition is prevented from passing
through the aperture in the screen mesh material. The backer
also allows the use of a slower hardening blocking composition
again since the blocking zomposition is prevented from passing
through the aperture.
Turning now to Fig. 5, an alternate embodiment of
the present invention generally designated 84 is illustrated
wherein the screen mesh material 86 is held on a drum 88 which
rotates along its central axis 90 relative to a drop ejection
head 92. The drop ejection head 92 moves along a rectilinear
path in an axial direction shown by arrow 94 to provide coor-
dinate movement between the head and the screen mesh surface
as the drum rotates. The rotation of the drum 88 and movement
of the drop ejection head 92 are controlled in accordance with
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1 an instruction set within a controller 96 similar to the
controller 66 of Fig. 1. As in thle embodiment illustrated in
Fig. 1, the drop ejection head 92 deposits a variable size or
volume drop at predetermined locations along the surface o~
the screen mesh material held by the drum 88. The blocking
composition can be ejected as the drum 8~ rotates a complete
revolution with the drop ejection head 92 being moved axially
a distance of one aperture for each complete revolution of the
drum until the complete surface of the screen mesh material is
scanned. ~lternatively, the drop ejection head 92 may make
one~complete axial scan of the drum depositing the ~locking
composition at the appropriate predetermined aperture loca-
tions with the drum 88 rotating one aperture distance each
time the drop ejection head makes one complete scan so that
the bloc~ing composition is deposited with bi-directional
movement of the head.
Turning now to Fig. 6, a further alternate embodi-
ment of the present invention generally designated 100 is
illustrated wherein the screen mesh material is held by a
frame 102 which is located such that a ~irst drop ejection
head 104 is located on one side 106 of the screen mesh mate-
rial and a second drop ej~ction head 108 is located on the
opposite side 110 of the screen mesh material. The movament
of the drop eject;ion heads as well as the depositing of block-
ing composition from the heads is controlled by a controller
tnot illustrated~ similar to the controllers of Fig. 1 and
Fig. 5. The heads 104,108 are moved in synchronization with
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l one another and drop5 o~ a blocking composition are ejected
substantially simultaneously ~rom each so that an aperture to
be occluded i5 occluded from both Isides of the screen mesh
material. The use of two drop ejection heads 104,108 disposed
opposite one another on either sidle of the screen mesh mate-
rial and operating in synchronization with one another ensures
that the aperture is occluded and aids in encapsulation oP the
screen mesh material. Also, drop ejected from one head acts
as a backer or stop for a drop ejected from the other head to
prevent one or the other of the drops ~rom passing through the
screen mesh material. The movement o~ the heads 104,108 rela-
tive to the surface of the screen mesh material is facilitated
with a drive belt and gantry system arrangement for each side
similar to the embodiment illustrated in Fig. 1 and may be
referred to above for its description.
Another feature of the present invention permits the
repair and reusage of a damaged print screen. Because a
graphic or image is stored as data in a storage device, the
graphic or image is readily reproducible. Thus a print screen
that has become damaged or otherwise unusable due to, for
example, loss o~ an occluded area due to blocking composition
becoming dislodged or some other such damage, can be re-
deposited in the damaged area to provide a once again viable
print screen. In such ~ircumstances the print screen is
repositioned on the work surface in its frame and the drop
ejection head is brought to the home position. A command is
input to the controller to activate an instruction set to
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l carry out the repair steps. The area to be repaired is
identified by incrementing movemen1: of the drop ejection head
to the beginning of the area to be repaired and this position
is noted and marked by input to the controller. The drop
ejection head is then incremented to the end o~ the area to be
repaired and this position is noted and marked by input to the
controller. Intermediate marginal positions along the area to
be repaired are likewise noted so that the area to be repaired
is "mapped out". The repair is carried out b.,v an input
command to the controller which activates the repair sequence
by moving the drop ejection head to the corresponding X-Y
coordinate positions along the mapped surface and depositing
or not depositing drops at the corresponding aperture loca-
tions as defined by the stored information retrieved from the
storage device.
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