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llVIPROVEMENTS RELATING TO PRINT HEADS
The invention relates to print heads. In particular, a first aspect relates to
wiper blades (such as wiper blades used in screen printing), and a second
aspect
relates to print heads (such as print heads employed in screen printing) .
Traditionally screen printing machines used a squeegee blade which was
dragged at an angle over a printing screen to press a substance to be printed
through
open apertures in the screen, a reservoir of the substance to be printed being
drawn
across the screen in the form of a bead maintained between the screen and the
wiper
blade.
The specification of co-pending Application PGTIEP97/05761 describes an
improved foam of screen printing wherein, instead of the substance to be
printed lying
on the screen and being dragged thereacross by a squeegee blade, the substance
is
contained within a chamber the open end of which is pressed against the
screen, the
substance being moved towards the screen by means such as a piston which can
place
the substance to be printed under pressure. At least one long flexible wiper
is
provided, the wiper extending transverse to the direction of displacement of
the
device over the stencil and transverse to the direction of displacement of the
substance
to be printed towards the screen, the wiper being inclined inwardly to extend
across
the open end of the chamber and arranged such that when the chamber is pushed
against the screen and the product is pushed under pressure through the
screen, the
wiper acts to provide a scraping action for lifting paste off the screen and
to provide
a "rolling" action of the paste. Thus rather than having a trailing angle, as
in
squeegees used for traditional printing, a wiper in the new form of printing
has a
leading angle.
It has been proposed that such wipers be formed of polyester material such as
those sold under the Tradcmark Mylar.
According to a first aspect of the invention, there is provided a print head
far
screen printing, the print head comprising: a chamber for substance to be
printed,
said chamber having an open end; means for driving substance to be printed
through
the open end of the chamber and onto a screen; and at least one wiper blade
provided
at the open end of the chamber, the wiper blade comprising a web of synthetic
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material which is mounted at a leading angle adjacent one longitudinal edge
thereof
to said print head; characterised in that: the other longitudinal edge of said
blade is
provided with a metallic reinforcing member which contacts the screen during a
printing operation; the wiper blade is mounted to said print head so that said
metallic
reinforcing member scrapes across said screen during a printing operation to
lift
substance to be printed from said screen; and in that the metallic reinforcing
member
acts as a wear strip to increase wear resistance of the wiper blade as the
blade is
scraped across the screen during a printing operation.
Preferably, the thickness of the metallic member is equal to or less than that
of the synthetic wiper blade.
Preferably, the width of the metallic member is equal to or less than that of
the synthetic wiper blade.
More preferably, the metallic member is of a thickness between 10 and 70%
of the thickness of the synthetic wiper blade, preferably around 20 % thereof,
and is
of a width of 10 to 40 % of the width of the wiper blade, preferably around 23
%
thereof. The metallic member can be flush with the free edge of the synthetic
wiper
blade (ie. zero projection), or can project from the free edge of the blade by
up to 30% of its width. Preferably the metallic reinforcement and wear strip
is
formed of stainless steel which increases the rigidity of the synthetic wiper
blade
while allowing the wiper blade still to flex and bend and increasing its
resistance to
wear. It can give greatly enhanced and consistent print quality from the
printing
operation. The metallic member could be of spring steel, titanium or a variety
of
other materials known to persons skilled in the art.
In one embodiment, given by way of example, the wiper blade was formed
of polyester and a metal reinforcement strip of stainless steel, the wiper
blade was of
an overall length of 340 millimetres, had six slots in one edge thereof
through which
screws could project to mount the wiper blade on a print head, was 22
millimetres
in width and 0.5 millimetres in thickness and the stainless steel blade was
secured by
adhesive to the opposite edge to said edge in which the slots were provided
for
mounting screws, had a length the same as that of the wiper blade, a width of
5 millimetres, a thickness of 0.1 millimetres and an overhang beyond the free
edge
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of the wiper blade of 0.5 millimetres.
A further aspect of the invention seeks to further enhance the screen printing
process described in the above-mentioned co-pending application.
As described above, the previously proposed process employs a chamber
within which printing product is provided. As the chamber is used, product is
dispensed and the quantity of product in the chamber is gradually reduced.
When the
chamber is exhausted, it must be replaced with a fresh chamber before the
printing
process can continue. The used chamber is then either returned to the
manufacturer
for replenishment, which is expensive, or thrown away, which is wasteful and
potentially environmentally damaging. In either case, the printing process
must be
halted to allow the chamber to be changed.
To alleviate at least some of these and other problems, the second aspect of
the invention provides a self-contained print head far a screen printer, the
print head
comprising: a body, a receptacle for printing product, said printing product
being
dispensable from said receptacle through an orifice, one or more charging
apertures
in said body which provide access to the receptacle and by means of which said
receptacle may be recharged as required with printing product; and a
respective
sealing means associated with each of said one or more charging apertures and
operable to allow or disable access to said receptacle; characterised in that
said
sealing means are operable to disable access to said receptacle to prevent
printing
product from being dispensed from a respective aperture during a printing
operation
and operable to allow the injection of printing product into said receptacle
when said
print head is not involved in a printing operation, said receptacle being.
rechargeable
by means of an external printing product injector temporarily engageable with
one or
more of said charging apertures and operable to inject printing product into
said
receptacle as required to recharge said receptacle.
This aspect of the invention allows the user of the chamber to inject printing
product into the chamber to recharge it, and therefore avoids the expense and
waste
associated with previously proposed arrangements.
The invention is diagrammatically illustrated by way of example in the
accompanying drawings, in which:-
'''CA 02321785 2000-08-22 ''~i'Y
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3a
Figures 1, 2 and 3 are views of a print head provided with wiper blades
according to the invention comprising respectively a perspective view from the
underside, one end and one side; an elevation from said one end; and a
perspective
view from the other end, above and the other side;
Figure 4 is a plan view of a wiper blade according to the invention;
Figure 5 is a sectional view on line V-V of Figure 4 to a greatly increased
scale;
Figure 6 is~an enlarged view of the lower part of Figure 5;
Figure 7 is a front elevational view of a print head;
~ Figure 8 is a cross-sectional view along the line X-X of Figure 7; and
Figure 9 is a cross-sectional view along the line A-A of Figure 7.
Referring to the drawings and firstly to Figures 1 to 3, a print head 1
comprises a body 2 with an underneath chamber 4 to receive product to be
printed,
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WO 99143500 PCTIGB99/00466
4
of the chamber 4, each wiper blade 6 being clamped by means of a mounting bar
7
having apertures therein through which screws 8 project, the screws 8 also
extending
through slots in the edge of the respective wiper blade 6 and engaging in
screw
threaded apertures in the body 2.
It should be noted that the one of the wiper blades 6 which is effective
during
a printing operation is the blade which is at the trailing side of the chamber
4 having
regard to the direction of movement of the print head 1, and that the wiper
blade fi
is at a leading angle to the screen which in use is provided below the print
head 1 but
which is not shown in Figures 1 to 3. The blade 6 which during a print
operation is
on the leading side of the head having regard to the direction of movement of
the
head acts only in a sealing sense. The print head is pressed downwardly to
force the
blade against the screen. Pressure is also applied to the substance to be
printed in the
chamber 4 to force substance through the screen. The pressure applied to the
substance also acts on the leading wiper blade 6 to press it downwardly
against the
screen and effect a seal, but that wiper blade 6 does not effect a scraping
action since
there is no substance on the upper surface of the screen in front of it. A
print head
of the kind shown in Figures 1 to 3 can be used to effect a printing operation
in either
of the two directions shown by the double-headed arrow 9 in Figure 2.
Referring to Figures 4 to 6, each wiper blade 6 is formed as a flat strip 10
of
polyester 21.5 millimetres in width, 0.5 millimetres in thickness and 340
millimetres
in length. In one edge l0a it has six slots 11 to receive the shanks of the
screws 8.
Adjacent its other edge lOb there is provided, adhered to the strip 6, a
metallic strip
12 of stainless steel of the same length as the wiper. blade 6, 5 millimetres
in width
and 0.1 millimetres in thickness. The metallic strip 12 projects, as shown in
Figure 6, by a portion 12a beyond the adjacent edge lOb of the polyester strip
10 by
approximately 0.5 millimetres and, as shown in Figure 1, the metallic strip 12
is
provided on the underside of the polyester strip 10 in its operative position
such that
it is the metallic strip 12 which contacts the screen during a printing
operation and
not the polyester strip 10. The metallic strip 12 thus gives rigidity to the
wiper blade
6 while still allowing the wiper blade 6 to flex and bend and also increases
the wear
resistance of the wiper blade 6 thereby to provide for consistent print
quality in
CA 02321785 2000-08-22
WO 99143500 PCT/GB99/00466
operation.
As mentioned above, Figure 7 shows a front elevational view of a print head
30 according to a second aspect of this invention. The print head 30 is
superFcially
similar to that described above with reference to Figures 1 to 6, and
comprises a body
5 32 which is hollow and defines a printing product receptacle from which the
printing
product may be dispensed through a printing product orifice 34. Either side of
the
printing product orifice 34, there is provided a pair of wiper blades 36 (only
one of
which is visible) which are each secured to the body 32 by means of screws or
other
suitable means 38.
With reference to Figure 8, the body 32 comprises an upper section 35, a
middle section 37 and a lower section 38 (although the body could alternative
comprise a pair of removable sections, if desired). Each of these sections are
removably detachable from one another. The sections 35, 37 and 38 together
define
the body 32 and the printing product receptacle.
In the preferred arrangement, the upper, middle and lower sections of the
body are held together by screws which attach the upper section to the middle
section,
and by further screws 39 (Figure 7) which attach the middle section to the
lower
section 38. An o-ring seal 50 is provided between the middle and lower
sections, and
a diaphragm 52 is provided between the upper and middle sections. The
diaphragm
52 may be of rubber or any other suitable material and preferably has a top-
hat-
shaped cross-section.
As shown in Figure 8, the diaphragm 52 extends into the cavity defined by the
upper section 35. Pressure is applied, in use, between the diaphragm and the
walls
of the upper section to drive the diaphragm, and hence the printing product,
towards
the orifice 34. A rubber diaphragm is preferred as rubber has elastic
properties
which, when the pressure is removed, cause the diaphragm to attempt to move
towards its initial position and therefore reduce the likelihood of printing
product
being dispensed from the orifice when it is not needed.
In the depicted embodiment, the printing product receptacle is defined by the
inner walls of the diaphragm 52 housed in the upper section 35, the middle
section
and the lower section 38. As shown in Figure 7, the body 32 includes one or
more
CA 02321785 2000-08-22
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sealing means 40 in a front wall thereof. In the preferred embodiment, three
sealing
means 40 are provided in the front wall of the print head 30, but a larger or
smaller
number may be provided, and they need not necessarily be located in the front
wall
of the print head 30. Preferably, the sealing means each comprises a valve
assembly
S such as a ball valve. Alternatively, the sealing means could each comprise a
cap for
closing a channel to the interior of the printing product receptacle.
As shown in Figure 9, in the preferred embodiment each valve assembly 40
comprises a channel 46 within which a ball 42 is movably retained. When the
print
head 30 is in use, pressure applied to the receptacle for printing product
forces the
ball 42 against a seat 46 thereby closing the passageway to the interior of
the grint
head 30. At other times (for example when the print head is being relocated in
the
printing apparatus (not shown) and pressure is released from the printing
product
receptacle), fresh printing product may be injected under pressure into the
print head
30 by means of one or more of the valve assemblies 40 to move the ball 42 to
open
the passageway 46. The print head 30 may remain in the printing apparatus or
may
be removed therefrom when the recharging of the print head 30 occurs.
As an alternative to a valve assembly, a simple cap or plug or bung could be
provided. The cap would, when in place, close the channel to the printing
product
receptacle and could be snap fittable with the channel, or could screw-
threadably
engage with the interior or exterior wall of the channel. The plug or bung
would be
push-finable into a friction fit with the walls of the channel. Further
alternatives will
also be apparent.
It will be understood that this second aspect of the invention may be employed
in combination with the above described first aspect, or alternatively, may be
employed in other types of print head - for example, such as those described
in the
above mentioned co-pending application.
Finally, it will be understood that various aspects of the invention have been
described herein by way of example only and that modifications may be made
within
the scope of the invention.
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