EP1058622B1

EP1058622B1 - Improvements relating to print heads

Info

Publication number: EP1058622B1
Application number: EP99905051A
Authority: EP
Inventors: Francis Bourrieres; Clément KAISER; Neil Macmillan Macraild; Andrew Charles Ricketts
Original assignee: DEK International GmbH
Current assignee: ASM Assembly Systems Switzerland GmbH
Priority date: 1998-02-24
Filing date: 1999-02-16
Publication date: 2003-07-02
Anticipated expiration: 2019-02-16
Also published as: GB9803873D0; GB9910940D0; EP1058622A1; GB2334481B; WO1999043500A1; DE69909253T2; JP2002504445A; CA2321785A1; GB2334481A; GB2334479A; GB2334479B; DE69909253D1

Description

The invention relates to print heads used 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.

WO-A-98/16387 discloses an improved form 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, with 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 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 trademark Mylar.

In this screen printing technique, a chamber is employed 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.

DE-A-2212920 discloses a rotary screen printing machine for printing dyestuffs onto heavy fabric materials. The screen printing machine includes a distribution pipe which has openings in the lower surface thereof through which the dyestuff is delivered to a chamber above the fabric material to be printed. In operation, dyestuff is fed to the distribution pipe through a tap which throttles the flow of dyestuff.

Accordingly, the present invention provides a print head for a screen printer, said print head comprising: a body; a receptacle for printing product, said printing product being dispensable from said receptacle through an orifice; at least one charging aperture in said body for providing fluid communication with said receptacle and by means of which said receptacle is rechargable with printing product; and at least one sealing means associated with each respective charging aperture and being operable selectively to allow fluid communication with said receptacle; characterised in that: said at least one sealing means is configured such that said respective charging aperture is closed during a printing operation and operable to allow the injection of printing product into said receptacle when said print head is not performing a printing operation.

This print head 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:

Figures 1, 2 and 3 are views of a print head 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;

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 in accordance with an embodiment of the invention;

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, end sealing blocks 5 for the chamber 4 and wiper blades 6 provided one on each side 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 6 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 10a it has six slots 11 to receive the shanks of the screws 8. Adjacent its other edge 10b 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 10b 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 operation.

Figure 7 shows a front elevational view of a print head 30 according to an embodiment of this invention. The print head 30 is superficially similar to that described above with reference to Figures 1 to 6, and comprises a body 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.

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 alternatively 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 0-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 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 comprise a valve assembly, ^-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 44, thereby closing the passageway to the interior of the print 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, 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-fittable into a friction fit with the walls of the channel. Further alternatives will also be apparent.

It will be understood that this embodiment of the invention may be employed in combination with the first-described print head, 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.

Claims

A print head (30) for a screen printer, said print head (30) comprising:

a body (32);

a receptacle (52, 37, 38) for printing product, said printing product being dispensable from said receptacle (52, 37, 38) through an orifice (34);

at least one charging aperture (40) in said body (32) for providing fluid communication with said receptacle (52, 37, 38) and by means of which said receptacle (52, 37, 38) is rechargable with printing product; and

at least one sealing means associated with each respective charging aperture (40) and being operable selectively to allow fluid communication with said receptacle (52, 37, 38);

characterised in that:

said at least one sealing means is configured such that said respective charging aperture (40) is closed during a printing operation and operable to allow the injection of printing product into said receptacle (52, 37, 38) when said print head (30) is not performing a printing operation.
A print head according to claim 1, further comprising:

a pair of blades (36) on respective sides of said orifice (34).
A print head according to claim 1 or 2, wherein said receptacle (52, 37, 38) is defined in part by at least some of inner walls of said body (32).
A print head according to claim 3, wherein said receptacle (52, 37, 38) is defined further by a diaphragm (52) clamped between opposed inner walls of said body (32).
A print head according to claim 4, wherein said diaphragm (52) has a "top-hat" shaped cross-section.
A print head according to claim 4 or 5, wherein said diaphragm (52) is of rubber.
A print head according to any of claims 4 to 6, wherein said body (32) comprises at least two portions (35, 37) separable from one another, said diaphragm (52) being clamped between two of said portions (35, 37).
A print head according to any of claims 1 to 7, wherein said receptacle (52, 37, 38) comprises a further body removable from said print head (30).
A print head according to any of claims 1 to 8, wherein said at least one sealing means comprises a valve having a ball (42) retained within a channel (46), said ball (42) being movable between an open position in which printing product may be injected under pressure into said receptacle (52, 37, 38) and a closed position in which said ball (42) is seated on a sealing surface (44) and prevents the dispensation of printing product from said at least one charging aperture (40).
A print head according to claim 9, wherein said ball (42) is spring biased to close said respective charging aperture (40).
A print head according to any of claims 1 to 8, wherein said at least one sealing means comprises a cap, plug or bung for closing said respective charging aperture (40).