The method and the apparatus of the invention allow in particular the engraving of one, or more screens at the same time, by means of a perfected positioning and centering system of the screen frames in order to eliminate the need of a subsequent registering of the same.

To the and of making the frames centering precision compatible with the printing quality of clothing, even for high resolutions, the invention comprises in combination, also an interlaced interpolation printing process for eliminating or minimizing engraving errors, not matchable with the preregistration of the screens during the engraving phase.

Scopes of the invention The polychromatic screen printing of clothing consists, essentially, in the positioning of the clothing element into a specific printing machine under a printing screen carrying the image to be transferred engraved on said screen by means of engraving machines.

The printing screens allow the passage of ink through the zones of the screen that define the image, whereas by means of suitable treatments of the screen, consisting normally in distributing a layer of photo sensitive emulsion over the screen, the ink does not pass through the zones outside the areas belonging and defining said image.

The image to be transferred is, in its turn, engraved on the relative zone of the printing screen supported by a frame which stretches it drumhead like, by means of techniques which provide a first phase of covering the entire surface of the screen with a not-exposed photosensitive emulsion, a second phase of drawing the image with opaque and removable means of the kind made of ink or wax, a subsequent phase of fixing of the photoemulsion by light exposure of, generally, the UV type, and a fourth phase of washing of the engraved screen which consists in the removal of the opaque means: the zones not exposed to ultraviolet because of the opaque means, after the removal and washing operations, are permeable to the printing inks, whereas the exposed zones are impermeable to the same.

The engraving of the image on the printing screen is made, preferably, as it is known, by printing by ink or by opaque wax; printing is generally made by using print heads of the ink-jet type.

The procedures and the machines adopted for engraving printing screens may differ according to whether the screen is supported by flat or by cylindrical frames.

For what concerns flat frames, the engraving process is conducted by positioning the flat frame on the machine working table by using means allowing to determine the relative position of the zone within the surface of the screen to be engraved with respect to the edges of said frame.

In case of polychromatic printing of clothing elements is necessary to engrave more than one screen concerning the same image, whereby is assigned each of said homologous screens to a different colors of the polychromatic image: the engraved patterns, although they must guarantee the polychromatic final effect of the same image, are different from each other, for they are specialized for the reproduction of a single color of the final image; the subsequent and sequential superposition during the printing phase of the individual monochromatic patterns will result into the desired polychromatic final image.

To the purpose of making the homologous patterns belonging to the same image perfectly at register, it is necessary that each following homologous pattern be perfectly superposed with the preceding one, at the printing phase of the clothing elements, in order to obtain that the resulting image of the superposition be not out of focus: so it is the more important, the greater is the resolution and quality of image requested.

The operation of setting at register, besides being long and tedious, for it is manually conducted, does not guarantee against the absence of misalignments among the homologous patterns of the image, with consequent penalisation of the quality of the final polychromatic image.

The setting at register of the composing patterns for obtaining the printed image becomes particularly critical when the image is shaded off and/or dithered, that is when the edges are not well defined: the decomposition of the image in single patterns for each colors does not allow to verify the perfect superposition, in particular in case of four-color printing where it becomes essential to superpose pixels of different colors in order to obtain the final chromatic effect.

One of the main target of the present invention is to provide for a method and an apparatus for engraving flat printing screens, supported by frames suitably shaped, so that the screens belonging homologously to the same image be already perfectly at register for the following printing phase.

An advantage of the present invention is to permit to engrave in a perfectly at register way one or more series of homologous screens at the same time, where each series of screens is relative to a different image to print.

The setting at register during the engraving of the homologous printing screens needs, especially for printing at high resolution (> 720 dpi), that the engraving operation be performed with the particular procedure which is object of the present invention, for eliminating or minimizing the common errors of the known engraving techniques, which use ink or wax jet printing heads.

In fact, the normally adopted techniques for engraving printing screens, which use heads for ink or wax jet, are not suitable to the perfect setting at register of the screens during the phase of printing clothing elements, for the possible errors would be amplified in this phase as it will be explained in the following.

The engraving technique which use ink or wax jet heads have in common the goal of covering the zone of the screen to be engraved with ink or wax, by means of nozzles which spray pixels of the said substances over the emulsion zone.

The nozzles are normally arranged one next to the other along a line, or along more parallel lines.

The most frequent problem for this sort of printing heads consists in the occlusion of one or more nozzles, either because of the drying of the ink or of the early hardening of the wax, either because of the dust existing in the working environments.

The occluded nozzles affect the engraving quality because of the missing pixels relative to said nozzles.

A further problem which can be found in the printing phase may be due to the different diameters of the opaque means jets, for example ink or wax, because of the tolerances of the diameters of the nozzles or because of their partial occlusion for lack of cleaning or maintenance or, additionally, because of the different ejection speed of each jet.

The above mentioned problems make completely or partially inapplicable, to the end of the present invention, the prior art engraving methods of printing screens.

One of said methods is disclosed in EP 0733951; in such a document a procedure for engraving, called by stripe, is claimed.

The said procedure consists in dividing the surface of the screen relative'to the image, to be covered by ink or wax, into adjacent vertical sections (stripes) ; said stripes are, by means of a computer program which controls the advancing of the printing heads and the opening of the nozzles according to the image to be engraved, entirely completed during a single vertical excursion of the printing head, so as to complete during said single excursion the entire portion of image within the stripe.

When one stripe is completed, the printing head is advanced in the direction orthogonal to the covered stripe by a step equal to the distance between the first and the last nozzle of the aligned series nozzles of the said printing head, in order to cover in the same manner the stripe following the preceding covered one; the said operation is repeated for each adjacent stripe till the completion of the last section of the image, which is coincident with the last stripe to be covered.

Such a covering of the single portion of image (stripe) by a single excursion of the printing head is made possible by means of the elimination of the gap existing between adjacent nozzles.

The teachings of EP 0733951 make possible the zeroing of the gap between the nozzles, in order to allow the performance of the printing method by stripes, by means of the expedient consisting in the inclination of the nozzles along a line forming an acute angle in respect to the base of the device supporting the printing head.

Said inclination of the line along which the nozzles are arranged, when it is seen as a projection on the screen to be engraved reduces the distance between the ejected pixels by a factor D=L cos a, where D is the distance between adjacent pixels, L is the distance between the nozzles and a is the inclination angle of the line of the nozzles in respect to the base of the printing head.

It is clear that when D becomes equal to the diameter of the nozzles, the adjacent pixels become contiguous (under the hypothesis that the diameter of the pixels is equal to that of the nozzles).

It is, also, evident that if one or more nozzles are occluded or out of order, the stripe will miss the respective pixels and such an error will be systematically repeated for each single stripe, the stripe being interested, correspondingly, in the same respective positions by the same nozzles.

In case, in example, of a printing head leaving 96 nozzles and under the hypothesis that the nozzles 11,12 and 85 are occluded, in each stripe the pixels corresponding to those nozzles will be missing and they will be positioned within the stripe at the same distance from the first nozzle.

Such inconvenient is frequent and as much serious as more adjacent nozzles, instead than single distant nozzles, become occluded.

Such a situation is not tolerable when high printing resolution of clothing elements are requested (>720 dpi).

In the present case of engraving screens, according to the method of the present invention, where high precision of presetting at register of the screens homologous to the same image is achieved, the procedure of engraving by stripes according to EP 0733951 would result, in case of errors due to the occlusion of nozzles, into a printed item of a quality not acceptable at all, for the zones of the homologous screens missing of pixels would superpose, so producing light lines or strips into the cloth drawing.

An other procedure used for'covering the printing screens is represented by the technique named as printing by interpolation.

According to such a technique the printing heads use nozzles arranged along a line parallel to the base.

For each vertical scan of the printing head, the nozzles eject the jets, consisting in dots (pixels) of opaque covering means, like i. e. ink or wax according to lines corresponding to each nozzle, but the lines are not contiguous like in the former case, being zero the inclination of the nozzles line with respect to the base: this makes the need, in order to trace a line which is contiguous to the preceding one, that the printing head advance along the horizontal direction of a step equal to the gap between the nozzles instead, as in EP 0733951, of a step equal to the distance between the first and the last nozzle of the printing head.

Such engraving technique does not allow, as for EP 0733951, to subdivide the image into printing sections, coincident with the stripes completed for each single scan of the printing head; it needs, instead, that the printing section be completed by a number of scans of the printing head sufficient to interpolate the gap between the adjacent nozzles.

In other words, said S the gap between adjacent nozzles and P the diameter of the dot of the covering means (supposing that is equal to the diameter of the nozzle), the requested advancing step of the printing head for tracing an adjacent line is P, while for covering the entire gap between two nozzles the printing head must complete (n-1) scans, where n is the whole number equal or immediately inferior to S/P.

It is evident that such engraving procedure, with respect to that disclosed in EP 0733951, although allowing greater resolutions (>720 dpi), is still not suitable for the present invention since it presents problems of printing quality when one or more nozzles are obstructed, due to the systematic lack of dots of covering means in correspondence of the occluded nozzles, whereby such a lack of dots is repeated at regular spacing within the engraving pattern.

Thus, the engraving method by interpolation, although is compatible with the resolution levels requested by the perfect setting at register of the screens, it shows the drawback of the systematic error caused by the malfunctioning of the nozzles.

Aim of the present invention is, also, to overcome the said drawbacks by mean of a procedure for controlling the nozzles in order to randomize the said systematic error, by achieving that the pixels (coincident with the dots of the covering means) engraved along the contiguous lines belonging to the pattern, be printed by different nozzles (cross-printing).

State of the art A method and an apparatus of the prior art which affords the problem of centering the image engraved on printing screens are disclosed by EP 0492351.

A polychromatic image needs, as above said, a plurality of engraved screens which must be in a precise correspondence and perfect superpositionability.

The solution offered by the above mentioned disclosure consists in positioning the screen to be engraved in a printing mechanism susceptible of movements along the axis W, Y and Z.

The printing screen is oriented with respect to said axis X, Y and Z while a zone within the same screen is aligned with respect to the engraving axis X and Y ; to the printing mechanism, further than the data defining the image to be engraved, are provided data defining the dimension of the screen and data defining the selected coordinates within the dimensions of the screen; said selected coordinates within the dimensions of said screen are set at register with the corresponding coordinates of the image to be engraved.

The drawback of such a method and of the apparatus which embodies it consists in that the screen must be positioned for each engraving operation with respect to the axis X and Y : the reference of such a positioning consists in the adjacent sides of the frame which supports the screen.

In practice, such a system does not allow the centering of the image with a sufficient precision such as to avoid the following setting at register in the following printing phase, especially when high resolution printing operations (>720 dpi) are requested: this happens for various orders of problems, among them the existing difference among frames, which being of wood or metal have large construction tolerances and are subject to possible deformations due to their re-utilisation, as well as the curvature of the sides of the frame when the screen is put under tension.

Furthermore, in case of frames of great dimensions, the above mentioned differences among the frames will comport an amplification of the centering error, whereas for frames of small dimension such an error, although not absent, could be tolerated for low resolution printing (>720 dpi).

An additional disadvantage of the disclosed prior art consists into the necessity to position the ink-jet printing head also along the Z axes, it being the axes perpendicular to the screen to be engraved, because the frames have different thickness and therefore it is necessary for each frame to position the printing head in relation to the screen always at the same distance from it: this obliges to a repositioning of the distance of the printing head from the screen for each new frame to be engraved.

Said drawback is due to the fact that the frame stands directly on the engraving table, whereas the present invention provides for a method and means for positioning the screen always at the same distance from the printing head, independently from the thickness variations within the normal tolerances, of the different frames; the positioning of the screen in accordance to the present invention guarantees the repetitiveness and precision of the engraving procedure in respect to the position of the printing head.

A further limitation of the process of EP 0492351 consists in that the positioning and the centering of the frames must be conducted with reference to the origin of the printing operation of the printing head, whose initial coordinates are fixed, thus obliging to move the screen with respect to the said printing head for positioning the coordinates of the selected area of the screen with reference to the fixed starting coordinates of said printing head, whereas the invention in object allows to position the starting of the printing operation in a manner independent from the initial coordinates of the printing head: this, beyond making easy the positioning of the frame and the centering of the zone to be engraved of the screen, frees from inputting to the computer controlling the printing head the data relative to the zone to be engraved with respect to the dimensions of the screen, relative data which, because of the dimensional differences of the different frames, would introduce further errors in the engraving of screens belonging to the same image.

Description of the figures Fig. 1 illustrates the apparatus which embodies the method of the present invention; Fig. 2 represents a different embodiment of the frame; Fig. 3 explicates the possibility of positioning more than one frame on the apparatus for the contemporary engraving of groups of screens, where each group is referred to an image ; Fig. 4 shows the construction details of the panels constituting the table of the engraving apparatus; Fig. 5 illustrates the construction details of the spacing blocks which support the frames and the screen; Fig. 6 represents. a method for engraving by simple interpolation; Figs. 7 and 8 represent methods for engraving by interlaced interpolation; Fig. 9 shows the position of the engraving dots with reference to the nozzles ejecting them in the case of massive interlace for an horizontal advancing of the printing head.

Description of the invention The claimed method and the apparatus as characterized in the principal claim is hereinafter illustrated, in a preferred embodiment, by means of the figures.

The apparatus is provided with an engraving table (1) constituted by structural honeycomb panels of aluminum (2) inserted between two sheets of aluminum (3,4), on the top surface thereon one or more sheets of zinc-plated steel or other paramagnetic material (10) are glued.

Parallelepiped form-like blocks (5) of expanded material, i. e. macrilene, are arranged on table (1), said blocks being characterized by having all the same thickness h, although being likely to take different width L and length 1 and provided at their base with magnetic elements (6) of cylindrical or other suitable shape apt to their quick and precise adhesion on and along the surface of table (1).

Said blocks (5) are suitable of being grouped in order to form a surface (7) which is parallel to the table (1) and distant therefrom of a height h, said surface (7) supporting and perfectly close-fitting to the internal side of the fine-mesh printing screen (8) of the frame (9) to be engraved.

By means of said blocks (5) it is possible to easily compose one or more surfaces (7) supporting the frames (9) to be engraved.

The function of the supporting surface (7) is to maintain constant the distance between the screen (8) and the printing head (16) independently from the screen to be engraved.

Every commercial flat frame is provided with hooking means acting as reference elements (25, 26, 28), for allowing the different screens to be superposed during the printing phase perfectly aligned with reference to said locking means (26,26,28).

The present invention uses the locking means (25,26,28) as reference elements of the frame either in the engraving than in the printing phase.

Accordingly, the two process being performed with regard to the same reference elements, it is obtained, by means of the centering of the engraved patterns, an absolute degree of setting at register of the different frames constituting the monochromatic elements of the same polychromatic image, by eliminating the following operation of manually setting at register during the printing phase of the clothing elements.

The locking means (25,28) belonging to the frame are provided with holes (23).

The engraving apparatus is provided at the lower end of the table (1) of a rail or guide (12) along which sliding weans (13), can freely slide; said sliding means (13) are susceptible of being locked at any position of the guide (12), by means of locking handles (14) which block said sliding means (13) to said guide (12).

Each sliding means (13), consisting of a parallelepiped and acting as a support, is provided at its internal face of a pivot (24) made of rectified steel, whose diameter corresponds to the hole (23) of the locking means of the frame (9) to be engraved.

After insertion of pivots (24) in holes (23) of the locking means (25,28), for the whole duration of the engraving operation, the sliding means (13) are locked, by means of locking handles (14), to the sliding guide (12) at the same position thereto the different frames of the screens concerning the same image to compose, are lodged in sequence.

The engraving head (16), being usually an ink or wax jet printing head, is mounted on a plate (17) ; said head is susceptible of movement along the guide (18), driven by a motor not shown in figure. The guide (18) is, also, susceptible of movement by means of a motor not shown along horizontal guide (19). The vertical guide supports, by means of supports, a tank (29) containing covering means, like ink or wax, connected by means of a duct (31) to the printing head (16).

Said printing head (16) can take on any position within the surface of table (1) of the apparatus by means of the sliding guides (18) and (19).

In order to assure the planarity of the head (16), this is connected to the plate (17) by four connecting means (20), i. e. Screws; four resilient means (21) are interposed between head (16) and plate (17).

The planarity of head (16) with respect to table (1) is adjusted by acting upon the connecting means (20): said adjustment is made necessary because of the requested precision of the printing method by interlaced interpolation, as it will be clearer in the following.

Frame (9) of the screen to be covered (engraved) is positioned on a series of pneumatic microcylinders (22).

Said microcylinders (22) exercise a light pressure towards the table (1) in order to allow frame (9) to adhere to the parallelepiped pad (7): this assures at the same time the locking of frame (9) and the perfect adhesion of the screen to the pad-spacer element (7).

Such an arrangement permits, furthermore, that the contour of frame (9) falls below the supporting plane (7), so to allow nead (16) to work without interference, in consideration of the fact that the head (16) is about 1 mm spaced from the supporting surface (7).

In fig. 2 frame (9) shows a single locking means (25) on its inferior side and a locking means (26) on its superior side: in such a case the superior locking means (26) is locked by the pneumatic microcylinder (27) standing on the superior side.

The invention, by means of the described embodying elements allows the positioning of more than one frame (9) at the same time on table (1), each frame supported by elements (5). By means of information means, as computer and software, it is possible to engrave during the same engraving operation the multiple positioned and centered frames, with consequent spare of time of the engraving phase.

Such a characteristics is particularly appreciated when the printing screens are of a small dimension, as in the case of production of printing frame for sample clothing elements or for the manufacturing of cloths of small dimension, like T-shirts.

An important application can be, also, found for stencils, where notoriously the frames are not of big dimensions.

The method and the apparatus embodying the printing screen centering, must be, as said, harmonized with the printing precision requested by the perfect setting at register of the screens, in order to avoid the drawbacks fore above mentioned: in fact, the invention provides for an engraving procedure by interlaced interpolation, in order to eliminate the problem of the possible obstruction of the nozzles with consequent production abortion of screens.

The principle on which is based the interlaced printing, in the case of ink or wax jet printing heads having a plurality of nozzles, is to avoid that adjacent pixels be printed by the same nozzle.

Different interlaced pixel methods are possible; some examples are disclosed in EP 0931669, EP0420399 and EP 0757328.

In the specific case, starting from the printing method by interpolation fore above mentioned, where patterns are produces by dots by means of multiple jets, the invention provides that the same nozzle ejecting pixels along a scanning line (Y is the scan direction) does not eject the jets for tracing the pixels of the adjacent line: this requires a line interlacing.

According to such a method, patterns where single pixels are mixed each other (interlaced) are produced and when the printing head advances with a constant step along the Y scanning direction the best mixing of lines is obtained.

Fig. 6 illustrates the case of engraving by interpolation by means of a head having 10 nozzles.

The references of the figure correspond to the scanning lines along which the corresponding dots of the nozzles are engraved.

Supposing that the gap between the nozzles is 480 micron, an image having a resolution of 51 dpi is obtained.

To achieve higher resolutions it is necessary to interpolate further lines between the nozzles gap, as in Fig. 7 for a resolution of 153 dpi.

Such a procedure, as aforesaid, although allowing high resolutions of engraving, has a great drawback: it happens quite often that not all lines are correctly traced. By means of such a procedure it happens that said bad lines are engraved more times one after the other.

Therefore, it can be that possible errors are visible and the engraved screen not usable.

In the specific case of the perfect centering for the presetting at register during the engraving phase of the screens of the present invention, said errors are even more serious because of the superposition of the homologous screens at the printing stage.

The present invention overcomes said problem by means of an engraving procedure so named as interlaced one.

Fig. 8 illustrates the procedure which consists in not tracing following lines by means of the same nozzles, while Fig. 9 shows the arrangement of the contiguous jets ejected by the nozzles of the printing head, where the numbers refer to the nozzles numbering, said nozzles being arranged regularly spaced along a horizontal line, in a case of interlacing called massive.

The massive interlace is a procedure with multiple jets for engraving screens, the jets being suitable to produce single dots of the pattern to be engraved in a wag such that the jets which are adjacent to the jet of a nozzle (corresponding to a pixel) are not produced by this same nozzle: this requires au interlacing of the jets and the pattern engraved has the best mixing of pixels, the printing head being constantly progressing along the X direction, whereas Y is the scan direction.

Every movement of the printing head and the opening of the nozzles are controlled by information means such as computers and computer programs.

In the case of simple interlacing, enough for obtaining the requested resolutions by the textile industry, the used algorithms are the following: I = a/r and V = n * a/I where n is the number of nozzles, I is the interlacing, r the resolution expressed in micron, a the gap between nozzles and V the advancing step of the printing head.

Obviously, on the ground of the present inventive concept, variation of the interlace process and mixed forms of simple and massive interlace are possible.

Also, for what concerns the centering aspects in order to achieve the presetting at register engraving of screens, the same inventive path conducts to different embodiments, without going away from the claimed protection limits.