IMPROVED UNIFORMITY INK JET SYSTEM

Field of the Invention

This invention relates to ink jet printing, and in particular to the use of multi-nozzle ink jet heads. The invention has application in printing and in building 3D objects.

Background of the Invention

Ink jet deposition is a low cost and effective method for depositing materials in liquid form onto a surface. Inkjet technology is widely used in printing and may also be used in other applications, such as applying coatings (as an alternative to spraying), building three-dimensional models, building flexographic printing plates, depositing electrical conductors and more. In order to achieve high throughput, ink jet heads typically contain from a few to a few thousand nozzles, each nozzle writing one track.

Two main problems resulting from having a large number of nozzles are non-uniformity and reliability. The human eye is very sensitive to slight non- unifonnities, particularly in areas of uniform tint. A stripe having an optical density differing by 1% from the surrounding area can be easily visible. It is very difficult to keep all nozzles balanced in color density (i.e. drop size) and position to a degree sufficient to create an area which appears to be uniformly tinted. Various methods are employed to minimize this visual non-uniformity, the most common ones are interleaving and overwriting. A disadvantage of overwriting is loss of throughput.

Interleaving is performed by spacing the tracks laid down by the individual nozzles at a pitch greater, by some integer multiple, than the desired pitch (as measured across the direction of the relative motion between the head and the material being scanned) and filling in the missing tracks on a second pass. For example, if the end result is to be printed at 600dpi, the nozzles are arranged to write 300 tacks per inch of width, across the scan direction. A second pass (or a second head) fills in the missing tracks. This tends to reduce the visibility of any non-uniformity among the tracks.

A main cause of non-uniformity is not the droplet size, which can be controlled, but the direction the droplets leave the nozzles. By overwriting the areas many times such directional variations can be averaged out at the expense of throughput. A second disadvantage of overwriting is loss of registration between passes. Such loss of registration causes blurring of edges. A related problem is nozzle reliability: with more nozzles there is a greater likelihood that one or more nozzles will become plugged or otherwise malfunction. Some prior art ink jet printing systems include printing heads having redundant nozzles. Since it is not known which nozzle(s) may become plugged, a complete second set of nozzles is required. This doubles the cost of the head.

Summary of the Invention

One aspect of this invention provides a method and apparatus which permit the writing of each tack by a plurality of nozzles, all part of a single head assembly, in order to average out any variations between nozzles, but without any throughput penalty. The invention achieves all the benefits of overwriting and interleaving without throughput and registration penalties. Apparatus according to the invention can also provide full redundancy for any failed nozzles without increasing the number of nozzles by a large amount. Apparatus according to preferred embodiments of the invention can automatically detect any non-uniformity in the output of the nozzle array and correct for it automatically. Further objects and advantages of this invention will become apparent by reading the disclosure in conjunction with the drawings.

A main feature of the invention is writing each tack by multiple nozzles but without overwriting the marks created by each nozzle. In order to achieve this, droplets deposited by a number of nozzles which form a single track are interleaved along the track direction (unlike conventional interleaving which is done across the direction of the tacks). If a track is made up of "n" nozzles, the droplets from each nozzle are spaced apart by "n" times the size of a mark made by one droplet. After adding together the droplets from all "n" nozzles, a continuous track is written. Since there is no overlap of droplets there is no loss of throughput. The position and density of the track is now the average of "n" nozzles, and any single nozzle which has the wrong position or droplet size will be averaged with all other nozzles. Furthermore, to achieve redundancy for any one of the "n" nozzles, the number of the nozzles only has to increase by one to "n" + 1. Since all writing can be done in a single pass, there is no registration loss and therefore no shaipness loss.

In certain applications, such as building 3D models using Inkjet heads (jetting a molten wax or photo-polymer), throughput is not limited by droplet rate but by the amount of deposited mass. In such a case it may be preferred to overlap the droplets to build up a thicker layer. In such an application, the droplets from each one of the "n" nozzles are not spaced apart, but superimposed. This provides high deposition rate without the loss of resolution larger drops would have caused.

Either mode can incorporate means of detecting non-uniform output or defective nozzles and automatically substituting a new nozzle as described in more detail below. Brief Description of the Drawings

In drawings which illustrate non-limiting embodiments of the invention,

Fig.1-a is a schematic depiction of a nozzle array in a typical prior art multi-channel ink j et head;

Fig. 1-b is a schematic depiction showing the nozzle array layout of Figure 1-a when the last track is overlapped with the first track of the next group of tracks, in order to minimize the effect of the boundary of a group of tracks;

Fig. 1-c is a schematic depiction showing the common mode of interleaving and the prior art way of using redundant nozzles;

Fig. 2 shows a nozzle array arrangement in an ink jet head according to the preferred embodiment of this invention;

Fig. 3 shows droplet interleaving in the direction of the track;

Fig. 4 shows droplet overwriting in the direction of the track when build-up is desired;

Fig 5 shows the use of an extra nozzle in each track to replace any failed nozzle; and,

Fig. 6 shows a method for detecting failed nozzles and controlling surface smoothness.

Description of the Preferred Embodiment

Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense. Prior Art

Multi-nozzle ink jet heads, particularly of the type known as piezo-activated, are typically configured in a two dimensional layout shown in Fig. 1-a. This layout allows the tracks to be written at a spacing closer than the physical spacing between the nozzles. Because of mechanical errors in moving such multi-nozzle heads relative to the material being written, there is a high likelihood of a registration error between each group of tracks and the next group, formed by the next pass of the head. Such an error will show up as a light or dark line at the boundary of groups of tracks.

A common remedy for such registration errors is to overlap at least one track at the boundary between passes. This is shown in Fig. 1-b, where 3 is a group of tracks written in one pass and 3' is a second group, written on a separate pass, with one track being overlapped. To further decrease track spacing and reduce visible non-uniformity interleaving is normally used, as shown in Fig. 1-c. Tracks 2 were written on the first pass while tacks V were written on the second pass between any two previously written tracks 2. Fig 1-c also shows the common way of achieving redundancy: behind every nozzle 1-a there is a spare nozzle 1-b which can be switched in.

This Invention

A preferred embodiment of the invention is illustrated in Fig. 2. A group of nozzles 3 form an assembly, typically package as a single head. By the way of example, each column is formed from three nozzles la, lb, and lc. The actual number of nozzles in each column may vary from 2 to over 10. The columns are also staggered to spread out the nozzles over a larger area in order to facilitate the manufacturing of the head. The staggering (explained in Fig. 1-a) is chosen according to the method of fabricating the head. The relative motion between the head and material being written is shown by arrow 7. For the purpose of this invention it makes no difference whether the head or material being written on is moving, as long as relative motion exists. The motion can be unidirectional or bi-directional. After a group of tacks 3 is written, a cross-scan motion, perpendicular to arrow 7 is also required, but it not part of the invention and will not be discussed here as it is well known in ink jet printing practice. The cross-scan motion can be eliminated if group 3 spans the full width of the material being written. This is known as "page wide" writing and requires thousands of nozzles. The staggering of nozzles in direction 7 can occur within one head or by using multiple heads .

In the first mode of the invention, each one of nozzles 1-a, 1-b, and 1-c create droplets separated from each other. For example if the nozzle rate is 24,000 droplets/second and the resolution is 600dpi, the scan speed should be 40"/sec to have a contiguous series of marks. When three nozzles are used per track, the scan speed can be increased to 120"/sec, as each nozzle only has to deposit every third droplet. The correct interleaving between droplets is controlled by electronic timing signals which take into account the distance between nozzles in direction 7. Such timing circuits are common in ink jet printers and need not be detailed here.

The advantages of the invention are shown in Fig. 3. Assuming that each nozzle has its own placement error, droplets coming from nozzle 1-a might not line up accurately with the desired tack position 4, but might form a displaced tack 5. In the same manner droplets from nozzles 1-b and 1-c form displaced tacks. The combined track has a position 6 which represents an average of the positions of the tracks formed by droplets 2-a, 2-b, and 2-c, which are respectively deposited by nozzles 1-a, 1-b and 1-c. This average position is closer to the desired track position 4 due to the averaging effect. For "n" nozzles the visual effect of an error "x" in the placement of droplets by an individual nozzle will be decreased by a factor of "n". Note that each nozzle is fed with different data, even if the nozzles form the same track.

In some applications it is desired to deposit the droplets on top of each other in order to build-up color density or thickness. An example of such an application is building three-dimensional objects by ink jet. The jetted material can be a molten wax or polymer which solidifies by cooling or a photo-polymer which is solidified by actinic radiation, such as UV light. In such an application the method shown in Fig. 4 is preferred, as it reduces the scan velocity compared to Fig. 3. Note that in Fig. 4 nozzles 1-a, 1-b, and 1-c carry the same data excerpt for a time delay t or 2 t, as the droplets from nozzle 1-b will overlap the droplets from nozzle 1-a and therefore have to carry the same data, delayed by t in order to compensate for the different time of jetting. The timing electronics is similar to the electronics used to compensate for staggered nozzles and need not be detailed here, as it is common in the art of ink jet printers.

In order to achieve redundancy an additional nozzle (or multiple nozzles) can be added to each track as shown in Fig. 5-a and 5-b. Nozzle 1-e is the spare nozzle while nozzles 1-a to 1-d are the active nozzles. When a faulty nozzle is detected, as shown by failed nozzle 1-c in Fig. 5-b, the spare nozzle 1-e is switched in and supplied with the data for the faulty nozzle. The data used to feed nozzle 1-c has to be timed correctly to compensate for the distance between nozzle 1-c and 1-e in the direction of scan 7. As before, such electronic timing is common in the art of ink jet printing, as the prior art redundancy shown in Fig. 1-a also requires such a timing change.

The main advantage of the invention over prior art (Fig. 5-d vs. Fig. 1-c) is that the number of nozzles needs to be only increased slightly, not double, to compensate for a single nozzle failure in each track. By the way of example, a 1000 nozzle array writing 1,000 tracks will require another 1,000 nozzles to allow any nozzle to fail on any track. This doubles the cost of the ink jet head. If the same 1,000 nozzles were arranged as 100 tracks. Each one containing 10 nozzles arranged in accordance to the invention, the throughput would be maintained (scan velocity will have to be increased tenfold, of course). To achieve full redundancy of every single nozzle only one extra nozzle would need to be added to each tack, for a total of 1,100 nozzles instead of

2,000 nozzles.. The savings are even more dramatic if it is desired to provide a system which can compensate for two consecutive failures in any tack. In a system according to this invention 1,200 nozzles would be required (2 extra nozzles per tack of 10 nozzles) instead of 3,000 nozzles (3 rows of 1,000 nozzles each) as would be required by a prior art system having similar throughput.

There are many well known methods for detecting failed nozzles. In general such methods can be divided into two groups: scanning the nozzles directly, using a laser or similar device to test for droplets, or scanning the output being produced using a CCD camera or similar devices. For a large number of nozzles it is more efficient to scan the printed output. A method of scanning not requiring any specialized test patterns is shown in Fig. 6. Electronic camera 11 (CCD based or other) is scanning scan lines 2 deposited on substrate 8 by ink jet array 3. An optional illuminator 10 can be used. By comparing image picked up by camera 11 to input data 13 any deviations caused by faulty tracks can be detected by image processor 12 and redundant nozzles switched in.

A specialized case of this closed-loop monitoring is when building three dimensional objects. In such an application the crucial paramount is not color or density but the smoothness of the surface crated by tracks 2. To determine the profile of the surface, illuminator 10 is a laser line illuminator, casting a thin line 9 on the surface created by tracks 2. When such a line is viewed by camera 11 from an oblique angle, any variations in height are translated to deviations in the staightness of the projected line. The image processor 12 can compute the height variations from such a line image and control the number of nozzles (or the droplet size) in each track to make line 9 as straight as possible. The electronics and software for such 3D surface processing based on image of line 9 are well known in the art of inspection systems and need not be detailed here.

The high uniformity and reliability made possible by the invention makes it particularly suitable for graphic arts applications such as high speed digital printers utilizing a "page wide" ink jet array as well as ink jet color proofers. Prior art ink jet color proofers could not produce a true dot-for-dot simulation of halftone printing (also known as "halftone proofing") due to lack of uniformity when reproducing halftone patterns. The uniformity and reliability offered by this invention makes such a device possible. The amount of control offered by multiple nozzles per track allows very uniform deposition even if each nozzle is binary, i.e. without control over the droplet size.

As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.