Method for imaging with UV curable inks

An inkjet printing method ejects fluid droplets onto a transfer surface. On the transfer surface the droplets are treated. The droplets are then transferred to a substrate. The treatment decreases the sizes of the dots and increases their viscosity. Adjacent dots in the pattern may be printed in separate passes to retain dot integrity. The droplets may comprise UV-curable inks. The droplets may be partially cured by exposure to UV radiation while on the transfer surface.

FIELD OF THE INVENTION

The invention pertains to the general field of printing and in particular to inkjet printing.

BACKGROUND OF THE INVENTION

Ink jet technology may be used to deposit fluid materials on substrates. Ink jet technology has numerous applications, mainly in printing. Ink jet printers function by depositing small droplets of fluid at desired positions on a substrate. There are various ink jet printing technologies. Many of these technologies can be classified in two general categories. Continuous ink jet printing involves electrically charging a stream of droplets and then deflecting the stream directly or indirectly onto a substrate. Drop on demand (DOD) inkjet printing has an actuator connected to an ink supply. The actuator creates ink droplets on demand. The actuator may comprise, for example, a piezoelectric actuator.

Ink jet printing suffers form a number of drawbacks. Ink jet printing is typically slower than traditional offset printing. This is especially true for process color printing. For example, the linear printing speed of inkjet printing is typically of the order of 10 times slower than can be achieved in offset printing. This represents a major issue limiting the implementation of inkjet technology in industrial printing systems. The inkjet printing speed limit is dictated by the rate at which inkjet nozzles can eject ink in discrete controllable amounts. This rate is at present on the order of 20,000 pulses per second for DOD inkjet printers. This limits state of the art DOD inkjet printers to print rates on the order of 2 pages per second. Continuous ink jet printing can be performed more quickly. However, at high speeds, the results tend to be poor. Quality may be improved by printing at slower speeds.

Inkjet printing typically cannot achieve printing quality as high as can be achieved using offset printing techniques. Inkjet printing is often characterized by a distinctive banding pattern that is repeated over the printed image. This may be traced to the arrangement of the inkjet nozzles in the printing head. Relatively small nozzle misalignments or off-center emission of droplets can cause banding. As the printing head is translated laterally across the width of the printing surface, the visual imperfections are periodically repeated. This produces banding or striping which is characteristic of inkjet printers. A number of approaches exist to control banding. These approaches reduce throughput of the printer.

Print quality of inkjet printers is also reduced by wicking or running . The low-viscosity water-based inks typically employed in ink-jet printers tends to run along the fibers of certain grades of paper. This phenomenon is also referred to as wicking and leads to reduced quality printing, particularly on the grades of paper employed in volume printing. Wicking can cause printed dots to become much larger than the droplet of ink emerging from the inkjet nozzle.

It is possible to reduce wicking by printing on specially treated paper. However, such paper tends to be undesirably expensive.

The matter of failure in inkjet nozzles is also deserving of attention. Various approaches exist for detecting faulty inkjet nozzles and for re-addressing the inkjet printing head to permit other nozzles to perform the tasks of faulty nozzles. This includes various redundancy schemes. Again, these usually have the effect of slowing down the net printing process speed. In many cases the redundancy is managed at printing head level, requiring backups for entire printing heads. This adds to the cost of the technology per printed page and again limits the industrial implementation of the technology.

The prior art describes various array inkjet print head designs aimed at reducing inkjet-printing artifacts such as banding. Examples are Furukawa in U.S. Pat. No. 4,272,771, Tsao in U.S. Pat. No. 4,232,771, Padalino in U.S. Pat. No. 4,809,016 and Lahut in U.S. Pat. No. 5,070,345. Considerable work has also been done in addressing reliability by providing inkjet nozzle redundancy. Examples are Schantz in U.S. Pat. No. 5,124,720, Hirosawa in U.S. Pat. No. 5,398,053 and Silverbrook in U.S. Pat. No. 5,796,418. Transfer rollers have also been described, both with and without the droplets deposited on them being processed in some way before final printing in order to reduce wicking. See for example Takita in U.S. Pat. No. 4,293,866, Durkee in U.S. Pat. No. 4,538,156, Anderson in U.S. Pat. No. 5,099,256, Sansone in U.S. Pat. No. 4,673,303 and Salomon in U.S. Pat. No. 5,953,034.

There is a need for inkjet printing methods which provide combinations of print quality, speed and cost which improve on the prior art.

SUMMARY OF THE INVENTION

This invention, provides an inkjet printing method in which inkjet droplets are deposited onto an intermediate transfer surface. On the transfer surface the droplets are treated to decrease their sizes and to increase their viscosities. The treated droplets are then transferred to a printing surface. Dots immediately adjacent to one another in the pattern may be printed in separate passes to retain dot integrity. The droplets may comprise droplets of a UV-curable material and the treatment may comprise exposing the droplets to ultraviolet light while on the transfer surface. The transfer surface may optionally be patterned.

Further aspects of the invention and features of specific embodiments of the invention are described below.

DETAILED DESCRIPTION

FIG. 1 shows an apparatus 100 which includes an inkjet head 1 comprising rows and columns of inkjet nozzles 2 arranged to deposit fluid droplets in a fluid droplet pattern 3 on a transfer surface 4 . Pattern 3 is set by the control signals provided to the inkjet nozzles 2 by a controller (not shown in FIG. 1 ). In this embodiment, transfer surface 4 comprises a continuous belt 4 moving in the direction indicated by the arrows. While inkjet nozzles are employed as sources of fluid droplets in preferred embodiments of the invention, the fluid droplet sources may be of other suitable types and the fluid may be an inkjet ink or another ink, a pigment or a resin or any fluid required to create an image or pattern. In the embodiment of FIG. 1 , the fluid droplet sources comprise inkjet nozzles 2 which eject droplets of ink.

Inkjet droplet pattern 3 is subjected to post-deposition processing by post-deposition processing unit 5 the processing changes properties of the ink droplets of pattern 3 . While the post-deposition treatment may comprise one or more of:

airflow

Heat treatment may comprise one or more of microwave heating, radiative heating or conduction heating.

As shown in FIGS. 2A through 2C , the post-deposition treatment reduces the size of the fluid droplets and changes their rheological properties. For example, the post-deposition treatment may increase a viscosity of the droplets in pattern 3 .

In the embodiment of FIG. 1 , continuous belt 4 rolls around rollers 6 and 8 . A printing medium 9 is compressed against roller 8 by an elastomeric roller 10 . Droplet pattern 3 is transferred from belt 4 to a surface of medium 9 at the location where medium 9 passes between rollers 8 and 10 . Medium 9 may comprise paper, plastic, polyester, a polymeric material or another material to be printed on or, in general, any substrate to which a fluid-droplet pattern may be transferred. The fluid used to create pattern 3 is chosen to be compatible with medium 9 . Medium 9 may be in the form of individual sheets or in the form of a continuous roll. Medium 9 could comprise a printed circuit board or a lithographic mask.

In embodiments where the post-deposition treatment comprises heating, surface 4 should be cooled to a temperature compatible with the type of medium 9 being printed upon before it comes into contact with medium 9 . In the embodiment of FIG. 1 , this is accomplished by providing surface 4 on an elongated belt 4 and also by providing a belt-cooling unit 7 .

The post-deposition treatment of the droplets of pattern 3 facilitates droplet transfer while preserving dot integrity. Dot integrity is preserved when the shape (i.e. the outline of a dot on the surface of medium 9 ) is preserved and is consistent from dot to dot. Dots that are deformed from a geometric shape anticipated by the design of the nozzles and the transferring surface, or droplets that have coalesced, therefore represent a loss in dot integrity.

Belt 4 is cleaned by a pre-cleaning unit 11 that removes any remaining ink in preparation for the deposition of more droplets by nozzle array 2 . If it is necessary or desirable to control the affinity of the surface of the continuous belt for the fluid droplets being deposited on it, pre-cleaning unit 11 may clean surface 4 using a liquid hydrophobic cleansing agent which may be sprayed on or wiped on.

The effect of a post-deposition treatment process on the dots of pattern 3 is illustrated in more detail in FIGS. 2A to 2 D. FIG. 2A shows droplets as deposited on surface 4 . FIG. 2B shows the same droplets after they have been heated. The droplets of FIG. 2B have shrunken because the heating has caused much of the solvent in the droplets (the solvent is water in most industrial inkjet inks) to turn to vapor. The heat treatment also changes rheological properties of the droplets. In particular, the viscosity of the droplets increases. The surface tension of the droplets ensures that they maintain integrity as they shrink due to the loss of solvent. In some embodiments, at least 40% or at least 50% of the solvent initially present in the droplets is evaporated in a post-deposition process.

The pattern of reduced-size, higher-viscosity droplets is then transferred to the surface of medium 9 . The increased viscosity of the droplets reduces the wicking or running of the droplets on medium 9 . In the transfer process, the droplets are flattened and therefore the dot size increases upon transfer. The dot size on the printing surface is controlled by the choice of processing temperatures and transfer pressures on the rollers and the paper. The result is shown in FIG. 2 C.

The increased viscosity of the droplets facilitates improved control over the inkjet printing process. The dot integrity of pattern 3 as deposited on surface 4 may be maintained on a wide range of media 9 . Standard high-volume printing paper of types used for offset-printing that has not specifically been treated for purposes of inkjet printing may be used as a medium 9 .

By way of example, surface 4 may comprise PEARLdry waterless printing plate supplied by the Presstek company of Hudson, N.H. Surface 4 may be coated with Scotchgard Leather Protector from the 3M company of St. Paul, Minn. to make it hydrophobic. The ink may be that employed in the HPC4844A cartridge supplied by the Hewlett-Packard company of Palo Alto, Calif. and it may be deposited as fluid droplets on the treated plate by means of an inkjet head from an HP 2000C inkjet printer supplied by the same company. A range of droplet sizes may be obtained.

In some embodiments the post deposition treatment causes the droplets to shrink from a first diameter to a second diameter. In some embodiments the second diameter is 85% or less of the first diameter. For example, with one choice of printing conditions, droplets which are 25 microns in diameter as deposited on surface 4 . The droplets are shrunk to 20 microns in diameter upon heating at 120 C. for 60 seconds. The droplets widen to 35 microns in diameter when printed onto regular paper, not specially treated for inkjet printing. When conventional inkjet printing is employed to print on the same regular paper, the same ink and head tend to print irregularly shaped dots on the order of 75 microns in diameter.

To achieve adequate coverage and a complete set of grey tones or color densities, it may be desirable to arrange droplet pattern 3 so that immediately adjacent nearest-neighbor droplets overlap to some degree on the surface of medium 9 . This overlap arrangement of immediately adjacent dots is shown in FIG. 2 D. If droplets occupying all possible positions in the pattern were deposited on the transfer surface at the same time, then some dots would likely touch and coalesce, with a consequential loss of dot integrity.

Print dot integrity may be enhanced by performing the printing process in two or more steps as shown in FIGS. 2A through 2C . In this embodiment, droplets intended to occupy immediately adjacent positions in the final printed pattern are deposited in separate steps. In the first step a first subset of droplets is deposited such that immediately adjacent nearest neighbor droplet positions are not occupied. In FIG. 2D , the dots so obtained are depicted as solid dots. In a second step an interleaved subset of dots, depicted by the hatched dots in FIG. 2 D and representing droplet positions in the final printed pattern that would be immediately adjacent nearest neighbors to the first subset, is printed. The two steps may be achieved by either running the paper through the same printing system twice or by having two entirely separated printing systems operating serially on the same medium 9 .

In one embodiment the fluid used to print with is water-based industrial inkjet ink and at least two printing units are employed. In a more general case any number of such printing units is used. The printing units deposit droplets as described above with reference to FIGS. 2A through 2C . No two droplets touch each other during the entire transfer process, unless they have first been through post-deposition processing. Therefore, the droplets have no opportunity to coalesce while in their un-processed states.

FIG. 3 shows a printing system comprising two printing units arranged in series. The two units may be substantially identical. In this embodiment, post-deposition treatment unit 5 comprises a thermal processing unit which comprises a heating system 5 a , and a vapor extraction unit 5 b . Vapor extraction unit 5 b forcibly removes solvents (such as water vapor) emitted by the heated fluid droplets during processing. Medium 9 is shown in FIG. 3 as being continuous and moving in the direction indicated by the arrow.

Belt-cooling unit 7 assists in maintaining registration between the patterns deposited by the two printing units of FIG. 3 when multiple printing units are employed. Belt-cooling unit 7 prevents excessive thermal expansion of belt 4 . Any suitable system may be used for maintaining synchronization between the belts 4 of the two printing units. Synchronization control systems for continuous belts are well known and will not be described here.

FIG. 4 shows an inkjet head 1 which may be used in this invention. Head 1 has inkjet nozzles arranged in rows and columns. The term column means a row of nozzles extending generally along the direction of motion of the transferring surface relative to the inkjet head as indicated by the arrow. The term rows means a row of nozzles in the remaining dimension. Ink jet head 1 has a primary array 2 a and a secondary array 2 b . Primary array 2 a may have any number of rows and columns. The specific embodiment of FIG. 4 has 24 columns of in-line nozzles arranged in 10 rows. Nozzles or fluid droplet sources in general are in-line or aligned when they are arranged in a straight line along the direction of motion of the transferring surface relative to the array of droplet sources. To this end the alignment of the nozzles need only be within the tolerance accepted for the printed line-width in the direction of motion of the transferring surface.

For the sake of simplicity and clarity, FIG. 4 depicts the nozzles of ink jet head 1 as being in straight rows. However, the invention presented here is not restricted to this arrangement. In the general case the rows of nozzles do not need to be perpendicular to the columns, nor do the rows need to be straight or the placement of the nozzles regular, as long as the nozzles in a column are placed directly in-line with the direction of motion of the transfer surface. It is common practice in industry to have the rows non-linear and in various staggered formats. Any of these variations are compatible with the invention presented here as long as a given column of nozzles prints in-line.

Secondary array 2 b comprises one or more rows of redundant nozzles. The embodiment of FIG. 4 shows a secondary array with one row of nozzles. The term redundant means for backup and does not mean superfluous. Should, for example, nozzle 2 c become blocked or intermittent or break, the control system of the print head will sense this failure and will cause the role of nozzle 2 c to be taken over by redundant nozzle 2 d with the timing signal appropriately adapted. The matter of timing management for inkjet nozzles is well established in the industry and will not be detailed here. Systems for detecting failing nozzles and automatically replacing them have also been described and will not be discussed here.

The redundant nozzles must be in-line with the nozzles they replace, even if nozzles within a redundant row are not arranged in a straight line. The placement of redundant nozzles in-line with the nozzles they are designed to replace, allows for the use of a single redundant nozzle to serve as back-up for a number of different main nozzles in-line with it without requiring the inkjet head to be laterally translated to bring the redundant nozzle into operation. Maximum printing speeds may therefore be retained despite there not being one redundant nozzle for every main nozzle. This arrangement allows redundancy to be implemented at very low cost whilst maintaining high printing speeds. As with the main nozzles, the alignment of the redundant nozzles with the main nozzles in the direction of motion of the transferring surface need only be within the tolerance accepted for the printed line-width in the direction of motion of the transferring surface.

The in-line arranged columns of inkjet nozzles in the primary array 2 a allow the writing of each printing track by a plurality of nozzles. The nozzles may all be part of a single head assembly. This averages out any variations between nozzles. Banding and striping, which are typical visual imperfections characterizing inkjet printing, are therefore greatly reduced without the throughput loss arising from more standard techniques such as interleaving and overwriting.

By placing the nozzles in a column aligned with the direction of motion of transferring surface 4 , the printing speed may be increased by a factor equal to the number of rows (or the number of nozzles in a column). The printing head 1 illustrated in FIG. 4 , permits the printing speed to be multiplied by a factor 10 (since primary array 2 a has 10 rows of nozzles).

In an alternative embodiment of the invention shown in FIG. 5 , inkjet heads 12 and 13 deposit inkjet patterns on drum roller 14 . Each of the patterns is a subset of the total pattern such that, when correctly combined, they constitute the complete pattern. As drum roller 14 rotates it transfers the subset droplet patterns to a printing surface 15 that is in the form of a looped continuous reel. The inkjet heads are controlled by a controller (not shown in FIG. 5 ), that ensures the appropriate programmed delay between the sets of data representing the patterns being printed. At any given moment in time the inkjet heads 12 and 13 will be printing subset patterns of different images, as determined by the extent of the loop in the continuous reel of paper. The programmed delay is timed to compensate exactly for the loop in the continuous reel 15 . Again, in keeping with standard practice in the industry, rollers 16 and 17 may be elastomeric.

In yet another alternative embodiment of the invention, transfer surface 4 has a patterned surface. This surface is chosen to be hydrophobic and has upon it a pattern of areas where water-based ink droplets preferentially locate themselves. This may be achieved by a variety of means including making these areas less hydrophobic, by creating a physical pattern on the surface that allows the droplets to locate there or any other means that will induce the droplets to locate there in order to minimize the surface energy. This includes the selective electrostatic charging of the surface. By this approach the droplets will self-correct their spatial registration when deposited onto transfer surface 4 and thereby automatically correct for any off-center droplet emission by the relevant inkjet nozzles and improve the quality of the printed image. This process need not be restricted to water-based inks. The requirement is merely that the affinity of the transfer surface for the fluid droplets vary in a pattern as described above, allowing the fluid droplets to locate at such positions as will minimize the surface energy.

FIG. 6 shows a still further embodiment of the invention which comprises a two-stage fluid droplet transfer unit. In the embodiment of FIG. 6 , a printing medium 9 is treated to improve the dot integrity. The treatment is applied by a paper treatment unit 18 which is located to treat medium 9 (typically regular paper) before it passes between rollers 8 and 10 . One example of a treatment of the paper is to spray it with a hydrophobic liquid.

In one embodiment of the invention, the fluid droplets comprise droplets of an ultraviolet (UV) curable ink. The ink may comprise a UV curable screen printing ink. The ink may comprise a relatively high viscosity UV curing oligomer in a volatile solvent. An oligomer is a polymer or collection of polymers and monomers that can be further reacted to form a larger polymer. The UV curable ink may comprise, for example, a dye or pigment and a mixture of UV pre-polymers and photoinitiators together with a mixture of one or more volatile solvents. The amount of solvent is chosen so that the ink has a viscosity suitable for inkjet printing. This is most typically in the range of about 2 to about 30 centipoise. The pre-polymers may comprise mixtures of acrylic oligomers and monomers and may also include diluents. Alternatively, cationic curing systems incorporating solvents may also be used.

The fluid droplets are applied to a medium 9 as described above. Before being applied to printing medium 9 , most of the solvent is removed from the droplets. Removing the solvent may comprise heating the droplets. This may be performed by post-deposition processing unit 5 . Optionally, the droplets of UV curable ink may be partially cured while on transfer surface 4 . Such partial curing may be initiated by exposing pattern 3 to ultraviolet light. This may be achieved, for example, by providing a UV light source 21 which illuminates pattern 3 on transfer surface 4 . Light source 21 may be considered to be a post-deposition processing unit. The partial curing of the fluid droplets of pattern 3 further thickens the ink before transfer and final curing. After being applied to printing medium 9 , the droplets are cured by exposing them to ultraviolet light from, for example, an exposure unit 20 (see FIG. 6 ). UV light from a single UV light source may be used both to partially cure the fluid droplets on transfer surface 4 and to cure the fluid droplets on substrate 9 .

The solvent is preferably collected. The collected solvent can either be removed from the machine or can be used to dilute an ink concentrate and re-used.

This choice of ink has a number of advantages including:

the solvent-depleted ink droplets which result from the post-deposition treatment have a high viscosity and therefore will retain a small dot size;

the UV curing of the ink on substrate 9 may occur almost instantaneously;

substrate 9 may comprise any of a wide range of media.