GB2438180A

GB2438180A - Ink jet printing methods

Info

Publication number: GB2438180A
Application number: GB0609608A
Authority: GB
Inventors: Matthew Pyne; Nick Geddes
Original assignee: Cametrics Ltd
Current assignee: Cametrics Ltd
Priority date: 2006-05-16
Filing date: 2006-05-16
Publication date: 2007-11-21
Also published as: GB0609608D0

Abstract

A method of colour ink jet printing an image onto a substrate, the image comprising a plurality of pixels of one or more colours, the printing using an ink jet print head having a plurality of nozzles, at least one nozzle for delivering droplets of coloured ink for each one of one or more process colours, the method comprising using a further one of said plurality of nozzles to deliver droplets of a solvent miscible with a solvent used for said droplets of said coloured ink, and printing a said pixel of said image using a combination of said process colours and said solvent to create a colour for the pixel.

Description

Ink Jet Printing Methods The invention is concerned generally with ink

jet printing, in particular to methods and apparatus for printing an image having a plurality of pixels of different colours onto a substrate.

Ink jet printers are known in the art for printing images onto a medium (e.g. paper) using a variety of different inks. They have many advantages over other forms of printing, in particular they may be configured to print large areas in colour or black and white relatively quickly and they are relatively inexpensive compared with other printing technologies.

Figure 1 shows a print head 2, as known in the art, positioned at a corner position over a piece of paper I with an exemplary image printed on it. The long axis of the print head is oriented at an angle of approximately 45 degrees to the long axis of the paper. In general, the print head may be oriented at any angle to the paper, the choice of angle providing a trade off between the area covered by the print head in one sweep of the paper and the resolution of an image printed by the print head.

Figure 2 shows process colours commonly used in the art for printing colour images. In addition to black (shown on the right as ink K), cyan (C), magenta (M) and yellow (Y) are used. These are the primary colours of subtractive colour mixing. Cyan ink absorbs red light and reflects green and blue light; magenta ink absorbs green light and reflects red and blue light, and yellow ink absorbs blue light and reflects red and green light.

These colours (which generally also include the colour black) may be termed process colours' in ink jet printing. Using these different colours in varying quantities, a large colour gamut may be obtained, containing all hues visible to the human eye. Saturation levels may vary, but the human eye is much less sensitive to differences in saturation than to differences in hue.

Black ink absorbs all the colours; this may be used in combination with the other process colours to reduce the quantity of process colour ink required to print a colour image and to ensure that the black portions of the image look properly black. The responsiveness of the human eye to light is logarithmic, and more sensitive to small variations in light levels at low intensities than at higher intensities; consequently small amounts of variation in the quantities of process colours used to mix a black colour would result in visible variations in the hue' of the black colour produced. By determining a quantity of ink needed for a process colour with the lowest ink requirement for a particular colour and subtracting this quantity from all the process colours, the requirement for the other process colours may be reduced and black ink may be used instead. Black ink may be cheaper than inks having other colours and it may be possible to guarantee a correct black' colour more easily with a single black ink than by mixing the three process colours. It may also be possible to replace a black ink cartridge separately from the other colours, possibly more regularly, especially if the printer is also used for printing black and white (or greyscale monochrome) images with no colour content. Frequently material for colour printing may comprise large black and white areas such as text, and comparatively smaller colour areas.

Figure 3 shows typical ink quantities for droplets of ink used in ink jet printing on paper. A quantity of ink deposited per pixel may vary, for example between 4 picolitres and 40 picolitres, and this may be used to produce a greyscale. At the lower end of the quantity range, the paper occupying one pixel space may not be completely covered by the ink as it is absorbed, leaving areas of paper in the original colour (for example white). This effect may be used, optionally in combination with dithering, to vary an intensity of a printed colour.

However, it flay be desirable to print on substrates other than paper. Textiles and fabrics such as cotton and man-man fibres are often used in the manufacture of clothing and garments such as T-shirts and sweatshirts, as well as bathroom items such as towels. These may be produced relatively inexpensively in a single colour, for example by dyeing or bleaching the fibres before manufacture, and it would be advantageous to print different colours on them using inexpensive techniques such as ink jet printing.

This would be useful for promotional materials, such as T-sbirts displaying company logos and trade marks, and would be much less expensive than weaving such logos and trade marks using separate coloured threads for each different coloured area.

However there is a problem with ink jet printing onto textiles and fabrics. These substrates are capable of absorbing much more ink than a piece of paper over a given area, so larger volumes of ink may be used to colour a fabric a given colour. Figure 4 shows an example of a typical quantity of ink in an ink droplet used to colour a fabric, and an example of the type of materials it may be deposited on. 20 nanolitres represents a quantity of ink 3 orders of magnitude greater for the droplets used in printing on fabrics, than the droplets used in printing on paper. This quantity may be absorbed by the fabric and run along the fabric pile. The figure shows an example of fabric piles used to make bath towels, where loops of fibre are supported by a fabric base. Ink droplets landing on the fibres land on the side of the pile facing the ink jet nozzle, and a sufficient quantity of ink may be used to run along the length of the pile until it just touches the substrate and colour the whole pile, not just the area where the droplet landed.

By this method it is possible to colour substantially the whole of a piece of fabric one colour, but it is not possible to print a range of colours or greyscale images using this method. Figure 5 shows the problem diagrammatically. If a smaller quantity of ink is used, for example to attempt to print a light grey colour, the ink lands at a certain point along the pile (roughly in a line according to the sweep of the ink jet head) and is absorbed by the fabric locally at that point, but there is not a sufficient quantity of ink to colour the whole pile. The effect is similar to dithering; at a distance the fabric may appear light grey, i.e. the colour intended, but close up the individual patches of colour are visible and the result looks messy. This may not be acceptable for promotional items, as companies may be highly protective of their brand logos and trade marks, and may not want inferior-looking products appearing on the market associated with their brand. Dithering may be difficult to employ because it relies on being able to print at high resolution, which is difficult on a fabric pile surface.

Furthermore, in articles such as bath towels, where the long piles of fabric may be moved by brushing against them, the side of the pile exposed to the print head may not always be the side visible to the viewer. If the piles are combed in the opposite direction, another side may be exposed which has less visible colouration on it due to the incomplete absorption of coloured ink into the fibre. This effect may be more pronounced with light colours than darker colours, due to the varying amounts of ink deposited to achieve the colour. Again, in general use this detracts from the quality of the image printed on the article, as dark areas appear relatively consistently dark, whereas lighter coloured areas vary in intensity depending on how the fibres are currently arranged relative to their arrangement when printed.

Current ink jet printing methods for fabric articles avoid this problem completely by the use of spot colours rather than process colours. These are inks chosen or pre-mixed together for the desired colour, so that they may be deposited on the article at constant density. However, this limits the variety of colours available for printing, as printing may only take place with the number of spot colours available. This leads to articles being printed with large areas of the same colour, and no colour fades between one colour and another. Furthermore, it may be more expensive, because a spot colour must be obtained for each colour in the desired image, leading to smaller quantities of a larger number of colours being consumed. It would be advantageous to provide a method of ink jet printing on fabrics using process colours that overcomes the above-mentioned problems.

According to an aspect of the present invention, there is provided a method of colour ink jet printing an image onto a substrate, the image comprising a plurality of pixels of one or more colours, the printing using an ink jet print head having a plurality of nozzles, at least one nozzle for delivering droplets of coloured ink for each one of one or more process colours, the method comprising using a further one of said plurality of nozzles to deliver droplets of a solvent miscible with a solvent used for said droplets of said coloured ink, and printing a said pixel of said image using a combination of said process colours and said solvent to create a colour for the pixel.

Preferably a substantially constant volume is maintained for a total volume of liquid (e.g. inlc and solvent) delivered to the substrate. This provides even coverage of the substrate over a range of colours obtainable with the method. The volume may be determined by a desired liquid loading for the substrate, for example in order to colour the substrate sufficiently without using excess ink, or by an ink penetration depth of the substrate (e.g. a distance to which a quantity of ink will penetrate, which may be determined by experimental methods).

Preferably the print head comprises a variable droplet volume print head, in order to provide droplets of different sizes to mix the desired colours. The volumes of the droplets of ink and solvent may be varied to achieve the substantially constant volume criterion. Alternatively the print head may be a constant droplet volume print head, and printing involves delivering droplets of ink and solvent from the print head a plurality of times over the same area of substrate, in order to achieve the desired colours and/or constant total volume. Preferably at least some of the droplets of ink and solvent are deposited in the same place on the substrate so that they mix on the substrate.

Alternatively they may mix in-flight from the print head nozzle to the substrate, or a combination of both. There may be a maximum volume constraint also for each droplet, for example to avoid too large droplets being produced and splashing off the substrate due to, for example, surface tension effects, and not being absorbed into the substrate, thereby affecting the colour produced away from the intended colour.

Preferably printing involves depositing a single droplet of each one or more process colours together with a single droplet of the solvent. Printing may also comprise dithering positions of ink and solvent, or of combinations of ink and solvent. The solvent may comprise water. A further nozzle may deliver droplets of a second solvent.

The second solvent may be quicker drying or slower drying than the first solvent, and the two solvents may be selected to have different drying times. The process colours may comprise cyan, magenta, yellow and black. Alternatively the method may be used with other process colours, for example half-tone cyan, magenta, yellow and black colours in addition, or other any other combination of colours. The gamut obtainable may vary depending on the colours available.

Preferably the method further involves calibrating the printing by printing combinations of the coloured inks and the solvent using selected droplet volumes onto the substrate.

Optionally the printer may be equipped with a Light source and one or more light detectors to measure light intensity of reflected light from the substrate, e.g. under pre-determined light conditions. Preferably the method may be used to print onto a textile substrate such as cotton or polyester.

The invention further provides processor control code to implement the above-described methods, for example on a general purpose computer system or on a digital signal processor (DSP). The code may be provided on a carrier such as a disk, CD-or DVD-ROM, programmed memory such as read-only memory (Firmware), or on a data carrier such as an optical or electrical signal carrier. Code (and/or data) to implement embodiments of the invention may comprise source, object or executable code in a conventional programming language (interpreted or compiled) such as C, or assembly code. The above described methods may also be implemented, for example, on an FPGA (field programmable gate array) or in an ASIC (application specific integrated circuit). Thus the code may also comprise code for setting up or controlling an ASIC or FPGA, or code for a hardware description language such as Verilog (Trade Mark), VHDL (Very high speed integrated circuit Hardware Description Language), or RTL code or SystemC. Typically dedicated hardware is described using code such as RTL (register transfer level code) or, at a higher level, using a language such as C. As the skilled person will appreciate such code and/or data may be distributed between a plurality of coupled components in communication with one another.

According to another aspect of the present invention, there is provided an apparatus for colour ink jet printing an image onto a substrate, the image comprising a plurality of pixels of different colours, the printing using an ink jet print head having a plurality of nozzles, at least one nozzle for delivering droplets of coloured ink for each one of a plurality of process colours, the apparatus comprising means for using a further one of said plurality of nozzles to deliver droplets of a solvent miscible with a solvent used for said droplets of said coloured ink, and means for printing a said pixel of said image a combination of said process colours and said solvent to create a colour for the pixel.

According to a further aspect of the present invention, there is provided an ink jet printer print head including a plurality of chambers in fluid communication with a plurality of respective nozzles, and wherein at least two of said chambers are loaded with inks of different process colours, and wherein at least a further one of said chambers is loaded with a preferably substantially uncoloured solvent miscible with a said ink.

According to a yet further aspect of the present invention, there is provided a method of monochrome ink jet printing an image Onto a substrate, the image comprising a plurality of pixels of different shades, the printing using an ink jet print head having a plurality of nozzles, a first nozzle for delivering droplets of ink, the method comprising using a second nozzle to deliver droplets of a solvent miscible with a solvent used for said droplets of said ink, arid printing a said pixel of said image using a combination of said ink and said solvent to create a shade for the pixel.

According to another aspect of the present invention, there is provided an apparatus for monochrome ink jet printing an image onto a substrate, the image comprising a plurality of pixels of different shades, the printing using an ink jet print head having a plurality of nozzles, at least one nozzle for delivering droplets of ink, the apparatus comprising means for using a further one of said plurality of nozzles to deliver droplets of a solvent miscible with a solvent used for said droplets of said ink, and means for printing a said pixel of said image a combination of said ink and said solvent to create a shade for the pixel.

According to a further aspect of the present invention, there is provided an ink jet printer print head including a plurality of chambers in fluid communication with a plurality of respective nozzles, and wherein at least one of said chambers is loaded with ink, and wherein at least a further one of said chambers is loaded with a substantially uncoloured solvent miscible with said ink.

Features of the above described aspects of the invention may be combined in any permutation.

These and other aspects of the invention will now be described in further detail, with reference to the accompanying drawings, in which: Figure 6 shows an example of ink colours and solvent usable with a method of ink jet printing according to the present invention Figure 7 shows an example of ink and solvent volumes for two composite colours obtainable with a method of ink jet printing according to the present invention.

Figure 8 shows a flowchart of a method of printing a range of colours on textiles according to the present invention.

Figure 9 shows a flowchart of a method of calibrating printing a range of colours on textiles according to the present invention We will describe mixing process colours in appropriate proportions and adding a solvent to provide an overall ink jet drop volume sufficient to penetrate the length of a fibre. Alternatively a white ink may be used instead of the solvent to provide sufficient volume of liquid.

Figure 6 shows chambers for printing materials which may be used with the present invention. In addition to the standard ink chambers for cyan, magenta, yellow and black inks, a chamber is provided for a solvent. Each chamber may be connected to a nozzle in an ink jet print head, and these may be configured to deposit ink or solvent in substantially the same place on a substrate, e.g. cotton fabric. Alternatively the chambers may be connected to a single print head nozzle for mixing the materials in the desired quantities before depositing them on the substrate. Examples of suitable inks (RTm) (inn) may be obtained commercially from SunJet / SunChemical of Sun Chemical Group ((Tm) A A B.V., Weesp, the Netherlands, DyStarTextilfarben GmbH of Frankfurt, Germany, and Seiko Epson Corportion of Nagano Japan. Examples of suitable solvents may be (RT obtained from SunJet/ SunChemical and DyStar-these may comprise the base solvents of the same inks provided by the above companies. It is preferable that the solvent(s) be miscible with the inks, so that the solvents and inks mix substantially completely and the ink pigment is carried with the solvent along the length of the fibre when they are deposited on the substrate.

Figure 7 shows ratios of process colours and solvent mixable to obtain two example colours. In the top example, a pinkish red colour is desired. This may be obtained by mixing one part magenta with one part yellow. However, the colour desired is not a deep, fully saturated red but a lighter, more pastel red. This may be achieved by adding solvent to the mixture, in order to increase the total volume of liquid deposited for that pixel to a value known to provide substantially full coverage of the fabric, in this example 2Onl. This value may vary depending on the type of substrate being printed on, the types of inks and solvent being used and the distance between pixels or print head positions used by the printer.

The bottom example shows an example of a mixture to print a grey colour with the present invention. A quantity of black ink (5n1) is mixed with solvent up to the desired overall volume (again 20n1), the end result appearing less black on the fabric than if 20n1 of black ink had been used, yet still covering the fabric to substantially the same extent.

Figure 8 shows a flowchart of a method of obtaining process colour and solvent volumes for a desired colour according to the present invention. Pixel colour data for the desired colour is input in step 81, and then in step 82 the process colour components for cyan, magenta and yellow are obtained. The pixel colour data may be a variety of different formats, for example RGB, HSV, YIQ, YUV or CIE. These may be converted to CMY format using conversions as known in the art, and then a further conversion may be performed to account for non-linearities in the input formats (for example gamma-corrected ROB values intended for driving a cathode ray tube) and/or variations in the strengths of the inks (e.g. the amount of pigmentation in the inks per unit volume). A further conversion may be performed to match the ink quantities to the response curve of the human eye, to reflect a desired amount of red, green and blue light from the printed article when the ink and solvent are absorbed. These conversions may be performed in a single step, for example using a look- up table, or if a mathematical formula or model for them is known, this may be calculated for each pixel colour. I0

In step 83 a black component is subtracted from the cyan, magenta and yellow process colours. For example, if a result of the previous step yielded ml cyan, 5n1 magenta and 8n1 yellow, a black component of In] may be subtracted from each of these leaving On! cyan, 4n1 magenta and 7n1 yellow and In! black. Depending on the relative strengths of the inks available, subtracting I ni from the process colours may be followed by adding more or less than ml black, and the quantities of ink subtractable from each process colour for a particular volume of black may vary between the individual process colours. A conversion may also be applied to compensate for the non-linearities cited in connection with step 82, and indeed both these steps may be combined in a single look-up table operation yielding all four parameters CMYK from the original input pixel colour data.

In step 84 an appropriate volume of solvent is determined, for example by calculating a total volume of ink provided by the CMYK parameters output from step 83 and subtracting this from a desired volume, for example 20n1. This may be pre-determined, or it may be determined on a continuous basis depending on, for example, a type of fabric being printed on at a particular moment and/or a thickness of said fabric. For example, when printing images on a pair of jeans or trousers, a greater total volume may be desirable over a pocket area or a seam area than over the rest of the fabric.

In step 85 the five parameters corresponding to cyan, magenta, yellow, black and solvent volume may be output as data to the print head for printing onto the substrate.

Alternatively these results may be stored in a memory for output to a print head at a later time, for example if a degree of processing power required to implement the method is too great for available systems to be able to output data at a speed of the print head. This is not necessary with appropriate use of lik-ip tables and/or the processing power available with modern processors such as Intel x86 series, MIPS and ARM processors, or with custom, dedicated hardware such as ASICs and programmable logic such as FPGAs. Suitable variable volume, drop-ondemand print heads which may be employed to implement embodiments of the invention are available from Printos / Videojet Technologies, Inc. of Illinois, USA. Ii

The printer may be self-calibrating in order to compensate for variations in ink strengths between different batches of ink. This may be achieved by printing test regions onto test substrates with different inks, and measuring reflected light intensities from the printed regions under test light conditions using a light detector. The light detector may be sensitive over a range of wavelengths, or a plurality of light detectors sensitive at specific wavelengths may be used.

Calibration may also be used instead of the method in Figure 8, for determining process colour and solvent volumes to achieve different colours. Starting with the target colour data in step 91, approximate values for cyan, magenta, yellow, black and solvent may be obtained, for example using the method of Figure 8, or using simple linear interpolation., in step 92. These values are used to print a test colour on a piece of test substrate (preferably the same material as that on which the image is to be printed). By measuring the actual colour resulting from these parameters, an entry in a look-up table may be updated indicating the actual colour resulting from that combination of parameters (step 94). If the resulting colour is close enough to the target colour (if, for example, there are no closer colours available with a digital, drop on demand print head) these parameters may be used to print that colour in the image (step 96). If not, the method loops to step 93. Simple linear interpolation may be used to determine the next test values for cyan, magenta, yellow, black and solvent. The constant volume criterion may be observed when determining the next values, so that (for example) cyan, magenta, yellow and black values are adjusted and the amount of solvent then adjusted to keep the overall volume the same.

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.

Claims

CLAIMS: I. A method of colour ink jet printing an image onto a

substrate, the image comprising a plurality of pixels of one or more colours, the printing using an ink jet print head having a plurality of nozzles, at least one nozzle for delivering droplets of coloured ink for each one of one or more process colours, the method comprising using a further one of said plurality of nozzles to deliver droplets of a solvent miscible with a solvent used for said droplets of said coloured ink, and printing a said pixel of said image using a combination of said process colours and said solvent to create a colour for the pixel.

2. A method as claimed is claim 1 wherein said printing of a pixel is subject to a substantially constant volume constraint on a total volume of liquid delivered to said pixel on said substrate by said printing.

3. A method as claimed in claim 2 wherein said constant volume constraint is determined by a desired liquid loading or ink penetration depth of said substrate.

4. A method as claimed in any claim 2 or 3 wherein said print head comprises a variable droplet volume print head, and wherein said printing comprises varying volumes of said droplets of coloured ink and of said solvent to meet said substantially constant volume constraint.

5. A method as claimed in any preceding claim wherein at least some of said droplets of ink and of said solvent for printing said pixel are deposited at substantially the same location on said substrate such that they mix on said substrate.

6. A method as claimed in any preceding claim wherein said printing is subject to a maximum volume constraint for each said droplet.

7. A method as claimed in any preceding claim wherein printing of a said pixel comprises depositing a single droplet each of said one or more coloured inks together with a single droplet of said solvent.

S. A method as claimed in any one of claims I to 6 wherein said printing further comprises dithering positions of combinations of droplets of said coloured ink and solvent.

9. A method as claimed in any preceding claim wherein said solvent comprises a base solvent of a said coloured ink.

10. A method as claimed in claim 9 wherein said solvent comprises water.

11. A method as claimed in any preceding claim further comprising using a still further one of said plurality of nozzles to deliver droplets of a second solvent.

12. A method as claimed in claim 11 wherein said two solvents are selected to have different drying times.

13. A method as claimed in any preceding claim wherein said process colours include at least cyan, magenta, yellow and black.

14. A method as claimed in any preceding claim further comprising calibrating said printing by printing combinations of said coloured inks and said solvent using selected droplet volumes onto said substrate.

15. A method of colour ink jet printing Onto a textile substrate using the method of any preceding claim.

16. A carrier carrying computer program code for use with the method of any preceding claim, said code being configured to: input calibration data derived from calibrating said printing by printing combinations of said coloured inks and said solvent using selected droplet volumes Onto said substrate; and to calculate droplet volumes for said droplets of coloured ink and solvent for printing at a pixel position to achieve a target said pixel colour.

17. Apparatus for colour ink jet printing an image Onto a substrate, the image comprising a plurality of pixels of different colours, the printing using an ink jet print head having a plurality of nozzles, at least one nozzle for delivering droplets of coloured ink for each one of a plurality of process colours, the apparatus comprising: means for using a further one of said plurality of nozzles to deliver droplets of a solvent miscible with a solvent used for said droplets of said coloured ink; and means for printing a said pixel of said image a combination of said process colours and said solvent to create a colour for the pixel.

18. An ink jet printer print head including a plurality of chambers in fluid communication with a plurality of respective nozzles, and wherein at least two of said chambers are loaded with inks of different process colours, and wherein at least a further one of said chambers is loaded with a solvent miscible with a said ink.

19. An ink jet printer print head as claimed in claim 18, wherein said solvent is substantially uncoloured.

20. A method of monochrome ink jet printing an image onto a substrate, the image comprising a plurality of pixels of different shades, the printing using an ink jet print head having a plurality of nozzles, a first nozzle for delivering droplets of ink, the method comprising using a second nozzle to deliver droplets of a solvent miscible with a solvent used for said droplets of said ink, and printing a said pixel of said image using a combination of said ink and said solvent to create a shade for the pixel.

21. Apparatus for monochrome ink jet printing an image onto a substrate, the image comprising a plurality of pixels of different shades, the printing using an ink jet print head having a plurality of nozzles, at least one nozzle for delivering droplets of ink, the apparatus comprising: means for using a further one of said plurality of nozzles to deliver droplets of a solvent miscible with a solvent used for said droplets of said ink; and means for printing a said pixel of said image a combination of said ink and said solvent to create a shade for the pixel.

22. An ink jet printer print head including a plurality of chambers in fluid communication with a plurality of respective nozzles, and wherein at least one of said chambers is loaded with inks and wherein at least a further one of said chambers is loaded with a substantially uncoloured solvent miscible with said ink.