Abstract:
The invention relates to a method of inkjet printing comprising providing a printhead, providing a controller for the printhead, providing a base ink formulation stream, introducing at least one first enhancer into the base ink stream to form a combined ink stream, adjusting the printhead setpoints and printing the combined ink onto a first substrate.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to inkjet printing, and particularly relates to a system for rapidly changing inks to allow printing on different substrates. 
       BACKGROUND OF THE INVENTION 
       [0002]    Inkjet printing is a non-impact method for producing images by the deposition of ink droplets on a substrate (paper, transparent film, fabric, etc.) in response to digital signals. Inkjet printers have found broad application across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging and large format printing for outdoor applications such as banners, signages, displays, posters, billboard and bus wraps. 
         [0003]    Continuous inkjet printing uses a pressurized ink source that produces a continuous stream of ink droplets. Conventional continuous inkjet printers utilize electrostatic charging devices that are placed close to the point where a filament of ink breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes. When no printing is desired, the ink droplets are directed into an ink-capturing mechanism (often referred to as a catcher, interceptor or gutter). When printing is desired, the ink droplets are directed to strike a print media. 
         [0004]    Typically, continuous inkjet printing devices are faster than drop-on-demand devices and produce higher quality printed images and graphics. However, each color ink printed requires an individual droplet formation, deflection, and capturing system. 
         [0005]    Even when black inks are being printed there is generally a need to reconfigure the print head and connect to another ink source when substrates are changed. This is because different inks are preferred for printing on different surfaces. For instance, inks that print well on absorbent paper do not print as well on papers such as coated papers that have a less absorbing surface. Further, there is a demand for inkjet printing on polymer sheets of materials such as polyvinyl chloride and polypropylene. These materials also require different preferred inks in order to adhere to the polymer surface. 
         [0006]    Water-based inks have been found to be preferred for environmental reasons, however they present problems with ink adherence to different types of substrates and with some formulations there is a short shelf life of the ink, prior to its no longer being useful for inkjet printing. Ultraviolet curing inks have been found to have excellent adhesion to a variety of surfaces. However they present environmental problems as they may be eye and skin irritants. There is a need water-based inks with the performance of UV inks and at lower cost than the ultraviolet curing inks. 
         [0007]    In U.S. Pat. No. 6,908,178 to Jeanmarie, there is disclosed a system for printing different colors onto a substrate. In the system disclosed each inkjet head is capable of being supplied with one of two inks. In U.S. Pat. No. 6,575,566 to Jeanmarie, there is disclosed a continuous inkjet printhead with selectable printing of different volumes of ink. 
         [0008]    There remains a need for a continuous inkjet printer that is able to quickly adjust inks to be compatible with a particular substrate fed through the printer without the necessity for stopping the printer or wasting a significant amount of ink as a changeover is carried out. 
       SUMMARY OF THE INVENTION 
       [0009]    The invention relates to a method of inkjet printing comprising providing a printhead, providing a controller for the printhead, providing a base ink formulation stream, introducing at least one first enhancer into the base ink stream, bringing the combined ink stream into the printhead and printing onto a first substrate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic illustration of the method and apparatus of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    The invention has numerous advantages over prior practices in the continuous inkjet art. The invention allows continuous variation of ink composition in a continuous inkjet printer. The ability to vary ink composition allows the effective printing on different substrates without a lengthy changeover to an ink compatible with a different substrate. The system in the invention also provides a low-cost way of shifting inks as opposed to having multiple large tanks of ink that the printer would switch between. The ink flow of the invention is generally uniform even as the compositions vary, thereby minimizing printhead blockage. In view of the ease of switching inks there is no need to use an ink that is not ideally matched for each substrate in order to avoid expensive ink changeovers. The invention allows the ink needs of different substrates to be easily satisfied. Another advantage is that some of the ink enhancers while improving performance of the ink significantly shorten the ink shelf life. In the invention the ink enhancing material is combined with the base ink immediately before printing and shelf life is not an issue. An advantage of the printing system of the invention is that it is possible to respond to workflow by adjusting not only the ink chemistry but also the print head set points to optimize jetting performance of the enhanced ink. Efficiency of the printing system will be enhanced as it is possible to make a fast and automatic response of the printing system to substrate changes. These and other advantages of the invention will be apparent from the specification and drawing. 
         [0012]    The invention provides a base ink formulation source  12 . The base ink formulation  16  is withdrawn from the source  12  by passing the ink stream  14  through tube  13 . The base ink formulation  16 , if not matched to the substrate  22 , will have an enhancer added prior to its reaching the inkjet head  24 . Four different enhancers  18 ,  26 ,  28 , and  32  are shown. The entry of these enhancers into stream  14  is by controlling of valves  34 ,  36 ,  38 , and  40 .  FIG. 1  shows the valve  36  open for the polypropylene substrate compatibility ink enhancer  26  to enter the base ink formulation stream  14  in tube  13 . These valves are electronically controlled by controller  42  through control means  44 . It is possible for the inkjet head  24  to receive ink comprising the base ink formulation and from one to four different enhancer materials for different substrates. 
         [0013]    In operation, the inkjet method and apparatus of the invention would in one instance be operated as follows. Substrate  22  would be selected as a substrate having a polypropylene surface, such as used for banners and food wrapping. The base ink formulation  16  would enter tube  13 . Polypropylene substrate compatibility enhancer  26  would be added through the valve  36  shown in  FIG. 1  in the open position. The blend of the base ink formulation  16  and the polypropylene substrate compatibility enhancer  26  are brought to the inkjet printhead  24 . The controller  42  would adjust the printhead set points such that they provide excellent drop formation of the ink that is formed by the combination of the base ink formulation  16  and the polypropylene substrate compatibility enhancer  26 . This change in set point would involve the control of the heaters adjacent the ink orifice of the printhead so as to form ink drops at the desired rate and size for printing. The ink with the polypropylene substrate compatibility enhancer  26  would have a different viscosity or weight requiring adjustment of the printhead. The ink would then be applied to the substrate  22  having a polypropylene surface and would adhere to that substrate, whereas the base ink formulation would have poor adhesion. 
         [0014]    Similarly, if substrate  22  was a glossy lithographic receiver then the glossy lithographic receiver compatibility enhancer  18  would be metered into stream  14  through valve  34  to combine with the base ink formulation to create an ink satisfactory for a glossy lithographic receiver. The other valves  36 ,  40  and  38  would be shut off by the controller  44 . The same procedure would be followed if a plain paper density enhancer  28  was desired or an image durability enhancer  32  was desired. In each of those instances the stream  14  of the base ink formulation  16  would be enhanced by addition of one of those materials to create an ink particularly desirable for a plain paper density enhancement or to improve durability. It is also possible that both the enhancers  32  and  28  could be added simultaneously to both improve the density and the durability of the base ink formulation  16 . In each instance the printhead set points would be adjusted to control the heaters in inkjet head  24  to effectively dispense droplets of the enhanced ink onto the compatible substrate. Further, while the invention has been illustrated with the combined ink stream containing enhancer prepared outside the inkjet head and brought to the head in one stream, it is possible that the base ink and enhancers could be mixed in the inkjet head. 
         [0015]    The base ink  16  of the invention in a preferred example would be an aqueous ink formed of a pigment dispersion, a dispersant, a humectant, and a biocide. In humectant would be a glycol based material to aid in drying of the ink. The dispersant could suitably be N-methyl-oleoyl taurate (OMT) or a commercial material surfactant such as Noveon, Solsperse, or surfactants available from Lubizol. The enhancer for glossy lithographic receiver compatibility could suitably be a acrylic latex polymer. The enhancer to improve the adherence to a polypropylene&#39;s substrate could suitably be a block copolymer of acrylic acid and ethylene. The enhancer to improve density when printed on plain paper suitably could be a calcium salt of oxalic acid. Enhancer to improve image durability by addition to the base ink formulation suitably could be a polyurethane polymer. 
         [0016]    The controllers for ink jet operation are well-known in the art. The control mechanism is shown in U.S. Pat. No. 6,883,904 to Jeanmarie, hereby incorporated by reference in its entirety and particularly with reference to columns 5-8 where the use of the controller to determine drop size and spacing is set forth. The use of a controller to control valves in a liquid configuration from several sources and mixing is well known in the art and its use with the valves of the ink enhancers is typical of such mixing and flow control. 
         [0017]    While the invention has been described with four ink enhancer&#39;s, the invention could be utilized with a greater number or fewer enhancers. Further, these valves could be provided a valve inlet for flushing fluid to clean the system. Further, other types of enhancers could be utilized. For instance, different colors could be added to a base ink to provide new colors by blending different colors into a base formulation. Further other enhancers, such as ultraviolet inhibitors, could be added for substrates that are intended for exterior use. Enhancers containing fungicides could also be added to form inks for use in the packaging and medical field. These and other variations will be apparent to one of ordinary skill in the art and are intended to be included by the invention as defined in the claims. 
       PARTS LIST 
       [0018]      
         [0000]    
       
         
               
               
             
           
               
                   
               
               
                 REFERENCE 
                   
               
               
                 NUMBER 
                 PART 
               
               
                   
               
             
             
               
                 12 
                 Source 
               
               
                 13 
                 Tube 
               
               
                 14 
                 Stream 
               
               
                 16 
                 Base 
               
               
                 18 
                 Enhancer 
               
               
                 22 
                 Substrate 
               
               
                 24 
                 Inkjet Head 
               
               
                 26 
                 Enhancer 
               
               
                 28 
                 Enhancer 
               
               
                 32 
                 Enhancer 
               
               
                 34 
                 Valve 
               
               
                 36 
                 Valve 
               
               
                 38 
                 Valve 
               
               
                 40 
                 Valve 
               
               
                 42 
                 Controller 
               
               
                 44 
                 Control