Patent Application
20120154473
The invention relates to the field of inkjet printing. More specifically, the invention relates to a process for controlling the composition of an atmosphere exposed to a curable ink in a radiation curing print process.
Inkjet printing involves producing a digital image on a substrate by propelling droplets of liquid material (ink) onto the substrate. Inkjet printing solutions can involve using base coats, electromagnetic radiation, curing, and inerting a print region with an inerting atmosphere.
Some printing solutions involve applying a base coat to a substrate before printing a desired image. For example, in order to print color images on non-white substrates, such as colored or transparent substrates, it is typically necessary to deposit a layer of white ink to serve as a backdrop for the color inks. Also, to print a multi-colored image on a black or colored substrate, the area of the substrate on which the image is to be printed is first pre-coated with a layer of white ink, and then the image is printed on top of the white pre-coat layer. The white background layer prevents the colors in the image from being distorted by the black or colored substrate
Additionally, when printing on a transparent substrate, the colored inks may be applied on the reverse side of the substrate, so that the image may be viewed through the front side of the substrate. Then, a layer of white ink is printed over the colored ink pattern in a post-coating step. The white “post coat” layer serves as a backdrop so that the colors of the image appear properly when viewed from the front side of the transparent substrate. In some cases, the transparent substrate is then laminated onto a second transparent substrate, such as a window, so that the color image is protected between the two transparent substrates.
The Applicants have developed methods and apparatus for printing a coating layer in co-pending United States Patent publication no. 20060158473, filed on Jan. 19, 2006, entitled Methods and apparatus for backlit and dual-sided imaging, which is incorporated herein in its entirety.
According to United States Patent publication no. 20060158473, an array of print heads arranged along a single print head axis is configured to print images and a coating layer on a substrate during a single printing step (i.e., without requiring separate pre-coat or post-coat processing). In particular, a print apparatus deposits a first image layer on a substrate, then deposits a coating layer over the first image layer, and then deposits a second image layer over the coating layer.
The coating layer may comprise a specialized printing fluid such as a substantially white ink. The substrate is oftentimes a substantially translucent or substantially clear material, such as glass or plastic media. Indeed, these printing techniques are useful for backlit imaging and dual-sided imaging.
Although basic base coating techniques have been previously developed, there is a need in the art for methods and systems for controlling the quality and characteristics of the base layer, wherein these characteristics affect the overlaid image. Currently, characteristics such as dot gain, interlayer adhesion and slip are controlled by using additives such as silicone based surfactants.
Additionally, an inert gas, such as nitrogen or carbon dioxide is commonly used in radiation curable processes to enhance cure speed, particularly surface cure by reducing oxygen that reduces cure speed as a result of competing triplet and radical quenching reactions.
Some printing solutions also involve light curing of inks. Known ink-curing techniques involve using a specific ink formulation and exposing it to energy from an electromagnetic radiation source. The goal in both conventional and inkjet printing is to enable cure with reduced dose and or power of actinic radiation. Curing of liquid chemical ink formulations has been an established practice for many years. In ultraviolet curing, a liquid chemical formulation comprising photoinitiators, monomers and oligomers, and possibly pigments and other additives is exposed to ultraviolet light, thereby converting the liquid chemical formulation into a solid state.
Curing ink involves directing photons, typically with wavelengths in the ultraviolet spectrum, onto an ink deposit. The photons interact with photoinitiators present within the ink, creating free radicals. The created free radicals initiate and propagate polymerization (cure) of the monomers and oligomers within the ink. This chain reaction results in the ink curing to a polymer solid. However, the presence of oxygen at the ink surface inhibits such a chain reaction from occurring within the ink. This is often referred to as oxygen inhibition.
In normal ultraviolet curing in an air environment, a high amount of ultraviolet energy and/or a high concentration of photoinitiator are needed to achieve full cure, compared to the ultraviolet power and photoinitiator concentration required in an oxygen free curing environment. Higher photoinitiator concentration may deleteriously affect the final film properties, and increase ink costs. Higher ultraviolet energy required to overcome oxygen inhibition increases power requirements and heat generated on the sample.
Common solutions for providing for less reactive curing include completely supplanting atmospheric oxygen with a less reactive gas such as nitrogen in the cure zone. For example, U.S. Pat. No. 6,126,095 to Matheson et al., entitled “Ultraviolet Curing Apparatus Using an Inert Atmosphere Chamber” teaches a curing apparatus comprising a curing chamber for accommodating a controlled atmosphere. The curing chamber includes inlets and nozzle assemblies for supplying less reactive gas into the chamber and maintaining a less reactive atmosphere therein.
The prior art involves specialized and expensive approaches to providing reduced oxygen curing conditions, but fall short of achieving feasibility for common inkjet printing systems. For example, curing chambers demand a large footprint and are typically expensive to obtain, operate, and maintain. Additionally, large curing chambers have unacceptable levels of power consumption and heat production, requiring the use of heat sinks and other cooling systems.
According to the current state of the art, while adding a surfactant to an undercoat such as a clear or white, enables sufficient spread and a smooth surface, the adhesion and print quality of the subsequent printed layer may be negatively impacted. This is particularly pertinent to inkjet printing where drops must spontaneously spread to cover the surface and there is no contact pressure to enhance spread that is found in many conventional printing processes. For ink jet printing, some of the above mentioned current practices, such as the use of particulate matting agents, are not accessible. This is because the size of the particulate, in order to be effective, exceeds the size that the print-head can accommodate.
Additionally, the majority of current ink-curing solutions utilize high pressure arc lamps for -curing. However, there are several drawbacks to these techniques.
First, typical light-curing systems that use arc lamps possess a very large physical footprint. In the case of a system for base coat printing followed by a top coat, a first printer having a UV curing station sets down and cures the base coat while an additional printer is required to set down the top coat. It would be highly beneficial to reduce the physical size of printers with light-curing stations. Likewise, it would be highly beneficial to eliminate the need for two printers in a two-step printing process.
Also, known current light-curing systems that use high pressure arc lamps produce a very high level of heat. This high level of heat prevents a traditional curing lamp from being placed in-line with other printing processes. Accordingly, heat sinks are required to remove excess heat. Likewise, traditional light-curing printing techniques release ozone which must be evacuated or otherwise removed.
Therefore, there is a need in the art for a solution that provides adequate curing, without requiring a large footprint, without requiring large amounts of power, and without producing unacceptable levels of heat while at the same time maintaining acceptable levels of print quality and interlayer adhesion.
In view of the foregoing, the invention provides a small footprint, in-line printing apparatus with an inerting station that delivers an atmosphere having an optimal composition to inert a deposit of ink such that light generated by an light emitting diode (LED) adequately cures the ink. Likewise, the invention provides a process for configuring a printing environment for delivering an atmosphere having an optimal composition to inert a layer of ink such that LED radiation adequately cures the ink.
The invention also provides a printing system with a pressurized air source and nitrogen source configured for controlling the levels of oxygen and inert gas in an interting region of a printer. Likewise, the invention provides a printing system having a compressed air source, a nitrogen generator for controlling the levels of oxygen and inert gas in an interting region of a printer.
The invention also provides a computer-operated printing environment that allows a user to control an inerting gas purity for delivering to an inerting station that delivers an atmosphere to inert a layer of ink in an LED curing system.
The invention also provides a method of dynamically controlling surface attributes in a print job by accepting instructions from a user-controlled computer for altering said at least printing method variable, wherein the alteration of said at least one printing method variable changes at least one print attribute of said print job.
Systems and methods are provided for introducing an at least partially inert gas in a curing region of a printing apparatus to support an ideal curing of the ink.
For the purposes of the invention, the term “inert” shall mean an atmosphere having a reduced level of any substance that inhibits a desired rate curing for ink. In the presently preferred embodiments, “inert” refers to an atmosphere having a reduced level of gaseous oxygen while this was done with increased levels of nitrogen, those with ordinary skill in the art having the benefit of this disclosure will readily understand that “inert” can refer to the reduction of oxygen by means of other non reactive gasses.
As explained above, the current state of the inkjet printing art utilizes high power lamps for curing of a base layer ink. As noted above, these systems prevent a two-step, base-coating and top-coating printing process from being performed in-line due to curing and heat concerns. In the presently preferred embodiments of the invention, light-emitting diodes (LEDs) are utilized to improve on the bulky, hot prior art systems. Additionally, LEDs increase curing uniformity and increased printer longevity. According to the invention, an improved curing process allows the design of low-profile, low-heat curing station that does not require a segmented, two-printer process.
In some embodiments of the invention an inert (reduced oxygen) atmosphere is introduced into a curing region of a printing apparatus to obtain sufficient cure when using inks that cure by means of a free radical mechanism that is initiated by actinic radiation. Surprisingly, we have found that using higher levels of purity does not yield the required surface characteristics and that controlling the level of the oxygen in the inert gas yields better results.
In the presently preferred embodiments of the invention, the level of oxygen in the inert gas is adjusted in order to control surface characteristics of the printed layers.
Also in the presently preferred embodiments, a white ultraviolet (UV) curable inkjet ink is printed on a substrate in an at least partially inerted atmosphere. In some embodiments of the invention, the white ink acts as a base layer for one or more subsequent layers of color ink.
According to
The substrate 101 receives at least some base-layer of ink before being transported to a curing region 106 of the inkjet printing apparatus 100. The curing region 106 includes a curing lamp 104 for exposing the base-layer with electromagnetic illumination, thereby curing the deposited ink. As explained above, in the presently-preferred embodiments of the invention, the curing lamp 104 comprises light-emitting diodes (LEDs). However, it will be readily apparent to those with ordinary skill in the art having the benefit of the disclosure that other types of lighting technology are equally applicable.
In presently preferred embodiments of the invention, the curing lamp 104 is configured to emit light in the ultraviolet (UV) range. However, those with ordinary skill in the art having the benefit of this disclosure will readily appreciate that a number of other visible and invisible colors and level of brightness are equally applicable to achieve the invention, as disclosed broadly herein.
Next, the substrate 101 with a cured base-layer is transported to a color printing region 105. As shown in
In the presently preferred embodiments of the invention, the white UV curable inkjet base-layer ink is printed on a substrate and cured using LED lights under a controlled level of an inert gas, such as nitrogen.
Although
Next, the substrate, with a base-layer applied, is transported to an inerting zone 204 of the printing apparatus where the substrate is exposed to an inerting gas 205. The substrate is then transported to a curing region 206 and illuminated 207, thereby curing the base-layer. Finally, the substrate having a cured base-layer is transported into a top-coat region 208 and a top-coat layer is applied thereon 209.
Using the system a process as generally described in
The inventors found that a high level of nitrogen purity gives a smooth white surface on which the subsequent layer of colored inks, when printed on that surface, spreads and gives a high quality image. On that surface, while the print quality is good, we found that the interlayer adhesion between the colored inks and the white layer is poor. On the other hand, curing the white layer without the use of an inert gas results in good interlayer adhesion. Good interlayer adhesion generally describes a printed substrate in which it is difficult to remove the colored layer of ink from the white layer by scratching it or by cross hatch and tape test. In these cases, while interlayer adhesion was sufficient, spread of the second layer of colored inks on the insufficiently cured white layer was poor, yielding a flawed image with observable lines between individual jets.
Therefore, it is desirable to have control over the amount of nitrogen and oxygen in a curing process in order to control the print quality. Indeed, the presently preferred embodiments of the invention involve a process whereby the inert gas which envelops the area where UV light is impinging on freshly printed ink has a controlled level of oxygen in order to obtain surface characteristics. In a particular embodiment, a white inkjet ink is printed on a substrate and an LED lamp is used to cure the ink under a controlled concentration of oxygen in order to obtain required characteristics, i.e. both sufficient spread of the subsequently printed inks and good interlayer adhesion.
In some embodiments of the invention, a static composition of inerting gas is established based on the resultant printing characteristics and that composition are used exclusively. In some other embodiments of the invention, a controller configured to adjust the composition of the inerting gas is dynamically configurable such that the resultant printing characteristics are adjustable.
In the presently preferred embodiments of the invention, a printing system includes an inerting gas controller for controlling the levels of oxygen and inert gas in an interting region of a printer.
According to
In some embodiments, a three-way connector 303 coupled the nitrogen source 301, the air source 302, and a nitrogen applicator (not shown) in the printer 305. The purity of the nitrogen source is fixed; therefore, as the air flow valve is opened, the purity of the nitrogen stream is lowered. In the presently preferred embodiments of the invention, the nitrogen applicator is placed before an LED lamp (not shown) as explained above.
In some embodiments of the invention, the air flow valve 304 is coupled with a user computer 306. The user computer 306 at least comprises a processor, a memory, a display, a user input device, and a graphical user interface. According to these embodiments, a user may adjust the levels for the composition of gas delivered to the printer 305. Accordingly, the user can adjust the resultant print quality. In some embodiments, the printer 305 receives a print job from a first computer and the inerting gas purity in controlled by an additional computer. In some other embodiments, the same computer initiates the print jobs and controls the purity level of the inert gas.
In some other embodiments of the invention, a membrane-based nitrogen generator is used to supply inerting gas, wherein incoming air pressure and flow are used to control the oxygen level of the inerting gas. These embodiments replace those embodiments using a nitrogen source, an air source, and a mixer. Indeed, eliminating nitrogen or oxygen tanks obviates the need for consumable nitrogen or oxygen tanks that constantly require replacement and that can be expensive. Furthermore, the elimination of tanks further reduces the footprint of the system.
In some embodiments of the invention, an adsorption gas separation process is used to generate nitrogen. In some other embodiments, a gas separation membrane is used to generate nitrogen. According to the embodiments in which a membrane is used, a compressed air source delivers air that is first cleaned to remove oil vapor or water vapor. The clean, compressed air is then driven through a series of membranes to separate oxygen out of the air, resulting in a gas having higher levels of nitrogen. The resultant amount of nitrogen in the resultant gas can be controlled by changing the system pressure and the flow rate of air through the system. Accordingly, the resultant inerting gas is controllable.
The compressed air source 398 is attached to the inlet of the nitrogen generator 397. The purity of the separated nitrogen exiting the generator is controlled by the pressure and flow rate of gas traveling through the membrane(s) of the nitrogen generator 397. As pressure is increased, the output nitrogen purity increases. As gas flow rate through the membrane is increased, the output purity decreases. The outlet of the nitrogen generator 397 is attached to the inlet of a flow-meter 396 to control the amount of nitrogen applied to the printer 395. The outlet of the flow-meter is attached to the nitrogen applicator (not shown). The nitrogen applicator is placed in the printer 395, before the curing lamp, so that curing takes place under a controlled atmosphere.
In any of the embodiments, the connection to the nitrogen applicator can be broken and an O2 sensor can be placed in line to determine its concentration of N2.
In some embodiments of the invention, nitrogen generator 397 is coupled with a user computer 394. The user computer 394 at least comprises a processor, a memory, a display, a user input device, and a graphical user interface. According to these embodiments, a user may adjust the levels for the composition of gas delivered to the printer 395. Accordingly, the user can adjust the resultant print quality.
As will be understood by those familiar with the art, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Likewise, the particular naming and division of the members, features, attributes, and other aspects are not mandatory or significant, and the mechanisms that implement the invention or its features may have different names, divisions and/or formats. Accordingly, the disclosure of the invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following Claims.
Examples of the printing process are described below. Representative examples of samples printed under various levels of oxygen are discussed herein with reference to
In prior art focus is on decreasing energy required for cure by decreasing oxygen to as low a level as possible in the curing environment. The example herein shows that extremely low oxygen levels do not give ideal print characteristics. Instead, there is an ideal range of oxygen concentration that will yield optimal print characteristics, including, but not limited to mar resistance, dot gain, and adhesion.
In this example, a printer is described that deposits a white ink formulated to cure under an LED light source. This white ink is comprised of acrylate monomers and oligomers, photoinitiator, dispersed pigment, and additives. Mixtures of acrylate monomers and oligomers are found in concentrations of 30 to 70% by weight, more ideally from 40-60% by weight. Mixtures of photoinitiators chosen to react under an LED light source are found in concentrations of 3-15% by weight, more ideally from 5-10% by weight. The dispersed pigment is comprised of monomers, oligomers, dispersants, and titanium dioxide pigment. The titanium dioxide pigment is found in concentrations of 10-40% by weight, more ideally 15-30% by weight.
In this example, the printer utilizes print heads to deposit the LED curable white ink to a transparent or colored substrate. Upon deposit, the printer's web drive moves the substrate with deposited ink into a nitrogen application region. The nitrogen application displaces the ambient atmosphere composition, replacing the space above the deposited white ink with a controlled oxygen atmosphere. The substrate and altered atmosphere continues to move into the LED curing region, where the LED lamp cures the white deposit. The web continues to the overprint color region, where print heads deposit additional colors to the cured white ink. The web continues to travel to a mercury vapor lamp in order to cure the additional colors.
