Oxygen inhibition for print-head reliability

Systems and methods of applying a gaseous inhibitor into a printing region to hinder the curing process of ink on the print heads caused by the presence of stray light in the printing environment.

BACKGROUND OF THE INVENTION

Technical Field

The invention relates to the field of inkjet printing. More specifically the invention relates to systems and methods of applying a gaseous inhibitor into a printing region to hinder the curing process of ink on the print heads caused by the presence of stray light in the printing environment.

Description of the Related Art

Using electromagnetic radiation to cure liquid chemical formulations has been an established practice for many years. Electromagnetic radiation curing involves a liquid chemical formulation comprising photoinitiators, monomers and oligomers, and possibly pigments and other additives and exposing the formulation to electromagnetic radiation, thereby converting the liquid chemical formulation into a solid state.

In printing applications, radiation-curable ink is jetted from a print head onto a substrate to form a portion of an image. In some applications, the print head scans back and forth across a width of the substrate, while the substrate steps forward for progressive scan passes. In some other applications, one or more blocks of fixed print heads are used to build an image.

In each of these printing settings, curing ink involves directing photons, typically with wavelengths in or near 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 into a polymer solid.

However, the use of curable inks has created negative side effects. In particular, standard ink curing designs have issues with the print heads being exposed to stray light and with ink hardening onto the print heads due to the exposure. Stray light enters the printing environment in a variety of ways. For example, environmental light enters even the smallest openings and reflects throughout the system. Additionally, printing systems are oftentimes opened to environmental light to access printer components. Furthermore, printing systems sometimes produce their own light by way of scanner functions or curing lamps.

Exposure to any stray light encourages ink to harden onto print heads. The hardened ink subsequently deflects the spray from the print head and causes poor print quality. Indeed, even a very small deflection in ink spray can cause ruinous results.

In all types of printers which use light-curing (i.e. wideformat, super wide format, single pass, etc.), similar methodologies have been applied to limit the impact of stray or ambient light. Some workarounds include the use of physical shutters and baffles to deflect the light coming from the lamps. However, no matter how much shielding is used, stray light still enters the printer. Another attempted solution involves configuring a curing lamp at such an angle that the light cannot deflect back at the print-heads. However, this technique detracts from the lamp's effectiveness in curing. Another attempted approach involved configuring a shield around the print zone that stops ambient light, especially UV, from entering the printer and reaching the heads. However, as explained above, stray light still enters the printer.

A number of other factors exacerbate the problems associated with stray light. Firstly, there are issues with inks curing on heads where the substrates being printed are very reflective, such as metallic finish substrates and even glossy white substrates. In these cases the amount of reflected light is much higher than usual. Secondly, with the increase in cure speed of the printers, both the ink sensitivity to UV light and the amount of light applied have increased substantially, thereby causing increased risk of ink curing on the heads. Thirdly, there are instances in printer design, where there is insufficient room to effectively shield the heads from stray light from the source.

Moreover, light emitting diodes (LEDs) are now predominately used for ink curing. The LEDS used operate at wavelengths in the upper band of the visible spectrum and into the ultraviolet spectrum and the ink is designed to be cured at these wavelengths. Accordingly, environmental light is particularly troublesome since environmental light contains a lot of energy in that band.

Yet another complication to the problem of stray light arises from the practice of using gaseous nitrogen in a print system to supplant oxygen. The presence of oxygen at the ink surface inhibits the curing reaction from occurring within the ink. This is often referred to as oxygen inhibition. Accordingly, the practice of supplanting oxygen in a curing region increases the efficiency of the cure process. However, nitrogen curing results in escaped nitrogen exposed to the print region, thereby exacerbating the problem of ink becoming cured to the printer heads.

SUMMARY OF THE INVENTION

In view of the foregoing the invention provides systems and methods of applying a gaseous inhibitor into a printing region to hinder the curing process of ink on the print heads caused by the presence of stray light in the printing environment.

Some embodiments of the invention involve single-layer and multi-layer single-pass printing systems involving oxygen applicators for supplying a blanket of oxygen to a substrate entering a printing region. Likewise, some embodiments of the invention involve a method of oxygen inhibition in single-layer and multi-layer printing systems.

Some embodiments of the invention involve a multi-pass scanning printing system having a carriage with a plurality of oxygen applicators, a plurality of curing lamps, a plurality of nitrogen applicators, and a hardware controller for selectively activating and deactivating the various applicators as the carriage sweeps back and forth across the substrate.

Some embodiments of the invention involve a method for selectively activating and deactivating various nitrogen and oxygen applicators as a print carriage sweeps back and forth across the substrate in a multi-pass scanning printing system.

DETAILED DESCRIPTION OF THE INVENTION

The invention solves the problem of inks curing on print-heads and nozzles in printing systems due to the effects of stray light from a curing lamp or from the outside environment by introducing curing inhibition zones around the print heads where curing effectively becomes much more difficult to occur. In the presently preferred embodiments of the invention, the inhibition zones comprise an application of oxygen to a print head region, thereby reducing the ability for ink to cure on the heads due to oxygen's inhibition effect on the free radical cure process.

FIG. 1Aillustrates a prior art single-pass printing system100involving the application of nitrogen in a process of ultraviolet (UV) curing. According toFIG. 1A, a transport surface101is directed over a series of rollers103and is configured to move a substrate102through the printing system100.

The substrate102is first transported through a printing region104beneath a block of print heads105configured for applying ink to the substrate102. According toFIG. 1A, the block of print heads105applies UV curable ink. Once the substrate102is exposed to the application of ink, it is subsequently passed through an inerting zone106comprising a region exposed to a blanket of nitrogen applied via a nitrogen applicator107. Environmental air contains about 20% oxygen and 78% nitrogen. Accordingly, the blanket of nitrogen replaces environmental air with a less reactive nitrogen gas composition—usually 95% up to 99.9% pure nitrogen. Oxygen is a natural inhibitor of free radical cure and the removal of the oxygen significantly increases the rate of cure at the surface of the ink.

Finally, the printed and inerted substrate is transported into a curing region109where the ink is exposed to light from a curing lamp108, thereby curing the ink.

Although the inertinq zone106is located after the printing region104in the transport process, a portion of the nitrogen disperses to the printing region104. As explained above, stray light enters the printing environment in a variety of ways and exposure to any stray light encourages ink to harden onto print heads. Therefore, the presence of nitrogen in the printing region104significantly increases the rate of cure of ink on the print heads.

The problem associated with the presence of nitrogen in a printing region is exacerbated in multilayer printing system. There are many instances where multilayer printing is advantageous. For example, two-sided images are printed on a transparent substrate using an intermediate white layer.FIG. 1Billustrates a prior art single-pass, multi-layer inkjet printing apparatus110configured to deposit two layers of ink on a substrate112.

According toFIG. 1B, a transport surface111is directed over a series of rollers113and is configured to move a substrate112through the printing system110.

The substrate112is transported through a first printing region114beneath a first block of print heads115configured for applying ink to the substrate112. After the substrate112is exposed to the application of ink, it is subsequently passed through an inerting zone116comprising a region exposed to a blanket of nitrogen applied via a nitrogen applicator117. Next, the printed and inerted substrate112is transported into a first curing region119where the ink is exposed to light from a first curing lamp118, thereby curing a first layer of ink.

The substrate112is then transported through a second printing region124beneath a second block of print heads125configured for applying ink to the substrate112. After the substrate112is exposed to a second application of ink, it is subsequently passed through a second inerting zone126comprising a region exposed to a blanket of nitrogen applied via a second nitrogen applicator127. Finally, the substrate112is transported into a second curing region129where the ink is exposed to light from a second curing lamp128, thereby curing a second layer of ink.

As previously mentioned, the problem associated with the presence of nitrogen in a printing region is exacerbated in a multilayer printing system like the one illustrated inFIG. 1B. This is due to the introduction of even more nitrogen into the second printing region124in addition to dispersed nitrogen. As the substrate112is transported through the stations, nitrogen gas from the inerting zones is “pulled” along with the substrate112. Therefore, the substrate112delivers nitrogen gas to the second printing region124. This excess nitrogen gas significantly increases the rate of cure of ink on the print heads due to stray light.

The presently preferred embodiments of the invention address the problems associated with the prior art solutions through oxygen inhibition in the printing regions.

FIG. 2Aillustrates a single-pass printing system200involving oxygen inhibition according to some embodiments of the invention. According toFIG. 2A, a transport surface201is directed over a series of rollers203and is configured to move a substrate202through the printing system200.

According toFIG. 2A, the substrate202is first transported through an oxygen inhibition region299in which a blanket of oxygen is deposited via an oxygen applicator298. This technique of oxygen inhibition protects the printheads from having ink cure on them due to stray or ambient light due to the fact that the oxygen rich feed is applied just before the heads and the motion of a substrate helps to create a blanket across the heads. In other words, the blanket of oxygen rich air is dragged along with the substrate and remains present near the print-heads while the printer is in operation.

The transport surface201moves the substrate202into the printing region204beneath a block of print heads205configured for applying ink to the substrate202.

As shown inFIG. 2A, the printing block205includes print heads defining the CMYK color model. However, it will be readily apparent to those with ordinary skill in the art having the benefit of the disclosure that other color models, now known or later developed, are equally applicable to accomplish the invention, as disclosed broadly herein.

In the presently preferred embodiments of the invention, the block of print heads205applies UV curable ink which is subsequently cured in a curing region209by a UV curing lamp208. However, the oxygen blanket must be deflected before it reaches the cure station209, otherwise the oxygen will inhibit cure of the print, as explained above. Therefore, once the substrate202is exposed to the application of ink, it is subsequently passed through an inerting zone206comprising an inerting region206exposed to a blanket of nitrogen applied via a nitrogen applicator207. In some other embodiments, the evacuation of oxygen is accomplished using baffles.

Finally, the printed and inerted substrate is transported into a curing region209where the ink is exposed to light from a curing lamp208, thereby curing the ink.

In some embodiments of the invention, the nitrogen gas supplied to the nitrogen applicator207and the oxygen supplied to the oxygen applicator298are delivered via separate nitrogen and air sources.

In the presently preferred embodiments of the invention, a membrane-based nitrogen generator297is used to supply the nitrogen gas and the oxygen gas. Indeed, eliminating separate 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.

In some embodiments of the invention, the purity of the oxygen stream into the oxygen applicator298ranges between 40% and 60%. In some other embodiments of the invention, the purity of the oxygen stream into the oxygen applicator298ranges between 60% and 80%.

In the presently preferred embodiments of the invention, the purity of the oxygen stream into the oxygen applicator298is greater than 80%. In some embodiments of the invention, a static elimination device is strategically positioned in the printing system200to avoid creation of ignition points, such as sparks in the oxygen rich atmosphere.

Also, in the presently-preferred embodiments of the invention, the curing lamp208comprises 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, such as incandescent lamps and fluorescent lamps, are equally applicable.

The problems associated with the presence of nitrogen in a printing region in a multilayer printing system explain in relation toFIG. 1Bare eliminated in a printing system220according toFIG. 2B.

FIG. 2Billustrates a single-pass, multi-layer inkjet printing apparatus210with multiple oxygen inhibition regions according to some embodiments of the invention.

According toFIG. 2B, a transport surface211is directed over a series of rollers213and is configured to move a substrate212through the printing system210.

The substrate212is first applied with a blanket of oxygen from an oxygen applicator295when the substrate212is passed into a first oxygen inhibition region292. The substrate212is then transported through a first printing region224beneath a first block of print heads225configured for applying ink to the substrate212. In some cases for printing two-sided images on a transparent substrate, the first block of print heads225is configured to apply white, or otherwise opaque, ink onto the transparent substrate.

After the substrate212is exposed to the application of ink, it is subsequently passed through a first inerting zone226comprising a region exposed to a blanket of nitrogen applied via a nitrogen applicator227. Next, the printed and inerted substrate212is transported into a first curing region229where the ink is exposed to light from a first curing lamp228, thereby curing a first layer of ink.

The substrate212is applied with a second blanket of oxygen from a second oxygen applicator294when the substrate is passed into a second oxygen inhibition region293. The substrate212is then transported through a second printing region214beneath a second block of print heads215configured for applying ink to the substrate112. In the case of printing two-sided images, the second block of print heads215is preferably the color print heads.

After the substrate212is exposed to a second application of ink, it is subsequently passed through a second inerting zone216comprising a region exposed to a blanket of nitrogen applied via a second nitrogen applicator217. Finally, the substrate212is transported into a second curing region219where the ink is exposed to light from a second curing lamp218, thereby curing a second layer of ink.

FIG. 2Cillustrates a method of oxygen inhibition250in a multi-layer printing system according to some embodiments of the invention. In the presently preferred embodiments of the invention, the method250begins with generating substantially pure oxygen and substantially pure nitrogen at step M1using a membrane-based nitrogen generator.

The method250continues with transporting a substrate through an oxygen blanketing zone at step M2. The substrate is then transported to a printing zone at step M3wherein ink is applied to the substrate in an oxygen rich atmosphere. Next, the substrate is transported through a nitrogen blanketing zone at step M4wherein the oxygen and other gases are supplanted by a blanket of nitrogen. The substrate is then transported to a curing region at step M5wherein the ink is illuminated with ultraviolet light in a nitrogen rich atmosphere.

The method250continues with transporting the printed substrate through a second oxygen blanketing zone at step M6. The printed substrate is then transported to a second layer printing zone at step M7wherein a second layer of ink is applied to the printed substrate in an oxygen rich atmosphere. Next, the twice-printed substrate is transported through a nitrogen blanketing zone at step M8wherein the oxygen and other gases are supplanted by a blanket of nitrogen. The twice-printed substrate is then transported to a curing region at step M9wherein the ink is illuminated with ultraviolet light in a nitrogen rich atmosphere.

The benefits of using oxygen inhibition in relation to the single-pass printing systems described above are also relevant to multi-pass, or scanning, printing systems.

FIG. 3Aillustrates a prior art multi-pass scanning printing system300configured to deposit ink onto a substrate302. According toFIG. 3A, a print carriage301moves back and forth across a substrate302(as indicated by the arrows) as the substrate302steps forward under the print carriage301(into the page). The carriage301includes a printing block303with print heads configured for applying liquid ink to the substrate302. The carriage301also includes two curing stations304,305positioned on either side of the printing block303. Curing station304comprises a curing lamp306and two nitrogen applicators307,308. Likewise, curing station305comprises a curing lamp309and two nitrogen applicators310,311.

The printing system300ofFIG. 3Ais a multi-pass printing system characterized by the fact that the printing block303applies ink to the same spot on the substrate302at least two times. Accordingly, as the print carriage301moves back and forth, the printing block303applies ink to the substrate302and the curing lamp (306or309) of the trailing curing station (304or305) partially cures the deposited ink. In the return traversal, the curing lamp (306or309) of the leading curing station (304or305) fully cures the previously partially-cured ink before the printing block303applies another deposit of ink.

The nitrogen applicators (307,308,310, and311) are somewhat directional in that the gas they emit is blanketed in a trailing fashion. Therefore, the leading curing station (304or305) deposits nitrogen gas directly to an area where the print heads of the printing block303will be moments after its deposit, thereby encouraging the curing of ink to the print heads.

Therefore, some embodiments of the invention involve oxygen applicators in a multi-pass, scanning printing system, thereby inhibiting the curing of ink on the print heads.

FIG. 3Billustrates a multi-pass scanning printing system320with a plurality of oxygen applicators399,398,397according to some embodiments of the invention.

According toFIG. 3B, a print carriage321moves back and forth across a substrate312(as indicated by the arrows) as the substrate312steps forward under the print carriage321(into the page). The print carriage321includes a plurality of printing blocks313,323with print heads configured for applying liquid ink to the substrate312.

The printing system320ofFIG. 3Bis a multi-pass printing system characterized by the fact that the printing blocks313,323apply ink to the same spot on the substrate312at least two times.

The print carriage321also includes two curing stations314,315positioned on either side of the print carriage321. Curing station314comprises a curing lamp316, two nitrogen applicators317,318, and an oxygen applicator399. Likewise, curing station315comprises a curing lamp319, two nitrogen applicators330,331, and another oxygen applicator397. A third oxygen applicator398is positioned between the two printing blocks313,323.

As the print carriage321moves back and forth, the printing blocks313,323apply ink to the substrate312, and the curing lamp (316or319) of the trailing curing station (314or315) partially cures the deposited ink. In the return traversal, the curing lamp (316or319) of the leading curing station (314or315) fully cures the previously partially-cured ink before the printing block (313or323) applies another deposit of ink.

The nitrogen applicators (317,318,330, and331) and the oxygen applicators (399,398, and397) are somewhat directional in that the gas they emit is blanketed in a trailing fashion. Therefore, the leading curing station (314or315) deposits nitrogen gas directly to an area where the print heads of the printing block (313or323) will be moments after its deposit.

The printing system310ofFIG. 3Balso includes a controller350configured to selectively activate and deactivate the nitrogen applicators317,318,330, and331and the oxygen applicators399,398, and397in such a way as to apply a steady blanket of oxygen around printing blocks313,323, thereby hindering ink curing on the print heads, while simultaneously applying a blanket of nitrogen in the curing regions, thereby ensuring a good cure.

In the presently preferred embodiment of the invention, the controller350is coupled with a membrane-based nitrogen generator345used to supply the nitrogen gas via supply tube346and the oxygen gas via supply tube347. Also in the presently preferred embodiments, the controller350comprises a processor (not shown) configured to selectively open and close a plurality of valves (not shown) for selectively allowing nitrogen flow from the nitrogen supply tube346to the nitrogen applicators317,318,330, and331and for selectively allowing oxygen flow from the oxygen supply tube347to the oxygen applicators399,398, and397. The selective allowance of nitrogen gas and oxygen gas is described in detail below.

FIG. 4illustrates a workflow400for the multi-pass scanning print system described inFIG. 3Baccording to some embodiments of the invention. Accordingly, the same reference numerals are used inFIG. 4as inFIG. 3Bto describe the workflow400.

The workflow400describes a multi-pass printing process that is mid-operational—in that the printing blocks313,323have already applied at least a first application of ink to the substrate312. For the purpose ofFIG. 4, suppose that the print carriage321starts on the right hand side of the substrate312and moves toward the left hand side at step W1.

At step W2, the print carriage321moves right-to-left, nitrogen applicator20317is active such that nitrogen passes beneath curing lamp316, thereby encouraging curing of ink previously printed and partially cured in a previous pass.

Next, at step W3, the leading oxygen applicator399is activated such that a blanket of oxygen supplants the nitrogen and passes beneath the printing block313as the print carriage321continues its right-to-left motion. Accordingly, the blanket of oxygen protects the print heads of printing block313, as the print heads apply ink to the substrate312in the oxygen rich atmosphere at step W4.

In some embodiments of the invention, the printing blocks313,323have a large profile such that the blanket of oxygen diffuses during the time the printing blocks move over a point on the substrate312. In these embodiments, a central oxygen applicator398is configured between the printing blocks313,323. Preferably, the central oxygen applicator398is active at all time during the workflow400. Accordingly, the central oxygen applicator398applies supplemental oxygen to the printing area at step W5after the leading printing block313passes over the area. Next, at step W6, the trailing printing block323applies ink to the substrate312in the oxygen rich atmosphere.

After the application of ink from printing blocks313and323, the workflow400continues as the trailing curing station315passes over the area of the substrate312recently printed on. At step W7, the leading oxygen application397remains inactive and the leading nitrogen applicator330is activated, thereby providing a blanket of nitrogen under the curing lamp319. At step W8, the curing lamp319illuminates the applied ink in a nitrogen rich atmosphere, thereby curing the ink.

Once the print carriage321reaches its left-most point in its traversal of the substrate312, the nitrogen applicators317,318,330,331and oxygen applicators399and397are toggled at step W9in preparation for the return pass. In some embodiments of the invention, the applicators are switched from active to inactive using a central valve control. However, it will be apparent to those having ordinary skill in the art that a variety of control mechanisms are equally applicable.

More specifically, at step W9, when the print carriage321travels left-to-right, the nitrogen applicator331is switched on and nitrogen applicator317is switched off; the nitrogen applicator330is switched off to keep nitrogen away from print heads; the oxygen applicator397is switched on to apply a blanket of oxygen for the printing blocks323,313; the nitrogen applicator318is turned on to provide a nitrogen blanket under the curing lamp316; and the oxygen applicator399is switched off.

In some embodiments of the invention the curing lamps316and319are standard Ultraviolet lamps. According to these embodiments, both curing lamps316and319remain active during the workflow400. In some other embodiments, the curing lamps316and319are Light Emitting Diode (LED) lamps. According to these embodiments, the LED curing lamps316and319are turned on and off when not positioned over uncured ink, thereby reducing system light.

According to the workflow400ofFIG. 4, a blanket of oxygen remains present in the printing regions while a blanket of nitrogen remains present in the curing regions, thereby optimizing the printing process and protecting the print heads.

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.