Patent Application
20130300801
Typically, a solid ink print head contains a reservoir into which molten ink is fed using a drip feed, or umbilical feed system. The print head also contains an array of jetting elements that are attached to a nozzle plate having an array of apertures through which ink exits in order to form an image on a print surface. Inside the print head, the ink flows from the reservoir to the jetting elements and nozzle plate through a series of channels or manifolds. These channels or manifolds within the print head are typically formed by a combination of discrete layers that are bonded together in order to form the overall fluidic structure.
Through the use of heaters, the print head is heated such that the solid ink within the print head melts, or becomes liquid during normal operation. During long periods of idleness, or after powering down, the heaters turn off. The associated cooling of the print head causes the ink within the print head to solidify and shrink. This, in turn, causes air to be introduced into the channels or manifolds within the print head. Upon the subsequent power-up, this air manifests itself as air bubbles within the fluidic structure. In order for the print head to perform correctly, all or substantially all of this air must be removed from the channels or manifolds internal to the print head.
One should note that the terms ‘printer’ and ‘print head’ apply to any structure or system that produces ink onto a print surface whether part of a printer, a fax machine, a photo printer, etc.
This discussion refers to the process by which the system removes the air from the fluidic structure as a purge cycle. Traditional air removal approaches generate waste ink that the system cannot reclaim or reuse. For example, in one approach, the system transports air bubbles to locations along the channels or manifolds, where they can exit the print head through vent holes that are not part of the nozzle plate. In another approach, the system forces the air through the jetting elements and associated nozzles themselves. In yet another approach, the system forces the air through vents or nozzles within the nozzle plate that are not associated with a jetting element. In each of these approaches, ink trapped between the air bubble and the vent or jetting elements also exits the print head. The printers cannot easily reclaim this ink, and it becomes waste.
With the advent of more stringent energy savings requirements, the printer will be required to power down more frequently than is currently required. Correspondingly, the need for purge cycles in order to remove air introduced into the print head during power down will also increase. This will contribute to more waste ink, resulting in less efficient print heads, higher user costs and unsatisfied customers.
In this example, the fluidic structure connects to a reservoir 12 that contains a fluid 14. In some instances, the reservoir receives a pressure that drives the fluid through channel 16 into chamber 18 within the fluidic structure 10. The fluidic structure may consist of multiple layers 22 that when stacked together form manifolds or channels to route ink from the reservoir to an array of jetting elements and their nozzles, also referred to as apertures such as 20. The stack-up of layers may consist of many more layers than shown here, but for purposes of this discussion the layer, or layers forming the chamber 18 and the layer containing the apertures are of the most interest. During printing, fluid is ejected from the nozzles by the jetting elements. In this example ink is ejected by the jetting elements in order to form images on a print substrate such as 24.
As discussed above, air may be introduced into the fluidic structure during power down cycles of the print head. One should also note that under certain circumstances it is possible for air to be introduced into the fluid structure during normal operation as well.
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Geometric parameters of various components of the fluidic structure affect the ability of the structure to expel air without generating excess waste ink. One such parameter includes the volume of the second chamber, or chambers. As discussed above, the volume of the second chamber needs to accommodate any collateral fluid forced into it during the purge process. Factors that determine the amount of fluid the chamber must accommodate include the amount of time it takes the last bubble to enter the chamber, and the time average flow rate of fluid entering the chamber during the purge process. The product of these two values will give the total volume flow into the chamber, which in turn determines how large the chamber or chambers need to be. For example, the different implementations shown in
Additionally, the cross section of the second chamber and vents should not exhibit large capillary action, or support large meniscus strength. This has several effects. First, it allows purged bubbles to float and escape as they approach the vents in the second chamber. Second, it allows purged fluid within the chamber to flow back into the primary fluidic structure without the need for a significant pressure differential between the chamber's vents and the primary fluidic structure, the first chamber. Third, it allows any residual bubbles within the chamber to coalesce and pop during the flow back.
Generally, the smallest dimension in the chamber cross-section will determine the chamber's meniscus strength. For current solid ink print head designs, the meniscus strength of the chamber needs to be less than about 0.25 inches of water. To achieve this, the smallest dimension needs to be greater than about 1 millimeter.
As with the chamber cross-section, the chamber vent or vents need to be sized in order to have low meniscus strength. For current solid ink print head designs, the meniscus strength of the vent or vents should less than about 0.25 inches of water. To achieve this, the smallest cross-sectional dimension of the vent needs to be greater than about 1 millimeter.
Another component of the fluidic structure that should have appropriate size is the flow path, or paths between the first and second chambers. Unlike the second chamber and vents, the flow path, or paths need to possess meniscus strength within a range that prevents draining of the first chamber during ordinary operation, but allows meniscus failure during purging. During ordinary operation, instances arise where negative pressure within the fluidic structure develops due to the action of the jetting elements. The meniscus strength of the flow path needs to resist breakage due to this negative pressure. Alternatively, a positive pressure is developed within the fluidic structure during the purge process. During the purge process, the meniscus strength of the flow path needs to allow for breakage of the meniscus in order for fluid and air to flow into the second chamber, or chambers. For current solid ink print head designs, this meniscus strength needs to fall within the range of 3 to 130 inches of water. Depending upon the shape of the flow path, this requires the smallest dimension to be less than about 125 micrometers but greater than 1.5 micrometers.
One should note that these consist of merely representative implementations and no intention exists to restrict the scope of the embodiments to these examples.
It will be appreciated that several of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.
