Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
2000-06-15
2001-09-25
Barlow, Jr., John E. (Department: 2853)
Incremental printing of symbolic information
Ink jet
Controller
C347S011000, C347S056000
Reexamination Certificate
active
06293644
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to ink jet printers.
2. Description of Related Art
A thermal ink jet print head selectively ejects droplets of ink from a plurality of drop emitters to create a desired image on an image receiving member, such as a sheet of paper. The print head typically comprises an array of the drop emitters that convey ink to the image receiving member. In a carriage-type ink jet print head, the print head moves back and forth relative to the image receiving member to print the image in swaths which are many pixels tall.
An ink jet print head typically comprises a plurality of ink passageways, such as capillary channels. Each channel has a nozzle and is connected to an ink supply manifold. Ink from the manifold is retained within each channel until, in response to an appropriate signal applied to a resistive heating element in each channel, the ink and a portion of the channel adjacent to the heating element is rapidly heated and vaporized. Rapid vaporization of some of the ink in the channel creates a bubble that causes a quantity of ink, i.e., an ink droplet, to be ejected from the emitter to the image receiving member. When an ink droplet is ejected at a velocity greater than approximately 5 m/s, the ink droplet is usually accompanied by one or more small sub-drops called satellites.
When a quantity of ink in the form of a ink droplet is ejected from the ejector to a receiving member, the resulting spot and any satellite drops become part of a desired image. Uniformity in spot size of a large number of droplets is very important for image quality. If the volume of droplets ejected from the print head over the course of producing a single document are permitted to vary widely, the lack of uniformity will have noticeable effects on the quality of the image. The most common and important cause of variance in the volume of droplets ejected from the print head is variations in the temperature of the print head over the course of use. The temperature of the liquid ink, before vaporization by the heating element, substantially affects both the bubble growth behavior and the viscosity of the ink. These two properties substantially influence the ejection volume of the ink droplets, and thus the resulting spot size on the image receiving member. The viscosity of the ink also directly affects the resulting spot size, by affecting the spread rate of the droplet after it contacts the image recording member. Control of temperature of the print head has long been of primary concerns in the art.
In order to maintain a constant spot size from an ink jet print head, various strategies have been attempted. One example is U.S. Pat. No. 4,899,180 to Elhatem et al., which is incorporated herein by reference in its entirety. In the′180 patent, the print head has a number of heater resistors and a temperature sensor that operate to heat the print head to an optimum operating temperature and maintain that temperature regardless of local temperature variations.
U.S. Pat. No. 4,791,435 to Smith et al., which is incorporated herein by reference in its entirety, discloses an ink jet system where the temperature of the print head is maintained by using the heating elements of the print head not only for ink ejection but also to maintain the temperature. The print head temperature is compared to thermal models of the print head to provide information for controlling the print head temperature. At low temperature, low energy pulses, i.e., pre-pulses, are sent to each channel, or nozzle, at a voltage that is below the voltage threshold that causes a ink droplet to be ejected. Alternatively, the print head can be warmed by firing some droplets of ink into an external chamber or “spittoon,” rather than onto the image receiving member.
SUMMARY OF THE INVENTION
As described above, the satellite drops are usually smaller than the main drop and travel at a lower speed than the main drop. As a result, the satellite drops can land on different locations of the image receiving member relative to the main drop during printing when the drop ejector and the image receiving member are moving relative to each other. Furthermore, the satellite drop can land at different locations from the main drop because the satellite drops generally have a velocity vector that is slightly different from that of the main drop. This satellite drop displacement can increase or decrease with increasing print head speeds and with an increased gap between the print head and the image receiving member.
Because the satellite drops occasional travel in slightly different directions relative to the main drop, the effect of the satellite drops on the image quality can differ based on a traveling direction of the print head. In one exemplary embodiment of a conventional print head, the print head transfers ink onto the image receiving member as the print head travels from right-to-left and left-to-right relative to the image receiving member, i.e., a bi-directional print head. Furthermore, while in a stationary position, the nozzles of this print head create satellite drops having velocity vectors which direct satellite drops to land slightly to the left of a main drop on the image receiving member.
As the print head passes from right-to-left and transfers ink onto the image receiving member, the velocity vector of the print head in conjunction with the velocity vectors of the satellite drops cause the satellite drops to land further away from the main drop on the image receiving member relative to that of the satellite drops of the print head when stationary. In other words, the satellite drops will lag even further behind the main drop due to the additional print head velocity vector.
On a return swath, as this print head moves from left-to-right and transfers ink onto the image receiving member, the velocity of the print head causes the satellite drops to land closer to, or actually within, the main drop on the image receiving member. The differing positions of the satellite drops relative to the main drop causes variations in the appearance between swaths formed by a left-to-right pass versus a swath formed by a right-to-left pass. Accordingly, this effect causes bidirectional banding in the final image because the patterns formed by drops printed in one direction are different from those printed in the other direction.
This invention counteracts the distorting effect of the bi-directional print head by reducing the volume of emitted ink when the print head is traveling in a direction that increases the satellite misplacement from the main drop. The emitted ink volume can be reduced by reducing an amount of pre-pulsing prior to ejection of the ink. Reducing the amount of pre-pulsing prior (generally a period of several micro-seconds) to the ejection of the ink can reduce the ink's temperature causing a smaller, less forceful, bubble to eject a more viscous ink from the channel. This combination of a smaller bubble and a more viscous ink in the channel can result in a smaller spot being formed on the receiving member and it can also affect the satellite displacement.
This invention can also counteract the distorting effect of the bi-directional print head by increasing the emitted ink volume when the print head is traveling in a direction that decreases the satellite displacement from the main drop. The emitted ink volume can be increased by increasing the amount of pre-pulsing prior to ejection of the ink. Increasing the amount of pre-pulsing prior to ejection of the ink can cause a larger, more forceful, bubble to eject a less viscous ink from the channel. This combination of larger bubble and less viscous ink can result in a larger spot being formed on the receiving member and it can also affect the satellite displacement.
As a result of these two effects, the overall appearance of right-to-left swaths more closely matches that of left-to-right swaths. Other objects, advantages and salient features of the invention will become apparent from the following detailed descri
Markham Roger Guy
Xie Yonglin
Barlow Jr. John E.
Loper, Jr. Robert D
Oliff & Berridg,e PLC
Xerox Corporation
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