The present invention is generally directed to ink jet printing devices. More particularly, the invention is directed to determining optimum characteristics of energy pulses provided to resistive heating elements in an ink jet print head, and to determining optimum characteristics of the resistive heating elements.
A thermal ink jet printer forms an image on a print medium by ejecting small droplets of ink from an array of nozzles in an ink jet print head as the print head traverses the print medium. The ink droplets are formed when ink in contact with a resistive heating element is nucleated due to heat produced when a pulse of electrical current flows through the heating element. Typically, there is one resistive heating element corresponding to each nozzle of the array. The activation of any particular resistive heating element is usually controlled by a microprocessor controller in the printer.
Once a bubble of ink begins to form due to heat energy transferred from the heating element into the ink, the ink is thermally isolated from the surface of the heating element. Thus, after the bubble forms, any additional energy provided to the heating element does not transfer into the ink, but is dissipated in the print head heater chip. This results in undesirable overheating of the chip.
One solution to this problem is to provide to the heating element only the minimum amount of energy necessary to nucleate the ink. This requires that the printer controller precisely control characteristics of the energy pulses provided to the heating element. Since the amount of heat energy transferred from the heating element into the ink depends upon characteristics of the ink and characteristics of the heating element, the characteristics of the minimum energy pulse should be determined taking into account the ink and heating element characteristics.
Therefore, a need exists for an ink jet printer that determines characteristics of a minimum energy pulse to be provided to a resistive heating element based on characteristics of the ink and the heating element.
The foregoing and other needs are met by a system for providing an optimum energy pulse to a resistive heating element in an ink jet print head. The optimum energy pulse generated by the invention provides an optimal energy density at a surface of the resistive heating element to cause optimal nucleation of ink near the surface of the resistive heating element. The system includes (a) storing in memory at least one heating element dimensional value that describes at least one physical dimension of the resistive heating element, (b) storing in memory at least one heating element electrical value that describes at least one electrical characteristic of the resistive heating element, and (c) storing in memory an expression that provides a mathematical relationship between the heating element dimensional value, the heating element electrical value, and a current value representing an optimum value of electrical current flowing through the heating element to generate the optimum energy pulse. The system also includes (d) retrieving from memory the heating element in dimensional value, the heating element electrical value, and the expression, (e) determining, based on the expression, the current value representing the optimum value of electrical current flowing through the heating element to generate the optimum energy pulse, (f) generating the optimum energy pulse corresponding to the value determined in step (e), and (g) providing the optimum energy pulse to the heating element.
In another aspect, the invention provides a system for providing an optimum energy pulse to a resistive heating element covered by a protective overcoat layer in an ink jet print head. The optimum energy pulse generated by the invention provides an optimal energy density at a surface of the resistive heating element to cause optimal nucleation of ink that is adjacent the surface of the protective overcoat layer. The system includes (a) storing in memory at least one protective overcoat dimensional value that describes at least one physical dimension of the protective overcoat, (b) storing in memory at least one heating element electrical value that describes at least one electrical characteristic of the resistive heating element, (c) storing in memory at least one ink-related coefficient that relates to at least one characteristic of the ink, and (d) storing in memory an expression that provides a mathematical relationship between the protective overcoat dimensional value, the heating element electrical value, the ink-related coefficient, and an optimum time duration of the optimum energy pulse. The system also includes (e) retrieving from memory the protective overcoat dimensional value, the heating element electrical value, the ink-related coefficient, and the expression, (f) determining, based on the expression, the optimum time duration of the optimum energy pulse, (g) generating the optimum energy pulse corresponding to the optimum time duration determined in step (f), and (h) providing the optimum energy pulse to the heating element.
Thus, by proper adjustment of the amplitude and duration of the energy pulse provided to the resistive heating elements in the print head, the present invention provides an optimum energy density at the surface of the heating elements. This optimum energy density is just large enough to cause the ink near the heating elements to form a bubble and a droplet. Little or no energy is wasted in excess energy that cannot be transferred into the ink after the bubble is formed. To adjust the amplitude and duration of the energy pulse in providing the optimum energy density, the invention takes into account several factors related to characteristics of the print head, characteristics of the resistive heating elements and the protective overcoat layer, and characteristics of the ink. By storing these factors in memory on the print head and on ink cartridges, and by expressing in mathematical form the relationship between these factors and the optimum pulse energy density, the invention can determine and provide the optimum pulse energy density for practically any combination of ink type and print head design.
In another aspect, the invention provides a system for determining a maximum optimal thickness of a protective overcoat layer covering a print head resistive heating element so that energy is optimally transferred into the adjacent ink. The system is implemented by a computer that includes a processor and a memory. The system includes (a) inputting one or more heating element dimensional values that describe one or more physical dimensions of the resistive heating element, (b) inputting one or more heating element electrical values that describe one or more electrical characteristics of the resistive heating element, (c) inputting one or more ink-related coefficients that relate to one or more characteristics of the ink, (d) inputting one or more print head thermal values relate to a thermal characteristic of the print head. The system also includes (e) retrieving from the memory an expression that provides a mathematical relationship between the one or more heating element dimensional values, the one or more heating element electrical values, the one or more ink-related coefficients, the one or more thermal values, and the maximum optimal thickness of the protective overcoat. The system further includes (f) determining, based on the expression, a thickness value representing the maximum optimal thickness of the protective overcoat.