Patent Application
20150054878
Ink in a print head of a printer is often heated and the temperature of the ink regulated. Temperatures of the ink should be within a recommended range of temperatures to ensure the highest print quality and the minimum risk of damage to the printer's components. Temperature measurement and monitoring is usually performed and incorporated into the print head itself to maintain the temperature of the print head in the recommended temperature range.
Conventional temperature measurement devices include thermistors placed at each side of the jet stack of a print head. Recent changes in high jet density print head designs have adopted flexible or “flex” circuit technology as the preferred method of including electronic components in the print heads. Further, space constraints for new print head designs provide little room for conventional thermistors.
Still further, thermistors experience frequent failure and are a major reason that print heads need maintenance or need to be replaced. Thermistors also are a separate component that needs to be attached to the print head during the manufacturing process, which presents separate failure issues. The failure rate, design and space constraints, cost, and difficult maintenance, make thermistors a poor design choice for the temperature measurement component for print head jet stacks. Embodiments of the disclosure address these and other limitations of the currently available methods and systems of temperature measurement in print head jet stacks.
Throughout the disclosure, some terms are used frequently and are defined as follows. A print head is an element of a printing apparatus that applies ink to media. A jet stack is the portion of the printing apparatus that includes ejectors for dispensing ink, which may include a silicon chip and associated channels, or layers of stainless steel or polyimide with piezoelectric ceramic actuators. A flexible circuit, or flex circuit, is one or more conductive layers, typically copper, adhered to a flexible substrate such as a plastic. A heat source layer or first layer having a heat source is a layer within the disclosed flex circuits that provides heat to the jet stack. A temperature measurement layer or second layer having a temperature sensing element is a layer within the disclosed flex circuits used to sense the temperature of the jet stack. An insulative layer is a layer of the flex circuit that prevents electrical conductivity and includes any suitable insulating material(s), typically polyimide. A print head controller is any suitable printing apparatus component that can control operations of the print head, such as an electronic circuit that includes a processor.
A single flex circuit includes a heat source, heat spreading, and thermal feedback, as described in this disclosure. The single flexible circuit component can be included in a print head of the printing apparatus in any suitable manner, serving as both a jet stack heating and temperature measuring element. The jet stack heating and temperature measuring element is thermally connected to the print head's jet stack.
The flex circuit 100 also includes a second, backside etched copper layer 104 formed by copper trace circuit components. The second, backside copper traces include a temperature sensing element that measures the temperature of the print head jet stack. Other suitable materials may be used in combination with or instead of copper, as discussed above regarding the first layer 102
An insulative third layer 106 is positioned between the first layer 102 and the second layer 104 of the example flex circuit 100 shown in
The flex circuit 100 can also include a top cover film 108 and a bottom cover film 110. Both of the top cover film 108 and the bottom cover film 110 have respective adhesive layers 112, 114 and insulative layers 116, 118. The adhesive layers 112, 114 of the top cover film 108 and the bottom cover film 110 can include an acrylic or modified acrylic adhesive, such as adhesives with an A381 designation. Any other suitable single- or double-sided adhesive can also be used. The insulative layers 116, 118 of the top cover film 108 and the bottom cover film 110 can include polyimide or any other suitable material having insulating properties.
The second layer 104 includes a temperature sensing element 134 typically connected to a voltage source 130 and a resistor 132 that are electrically connected in series, as shown in
In the above described examples of the disclosed flex circuits, copper is used exclusively or in combination with gold or another material to form the traces for the circuit elements. Copper traces have known electrical properties in which the resistance of the copper (R) changes approximately 0.4% for every degree Celsius (° C.). Therefore, the resistance of the copper (R) at a particular temperature (T)=Rref[1+α(T−Tref)]. The reference resistance (Rref) is a reference resistance of the copper at a reference temperature (Tref). Frequently, Tref is 20° C., but can alternatively be 0° C. A temperature coefficient (α) of R, the resistance of the copper, is a measurement of the change in physical property, in this case the R of the copper, as the temperature increases by a set amount, usually 1 Kelvin (K). The equation described here is not unique to copper and can be calculated for any conductive material used in the flex circuit, including gold, a gold and copper combination, or the like.
The above disclosed flex circuits can be used to measure the temperature of a print head jet stack. The temperature measurements can be sent to a print head controller that can adjust the temperature of the jet stack based on the received measurements. Oftentimes, the desired operation of the print head requires the jet stack to maintain a temperature within a defined range of temperatures.
The print head jet stack can be heated by the heat source of the first layer of the flex circuit examples discussed above. A value of the resistance of the second layer of the flex circuits described above is measured. The temperature sensing element of the second layer of the above described flex circuits define a resistance that changes in accordance with the temperature of the second layer, based on the properties of the material used in the second layer. The above examples include copper and/or gold in the second layer. The second layer serves as a temperature measurement layer of the flex circuit. As discussed above, the second layer is separated from the heat source or first layer by an insulative layer that prevents electrical conductivity between the first, heat source layer and the second, temperature measurement layer.
A predetermined temperature scale is created or is already known based on the properties of the materials used in the second layer to form the circuit elements of the temperature sensing elements. The measured resistance values of the second, temperature measurement layer are compared to the predetermined temperature scale. From the compared resistance values, a corresponding temperature of the second, temperature measurement layer is determined. In the example flex circuit in which the second layer includes copper traces, the resistance of the copper is measured and compared to a known temperature scale for copper to determine the associated temperature of the second layer at any given time.
The above described systems and methods may require a print head calibration step that includes measuring both the temperature of the jet stack and the resistance value of the second layer of the flex circuit to determine if any offset or gain is required. If the calibration measurements differ from the known temperature measurement scale, an offset or gain can be calculated and then applied to the resulting measured resistance when the temperature measurement system is operating.
The disclosed flex circuits reduce the number of materials required for manufacturing a print head because the flex circuits rely on an existing layer of copper (or other conductive material) on which the traces are formed. The copper traces in the second layer on the backside of the heater provide heat spreading capabilities and thus no conventional thermistor is required. Because of the simplified manufacturing and reduction in parts, both the reliability of the print heads and the cost of manufacturing the print heads improve.
It will be appreciated that variations of the above-disclosed systems and methods for measuring the temperature of print head jet stacks and other features and functions, or alternatives thereof, may be desirably combined into many other different systems, methods, or applications. Also various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art.
