Printing apparatuses such as printers, copying machines, and facsimile apparatuses are designed to print an image formed from a dot pattern on a printing material such as a paper sheet or thin plastic plate on the basis of image information. Such printing apparatuses can be classified into inkjet printing apparatuses, wire dot printing apparatuses, thermal printing apparatuses, laser beam printing apparatuses, and the like in accordance with their printing schemes. Of these schemes, the inkjet scheme (inkjet printing apparatus) is designed to print an image by discharging ink (printing liquid) droplets from the orifices of a printhead and causing the droplets to fly and land on a printing material.
As many printing apparatuses are used in recent years, they are required to realize high-speed printing, high resolution, high image quality, and low noise level. An inkjet printing apparatus can meet these requirements. In an inkjet printing apparatus which prints an image by discharging ink from a printhead, the temperature of ink at the discharge section has a strong influence on stable ink discharge and stable ink discharge amount, which are necessary for meeting the above requirements. More specifically, if the ink temperature is too low, the ink viscosity excessively increases. This makes it impossible to discharge the ink by normal discharge energy. Conversely, if the ink temperature is too high, the discharge amount excessively increases to cause ink overflow on a printing paper sheet, resulting in poor image quality.
To prevent this, conventional inkjet printing apparatuses have a temperature sensor at the printhead portion and employ a method of controlling the ink temperature at the discharge section within a desired range on the basis of the detected temperature of the printhead or a method of controlling a discharge recovery process.
As a heater for temperature control, a heater member joined to the printhead portion is used. Some inkjet printing apparatuses which print an image by forming flying droplets using thermal energy, i.e., some apparatuses which discharge ink droplets by growing bubbles by film boiling of ink, use a discharge heater itself as a heater for temperature control. Especially, when the discharge heater is used, the heater must be energized not to grow ink bubbles.
In a printing apparatus which obtains ink droplets to be discharged by forming bubbles in solid ink or liquid ink using thermal energy, the discharge characteristic largely changes depending on the temperature of the printhead. It is therefore particularly important to manage the ink temperature at the discharge section and the printhead temperature that greatly influences the ink temperature.
In managing the printhead temperature, it is important to measure the ink temperature at the discharge section which has a large influence on the discharge characteristic. However, it is very difficult to measure the ink temperature because the discharge section also generates heat, and this makes the detection temperature of the temperature sensor largely vary more than the variation in ink temperature necessary for management, and also because the ink itself moves. For these reasons, even when a temperature sensor is simply arranged near the printhead to accurately measure the ink temperature at the time of discharging, the variation in temperature of the ink itself can hardly be measured.
An inkjet printing apparatus has been proposed, which stabilizes the ink temperature indirectly by stabilizing the printhead temperature as an ink temperature management means. U.S. Pat. No. 4,910,528 discloses an inkjet printer which has a means for stabilizing the printhead temperature by predicting a subsequent discharge heater driving amount in a predetermined time using, as a reference, the detection temperature of a temperature sensor arranged near the discharge heater.
More specifically, the apparatus stabilizes the printhead temperature by controlling, in accordance with the predicted temperature, a printhead heating means, a discharge heater energization means, a carriage driving control means for maintaining the printhead temperature to a predetermined value or less, a carriage scanning delay means, a means for reducing the carriage scanning speed, a means for changing the printing sequence of ink droplet discharge from the printhead, and the like.
However, the inkjet printer disclosed in U.S. Pat. No. 4,910,528 has a measurement error of the temperature sensor and a time lag between reading by the temperature sensor and reflection of the reading result on driving. It cannot sufficiently stabilize discharge in recent high-speed printing. In addition, since the printhead incorporates the temperature sensor, the cost of printhead increases. Hence, no inexpensive printing apparatus can be provided.