Based on a special coating-processing method, a thermal imaging layer and a protective layer are coated on the surface of the blue polyester substrate. After drying, a thermal medical dry film is formed. Under the action of the thermal imager, the thermal film produces graphics according to the different temperature effect of thermal print head.
Direct printing on heat sensitive film, where a printhead is used to supply heat by contact to a heat sensitive film. A pulse of current is applied to one or more of the heating elements to cause rapid heating of the resistors; heat conducted to the film via the region of contact produces a significant temperature rise on the surface of the heat sensitive film. The resistor element must be able to heat the thermally sensitive film up to the coloration temperature, and its surface must be highly resistant to friction from the film. The heat sensitive film contains a coating of dye forming chemicals held within a binder material. At room temperature the dyes are in a leuco (i.e., colourless) form, and the film has the appearance and feel of smooth film. The dye precursor agents react at the printing temperature to generate a light absorbing dye resulting in the visual printed image.
It is important that the temperature excursion of the heater elements be kept within a reasonable range (typically several hundred degrees Centigrade) in order to ensure long head life and avoid direct thermal damage to the material in contact with the head. The width of the excitation pulse applied to the heating elements is then chosen to be long enough to allow sufficient energy to migrate into the film to develop the required level of blackness, typically, a few joules/cm2. Following the heating pulse, the resistor heating elements are allowed to cool prior to printing at the next desired location upon the film. Most of the energy generated flows into the printhead, with only a small amount actually transfers to the film. Hence, the thermal time constant of the printhead structure is therefore of major importance in determining the rate at which printing may be achieved, typically results in thermal cycle times on the order of a millisecond. This is sufficient to allow printing at several tens of characters per second in a serial printing mode.
The page wide configuration printhead makes possible printing at much higher rates and permits printing of graphic images at a higher than 200 dots per inch. The attractive feature of this arrangement is that the head extends completely across the page and is stationary during printing. Since the printhead contains several thousand resistor elements, printing may be accomplished at high resolution with a throughput of several pages per minute. The data handling and driver circuits are integrated upon the ceramic printhead so that relatively few electrical connections to the head are required. Speed in this case is limited by the energy dissipation capability of the printhead rather than by the thermal cycle time of the print elements.
Thermal printing on heat sensitive film provides a quiet, compact, reliable printing method that is of importance today for instrumentation applications or where simplicity is a prime requirement. Thermal films are smooth to ensure good thermal contact to the printhead, and are available for printing in a limited range of colours. Improvements in shelf life, resistance to unwanted coloration due to fingerprints or other contaminating materials, and in appearance, coupled with the availability of cost-efficient integrated printheads make direct thermal printing of continuing interest for a number of applications.
Due to the simple nature of the printing process, there is none of the whir, clatter and buzz inherent in other printers. What’s more, there is no toner, ink or ribbons. Since there are at least a few hundred dots to the inch, a thermal imager can deliver an image that is sharper and clearer than the standard inkjet printer. And it’s a more consistent image because there is no ribbon to wear out. A wide variety of fonts can be used to quickly and clearly print many logos and graphical images that are beyond the capability of most current printers.
Thermal printing also creates less dust than the typical printing unit while the film coatings prevent abrasive wear to printheads and rollers. And, since the printhead has few moving parts, it requires far less interim maintenance than the more complicated devices that currently dominate the market.
Thermal benefits include excellent, crisp print quality, consistent print quality, high reliability, easy handling without ink or ribbon, high speed printing, lower maintenance costs, compact mechanism, flexible use of fonts, high quality graphics/ and images, and easier film loading.
Now thermal film is widely used in medical imaging such as CT examination, magnetic resonance, CR, DR and other records, as a doctor’s diagnosis and treatment basis.