Printing Speed

Often the maximum printing speed of a printhead will be referred to. Speed limitation comes from several factors as follows. For background, please understand that there is a repeating period of time within which the thermal printer prints a line, then moves the media to the next line. This is called the cycle time or pulse cycle. The cycle time is the sum of pulse width, when electrical current flows through the heater element, and cooling time, when no current flows. A shorter cycle time means higher printing speed.

 1. Applied Power Limitation

If we can apply higher power to the thermal printhead, a shorter pulse width can achieve enough energy to darken the media. A shorter pulse width can support a shorter cycle time. Power is given by the product of current and voltage, and can be increased by increasing the voltage. The capability of the driver IC on the thermal printhead places limits on the current and voltage. The resistance of the heater element is fixed, so that the electrical current limit is not exceeded when the printhead is operated at the specified applied voltage.

heater surface temperature chart

 2. Thermal Response Limitation

On the other hand, a short cooling time is required to get high printing speed. If the heater does not lose heat quickly enough, the next pulse may be applied before the temperature has dropped to the base level. The result of too short a cooling time is too much heat for the next dot. This will lead to poor print quality and, in the worst case, shorter printhead life. Thin film printheads usually have much better heat response than thick film printheads. However, even with thin film printheads, history control is required when print speed is more than a certain limit. History control means using a shorter pulse width to print a dot when the same heater retains heat from printing a previous line. It can be provided on the printhead by history control driver ICs or it can be provided by the printer controller by using multiple pulse widths within the cycle time. Thin glaze raises the limit at which history control is required.

The charts show the effects of the glaze under the heater line. The horizontal axis is time and the vertical axis is the temperature of the heater surface. Thin glaze more efficiently dissipates heat so that the heater surface returns to the base level and no further heat accumulates after 5 pulses. Under the same power conditions, heat builds up in the partial glaze printhead. These effects are relative. At a longer cycle time, partial glaze could be shown to dissipate heat more efficiently than flat glaze. With dye sublimation ribbons, heat accumulation is desirable.

 3. Data Transfer Time Limitation
In some cases, the time required to send data from the printer controller to the thermal printhead may take longer than the desired pulse cycle. When the printer controller generates history control or gray-scale, full print lines of data are transferred multiple times per printed line. In these cases printheads have multiple data-in lines. For example the KGT-217-12MPL20 has 20 data-in lines to provide reasonable performance for fine gray scale graphics. The need for multiple data-in increases with the density and width of the print line, simply because there is more data to transfer. Of course using a higher clock speed will also increase the data transfer rate, but the limits of the driver ICs on the printheads are reached rather quickly.

 4. Media Limitation
Printing speed is related to media sensitivity. Printing on high sensitivity media requires less energy than on low sensitivity media. Less required energy can mean reduced pulse width and reduced cycle time.

 The Fastest Thermal Printhead
A Kyocera KCE-series printhead has achieved 70ips in an actual customer's product. Is this the speed limit of thermal printing? We cannot be certain. Kyocera is working with printer and media manufacturers to continually raise the speed limit, as required by the marketplace.
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