1. Field of the Invention
The present invention relates to thermal printers wherein the selective energization of thermal pixels causes the transfer of dye to a receiver member.
2. Description of the Prior Art
In a thermal printer, a carrier containing dye is disposed between a receiver, such as paper, and a print head assembly formed of, for example, a plurality of individual thermal elements often referred to as thermal pixels or dots. When a particular thermal pixel is energized, it is heated and causes dye from the carrier to transfer to the receiver. The density, or darkness, of the printed dye is a function of the temperature of the thermal pixel and the time the carrier is heated, in other words the energy delivered from the thermal pixel to the carrier.
Thermal dye transfer printers offer the advantage of true "continuous tone" dye density transfer. This result is obtained by varying the energy applied to each thermal pixel, yielding a variable dye density image pixel on the receiver.
A conventional method of energizing thermal pixels employs pulse width modulation as will now be explained. A print head is organized into a plurality of groups of thermal pixels. The thermal pixels in each group are simultaneously addressed in parallel. Each group is addressed sequentially one at a time. The reason groups are used is that if all the thermal pixels were energized at the same time, a large and more expensive power supply would be needed. For example, if a thermal pixel were to draw 68 millamperes and 512 thermal pixels were used, the power supply would, if all thermal pixels were energized, have to produce 33.3 amperes. Therefore, the group arrangement is preferred.
When a group of thermal pixels are addressed, they are selectively energized. The thermal pixels are driven by a constant voltage. FIG. 1 shows a prior art pulse width modulation scheme used to drive a thermal pixel. The maximum time a current pulse can be provided to a thermal pixel is (t.sub.1 -t.sub.0). This will produce the maximum density dye image pixel. If the pulse width is made smaller (t.sub.b -t.sub.0), then a less dense image pixel will be formed. If a still smaller pulse width (t.sub.a -t.sub.0) is used then an even lower dye density image pixel will be formed. Such an arrangement presents several drawbacks. Generally all thermal pixels in a print head are not driven simultaneously, but are addressed in separate "groups". Thus when a group is undergoing a heat cycle, all other groups are either "cold" or cooling. Carrier heat resisting or slipping layers applied to the head side of the carrier must therefore continue to perform well under both hot and cold conditions. This makes the design of the slipping layer difficult since a designer must optimize both "hot" and "cold" slipping performance. Layers that may function well under hot conditions may tend to aggregate or stick to the print head when run with cold surfaces. This difficulty complicates the design of the slipping layer.
A second problem involves the surface temperature of the thermal pixels along the print head. A thermal pixel in the middle of a group that is being addressed and energized generally has neighbors on both sides that are warm. As such, the temperature profile of the thermal pixel itself as well as any interpixel gap in this group tends to average to some level, since the temperature gradients in the print head will tend to equilibrate. The temperature of a thermal pixel on the end of a group can be significantly reduced due to the heat flow to the cold thermal pixels of the adjacent group which is not being addressed and energized. When dye images are transferred with such a printer, low density streaks, or "group lines" can often appear due to cold end thermal pixels.
An obvious solution to this problem would be to run all the thermal pixels of the print head at once, eliminating the existence of these cold spots. However, two factors make such a scheme rather impractical. First, as discussed above, thermal pixels draw significant currents. If one were to drive all thermal pixels simultaneously, head currents would be increased by a multiplicative factor equal to the number of groups. This causes difficult design constraints on both the power supply as well as the design of power buses within the head.
When a thermal pixel is energized for an extended period of time (say 4 milliseconds or more), limited thermal conductivity within the pixel itself results in rather large temperature gradients across its surface. Thus very high peak temperatures (hot spots) are experienced in the center of the thermal pixel, while the outside regions of the pixel remain relatively cool, due to thermal lag. As a result, damage to the thermal pixel or the carrier or the receiver can result since very high peak temperatures can be reached. For example, such hot spots can cause melted regions of the carrier or receiver or reduced life of the thermal pixels.