Thermal Printhead

The present invention relates to the structure of a thermal head applicable to a thermal printer of the type using a paper to which thermally fusible or sublimable ink is applied or of the type using a heat-sensitive paper.

A prior art thermal head mounted on a thermal printer includes a heat-generating resistance body having one or more heat-generating portions, and a pair of electrode portions which are associated with the resistance body, a plurality of such resistance body and electrode units being arranged in an array. A problem with this kind of prior art thermal head is that not more than one kind of printing patterns is available, that is, it is impossible to vary a pattern configuration for the purpose of controlling density or thickness of printing.

It is an object of the present invention to provide a thermal head for a thermal printer which is capable of printing out various kinds of printing patterns.

It is another object of the present invention to provide a thermal head for a thermal printer which is capable of printing out patterns in a variable degree of printing density.

It is another object of the present invention to provide a thermal head for a thermal printer which is capable of varying a printing pattern controlled to the density of printing.

It is another object of the present invention to provide a generally improved thermal head for a thermal printer.

In a thermal head having an array of units each comprising a single resistance body and a pair of electrode members respectively provided for at opposite sides thereof to feed a current flowing through the resistance body, at least one of the electrode members in each of the units comprises a plurality of divided electrodes, and the resistance body comprises an island and a plurality of first heat-generating portions branching out from the island and connecting thereof with the plurality of divided electrodes respectively, and a second heat-generating portion branching out from the island and connecting thereof with another of the electrode members.

The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings.

FIG. 1 is a fragmentary plan view of a thermal head for a thermal printer in accordance with the present invention; and

FIGS. 2 and 3A, 3B, 3C are diagrams showing printing patterns available with the thermal head of FIG. 1.

Referring to FIG. 1 of the drawings, a thermal head embodying the present invention is shown and generally designated by the reference numeral 10. The thermal head 10 includes a plurality of heating units which are arranged in an array, although only two heating units 10a and 10b are shown in the drawing. The two heating units 10a and 10b have resistance bodies 20a and 20b respectively. Each of the resistance bodies 20a and 20b has uniform thickness and material. Bisected electrodes 30a and 40a are electrically interconnected to one end of the resistance body 20a, and bisected electrodes 30b and 40b to one end of the resistance body 20b. Electrically interconnected to the other end of the resistance bodies 20a and 20b, respectively, are non-divided or unitary electrodes 50a and 50b. The resistance body 20a is provided with heat-generating portions 60a, 70a and 80a while the resistance body 20b is provided with heat-generating portions 60b, 70b and 80b. The heat-generating portions 60a and 60b are associated with the electrodes 30a and 30b, respectively, and the heat-generating portions 70a and 70b are associated with the electrodes 40a and 40b, respectively. Further, the heat generating portions 80a and 80b are associated with the electrodes 50a and 50b, respectively. While the heat-generating portions 60a, 60b, 70a and 70b are equal in size to each other, each of the heat-generating portions 80a and 80b is provided with an approximately two times greater width but 1/2 times shorter length than the heat-generating portion 60a, 60b, 70a or 70b so that the electric resistance of the heat-generating portion 80a or 80b is approximately a quarter of that of the heat-generating portion 60a, 60b, 70a or 70b. The electric resistance of other parts of the resistance body than the heat-generating portions is substantially small compared with that of the heat-generating portions. Accordingly, heat generation from the other parts can be deemed minor for a given current.

In the thermal head 10 constructed as described above, assume that a voltage is applied across the electrode portions 30a and 30b and associated electrodes 50a and 50b so as to cause a current to flow through the resistance bodies 20a and 20b. Then, heat is generated substantially at the heat-generating portions 60a and 60b resulting that dots are printed in a particular pattern as represented by solid circles in FIG. 2. On the other hand, when a voltage is applied across the electrodes 40a and 40b and the electrodes 50a and 50b, the heat-generating portions 70a and 70b generate heat so that dots are printed out in a pattern as represented by phantom circles in FIG. 2. It is to be noted that in FIG. 2 the large circles and the small circles respectively correspond to a case wherein the amount of current is small and a case wherein it is large, that is, the area of the circle is variable with the amount of current. When the amount of current is large, although the printing area spreads rapidly, nearby dots are prevented from joining each other since every other heat-generating portion generates heat at a time to print out a dot.

When a voltage is applied across the electrodes 30a, 30b, 40a and 40b and the electrodes 50a and 50b at the same time, a current which then flows through each of the heat-generating portions 80a and 80b is equal to the sum of currents which flow through its associated heat-generating portions 60a and 70a or 60b and 70b. In this condition, all the heat-generating portions 60a, 60b, 70a, 70b, 80a and 80b generate heat resulting that a pattern of dots is printed out as indicated by solid circles in FIGS. 3A and 3B. Thereafter, when a voltage is applied in the same manner after moving the thermal head 10 by a distance L between the heat-generating portions 60a, 60b, 70a and 70b and the heat-generating portions 80a and 80b, dots are printed out in a pattern as represented by phantom circles in FIGS. 3B and 3C. Specifically, a dot printed out by the heat-generating portion 80a is interspersed between those dots which are printed out by the heat-generating portions 60a and 70a, thereby substantially filling the space between the dots. Such interpolation allows information to be printed out with high density. In addition, the number of dots is increased to enhance smooth appearance of image printed out.

A circuit for driving the thermal head 10 as shown and described may be implemented with, for example, a drive circuit which is shown in Japanese Laid-open Patent Publication No. 59-107681, which was filed by the same assignee as the present application.

While one electrode in each electrode pair of the thermal head 10 has been shown and described as being bisected, it may be divided into three portions or even to 2.sup.n (n being 2 or greater integer) portions. Further, both of the electrodes in each electrode pair may be divided into a plurality of electrode portions, so that the number of the divisions is always greater at one side than at the other side.

In summary, it will be seen that the present invention provides a thermal head which is capable of printing out large dots and small dots as desired. Large dots printed out by the thermal head are prevented from joining each other, thereby promoting high clarity printing. Small dots, on the other hand, are printed out without any significant spacing left therebetween, so that information may be printed with high density and clarity. This means an increased number of dots and thereby provides information printed out with smooth appearance.

Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.