1. Field of the Invention
This invention relates to a thermal printing head. More specifically, the present invention relates to a line-type thermal printing head in which connection terminals used for electrical connection to an external control circuit are arranged locally only in a limited central portion of the head.
2. Description of the Prior Art
As is well known, line-type thermal printing heads are widely used in facsimile machines to print transmitted information on thermosensitive paper. The line-type thermal printing head is also used in printers wherein the ink of a transfer ink ribbon or film is thermally caused to be transferred onto printing paper.
An example of prior line-type thermal printing head is disclosed for example in U.S. Pat. No. 4,963,886 to Fukuda et al. For the convenience of explanation, reference is now made to FIGS. 6-9 of the accompanying drawings which show a line-type thermal printing heat similar to the one disclosed in the above U.S. patent.
As shown in FIGS. 6-9, the prior art line-type thermal printing head includes an elongate metallic heat sink plate 1" and an elongate insulating head substrate 2" attached onto the heat sink plate. The heat sink plate 1" and the head substrate 2" have an equal length LO" which is an overall length of the thermal printing head.
The head substrate 2" has an upper surface formed with a heating resistor line 3" extending longitudinally of the substrate adjacent to one longitudinal edge thereof for linear contact with a platen 19", and an array of drive IC's 4" also extending longitudinally of the substrate. The upper surface of the head substrate 2" is further formed with a plurality of connection terminals 5" arranged adjacent to the other longitudinal edge of the substrate within a limited central portion thereof.
The thermal head further comprises a connector board 6" which includes a flexible film 7" and a reinforcing plate 8". The connector board 6" has a length L1" which is sufficiently smaller than the length LO" of the head substrate 2" because the connection terminals 5" of the head substrate 2" are arranged locally only within the limited central portion.
The flexible film 7" of the connector board 6" has a marginal portion 7a" projecting beyond the reinforcing plate 8". The underside of the marginal portion 7a" is formed with a plurality of connection terminals 9" corresponding to the connection terminals 5" of the head substrate 2".
The underside of the reinforcing plate 8" carries a connector 10" for connection to an external control circuit (not shown). The connector 10" is electrically connected to the connection terminals 9" through a conductor pattern (not shown) formed on the connector board 6".
The connector board 6" is held in place on the heat sink plate 1" by means of an elongate cover member 11" which is substantially equal in length to the head substrate 2". The cover member 11" covers the array of drive IC's 4" for protection, as shown in FIG. 7.
The cover member 11" is provided with non-threaded central holes 12" and non-threaded end holes 15", whereas the heat sink plate 1" is provided with threaded central holes 13" and threaded end holes 16". Central and end screws 14", 17" are inserted through the respective non-threaded holes 12", 15" into engagement with the respective threaded holes 13", 16", thereby fixing the cover member 11" together with the connector board 6" relative to the heat sink plate 1".
The underside of the cover member 11" is formed with a longitudinal groove 11a" at a position corresponding to the projecting marginal portion 7a" of the flexible film 7" for receiving a rod-like pressing rubber member 18" of a constant diameter. Thus, when the cover member 11" is fixed onto the heat sink plate 1", the pressing member 18" presses the marginal portion 7a" to bring the connection terminals 9" of the connector board 6" into intimate electrical contact with the connection terminals of the head substrate 2".
With the arrangement described above, since the connection terminals 5" of the head substrate 2" are arranged locally only in the limited central portion of the substrate, there are at least three advantages, as described in U.S. Pat. No. 4,963,886. First, the length of the connector board 6" can be correspondingly reduced to realize material saving. Secondly, only the two central screws 14" need be tightened with a strictly controlled force because electrical contact between the head substrate 2" and the connector board 6" is needed only in the central portion, so that an assembling operation for the cover member 11" can be facilitated. In the third place, the thermal head can be effectively prevented from thermally bending during operation, thereby avoiding deterioration of the printing quality. The operating mechanism for the prevention of thermal bending is clearly described in U.S. Pat. No. 4,963,886 and therefore not described here.
However, since the pressing rubber member 18" for the prior art thermal head has a constant diameter, the cover member 11" will have a tendency to bulge upward at the portions between the central screws 14" and the end screws 17". More specifically, when the respective screws 14", 17" are tightened up, the rubber member 18" is elastically compressed more at the positions of these screws than at the portions between the respective screws. Since the distance between the two central screws 14" is relatively small, upward bulging of the cover member 11" is less likely to occur at the central portion of the cover member between these two central screws. On the other hand, since the distance between each central screw 14" and each end screw 17" is much larger, the cover member 11" is likely to bulge upward at the portion between the central screw 14" and the end screw 17" due to the reaction force of the rubber member 18" and the inevitable elasticity of the cover member 11", as indicated by phantom lines in FIG. 8.
Once the cover member 11" bulges or bends upward, it may come into interfering contact with the platen 19", consequently hindering proper printing or paper feeding. Further, such an interfering contact may damage the platen 19" due to abrasion.