Abstract:
When the support member and cooling member of the thermal head are made of aluminum, and the head substrate is made of ceramic, the difference in the coefficient of thermal expansion of the two is very large, and the aluminum is expanded in use, and the gap to the adjacent head substrate is increased to cause white stripes. In the invention, the materials are properly selected so that the coefficient of thermal expansion of the support member and cooling member and the coefficient of thermal expansion of the head substrate may be close to each other, and the head substrate is adhered to the cooling plate with a soft adhesive. Therefore, if a difference is caused in the amount of expansion between the head substrate and the cooling plate during use, it is suppressed to such an extent as to be absorbed by the elastic deformation of the soft adhesive.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a thermal head, and more particularly to a long-spanning thermal head constructed by combining a plurality of head substrates matching with the alignment direction of heating resistance elements arranged linearly on head substrates. 
     2. Description of the Prior Art 
     FIG. 1 is a sectional view of a typical conventional thermal head. For example, when thermal printing is attempted in the longitudinal direction of recording paper of A2 format of Japan Industrial Standards as the principal scanning direction, about 600 mm is required as the overall length of the thermal head 1, that is, the printing with W1 in the principal scanning direction. It is difficult to form a small heating resistance element 5 with uniform temperature characteristic over 600 mm on a single head substrate made of, for instance, ceramic. Hitherto, therefore, for example, two head substrates 2, 3 having a printing width W2 of about 300 mm are combined. 
     Such head substrates 2, 3 are composed of, for example, ceramic, of which coefficient of thermal expansion is 
     
         αA=0.73×10.sup.-5 ·C.sup.-1           . . . ( 1) 
    
     Such head substrates 2, 3 are individually affixed to cooling plates 6, 7 in a size corresponding to the head substrates 2, 3 through a soft adhesive layer 4. The cooling plates 6, 7 are composed of a metallic material excellent in thermal conduction, and in the case of aluminum, the coefficient of thermal expansion is 
     
         αB=2.37×10.sup.-5 ·C.sup.-1           . . . ( 2) 
    
     The end face 2a at the head substrate 3 side of the head substrate 2 and the end face 6a of the cooling plate 7 side of the cooling plate 6 are composed flush, and the end face 3a of the head substrate 2 side of the head substrate 3 and the end face 7a of the cooling plate 6 side of the cooling plate 7 are also formed flush. 
     Such cooling plates 6, 7 are fixed in the central position in the principal scanning direction of the cooling plates 6, 7, to support plate 8 made of aluminum or the like by screws or the like. That is, the fixing positions 9, 10 of the cooling plates 6, 7 to the support plate 8 are located at a distance of W2/2 from the central position CN relating to the principal scanning direction of the support plate 8 at the location of the end faces 6a, 7a of the cooling plates 6, 7. 
     The support plate 8 is made of aluminum, and possesses the coefficient of thermal expansion αB. Here, the difference in the coefficient of thermal expansion is 
     
         αB-αA=1.64×10.sup.-5 ·C.sup.-1  . . . ( 3) 
    
     Or the cooling plates 6, 7 and support plate 8 may be made of iron, and the coefficient of thermal expansion αC of such iron is 
     
         αC=1.40×10.sup.-5 ·C.sup.-1           . . . ( 4) 
    
     and the difference from the coefficient of thermal expansion αA is 
     
         αC-αA=0.67×10.sup.-5 ·C.sup.-1  . . . ( 5) 
    
     FIG. 2 is a sectional view for explaining the problems of this prior art. For example, the thermal head 1 changes from ordinary temperature of 25° C. to high temperature to 90° C. along with the thermal printing action, assuming that the cooling plates 6, 7 and support plate 8 are made of iron, the fixing positions 9, 10 of the support plate 8 and the cooling plates 6, 7 are dislocated in the mutually departing directions by the variations x3a, x3b as shown in FIG. 2. On the other hand, the cooling plates 6, 7 fixed in the fixing positions 9, 10 of the support plate 8 is dislocated in the mutually departing direction from the central position CN are expanded by the variations x2a, x2b from the fixed positions 9, 10. The head substrates 2, 3 are similarly expanded by the variations x1a, x1b. 
     According to the experiment by the present inventor, in such prior art, the results of measurement as shown in Table 1 were obtained. 
     
                       TABLE 1______________________________________  Substrate Cooling plate                        Support plate  x1a   x1b     x2a     x2b   x3a   x3b______________________________________Coefficient of    0.73 × 10.sup.-5                1.40 × 10.sup.-5thermal expan-sion (°C..sup.-1)Elongation    -71.2   -71.2   -136.5                          -136.5                                +136.5                                      +136.5(μm)______________________________________ 
    
     It is known from Table 1 that the interval W3 between the head substrates 2, 3 is extended by 
     
         W3=x3a+x1a+x3b+x1b=130.6 μm                             . . . (6) 
    
     This extending amount is a length more than 1 dot if the density of the small heating resistance element formed along the principal scanning on the head substrates 2, 3 by the thermal head is 8 dots per 1 mm, that is, 125 μm pitch, and therefore an unprinted white stripe is left over on the recording paper, or a white-out phenomenon occurs. Such white-out seriously lowers the printing quality. 
     FIG. 8 is a plane view of a head substrate 2. On the head substrate 2, the heating resistance element 5 is formed linearly, and the length W2 along the principal scanning direction of this heating resistance element 5 is the printing width W2. The present inventor measured the degree of warp in the thicknesswise direction (the vertical direction to the sheet of paper in FIG. 8) of the head substrate 2 due to temperature rise at five observation points P1 to P5 at mutually equal intervals. The results of observation are shown in Table 2. 
     
                       TABLE 2______________________________________                Variation of warp due toCooling plate     Temperature                temperature change (μm)material  change     P1      P2   P3   P4   P5______________________________________Aluminum  25° C.                0       -29  -53  -33  0cooling plate     ↓     90° C.______________________________________ 
    
     The mode of change is as indicated solind line L1 in FIG. 9. Generation of such warp is same as in the head substrate 3. 
     When such head substrates 2, 3 are used, the head substrates 2, 3 are warped due to temperature rise in use, and when the head substrates 2, 3 perform thermal printing, the pressing force of pressing the platen roller to the thermal paper, for example, is not uniform in the principal scanning direction, which may result in uneven printing. 
     SUMMARY OF THE INVENTION 
     It is hence a primary object of the invention to solve the above technical problems and present a thermal head capable of equalizing thermal printing action of high quality regardless of temperature changes. 
     To achieve the above object, the invention presents a thermal head characterized by fixing a plurality of cooling members on a support member, and fixing head substrates on which multiple heating resistance elements are arranged individually to the cooling members, in which the relation between the coefficient of thermal expansion α of the head substrate and the coefficient of thermal expansion γ of the support member is as follows: 
     
         0.8≦ατ≦1.2 
    
     According to the invention, using plural head substrates having multiple heating resistance elements disposed on the surface, cooling members are adhered to the opposite side surface of the heating resistance elements of the head substrates. The cooling plates are disposed on the support member along the alignment direction of the heating resistance elements. Here, the coefficient of thermal expansion α of the head substrate and the coefficient of thermal expansion γ of the support member are selected in the relation of 
     
         0.8≦2/α≦1.2 
    
     The interval of the plural head substrates is determined by the expansion of contraction amount due to temperature changes of the support member, that is, by the moving extent of each head substrate due to this expansion or contraction and the expansion or contraction amount due to thermal changes of the head substrate itself. Therefore, when the coefficients of thermal expansion α, γ of the head substrate and support member fulfill the above relation, the interval of the plural head substrates maintains an interval nearly equal to the interval set preliminary at ordinary temperature. It is therefore effect to prevent occurrence of the region not responsible for printing due to expansion of the interval of the head substrates due to, for example, temperature rise. Or, if a gap is produced at the cooling plate side of the head substrate due to temperature rise, it is possible to prevent printing with lowered contrast due to insufficient cooling action of the heating resistance elements on the end substrate corresponding to such gap. 
     Thus, according to the invention, the head substrates are disposed on the support member along the alignment direction of heating resistance elements. Here, the first coefficient of thermal expansion of the head substrate and the third coefficient of thermal expansion support member are selected to be nearly equal to each other, while the second coefficient of thermal expansion of cooling member is nearly equivalent or superior to the first coefficient of thermal expansion. The interval of plural head substrates is determined by the expansion or contraction amount due to temperature changes of the support member, that is, by the expansion or contraction amount by thermal changes of the head substrate itself and the moving amount of each head substrate due to expansion or contraction. Therefore, when the coefficient of thermal expansion of the head substrate and support member is defined in the above expression, the interval of plural head substrates maintains the interval nearly equal to the interval set preliminarily at ordinary temperature. Accordingly, it is effective to prevent occurrence of region not responsible for printing due to expansion of the interval of the head substrates due to, for example, temperature rise. For example, if the gap is reduced at the cooling member side of the head substrate at the time of temperature rise, it is effective to prevent printing action with lowered contrast due to insufficient cooling action of the heating resistance elements on the head substrate corresponding to the gap. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other and further objects, features, and advantages of the invention will be more explicit from the following detailed description taken with reference to the drawings wherein: 
     FIG. 1 is a sectional view of a typical conventional thermal head 1, 
     FIG. 2 is a front view near the central position CN of the thermal head 1, 
     FIG. 3 is a perspective view of a thermal head 11 in an embodiment of the invention, 
     FIG. 4 is a magnified sectional view near an abutting position 34 of the thermal head 11, 
     FIG. 5 is a sectional view near the thermal head 11, 
     FIG. 6 is a sectional view over the entire length of the printing width of the thermal head 11, 
     FIG. 7 is a diagram for explaining the principle of setting the projection length d in the dame embodiment, 
     FIG. 8 is a plan view showing the warp measuring position of the thermal heads 1, 11, 
     FIG. 9 is a graph showing the state at various temperatures of the thermal head 1, 11a, 
     FIG. 10 is a perspective view of a thermal head 11a in other embodiment of the invention, 
     FIG. 11 is a sectional view of a thermal head 11a, and 
     FIG. 12 is a partial sectional view of the thermal head 11a. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now referring to the drawing, preferred embodiments of the invention are described below. 
     FIG. 3 is a perspective view of a thermal head 11 in an embodiment of the invention, and FIG. 4 is a sectional view near the central position 34 along the principal scanning direction of the thermal head 11. The thermal head 11 is made of, for example, ceramics such as aluminum oxide Al 2  O 3 , in a rectangular form in a width of W6 along the principal scanning direction (for example, 300 mm), and comprises head substrates 12a, 12b with the coefficient of the ram expansion of αA=0.73×10 -5  ·C -1 . On each one of the head substrates 12a, 12b, for example, plural heating resistance elements 13 made of tantalum nitride Ta 2  N, nichrome Ni--Cr, and ruthenium oxide RuO 2  are formed by thin film technology such as evaporation and sputtering, thick film technology such as screen printing, or etching technology, linearly at alignment interval of δ1 (for example, about 20 μm) and in alignment pitch of δ3 (for example, about 40 μm), at a distance of δ4 (for example, 10 μm) from the end faces 31a, 31b of the head substrates 12a, 12b. 
     The heating resistance element 13 performs thermal printing on thermal recording paper or thermal film and recording pater, and raises the temperature to 400° C. when powered. The thermal head 11 is composed in an overall width W7 of about 600 mm along the principal scanning direction, when thermally recording in a recording paper in, for example, A2 format of the Japanese Industrial Standard in the longitudinal direction as the principal scanning direction. At this time, the heating resistance elements 13a, 13b at the outermost end portion on the head substrates 12a, 12b are defined to be spaced by a distance δ2 (for example 20 μm). 
     The heating resistance elements 13 are connected parallel to common electrodes 14 on every head substrate 12a, 12 b, and individual electrodes 15 are connected to the opposite side of the common electrodes 14 of the heating resistance elements 13. The individual electrodes 15 are connected to driving circuit elements 16 in every predetermined number of pieces, and plural signal lines 17 are connected to the plural driving circuit elements 16 for feeding the image data and control signals for printing on the heating resistance elements 13. The common electrodes 14, the individual electrodes 15, and the signal lines 17 are composed of metal such as aluminum Al and gold Au, and are formed by thin film technology or thick film technology. 
     Such head substrates 12a, 12b are installed, by a soft adhesive 18, in the configuration as described below, at the central positions 37, 38 along the principal scanning direction of the cooling plates 19a, 19b, to the cooling plates 19a, 19b formed in a rectangular plate form with a width W8 in the principal scanning direction, of a metal material having the coefficient of thermal coefficient relatively close to that of the head substrates 12a, 12b and an excellent thermal conductivity, such as Fe-Ni alloy of JIS C 2351 PB with the coefficient of thermal expansion of 
     
         αD=0.70×10.sup.-5 ·C.sup.-1           . . . (7) 
    
     and the cooling plates 19a, 19b with an overall width W9 along the principal scanning direction are fixed on the support plate 20 made of metal material such as Fe-Ni alloy. 
     FIG. 5 is an overall sectional view of the thermal head 11. The thermal head 11 has, in addition to the constitution described above, the driving circuit elements 16 covered with a protective layer 21. The area near the opposite ends of the driving circuit elements 16 of the signal lines 17 is connected to an elastic wiring substrate 24 having a circuit wiring 23 formed on an elastic film 22. This elastic wiring substrate 24 is disposed on the cooling plates 19a, 19b by soft adhesive 18 through a spacer 25. Besides, to cover a range from the individual electrodes 15 to the elastic wiring substrates 24, a head cover 26 is disposed, and the head cover 26, the elastic wiring substrate 24, and spacer 25 are fixed to the cooling plates 19a, 19b with screws 27. 
     Such thermal head 11 is disposed closely to a platen roller 29, and the heating resistance element 13 presses the thermal recording paper 30 on the platen roller 29 against the platen roller 29, while the heating resistance elements 13 are selectively energized and de-energized, desired printing is made. 
     FIG. 6 is a sectional view along the entire principal scanning direction of the thermal head 11, and FIG. 7 is a sectional view near the central position 33 of the thermal head 11 for explaining the action of this embodiment. They are mutually fixed at the central positions 37, 38 in the principal scanning direction of the cooling plates 19a, 19b remote from the central position 33 of the thermal head 11 by distance W11 (=W6/2). Therefore, along with the use of the thermal head 11, when the temperature changes from ordinary temperature of 25° C. to high temperature of 90° C., the expansion amounts x1a, x1b of the head substrates 12a, 12b, the expansion amounts x2a, x2b of the cooling plates 19a, 19b, and the expansion amounts x3a, x3b of the support plate 20 will be as shown in Table 3. 
     
                       TABLE 3______________________________________  Substrate Cooling plate                        Support plate  x1a   x1b     x2a     x2b   x3a   x3b______________________________________Coefficient of    α A = α D = 0.70 × 10.sup.-5thermal expan-    0.73 × 10.sup.-5sion (°C..sup.-1)Elongation    -71.2   -71.2   -68.3 -68.3 +68.3 +68.3(μm)______________________________________ 
    
     It is known from Table 3 that the interval W10 of the head substrates 12a,12b is contracted by 
     
         ΔW10=x3a+x1a+x3b+x1b=-5.8 μm                      . . . (8) 
    
     Therefore, between the head substrates 12a, 12b, and the cooling plates 19a, 19b, a shearing force parallel to the principal scanning direction acts, but this shearing force is small enough as to be absorbed as an elastic deformation of the soft adhesive layer 18 intervening between the two. At this time, the ratio of the coefficients of thermal expansion αA, αD is 
     
         αA/αD=1.043                                    . . . (9) 
    
     Thus, in this embodiment, widening of the gap between the head substrates 12a, 12b may be prevented, and even if the thermal head 11 is heated to a high temperature, white-out due to widening of the gap as explained in relation to the prior art may be prevented. 
     In this embodiment, the head substrates 12a, 12b and the cooling plates 19a, 19b are only mutually affixed with soft adhesive layer 18, while the cooling plates 19a, 19b and the support plate 20 are fastened with screws. Therefore, as compared with the prior art of fixing the head substrate 12a, 12b with hard adhesive or mechanism so as not to separate, the manufacturing process is saved and the constitution is simplified. 
     FIG. 8 is a drawing referred to in the prior art, and it is also referred to in this embodiment. Concerning the linear heating resistance elements 13 on the head substrate 12a, same as in the prior art, the warp of the head substrate 12a was measured at five positions of equal intervals including the both ends in the principal scanning direction of the heating resistance elements 13. The results are shown in Table 4. 
     
                       TABLE 4______________________________________                Wrap change due toCooling plate     Temperature                temperature change (μm)material  change     P1      P2   P3   P4   P5______________________________________Fe--Ni alloy     25° C.                0       +1   -3   -2   0     ↓     90° C.______________________________________ 
    
     Meanwhile, the warp state is indicated by broken line L2 in FIG. 9. 
     Thus, in the embodiment, the head substrates 12a, 12b can prevent uneven concentration due to uneven pressure in the principal scanning direction on the thermal recording paper 30 against the platen roller 29 by warping. 
     In this embodiment, relating to the coefficients of thermal expansion of the head substrates 12a, 12b, cooling plates 19a, 19b, and support plate 20, α (=αA), β (=αD), γ (=αD), the materials are selected so as to satisfy the relation of 
     
         α≈β≈γ                     . . . (10) 
    
     but the materials are not limited to Fe-Ni alloy, but Cu-W alloy or Ti alloy (for example, JIS H 4600) may be used. 
     In this embodiment, it is intended to control the distance δ2 of the heating resistance elements 13a, 13b at the outermost end parts on the head substrates 12a, 12b shown in FIG. 4 at less than the alignment pitch δ3 of the heating resistance elements 3. As a result of the thermal head 11 of this embodiment, as far as the distance δ2 of the heating resistance elements 13a, 13b is less than the alignment pitch δ3 of the heating resistance elements 13, the white stripe (white-out) in thermal printing is not so obvious, and it is found to be obvious when becoming about 50 μm, exceeding the alignment pitch δ2 (for example, 40 μm). 
     If, the distance of 4 between the heating resistance elements 13a, 13b and the respective outermost end parts of the head substrates 12a and 12a, and the separation between the end faces 31a, 31b is about, for example, 10 μm respectively, white stripe is not so obvious until the separation between the end faces 31a and 31b is widened by about 20 μm. In such variations of the interval W10, it has been known that the coefficient of the thermal expansion γ of the support plate 20 is increased about 15% with respect to the coefficient of thermal coefficient α of the head substrates 12a, 12b. 
     On the other hand, when the distance δ4 between the heating resistance elements 13a, 13b and the respective outermost end parts of the head substrate 12a, 12 b and the separation between the end faces 31a, 31b is substantially zero in the thermal head, it has been confirmed that the ratio of the coefficient of thermal expansion α and the coefficient of thermal expansion δ of the support plate 20 is permitted up to 20% increase. Therefore, corresponding to the type of structure of the thermal head, the ratio of the coefficient of thermal expansion α of the head substrates 12a, 12b and the coefficient of thermal expansion γ of the support plate 20 is known to be selected as 
     
         0.8≦α/τ≦1.2                        . . . (11) 
    
     
         0.85≦α/τ≦1.15                      (12) 
    
     FIG. 10 is a perspective view of a thermal head 11a in other embodiment of the invention, and FIG. 11 is a sectional view along the overall length of the printing width of the thermal head 11a. This embodiment is similar to the foregoing embodiment, and corresponding parts are identified with same reference number. What is of note in this embodiment is that the thermal head 11a is composed of three head substrates 12a, 12b, 12c, and three cooling plates 19a, 19b, 19c, which are mutually adhered with a soft adhesive layer 18. The thermal head 11a of this embodiment is a combination of three head substrates 12a to 12c having the printing width W6 of the foregoing embodiment, and is intended to print on a recording paper of, for example, AO format of JIS, realizing an overall width of W12 (for example, 1000 mm). The cooling plates 19a to 19c are affixed to the support plates 20 with, for example, screws at the central positions 38, 39, 40 in the principal scanning direction. In this embodiment, the coefficient of thermal expansion α of the head substrates 12a to 12c, the coefficient of thermal expansion β of the support plate 20, and the coefficient of thermal expansion γ of the cooling plates 19a to 19c are selected to satisfy the relation of 
     
         α≈γ&lt;γ                            . . . (13) 
    
     As such materials, the head substrates 12a to 12c are made of ceramic material having the same coefficient of thermal expansion as the foregoing embodiment, the cooling plates 19a to 19c are made of aluminum (JIS H 4100, A6063-T5), and the support plate 22 is made of Fe-Ni alloy (JIS C 2531, PB), of which case is explained below. 
     FIG. 12 is a sectional view for explaining the action of this embodiment. In this embodiment, the variation ΔW3 due to temperature change of the gap W between the head substrates 12b, 12c is determined as follows 
     
         ΔW3=x3c+x1c+x1b                                      . . . (14) 
    
     on the basis of the variation amounts x1b, x2b, x3c of the head substrate 12b, cooling plate 19b and support plate 20 from the central position 33, and the variation amounts x1c, x2c of the head substrate 12c and the cooling plate 19c from the fixed position 39. Therefore, as a result of the experiment by the present invention on the change of the gap W3 same as in the foregoing embodiment, the findings are obtained as shown in Table 5. 
     
                       TABLE 5______________________________________            Coefficient of   Temperature            thermal expansion                          Elongation   change (°C.)            (1/°C.)                          Δx(μm)______________________________________Substrate  x1b    65.0       0.73 × 10.sup.-5                                 71.2  x1cCooling  x2b               2.37 × 10.sup.-5                                231.1plate  x2cSupport  x3c               0.70 × 10.sup.-5                                136.5plate______________________________________ Elongation = coefficient of thermal expansion × temperature change × distance from reference position 
    
     In this example, the variation of the gap W3 is ΔW3 =-5.9 μm, and it means that 5.9 μm is contracted. The change of this extent is small enough to be absorbed by the elastic deformation of the soft adhesive layer 18 between the head substrate 12a to 12c and the cooling plates 19a to 19c. In such embodiment, too, the same effects as in the foregoing embodiment are achieved. 
     In the thermal head 11a of this embodiment, it is intended to suppress the gap W3 of the head substrates 12a  to 12c at less that the alignment pitch δ3 of the heating resistance elements 13. Accordingly, the variation amount ΔW3 of the gap W3 is controlled under the alignment pitch δ1 of the heating resistance elements 13. The principle for realizing such action is explained below. Concerning the coefficient of thermal expansion α of the head substrates 12a to 12c and the coefficient of thermal expansion γ of the support plate 20, when the temperature changes from 25° to 90° C. by 65° C., the variation amounts x1b, x1c or the head substrates 12b, 12c shown in FIG. 12 are ##EQU1## and the variation amount x3c of the support plate 20 is ##EQU2## 
     Therefore, the variation amounts ΔW3 of the gap W3 is ##EQU3## Here, seeing α=0.73×10 -5  ·C -1 , and supporting 
     
         ΔW3≦20 μm                                  . . . (18) 
    
     equation (18) us modified as 
     
         19500(γ-0.73×10.sup.-5)≦20×10.sup.-3 . . . (19) 
    
     so that 
     
         γ≦0.832×10.sup.-5 ·C.sup.-1    . . . (20) 
    
     is obtained. That is, as the coefficient of thermal expansion of the support plate 20, it is confirmed that 
     
         0.73×10.sup.-5 ≦γ≦0.832×10.sup.-5 . . . (21) 
    
     is desired. 
     The invention may be executed in a wide range of metal materials, not limited to the materials presented in the foregoing embodiment, as fast as the material possesses the coefficient of thermal expansion γ expressed in equation (21). 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and the range of equivalency of the claims are thereof intended to be embraced therein.