Wire dot print head

Abstract: In a wire dot print head in which each of armatures (2) are attracted toward a core (9) to resiliently deform a plate spring (3) when an associated coil is not energized, and is released and moved forward by the action of the plate spring when the coil is energized, with the rear surface of the armatures (2) being kept in contact with the front end of the fulcrum members; a partition sheet is disposed between the rear surfaces of the armatures and the front ends of the cores and the front ends of the fulcrum members to interrupt transfer of fluid, and to prevent direct contact between the cores and the fulcrum members, and the armatures. The partition sheet comprises a front plastic film (14), a metallic residual sheet (13), a rear plastic film (15) which are stacked in the stated order from the front side to the rear side; and the front film (14) is bonded to the metallic residual sheet (13) over the entire surface of the metallic residual sheet. By bonding the front film and the metallic residual sheet, migration of grease (20) from the rear side to the front side of the partition sheet is prevented, and adhesion of the grease component to the plate spring is also prevented. Moreover, because of the bonding, the metallic residual sheet (13) is prevented from contacting the air and hence from rusting.

Description: FIELD OF THE INVENTION The present invention relates to a wire dot print head, and is suitable when used as a wire dot print head provided with a metallic residual sheet. BACKGROUND OF THE INVENTION Impact printers are known in which, according to the print information, print wires are driven so that the tips of the print wires are pressed against the print medium to achieve printing. In such an impact printer, a wire dot print head of the plunger type, of the spring charge type, of the clapper type or the like is used. Fig. 1 is a sectional view showing an example of a prior art spring charge type print head. In Fig. 1, print wires 1 are fixed to tips of armatures 2 and are moved toward and away from a platen PL. When the print wires 1 are moved forward, i.e., toward the platen PL, the tips 1a of the wires press the ink ribbon IR against a printing paper PP passing over the platen PL, thereby printing dots on the printing paper PP. It is noted here that the terms "forward" and "front" in connection with the print head are used to mean "toward the platen" and "the side facing or closer to the platen". The terms "rearward" and "rear" are used to mean "away from the platen" and "the side facing opposite to or farther from the platen". The armatures 2 are disposed between radial parts 4a of a front yoke 4 which also has an annular part 4b to which the outer ends of the radial parts 4a are connected. The armatures 2 are mounted to inner ends 3c of radial parts 3a of a biasing plate spring 3 having its annular base part 3b clamped between an annular spacer 5 and an annular hinge plate 6. The armatures 2 are normally magnetically attracted to cores 9. The magnetic attraction force is generated in the core 9 due to a magnetic flux generated by an annular permanent magnet 7 and passing through a generally cup-shaped rear yoke 8 comprising a disk-shaped base 8b and a cylindrical side wall 8c. Each of the radial parts 3a of the plate spring 3 functions as a plate spring independently of each other, so each radial part 3a is also called a plate spring. The cylindrical side wall 8c, the annular permanent magnet 7, the annular hinge plate 6, the annular part 3b of the plate spring 3, the annular spacer 5, the annular part 4b of the front yoke 4 form an annular wall of the print head, while the disk-shaped base 8b of the rear yoke 8 forms the bottom wall of the print head. A guide member 40 comprises a flange part 40a connected to the annular part 4b, and a nose part 40b provided with guide conduit 40c with notches 40d at which wire guides 16 are received. Each of the wire guides 16 have holes through which the print wires 1 are passed such that they are slidably guided for movement toward and away from the platen PL. Cores 9 extend from the front surface 8d of the disk-shaped base 8b of the rear yoke 8. Fulcrum members 8a also extend from the front surface 8d of the disk-shaped base 8b of the rear yoke 8. As is better illustrated in Fig. 2 the fulcrum members 8a are provided in association with respective cores 9 and disposed between the associated cores 9 and the annular wall of the print head. A demagnetizing coil 10 is wound on each of the cores 9 to form an electromagnet, and when a demagnetizing current is made to flow through the demagnetizing coil 10, the electromagnet generates a magnetic flux canceling the magnetic flux due to the permanent magnet 7. Because the magnetic force which attracts the armature 2 to the core 9, bending the plate spring radial part 3a, diminishes, so because of the resilient force of the plate spring, the armature 2 swings forward, i.e., toward the platen PL. Due to the swinging, the print wire 1 moves forward of the print head, being guided by the wire guides 16, and strikes the ink ribbon IR and the printing paper PP to achieve printing. Energization of the demagnetizing coil 10 is terminated at an appropriate time, and a bounding force responsive to the impact on the platen PL acts on the print wire 1. Accordingly, the print wire 1 begins to return backward, i.e., away from the platen PL. Due to the magnetic flux from the permanent magnet 7, the armature 2 is again magnetically attracted to the core 9, and a printing operation of one cycle is thus completed. The swinging motion of the armature 2 is made about the tip 8e of the fulcrum member 8a, as a fulcrum part, so a sliding friction occurs at the fulcrum part 8e. At the time of returning, the armature 2 collides, so the wear due to collision occurs on the core 9. In order to the prevent the wear, a partition sheet including a circular metallic residual sheet 13 is inserted between the core 9 and the plate spring 3, as shown in Fig. 1. Moreover, a front plastic film 14 is inserted between the metallic residual sheet and the plate spring radial parts 3a. More specifically, as shown in Fig. 4, an exploded view, the front plastic film 14 is circular, has the same diameter as the metallic residual sheet 13, and is superimposed with the metallic residual sheet 13 to cover the entirety of the front surface of the metallic residual sheet 13. A rear plastic film 15 is inserted between the metallic residual sheet 13 and the fulcrum parts 8e. More specifically, as shown in Fig. 4, the rear plastic film 15 is annular, has the same outer diameter as the metallic residual sheet 13, and is superimposed with the metallic sheets 13 to cover the entire peripheral part of the rear surface of the metallic residual sheet 13. The metallic residual sheet 13 is formed of a magnetic material, such as silicon steel containing 1 % of silicon. The plastic films 14 and 15 are formed of abrasion-resistant resinous film. The plastic films 14 and 15 are formed with a thickness of several microns. They are therefore difficult to assemble. In order to keep them in the desired shape during assembly, plastic rings 14a and 15a are bonded to the edges of the plastic films 14 and 15. Furthermore, as shown in Fig. 3, the sectional view of the main part, grease 20 for lubrication is applied on the surface of the fulcrum part 8a and the core 9 which face the plate spring radial parts 3a to prevent the wear of these parts. Because of the repeated application of heat and vibration accompanying the printing operation, grease 20 may penetrate between the rear film 15 and the residual sheet 13, and then between the residual sheet 13 and the front film 14, following the path indicated by arrows Pa and Pb in Fig. 3. If a pinhole is formed in the front film 14 due to wear, the grease 20 may ooze out as indicated by arrow PC, and enter the space between the plate spring 3 and the front film 14, and may adhere to the plate spring 3. Thus, a sticking due to grease occurs. The sticking will act as a load when the armature 2 rotates during printing. Accordingly, when such sticking occurs, the pixels are not fully printed. The metallic residual sheet is formed of a ferromagnetic material, and very easily rusts. The metallic residual sheet is therefore given rust-proof treatment. But the required accuracy of the thickness of the residual sheet is very high, and when the variation in thickness of the metallic residual sheet is on the order of several microns, it gives an adverse effect on the operation characteristic of the print head. For this reason, it is difficult to give a complete rust-proof treatment. When, therefore, the humidity is high, or the residual sheet is touched by hand during assembly of the print head, leaving a fingerprint, the life of the print head is shortened. JP-A-1 249 357 discloses a printing head of dot impact printer wherein a film comprising a thin metal plate and a polyester film is inserted between a yoke and an armature. SUMMARY OF THE INVENTION The invention has been made in view of the above, and an object of the invention is to prevent adherence of the grease component to the plate spring, and rusting of the metallic residual sheet. This object is solved by a wire dot print head according to claims 1, 2 and 5. The subclaims show advantageous developments of the invention. By bonding the front film and the metallic residual sheet, migration of grease from one side to the other side of the partition sheet is prevented, and adhesion of the grease to the plate spring is also prevented. Moreover, because of the bonding, contact with the air is prevented, and rusting of the metallic residual sheet is prevented. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view showing the structure of a spring-charge type print head. Fig. 2 is a perspective view showing the fulcrum members. Fig. 3 is a sectional view of the main part of a known wire dot print head. Fig. 4 is an exploded view showing the front film, a metallic residual sheet, and a rear film. Fig. 5 is a sectional view of the main part of the wire dot print head showing an embodiment of the invention. Fig. 6 is an exploded view showing the front film, a metallic residual sheet, and a rear film according to the invention. Fig. 7 is a sectional view of the main part of the wire dot print head of the embodiment of Fig. 5. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 5, Fig. 6 and Fig. 7 show an embodiment of the invention. As illustrated, the wire dot print head of this embodiment is similar to that shown in and described with reference to Fig. 1 to Fig. 3. The following description is directed mainly to the difference of the present embodiment from that of Fig. 1 to Fig. 3. The difference resides in the structure of the partition sheet. In the partition sheet of this embodiment, the front film 14 and the metallic residual film 13 are bonded by an adhesive agent 21, such as heat-resistant epoxy adhesive agent, over the entire surface. Such a configuration enable the method of fabrication in which the front film 14 and the metallic residual sheet 13 are bonded to each other at the stage of materials, and are then cut into circular forms. Accordingly, compared with the prior art method in which the front film 14 and the metallic residual sheet 13 are made into circular forms separately and are then assembled together, the number of process steps is reduced, and the cost of production is lowered. Moreover, by being integrally bonded with the metallic residual sheet 13, the front film 14 maintains the circular form, and the plastic ring along the outer edge is no longer required. In the metallic partition sheet of the embodiment so constructed, the grease component of the lubricating grease 20 penetrates into the interstice between the rear film 15 and the metallic residual sheet 13, due to a capillary phenomenon, as indicated by arrows 23 and 24 in Fig. 5. But, the front film 14 and the metallic residual sheet 13 are blocked by the adhesive agent 21, the grease component cannot get in between the front film 14 and the metallic residual sheet 13. Accordingly, the grease component that has migrated up to the peripheral edge of the metallic residual sheet 13, as indicated by arrow 25, cannot migrate further in the direction of the plate spring 3. The adhesion of grease component to the plate spring 3 is avoided, and good printing is therefore ensured. Moreover, by bonding the front film 14, protection is made such that the front surface of the metallic residual sheet 13 does not contact the air. Accordingly, rusting of the front surface of the metallic residual sheet can be prevented. On the other hand, the rear surface of the metallic residual sheet 13 is protected by the grease 20, and is prevented from rusting. Rusting of the metallic residual sheet 13 is thus avoided without the rust-proof treatment. The life of the print head is lengthened, and a stable printing operation is ensured for a long period. As has been described, according to the invention, the front film and the metallic residual sheet are bonded, so the grease component does not penetrate between the metallic residual sheet and the front film, and the front surface of the metallic residual sheet is protected such that it does not contact the air. Accordingly, the grease component that is applied to the rear side of the metallic residual sheet is prevented from migrating to the front side and adhering to the plate spring. Sticking due to the grease component is eliminated, and undesirable printing operation is prevented. Furthermore, the front surface of the metallic residual sheet is prevented from rusting, and shortening of the life of the print head due to rust is avoided, and a stable operation for a long period is ensured.

Claims: A wire dot print head for printing on a print medium on a platen, comprising: a print wire (1); a plate spring (3); an armature (2) which is fixed to the plate spring, and to which the print wire is fixed such that it can be moved toward and away from the platen; and a wire drive member including a core (9) for attracting the wire support member to the core; said wire drive member further comprising a coil (10) which is wound on the core (9) and which, when energized, generates a magnetic flux in the core, and a permanent magnet; and a partition sheet (13, 14, 15) disposed between the rear surface of the armature (2), on the one hand, and the front end of the core (9) and the front end of a fulcrum member (8a) positioned between the permanent magnet and the core, on the other hand, to interrupt transfer of fluid from the front end of the core (9) and the front end of the fulcrum member (8a) to the plate spring (3); and to prevent direct contact between the core (9) and the fulcrum member (8a) on the one hand, and the plate spring (3) on the other hand; said partition sheet comprising a front plastic film (14), a metallic residual sheet (13), and a rear plastic film (15) which are stacked in the stated order from the front side to the rear side; wherein the front film (14) is bonded to the metallic residual sheet (13) over the entire surface of the metallic residual sheet. A wire dot print head for printing on a print medium on a platen, comprising: a print wire (1) extending forward to the platen; an armature (2) to which a rear end of the print wire is fixed; a core (9) having a forward end thereof adjacent to a rear surface of the armature; a coil (10) wound on the core; an annular side wall (8c); an annular permanent magnet (7) forming part of said annular side wall; a plate spring (3) having a first end fixed near the permanent magnet; said armature being fixed to a second end of the plate spring; a fulcrum member (8a) positioned between the permanent magnet and the core, and having a forward end thereof adjacent to and cooperating with the rear surface of the armature; said armature being attracted toward the core to resiliently deform the plate spring when the coil is not energized; said armature being released and moved forward by the action of the plate spring when the coil is energized; the rear surface of the armature cooperating with the front end of the fulcrum member so that the front end of the fulcrum member forms a fulcrum point for swinging of the armature; and a partition sheet (13, 14, 15) disposed between the rear surface of the armature (2), on the one hand, and the front end of the core (9) and the front end of the fulcrum member (8a), on the other hand, to interrupt transfer of fluid from the front end of the core (9) and the front end of the fulcrum member (8a) to the plate spring (3); and to prevent direct contact between the core (9) and the fulcrum member (8a) on the one hand, and the plate spring (3) on the other hand; said partition sheet comprising a front plastic film (14), a metallic residual sheet (13), and a rear plastic film (15) which are stacked in the stated order from the front side to the rear side; wherein the front film (14) is bonded to the metallic residual sheet (13) over the entire surface of the metallic residual sheet. A print head according to claim 1 or 2, further comprising: a base (8b) connecting the permanent magnet (7) and the core (9); and a front yoke (4) having one end adjacent to the armature (2) and having another end magnetically coupled to the permanent magnet (7). A print head according to claim 3, wherein the fulcrum member (8a) extends from the base (8b). A wire dot print head for printing on a print medium on a platen, comprising: printing wires (1) extending forward toward the platen, generally parallel with each other; armatures (2) in association with the respective print wires; a rear end of each print wire being fixed to a respective one of the associated armatures; cores (9) in association with the respective armatures, each core having its forward end adjacent to a rear surface of a respective one of the armatures; coils (10) in association with the respective cores, each of the coils being wound on the associated core; an annular side wall (8c); an annular permanent magnet (7) forming part of said annular side wall; plate springs (3) in association with the respective armatures, each plate spring having a first end fixed near the permanent magnet and a second end fixed to the associated plate armature; fulcrum members (8a) in association with the respective cores, each fulcrum member being positioned between the permanent magnet and the associated core, and having a forward end adjacent to a rear surface of the respective associated armature; said permanent magnet being in the form of a ring surrounding said armatures, said core, said plate springs and said fulcrum members; each of said armatures being attracted toward the associated core to resiliently deform the plate spring when the associated coil is not energized; each of said armatures being released and moved forward by the action of the associated plate spring when the associated coil is energized; the rear surface of each of the armatures cooperating with the front end of the associated fulcrum member so that the front end of the fulcrum member forms a fulcrum point for swinging of the armature; and a partition sheet (13, 14, 15) disposed between the rear surfaces of the armatures (2), on the one hand, and the front ends of the cores (9) and the front ends of the fulcrum members (8a), on the other hand, to interrupt transfer of fluid from the front end of the cores (9) and the front end of the fulcrum members (8a) to the plate springs (3); and to prevent direct contact between the cores (9) and the fulcrum members (8a) on the one hand, and the plate spring (3) on the other hand; said partition sheet comprising a front plastic film (14), a metallic residual sheet (13), and a rear plastic film (15) which are stacked in the stated order from the front side; wherein the front film (14) is bonded to the metallic residual sheet (13) over the entire surface of the metallic residual sheet. A print head according to claim 5, further comprising: a substantially disk-shaped base (8b) connecting the permanent magnet (7) and the cores (9); a front yoke (4) having one end adjacent to the armatures (2) and having another end magnetically coupled to the permanent magnet (7). A print head according to claim 6, wherein the fulcrum members (8a) extend forward from the base (8b). A print head according to any of the claims 5 to 7, wherein said annular side wall (8c) surrounds said print wires (1), said armatures (2), said plate springs (3), said cores (9) and said fulcrum members (8a). A print head according to any of the claims 5 to 8, wherein said partition sheet (13, 14, 15) is held between the front ends of the fulcrum members (8a) and the rear surfaces of the plate springs (3).

Inventors: IKEDA KIYOSHI C O OKI ELEC IND, KAMIMURA KATSUYA C O OKI ELEC, KOYAMA TATSUYA C O OKI ELEC IN, IKEDA, KIYOSHI, C/O OKI ELEC. IND. CO. LTD., KAMIMURA, KATSUYA, C/O OKI ELEC. IND. CO. LTD., KOYAMA, TATSUYA, C/O OKI ELEC. IND. CO. LTD.
Applicants: OKI ELECTRIC IND CO LTD, OKI ELECTRIC INDUSTRY CO., LTD.
Application year: 1991
Main classification: B41J2
IPC codes: B41K1, B41J2