Serial impact printer

Abstract: An object of this invention is to provide a serial impact printer which has a high sheet conveyance accuracy and is relatively quiet during printing. In a serial impact printer, tension rollers 2 and a sheet retaining roller 5 confronted with the latter are provided downstream of a platen 1. A guide member 20 is provided to guide a printing sheet P to those rollers. The guide member 20 has a downstream end portion 202 which is protruded towards the platen beyond the tangental line from the platen to the tension rollers.

Description: The present invention relates to a serial impact printer and more particularly to, but not exclusively to, the structure of a serial impact printer in which a tension roller is used to give the tension to a sheet to be printed and to the structure of a sheet guide for the purpose of reducing the volume of noise produced in the printer. A conventional serial impact printer concerning the present invention will be described with reference to figure 4. This kind of apparatus is also disclosed in e.g. DE-A-34 21 936. Figure 4 is a sectional view outlining the conventional serial impact printer. In figure 4, reference numeral 1 designates a platen; 2, tension rollers (only one is shown); and 3, 4 and 5, sheet retaining rollers. The platen 1 and the tension rollers 2 are rotated in the directions of the arrows A and B, respectively, to convey a printing sheet P in the direction of the arrow C. Further in figure 4, reference numeral 6 designates a sheet retaining plate adapted to prevent a printing sheet P from being raised when it is conveyed to a printing region; 7, a sheet guide; 8 and 9, discharging sheet guides for guiding the front end portion of a printed sheet to the tension rollers 2; 10, an impact dot head; and 11, a carriage on which the head 10 is mounted. The carriage 11 is guided by guide shafts 12 and 13, to move over the printing sheet and vertically along the platen 1. Further in figure 4, reference numeral 14 designates a guide member mounted on the carriage 11. The guide member 14 is adapted to guide the printing sheet P along the outer cylindrical wall of the platen 1 near the printing region where the platen 1 is presented to the head 10. In order to guide the front end portion of the sheet P along the platen 1 and towards the tension rollers 2, the guide member 14 is so shaped that a small gap is formed between its guide surface and the tangent line from the outer cylindrical wall of the platen 1 to the tension roller 2. In the conventional serial impact printer thus constructed, the tension rollers 2 are rotated at a peripheral speed slightly higher than the platen 1 so that the conveyance of the sheet P by the platen 1 is assisted. Also by this higher peripheral speed, tension is suitably applied to the sheet P to prevent the latter from being raised and to reduce the volume of noise produced during printing. Although only one tension roller 2 is shown in figure 4, in fact a plurality of tension rollers 2 are arranged along the direction of axis of the platen 1 and each in parallel with the latter. During printing, the guide member 14 is not in contact with the sheet P as shown in figure 4. The above-described serial impact printer suffers from the following problems to be solved: In general, in a serial impact printer, sheets having a width of 10.16 to 40.64 cm (four to sixteen inches) are used. As was described above, a plurality of tension rollers 2 are arranged in the direction of axis of the platen 1. Hence, in the sheet P, the tension is distributed in the direction of width. That is, tension applied to the part of the sheet in contact with the tension rollers 2 is satisfactory. However, the tension applied to the part of the sheet where none of the tension rollers are in contact is unsatisfactory in that the sheet may become raised and it is difficult to prevent the production of noises during printing. In order to uniformly distribute tension over the sheet, a tension roller may be employed which is a wide as the platen 1. However, the method is still disadvantageous in that such components increase the manufacturing cost, and accordingly it is difficult to reduce the manufacturing cost of the printer. Moreover, since it is essential to polish the outer cylindrical wall of the tension roller, it is impossible to reduce the diameter of the tension roller; that is, it is difficult to achieve miniaturisation of the tension roller. In order to maintain a suitable tension at the part of the sheet where none of the tension rollers 2 are in contact, the sheet conveyance force of the tension rollers 2 should be increased. However, in this case, since the peripheral speed of the tension rollers 2 must be higher than that of the platen 1, the sheet P is caused to slip on the platen 2, and it is therefore difficult to convey the sheet with high accuracy. In the case where a sheet of continuous form is used with a push type tractor, the printer is operated with the sheet retaining rollers 3 and 4 released, and therefore, the sheet P is more liable to slip on the platen 1, and at worst it may go over the tractor pins. Accordingly, an object of this invention is to eliminate the above-described difficulties accompanying a conventional serial impact printer. More specifically, an object of the invention is to provide a serial impact printer which is low in manufacturing cost and can be miniaturised, and which can convey a printing sheet with high accuracy and produces less noise. The foregoing object of the invention has been achieved by the provision of a serial impact printer comprising: a platen; an impact dot head facing said platen; one or more tension rollers located downstream of said platen and said impact dot head in a conveyance direction of a printed sheet; a guide member which moves with said impact dot head for guiding said sheet over said platen after said sheet passes said impact dot head, characterised in that said guide member has an end portion protruding beyond the tangential path between the platen and the or each tension roller towards the platen for improved guidance of said sheet to the or each tension roller. Also there is provided a serial impact printer comprising: tension rollers positioned downstream of a region, in the direction of conveyance of a printing sheet, where a platen and an impact dot head confront with each other; and a guide member which is moved together with said head, to guide said sheet along the platen near the region where the platen and the head confront with each other; in which, according to the invention, the guide member has an end portion which is protruded towards the platen through a plane which is defined by the portions of the tension rollers and platen which are brought into contact with the sheet. In the printer, the peripheral speed of the tension rollers is higher than that of the platen. Embodiments of the present invention will now be described with reference to the accompanying figures, of which; figure 1 is a sectional view illustrating an embodiment of a serial impact printer according to the present invention; figure 2 is a perspective view of a guide member in the printer shown in figure 1; figure 3 is a sectional view illustrating a sheet path in the printer shown in figure 1; and figure 4 is a sectional view showing a conventional serial impact printer. One preferred embodiment of this invention will be described with reference to figures 1 through 3. Figure 1 is a sectional view outlining a conventional serial impact printer according to the invention. In figure 1, reference numeral 1 designates a platen; 2, tension rollers (only one shown); and 3, 4 and 5 sheet retaining rollers. The platen 1 and the tension rollers 2 are rotated in the directions of the arrows A and B, respectively, to convey a printing sheet P in the direction of the arrow C. The tension rollers 2 are rotated at a peripheral speed slightly higher than the platen 1, and are arranged in the direction of axis of the platen 1 and in parallel with the latter. Further in figure 1, reference numeral 6 designates a sheet retaining plate adapted to prevent a printing sheet P from being raised which is being conveyed to a printing region; 7, a sheet guide; 8 and 21, discharging sheet guides for guiding the front end portion of a printed sheet to the tension rollers 2; 10, an impact dot head; and 11, a carriage on which the head 10 is mounted. The carriage 11 is moved in a direction perpendicular to the guide shafts 12 and 13. Further in figure 1, reference numeral 20 designates a guide member mounted on the carriage 11. The guide member 20 is adapted to guide the printing sheet P along the outer cylindrical wall of the platen 1 near the printing region where the platen 1 confronts the head 10. The guide member 20 has a guide surface 201 which is curved where it is adjacent to the cylindrical surface of the platen 1. That is to say, the guide surface 201 is so shaped as to guide the front end portion of the sheet P along the platen 1 and towards the tension rollers 2. The guide member 20 has an end portion 202 on the side of the guide member 20 towards the platen 1. The end portion 202, as shown in figure 1, is slightly protruded towards the platen 1 beyond a sheet path between the platen 1 and the tension rollers 2. That is to say, beyond a tangent line T (cf. figure 3) from the platen to the tension rollers. The end portion 202, as shown in figure 2, is positioned over the printing wire section (not shown) of the printing head, and protrudes towards the platen 1. As shown in figure 3, the discharging sheet guide 21 has a guide surface 211 which is so shaped as to guide the front end portion of the sheet P towards the tension rollers 2 after having been guided round the platen 1 by the guide surface 201 of the guide member 20. Hence, the front end portion of the sheet P is guided as indicated by the broken line P1 in figure 3. In figure 3, the one-dot chain line T is the aforementioned tangent line of the platen 1 to the tension rollers 2. In the serial impact printer thus constructed, the printing sheet P behaves as follows when printed. The portion of the sheet P which is not confronted with the end portion 202 of the guide member 20, forms a sheet path which is substantially similar to the tangent line T between the platen 1 and the tension rollers 2. However, the portion of the sheet P which is confronted with the end portion 202 of the guide member 20 forms a sheet path curved slightly counterclockwise as indicated by the solid line in figure 3 by the sheet P contacting the end portion 202. Hence, even the portion of the sheet P which is not confronted with the tension rollers 2 and heretofore became raised, forms a sheet path curled slightly counterclockwise by being brought into contact with guide member 20 when the printing head 1 is moved over it. Thus, the sheet path is slightly increased in length and the tension of the sheet P is increased. That is to say, tension is suitably applied to the sheet P, thus preventing the sheet from being raised. As is apparent from the above description, the area of the part of the sheet which is brought into contact with the platen is increased, the sheet flutters less when struck by the printing wires of the impact dot head, and accordingly the printing is quieter. The following Table 1 indicates the results of noise measurements produced when letters of the alphabet were printed at a rate of about ninety characters per second. In the measurement, the tension rollers 2 were spaced about 50mm from one another, and the tension was about 20 g per 50 mm, and the amount of protrusion of the guide member end portion 202 was changed from 0 to 3 mm, and the width of the guide member end portion 202 was changed to 10 mm, 25 mm, and 50 mm. As is seen from the above Table 1, in order to decrease the volume of noise, it is preferable that the width of the guide member end portion 202 is at least half of the space between the tension rollers 2, and the amount of protrusion of the guide member end portion is 0.5 mm or more. When the amount of protrusion of the guide member end portion is 3.0 mm or more, the guide member end portion strongly rubs the surface of the sheet which has been just printed, thus making the resultant print dirty. Hence, it is preferable that the amount of protrusion of the guide member end portion 202 is in a range of 0.5mm to 3.0mm but most preferably in the range of 1.0 mm to 2.5 mm. The serial impact printer thus constructed according to the invention has the following merits or effects: The part of the guide member, which is moved together with the impact dot head, is protruded towards the platen to contact the sheet, so that, at the contacting point, the sheet path is bent outwardly to increase the tension applied to the sheet. This prevents the portion of the sheet from being raised which is locate downstream of the printing section. Hence, although a plurality of tensions rollers are arranged in the direction of axis of the platen and in parallel with the latter, tension is suitably applied to the part of the sheet which is located downstream of the printing region. Therefore, even though tension rollers small in diameter can be used, the volume of noise can be decreased. Thus, the serial impact printer can be low in manufacturing cost, small in size, and relatively quiet. Furthermore, with the printer of the present invention, the sheet is prevented from being raised by only slightly increasing the sheet conveying force of the tension rollers. Hence, the printer of the present invention does not have a lowered sheet conveying accuracy, and the sheet goes over the pins when a push type tractor is used. Thus, the serial impact printer of the invention is excellent in the conveyance of a printing sheet and relatively quiet.

Claims: A serial impact printer comprising; a platen (1); an impact dot head (10) facing said platen; one or more tension rollers (2) located downstream of said platen (1) and said impact dot head in a conveyance direction of a printed sheet; a guide member (20) which moves with said impact dot head (10) for guiding said sheet over said platen after said sheet passes said impact dot head, characterised in that said guide member has an end portion (202) protruding beyond the tangential path (T) between the platen (1) and the or each tension roller towards the platen for improved guidance of said sheet to the or each tension roller. A serial impact printer as claimed in claim 1, in which said end portion (202) protrudes beyond the tangential path by an amount x in the range O.5 < x < 3mm. A serial impact printer as claimed in claim 1, in which said end portion (202) protrudes beyond the tangential path by an amount x in the range 1 < x < 2.5mm. A serial impact printer as claimed in claim 1, 2 or 3, in which there are two or more tension rollers (2) sharing a common axis and one or more guide members (20) such that each guide member (20) is disposed between adjacent tension rollers (2). A serial impact printer as claimed in claim 4, in which the lateral dimension of the or each guide member parallel to said common axis of the tension rollers is greater than or equal to half the distance between the respective tension rollers (2). A serial impact printer comprising: tension rollers positioned downstream of a region, in the direction of conveyance of a printing sheet, where a platen and an impact dot head confront with each other; and a guide member which is moved together with said head, to guide said sheet along said platen near said region where said platen and said head confront with each other, CHARACTERISED in that said guide member has an end portion which is protruded towards said platen through a plane which is defined by the portions of said tension rollers and platen which are brought into contact with said sheet. A serial impact printer as claimed in claim 6, CHARACTERISED in that said tension rollers are higher in peripheral speed than said platen.

Inventors: HIRANO SEIICHI C OM SEIKO EPSO, HIRANO, SEIICHI, C/OM SEIKO EPSON CORP.
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1992
Main classification: B41J13
IPC codes: B41J13, B41J11