Printer having ribbon mask for reducing interference with recording sheet
FIELD OF THE INVENTION AND RELATED ART STATEMENT 
     The present invention relates to a printer having a platen around which a recording sheet is wound for printing and, more particularly, to an impact serial printer. 
     A conventional impact serial printer will be described with reference to FIGS. 5 to 9. A cylindrical platen 1 is driven for rotation by a platen driving unit, not shown. A carriage guide shaft 4 is extended with its axis in parallel to that of the platen 1, and a carriage 3 mounted with a print head 2 is supported on the carriage guide shaft 4 for sliding movement along the carriage guide shaft 4. The carriage 3 is driven for back-and-forth movement by a carriage driving unit, not shown. The print head 2 is of a dot matrix print type provided on its front end with a plurality of wires 2a having front ends arranged in a single column. A wire driving unit moves the wires 2a back and forth. The print head 2 is disposed with the front ends of the wires 2a opposite the platen 1 with a small gap therebetween. The print head 2 may be provided with a plurality of wires having front ends arranged in two or more columns instead of a single column. 
     A ribbon mask 10 is attached to the carriage 3 so as to be positioned between the platen 1 and the print head 2. The ribbon mask 10 consists of a frame 10a screwed to the carriage 3, and a very thin, filmy masking plate 10b attached to the frame 10a. The masking plate 10b is provided with an aperture 11 through which the front ends of the wires 2a provided on the front end of the print head 2 are allowed to project. The shape of the aperture 11 is a rhombus (FIG. 7), an ellipse (FIG. 8) or a hexagon (FIG. 9). The aperture 11 of either shape is symmetric with respect to a platen center C, which is a normal to the platen 1 at a position on the platen nearest to the front end of the print head 2. Accordingly, a portion of the ribbon mask 10 corresponding to the platen center C is nearest to the platen 1 as well as the front end of the print head 2. The platen center C in FIGS. 7, 8 and 9 is a path along which a point on the print head 2 corresponding to the platen center C moves as the print head 2 moves. 
     A ribbon feed mechanism, not shown, is mounted on the carriage 3 to feed an ink ribbon 5 through a space between the platen 1 and the print head 2. As shown in FIG. 6, the ink ribbon 5 extends behind the ribbon mask 10 on the side of the print head 2. 
     A recording sheet P is fed between the platen 1 and the ribbon mask 10, and then the wires 2a of the print head 2 are driven selectively to impact the platen 1 with the front ends of the wires 2a; consequently, portions of the ink ribbon 5 corresponding to the wires 2a impacting on the platen 1 are transferred to the recording sheet P to print dots. The carriage 3 is driven by the carriage driving unit to print a line of characters by moving the print head 2 along the axis of the platen 1 in a direction indicated by double-head arrow X in FIG. 5. Such a printing operation is repeated every time the platen 1 is turned through an angle corresponding to the line spacing to print all the lines of characters on the recording sheet P. 
     Since the recording sheet P is shielded with the ribbon mask 10 from the ink ribbon 5, the recording sheet P can surely be prevented from being smeared with the ink of the ink ribbon 5 as the print head 2 is moved back and forth along the platen 1. 
     However, this prior art has the following drawbacks. It is possible that a portion of the recording sheet P wound round the platen 1 corresponding to the platen center C floats slightly from the platen 1 and, in some cases, the edge of the aperture 11 of the ribbon mask 10 interferes with the side edge of the recording sheet P while the print head 2 is moved back and forth. The interference between the edge of the aperture 11 and the side edge of the recording sheet P may cause jamming or may damage the ribbon mask 10. 
     OBJECT AND SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a printer having a ribbon mask, capable of preventing the interference between the aperture of the ribbon mask and the recording sheet. 
     To achieve the object, the present invention disposes a platen and a print head supported for sliding movement along the platen opposite to each other, extends an ink ribbon through a space between the platen and the print head, fixedly provides a ribbon mask having an aperture so as to expose only the front end of the print head to the platen provided on the print head and so that the inclined edges thereof inclined to a direction perpendicular to the direction of movement of the print head extend across the platen center. The platen center is a normal to the platen at a position nearest to the front end of the print head. Characters are printed on a recording sheet wound round the platen while the print head is moved back and forth and the platen is turned for feeding the recording sheet. Since the recording sheet is shielded from the ink ribbon by the ribbon mask, the recording sheet is not smeared accidentally by the ink. Since the floating edges the recording sheet are able to slide along the inclined edges of the aperture even if the floating edges of the recording sheet are caught by the edges of the aperture, the recording sheet is not caught by the aperture and hence jamming and breakage of the ribbon mask can be prevented. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front view of a masking plate for a ribbon mask employed in a printer in a preferred embodiment according to the present invention; 
     FIG. 2 is a front view of a masking plate in a modification included in the ribbon mask; 
     FIG. 3 is a front view of a masking plate in another modification for the ribbon mask; 
     FIG. 4 is a front view of a masking plate in a further modification for the ribbon mask; 
     FIG. 5 is a perspective view of a portion of a conventional printer; 
     FIG. 6 is a side view of assistance in explaining the positional relation between a platen, a print head, an ink ribbon and a ribbon mask; 
     FIG. 7 is a front view of a masking plate for a ribbon mask; 
     FIG. 8 is a front view of another masking plate for a ribbon mask; and 
     FIG. 9 is a front view of a further masking plate for a ribbon mask. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A printer in a preferred embodiment according to the present invention will be described with reference to FIG. 1, in which parts like or corresponding to those previously described with reference to FIGS. 5 to 9 are denoted by the same reference characters and the description thereof will be omitted. A ribbon mask 10 has a masking plate 10a provided with a parallelogrammatic aperture 11 having opposite edges 11a inclined to a direction perpendicular to the direction of movement of the print head 2 indicated by a double-head arrow X in FIG. 1 and formed so as to extend across a platen center C, namely, a normal to the platen 1 at a position nearest to the front end of the print head 2. A point on the print head 2 coinciding with the platen center C moves along the platen center C as the print head 2 moves. Platen centers C shown in FIGS. 2 to 4 are of the same definition. 
     In case a portion of the side edge of a recording sheet P slightly floating from the platen 1 enters the aperture 11 during printing operation in which the print head 2 is moved back and forth, the side edge of the recording sheet P slides along the inclined edge 11a past the platen center C in the direction of an arrow A toward a corner 11b of the aperture 11. Thus, the recording sheet P escapes from the aperture 11, so that printing operation is continued smoothly without entailing the jamming of the recording sheet P and without damaging the ribbon mask 10. 
     Since the aperture 11 has a polygonal shape, the least necessary area of the front end of the print head 2 is exposed to the platen 1 to shield the recording sheet P effectively from the ink ribbon 5, the inclined edges 11a inclining at a large angle to the platen center C ensures the escape of the recording sheet P from the platen center C. 
     Since the aperture 11 is designed in a shape effective for preventing the recording sheet P being caught by the aperture 11, the ribbon mask (hence the printer) is simple in construction and small in size. 
     The shape of the aperture 11 of the ribbon mask 10 need not necessarily be a parallelogram; the aperture 11 may be of any shape provided that the aperture 11 has inclined edges 11a intersecting the platen center C. 
     For example, the ribbon mask 10 may employ a masking plate 10a provided with an aperture 11 having the shape of a rhombus and formed with its horizontal axis of symmetry vertically dislocated from the platen center C or inclined at a predetermined angle to the platen center C as shown in FIG. 2 so that the inclined edges 11a thereof intersect the platen center C obliquely. 
     The ribbon mask 10 may employ a masking plate 10a provided with an aperture 11 having the shape of an ellipse formed with its axis of symmetry dislocated vertically from the platen center C or inclined at a predetermined angle to the platen center C as shown in FIG. 3 so that a portions 11a of the elliptic aperture 11 intersect the platen center C obliquely. Since the edge of the aperture 11 is a smooth curve, the recording sheet P being caught by the aperture 11 can be prevented even if the angle of inclination of the inclined portions 11a is small. 
     Furthermore, the ribbon mask 10 may employ a masking plate 10a provided with an aperture 11 having the shape of a hexagon formed with its axis of symmetry dislocated vertically from the platen center C or inclined at a predetermined angel to the platen center C as shown in FIG. 4 so that inclined edges 11a thereof intersect the platen center C obliquely.