Ribbon mask and guide for dot matrix impact printers

A ribbon mask for a wire or stylus-type dot matrix print head is carried with the print head, positioned between the print ribbon and the printing surface, and has a forwardly-angled integral flange extending from along a portion of the top of the main body of the ribbon mask. The flange rigidifies the intersection where the main body of the mask meets the flange, so that curving the ribbon mask along its longitudinal axis forms a generally triangular, forwardly-biased face adjacent the printing surface. Centrally located within the face is a vertically-oriented oval opening through which pass the printing wires as they move toward and strike the ribbon against the printing surface to form dot characters.

BACKGROUND OF THE INVENTION 
This invention relates to printers and, in particular, to means for masking 
the inked printing ribbon from the printing surface. 
The prior art teaches using a pair of fork-like brackets, one on either 
side of the printing wires of a wire matrix dot print head, to position or 
guide the printing ribbon as it passes between the printing surface and 
the printing styli. The longitudinal travel of each printing stylus, or 
wire is a very short distance requiring the ends of the printing wires to 
be placed in close proximity to the printing surface (usually, paper 
entrained over a platen or roller). Accordingly, an inked printing ribbon 
positioned between the ends of the printing wires and the printing surface 
is positioned in a very narrow opening, and this typically results in 
brushing contact between the printing ribbon and the printing surface. 
Such contact produces ink smears on the printing surface and an 
undesirable finished appearance, and it also may add undesirable friction 
loading on the ribbon and its drive mechanism. 
Additionally, snagging or even wedging may occur between the printing 
ribbon and the printing surface. This is particularly true during vertical 
feed of the paper. For example, when either the top edge of the paper or 
horizontal perforations or folds in the paper move vertically past the 
ribbon, the printing ribbon and the paper may catch one another, thereby 
very adversely affecting operation of the printing machine. For example, 
the ribbon is likely to be moved out of its proper position, causing loss 
of printing even though the print styli strike the paper; further, sheets 
of paper may become wrinkled or torn and have to be replaced, and the 
printing ribbon may have to be rethreaded or replaced. Not only does this 
cause material loss, but even more importantly, loss of time and delay in 
printing. Such a drawback is particularly significant, and all the more 
likely to occur, when the printer is designed for high speed printing. 
These are some of the drawbacks this invention overcomes. 
SUMMARY OF THE INVENTION 
This invention recognizes that the printing quality and reliability of a 
printer can be improved by the use of a ribbon mask positioned 
intermediate the printing surface and the printing ribbon. An opening 
through the ribbon mask is aligned with the printing means, such as wire 
styli, so the printing ribbon can be selectively struck against the 
printing surface. Except for this relatively small opening, the printing 
surface is substantially completely shielded from the printing ribbon, 
thus protecting against smudging and catching between the printing ribbon 
and the printing surface. Also, the ribbon is smoothly and evenly guided 
along its path, thereby providing smooth and consistent ribbon movement 
and, as a result, improved printing quality. 
In accordance with an embodiment of this invention, the ribbon mask has a 
generally planar main body and an angled, forwardly-projecting flange from 
a portion of the top longitudinal edge, including the edge above the 
opening. The use of the flange structurally rigidifies the mask along the 
area between the flange and the main body, thereby keeping that area 
relatively straight when the ribbon mask is mounted on a print head with 
the mask curved along its longitudinal axis, across the face of the print 
head, by wrapping the mask across the curved print head face and securing 
its ends to hold the mask in such position. The flange and its area of 
angular intersection with the main body prevents uniform curving of the 
ribbon mask so that a flat face is formed. The boundaries of this flat 
area face include the flange edge and two face edges running downward from 
the extremities of the flange toward each other, thus including the stylus 
opening between them. As a result, the generally flat face is biased 
outward toward the printing surface by the remainder of the ribbon mask. 
Advantageously, the face edges protrude outwardly slightly more than the 
opening in the face. Accordingly, the small portion of printing ribbon 
exposed through the opening is purposely and slightly spaced from the 
printing surface to prevent smudging. 
Further, in accordance with an embodiment of this invention adapted for a 
wire matrix printer, the opening through the mask advantageously has an 
oval or elliptical shape, with the major longitudinal axis being vertical. 
The elliptical shape is high enough to permit all the print wires in a 
vertical column to pass through the ribbon mask. Yet, use of the 
elliptical shape recognizes that a curved boundary and the narrowest 
possible opening width are advantageous to reduce snagging effects between 
the printing surface and the ribbon mask, and that it is advantageous to 
expose a minimum amount of ribbon which is not directly in front of the 
printing wires. 
A ribbon mask in accordance with an embodiment of this invention provides 
for protective partial encirclement of the printing ribbon, thereby 
producing clearer printing and reducing snagging between the printing 
ribbon and the printing surface. As a result, the quality of the printing 
improves and printing can even be done on irregular surfaces. Further, 
with either vertical or horizontal movement of the printing surface the 
printing ribbon is protectively guarded. For example, the lifting of a 
ribbon by the upward movement of either an upper edge or a horizontal 
perforation of the printing surface is eliminated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a print head 10 shown for purposes of general 
illustration includes a plurality of print wires 11 extending from a drive 
assembly 12 through an intermediate guide 15 to end guide 14 for 
positioning the extremities of print wires 11 in a closely-spaced vertical 
column 17. A frame 13 extends longitudinally between drive assembly 12 and 
end guide 14 and also supports intermediate guide 15 and a ribbon guide 
20. 
Ribbon guide 20 includes a pair of ribbon positioners 25 and 26 on one side 
of end guide 14 and a pair of ribbon positioners 25' and 26' on the other 
side of end guide 14. A ribbon 16 is guided between each pair of ribbon 
positioners from one side of end guide 14 to the other side thereof so 
ribbon 16 passes in front of the vertical column of needles 17. Ribbon 
guide 20 further includes an integral base member 21 extending between and 
supporting ribbon positioners 25, 26, 25' and 26', and connected by such 
means as screws to frame 13. 
An elongated ribbon mask 30 is supported by ribbon guide 20 at both ends 
and along the bottom side. Base member 21 has an upwardly-opening, 
outwardly-bowed longitudinal slot 22 along its length for receiving the 
bottom edge of ribbon mask 30. Ribbon positioners 26 and 26' are located 
furthest outward of each pair of ribbon positioners and have vertical end 
slots 23 and 24, respectively, for receiving the ends of ribbon mask 30. 
Referring to FIGS. 2, 3 and 4, ribbon mask 30 has a generally elongated 
rectangular main body 31 integrally coupled at each end to end mounts 35 
and 36, which are in the nature of flaps, or flanges, angled rearwardly to 
be received within end slots 23 and 24 of ribbon guide 20 when ribbon mask 
30 is mounted on the ribbon guide. When so mounted, the bottom 
longitudinal edge 43 of ribbon mask 30 is received within the bowed or 
curved longitudinal slot 22 of ribbon guide 20. Consequently, when the 
mask is mounted it may be described as having a pair of side portions 
(i.e., the parts of the mask outboard of the center, where the generally 
triangular face 37 is located, described hereinafter) extending angularly 
away from, and diverging with respect to one another (note FIG. 1). An 
integral top flange 33 of the ribbon mask has a trapezoidal shape whose 
bottom parallel side is integrally coupled to the top edge 44 of main body 
31 along a structural chord, or edge, 34. When mounted on the ribbon guide 
20, the top edge of flange 33 is offset forwardly so flange 33 tilts 
angularly forward from the plane of main body 31, along structural chord 
34. Structural chord 34 extends only a portion of the length of top edge 
44. Positioned below the mid-point of structural chord 34 in main body 31 
is an elliptical opening 32 having its longitudinal axis vertically 
aligned. To secure the ribbon mask in place upon the ribbon guide, with 
bottom edge 43 in longitudinal slot 22, end mount 35 in end slot 23, and 
end mount 36 in end slot 24, adhesive is preferably applied at these 
mounting points, even though the shape, mounting curvature and 
tab-and-slot arrangement tends to make the mask self-retaining. 
As already noted, longitudinal slot 22 has a forwardly-bowed configuration. 
Further, end slots 23 and 24 are closer to each other than the distance 
normally separating end mounts 35 and 36 when ribbon mask 30 is forced 
into a curved shape. As a result, when ribbon mask 30 is installed upon 
ribbon guide 20, main body 31 is bowed forwardly so that it curves along 
its longitudinal axis. Because structural chord 34 is relatively more 
rigid (i.e., stronger in bending) than the remainder of main body 31, such 
longitudinal curving of the body causes a generally triangular flat face 
37 to be formed directly below structural chord 34. More specifically, the 
boundaries of face 37 include face edges 38 and 39 extending downwardly 
and toward each other from the extremities of structural chord 34. The 
point where face edges 38 and 39 intersect is about two-thirds of the way 
down main body 31 from structural chord 34. Face edges 38 and 39 are each 
of approximately the same length as structural chord 34, so that face 37 
is approximately an equilateral triangle. 
Face edges 38 and 39 are not creased, or abruptly angled, but have a gentle 
curve which contacts the printing surface. The forwardmost portions of 
face 37 are face edges 38 and 39 (Note FIG. 1). That is, opening 32 is 
somewhat recessed rearwardly relative to face edges 38 and 39 whose 
position is of course immediately adjacent the center of the curved part 
of the mask, i.e., the center of slot 22, below the triangular face 37. 
Accordingly, the portion of ribbon 16 exposed through opening 32 is spaced 
rearwardly from the printing surface. This is advantageous so that any 
smudging due to the exposed ribbon 16 is minimized. Flange top 42 of 
flange 33 does not typically contact the printing surface because the 
printing surface is constrained to move around a platen which is typically 
cylindrical and curves away from flange 33 as the printing surface moves 
upward. 
If the longitudinal length of main body 31 between end mounts 35 and 36 is 
too long, in relation to the length of chord 34, face edges 38 and 39 
become very pronounced in curvature and two dimples result. This is 
undesirable because as ribbon mask 30 rides along on the printing surface 
there is a tendency to wear holes in the ribbon mask at the dimples and 
expose ribbon 16, thereby causing smudging on the printing surface. If the 
longitudinal dimension of main body 31 in relation to the length of chord 
34 is too small, ribbon mask 30 is not sufficiently bowed and a generally 
planar face like that designated 37 is not formed; thus, opening 32 is not 
recessed from the printing surface. This is undersirable because ribbon 16 
will then be directly and closely adjacent to the printing surface and may 
cause smudging. As an example, the longitudinal length of main body 31 can 
be about 1.8 inches and the length of chord 34 about 1.0 inch. A typical 
material for ribbon mask 30 is the lubricous polymeric sheet material 
known as "Mylar". 
In the illustrated embodiment, print wires 11 are arranged in a single 
column at end guide 14 and opening 32 has an oval shape. Such a shape has 
decided and unforeseen advantages, since investigation shows that 
rectangular shape, for example a narrow vertical window which just exposes 
the ends of print wires 11, will tend to cause paper tears because if a 
pucker of paper enters such an opening, it cannot easily come out because 
of the right-angle corners of a rectangular opening. Although a circular 
opening will reduce paper snagging, it exposes an undesirably large 
surface area of ribbon 16 to the printing surface, thereby causing 
undesirable smudging. An oval opening 32 is advantageous because it 
simultaneously reduces smudging and reduces paper tearing. That is, if 
paper enters an oval opening such as opening 32, the curved boundaries of 
the opening will guide the paper back out of the opening until there is 
just a small pucker which has insignificant strength and withdraws easily 
from the opening. At the same time, the least possible amount of ribbon is 
exposed to the paper, thereby reducing smudging to the point of 
elimination. 
The shape of flange 33 also operates to reduce the possibilities of paper 
catching on ribbon mask 30. That is, flange edges 40 and 41, i.e., the 
non-parallel sides of the trapezoidal flange, have an acute angle with 
respect to structural chord 34 so paper travelling along top edge 44 of 
main body 31 easily passes to flange edge 40 or 41 and off flange 33. 
Nevertheless, in the broader aspects of the invention flange 33 can have a 
shape other than trapezoidal, but it should not extend too far outwardly 
from top edge 44, and it must have sufficient support at the extremes of 
structural chord 34 to provide the necessary rigidity along the entire 
length of structural chord 34. If structural chord 34 does not have 
sufficient rigidity at its extremes, then face 37 will not be desirably 
flat and opening 32 will not be sufficiently recessed from the printing 
surface. The forward angle of flange 33 is particularly advantageous when 
loading ribbon 16 across the face of print head 10, since it forms a 
downwardly-conversing slot which receives the ribbon easily and without 
wrinkling, the rear face of flange 33 acting as a guide in cooperation 
with end guide 14 to correctly and easily position the ribbon in front of 
print wires 11. A typical angle advantageous for performing this guiding 
function is about 30.degree. forward of the plane of the main body of the 
ribbon mask. 
OPERATION 
Ribbon mask 30 is mounted on ribbon guide 20 as described above, so that 
face 37 is generally flat and disposed chord-like behind protruding face 
edges 38 and 39. Ribbon 16 is loaded between ribbon positioners 25, 26, 
25' and 26' and guided by flange 33 into position in front of print wires 
11. Print head 10 is positioned so the distance from the ends of print 
wires 11 to the platen takes into account the thickness of the printing 
surface. That is, if the printing surface is part of a relatively thick 
material, print head 10 will be positioned further back from the platen 
than when the printing surface is part of a relatively thin material. 
The printing surface, such as paper, is then positioned in front of print 
head 10, being vertically and horizontally located at the desired 
position. Advantageously, face edges 38 and 39 lightly contact the 
printing surface, and opening 32 is spaced at least slightly from the 
printing surface, thus maintaining a space between the printing surface 
and ribbon 16. It is typical of this invention that during vertical and 
horizontal movement in the positioning of the printing surface, neither 
the edges nor any perforated lines in the printing surface will snag on 
the ribbon mask or the ribbon. Indeed, because of ribbon mask 30 all such 
ribbon problems are eliminated. 
To print a desired character, print wires 11 are selectively activated 
(i.e., impelled endwise toward the ribbon and the paper) and print head 10 
is moved across the printing surface. Ribbon mask 30 permits contact 
between ribbon 16 and the printing surface whenever ribbon 16 is driven 
forward by one or more of the print wires 11, and substantially eliminates 
contact between ribbon 16 and the printing surface when none of the print 
wires 11 are activated. As will be understood, to present freshly-inked 
ribbon 16 before print wires 11, ribbon 16 is usually advanced as print 
head 10 proceeds across the printing surface. 
After becoming familiar with the foregoing, various modifications and 
variations will no doubt occur to those skilled in the various arts to 
which this invention pertains. For example, the particular coupling of the 
ribbon mask 30 to print head 10 may be varied from that disclosed herein. 
Further, the particular shape of the projecting flange may be varied from 
that disclosed herein. These and all variations which basically rely on 
the teachings through which this disclosure has advanced the art are 
properly considered within the scope of this invention as defined by the 
appended claims.