IMPROVED INK AND MU TISTRIKE RIBBON INCORPORATING THE SAME TECHNICAL FIELD This invention relates to an improved ink and to multistrike ribbons incorporating the same. BACKGROUND ART Multistrike ribbons are used in connection with dot matrix, daisy wheel, and other impact printers, and are usually packaged in cartridges that are characterized by the number of impacts that can be obtained. Such number depends on various factors including the volume of ink on the ribbon in the cartridge, and the transfer characteristics of the ink/ribbon base combination. Knowing the cost of a cartridge and the number of characters that can be printed, one can compute the cost per impact which is a factor of considerable importance to a user in comparing cartridges. When a print pin of a printer impacts a ribbon, the change in momentum of the ink in the ribbon due to the impulse of the print pin induces a flow of ink from the ribbon to the paper. If the ink is Newtonian (i.e., its viscosity is independent of the rate of shear) , the viscosity of the ink, or its resistance to flow, is independent of the impulse applied to the ribbon by the print pin. Inks used in multistrike ribbons may also; be thixotropic (i.e., their viscosity changes inversely with rate of shear as shown in Fig. 1) . At rest, prior to being struck by a print pin, the ink in the ribbon is viscous and does not ooze from the ribbon. On being struck by a print pin, the ink is subject to shear and becomes less viscous and flows easily from the ribbon to the paper. Multistrike ribbons are conventionally made using a carrier that includes either a plastic film or woven fabric. If the carrier were to include only a plastic film coated with thixotropic ink, all or nearly all of the ink on the film, within the projected area of a print pin, would be transferred from the film to the paper when the ribbon is struck by the print pin. The result would be a ribbon that lacks multistrike capability. Conventionally multistrike
ribbons in which the carrier includes a plastic film are therefore provided with a porous layer on the side of the film facing the paper. Such layer has a large number of interstices that are filled with ink, only a measured amount of which is transferred to a piece of paper when the ribbon is struck. The structure of the layer serves to meter the flow of ink allowing multiple strikes on the same place on the ribbon to produce ink deposits on the paper. Typically, the film is polyester and is coated with an open "sponge" of polymeric material filled with ink. In general, the polyester body is 10-30 microns thick and the "sponge" is from 8-50 microns thick with openings about 10 microns in diameter. These openings act to modify the flow of ink during impact thus metering the amount of ink transferred in response to striking the ribbon, enabling the ribbon to continue to transfer ink when struck a number of times in the same spot. The deficiencies of this type of ribbon include reduced density uniformity for letters printed on previously struck areas of the ribbon and rapid degradation of the density and sharpness of the printed characters, the added cost of the sponge layer, and the large volume of cartridge taken up by the structure of the sponge. Other conventional multistrike ribbons use special fabrics that are woven to provide interstices for holding ink. Crimped nylon and silk are the materials presently favored for this purpose. The deficiency with this arrangement is the cost of the fabrics, the low number of strikes due to the volume taken up by the ribbon base for a ribbon of this type, and the low print quality. It is therefore an object of the present invention to provide a new and improved ink and multistrike ribbon which increases the number of usable characters printed b the ribbon and reduces the cost per impact.
BRIEF DESCRIPTION OF THE INVENTION The present invention provides a ribbon ink having a viscosity that increases with increasing rate of shear beyond a predetermined rate of shear. Such an ink is termed herein as a shear thickening or dilatant type of ink. A shear thickening ink according to the present invention may be Newtonian or even thixotropic below a threshold rate of shear; but at and beyond this threshold, the material stiffens and requires a large stress to effect flow. That is to say, a shear thickening ink according to the present invention flows under conditions of low stress, but strongly resists movement when stressed beyond a shear threshold. The increase in the viscosity above the threshold may be abrupt or it may be gradual with increasing shear. According to the present invention, particular shear thickening inks include pigment particles such as carbon black having a specific area less than about 50 m 2 /gm. Inks containing carbon black with a specific area in the range of about 8-50 m 2 /gm can be made to exhibit shear thickening properties while inks containing carbon black with a specific area greater than about 50 m 2 /gm fail to exhibit shear thickening properties. In addition, it has been found that carbon blacks having a specific area in the range 8-50 m 2 /gm have a relatively large particle size, with diameters greater than about 50 nanometers, and generally less than about 700 nanometers. In addition to particles having the necessary physical properties, a shear thickening ink according to the present invention further comprises a mixture of a mineral oil and a dispersant. The choice of dispersant must be made experimentally to match the characteristics of the carbon black and the mineral oil, and it is believed that the degree of dispersion plays an important role in the properties of the ink. The percent weight of mineral oil to dispersant preferably ranges from about 85:15 to 80:20. The percent weight of carbon black to liquid (mineral oil and dispersant) is in the range of about 44% to 75%.
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According to the present invention, a printer ribbon is provided for use with an impact printer comprising a substrate containing a shear thickening ink. In one embodiment of the invention, the substrate comprises a polyester film having, on one surface thereof, a layer containing interstices containing the shear thickening ink. The layer may be formed of precipitated polymer. In another embodiment, the layer is formed of small diameter spheres adhered to one surface of the film. In such case, the spheres may be 30-50 micron diameter acrylic balls. In another embodiment of the invention, the layer may be in the form of a pattern of channels on one surface of the film formed by embossing or by making depressions on the film. The width of these channels may vary from about 15-75 microns. Alternatively, the pattern may be in the form of elongated channels that are oriented at an acute angle, preferably about 45°, to the longitudinal direction of the ribbon. The cross-section of the channels may be V- shaped with a width of about 15 to 75 microns, or may be rectangular with a width and spacing of about 8-75 microns. In another embodiment of the invention, the layer may be a polyurethane foam impregnated with a shear thickening ink. The foam may be bonded to the film. Alternatively, no separate bonding of the foam to the film is required when the wetting of the ink under static conditions is sufficient to maintain the mechanical connection of the foam to the film. The invention also includes a ribbon having more than one layer, at least one of which is a porous layer, and contains printing ink, preferably a shear thickening ink. These layers may be attached to each other but need not be laminated. The interface between the layers contains a film of ink that acts as a capillary layer, or the interface has capillary channels therein for the transport of ink in directions transverse to the thickness of the layers. The film of ink in the interface is effective in maintaining the layers attached to each other, and acts as lateral ink transfer means for effecting the transfer of ink from one
portion of the ribbon to another, laterally spaced portion of the ribbon. The invention also includes a ribbon in which the layer is a non-woven fabric backed by a film. Preferably no separate bonding of the non-woven fabric to the film is required when the wetting of the ink under static conditions is sufficient to maintain the mechanical connection of the non-woven fabric to the film. In a modification of this embodiment, the fabric may be coated with a thin layer of polyurethane on one side and the film omitted. In a further embodiment, the layer may be a sheet of melt blown fibers deposited on a polyester base that is calendered smooth and attached by bonding to the film. In a preferred embodiment of the invention the ribbon may comprise two layers of non woven fabric preferably of nylon. In an alternative embodiment the two layers are separated by a layer of permeable tissue paper or of polyethylene, in another the layers are backed by a thin layer of nylon or polypropylene. Alternatively, the non- woven fabric may be formed of polyester fibers. In a preferred embodiment of the invention the non- woven fabric is formed of fibers of diameter greater than about 5 microns in diameter. In a preferred embodiment of the invention the non-woven fabric is formed of fibers of diameter less than about 30 microns in diameter. Preferably the fiber diameter is in the range 10-25 microns. In a preferred embodiment of the invention adhesion between the fibers is enhanced for example by an adhesive or by heat and pressure during manufacture. The invention also comprises making a printing ribbon for use with an impact printer by applying a shear thickening ink to a substrate. The method may include the application of a coating of polymeric particles to a plastic film, and heating the coated film to sinter the particles into a sponge-like substrate for holding the ink in the interstices in the substrate. Alternatively, ink may be mixed with particles and applied to the film before heating, or may be applied to the substrate after heating.
DESCRIPTION OF DRAWINGS Embodiments of the present invention are shown in the accompanying drawings wherein: Fig. 1 is a graph showing the relationship between viscosity and shear rate for thixotropic (shear thinning) , Newtonian and shear thickening liquids; Fig. 2 is graph showing the relationship of stress to shear rate for the materials shown in Fig. 1; Fig. 3 is a schematic showing of a print pin covered with paper about to strike a pool of ink on an anvil; Fig. 4 is a graph showing the relationship between viscosity and shear rate for inks having different specific areas of the carbon black particles, but having the same concentration of carbon black to liquid; Fig. 5 is a chart like Fig. 4 but showing the effect of increasing the concentration of solid particles to liquid for particles having the same specific area; Figs. 6 - 11 are cross-sections of various embodiments of a ribbon according to the present invention; and Figs. 12-13 illustrate two techniques for applying shear thickening ink to ribbons according to the present invention. Figs. 14-15 illustrate two preferred alternative configurations of ribbon substrate according to the present invention.
DETAILED DESCRIPTION Referring now to the drawings. Fig. 1 shows the qualitative differences between liquids that are termed Newtonian, shear-thinning (sometimes referred to herein as thixotropic) , and shear-thickening (sometimes referred to herein as dilatant) . A liquid that is Newtonian (such as water) , has a viscosity at a given temperature which is independent of the rate of shear of the liquid (i.e., the rate at which a strata of liquid moves relative to an adjacent strata) . A liquid that is shear thinning has a viscosity, at a given temperature, which decreases with the rate of shear. A thixotropic liquid (such as latex paint) will thus coalesce and not run under static conditions; yet such liquid can be spread easily by inducing shear in the liquid, by brushing, for example. Finally, a shear-thickening (dilatant) liquid has a viscosity at a given temperature which increases with the rate of shear above a particular rate of shear. Such a liquid flows relatively easily when gently stirred, but becomes very stiff when an attempt is made to stir the liquid rapidly. The point at which the viscosity changes very rapidly is referred to herein as the dilatancy point. In this sense, a shear thickening liquid responds to shear in a manner analogous to a viscoelastic material such as "silly putty" which is easily kneaded by hand, yet is elastic when bounced against a surface, and is brittle in reaction to an applied impulse. For shear rates below the dilatancy point, the material may be Newtonian or thixotropic. Fig. 2 is a graph similar to Fig. 1 but showing the stress response of Newtonian, thixotropic, and dilatant liquids to the shear rate applied to the liquids. This graph merely illustrates another parameter of the liquids. The response of a shear thickening ink as compared to the response of conventional thixotropic and Newtonian inks in an impact printing environment is illustrated in Fig. 3 which shows pin 10 moving downwardly towards anvil 11 having
pool 12 of ink in the path of end surface 13 on the pin. Sheet of paper 14 is interposed between surface 13 and the ink on the anvil. Regardless of the nature of the ink, the initial impact of the pin and paper with the ink induces a shear in the ink in the impact area defined by surface 13, the rate of shear being low initially but increasing rapidly. If the ink is thixotropic, its initial viscosity will be relatively high; but as the rate of shear increases with terminal movement of the print head against the anvil, the shear-thinning property of the ink will manifest itself. Consequently, with both thixotropic and Newtonian ink, the ink will splatter over a large region surrounding the impact region, and a considerable quantity will be absorbed by the paper over a region much larger than the projected area of the pin. On the other hand, if the ink is shear thickening, it will have a low viscosity at the instant of impact thereby allowing for the wetting of the paper in a region defined by the projected area of the pin. As the impact proceeds, however, and the ink tends to move outwardly of the impact region, the shear-thickening property of the ink will manifest itself as the ink stiffens and stops moving. When the impact is removed, a thin film of ink is left on the paper essentially confined to the projected area of the pin, and the pool of ink on the anvil is relatively undisturbed. This operation illustrates the basic idea behind the use of a dilatant ink for impact printing: the shear- thickening property of the ink itself limits the transfer of ink to paper. In conventional impact printing using thixotropic or Newtonian ink, an external structure such as the interstices of a woven fabric is necessary to limit ink transfer on impact. As a consequence, a ribbon carrying dilatant ink may be very much more open than a ribbon carrying Newtonian or thixotropic ink, allowing a given volume of ribbon using shear thickening ink to contain a higher percentage of ink than a conventional ribbon. A particular shear thickening ink according to the
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present invention comprises a mixture of mineral oil, a dispersant, and carbon black having a critical surface area per unit weight. The mineral oil should have a low viscosity, so that the resultant viscosity of the ink below the dilatancy point is as low as possible, a high boiling point, and a low vapor pressure (to inhibit drying of the ink in the ribbon) . The dispersant is chosen to be compatible with the mineral oil and the carbon black as is known in the art. Pigment, such as carbon black, characterized by a small specific area (defined as the surface area per unit weight in square meters per gram, m /gm) causes the ink to exhibit dilatant properties. It has been found that carbon black having a specific area of about 8 m 2 /gm has a sharp dilatancy point and works very well in ribbons of various configurations. Carbon black having a specific area in the range 20-45 m 2 /gm also works satisfactorily, but the dilatancy point is less sharp. When the specific area exceeds about 50 m 2 /gm, the resultant ink does not appear to exhibit substantial dilatancy for the carbon blacks tested. Another physical parameter associated with the carbon blacks which have been found to form a shear thickening ink is the particle size. It has been found that inks prepared with carbon blacks having particles with diameters greater than about 200 nanometers appear to have high dilatancy. Carbon blacks with diameters in the range of 50 to 100 nanometers also work satisfactorily, but the dilatancy point is less sharp. Dilatant inks have been produced using the following types of carbon black:
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Approx. Approx.
Brand Manufacturer Specific Area Diameter Sevcarb MT-LS Sevalco, Bristol UK 8 πr/gm 200-700 n Printex G Degussa, FRG 30 51 nanometer Elftex 180 Cabot 37 50 nanometer Raven 14 Columbian Chemicals 45 m 2 /gm 59 nanometer Raven 410 Columbian Chemicals 24 m 2 /gm 70 nanometer Raven 16 Columbian Chemicals 25 m 2 /gm 61 nanometer Raven 22 Columbian Chemicals 20 m /gm 62 nanometer Lampblack Degussa, FRG 20 m 2 /gm 95 nanometer At Present the preferred materials are Sevcarb MT-LS and Elftex 180. Shear thickening inks have been produced using the following mineral oils: (1) PAZDINA 15 manufactured by PAZ Oil Co., Israel. This is a "medical quality paraffin oil" with a viscosity of about 13 centipoise at 40 * C and is the preferred oil. (2) MARCOL 62 manufactured by Exxon, with a viscosity of about 9.4 centipoise at 40'C. (3) ISOPAR, especially ISOPAR M, manufactured by Exxon, with a viscosity of about 2.5 centipoise at 25*C. (4) NORPAR 15 manufactured by Exxon, with a viscosity of about 2.5 centipoise at 25 β C. (5) MARCOL 52 manufactured by Exxon, with a viscosity of about 6.92 centipoise at 40*C. Shear thickening inks have been produced using a number of dispersants such as OLOA 1200 and OLOA 374Q manufactured by Chevron, and AMOCO 9251 manufactured by Amoco Petroleum Additives Company. At present, the exact nature of the dispersant does not appear to be critical, and it is expected that other types of dispersants will work as well, although there do appear to be many combinations of carbon black, dispersant and mineral oil which give good results. Inks which are more nearly Newtonian at low shear rates are generally more desirable than inks which are thixotropic at these levels. Extensive tests have been made on dilatant inks made from the above constituents. Typically, dilatant inks have
been produced using a liquid base comprising a mixture of 85% weight mineral oil (PAZDINA 15) and 15% weight dispersant (OLOA 374Q) and the following amounts of carbon black: (1) 75% weight SEVCARB MT/LS and 25% weight liquid base; (2) 50% weight PRINTEX G and 50% weight liquid base; (3) 44.4% weight ELFTEX 180 and 55.6% weight liquid base. Additional shear thickening inks have been made using a liquid base comprising a mixture of 80% weight mineral oil (NORPAR 15) and 20% weight dispersant (OLOA 374Q) and the following amounts of carbon black:
(4) 50% weight RAVEN 14 and 50% weight liquid base;
(5) 40% weight RAVEN 410 and 60% weight liquid base;
(6) 50% weight RAVEN 16 and 50% weight liquid base;
(7) 50% weight RAVEN 22 and 50% weight liquid base;
(8) 55.6% weight ELFTEX 180 and 44.4% weight liquid
(9) 47.7% weight PRINTEX G and 52.3% weight liquid
It has been found that higher solids proportions lead to a very viscous ink under low shear conditions, and that higher particle size and lower specific area carbon blacks require less liquid base. Moreover, all of these shear thickening inks are thixotropic at shear rates below the dilatancy point. A preferred ink is produced by using a liquid base comprising 80% weight mineral oil (MARCOL 52) and 20% weight dispersant (AMOCO 9251) . The ink is composed of one part liquid base and preferably 0.8-0.95 parts of Elftex 180 carbon black. Experimental results are shown quantitatively in Figs. 4 and 5. Fig. 4 shows that increasing the specific area of the carbon black while maintaining the same concentration of carbon black to liquid base reduces the sharpness of the dilatancy effect, and reduces the low shear viscosity. Fig. 5 shows that increasing the concentration of carbon black in
1 the liquid base while maintaining the same specific area
2 increases the sharpness of the dilatancy effect which occurs
3 at a lower shear rate, and increases the low shear
4 viscosity.
5 Shear thickening inks have also been produced
6 comprising mineral oil, dispersant and solid materials other
7 than carbon black. These solid materials included: Lake
8 Black H (manufactured by Bayer) , barium carbonate, calcium
9 carbonate, zirconium oxide, iron oxide, Volstonyt, antimony
10 oxide and aluminum oxide. Optionally, inks may include a dye
11 or dyes known in the art dissolved in the liquid.
12 The present invention also consists in ribbons
13 incorporating dilatant ink. Referring to Fig. 6, reference
14 numeral 20 designates the first embodiment of what is termed
15 herein a "small-pore" ribbon, so called because it has
16 openings comparable to the openings in ribbons impregnated
17 with conventional ribbon ink. Ribbon 20 comprises film base
18 21 of polyester much like a conventional impact ribbon film,
19 and separate layer 22 of material containing shear
20 thickening ink. A ribbon according to the present invention
21 was constructed following the procedure described below. A
22 mixture of approximately 20% polymeric particles (SURLYN
23 1706 manufactured by DuPont) and 80% shear thickening ink
24 according to the present invention was prepared and appDied
25 to one surface of the film. The coated film was heated to a
26 temperature at which the sintering of the polymeric
27 particles occurred thus creating an ink filled sponge layer.
28 In a variation of this procedure, only the polymeric
29 material was applied to a polyester film and sintered as
30 before. A shear thickening ink was applied to the sintered
31 layer after the sintering process. In both cases, the
32 resultant ribbon can be described as a film base having a
33 layer with small pores.
34 Another small-pore ribbon according to the invention
35 was produced by dissolving 20% by weight of ELVAX 5720 resin
36 manufactured by DuPont, in heated ISOPAR, manufactured by
37 EXXON, to produce a mixture that was coated on one surface
38 of a polyester film. Upon cooling, the ELVAX 5720 resin
precipitates forming an open cell sponge layer whose interstices are subsequently filled with shear thickening ink. Because the shear thickening ink has a relatively low viscosity under low shear conditions, procedures like those described below can be used to fill the sponge-like layer on the polyester film. The shear thickening nature of the ink according to the present invention also allows the use of "large-pore" ribbons, so called because the size of the pores are considerably larger than the size of the pores found in conventional multi-strike ribbons into which conventional ribbon ink is incorporated. So-called large pore ribbons can be manufactured following the steps described below. In the first of the large-pore ribbons designated by reference numeral 25 in Fig. 7, one surface of polyester film 26 is coated with an adhesive, preferably thermoplastic. A single layer of 30-50 micron diameter acrylic balls 27 is then adhered to the adhesive covered surface of the film. Ribbon 25 is then inked by applying dilatant ink to the ball- covered surface of the film. The ink may be applied in the form of drops or in a rivulet. Because of the relatively low viscosity of the dilatant ink under low shear conditions, the ink will flow into and fill the interstices created by the balls. In the second of the large pore ribbons designated by reference numeral 30 in Fig. 8, a pattern of closely spaced projections or bumps 31 is formed in one surface of film 32. This may be achieved by a forming operation that permits the opposite side of the film to remain flat. Bumps 31 form a plurality of interconnected depressions 34 whose width is in the range of 15-75 microns, and which are filled with shear thickening ink. The bumps may be cubic with each side being in the range of 15-75 microns. The shape and size of the bumps is not believed to be critical consistent with being small compared to the cross-section of the print pin with which the ribbon is to be used. Alternatively, as shown in Fig. 9, a pattern of elongated, preferably V-shaped depressions can be formed on
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one surface of a film as shown by troughs 33 in film 34 of ribbon 35. Preferably, troughs 33 are arranged in rows that make a 45 β angle to the longitudinal direction of film 34. Other inclinations are of course possible. This configuration effects the flow of ink from the periphery of the ribbon towards the center where the pins of a printer contact the ribbon thus replenishing ink removed from the active region of the ribbon. Furthermore, the angularity of the pattern of troughs enhances the strength of the ribbon in the direction of its stress during use, which is the direction of movement. Fig. 10 shows a variation of the embodiment shown in Fig. 9. In Fig. 10, the grooves or depressions in the ribbon are rectangular in cross-section rather than V- shaped. The grooves may be 8-75 microns across. A ribbon with 10 micron grooves and 10 micron lands between adjacent grooves may be suitable. The angularity of the grooves relative to the longitudinal direction of the ribbon is preferably about 45*. This angularity is a compromise between ribbon strength and allowing flow of ink from the (unstruck) margins of the ribbon to replenish the ink transferred to the paper on impact. Another embodiment of ribbon according to the present invention includes a polyester film on one surface of which is bonded a layer in the form of a thin foamed open celled filmless polyurethane sponge with large (i.e., greater than 10 micron) pores, and preferably with pores greater than about 50 microns. This type of ribbon is illustrated in Fig. 6. After foaming, a sponge is formed without a skin; and the interstices of the sponge are then filled with ink. Material of this type is available from W.R. Grace and Company, Organic Chemicals division, under the trade name Hypol, foamable hydrophilic urethane polymers. Experiments with this type of ribbon using conventional inks, impacting of the ribbon with printer pins invariably resulted in splattering of the ink. When shear thickening inks of the invention were used, no splattering was evident. A variation of the foam sponge/film ribbon results when
the sponge is not bonded to the film. In such case, the foam is retained on the film by the wetting of the ink which intimately contacts the film at the interface between the sponge and the film. In a preferred embodiment of the invention the surface of the film facing the ribbon is roughened. This has been found to improve the lateral flow of the ink, especially between impacts. It is believed that the interface between the sponge and the roughened surface give improved capillary forces to transfer ink from regions which have not been struck to regions which have been struck, thereby to replenish the ink transferred to the paper in the struck regions. In preferred embodiments of the invention, the ribbon comprises at least two overlying layers, at least one of which is porous and contains printing ink. By roughening one or both of the facing surfaces, by forming grooves or other protrusions in one or both facing surfaces of the layers, or by utilizing materials which do not form a continuous bond, such as non- woven fabric, capillary channels are formed in the interface between the layers. These capillary channels enhance the lateral flow of the ink and allow for improved replenishment of ink in a region of the ribbon that is struck during printing. The overlying layers contact through the ink interface between the layers. An embodiment of ribbon according to the invention is designated by reference numeral 40 in Fig. 11. Ribbon 40 comprises polyester film 41 to which non-woven fabric 42 is adhered. Fabric 42 is formed of spun fibers, preferably polyester fibers about 10-15 microns in diameter. The resultant fabric has a thickness in the range 40-100 microns, and has an open structure with spaces ranging in size up to 100 microns. These spaces are filled with dilatant ink. A preferred variation of the non-woven fabric/film ribbon results when the fabric is not bonded to the film. In such case, the fabric is retained on the film by the wetting of the ink which intimately contacts the film at the interface between the fabric and the film.
In a variation of ribbon 40, one side of fabric 42 is coated with a thin layer (2-10 microns) of polyurethane which stretches and then recovers when impacted. The layer thus forms a a backing for the fabric, replacing the film; the cross section of the ribbon so produced is similar to that shown in Fig. 11. Another embodiment of ribbon according the present invention includes a polyester film to which is bonded a layer comprising melt-blown fibers. The layer is formed by blowing fibers of 1-10 microns diameter onto the base, and then calendering the combination to form a non-woven fabric which can be bonded to the film. The fabric so formed is even less dense than fabric 42. In an alternative arrangement, the layer need not be bonded to the film because the dilatant ink in the interface between the film and the layer serves to retain the two. Other embodiments of ribbon may utilize a non-woven polyester or nylon fabric or even paper which is of course non-woven. The term non-woven fabric as used herein include nylon, polyester, or paper materials, unless otherwise indicated. In alternative embodiments of the invention the polyester films used may be replaced by film produced of material which remains essentially elastic under impact printing. Such elastomeric materials, as for example polyurethane, stretch and recover when impacted, which can cause less distortion of the ribbon under the large number of impressions possible with ribbons of the present invention. Another embodiment of the invention is shown in Fig. 14 as ribbon 80 comprising at least two layers 82, 84 of non- woven fabric without any film backing. The spacing shown in the drawing between the layers is exaggerated, and the layers actually overlie and contact each other. Dilatant ink is applied to each of the layers following the techniques disclosed below, and the ink permeates both layers in the interstices of the fabric. Moreover, the layers tend to adhere to each other by reason of the ink
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that is present. A suitable ribbon may comprise two layers of CEREX 0.5 non-woven nylon fabric (James River Corporation) . This material utilizes 25 micron diameter threads and is approximately 80 microns thick (each layer) . It weighs about 5 oz/sg. yard. Thicker material may be used in a single layer, but it has been found that the double layer provides higher recovery (i.e., lateral ink transfer) than a single layer. This is believed to be caused by capillary channels and/or a capillary layer formed between the two layers. Additionally, and/or alternatively, a thin plastic layer may be placed on the "pin" side of the ribbon. In this case it may be desirable to coat the pin side of the film with a suitable lubricant to lubricate the pins during use. In operation, the ribbon is placed between the print head of a dot-matrix printer and a carrier sheet held on a platen operatively positioned with respect to the print head. When a print pin is actuated, forward movement of the pin results in direct contact of the pin with one of the layers of the ribbon. The impulse delivered by the engagement of the pin with the ribbon is transmitted though both ink impregnated layers and results in the application of a spot of ink on the carrier sheet. The dilatant nature of the ink limits the cross-sectional area of the spot, -and effectively meters the ink from the ribbon to the carrier sheet while permitting repeated engagement of a pin at the same point to deposit the same amount of ink on the carrier sheet. Improved performance is obtained when ribbon 86 shown in Fig. 15 is used. Ribbon 86 includes intermediate layer of material 88 between layers 82, 84. Layer 88 may be tissue paper placed between the two layers of non-woven fabric, such as nylon fabric. Optionally, the sandwich of nylon non-woven fabric and tissue paper may be calendered to give approximately 60 microns of CEREX 0.5 on each side and 50 microns of tissue paper in the center. In an alternative embodiment of the invention the tissue paper is replaced by a 10 micron thick film of porous polyethylene.
In a further embodiment, the layers of CEREX 0.5 are individually dipped in a thin glue, for example a 2-1/2% solution of SCRIPSET 910 (MONSANTO) in water. This treatment improves the integrity of the non-woven fabric, since the glue improves the adhesion of fibers at their joining points. After the glue is set, the layers are inked and placed together. Alternatively, the integrity may be improved by rolling the two layer material between two rough rollers at 240 β C to cause the fibers to adhere. In a preferred embodiment of the invention, the non- woven fabric is formed of 10 micron diameter threads of plastic preferably nylon material. In a two layer case each layer may be 50 microns thick. The use of thinner threads give a more uniform material, especially with regard to open space sizes and results in improved resolution. Preferably, the diameter of the fibers is between 5 microns and 30 microns. It is preferred that openings in the non-woven fabric be smaller than the size of the pins in the printer. Alternatively, open weave nylon fabric could replace the non-woven fabric in the embodiments which employ non- woven fabric. In order to ink the ribbons of the invention, ink, preferably shear thickening ink may be placed on the surface of the ribbon to be inked in drops or in rivulets as described above. A preferred manner of applying the ink to a ribbon is to use a gravure roller which applies spots of shear thickening ink to the ribbon surface. Subsequently the ink flows by capillary action into the main structure of the ribbon. In a modification of this technique, the dilatant ink may be diluted with a highly volatile solvent to reduce the viscosity of the ink as it is applied to the ribbon. After the ink penetrates into the ribbon, the solvent will evaporate. Alternatively, the dilatant ink may be heated to reduce its viscosity during application. This permits freer flow and permits the use of mechanical applicators that
produce higher shears in the ink during application. Fig. 12 shows one method for by which shear thickening ink can be forced into a foam or sponge. Ribbon 60, having plastic backing 61 and foam or sponge-like resilient layer 62 is fed between backing roller 63 and application roller 64. The spacing or bite between these rollers is such as to compress layer 62 as one or the other or both of the rollers is rotated thus drawing the ribbon downwardly between the bite of the rollers. Shear thickening ink 65, in a reservoir formed between compressed layer 62 and roller 64, is deposited on the surface of the foam and is drawn thereinto as the latter expands below the bite. The rate of rotation of the rollers must be such that the ink does not "freeze" from the induced shear. Fig. 13 shows another method suitable for use with layers that need not be bonded to the polyester film. In this approach, one surface of film 70 is coated with ink as indicated by reference numeral 71; and the uncoated surface engages roller 72 which draws the film in the direction of the arrow. One surface of layer 73 engages inked layer 71 on film 70 as a backup roller (not shown) cooperates with feed roller 72 and feeds the combination of layer 73 and film 72 in the direction of the arrow. The ink flows into the interstices in the layer; and a force is created that holds the layer to the film. The advantages of the present invention lie in the increased amount of ink per unit volume of ribbon that can be achieved as compared with ribbons of the prior art, the self-metering capability of the ribbons according to the invention and the high rate of ink replenishment from adjacent areas. Thus, ribbons according to the invention permit more strikes to be achieved on the same area of ribbon. In addition, the optical density of characters printed by re-striking the same area of the ribbon is more consistent for a larger number of strikes than for ribbons with conventional inks and structures. Finally, many embodiments of the present invention have a structure that is simpler than the structure of prior art ribbons, and are
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thus less expensive to produce and provide more strikes per ribbon. The advantages and improved results furnished by the method and apparatus of the present invention are apparent from the foregoing description of the preferred embodiments of the invention. Various changes and modifications may be made without departing from the spirit and scope of the invention as described in the claims that follow.
