This invention relates generally to the art of manufacturing ribbons for use with printers associated with equipment such as computers and word processors. More particularly, the present invention provides a method and apparatus for efficiently and accurately adjoining the ends of a length of a ribbon so as to form a continuous loop supported within a ribbon cartridge.
Present society is highly dependent upon the use of microprocessors, minicomputers, and mainframe computers. In addition to the more traditional use of computers as a research tool, computers are now often found in the home environment and work environment of individuals who, until recently, never before came into contact with a computer. Computers are used to automate equipment in a manufacturing facility, to automate secretarial duties at the office (word processors), and to automate a myriad of toys and tools around the home.
All computers, whether mainframe computers, minicomputers, or microcomputers, share a common requirement for some means of communication with users. Such means of communication may vary from the simple light-emitting diode (LED) to the complex cathode ray tube (CRT) terminal. A common means for communication of information from a computer to a user is a printer. Printers typically receive from the computer a stream of information in the form of digital electronic signals and convert the electronic signals to a series of symbols (letters and words) printed on paper.
A first common type of printer is an impact printer, so named for the means by which characters are printed on the paper. Each symbol which the impact printer is capable of generating is permanently embossed on a striking surface (analogous to the ball within a typewriter) within the printer. The printing of a particular character is accomplished by aligning the embossed image of that character on the striker with the paper surface onto which the character is to be printed, positioning a printer ribbon having ink thereon between the striker and the paper, and causing the striker to impact the ribbon against the paper surface so as to result in an ink image of the embossed character on the surface of the paper. Printer ribbons are typically made of fabric, such as nylon, or a polyester film coated with a carbon-type surface.
A second common type of printer is a dot matrix printer. Such a printer includes a group of small-diameter wires oriented parallel to one another and arranged in a two dimensional matrix. Each wire is arranged for independent extension from its normal matrix position to contact a printer ribbon, thereby printing a dot image on an adjacent paper surface. By selectively extending particular patterns of wires, dot images can be combined to form alphanumeric or other legible characters on the paper surface.
In approximately 1970 there first appeared on the market a printer ribbon having the ends thereof adjoined to form a continuous loop. The continuous loop ribbons are supported within cartridges which easily may be removed from and inserted within the printers. Such continuous loop cartridges mechanically simplify the ribbon control portion of printers and thereby eliminate a potential source of printer failure.
The first continuous loop ribbons were adjoined by a simple end-to-end overlap-weld process. To accomplish this process the two ribbon ends are placed in an overlapping configuration with one end overlapping the other by approximately 0.025 inch. The two ends are then welded together by means of ultrasonic vibration, which heats the overlapping fabric material to its melting point and thereby adjoins the two ends. Alternatively, a small diameter, resistance-heated wire may be oriented adjacent to the overlapping ends and properly spaced a short distance therefrom to weld the two ends together.
The end-to-end overlap weld presented several problems. First, the density of the ribbon in the region of the overlap is increased by compression of the overlapped ribbon ends during the welding process. The increased density within the overlap region hardens the ribbon and thereby diminishes the quality of the printed image on paper. The printed image appears blurred or smudged and, consequently, often illegible.
The second problem with the end-to-end overlap weld is its susceptibility to tearing due to the weakened state of ribbon fibers in the overlap region. A ribbon cartridge typically includes a pair of drive gears and a tensioner mechanism. The loop of ribbon passes between the drive gears, which are used to import motion to the ribbon, while the tensioner mechanism maintains the ribbon taut within the ribbon cartridge. Because the ribbon fibers in the region of the overlap were substantially weakened or destroyed by excessive heat during the welding process, the overlap region of the ribbon is especially susceptible to the increased pressure experienced by the ribbon as it passes between the drive gears. After a period of time, the increased pressure causes the ribbon to begin to tear in the overlap region. The combination of drive gears and the tensioner mechanism eventually results in a separation of the ribbon loop in the region of the overlap.
The process of heating the overlap region by means of a hot wire includes the further problem of controlling the position of the hot wire relative to the ends of the ribbon to be welded. A hot wire positioned too close to the ribbon is likely to sever the ribbon rather than weld it.
In an effort to alleviate the problems associated with the end-to-end overlap weld, the angular overlap weld process was developed. The process for forming an angular overlap weld begins by trimming the two ends of the ribbon along a diagonal line to form opposing forty-five degree angles. The two trimmed ends are then overlapped by approximately 0.025 inch and welded according to one of the methods described above for the end-to-end overlap weld. The angular overlap process thereby results in a weld oriented at forty-five degrees relative to the direction of movement of the ribbon, rather than normal thereto as in the case of the end-to-end overlap weld. The angularly oriented weld tended to improve the quality of the printed characters and also improved slightly the durability of the weld. Nonetheless, the problems described above persisted.
The next major improvement in continous loop weld technology was the development of the angular butt weld process. The angular butt weld process requires use of a base, known as the welding anvil, positioned beneath the two ends of the ribbon. The welding anvil includes upwardly sloping sides converging on a flat upper surface of land having a width of 0.005 to 0.020 inch and a length of an inch or more. For the purpose of illustrating the angular butt weld process, assume that the welding anvil is positioned in front of an operator with its lengthwise dimension extending from side-to-side. The operator grasps the right end of the ribbon and positions it across the welding anvil at forty-five degrees with respect to the lengthwise dimension of the anvil. The operator then grasps the left end of the ribbon, twists it to form a 180-degree counterclockwise spiral therein (as viewed by looking toward the end), and positions the left end in crisscross fashion across the top of the right end of the ribbon and the anvil. The two ends of the ribbon are now oriented at ninety degrees with respect to one another and at forty-five degrees with respect to the lengthwise dimension of the anvil. The land surface of the anvil now extends from corner to corner of the intersection of the crisscrossed ribbon ends.
Next, the active mechanism, or horn, of an ultrasonic welder is lowered on top of the crossed ribbons to press the ribbons firmly against the land surface of the anvil. Ultrasonic vibration generated by the welder heats the fabric ribbon and forms a weld bead, which conforms substantially to the dimensions of the land surface of the anvil against which the ribbons are pressed. The force of the welder horn against the anvil is also used to cut the ribbon along the weld bead. The two ribbon ends which were severed by this process are discarded. The operator then grasps the left side of the ribbon and rotates its clockwise 180 degrees to return it to its original orientation. The result is a continuous length of ribbon adjoined by a welded bead oriented at 45 degrees with respect to the direction of travel of the ribbon.
Subsequent to the reorientation of the ribbon, the welding anvil is replaced by an ironing anvil, which has a flat upper surface of land substantially greater in width than the land surface of the welding anvil. The welder is then lowered once again and the weld bead reheated and pressed between the welder horn and the ironing anvil to cause the ribbon in the region of the weld bead to assume a thickness substantially the same as that of the ribbon elsewhere.
The angular butt weld constituted a dramatic improvement over the overlap welds. Because the thickness of the ribbon in the region of the weld was reduced, the quality of the printed character generated by striking the weld region was much enhanced. In addition, the reduction in thickness diminished the tendency of the weld region to tear and thereby increased the average life of a continuous loop ribbon cartridge. Shortly after introduction of the forty-five degree angular butt weld, it was discovered that a sixty-degree butt weld constituted a further improvement in both the strength and the life of the weld. The improvements resulting from the angular butt weld were, however, accompanied by several new problems.
A first problem is controlling the width of the weld bead. As noted above, the land surface of the welding anvil may be from 0.005 inch to 0.020 inch in width. Prior to cutting the ribbon along the weld bead, the weld bead is typically 0.002 inch wider than the land surface of the welding anvil. The opposing forces of the welder horn and the welding anvil may generate a cut in the weld bead in any position along the width of the land surface. Thus, the weld bead adjoining the ends of the ribbon may be as little at 0.002 inch wide or as great as 0.022 inch wide, depending on the path of the cut along the length of the weld bead. The width of the final weld bead is important because the strength of the weld changes proportionately with the width of the bead. For example, a 0.002 inch bead may result in a one to two pound break test, whereas a 0.010 inch bead may produce a break at four to fifteen pounds, the exact break point being dependent upon the width of the ribbon.
In an effort to control the path of the cut along the weld bead, the land surface of the welding anvil has been inclined to form a cutting edge along one side approximately 0.001 inch higher than the lower edge. This configuration results in a consistent weld bead cut pattern until such time as the contacting surface on the welder horn wears in conformance to the cutting edge, whereupon the inconsistent cutting pattern resumes. Such conforming wear on the welder horn typically occurs within one week of single shift production using the welder, thereby necessitating frequent precision machining of the welder horn. Because a typical welder horn can be resurfaced only approximately six times, and because welder horns range in price from $300.00 to $800.00 each, it is not uncommon for a manufacturer to spend as much as $4,000.00 per year per welding operation on new welder horns alone.
The inclined land surface configuration creates additional problems with the width of the weld bead, which recedes in width from the edge of the land surface receiving the least amount of cutting-welding pressure. The break test strength of such a weld varies almost as much as that of a weld formed on the flat land surface. Even in the absence of an inclined land surface, use of the welder horn to cut the ribbon along the weld bead results in significant and rapid deterioration of the horn, with its attendant costs.
The process of continuous loop fabric ribbon welding, as described above, requires the use of two distinct pieces of equipment. The first is a welding apparatus, preferably an ultrasonic welder, such as is manufactured by Branson Welding Company. The process also requires a welding fixture which provides, at a minimum, means for holding the ribbon in the desired crisscross pattern during the welding operation, a welding anvil, and an ironing anvil.
The welding fixture is positioned on a welder table, above which is suspended the welder horn. The welder horn is supported on a welder carriage by a carriage arm which extends upward from the rear of the welder table. On actuation of the welder, the welder horn travels downward along the carriage arm until it is stopped by an adjustable micrometer downstop.
In approximately March of 1979, Branson Welding Company began to manufacture a welding fixture for use in cooperation with its ultrasonic welders. The fixture includes a pair of crossing tracks with spring clips at the ends thereof to provide the proper orientation and gripping of the fabric ribbon ends. The crossing tracks are wide enough to handle large-width ribbons, such as one inch ribbons and, hence, do not conform precisely to the width of smaller ribbons, thereby introducing the possibility of error in their alignment. In addition to the welder horn, the welder carriage on the Branson machine supports four spring-loaded legs extending outwardly and downwardly from the welder carriage in alignment with the ends of the tracks on the welding fixture. As the welding carriage is lowered onto the welding fixture, pads at the base of each of the four legs contact the ribbon for the purpose of holding it in place while the horn welds and cuts the ribbon. Thereafter, the operator discards the waste ends of the ribbon, rotates the left end of the ribbon to return it to proper alignment, replaces the welding anvil with the ironing anvil, and initiates the second phase of the welding process, called the ironing stroke, to flatten the weld bead. In addition to flattening the weld bead, the ironing stroke widens the bead by as much as 50 to 100 percent.
Because of the previously discussed problems inherent in the angular butt weld process and because of the lack of precision in alignment and clamping of the ribbon ends, the Branson welding fixture provides an undesirably high rejection rate of finished continuous loop ribbons. As a result, most manufacturers have, either directly or indirectly, undertaken the design and manufacture of their own welding fixture. Typically, these fixtures include a pair of crossing tracks machined into a steel plate. The plate may, for example, be hardened steel which is heat treated and ground to the desired configuration. The width of the machined tracks are typically one inch, representing the widest ribbon normally handled by the fixtures, and are adjustable to the width of smaller ribbon by means of small metal blocks or magnets of various sizes.
The ribbon is typically positioned within the crossing tracks of the custom fixtures by grasping a first length of ribbon from one end of the cartridge, stretching the first length of ribbon across the length of a first track, releasing one end of the first length of ribbon while the other end is being clamped under a first magnet, tensioning the first length of ribbon, and clamping the loose end of the first length of ribbon between a second magnet and the welding fixture. A second length of ribbon from the opposite side of the cartridge is clamped in crossing relationship with respect to the first length of ribbon in a similar manner, after the operator twists the ribbon end 180 degrees. The welder is then actuated to weld and cut the ribbon. Next, the operator removes the magnets from the rear side of the fixture to free the waste ends of the ribbon, removes the front magnet from the second length of ribbon, rotates the second length of ribbon to remove the twist and return it to its proper alignment, reclamps the left length into position at the rear side of the fixture, and actuates the welder to perform the ironing operation.
Even with the additional apparatus for aligning the ribbon within the crossing tracks, the ribbons may be as much as 0.125 inch off the centerline of the track, causing a misalignment or bend in the ribbon at the point of the bead. With as little as 0.031 inch misalignment, the ribbon is likely to hang within the ribbon cartridge.
With a combination of the problem associated with the Branson welding fixture, the custom-designed fixtures, and welder horn cutting, and the problems described above for angular butt welds, i.e., variable weld strength and rapid deterioration of welding equipment, it is hardly surprising that most continuous loop manufacturing operations have a high rejection rate for completed printer ribbon cartridges as well as a needlessly high overhead equipment expense. Hence, it should be apparent that certain inadequacies exist in the present method and apparatus for adjoining the two ends of a fabric ribbon.