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THERMAL PRINTING APPARATUS
Matter enclosed in heavy brackets [ ] appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions 5 made by reissue.
This is a continuation of application Ser. No. 08/241,756 filed May 12, 1994, now U.S. Pat. No. 5,818,492.
TECHNICAL FIELD 10
This invention pertains to a method and system for applying a thermally-printed indicia to a sheeting in a direction normal to the direction of movement of the sheeting past a print head.
BACKGROUND OF THE INVENTION
Signs are commonly used along roadways to display information to motor vehicle drivers. A highway sign typically includes a retroreflective sheeting that has characters placed thereon. The characters form information that is of 20 interest to motor vehicle drivers, and the retroreflective sheeting allows the information to be vividly displayed by the sign at nighttime. A retroreflective sheeting has the ability to return a substantial portion of incident light in the direction from which the light originated. Light from motor vehicle headlamps is retroreflected by the signs, allowing the information to be read more easily by passing motorists.
Highway signs tend to be fairly large in size to accommodate large characters. The characters are applied to the 3Q signs, typically, by screen printing or by use of cut-out characters. In screen printing, a positive or negative image of the characters is first provided on the screen. This often is accomplished by exposing non-masked portions of a photosensitive screen to light and removing the 3J un-sensitized, masked regions by scrubbing. Ink is then forced onto the retroreflective sheeting through the openings in the screen where the photosensitive material was removed. Screen printing is the method of choice for making the more common signs such as stop and yield signs. 40
When a custom sign is needed, the cut-out character method frequently is used. Cut-out characters are made by die cutting each character or by electronically cutting the characters from a stock material such as ScotchliteTM electronic cuttable film. The cut-out characters typically are 45 secured to the underlying retroreflective sheeting by use of an adhesive or rivets. Although the screen printing and cut-out character methods provide suitable ways of placing characters on highway signs, these methods tend to be time-consuming and somewhat cumbersome. 50
Thermal printing has become a popular and commercially successful technique for forming characters on a substrate. Also referred to as thermal transfer printing, non-impact printing, thermal graphic printing, and thermography, thermal printing is a process by which a colorant is transferred 55 with the aid of heat from a carrier to a thermal print receptive substrate. Thermal printing is more rapid than screen printing or cut-out characters, and it is less cumbersome and relatively simple to carry out in practice.
While thermal printing provides a rapid, wielding means 60 for placing information on a sheeting, this printing method also has its drawbacks. A major drawback is that known thermal printing apparatuses are unable to handle large sheetings. The presently known apparatuses generally are unable to print on sheetings greater than 16 cm wide. If a 65 sign larger than 16 cm wide is desired, separate sheets must be printed on and those sheets must be subsequently joined
together to produce the whole sign. Thermal printing has been used to place information on a retroreflective sheeting, however, the information that has been printed has been limited in size to images such as bar codes; see, for example, U.S. Pat. No. 5,118,930 to Takada.
FIGS. 1 and 2 illustrate an example of a known thermal printing apparatus 10 having a print head arrangement 11 that comprises driven roller 14 and a thermal print head 16. A thermal print receptive sheeting 12 is shown disposed therebetween. Thermal print head 16 may comprise heatable resistive elements in a thermal heating system.
If roll sheeting stock is used as depicted in FIG. 2, sheeting 12 is held upon a sheeting supply reel 26 and is collected at a sheeting take-up reel 28. Dancer rollers 25 along with supply reel 26 comprise suitable tensioning means for sheeting 12. Ribbon 24 is held upon and tensioned by reel 22 and is collected on driven reel 23. Sheeting 12 is transported in the direction generally indicated by arrow y across roller 14 by sheeting transport means known in the art, for example, a friction drive mechanism using a stepper motor. Print head 16 remains stationary and makes contact with thermographic ribbon 24 and transfers colorant from ribbon 24 to sheeting 12 as the sheeting 12 moves past the print head 16. When transfer of colorant is completed or is not to be applied, print head 16 may be retractably disengaged from ribbon 24 in the direction generally indicated by arrow z. Currently available thermal print arrangements may be referred to as "down web" systems, because indicia are applied down the length of the sheeting while the sheeting is in motion. The thermal print head 16 is rectangular in shape and typically has a dimension S of about 10 to 16 centimeters, but thermal print heads having a dimension S of up to about 46 centimeter also are known. Ribbon 24 has the same length S, shown in FIG. 1, as print head 16. Dimension S determines the maximum printing width of sheeting 12 that can be printed upon with print head arrangement 11 in a single pass.
Because dimension S is limited in size, thermal printing has not found great commercial success in providing images on large sheetings such as on retroreflective sheetings used in highway signs. When a sign larger than S is desired, separate sheets must be printed on and those sheets must be joined together in registration to produce the whole sign. Another disadvantage of known thermal printing systems is that the wide ribbon has a tendency to wrinkle, causing an uneven transfer of colorant and poor quality indicia. Further, known systems do not use the ribbon in a very efficient manner. Thermal printing in a region having a width less than S results in using only the portion of the ribbon corresponding to the width of the printed image. The unused portion of the ribbon becomes discarded with the used portion and therefor results in unnecessary waste.
SUMMARY OF THE INVENTION
The present invention provides a method and systems which overcome the aforementioned drawbacks. Briefly, the method of the invention comprises thermally transferring colorant from a movable ribbon onto the printable areas of a thermal print receptive sheeting by the steps of:
a) moving a thermal print receptive sheeting past a thermal print head, where the thermal print head is elongate and has a length L of at least one centimeter that extends substantially in a first direction; and
b) transferring the colorant from the movable ribbon to the thermal print receptive sheeting while the thermal print head is moved in a second direction substantially normal to the first direction.
In one aspect, the system of the invention comprises: a) an elongate thermal print head for transferring colorant, having a length L of at least one centimeter arranged substantially in a first, "down web", direction that is parallel to the direction of travel of the thermal print receptive sheeting; b) 5 a transport for moving the thermal print receptive sheeting past the thermal print head in the first direction and operably positioning the sheeting to receive colorant; c) a mechanism that moves the print head in a second, "cross web", direction substantially normal to the first direction when the print head is transferring colorant; and d) at least one control device for coordinating print head engagement, colorant transfer, sheeting transport, and print head linear motion. The system may further comprise a mechanism for disengaging the thermal print head when the thermal print head is moved in the second direction and when no colorant is being applied 15 to a printable area of the sheeting.
In another aspect, the system of the invention comprises a modular and transportable thermal printing system for transferring colorant from a ribbon to the printable area of a thermal print receptive sheeting. The modular and transport- 20 able thermal printing system has a frame assembly, a plurality of reel assemblies, a thermal printing mechanism, a thermal print moving mechanism, and at least one control device. The frame assembly includes horizontal rail members and vertical rail members assembled in an open frame 2s structure. The frame members have walls defining apertures suitable for receiving, in modular fashion, system components as needed. The reel assemblies operably hold, position, and rewind the thermal print receptive sheeting. Each of the reel assemblies has a mechanism for mounting onto the frame assembly. The reel assemblies transport the sheeting in a first direction. The thermal printing mechanism comprises an elongate, disengageable thermal print head having a length L of at least one centimeter arranged substantially parallel to the first direction for transferring colorant. The print head moving mechanism moves the print head in a 35 second direction substantially normal to the first direction when the print head is transferring colorant. The system control mechanism operably controls print head actuation, print head retraction, colorant transfer, sheeting transport, and print head linear motion. 40
The methods and systems of the invention are advantageous in that sheetings having a width greater than S (FIG. 1) now can receive a thermally printed indicia. The thermal printing method and systems of the invention also overcome the problem of ribbon wrinkling because they are able to use 45 a ribbon having a narrow width. This is accomplished by having the sheeting stationary while the ribbon moves over the sheeting as the indicia is being printed thereon. Further, the method and systems of the invention provide more efficient use of ribbon and place less stress on the ribbon 50 when in use. Also, a single ribbon can be used to print on sheetings of varying widths. No longer does the ribbon need to be changed to accommodate varying sheeting widths.
The above and other advantages of the invention are more fully shown and described in the drawings and detailed 55 description of this invention, where like reference numerals are used to represent similar parts. It is to be understood, however, that the drawings and description are for the purposes of illustration only and should not be read in a manner that would unduly limit the scope of this invention. 60
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a top schematic view of a print head arrangement 11 in a known thermal printing system.
FIG. 2 is a side schematic view taken along line 2—2 of 65 FIG. 1, depicting the print head arrangement 11 in a known thermal printing system 10.
FIG. 3 is a top schematic view of a print head arrangement 40 in a thermal printing system in accordance with the present invention.
FIG. 4 is an end schematic view taken along line 4—4 of FIG. 3 of one embodiment of a printer 41 having a print head arrangement in accordance with the present invention.
FIG. 5 is a top schematic view of a thermal print receptive sheeting 46 illustrating printed areas 140a printed in accordance with the present invention.
FIG. 6 is a schematic view of a thermal printing system 110 in accordance with the present invention.
DETAILED DESCRIPTION OF PREFERRED
EMBODIMENTS OF THE INVENTION
In this invention, "thermal printing" refers to those processes that transfer colorant from a ribbon to a substrate by use of localized heat. Typically, thermal printing is accomplished by a non-impact system that transfers colorant by the simultaneous application of localized heat and pressure. A system that transfers colorant predominantly by impact would not be considered to generate a significant amount of localized heat during the impact to qualify as a thermal printing system. The term "colorant" is used herein to mean a media capable of providing an image or indicia on the surface of a thermal print receptive sheeting. The colorant may be a binder media that contains a pigment(s), a dye(s), or a combination thereof. The colorant is transferred to a thermal print receptive sheeting by a thermal print head that contains, for example, resistive elements, ribbon-contacting elements in a laser system, electronic elements, thermally activated valve elements, inductive elements, thermopile elements, and the like. The term "thermal print head" refers to the mechanism or mechanism that provide the localized heat for the transfer of colorant. A preferred mechanism for transferring colorant comprises heatable resistive elements in a thermal print head of a thermal mass transfer printing system. Using the method and systems of the invention, indicia may be formed which include alphanumeric characters, logos, or graphic information upon the thermal print receptive substrate.
FIG. 3 schematically illustrates a thermal print head arrangement 40 of the invention. Print head arrangement 40 is shown to comprise a platen 42 and an elongate thermal print head 44. A thermal print receptive sheeting 46 is shown disposed therebetween. Sheeting 46 may take the form of, for example, a continuous roll of sheeting or a number of individual sheets each fed individually past print head 44. Print head 44 may comprise [ribbonengaging] ribbon engaging elements which place the ribbon in contact with the thermal print receptive sheeting by applying a slight degree of pressure thereon. Heatable resistive elements or any other suitable means for providing localized heat may then operate to transfer the colorant to the sheeting. Elongate print head 44 preferably includes a row of discrete heating elements that operably transfer colorant from a ribbon to a thermal print receptive sheeting by heating means known in the art. The length of the heating element row defines a dimension L, which is substantially parallel to a first direction generally indicated by arrow y in FIG. 3. L is a maximum length of sheeting 46 that can be printed upon by print head arrangement 40 in one print operation cycle, described more particularly below. Length L of print head 44 may be any reasonable dimension, but generally is from about 1 to 38 centimeters, preferably about 4 to 27 centimeters, more preferably about 10 or about 16 centimeters. Print head 44 may be the same print head 32 used in known print head
arrangement 11 (FIG. 1), in which case, dimensions S and L of FIGS. 1 and 3, respectively, may be equal.
Thermal print head 44 operates to transfer discrete areas of colorant to a thermal print receptive sheeting 46. The size of the colorant transfer area, or dot, can be determined by the area of each discrete heating element in print head 44, as is known in the art. Such dots are generally about 3.76xl0~6 square centimeters, which is the area of a pixel. The resolution of indicia printed with print head 44 generally is from about 75 to about 250 dots per lineal centimeter.
FIG. 4 illustrates an embodiment of a cross web thermal printer 41 having a thermal print head assembly 50. In this embodiment, print head assembly 50 comprises a print head frame 52 supporting print head 44, head actuator solenoid 58 projecting through head tensioning spring 56 and bushing 60 and mounting on alignment bearing carriage 61. Print head assembly 50 also has a ribbon travel mechanism, comprising back tensioning supply reel 62 dispensing ribbon 66 under print head 44 onto powered ribbon take-up reel 64. Ribbon 66 travels from supply reel 62 to take-up reel 64 when print head 44 is operably transferring colorant and is moving across sheeting 46. Print head 44 may be retractably disengaged from contact with ribbon 66 while moving across sheeting 46, as described more particularly below.
An example of a print head that can be incorporated into print head assembly 50 of FIG. 4 is the print head incorporated into an apparatus sold under the trade name Model T1006, manufactured by Printronix of Irvine, Calif. This apparatus combines a frame, print head, ribbon transport mechanism, a mechanism for data communication, a mechanism for heating head elements and the like, in one modular, readily obtainable item. The print head in the Model T1006 apparatus has a dimension L of about 16 centimeters. Other, similar apparatuses that incorporate a print head suitable for the invention are the apparatus sold under the trade name Tec B472, by Tech Corporation, Los Angeles, Calif, having a print head dimension L of about 10 centimeters, and the apparatus sold under the trade name Zebra 140, by Zebra Technologies Corporation, Vernon Hills, 111., having a print head length L of about 13 centimeters.
Ribbon 66 may have a wax-based, resin-based or a combined wax/resin-based binder, although a preferred ribbon comprises a resin-based binder. The width of ribbon 66 may be substantially the same as or narrower than length L of print head 44. Ribbon 66 may be, for example, a ribbon sold under the trade name Printronix 2150 or 2200, or ribbon sold under the trade name Zebra No. 5030/5099 by Zebra Technologies Corporation of Vernon Hills, 111. Alternatively, ribbons may be used which are sold under the trade names Sony brand No. 3021/3022/3023 by Sony Chemical Corporation of America, Wood Dale, 111.
In the embodiment shown in FIG. 4, a print head assembly alignment mechanism comprises a plurality of linear motion bearings 68, two or which are shown in FIG. 4. Linear motion bearings 68 are secured to alignment bearing carriage 61 and slidably mounted upon fixed shaft 70 and fixed shaft 71 (FIG. 3). Print head assembly stop 76 is positioned stationary on shaft 70 or shaft 71.
Aprint head linear motion mechanism moves a print head in a second direction (indicated by arrow x in FIGS. 3 and 4) that is substantially normal to the first direction (indicated by arrow y in FIG. 3). As shown in the embodiment of FIG. 4, a print head linear motion mechanism may comprise a continuous drive belt 74, that engages a drive wheel of a print head motor 72, and that is responsive to actuation of the print head motor 72. Tension may be maintained on drive
belt 74 by an idler wheel 73. Aprint head assembly 50 may be secured to a drive belt 74 and may be linearly movable along a fixed shaft 70 and a fixed shaft 71 in response to movement of a drive belt 74 by a motor 72.
Aprint head motor may be, for example, a motor made by Airpax Corp., Chesire, Conn, and sold under the trade name Airpax 82900. Other suitable motors for actuating print head linear motion are, for example, a stepper motor, a DC brushless motor with an encoder, or an AC synchronous motor with encoder. Print head linear motion mechanisms that comprise one of these motors are known in the art.
A linear motor system, which combines a track mechanism, drive mechanism and positioning mechanism, may conveniently provide both linear motion mechanisms as well as position tracking devices, described below. Alternatively, a lead screw drive assembly actuator apparatus comprises linear motion mechanisms and print head alignment mechanisms in one readily obtainable item.
A print head sensor 78 may be mounted on print head assembly 50 by attachment to print head frame 52 that slidably engages a fixed, etched glass bar 80. Print head sensor 78 may be, for example, an optical sensor or a magnetic sensor, and is operably connected by a data line (not shown) to a mechanism such as computer 112 for comparing the position of print head 44 relative to the position of sheeting 46. Alternatively, print head sensor 78 may be a linear scale system made by Sony Corporation, which system comprises a bar and suitable sensor. Other print head position tracking devices known in the art also may be used.
A transport advances or transports thermal print receptive sheeting past a print head in a first direction, (indicated by arrow y in FIG. 3). In the embodiment depicted in FIG. 4, the transport is shown to comprise a platen 42 having a shaft 98 projecting at each end therefrom. Shaft 98 extends at one end through sheeting positioning mechanism 96 to axially rotatably engage platen stepper motor 94. Sheeting positioning mechanism 96 is rotatably coupled to shaft 98 and may be, for example, adjustable pin feed wheels of tractor drive mechanisms. Thermal print receptive sheeting 46 rests upon platen 42 and engages sheeting positioning mechanism 96 through drive holes in sheeting 46. Platen 42 provides a platform against which print head 44 can maintain a uniform contact pressure as print head 44 and ribbon 66 travel across sheeting 46. Platen 42 generally is made of a material having medium hardness and moderate resilience. The material selected to form platen 42 depends to some extent upon the flexibility and resilience of sheeting 46, in order to allow suitable uniform movement of ribbon 66 under head 44.
A preferred sheeting transport can comprises a tractor drive mechanism sold under the trade name Model ST-611, by Seitztec, Torrington, Conn. A friction drive assembly, however, may be substituted for a tractor drive mechanism if less graphic accuracy in the formed indicia is acceptable. A friction drive assembly has a lower cost and eliminates the need for drive holes in sheeting 46. It is possible in this invention, although typically less preferable, to use a manual transport of the sheeting.
An optional sheeting sensor may be employed to receive positional information regarding sheeting 46. In the embodiment shown in FIG. 4, a sensor 100 is located near a graphic edge of sheeting 46. Sheeting sensor 100 may comprise a light actuated sensor providing accurate positional information suitable for identifying the down web position of sheeting 46 in the y direction. Other types of sensors are also suitable, such as a tactile sensor or a gap hole sensor. A