Variable platen pressure control for a thermal transfer printer

A multi-color thermal transfer printer for printing multi-color license plate images on a web of retroflective sheeting material, the printer having a plurality of printing stations. The printer, at each of the printing stations, has a thermal transfer printing head, a printing ribbon having an ink of a pre-selected color, a pivotally-mounted roller platen assembly for pressing a respective one of the printing ribbons and a web of material to be printed upon, against a respective one of the printing heads at a variably selectable pressure to transfer the ink from the ribbon to the web of material. A cylinder device is provided at each of the stations. Each cylinder device is adjustably pressurized by a fluid medium and engaged with a respective one of the roller platen assemblies for the purpose of applying a variably selectable pressure to the pivotally-mounted roller platen assembly in order to control roller platen pressure.

FIELD OF THE INVENTION 
The present invention relates to thermal transfer printers and more 
specifically, to a thermal transfer printer having variably adjustable 
roller platen pressure control. 
BACKGROUND OF THE INVENTION 
Currently available thermal transfer printing heads generally comprise a 
plurality of thermal elements. The heating elements are generally provided 
on one side of flat substrate of ceramic or like material along with the 
requisite electronic circuitry for controlling the activation of the 
heating elements. 
The thermal transfer printing head is typically used in conjunction with a 
roller platen assembly and an ink transfer printing ribbon which carries a 
thermally transferred printing ink. During printing, a web of material to 
be printed and the printing ribbon are oriented between the thermal 
elements of the printing head and the roller platen of the roller platen 
assembly such that the printing ribbon is adjacent to the print head and 
the material to be printed is adjacent to the roller platen. The roller 
platen and printing head are moved against each other so that the printing 
ribbon and the material to be printed are pressed against the printing 
head such that when selected thermal elements of the printing head are 
heated, ink from the ink transfer printing ribbon is transferred to the 
material's surface. 
Typical thermal transfer printers can include one or more printing heads 
each of which are capable of printing in a different color. If more than 
one printing head is employed, the printing heads are generally arranged 
in line with each other, with each printing head having a roller platen 
assembly associated therewith. 
In order to produce high quality printing, the proper pressure and thermal 
energy must be applied by the printing head to the material to be printed 
and ink printing ribbon. The proper printing pressure is dependent upon 
many factors including the type of printing ribbon used, the material to 
be printed, and the width and thickness of the printing ribbon and/or 
material to be printed. For example, the printing pressure requirements 
for a waxed based printing ribbon are generally different than the 
printing pressure requirements for a resin based printing ribbon. The 
printing pressure requirements would also be different for a paper based 
material to be printed and a material to be printed comprised of vinyl 
coated retroflective sheeting. 
When the transfer printer includes multiple printing heads and the material 
to be printed being requires the use of less than all the printing heads, 
the platen rollers must be moved away from the print heads to preserve the 
printing ribbon. This is typically accomplished by manually inserting some 
type of block or like means which moves the roller platen assembly away 
from the printing heads that are not being used in a particular printing 
job. 
The prior art has attempted to address the issue of printing pressure. For 
example, U.S. Pat. No. 4,228,441, issued to Rhine entitled PRINTER HEAD 
BIASING APATUS, discloses a cantilevered leaf spring arrangement which 
maintains a constant and preset pressure between the print head and the 
material to be printed. The cantilevered spring is adjustable to provide 
the desired amount of pressure between the printing head and the material 
to be printed. 
U.S. Pat. No. 4,844,632 issued to Minowa entitled HEAD BIASING MECHANISM IN 
A THERMAL PRINTER, discloses an elastic member for moving the printing 
head toward and away from the platen. The elastic member urges the 
printing head against the platen at least two levels of biasing force. The 
Minowa patent also addresses the issue of printing head disengagement by 
providing a release mechanism for selectively releasing the biasing force 
on the printing head. 
U.S. Pat. No. 4,949,098 issued to Cluck et al. entitled THERMAL PRINTHEAD 
CONTROLLING MEANS, discloses a printing head controlling structure for 
disposing the printing head in a non-printing position and for resiliently 
supporting the printing head in the printing position. 
U.S. Pat. No. 5,206,662 issued to Fox et al. entitled METHOD AND APATUS 
FOR ADJUSTING CONTACT PRESSURE OF A THERMAL PRINTHEAD, discloses a spring 
mechanism for applying a torque against an arm for rotating a shaft to 
make printhead contact with the printing ribbon and roller platen. The 
spring mechanism allows the a wide variety of printer media to be 
accommodated since the pressure between the platen roller an the printing 
head is variably adjustable. 
U.S. Pat. No. 5,328,281 issued to Narita et al. entitled RECORDING MEDIUM 
FEED MECHANISM FOR A PRINTER AND METHOD OF MEDIUM FEED CONTROL, discloses 
a printer for an electronic calculator or electronic cash register. The 
printer disclosed therein employs solenoids for engaging and disengaging 
contact pressure rollers which permit the separation or disengagement and 
contacting or engagement with the printing medium. 
U.S. Pat. No. 5,448,281 issued to Walter et al. entitled PRINT HEAD 
PRESSURE ADJUSTING MECHANISM, discloses a printing pressure adjustment 
system. The adjustment system employs a steel spring for adjusting the 
pressure of the printing head. 
The mechanisms described above still don't offer the precision necessary to 
optimize printing pressure for various types of printing ribbons and 
materials to be printed which are available. 
The pressure required for closing of the top door is directly related to 
the pressure applied on the platen. The pressure required for a reflective 
media could not be implemented without the necessity of a complex latch 
assembly. 
Accordingly, the is a need for a thermal transfer printer which employs a 
roller platen assembly having a variably adjustable roller platen pressure 
that can be adjusted in substantially fine increments in order to optimize 
the printing process for various types of printing ribbons and printing 
materials. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided a thermal 
transfer printer comprising a thermal transfer printing head, a printing 
ribbon having an ink of a pre-selected color, a pivotally-mounted roller 
platen assembly for pressing the printing ribbon and a web of material to 
be printed upon, against the printing head at a variably selectable 
pressure to transfer the ink from the ribbon to the web of material. A 
cylinder device adjustably pressurized by a fluid medium and engaged with 
the roller platen assembly, applies a variably selectable pressure to the 
pivotally-mounted roller platen assembly to control the roller platen 
pressure. 
In another embodiment of the present invention, the printer comprises 
multiple printing stations, each of which includes the print head and 
pressure controlled roller platen arrangement described above.

DETAILED DESCRIPTION OF VARIOUS ILLUSTRATIVE EMBODIMENTS 
Referring to FIG. 1, a typical prior art multi-color thermal transfer 
printer 10 is schematically depicted. The printer 10 shown was originally 
intended for printing onto paper using wax ribbons. However, recent 
advances have been made in the sign art and especially, in the manufacture 
of license plates. These advances involve the manufacture of license 
plates from webs of vinyl coated retroreflective sheeting material, which 
are first printed with the desired license plate images and then, 
laminated to rigid substrate material. The printed images can include 
logos, trademarks, scenes, designs, artworks, and alphanumerics. The use 
of such images afford the license plates with a personalized character. 
Moreover, the images can also include one or more machine readable 
identification codes which allow each of the license plates to be 
manufactured more efficiently. Further, the identifications codes also 
function as distribution markers, for automatic toll collection and other 
similar uses, and allow for subsequent counterfeit protection and license 
plate verification. 
The license plates described above are presently manufactured using the 
multicolor thermal transfer printer of FIG. 1. Although the printer was 
specifically designed for printing on paper, the printer 10 is also 
capable of printing the multi-color logos, trademarks, scenes, designs, 
artworks, alphanumerics, and ID codes onto the unprinted webs of vinyl 
coated retroreflective sheeting material. 
The printer 10 includes a series of essentially identical in line printing 
stations 12 for printing onto an unprinted web 22 of vinyl coated 
retroreflective sheeting material. Each of the printing stations 12 has a 
thermal printing head 14, a pivotally-mounted roller platen assembly 16 
which includes a roller platen 17, and a resin-based printing ribbon 24 of 
a pre-selected color. In order to maintain the appropriate amount of 
printing pressure, each roller platen assembly 16 is coupled to a 
coil-like spring 18 which presses the roller platen 17 of the roller 
platen assembly 16 against the printing ribbon 24 and the web 22 of 
retroreflective sheeting material, thereby forcing the printing ribbon 24 
and the web 22 of retroflective sheeting material against the printing 
head 14. Since the retroreflective sheeting material used for printing 
license plates requires substantially higher roller platen pressure than 
the paper medium that the printer 10 was designed to print upon, higher 
rate coil springs are employed. 
The use of the higher rate coil springs causes problems with the top door 
closure. A further problem inherent in the design of the printer 10 
involves the method for adjusting the roller platen pressure. More 
specifically, when the roller platen pressure needs to be adjusted in 
order to compensate for the type of printing ribbon used, the type of 
sheeting material being printed upon, etc., the coil springs 18 must be 
removed and replaced with a different set of coil springs having a 
different spring rate. Accordingly, many different sets of coil springs of 
differing spring rates must be maintained in order to allow for fine 
adjustments in roller platen pressure. Further, determining which spring 
set is appropriate for a given type of sheeting material, printing ribbon, 
etc., is a hit and miss process which requires costly printer down time. 
Additionally, the coil springs can fatigue and thus, exhibit a lower 
spring rate which lowers the roller platen pressure and detrimentally 
effects the printing process. 
Another problem inherent in the design of printer 10 involves the method 
for deactivating one or more of the printing stations 12. This is 
presently accomplish by a block device 20 which is manually inserted under 
the roller platen assembly 16 to lift the roller platen 17 away from the 
printing head 14 when the image being printed on the web 22 of 
retroflective sheeting material, requires the use of less than all the 
printing stations 12. 
Referring to FIG. 2, there is shown an embodiment of a multi-color thermal 
transfer printer 30 according to the present invention. The printer 30 is 
somewhat similar to the printer 10 of FIG. 1 in that it includes a series 
of essentially identical in line printing stations 32 for printing onto an 
unprinted web 52 of vinyl coated retroreflective sheeting material. Each 
printing station 32 employs a thermal printing head 34, a 
pivotally-mounted roller platen assembly 36 and a resin-based printing 
ribbon 50 of a pre-selected color. However, the printer 30 of the present 
invention doesn't employ the prior art's coil spring arrangement for 
roller platen pressure control. Instead, each printing station 32 of the 
printer 30 of the present invention comprises a cylinder device 38 which 
is pressurized by a fluid medium. The cylinder device 38 applies a 
constant, finely adjustable force to the roller platen assembly 36, for 
controlling the roller platen pressure and thus, ensures the proper 
transfer of ink from the printing ribbon 50 to the web 52 of 
retroreflective sheeting material. As will be explained, the pressurized 
cylinder devices 38 can be selectively activated and deactivated as needed 
for a given license plate image. 
In a preferred embodiment of the present invention, the fluid medium used 
for pressurizing the cylinder devices 38 is a compressible, gas such as 
air. Accordingly, the pressurized cylinder devices 38 comprise 
conventional pneumatically pressurized cylinders 38. Each pneumatically 
pressurized cylinder 38 includes a piston 78 (FIGS. 4A and 4B) which 
applies a variably adjustable force to an associated roller platen 
assembly 36 in accordance with the air pressure at the cylinders 38. In 
other embodiments of the present invention, the fluid medium used for 
pressurizing the cylinder devices 38 comprise a substantially 
non-compressible fluid such as oil. Accordingly, in these embodiments, 
hydraulically pressurized cylinders or the like are employed in place of 
the pneumatically pressurized cylinders 38. 
In any case, all the pressurized cylinders 38 are coupled to a mechanical 
or electrically controlled fluid pressure regulator 44 that allows for 
very fine adjustments of the fluid medium pressure supplied to the 
pressurized cylinders 38 to variably adjust and maintain proper roller 
platen pressure. When pneumatically pressurized cylinders are employed, 
the pressure regulator 44 comprises a conventional air pressure regulator. 
Accordingly, optimal roller platen pressures for printing can be easily be 
determined during process development by simply adjusting the air pressure 
of the pneumatically pressurized cylinders 38 via the air pressure 
regulator 44. In addition, the pneumatically controlled roller platen 
pressures can be easily maintained during regular production since, simple 
air pressure adjustments can be made for proper ink transfer with lot to 
lot variations in printing ribbons, reflective sheeting or when printing 
ribbon and reflective sheeting vendors are changed. 
Referring still to FIG. 2, an electrically activated fluid medium control 
valve 42 is coupled between each of the pressurized cylinders 38 and the 
pressure regulator 44. When pneumatically pressurized cylinders are 
employed, the control valve 42 comprises a conventional air control valve. 
The electrically activated air control valves 42 allow their respective 
pneumatic cylinders 38 to be selectively deactivated when a particular 
license plate image requires less than all the printing stations 32. A 
roller platen return spring 46 is provided for moving the deactivated 
roller platen assembly 36 away from the printing head 34 when the printing 
station 32 is not in use. This would be desirable for example, in the 
situation where the license plate image being printed is only 2 or 3 
colors, and thus, requires less than all the printing stations 32. This 
automatic feature eliminates the use of the manually inserted block 
devices of the prior art. 
In order to allow all the pressurized cylinder devices 38 to be 
simultaneously deactivated without having to shut down the fluid medium 
supply or change the fluid medium pressure regulator setting, the present 
invention provides an electrically activated main fluid medium control 
valve 48 coupled between the fluid medium supply 49 and the fluid medium 
pressure regulator 44. When pneumatically pressurized cylinders are 
employed, the main fluid medium control valve 48 comprises a main air 
control valve. 
It should be understood, that when hydraulically pressurized cylinders or 
the like are employed, the fluid medium pressure regulator, the fluid 
medium control valves, and the main fluid medium control valve are 
appropriately selected for use with a hydraulic fluid medium such as oil. 
Such hydraulic components are well known in the art. 
The air pressure regulator 44, the electrically activated air control 
valves 42 and the electrically activated main air control valve 46 are all 
coupled to a process control computer 56. The process control computer 56 
includes a database which stores information about the printing ribbon, 
the printing medium, image, etc., and uses this information to 
automatically select the appropriate pneumatic cylinders 38 and pneumatic 
cylinder control pressure via the air pressure regulator 44 and the 
electrically activated air control valves 42 and 48. This can be also 
applied to each print station. If desired, the process control computer 56 
can be used to manually by an operator, to select the pneumatic cylinders 
38 and control the air pressure via the air pressure regulator 44 and the 
electrically activated air control valves 42 and 48. In an alternate 
embodiment the air control valves 42 and 48 can be manually actuated. If 
during the printing process, a problem occurs, the computer 56 can 
automatically deactivate all the pneumatic cylinders 38 via the main air 
control valve 48, to stop the process until the problem can be corrected. 
In an enhancement to the present invention, each license plate image may 
include at least one machine readable identification code (bar code) or, 
the bar codes may be preprinted on the vinyl coated retroreflective 
sheeting material 52. The bar codes can be scanned by an optical scanner 
to gain access to a database in the process control computer 56 which 
stores information about the image to be printed on the license plates by 
the printer of the present invention. When the bar codes are preprinted on 
the sheeting material 52, additional automatic control over the printing 
process can be had by providing an optical scanner 54 for reading the 
preprinted identification codes at each printing station 32. In such an 
embodiment of the present invention, the optical scanners 54 are also 
coupled to the process control computer 56 along with the air pressure 
regulator 44, the electrically activated air control valves 42 and the 
electrically activated main air control valve 48. As the sheeting material 
enters each printing station, the optical scanner 54 reads the 
identification code and signals the computer to select the appropriate 
pneumatic cylinders 38 and pneumatic cylinder control pressure via the air 
pressure regulator 44 and the electrically activated air control valves 42 
based on the image information stored in the database of the process 
control computer 56. 
Referring to FIG. 3, there is shown an embodiment of one of the pivoting 
roller platen assemblies 36 employed in the present invention. The 
pivoting roller platen assembly 36 generally includes a pair of parallel 
spaced linkage arms 58 which rotatively retain a cylindrically-shaped 
rubber roller platen 60 which extends there between. Each linkage arm 58 
includes a first end 62 and a second end 64. The first end 62 of each 
linkage arm 58 has a bushing 66 installed therein for receiving a spindle 
68 extending from each end of the roller platen 60. The second end 64 of 
each linkage arm 58 defines an aperture 70 for receiving a set screw pivot 
pin 72. A plate member 74 extends between the linkage arms 58 immediately 
adjacent to the roller platen 60 and includes an enlarged section 76 which 
is engaged by the piston 78 of a respective pressurized cylinder 38. 
FIG. 4A illustrates how the pressurized cylinder 38 controls roller platen 
pressure. As shown, the pressurized cylinder device 38 is mounted above 
the roller platen assembly 36 so that the piston 78 of the cylinder device 
38 engages the enlarged plate section 76 of the roller platen assembly 36. 
As the air pressure is increased or decreased to the cylinder device 38, 
the piston 78 applies more or less pressure respectively, to the roller 
platen assembly 36 via the enlarged plate section 76 accordingly, 
increasing or decreasing the roller platen pressure. When air pressure is 
completely removed from the cylinder device 38, the roller platen return 
spring 46 pivots the roller platen assembly 36 up and away from the 
printing head 34 which forces the piston 78 into the cylinder device 38 as 
shown in FIG. 4B, and thus, deactivates the printing station. 
Alternatively, the roller platen return spring 46 may be omitted, as the 
media path will raise the roller platen assembly 36 up and away from the 
printing head 34. 
Numerous modifications and alternative embodiments of the invention will be 
apparent to those skilled in the art in view of the foregoing description. 
Accordingly, this description is to be construed as illustrative only and 
is for the purpose of teaching those skilled in the art the best mode of 
carrying out the invention. Details of the structure may be varied 
substantially without departing from the spirit of the invention and the 
exclusive use of all modifications which come within the scope of the 
appended claims is reserved.