Transfer-type thermal printer

A transfer type thermal printer includes a heat sensitive ink ribbon which is fed along an ink ribbon feeding path, a part of which is defined as a recording section, a thermal printhead and a platen roller, both located at the recording section in pressure contact with the ink ribbon sandwiched therebetween. A paper transporting path is defined in the printer for transporting a sheet of recording paper through the recording section where the recording paper is passed between the printhead and the platen roller in surface contact with the ink ribbon. A main feature of the present printer includes a housing which is generally divided into two: upper housing half and lower housing half, and the upper housing half is pivotted to the lower housing half at one end so that the upper housing half may be pivotted open or closed with respect to the lower housing half with the printhead being mounted in the upper housing half and the platen roller being disposed in the lower housing half.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention generally relates to a thermal printer for printing an 
image, such as a character and symbol, by applying a heat pattern of an 
image to be printed to a recording medium, and, in particular, to a 
transfer type thermal printer in which a heatsensitive ink ribbon is used 
to transfer ink in the form of a desired heat pattern applied by a thermal 
printhead to a recording medium. 
2. Description of the Prior Art 
A transfer type thermal printer is well known in the art. In such a 
printer, a heatsensitive ink ribbon is placed as sandwiched between a 
recording medium, typically plain paper, and a thermal printhead, and, 
according to a heat pattern created on the thermal printhead in accordance 
with an image signal supplied thereto, the ink on the ink ribbon is 
selectively melted and transferred to the recording medium thereby forming 
a printed image on the recoding medium by the transferred ink. Such a 
transfer type thermal printer has numerous advantages, including 
capability of providing a printed image of excellent quality, high 
printing speed and quiet operation. Accordingly, it has been and is being 
actively applied as an output device of computer and/or wordprocessor 
system, or as a recording section of facsimile machine. 
An ink ribbon or sheet used in such a transfer type thermal printer 
includes a base of thin resin film, paper or the like and an ink layer 
formed on the base. When manufacturing such an ink ribbon, the ink is 
first applied to the base as being heated to be in a melted condition and 
then it is cooled to the room temperature to solidify. The ink forming the 
ink layer is thus in a solid state at room temperature, and, thus, the ink 
is not transferred to any object even if it is brought into contact 
therewith. On the other hand, if the ink is heated above a predetermined 
temperature, it melts and becomes easily transferred to an object which is 
brought into contact therewith. 
The thermal printhead contacts the ink ribbon at its base side so that a 
heat pattern created by the thermal printhead is applied to the ink layer 
as conducted through the base. Thus, in order to maintain a high printing 
speed and to reduce energy consumption, it is desirous to make the base of 
ink ribbon as thin as practically possible thereby allowing to increase 
the rate of heat transfer to the ink layer through the base under a given 
condition. Since the ink layer itself is already substantially thin, when 
the base is made thinner, the entier ink ribbon is made thinner. For this 
reason, extremely thin heatsensitive ink ribbons have recently become 
commercially available. 
However, in prior art transfer type thermal printers, it has been noted a 
difficulty in setting such a heatsensitive ink ribbon ready for operation. 
It is more often than not that the ink ribbon becomes creased or twisted 
while it is being set in position, which could then cause malfunctioning 
in ink ribbon feeding operation and/or printed image of poor quality. 
Moreover, in such prior art printers, when the ink ribbon jams during 
operation, it is not easy to remove a sheet of recording paper on which 
printing has been carried out from the printer and the ink ribbon could be 
easily damaged while this sheet of recording paper is being removed. Thus, 
if such jamming occurs in a prior art printer, it could be rectified only 
with a great difficulty. 
SUMMARY OF THE INVENTION 
It is therefore a primary object of the present invention to obviate the 
disadvantages of the prior art as described above and to provide an 
improved transfer type thermal printer. 
Another object of the present invention is to provide an improved transfer 
type thermal printer which is easy for an operator to handle. 
A further object of the present invention is to provide an improved 
transfer type thermal printer capable of providing a printed image of high 
quality and fast and quiet in operation. 
A still further object of the present invention is to provide an improved 
transfer type thermal printer which is so structured to facilitate setting 
of ink ribbon in position and removal of jammed recording medium. 
A still further object of the present invention is to provide an improved 
transfer type thermal printer which is reliable in operation, increased in 
convenience in usage and easy in maintenance. 
Other objects, advantages and novel features of the present invention will 
become apparent from the following detailed description of the invention 
when considered in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A transfer type thermal printer according to the present invention has 
numerous advantages over the prior art printers of the same kind, which 
include the following features. 
In the first place, the entire structure of the present printer is 
generally divided into upper and lower halves with a separation line being 
generally defined by a passage for ink ribbon and the upper half is 
pivotted to the bottom half at one end thereby allowing the upper half to 
be pivotted open or closed with respect to the bottom half. Second, the 
present printer includes a platen roller and a thermal printhead with a 
laminate of ink ribbon and recording medium sandwiched therebetween, in 
which the thermal printhead is provided in the upper half of the printer 
with the platen roller being provided in position in the lower half. The 
platen roller is driven to rotate intermittently thereby causing the 
laminate of ink ribbon and recording medium to move with respect to the 
thermal printhead. 
With such a structure, in which the upper half of the printer may be 
pivotted open or closed with respect to the bottom half at a separation 
line defined by the ink ribbon passage, the ink ribbon may be set in 
position or removed easily as well as securely, and, moreover, a jammed 
recording medium within the printer may be easily removed. It should also 
be noted that the platen roller is provided in the lower half of the 
printer. The platen roller is driven to rotate in an intermittent manner 
to have a laminate of ink ribbon and recording medium advance with respect 
to the thermal printhead. For this reason, high accuracy is required in 
implementing the intermittent rotation of platen roller, which then 
requires a structure in which the platen roller is driven by a driving 
motor directly. Thus, if the platen roller were to be provided in the 
upper half of the printer, then it would also require that its driving 
motor be provided in the upper half. This is apparently disadvantageous 
because such a driving motor is commonly heavy in weight thereby causing 
the pivotal motion of the upper half with respect to the lower half to be 
difficult to carry out. No such a problem arises if the platen roller is 
provided in the lower half according to the teachings of the present 
invention. 
Referring now to FIG. 1, there is shown in perspective a transfer type 
thermal printer constructed in accordance with one embodiment of the 
present invention. As shown, the present thermal printer includes an upper 
cover 1 generally defining an upper half, a main housing 2 generally 
defining a lower half, a tray 3, a cassette 4 detachably mounted in the 
printer for storing a stack of recording paper S and a door member 5. As 
will become clearer later, the upper cover 1 is pivotted to the main 
housing 2 at one end so that the upper cover 1 may be pivotted open or 
closed with respect to the main housing 2. The tray 3 is also detachably 
mounted on the upper cover 1. 
As the recording paper S, use may be preferably made of plain paper; 
however, use may, of course, be made of any other appropriate material, 
such as a film of resin. Various sizes of recording paper S may also be 
used and a number of cassettes 4 are preferably prepared for storing 
differently sized sheets of recording paper S, one cassette for each size. 
It should be noted that throughout the present specification the length of 
a sheet of recording paper S in the direction perpendicular to the 
direction of transportation of the recording paper S within the printer 
will be referred to as the width of recording paper S and this direction 
will be referred to as the widthwise direction of recording paper S 
irrespective of the size of recording paper S used. It is so structured 
that the recording paper S is always positioned with its center in its 
widthwise direction aligned with a predetermined position when its 
cassette 4 is detachably mounted in position irrespective of the size of 
recording paper S used. That is, in the illustrated embodiment, the center 
of a sheet of recording paper S used is used as a reference in operation, 
which may be termed as a center reference system. 
Although not shown specifically, it should be understood that each cassette 
4 is provided with a means for producing a particular magnetic field 
pattern depending on the size of a sheet of recording paper S to be stored 
therein. And, thus, when the cassette 4 is mounted in position, this 
particular magnetic field pattern may be detected by a detector provided 
in the main housing 2. Accordingly, the size of a sheet of recording paper 
S set ready for operation may be automatically detected and the operating 
or scanning range of a thermal printhead is automatically set in 
accordance with the detected with of recording paper S set ready for 
operation. As with become clearer later, it is to be noted that a separate 
sheet of recording paper S other than those stored in the cassette 4 
mounted in position may be manually fed for use in printing operation with 
the cassette 4 set in position. In this case, the door member 5 must first 
be pivotted open to allow manual insertion of recording paper S sheet by 
sheet. 
Referring now to FIG. 2, which illustrates the overall arrangement of 
various components provided in the present printer, a feed roller 11 is 
disposed such that it comes into contact with the topmost sheet of 
recording paper S when the cassette 4 is detachably mounted in position 
and it is driven to rotate in the clockwise direction intermittently to 
feed the topmost sheet of recording paper S. The feed roller 11 is 
preferably comprised of a plurality of roller segments fixedly supported 
on a common shaft as spaced apart from one another along the shaft. 
Besides, the feed roller 11 is preferably comprised of rubber at least at 
its peripheral surface, thereby allowing to secure a sufficient frictional 
force against the topmost sheet of recording paper S when brought into 
contact therewith. 
Downstream of the feed roller 11 with respect to the direction of 
advancement of recording paper S is disposed a transportation roller 12 
which is driven to rotate clockwise to have the recording paper S 
transported along a predetermined transportation path defined in the 
printer. Also disposed generally below the transportation roller 12 is a 
back-up roller 13. The rollers 12 and 13 are also each preferably 
comprised of a plurality of roller segments fixedly supported on a common 
shaft as spaced apart from one another similarly with the feed roller 11. 
Furthermore, these rollers 12 and 13 are also each preferably comprised of 
rubber at least at its peripheral surface in order to secure a sufficient 
friction against the recording paper S when in contact. It is to be noted 
that the back-up roller 13 rotates couterclockwise when driven and this is 
the roller which causes the accompanying sheets of recording paper S to 
return to the cassette 4 thereby insuring that sheets of recording paper S 
may be fed and transported one by one. This aspect will be described more 
in detail later. 
A guide plate 14 is provided as extending from one end of the cassette 4 in 
position to the vicinity of the location where registration rollers 18A 
and 18B are disposed, thereby defining part of the passage for 
transporting the recording paper S within the printer. Also provided is a 
pair of guide plates 15 and 16 which extend in parallel between the 
pivotal point of door member 5 and the location where the registration 
rollers 18A and 18B are disposed, thereby defining another passage for 
transporting a sheet of recording paper S which is inserted into the 
printer manually. Thus, the forward ends of guide plate 14 and the paired 
guide plates 15 and 16 meet at a point between the rollers 18A and 18B 
thereby defining an inlet point to the paired rollers 18A and 18B. 
A paper sensor 17 is disposed near the point where the above-described two 
paper transporting passages meet and it has a feeler or actuator 17A 
extending generally upwardly across the two paper passages. Of course, the 
guide plates 14-16 are suitably cut-away to allow the actuator 17A to 
pivot around the sensor 17 when it is pushed forward through engagement 
with the leading edge of a sheet of recording paper S in transportation. 
It should also be appreciated that the actuator 17A extends long enough to 
be actuated by a sheet of recording paper S which is transported either 
through the lower passage from the cassette 4 or through the upper passage 
as inserted manually. 
The registration rollers 18A and 18B are normally in contact with the 
roller 18A being used as a driving roller and the roller 18B as a follower 
roller. The roller 18A at the driving side is comprised of rubber at least 
at its peripheral surface, whereas the roller 18B at the follower side is 
comprised of stainless steel. As shown in FIG. 2, a center-to-center line 
between the rollers 18A and 18B is inclined with respect to a vertical 
line, and the forward end of the guide plate 14 is located somewhat below 
the nip between the two rollers 18A and 18B thereby insuring that a sheet 
of recording paper S may be smoothly inserted into the nip between these 
rollers. The guide plate 14 extends as inclined rising gradually from its 
end adjacent to the feed roller 11 toward its forward end adjacent to the 
nip between the registration rollers 18A and 18B. 
Downstream of the registration rollers 18A and 18B with respect to the 
direction of advancement of a sheet of recording paper S is disposed 
another pair of guide plates 19 arranged convergent toward their forward 
ends to define part of passage for transporting a sheet of recording paper 
S from the registration rollers 18A and 18B toward a platen roller 20, 
which is rotatably supported on the main housing 2. The platen roller 20 
is comprised of rubber at least at its peripheral surface and coupled to a 
step motor (not shown) for intermittent rotation in either direction 
selectively. 
Opposite to the platen roller 20 is disposed a thermal printhead 25 which 
is generally elongated in shape extending in the direction perpendicular 
to the plane of the drawing. The thermal printhead 25 has a structure 
which is well known for one skilled in the art and thus its detailed 
description will be omitted here. Briefly stated, the thermal printhead 25 
includes an elongated rectangular substrate on which a plurality of 
heat-producing elements, e.g., electrical resistors, are arranged in the 
form of a single array at a predetermined pitch, and the heat-producing 
elements are selectively activated in accordance with an image signal 
supplied thereto to produce a heat pattern, which is then applied to an 
ink ribbon for recording as will become clear later. Here, that portion of 
the thermal printhead where the array of heat-producing elements is 
provided will be called write-in section. 
The thermal printhead 25 is supported on a support bracket 26 as fixedly 
attached thereto, and during recording operation, the platen roller 20 is 
pressed against the thermal printhead 25, more precisely against the 
write-in section of printhead 25, across its full width with a laminate of 
ink ribbon IS and recording paper S sandwiched therebetween. It is to be 
noted that a pressure contact section between the platen roller 20 and the 
thermal printhead 25 will be called recording section in this 
specification. 
A further guide plate 28 is disposed at the downstream side of the 
recording section and it defines a passage for transportation of recording 
paper S from the recording section to a nip between a pair of paper 
discharge rollers 30A and 30B. A separating pawl 29 is disposed adjacent 
to the guide plate 28 and it serves as an auxiliary means for securing 
separation of a sheet of recording paper S from the ink ribbon IS after 
recording. As shown in FIG. 2, a plurality of pairs of paper discharging 
rollers 30A-30B, 32A-32B and 34A-34B and a plurality of pairs of guide 
plates 31A-31B and 33A-33B are disposed at appropriate positions to define 
a paper discharging passage extending from the forward end of guide plate 
28 to the tray 3, along which a sheet of recording paper S on which a 
desired image has been printed is transported. 
The paper discharging rollers 30A-30B, 32A-32B and 34A-34B are preferably 
each comprised of a plurality of roller segments generally in the shape of 
discs fixedly supported on a common shaft spaced apart from one another 
along the shaft. The rollers 30A, 32A and 34A are follower rollers and 
preferably comprised of a material, such as resin and aluminum, which is 
difficult to be contaminated with ink. On the other hand, the rollers 30B, 
32B and 34B are driving rollers and they are preferably comprised of a 
material, such as rubber, which has a sufficiently large frictional 
coefficient against the recording paper S used. In the preferred 
embodiment, all of these paper discharging rollers 30, 32 and 34 are 
driven to establish an equal paper transportation speed at each position, 
which is faster than the paper transportation speed established by the 
platen roller 20 or a relay roller 27. 
Thus, a sheet of recording paper S after transporting along the upper feed 
passage if inserted manually or along the lower feed passage if fed from 
the cassette 4 is transported toward the recording section defined between 
the platen roller 20 and the thermal printhead 25 as driven by the 
registration rollers 18A and 18B and guided by the guide plates 19, and 
recording is effected to the sheet of recording paper S as it moves past 
the recording section. Then, the recording paper S is transported to the 
paper discharging passage as guided by the guide plate 28, and, 
thereafter, the recording paper S is discharged onto the tray 3 after 
having been transported along the passage defined by the rollers 30, 32 
and 34 and the guide plates 31 and 33. The paper transportation passage 
from the cassette 4 to the tray 3 is indicated by the dotted line in FIG. 
2. 
A second paper sensor 38 is disposed immediately downstream of the paper 
discharging rollers 30A and 30B for detecting entrance of recording paper 
S into the paper discharging passage, and the sensor 38 has a pivotally 
supported actuator 38A which traverses the paper discharging passage so as 
to detect the passage of recording paper S by physical engagement 
therewith. For this purpose, the guide plates 31A and 31B are partly 
cut-away to accommodate the pivotal motion of actuator 38A when it is 
pushed forward by coming into engagement with the advancing recording 
paper S. 
The ink ribbon IS is originally stored in the form of a roll and it is 
rotatably supported in the upper half of the printer. The ink ribbon IS 
lead out of the roll passes around a first guide pipe 21 at its underside, 
around a second guide pipe 23 at its top side and then around a third 
guide pipe 24 at its underside, and, after passing through the recording 
section, it passes around the relay roller 27 at its underside to reach a 
take-up spool 39 fixedly mounted on a take-up shaft 40. Thus, the ink 
ribbon IS is wound around the take-up spool 39 as the take-up shaft 40 is 
driven to rotate counterclockwise. It is to be noted that the take-up 
shaft 40 is provided in the lower half or main housing of the present 
printer. The transporting path for ink ribbon IS from the supply roll to 
the take-up spool 39 is indicated by the one-dotted line in FIG. 2. 
The guide pipe 21 is rotatably mounted on the printer upper half. On the 
other hand, the guide pipe 22 is rotatably supported at the free end of an 
arm (not shown) whose base end is pivotally supported at a shaft for 
rotatably supporting the guide pipe 21. As a result, the guide pipe 22 is 
not only rotatale around its own rotating axis, but also is pivotally 
movable between the position indicated by the dotted line and the solid 
line. The guide pipe 22 thus contacts the ink ribbon IS at its top 
surface, i.e., the back surface of the ink ribbon where the base is 
provided. The guide pipe 23 on the other hand is rotatably mounted on the 
printer lower half or main housing 2; however, the guide roller 24 is 
rotatbly mounted on the printer upper half and it is preferably comprised 
of foam rubber to be low in hardness but larger in outer diameter. 
The relay roller 27 is preferably comprised of rubber or foam rubber at 
least at its surface so as to provide a sufficient power transmitting 
ability due to frictional contact with the base of ink ribbon IS, and it 
is preferably driven to rotate to establish the paper transportation speed 
which is larger than the paper transportation speed established by the 
platen roller 20 by 1-10%. A control unit 35 is mounted on the bottom 
plate of main housing 2 and it includes various electronics components, 
such as CPU, which are mounted on printed circuit boards. It should be 
understood that the present printer includes two driving motors (not 
shown), one of which is the previously described step motor and the other 
is a common motor for use in continuous rotation. 
Now, the above-described various components of the present printer will be 
described as divided between the printer upper and lower halves. As 
described previously, the printer upper half, which is mainly defined by 
the upper cover 1 and its frame structure (not shown), is pivotted at a 
pivot X to the printer lower half, which is mainly defined by the main 
housing 2 and its frame structure (not shown), so that the upper half may 
be pivotted open or closed with respect to the printer lower half. The 
printer upper half is provided with the guide pipe 21, tension pipe 22, 
guide roller 24, thermal printhead 25, relay roller 27, paper discharging 
rollers 32A, 32B, 34A and 34B and guide plates 33A and 33B. It should be 
noted that the ink ribbon IS is operatively set in the printer upper half 
as described previously. The remaining components, including the platen 
roller 20, are all provided in the printer lower half. 
Thus, when the printer upper half or upper cover 1 is pivotted open around 
the pivot X as if an alligator yawns, as shown in FIG. 3, the printer 
separates into the upper and lower halves along the ink ribbon 
transporting passage defined within the printer as indicated by the 
one-dotted line in FIG. 2. An auxiliary cover 37 is pivotted to the lower 
half at a pivot 37A and its free end rests on the upper cover 1, so that 
when the upper cover 1 is pivotted open, the auxiliary cover 37 also 
pivots following the movement of the upper cover 1, as shown in FIG. 3. 
It will now be described as to a mechanism for driving the various 
components of the printer. As described previously, the present printer is 
provided with a step motor as well as a common motor, though these motors 
are not specifically shown in the drawings. It should, however, be 
understood that these motors are mounted in the main housing 2. The step 
motor is used for transmitting driving power to the platen roller 20, 
relay roller 27 and take-up shaft 40, wherein the platen roller 20 and 
relay roller 27 are directly driven by the step motor but the take-up 
shaft 40 is driven at constant torque by the step motor through a 
well-known frictional coupling. On the other hand, the common motor is 
used for transmitting driving power to the feed roller 11, transporting 
roller 12, back-up roller 13, registration roller 18A at the driving side 
and paper discharging rollers 30B, 32B and 34B at the driving side. In 
this case, the back-up roller 13 and paper discharging rollers 30B, 32B 
and 34B are driven at constant torque by the common motor through a 
well-known frictional coupling. 
As is obvious for those skilled in the art, the operation of each of the 
rollers is controlled at predetermined timing using electromagnetic 
clutches or the like. Furthermore, control over printing process is 
provided by the control unit 35, which has a structure schematically shown 
in block form in FIG. 4. 
Referring now to FIG. 4, a section enclosed by the solid line 4-1 indicates 
the present transfer type thermal printer and it is shown to be connected 
to a host system, which is a system for supplying an image signal to be 
printed to the present printer 4-1 and typically comprised of computer, 
wordprocessor, or any other type of communication unit. The printer system 
4-1 includes a video interface through which the host system transmits and 
receives data to and from the printer system 4-1. A sub-section indicated 
by the dotted line 4-2 within the printer system 4-1 corresponds to the 
control unit 35, which is shown to include a CPU, video interface, bit 
unit energy control circuit, RAM, printhead driver, pulse width 
determining circuit, amplifier and mechanical structure driver. The 
printhead driver is connected to the thermal printhead 25 for selectively 
driving its array of heat-producing elements in accordance with an image 
signal to be printed. On the other hand, the mechanical structure driver 
is connected to drive such components as motors, clutches and solenoids. 
The bit unit energy control circuit and pulse width determining circuit 
will be described in detail later. 
Now, a printing process carried out by the present transfer type thermal 
printer will be described in detail below. 
In the first place, the upper cover 1 or printer upper half is pivotted 
open with respect to the main housing or printer lower half as shown in 
FIG. 3, and the ink ribbon IS in the form of a roll is set in position in 
a holding mechanism (not shown) provided in the printer upper half with 
the take-up spool 39, to which the leading end of the ink ribbon IS is 
fixedly attached, being fitted onto the take-up shaft 40. Upon completion 
of setting the ink ribbon IS in position in this manner, the upper cover 1 
is pivotted closed around the pivot X, thereby establishing the condition 
shown in FIG. 2. 
Under the condition, when a printing mode is turned on, a lift mechanism 
(not shown) is set in operation to lift the forward end portion of a stack 
of recording paper S stored in the cassette 4 to bring the topmost sheet 
of recording paper S into pressure contact with the feed roller 11. 
Subsequently, the feed roller 11, transporting roller 12 and back-up 
roller 13 are set in operation to be driven to rotate in the respective 
directions indicated by the arrows in FIG. 2. It is to be noted that since 
a frictional coupling is provided between the back-up roller 13 and its 
driving source, when the back-up roller 13 is in contact with the paper 
transporting roller 12, the back-up roller 13 rotates counterclockwise due 
to contact with the transporting roller 12 with a slippage produced in the 
frictional coupling. 
The clockwise rotation of feed roller 11 causes the topmost sheet of 
recording paper S to be discharged out of the cassette 4. The topmost 
sheet of recording paper S thus discharged from the cassette 4 comes into 
engagement with the transporting roller 12 which then causes this 
recording paper S to be transported along the guide plate 14. During this 
operation, there is slippage in the frictional coupling connected to the 
transporting roller 12. 
Although it does not happen often, two or more sheets of recording paper S 
may be fed at the same time by the feed roller 11. Even so, through a 
cooperation between the transporting roller 12 and back-up roller 13, it 
is insured that only the topmost single sheet of recording paper S is 
allowed to be fed toward the registration roller 18. Described more in 
detail in this respect, designating a frictional force between the paper 
transporting roller 12 and recording paper S by F.sub.F-P, a frictional 
force between two sheets of recording paper S by F.sub.P-P and a 
frictional force between the back-up roller 13 and recording paper S by 
F.sub.R-P, in the preferred embodiment of the present invention, it is so 
structured to hold that F.sub.F-P is larger than F.sub.P-P, F.sub.R-P is 
larger than F.sub.P-P and F.sub.F-P is larger than F.sub.R-P. In the 
preferred embodiment, the relation of F.sub.F-P being larger than 
F.sub.P-P and F.sub.R-P being larger than F.sub.P-P is realized by forming 
each of the transporting roller 12 and back-up roller 13 with rubber at 
least at its peripheral surface. Besides, the other relation of F.sub.F-P 
being larger than F.sub.R-P is realized by setting the maximum power 
transmission torque of the frictional coupling connected to the back-up 
roller 13 to satisfy this relation. That is, when only a single sheet of 
recording paper S is transported as driven by the transporting roller 12, 
the back-up roller 13 rotates counterclockwise as driven by the recording 
paper S in contact therewith, in which case slippage is produced in the 
frictional coupling connected to the back-up roller 13. 
With such a structure, even if two or more sheets of recording paper S 
happen to be discharged out of the cassette 4 to become pinched between 
the rollers 12 and 13, the back-up roller 13 changes its direction of 
rotation to rotate clockwise as indicated by the arrow in FIG. 3 thereby 
causing the half discharged sheets of recording paper S, excepting the 
topmost sheet, to be returned to the cassette 4. Accordingly, in the 
present structure, it is always insured that sheets of recording paper S 
stored in the cassette 4 are fed one by one toward the recording station. 
In the most preferred embodiment in order to establish a stable 
transporting operation of recording paper S, the paper transportation 
speed determined by the transporting roller 12 is set faster than the 
speed determined by the feed roller 11 and yet the speed determined by the 
back-up roller 13 is set faster than these two speeds. 
The recording paper S in engagement with the transporting roller 12 moves 
gradually upward along a slope defined by the guide plate 14 until its 
leading edge reaches the nip between the registration rollers 18A and 18B. 
During this movement, the recording paper S comes into contact with the 
actuator 17A thereby causing it to pivot to be kept out of the way. As 
described previously, the driver registration roller 18A is comprised of 
rubber at least at its peripheral surface and the follower registration 
roller 18B is comprised of stainless steel. These registration rollers 18A 
and 18B are not yet set in rotation when the leading edge of recording 
paper S comes into contact therewith. Since the forward end of guide plate 
14 is located somewhat below the nip between the registration rollers 18A 
and 18B, the leading edge of recording paper S being transported first 
comes into contact with the roller 18A of stainless steel and it slides 
along the peripheral surface of this roller to finally reach the nip 
between the two rollers 18A and 18B smoothly. 
The registration rollers 18A and 18B are driven to rotate after elapsing a 
predetermined time period as from the time when the sensor 17 is turned on 
by the pivotal motion of its actuator 17A. However, it is so structured 
that the registration rollers 18A and 18B start to rotate slightly after 
the leading edge of recording paper S having reached the nip between the 
rollers 18A and 18B. During this, the transporting roller 12 remains 
driven to rotate so that the recording paper S, which is obstructed in its 
forward movement with its leading edge in abutment against the nip between 
the tow rollers 18A and 18B, warps between the rollers 18A and 18B and the 
transporting roller 12. 
Subsequently, the registration rollers 18A and 18B start to rotate so that 
the recording paper S resumes its advancing motion toward the recording 
section defined between the thermal printhead 25 and the platen roller 20. 
With such a structure, the recording paper S is once restrained its 
advancing motion thereby becoming warped, and, then, the registration 
rollers 18A and 18B are driven to rotate to resume the advancing motion of 
recording paper S, irregularities in orientation, such as skew, and timing 
of transportation, which may arise during the movement from the cassette 4 
to the registration rollers 18A and 18B, can be properly absorbed and the 
recording paper S can be transported toward the recording section at 
proper timing and orientation. 
It is to be noted that when the registration rollers 18A and 18B are set 
into rotation, the transporting roller 12 is set in a free state so that 
it rotates following the movement of recording paper S. 
In the case where a sheet of recording paper S is to be inserted manually, 
it is fed into the passage defined between the guide plates 15 and 16 
through an opening in the main housing 2, which may be opened or closed by 
the door member 5. The recording paper S thus inserted then comes into 
contact with the actuator 17A at its leading edge to cause it to pivot to 
move out of the way, and, then, advances until its leading edge reaches 
the nip between the registration rollers 18A and 18B. It is so structured 
that the registration rollers 18A and 18B are set into rotation after 
elapsing a predetermined time period as from the time when the sensor 17 
is turned on due to the pivotal movement of the actuator 17A. In the 
preferred embodiment, if the length of passage from the position where the 
sensor 17 is turned on by the leading edge of recording paper S being 
transported to the nip between the registration rollers 18A and 18B is 20 
mm, this predetermined time period is preferably set in a range between 
0.5-1.5 seconds. 
It is important that the time period from the point in time when the sensor 
17 is turned on to the point in time when the registration rollers 18A and 
18B are set in rotation should be set differently depending on whether the 
recording paper S is fed manually or from the cassette 4, and, thus, it is 
important that these two different paper feeding modes be discriminated 
accurately so as to insure proper operation. This aspect of the present 
invention will be described in detail later. 
When the registration rollers 18A and 18B are set in rotation thereby 
causing the recording paper S to be transported toward the recording 
section, the platen roller 20 is driven to rotate counterclockwise thereby 
setting the leading edge of recording paper S at a start line of recording 
station, whereby the recording paper is brought into contact with the ink 
ribbon IS at least partly. When the recording paper S is so set at the 
recording station, the registration rollers 18A and 18B are set in a free 
state as being disconnected from the driving source. Then, the platen 
roller 20 is driven to rotate counterclockwise intermittently thereby 
causing a laminate of recording paper S and ink ribbon IS to advance along 
the thermal printhead 25 while being maintained as sandwiched between the 
platen roller 20 and the thermal printhead 25 under pressure. 
At the same time, an image signal is supplied to the thermal printhead 25 
so that the plurality of heat-producing elements provided in the printhead 
25 are selectively activated to form a heat pattern according to the image 
signal supplied, which is then applied to the laminate of recording paper 
S and ink ribbon IS. In this case, when activated, the heat-producing 
element produces heat, temperature of which is approximately 300.degree. 
C., momentarily. Thus, the heat thus produced is applied to the ink layer 
through the base, the ink layer selectively melts and becomes transferred 
to the recording paper S. 
As described previously, variously sized sheets of recording paper S may be 
used in the present printer and each of these differently sized sheets is 
transported along the passage defined in the printer with its center line, 
extending in the direction of advancement, as a reference. Accordingly, in 
the case where the width of a sheet of recording paper S used is shorter 
than the total width or length of write-in section of thermal printhead 
25, the active region of write-in section is so adjusted that an image 
signal is applied to those heat-producing elements which are located 
within the width of recoding paper S used. In the preferred embodiment, 
the image signal to be supplied to the thermal printhead 25 is masked 
corresponding to the width of recording paper S used, and timing of 
latching the signal into the printhead 25 is measured so as to carry out a 
proper printing operation in accordance with the width of recording paper 
S used. 
When emerging from the recording section, the recording paper S is in 
adhesive contact with the ink ribbon IS through the melted ink of the ink 
ribbon which has been transferred to the recording paper S, but they are 
separated from each other at the relay roller 27. Described more in detail 
in this respect, since the ink ribbon IS is extremely thin, a significant 
difference in stiffness exists between the ink ribbon IS and recording 
paper S. Thus, by suddenly changing the direction of advancement of ink 
ribbon IS by means of the relay roller 27, since the recording paper S is 
larger in stiffness, it cannot follow the sudden change in the direction 
of movement of ink ribbon IS so that it separates away from the ink ribbon 
IS and moves along the guide plate 28 toward the nip between the paper 
discharging rollers 30A and 30B. The paper separating pawl 29 is disposed 
adjacent to the guide plate 28 and it serves to separate the recording 
paper S from the ink ribbon IS securely if such a separation fails to take 
place at the relay roller 27. 
It is preferable to form the relay roller 27 as small as practically 
possible because it is intended to have the recording paper S separated 
away from the ink ribbon IS. The smaller the diameter of relay roller 27, 
the more secure in the operation of separating the recording paper S from 
the ink ribbon IS, so that it becomes possible to use recording paper S 
having less stiffness thereby allowing to increase the range of selection 
of recording paper S usable. After such separation, the ink ribbon IS is 
wound around the take-up spool 39. On the other hand, the recording paper 
S is transported along the paper discharging path defined by the rollers 
30A-B, 32A-B and 34A-B and guide plates 31A-B and 33A-B and discharged out 
onto the tray 3. In this manner, there is obtained a sheet of recording 
paper S on which a desired image is printed by the thermally transferred 
ink. 
It is to be noted that a common tangential plane defined at the nip between 
the paper discharging rollers 34A and 34B is not horizontal but somewhat 
inclined such that it gradually rises toward the right as viewing into 
FIG. 2. For this reason, the recording paper S is directed slightly 
obliquely upwardly when it is discharged out of the rollers 34A and 34B. 
Such a structure is advantageous since it can insure the formation of an 
excellent stack of printed recording paper S on the tray 3. 
As described previously, when the actuator 38A is pivotted downward due to 
engagement with the recording paper S in transportation, the sensor 38 
detects the leading edge thereof; on the other hand, when the actuator 38A 
is pivotted upward thereby returning to its original position due to 
disengagement with the recording paper S, the sensor 38 detects the 
trailing edge thereof. These detection signals are used to compare with 
predetermined values to determine as to whether jamming of the recording 
paper S has taken place or not and the time of completion of printing 
operation. 
Upon completion of printing operation at the recording section, the ink 
ribbon IS, together with the recording paper S, advances to the separating 
position. And, if the next printing operation were carried out under the 
condition, that portion of the ink ribbon IS extending between the 
recording section and the separating section would be unused. Thus, in 
accordance with the present invention, after completion of separation of 
the recording paper S from the ink ribbon IS, the platen roller 20 is 
driven in the reversed direction thereby causing that portion of the ink 
ribbon IS extending between the separating portion and the recording 
portion to move backward until that portion of the ink ribbon IS currently 
located at the separation position returns to the recording position. In 
the case of a continuous recording operation, this partial backward 
feeding is carried out after separation of the last sheet of recording 
paper S. This aspect of partial backward feeding will be described further 
in detail later. 
Now, various distinctive aspects of the present invention will be described 
in detail hereinbelow. 
The first aspect relates to the manner of supplying a sheet of recording 
paper S. In the embodiment described above, a sheet of recording paper S 
may be fed in either of two ways: manual insertion and automatic feeding 
from the cassette 4. There is defined a pair of passages, one for a 
manually inserted recording paper S and the other for automatically fed 
recording paper S from the cassette 4. These passages extend into the 
interior of the main housing 2 in a convergent manner, and in the vicinity 
of a point where these passages meet is disposed the paper sensor 17. It 
is so structured that the registration rollers 18A and 18B are set into 
rotation after elapsing a predetermined time period as from the time when 
the sensor 17 is turned on by the downward pivotal motion of the actuator 
17A through engagement with the leading edge of recording paper S in 
transportation. 
However, since the transportation speed of recording paper S generally 
differs between the case when the recording paper S is manually fed and 
the case when the recording paper S is automatically fed from the cassette 
4, there is a difference in timing for the leading edge of recording paper 
S to reach the nip between the registration rollers 18A and 18B as from 
the time when the sensor 17 has been turned on. Accordingly, it is 
necessary to change the length of time delay depending on whether the 
recording paper S has been fed manually or automatically from the cassette 
4. If the length of time delay is to be changed depending on the manner of 
feeding recording paper S as in this case, it is necessary to select an 
appropriate delay time length by detecting the manner of feeding recording 
paper S in use. In the illustrated embodiment, detection of such manner of 
feeding recording paper S is carried out such that the feed roller 11 is 
examined as to whether it is being driven or not and a determination is 
made by combining the result of this examination and the state of sensor 
17. 
Explained more in detail in this respect, in the case where recording paper 
S is fed automatically from the cassette 4, the feed roller 11 is 
necessarily driven to rotate, and, therefore, whenever the feed roller 11 
is driven to rotate, it is detected that recording paper S is fed from the 
cassette 4. On the contrary, in the case of manual feeding mode, the feed 
roller 11 is not driven to rotate. And, thus, if the sensor 17 is turned 
on without the feed roller 11 having been driven to rotate, it is 
immediately known that recording paper S has been fed manually. 
As a result, a combination of feed roller 11, transporting roller 12 and 
back-up roller 13, in effect, constitutes an automatic paper feeding 
mechanism, and a driving signal to drive this automatic paper feeding 
mechanism is generated from the CPU in control unit 35 (see FIG. 4). When 
such an automatic feeding mode driving signal is generated, the automatic 
paper feeding mechanism is set in operation and the recording paper S 
stored in the cassette 4 is fed from the cassette 4 one by one 
automatically. Accordingly, the paper feeding mode may be determined as 
the automatic mode when such a driving signal is generated. On the other 
hand, in the case of manual feed mode, the sensor 17 is turned on without 
generation of such driving signal. Thus, the manual feed mode may be 
identified when the sensor 17 has been turned on without generation of 
automatic feeding mode driving signal. 
The second aspect of the present invention relates to the relay roller 27. 
As described previously, the relay roller 27 is comprised at least at its 
peripheral surface of rubber or foam rubber, which has a sufficient 
frictional force against the ink ribbon IS, and it is driven to rotate by 
the step motor similarly with the platen roller 20 with its peripheral 
speed being set faster than the paper transportation speed set by the 
platen roller 20 by 1-10%, e.g., 5%. The following effects result with 
such a relay roller 27. 
In the first place, it will contribute to feed the ink ribbon IS. That is, 
as described above, feeding of a laminate of recording paper S and ink 
ribbon IS through the recording section is effected by the platen roller 
20. In other words, the platen roller 20 contacts the back side of 
recording paper S to have it transported through the recording section. In 
this case, the ink ribbon IS is transported together with the recording 
paper S through a frictional force between the ink ribbon IS and recording 
paper S. In this manner, since feeding of ink ribbon IS through the 
recording section relies on the friction against the recording paper S, if 
use is made of recording paper S having a small frictional coefficient 
against the ink ribbon IS, there may arise malfunction in feeding the ink 
ribbon IS due to slippage between the ink ribbon IS and recording paper S. 
However, since the relay roller 27 possesses a sufficient frictional force 
transmitting capability against the ink ribbon IS and yet its peripheral 
speed is set faster than the paper transportation speed established by the 
platen roller 20, the relay roller 27 produces a force which tends to pull 
the ink ribbon IS forward, which contributes to guarantee secure feeding 
of ink ribbon IS through the recording section. Since the peripheral speed 
of relay roller 27 is larger than the transportation speed of ink ribbon 
IS at the recording section, the relay roller 27 slips on the ink ribbon 
IS thereby applying a force tending to pull the ink ribbon IS forward. 
Second, due to the force tending to pull the ink ribbon IS forward, the ink 
ribbon IS, together with the recording paper S, is set in tension between 
the relay roller 27 and the recording section. Such a structure 
contributes to prevent formation of creases in ink ribbon IS and/or 
recording paper S and to enhanced separation of recording paper S from the 
ink ribbon IS. 
Third, it may prevent irregularities in feeding of recording paper S and/or 
ink ribbon IS from occurring. That is, after printing, the ink ribbon IS 
is wound around the take-up spool 39; however, as described above, since 
the take-up spool 39 is driven at constant torque through the frictional 
coupling by the step motor, the force for pulling the ink ribbon IS to be 
wound around the take-up spool 39 is larger when the diameter of the ink 
ribbon IS wound around the take-up spool 39 is smaller, but this force 
becomes smaller as the diameter of the ink ribbon IS wound around the 
take-up spool 39 becomes larger. In the case of absence of relay roller 
27, such a change in pulling force is directly transmitted to the 
recording section thereby causing irregularities in feeding. However, 
provision of relay roller 27 allows to prevent such irregularities from 
occurring. It is to be noted that instead of driving the relay roller 27 
directly by the step motor, it may be driven to rotate indirectly through 
a suitably friction coupling mechanism. 
The third distinctive aspect of the present invention relates to paper 
discharging rollers 30A and 30B. As described previously, the paper 
discharging rollers 30B, 32B and 34B at the driving side come into contact 
with the back side of recording paper S and they are comprised of a 
material having a sufficient frictional force transmitting capability 
against the recording paper S, such as rubber or the like, at least at 
their peripheral surfaces. Each of these driver side rollers is driven to 
rotate by means of the common motor through a frictional coupling and its 
peripheral speed for transportation of recording paper S is set faster 
than the peripheral speed of relay roller 27. The peripheral speeds of 
these rollers 30B, 32B and 34B are identical. On the other hand, follower 
side rollers 30A, 32A and 34A are comprised of a material, such as resin 
and aluminum, which is difficult to be contaminated by ink. 
Since the roller 30B is larger in peripheral speed than the platen roller 
20 and relay roller 27, the recording paper S is maintained in tension on 
the guide plate 28 so that any formation of crease in this section is 
prevented from occurring. In addition, since the rollers 30A, 32A and 34A 
are difficult to be contaminated by ink, the ink on the recording paper S 
hardly sticks to these rollers so that the so-called offset printing 
phenomenon of the ink on the recording paper S once sticking to one of 
these rollers and again back to the recording paper S is advantageously 
prevented. Moreover, since each of the rollers 30A, 32A and 34A causes the 
recording paper S to be transported at the same speed, there is no danger 
that the printed image formed on the recording paper S by the transferred 
ink is scrubbed against the guide plates 31A and 33A. 
In general, in the case of transporting a sheet type object, it is common 
practice to set the speed of transportation faster as it goes further 
downstream so as to keep the sheet type object in tension thereby 
attaining stability in transportation. If this were applied to the present 
paper discharging passage for recording paper S, since the passage is 
curved significantly, the recording paper S would be pulled in tension, 
for example, when extending between the rollers 30A-30B and 32A-32B, 
whereby the printed image on the recording paper S would be scrubbed 
against the guide plate 31A thereby smearing or damaging the printed 
image. No such problem arises in the present invention because all of the 
paper discharging rollers 30, 32 and 34 are driven to rotate at the same 
speed. 
It is to be noted that this aspect of the present invention is universally 
applicable to the situation where a curved passage for transporting a 
sheet or recording paper is provided. 
The fourth distinctive aspect of the present invention relates to the 
buffer guide pipe 22. As described previously, the buffer guide pipe 22 is 
rotatably supported at the free end of a bracket which is pivotally 
supported at the rotating shaft for the guide pipe 21. Thus, the buffer 
guide pipe 22 may pivot between the advanced position indicated by the 
dotted line and the retracted position indicated by the solid line around 
the pivot or rotating shaft for the guide pipe 21. The buffer guide pipe 
22 rests on the ink ribbon IS extending between the guide pipes 21 and 23 
by its own weight. And, the buffer guide pipe 22 may take any position 
between the advanced and retracted positions depending on the degree of 
tension acting on the ink ribbon IS. The following advantages may be 
obtained by using such a buffer guide pipe 22. 
That is, in the first place, there is obtained a buffer effect in feeding 
the ink ribbon IS toward the recording section. In other words, the mass 
of the ink ribbon IS wound in the form of a roll is prevented from 
applying the effect of inertia load to the feeding force at the recording 
section. Second, including the occasion of ink ribbon replacement, since 
the buffer guide pipe 22 always keep the ink ribbon IS in tension, creases 
are prevented from occurring in the ink ribbon IS. Third, when the unused 
portion of ink ribbon IS is pulled backward due to reversed rotation of 
platen roller 20 upon completion of printing operation, the length of ink 
ribbon IS pulled backward is absorbed by the pivotal movement of buffer 
guide pipe 22 so that occurrence of a slack or twisting in the ink ribbon 
IS is prevented. 
It is to be noted that such a buffer guide pipe 22 may be applied to any 
other types of transfer type thermal printers. The guide roller 24 also 
has a similar buffer effect and it serves to guide the recording paper S 
to be smoothly lead into the recording section. 
The fifth distinctive aspect of the present invention relates to the manner 
of mounting the ink ribbon IS. As described previously, the ink ribbon IS 
is originally wound in the form of a roll and it is detachably mounted in 
position in the printer upper half. The roll of ink ribbon IS is then 
unwound to be fed toward the recording section. It is important, however, 
that the ink sheet IS be easy for replacement and mounting and detachment 
of ink ribbon IS to or from the printer be easy. In the illustrated 
embodiment, the ink ribbon IS is set in position with a supply spool SP on 
which the ink ribbon IS is wound is inserted to be supported by a support 
member 401 and a leaf spring 402, as shown in FIG. 5. 
The supply spool SP is slightly larger in width than the ink ribbon IS and 
thus it projects slightly on both ends of the ink ribbon IS wound in the 
form of a roll. Both ends of the supply spool SP are rotatably supported 
by a support structure shown in FIG. 5. Thus, in order to mount the ink 
ribbon IS in position for operation, it is only necessary to push the 
supply spool SP into the support member 401 through its opening. The 
supply spool SP thus pushed into position may be maintained in position 
stably as urged by the leaf spring 02. In order to detach the supply spool 
SP from the printer, it is only necessary to pull the supply spool SP from 
the support member 401 through its opening against the force of spring 
402. When the supply spool SP is set in position inside the support member 
401 as shown in FIG. 5, it may rotate with respect to the support member 
401. Thus, as the platen roller 20 is driven to rotate to pull the ink 
ribbon IS forward, the supply spool SP rotates counterclockwise as 
indicated by the arrow in FIG. 5 to supply the ink ribbon IS as unwound 
from the roll. 
In the embodiment illustrated in FIG. 5, that portion of the support member 
401 on which the supply spool rests is provided with a frictional element 
401A, such as cork. The remaining portion of support member 401, spring 
402 and supply spool SP are all quite slippery. For this reason, when the 
supply spool SP rotates counterclockwise, it experiences friction against 
the frictional element 401A; however, this friction serves to cause the 
supply spool SP to be pressed against the support member 401 so that the 
supply spool SP may be securely maintained in the printer upper half quite 
stably while it is being held in rotation. It is to be noted that such a 
supply spool supporting structure may be applied to any appropriate type 
of printers. 
The sixth distinctive aspect of the present invention is concerned with the 
bracket 26 for supporting thereon the thermal printhead 25. In order to 
carry out a proper transfer type thermal printing operation, it is 
important that a relative positional relation between the platen roller 20 
and thermal printhead 25 be accurately maintained. This is the problem of 
alignment among components. 
In order to insure a proper transfer type thermal printing operation, it is 
also important that the platen roller 20 be brought into pressure contact 
with the thermal printhead uniformly along its longitudinal direction. 
However, since the thermal printhead 25 is elongated in shape, when a 
pressure force is applied at each end thereof in the direction of its 
thickness, it tends to become deflected such that the pressure force is 
smaller at the center as compared with the end portions, thereby hindering 
to obtain a uniform pressure force along the entire contact line between 
the platen roller 20 and thermal printhead 25. 
In the illustrated embodiment, the bracket 26 to support thereon the 
thermal printhead 25 is so structured to solve these problems relating to 
alignment and uniformity in pressure force as in the following manner. 
FIG. 6 shows the overall structure of bracket 26 embodying the present 
invention. As shown, the bracket 26 includes a flat plate portion 261, a 
pair of support portions 262, a bent portion 263 and a pair of engaging 
portions 264, which are formed integrally as a unit. The thermal printhead 
25 is fixedly attached to the bottom surface of the flat plate portion 
261. When so attached, the thermal printhead 25 has its lengthwise 
direction in parallel with the Y direction indicated in FIG. 6. It is to 
be noted that in FIG. 6 the bracket 26 is shown to be reduced in size in 
the Y direction for convenience in drawing, but the actual bracket 26 is 
substantially elongated in the Y direction so as to allow the thermal 
printhead 25 to be fixedly attached thereto. 
Each of the support portions 262 is formed with a slot 2621, into which a 
pin (not shown) fixedly planted in the frame of upper cover or printer 
upper half is loosely fitted, so that the bracket 26 may be pivotally 
mounted in the printer upper half. The bent portion 263 is inclined 
upwardly at an angle .theta. with respect to the flat plate portion 261. 
Here, regarding the bent portion 263, the Z direction indicated in FIG. 6 
will be called widthwise direction of bent portion 263. It is to be noted 
that the angle .theta. is an acute angle, preferably in the range between 
approximately 60.degree. and 80.degree.. The bent portion 263 is formed 
with projections 2631 on both ends in the Y direction, and each of these 
projections 2631 will be used for engagement of one end of a coil spring, 
as will be described further in detail later. The engaging portions 264 
are defined on both ends of the flat plate portion 261 in the Y direction 
as bent downwardly. 
Referring now to FIG. 7, there is shown the condition in which the printer 
upper half is pivotted closed with respect to the printer lower half and 
the platen roller 20 is pressed against the thermal printhead 25. As 
shown, there is provided a coil spring 265 having its one end engaged with 
the projection 2631 of bracket 26 and the other end engaged with an 
appropriate point in the printer upper half at each end of the bracket 26. 
Thus, the coil springs 265 normally apply a force to bring the thermal 
printhead 26 into pressure contact with the platen roller 20. The platen 
roller 20 includes its rotating shaft 20A onto which is rotatably fitted 
an aligning ring 200, which, in turn, is fixedly attached to the printer 
lower half. 
Now, as the printer upper half is gradually brought into the closed 
position from the open position, the thermal printhead 25 comes into 
contact with the platen roller 20 at its top peripheral surface and the 
engaging portions 264 of bracket 26 are also brought into contact with the 
respective aligning rings 200. As the printer upper half is further moved 
and finally brought into its closed position, the thermal printhead 25 
receives an upward pressure force from the platen roller 20 so that the 
bracket 26 slightly pivots clockwise around the pins PN thereby 
establishing the condition shown in FIG. 7. In this instance, the springs 
265 become extended and its spring forces cause the thermal printhead 25 
to be pressed against the platen roller 20. Of course, under normal 
circumstances, the ink ribbon IS is present between the thermal printhead 
25 and platen roller 20. In the illustrated embodiment, through the 
engagement between the engaging portion 264 of bracket 26 and the 
corresponding aligning ring 200, there may be obtained an accurate 
relative positional relation between the platen roller 20 and thermal 
printhead 25. 
The coil springs 265 are provided to extend in the Z direction or the 
widthwise direction of bent portion 263 of bracket 26 so that the forces 
of these springs 265 act in the Z direction. Since the forces imparted by 
the springs 263 on the bent portion 263 are directed in the widthwise 
direction of bent portion 263, a relatively strong rigidity against 
bending is exhibited by the bent portion 263. Thus, even if forces are 
applied at both ends of bent portion 263 in the Y direction, no bending 
deflection is produced in the bent portion 263. Accordingly, the thermal 
printhead 25 may be securely brought into contact with the platen roller 
20 uniformly along its full length. 
As mentioned previously, the bent portion 263 is bent to define an acute 
angle with respect to the flat plate portion 261. Thus, when a pressure 
force is applied by the coil springs 265 as described above, it produces a 
force component acting in parallel with the flat plate portion 261 and 
directed perpendicular to the Y direction. This force component serves to 
cause the engaging portion 264 of bracket 26 to be pressed against the 
aligning ring 200 of platen roller 20. With this structure, engagement 
between the engaging portion 264 and aligning ring 200 is secured, which 
contributes to maintain the relative positional relation between the 
thermal printhead 25 and platen roller 20 properly at all times. 
The seventh distinctive aspect of the present invention relates to a 
structure for feeding a sheet of recording paper S to the recording 
section. Although the platen roller 20 is driven to rotate intermittently 
during printing operation, it is driven to rotate at its maximum speed 
during a process to set the recording paper S at the recording section. 
However, to drive the platen roller 20 and the registration rollers 18A 
and 18B all at the same speed is not advantageous from the viewpoint of 
driving efficiency. Accordingly, in the present embodiment, it is 
structured that the transportation speed for supplying the recording paper 
S to the recording section is set faster than the transportation speed 
established by the platen roller 20, thereby allowing to carry out 
printing for the very first sheet of recording paper S sooner. In the 
preferred embodiment, the transportation speed of recording paper S 
established by the registration rollers 18A and 18B is set approximately 
2.5 times of the transportation speed established by the platen roller 20. 
As described previously, the registration rollers 18A and 18B cease to be 
driven as soon as the leading edge of recording paper S reaches the 
recording section as transported by the platen roller 20. However, as 
described above, since the transportation speed differs significantly 
between the registration and platen rollers, an inventive concept must be 
introduced so as to carry out the above-mentioned setting of recording 
paper S at the recording section. That is, for an excellent setting of 
recording paper S, it is ideal if the transportation speed is identical 
between the registration and platen rollers. 
The preferred manner of carrying out such setting of recording paper S in 
accordance with the present invention will now be described with 
particular reference to FIG. 8. As shown, at first, while maintaining the 
platen roller 20 in a non-rotating state, the registration rollers 18A and 
18B are driven to rotate for 340 msec, so that the recording paper S is 
caused to move toward the recording section by means of the registration 
rollers 18A and 18B and its leading edge reaches the contact line between 
the platen roller 20 and thermal printhead 25 with the ink ribbon IS 
sandwiched therebetween, whereby the recording paper S becomes somewhat 
warped as it is further driven by the registration rollers 18A and 18B. 
Then, the registration rollers 18A and 18B cease to be driven to rotate, 
and, at the same time, the platen roller 20 starts to be driven so that it 
rotates counterclockwise for 180 msec. During the first half (90 msec) of 
this 180 msec period, the platen roller 20 causes the leading edge of 
recording paper S to be fed into the recording section thereby 
substantially absorbing the warp of recording paper S. During the next 
half (90 msec), the registration rollers 18A and 18B are again driven to 
rotate and the platen roller 20 is also driven to rotate to have the 
recording paper S properly and completely set in the recording section. 
The time period of 90 msec from the time when the registration rollers 18A 
and 18B once cease to be driven to rotate to the time when the 
registration rollers 18A and 18B are again driven to rotate is a time 
period to absorb a difference in transportation speed between the platen 
and registration rollers. In the case of absence of such an intermediate 
time period, the recording paper S will warp more appreciably thereby 
hindering to carry out smooth setting of recording paper S. 
The mechanism of above-described setting of recording paper may be 
explained as in the following manner. That is, as described previously, it 
is again assumed that the transportation speed established by the platen 
roller is V and that by the registration rollers is 2.5 V. Under the 
condition, focusing on the time period of 180 msec during which the platen 
roller 20 is driven to rotate, the registration rollers 18A and 18B stop 
rotation for the first half 90 msec and then resume rotation for the next 
half 90 msec. Accordingly, the average transportation speed established by 
the registration rollers 18A and 18B for this time period of 180 msec is 
(90 msec.times.2.5 V)/180 msec=1.25 V. A difference between this average 
transportation speed and the transportation speed V of platen roller 20 is 
0.25 V. That is, the above-mentioned setting operation is directed to 
carry out the desired paper setting operation as smoothly as possible by 
causing the average transportation speed of recording paper S to approach 
the transportation speed by the platen roller 20 by driving the 
registration rollers 18A and 18B intermittently. Thus, if at all possible, 
a more ideal paper setting operation may be carried out by controlling to 
drive the registration rollers 18A and 18B more finely thereby allowing 
the average transportation speed to be more closer to the transportation 
speed established by the platen roller 20. However, from practical 
viewpoint, the control scheme shown in FIG. 8 is more than sufficient. 
The eighth distinctive aspect of the present invention relates to timing in 
feeding recording paper S during continuous printing operation. That is, 
in the present transfer type thermal printer, printing operation is 
initiated by a print command supplied from the host system (FIG. 4). In 
the case of a single page printing operation, this print command is turned 
off when a "print in progress" signal is supplied from the printer. 
However, in the case of a continuous printing operation, this command 
remains on. During such a continuous printing operation, if the print 
command is on at the time when the trailing edge of the last preceding 
recording paper S has been detected by the sensor 17, the next following 
sheet of recording paper S is fed. This recording paper S is set in a 
standby state with its leading edge in abutment against the nip between 
the registration rollers 18A and 18B and it is transported to the 
recording section at such a timing that its leading edge portion does not 
overlap the trailing edge portion of the last preceding recording paper S. 
This operation is carried out under the control of CPU provided in the 
control unit 35. 
The ninth distinctive aspect of the present invention relates to timing in 
carrying out the pull-back operation of ink ribbon. As described 
previously, in order to minimize the waste of ink ribbon IS, the ink 
ribbon IS is preferably pulled backward. That is, upon completion of 
printing for the last page, a laminate of recording paper S and ink ribbon 
IS is advanced further, and after the recording paper S having been 
separated from the ink ribbon IS by the relay roller 27, that portion of 
ink ribbon IS which extends between the point of separation and the 
recording section and thus has not been used is pulled backward until the 
unused portion becomes located at the recording section. In order to 
implement this, it is necessary to detect the timing when the trailing 
edge of recording paper S has reached the point of separation. It is 
conceivable to use the length of time period, e.g., from the point in time 
when the sensor 17 has detected the trailing edge of recording paper S to 
the point in time when that trailing edge reaches the point of separation, 
in order to carry out such a detection method. 
However, in the present embodiment, a method is adopted in which lines 
having no image information, or all "white", are skipped so as to increase 
the printing speed. Presence and absence of skips differ for each image 
information, and depending on the number of skips, the length of time 
period from the time of detecting the trailing edge of recording paper S 
by the sensor 17 to the time of the trailing edge reaching the point of 
separation differs. For this reason, detection by time period as described 
above is not possible. Under the condition, in accordance with the 
preferred embodiment of the present invention, such detection is carried 
out by counting the number of drive lines of platen roller 20 from the 
time when the sensor 17 has detected the trailing edge of recording paper 
S to the time when the trailing edge reaches the point of separation. This 
operation is also carried out under the control of CPU in the control unit 
35. 
The tenth distinctive aspect of the present invention relates to monitoring 
of temperature. In the illustrated embodiment, the components producing 
significant heat include the thermal printhead 25 and a power supply. The 
amount of heat produced depends on the contents of image information, 
i.e., more white dot information or more black dot information, so that 
the instantaneous power supply capacity must have an upper limit (in the 
case where a single line is all black), but if thermal designing is 
carried out in compliance with this upper limit, the entire device becomes 
bulky and thus not advantageous. 
Under the condition, in the illustrated embodiment, for the maximum power 
consumption of 300 W, thermal designing of power supply is carried out 
with 120 W, but the thermal printhead 25 is not provided with special 
heat-releasing plates or the like. However, a thermistor is mounted as a 
heat level detecting element in the thermal printhead 25 and the power 
transistor of power supply to monitor temperature by the CPU of control 
unit 35. This thermistor is also used to determine the pulse width of 
image signal as will be described in detail later. 
During printing operation, if the temperature of either of thermal 
printhead 25 or power supply transistor (not shown) has exceeded a 
predetermined level, i.e., 60 .degree. C. for thermal printhead 25 and 110 
.degree. C. for power supply transistor, the on-going thermal printing 
operation is allowed to continue until it is completed, but, thereafter, 
the printer is set in a "standby" state for a predetermined time period, 
during which the printer is allowed to cool down without carrying out 
printing operation. Experimentally, it has been found that, for typical 
operation of printing characters, the "standby" state has not been entered 
even for 2 hours of continuous printing operation. It is thus considered 
that under normal printing conditions the "standby" state is hardly 
established. 
The eleventh distinctive aspect of the present invention relates to the 
control over the level of energy applied to the thermal printhead 25. As 
described previously, each of the individual heat-producing elements 
constituting the write-in section of thermal printhead 25 becomes 
momentarily heated to the temperature of approximately 300 .degree. C. 
when a pulse signal is applied thereto as an image signal. Thereafter, due 
to natural cooling, the heat is dissipated and the temperature gradually 
goes down. These rising and falling characteristics in temperature change 
depending on the temperature of substrate of thermal printhead 25 and the 
pulse width of pulse current. 
As described above, the temperature falling characteristic after having 
been heated to a higher temperature with the application of a pulse 
current is not so steep as compared with the temperature rising 
characteristic. Thus, if the same heat-producing element is driven 
repetitively, the next following heating step starts before it has cooled 
down sufficiently. If this happens, the temperature of heat-producing 
element increases cumulatively, which then could cause a damage to the ink 
ribbon IS and/or deterioration in image resolution due to excessive 
transfer of ink. 
On the other hand, in the illustrated embodiment, the above-mentioned 
problem has been solved by controlling the level of energy applied to the 
thermal printhead 25. That is, in the present embodiment, the write-in 
section of thermal printhead 25 is divided in the longitudinal direction 
into four blocks and the write-in operation is carried out twice for each 
line. Furthermore, every time when write-in operation is carried out, it 
is compared with the information for the last preceding line, in which the 
write-in operation is carried out twice for a bit or pixel whose last 
preceding bit is a white bit and the write-in operation is carried out 
only once for a bit whose last preceding bit is black. The control of this 
operation is implemented by the bit unit energy control circuit shown in 
FIG. 4. 
That is, when the bit unit energy control circuit receives one line of 
image signal from the video interface, the head driver is activated with 
this image signal and this image signal is stored into the RAM. Then, when 
an image signal for the next following line is received, the image signal 
of the last preceding line is read out and after comparison between the 
signals, the write-in operation is carried out only for those bits whose 
last preceding bits are all white. 
Incidentally, as described previously, since the rising and falling 
characteristics of a heat-producing element also depends on the substrate 
temperature of thermal printhead 25, if the substrate temperature itself 
increases, even with the control over the level of energy applied for each 
bit, there will be a cumulative accumulation of heat to present problems, 
such as damage to the ink ribbon IS. Under the condition, in accordance 
with the illustrated embodiment, use is made of the pulse width 
determining circuit (FIG. 4) to control the pulse width of current pulse 
to be applied to each heat-producing element in response to the substrate 
temperature to obviate the above-mentioned problem. That is, as the 
substrate temperature goes up, it is so controlled to make the pulse width 
smaller. Of course, such a control may be carried out using the CPU in the 
control unit 35. The detection of substrate temperature may be carried out 
using a thermistor as mentioned previously. 
The twelfth distinctive aspect of the present invention relates to a scheme 
of shifting the application of image signal to the write-in section for 
each sheet of recording paper S in the case of continuous printing 
operation for the same formatted image. Among images to be thermally 
printed, there is an image, such as a slip, which contains ruled lines. In 
the case of printing images of the same format having such ruled lines 
repetitively in a continuous manner, particular ones of heat-producing 
elements are used repetitively so as to print the ruled lines, which could 
be detrimental to the life of thermal printhead because of concentrated 
usage of particular heat-producing elements. In view of this, in 
accordance with the present embodiment, it is so structured that the 
application of image signal is shifted by one bit at the write-in section 
for each sheet of recording paper S over an 8-bit range. With such a 
structure, the image printed on a sheet of recording paper S comes to be 
shifted in position one sheet from another; however, since such a shift is 
limited to 8 bits at maximum, which corresponds to 1 mm, this may be 
practically neglected. 
While the above provides a full and complete disclosure of the preferred 
embodiments of the present invention, various modifications, alternate 
constructions and equivalents may be employed without departing from the 
true spirit and scope of the invention. Therefore, the above description 
and illustration should not be construed as limiting the scope of the 
invention, which is defined by the appended claims.