EP1338423A2

EP1338423A2 - Thermal printer

Info

Publication number: EP1338423A2
Application number: EP03250982A
Authority: EP
Inventors: Akio c/o SII P & S Inc. Naito
Original assignee: SII P and S Inc
Current assignee: Seiko Instruments Inc
Priority date: 2002-02-21
Filing date: 2003-02-18
Publication date: 2003-08-27
Anticipated expiration: 2023-02-18
Also published as: CN1325276C; DE60307530D1; DE60307530T2; EP1338423A3; CN1439525A; JP2003237118A; EP1338423B1

Abstract

To provide a thermal printer that is capable of simplifying the assembling work to reduce the manufacturing cost.

A frame (F) is comprised of a lower frame member (F1) and an upper frame member (F2) that is dividable up and down, and receiving portions (receiving portion A, gear receiving portion B) for receiving a line thermal head (H), a platen roller (1), a motor (M), and a gear (G1) are formed in the lower frame member, respectively, and at the same time, a plurality of holder portions (C1 to C4, D1 to D4) for rotatably clamping each rotary shaft of the platen roller and the motor when both frames are coupled together are formed in side wall portions of the lower frame member and side wall portions of the upper frame member.

Description

The present invention relates to a miniature thermal printer mounted on a variety of portable information equipments or the like, and more particularly to a thermal printer that has a simplified structure of a pressure means for pressing a line thermal head so that a line thermal head may be depressed against a surface of a platen roller.
A general conventional line dot type thermal printer is composed of a line thermal head and a platen roller assembled into a frame to face each other, a drive means therefor, a transport mechanism for transportation of thermal recording paper, and the like.
A structural example of the conventional thermal printer will herewith be described with reference to Fig. 10 which is an exploded perspective view.
As shown in Fig. 10, a platen roller 102, a thermal head H, two coiled springs 104 for biasing this thermal head H against the platen roller 102, a lock arm member 105 for rotatably supporting both end portions of a rotary shaft 102A of the platen roller 102, a motor M as a drive source, a gear transmission mechanism G2 for transmitting a rotational drive force of the motor M to a driven gear g10 fixed to one end 102A of the platen roller 102 and the like are mounted on a frame 101 molded of resin to form the thermal printer P2.
In Fig. 10, reference numeral 200 designates a head support member serving also as a heat radiation member of the thermal head H, reference numeral 300 denotes a bearing member of the rotary shaft 102A of the platen roller 102 and reference numeral 400 denotes a sensor for detecting an end portion of the recording paper.
The lock arm member 105 is provided with a pair of hook-shaped rotary shaft support portions (latch portions) 105a for engaging with and dragging both end portions of the rotary shaft of the platen roller 102 so that the surface of the platen roller 102 is brought into pressing contact with the thermal head H side, a pair of arm portions 105b for leading the latch portions 105a to the rear side of the thermal head H through both sides of the thermal head H, and an elastic member support portion 105c arranged in the widthwise direction of the back side of the thermal head H provided with arm portions 105b at both sides thereof for supporting the above-described coiled springs 104. Note that the lock arm member 105 and the thermal head H are rotatably supported to both side surfaces of the frame 101 through the rotary shaft 500. Incidentally, reference numeral 700 denotes a screw for fastening the motor M to the frame 101.
The thermal printer with such a structure is mounted on, for example, a portable information terminal or the like. These information terminals and the like are required to be further miniaturized in order to enhance the convenience or the portability and to be produced at lower cost in order to enhance the competitive power with respect to the cost.
Such a requirement is also applied to the thermal printer per seas constituents of the information terminals. The thermal printer per se is also required to be further miniaturized and produced in lower cost.
It is considered that in order to miniaturize the thermal printer and to reduce the cost therefor, it is effective to reduce the number of parts and to simplify the assembling work to reduce the manufacturing cost.
However, in the above-described conventional thermal printer P2, as shown in Fig. 10, the various constituents (platen roller 102, thermal head H, head support member 200, gear transmission mechanism G, and the like) have various mounting positions and mounting directions. Thus, the conventional thermal printer has a disadvantage that the assembling work is troublesome and needs high skill.
In order to solve the above-noted defects, an object of this invention is to reduce the number of parts in order to attain the miniaturization and to simplify the assembling work to reduce the manufacturing cost.
In order to achieve the above-mentioned object, according to the present invention, there is provided a thermal printer including at least: a frame (F) having a pair of side wall portions arranged to face each other at a predetermined interval in a paper width direction; a thermal head (H) for effecting printing; a platen roller (1) whose circumferential surface is brought into contact with the line thermal head; a pressure means for pressing the platen roller so that a back surface of the line thermal head is brought into pressing contact with the surface of the platen roller; a motor (M) as a drive source; and a gear transmission mechanism (G1) for transmitting a rotational drive force of the motor to the platen roller, in which the pressure means is constituted by a part of constituents of the frame that is brought into pressing contact with a back surface of the line thermal head.
Thus, the pressure means (members such as coiled springs) provided in the conventional printer may be dispensed with to reduce the number of parts so that the printer may be miniaturized and the manufacturing cost may be reduced. Also, since the work for assembling the coiled springs or the like may be dispensed with, the assembling work may be simplified so that the manufacturing cost may be further reduced.
Also, the frame may be composed of a lower frame member and an upper frame member which is dividable up and down, a part of the constituents of the frame constituting the pressure means may be a pressure portion formed integrally with the upper frame portion and having elasticity and when the upper frame member is engaged with the lower frame member, the pressure portion is formed in a position where an end portion of the pressure portion is in contact with a back surface of the line thermal head. Thus, it is possible to constitute the pressure means with a simple structure to thereby reduce the manufacturing cost.
Also, the frame may be composed of a lower frame member and an upper frame member which is dividable up and down, a part of the constituents of the frame constituting the pressure means may be a pressure portion formed integrally with the upper frame portion and having rigidity and when the upper frame member is engaged with the lower frame member, the pressure portion is formed in a position where an end portion of the pressure portion is in contact with a back surface of the line thermal head. Thus, it is possible to constitute the pressure means with a simple structure to thereby reduce the manufacturing cost.
Also, the end portion of the pressure portion may also serve as a heat radiating portion for conducting and radiating heat of the line thermal head. Thus, the head support member is made of, for example, metal having a high thermal conductivity so that it is unnecessary to separately provide a heat radiation member to thereby make it possible to reduce the number of the parts and to reduce the manufacturing cost.
Also, a thermal printer according to another embodiment includes at least: a frame having a pair of side wall portions arranged to face each other at a predetermined interval in a paper width direction; a thermal head for effecting printing; a platen roller whose circumferential surface is brought into contact with the line thermal head; a pressure means for pressing the platen roller so that the line thermal head is brought into pressing contact with the surface of theplaten roller; a motor as a drive source; and a gear transmission mechanism for transmitting a rotational drive force of the motor to the platen roller, in which the pressure means is composed of a support member for rotatably supporting both ends of a rotary shaft of the platen roller for pressing and compressing the surface of the platen roller in parallel with the line thermal head.
Thus, the pressure means (members such as coiled springs) provided in the conventional printer may be dispensed with to reduce the number of parts so that the printer may be miniaturized and the manufacturing cost may be reduced. Also, since the work for assembling the coiled springs or the like may be dispensed with, the assembling work may be simplified so that the manufacturing cost may be further reduced.
Also, it is preferable that the support member is composed of: longitudinal grooves formed on both side wall portions of the frame and having open portions on the above thereof and retainer portions on a lower end side located in the vicinity of the line thermal head; and a bearing member for rotatably supporting the rotary shaft of the platen roller inserted into the longitudinal grooves through the open portions of both longitudinal grooves, and each retainer portion of the longitudinal grooves is provided in a position where the surface of the platen roller is depressed in parallel against the line thermal head so that the platen roller may be compressed. Thus, it is possible to simplify the assembling process and to reduce the manufacturing cost.
Embodiments of the present invention will now be described by way of further example only and with reference to the accompanying drawings, in which:-
Fig. 1 is an exploded perspective view showing a structure of a thermal printer according to a first embodiment;
Fig. 2 is a perspective view showing an outer appearance of the thermal printer according to the first embodiment;
Fig. 3 is a side cross-sectional view showing a structure of a thermal printer according to another example of the first embodiment;
Fig. 4 is a perspective view showing an outer appearance of a thermal printer according to a second embodiment;
Fig. 5 is a side cross-sectional view showing the structure of the thermal printer according to the second embodiment;
Fig. 6 is a perspective view showing an outer appearance of a thermal printer according to a third embodiment;
Fig. 7 is a schematic side cross-sectional view showing the structure of the thermal printer according to the third embodiment;
Fig. 8 is an exploded perspective view showing a structure of a thermal printer according to a fourth embodiment;
Fig. 9 is a schematic side cross-sectional view showing the structure of the thermal printer according to the first embodiment; and
Fig. 10 is an exploded perspective view showing a structure of a conventional thermal printer.
A preferred embodiment of the present invention will now be described with reference to the accompanying drawings.
Fig. 1 is an exploded perspective view showing a first embodiment of a thermal printer according to the present invention. Fig. 2 is a perspective view showing an outer appearance of the thermal printer that has been assembled.
As shown in Fig. 1, the thermal printer P1a according to the first embodiment is provided with a frame F composed of a lower frame member F1 and an upper frame member F2 having a pair of side wall portions arranged to face each other in a paper width direction at a predetermined interval.
The frame F is comprised of, for example, the injection-molded products of plastic such as a polycarbonate.
An upper surface of the lower frame member F1 and a lower surface of the upper frame member F2 are formed into a coupling structure (that is, a structure in which end faces correspond to each other and when coupled, there is generated no gap) and in the coupled condition shown in Fig. 2, both frame members F1 and F2 are fixed together by engagement claws or screws which can be fitted in.
In the lower frame member F1, a receiving portion A for receiving a platen roller 1, a thermal head H, a motor M as a drive source, and the like is formed.
Although not shown in the drawing, a groove, a cutaway portion, or the like for positioning the thermal head H is formed in the receiving portion A. The thermal head H may be bonded to a heat radiation portion 11 to be described later in the upper frame member F2. Thus, it is possible to effectively perform the assembling work.
Also, a support portion E1 for fixing a rear end portion of the motor M is formed in the receiving portion A. Also, a connector 30 connected to signal lines for transmitting printing signals is provided in the thermal head.
A divisional surface is determined in the lower frame member F1 and the upper frame member F2 so that it is divided through a plane passing through each bearing portion. In the case where each bearing portion is completely overlapped in the vertical direction, it is impossible to cope with the divisional structure of the lower frame member F1 and the upper frame member F2. Accordingly, the structure is designed so that each rotary shaft of each gear g1, g2 is not overlapped in the vertical direction and displaced in the horizontal direction.
A gear receiving portion B for receiving each gear g1, g2 of a gear transmission mechanism G1 for transmitting a rotational drive force of the motor M to a driven gear g20 fixed at one end of the rotary shaft 1A of the platen roller 1 and a gear receiving portion B for receiving the driven gear g20 is formed on one side wall side (on the right side wall side in Fig. 1) of the lower frame member F1.
Semicircular holder portions C1 to C3 for rotatably holding the rotary shaft 1A of the platen roller 1 and the rotary shafts g1a and g2a of the respective gears g1 and g2 and a semicircular holder portion C4 for holding the bearing portion Ma of the motor M are formed on the upper edge side of the gear receiving portion B and the side wall of the lower frame member F1. In the lower frame member F1, reference numeral 20 denotes a guide portion that serves as a guide when a roll paper (not shown) as the recording paper is fed.
Also, semicircular holder portions D1 to D3 (incidentally, D4 does not appear in the drawing) formed in confronting positions with the holder portions C1 to C3 of the lower frame member F1 are formed in the lower surface side of the upper frame member F2.
As shown in Fig. 9, a V-shaped pressure portion 10a having elasticity for pressing a back surface of the line thermal head H so that the line thermal head H is brought into pressing contact with the surface of the platen roller 1 is formed integrally with the upper frame member F2 in the upper surface side of the upper frame member F2.
Also, in Figs. 1 and 9, reference numeral 11 denotes a heat radiation portion for radiating heat of the thermal head H. This heat radiation portion 11 is formed integrally with the pressure portion 10 and is provided in a position where the end face of the heat radiation portion 11 is in contact with the back surface of the thermal head H when the upper frame member F2 is fitted with the lower frame member F1.
Then, the pressure portion 10a flexed rearward to cause the thermal head H to come into pressing contact with the surface of the platen roller 1 with a predetermined pressing force under the condition that the pressure portion 10a is in contact with the back surface of the thermal head H. Thus, it is possible to apply a suitable pressure to the recording paper clamped between the line thermal head H and the platen roller 1 in printing.
An assembling order of the thermal printer P1a with such a structure will now be described.
First of all, the motor M of the lower frame member F1 is mounted in the receiving portion A and the rotary gears g1a and g2a of the gears g1 and g2 are mounted in the holder portions C3 and C2 of the gear receiving portion B. Subsequently, the driven gear g20 is mounted on one end portion of the rotary shaft 1A of the platen roller 1, and both end portions of the rotary shaft 1A of the platen roller 1 are mounted on the holder portion C1 of the lower frame member F1. In this case, the driven gear 20 is received in the gear receiving portion B.
Subsequently, the line thermal head H is mounted in the receiving portion A and finally, the upper frame member F2 is fitted with the lower frame member F1. Thus, the platen roller 1 and the rotary shafts of the gears g1 and g2 and the like are rotatably held by the holder portions C1 to C4 and the holder portions D1 to D4, and at the same time, the line thermal head H is brought into pressing contact with the surface of the platen roller 1 by the pressure portion 10a of the upper frame member F2.
Then, the upper frame member F2 is fixed to the lower frame member F1 by screw fastening with engagement claws or screws to thereby assemble the thermal printer P1a as shown in Fig. 2.
In the thermal printer P1 according to this embodiment, since the coiled springs or the like as the pressure means may be dispensed with, the embodiment is advantageous in that the number of parts is reduced and the cost may be reduced more.
Also, it is possible to complete the assembling by a simple work for mounting each constituent on the lower frame member F1 and coupling the lower frame member F1 with the upper frame member F2, and it is possible to dispense with the work for assembling the coiled springs as the pressure means as in the conventional case to thereby make it possible to shorten the assembling time and reduce the manufacturing cost of the printer.
The pressure portion is not limited to the V-shape as described above. However, for example, as in a thermal printer P1b shown in Fig. 3, it is possible to form the pressure portion as a planar piece 10b curved upwardly. In this case, under the condition that the pressure portion 10b is in contact with the back surface of the line thermal head H, it is flexed upwardly to cause the thermal head H to be in pressing contact with the surface of the platen roller 1. Thus, in printing, it is possible to apply a suitable pressure to the recording paper clamped between the line thermal head H and the platen roller 1.
A thermal printer P1c according to a second embodiment will now be described with reference to Figs. 4 and 5. The basic structure of the thermal printer P1c is substantially the same as the thermal printer P1a (P1b) according to the first embodiment. The different point is that the pressure portion 10a (10b) provided in the upper frame member F2 according to the first embodiment has elasticity whereas the pressure portion 10c according to the second embodiment has rigidity. For this reason, the pressure portion 10c is designed to have rigidity by thickening the pressure portion in comparison with the pressure portion 10b (see Fig. 3) according to the first embodiment. In the case where the pressure portion 10c is made of resin having high degree of hardness or metal, it is not always necessary to thicken the portion if a predetermined strength may be ensured.
Then, as shown in Fig. 5, under the condition that the pressure portion 10c is in contact with the back surface of the line thermal head H, the pressure portion is no longer flexed and causes the line thermal head H to come into pressing contact with the surface of the platen roller 1. In this case, the surface of the platen roller 1 (normally made of hard rubber or the like) is suitably compressed and deformed to generate a predetermined pressure. Accordingly, in printing, it is possible to apply a suitable pressure to the recording paper clamped between the line thermal head H and the platen roller 1.
A thermal printer P1d according to a third embodiment will now be described with reference to Figs. 6 and 7. Since the basic structure of the thermal printer P1d is the same as that of the first and second embodiments, the same reference numerals are used to indicate the same members and the explanation therefor will be omitted.
The different point of the thermal printer P1d according to the third embodiment from the thermal printer according to the first and second embodiments is that the pressure of the line thermal head H to the platen roller 1 is performed by means of a pair of lock pin mechanisms 50 provided in the upper frame member F2.
In Figs. 6 and 7, reference character 10d denotes a pressure member rotatably supported by the upper frame member F2. The end portion 10d1 of the pressure member 10d is adapted to come into contact with the back surface of the line thermal head H. The end portion 10d1 may also perform the function as a heat radiation member of the line thermal head H.
The lock pin mechanisms 50 are provided on both sides to face the line thermal head H and the platen roller 1 of the upper frame member F2 and are composed of pin support portions 50a and lock pins 50b. The pin support portions 50a are formed so as to be bendable right and left through a predetermined distance by the manual operation as indicated by the arrow S in Fig. 6, whereby the lock pins 50b and both right and left end portions of the end portion 10d1 of the pressure member may be engaged and disengaged. Also, the mounting position of the lock pins 50b are designed so that the lock pins are engaged with both right and left end portions of the end portion 10d1 of the pressure member when the line thermal head H can be depressed with a predetermined pressure to the surface of the platen roller 1.
Then, in the case where the recording paper is set in the thermal printer P1d, first of all, the lock pin mechanisms 50 are operated so that the engagement condition between the lock pins 50b and the end portion 10d1 of the pressure member is cancelled. Subsequently, the recording paper is inserted between the line thermal head H and the platen roller 1 and the lock pin mechanisms 50 are again operated so that the lock pins 50b and the end portion 10d1 of the pressure member are brought into the engagement condition. Thus, in printing, it is possible to apply a suitable pressure to the recording paper clamped between the line thermal head H and the platen roller 1.
It is possible to change the arrangement position of the lock pins 50b to a position 50c indicated by the two-dot-and-dash line in Fig. 7. In this case, it is possible to apply a suitable pressure to the recording paper clamped between the line thermal head H and the platen roller 1 also by utilizing the elasticity of the pressure member 10d.
A thermal printer P1e in accordance with a fourth embodiment will now be described with reference to Fig. 8.
In the thermal printer P1e, the respective members are mounted in a frame F3 provided with a pair of side wall portions 900a and 900b and a thermal head support portion 901 provided between side wall portions 900a and 900b with a predetermined slant.
A motor M is arranged inside and on the lower side of one of the side walls 900a of the frame F3 and a drive gear g30 and a driven gear g40 are provided.
Longitudinal grooves 800 having opening portions 800a on the upper side and retainer portions 801 on the lower side are formed in the upper end portions of both side wall portions 900a and 900b.
Also, a line thermal head H is bonded to the top surface of the thermal head support portion 901.
Each retainer portion 801 of the longitudinal grooves 800 is provided in a position where the surface of the platen roller 1 to be mounted can be depressed in parallel with the line thermal head H to compress the platen roller. Also, reference numeral 810 denotes a bearing member to be engaged with the retainer portions 801 of the longitudinal grooves 800 while passing through an end portion of the rotary shaft 1A, 1B of the platen roller 1.
Then, as shown in Fig. 8(a), first of all, after the bearing member 810 is caused to pass through an tip end of the rotary shaft 1B of the platen roller 1, the rotary shafts 1A and 1B are inserted through the open portions 800a of the longitudinal grooves 800 to the longitudinal grooves 800.
Subsequently, under the condition that the surface of the platen roller 1 is kept in pressing contact with the line thermal head H (that is, under the condition that the roller made of hard rubber or the like is somewhat deformed and compressed), the bearing member 810 that has been inserted into the tip end portion of the rotary shaft 1B of the platen roller 1 is engaged from the inside with the retainer portions 801 of the longitudinal grooves 800.
Subsequently, the bearing member 810 is engaged from the outside of the retainer portions 801 of the longitudinal grooves 800 into the tip end portion of the rotary shaft 1A of the platen roller 1.
Next, the driven gear g50 is mounted on the tip end portion of the rotary shaft 1A and finally, the driven gear g60 is mounted on the side wall portion 900a of the frame F3.
Thus, the pressure means (members such as coiled springs) provided in the conventional printer may be dispensed with to reduce the number of parts so that the printer can be miniaturized and the cost can be reduced. Also, since the work for assembling the coiled springs or the like can be dispensed with, the assembling work may be simplified so that the manufacturing cost may be further reduced.
The invention made by the present inventors has been described specifically based upon the embodiments. However, the present invention is not limited to the above-described embodiments and it is possible to effect the change within the scope of the claims.
For example, it is possible to design the pressure member (pressure portion) according to the first to fourth embodiments so as to also serve as the radiation plate. In this case, this is more effective to form the pressure member of metal such as zinc die cast that is superior in heat conductivity and heat radiation.
Also, in the embodiments, the case where the frame F is injection molded of plastic such as polycarbonate has been described. However, the invention is not limited thereto and it is possible to make the frame of metal. In particular, it is effective that the pressure portion 10a (10b) according to the first embodiment is made of metal having suitable elasticity. Also, it is effective that the pressure portion 10c according to the second embodiment is made of metal having a suitable strength (hardness).
Also, the invention is not limited to the case where the pressure members 10a to 10c are formed integrally with the frame (upper frame member F2); it is possible to apply the invention to the case where the pressure portions are made as discrete members.
Also, in the first to third embodiments, the frame F may be divided into two parts, i.e., the lower frame member F1 and the upper frame member F2. However, this is not always necessary for the present invention; as long as the line thermal head H may be depressed against the platen roller 1 by a part of the constituents of the frame, the type of the frame does not matter.
As described above, a thermal printer according to the present invention includes at least: a frame having a pair of side wall portions arranged to face each other at a predetermined interval in a paper width direction; a thermal head for effecting printing; a platen roller whose circumferential surface is brought into contact with the line thermal head; a pressure means for pressing the platen roller so that a back surface of the line thermal head is brought into pressing contact with the surface of the platen roller; a motor as a drive source; and a gear transmission mechanism for transmitting a rotational drive force of the motor to the platen roller, in which the pressure means is constituted by a part of constituents of the frame that is brought into pressing contact with a back surface of the line thermal head. Accordingly, it is possible that the member such as a coiled spring provided in the conventional printer may be dispensed with, thereby making it possible to reduce the number of the parts, to miniaturize the printer, and to reduce the manufacturing cost.
Also, it is possible to simplify the assembling work since the work for assembling the coiled spring or the like may be avoided. Thus, it is possible to further reduce the manufacturing cost.

Claims

A thermal printer comprising:

a frame having a pair of side wall portions arranged to face each other at a predetermined interval in a paper width direction;

a thermal head for effecting printing;

a platen roller whose circumferential surface is brought into contact with the line thermal head;

a pressure means for pressing the platen roller so that the line thermal head is brought into pressing contact with the surface of the platen roller;

a motor as a drive source; and

a gear transmission mechanism for transmitting a rotational drive force of the motor to said platen roller,

wherein the pressure means is constituted by a part of constituents of the frame being brought into pressing contact with a back surface of said line thermal head.
The thermal printer according to claim 1, wherein
the frame is composed of a lower frame member and an upper frame member which is dividable up and down,
a part of constituents of the frame constituting the pressure means is a pressure portion formed integrally with the upper frame portion and having elasticity and when the upper frame member is engaged with the lower frame member, the pressure portion is in a position where an end portion of the pressure portion is in contact with a back surface of the line thermal head.
The thermal printer according to claim 1, wherein
the frame is composed of a lower frame member and an upper frame member which is dividable up and down,
a part of constituents of the frame constituting the pressure means is a pressure portion formed integrally with the upper frame member and having rigidity and when the upper frame member is engaged with the lower frame member, the pressure portion is in a position where an end portion of the pressure portion is in contact with a back surface of the line thermal head.
The thermal printer according to claim 2 or 3, wherein
the end portion of the pressure portion also serves as a heat radiating portion for conducting and radiating heat of the line thermal head.
A thermal printer comprising:

a frame having a pair of side wall portions arranged to face each other at a predetermined interval in a paper width direction;

a thermal head for effecting printing;

a platen roller whose circumferential surface is brought into contact with the line thermal head;

a pressure means for pressing the platen roller so that the line thermal head is brought into pressing contact with the surface of the platen roller;

a motor as a drive source; and

a gear transmission mechanism for transmitting a rotational drive force of the motor to the platen roller;

wherein the pressure means is composed of a support member for rotatably supporting both ends of a rotary shaft of the platen roller for pressing and compressing the surface of the platen roller in parallel with the line thermal head.
The thermal printer according to claim 5, wherein
the support member is composed of:

longitudinal grooves formed on both side wall portions of the frame and having open portions on the above thereof and retainer portions on a lower end side located in the vicinity of the line thermal head; and

a bearing member for rotatably supporting the rotary shaft of the platen roller inserted into the retainer portions of the longitudinal grooves through the open portions of both longitudinal grooves, and

each retainer portion of the longitudinal grooves is provided in a position where the surface of the platen roller is depressed in parallel against the line thermal head so that the platen roller is compressed.