Patent Publication Number: US-10759200-B2

Title: Thermal printer module and thermal printer

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2018-059223 filed on Mar. 27, 2018, the entire content of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a thermal printer module and a thermal printer. 
     2. Description of the Related Art 
     Hitherto, thermal printers are configured to perform printing by heating a printing surface of a recording sheet with heating elements of a thermal head to develop a color on the printing surface while feeding the recording sheet through rotation of a platen roller under a state in which the recording sheet is nipped between the platen roller and the thermal head. In those thermal printers, the platen roller is removable to facilitate work of replacing the recording sheet. 
     A thermal printer includes a main body frame, a thermal head, platen bearings, bearing insertion grooves formed in the main body frame, a lock lever (lock arm) movable from a locking position where the platen bearings are locked so as not to detach from the bearing insertion grooves to an unlocking position where the platen bearings are detachable from the bearing insertion grooves, and a lever biasing member configured to bias the lock lever constantly toward the locking position. The platen bearings are pressed against inner peripheral end surfaces of the bearing insertion grooves by the lock lever located at the locking position, to thereby fix the positions of the platen bearings with respect to the main body frame. 
     In a thermal printer module of the above-mentioned thermal printer, however, there is a case in which an external force caused by drop impact or the like may be applied to the main body frame or the lock arm to distort the respective components, resulting in decrease in holding force of the lock arm for the platen roller. In such a case, there is a risk of such trouble that the platen roller may drop off from the main body frame. This type of thermal printer module is generally mounted in a housing having an openable and closable cover, but the strength of the housing cannot be set extremely high in consideration of weight and cost of the thermal printer. Thus, when the thermal printer drops off, a force caused by distortion or twist is generated in the housing due to the drop impact, and the force is applied to the main body frame or the lock arm so that the above-mentioned trouble may occur. Further, when the platen roller drops off, the cover is opened so that the device may be damaged or the recording sheet may drop off, resulting in inconvenience to the user of the printer. 
     In view of the above-mentioned matters, in this type of thermal printer, it has been required that the platen roller can be stably held. 
     SUMMARY OF THE INVENTION 
     According to one embodiment of the present invention, there is provided a thermal printer module, including: a housing including a roll sheet receiving portion; a cover, which is mounted to the housing so as to be pivotable, and is configured to open and close the roll sheet receiving portion; a platen roller provided to the cover so as to be freely rotatable; a support frame, which is provided in the housing, and has a groove configured to receive a shaft of the platen roller to be inserted into the groove when the cover is closed; a lock arm, which is provided to the support frame so as to be pivotable, and is configured to hold down, by a platen roller engagement portion, the shaft of the platen roller inserted into the groove; a biasing member configured to apply a biasing force to the lock arm in a direction of causing the lock arm to pivot so as to maintain holding of the shaft by the platen roller engagement portion; and a printing head provided at a position opposed to the platen roller having the shaft held in the groove, wherein, when viewed in a direction extending along an axis of the shaft, the platen roller engagement portion and a tangential line of a track obtained when the axis of the shaft held in the groove moves along with an opening operation of the cover form an intersection angle θAB in a closing direction of the lock arm, and the intersection angle θAB satisfies a relation of 90°≤θAB≤110°, and wherein, when viewed in the direction extending along the axis of the shaft, the platen roller engagement portion and a tangential line of a track obtained when a contact point between the shaft held in the groove and the platen roller engagement portion pivots in an unlocking direction of the lock arm form an intersection angle θBC in the closing direction of the lock arm, and the intersection angle θBC satisfies a relation of 0°≤θBC≤10°. 
     In the above-mentioned printer according to the one embodiment of the thermal printer module, wherein when viewed in the direction extending along the axis of the shaft, the platen roller engagement portion and the tangential line of the track obtained when the contact point between the shaft held in the groove and the platen roller engagement portion pivots in the unlocking direction of the lock arm are parallel to each other. 
     In the above-mentioned printer according to the one embodiment of the thermal printer module, wherein when viewed in the direction extending along the axis of the shaft, the platen roller engagement portion and the tangential line of the track obtained when the axis of the shaft held in the groove moves along with the opening operation of the cover are orthogonal to each other. 
     According to one embodiment of the present invention, there is provided a thermal printer, including the above-mentioned thermal printer module. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a thermal printer according to one embodiment of the present invention, for illustrating a state in which a paper cover is closed. 
         FIG. 2  is a perspective view of the thermal printer, for illustrating a state in which the paper cover is opened. 
         FIG. 3  is a perspective view for illustrating a thermal printer module of the thermal printer. 
         FIG. 4  is a perspective view of the thermal printer module when viewed in a direction different from that in  FIG. 3 . 
         FIG. 5  is an explanatory view for illustrating an operation of a lock arm of the thermal printer module, and is a partial enlarged view of the thermal printer module when viewed in a direction indicated by the line X-X of  FIG. 3 . 
         FIG. 6  is a perspective view for illustrating the lock arm. 
         FIG. 7  is an explanatory view for illustrating the operation of the lock arm, and is a partial enlarged view for illustrating an unlocked state. 
         FIG. 8  is a view for illustrating a state in which a bearing of a platen roller is inserted into a roller insertion groove and locked by the lock arm, and illustrating a desirable relative positional relationship between a platen roller engagement portion of the lock arm and a tangential line of pivot of the platen roller. 
         FIG. 9  is a partial enlarged view for illustrating a state in which the bearing of the platen roller is inserted into the roller insertion groove and locked by the lock arm, and illustrating an undesirable relative positional relationship between a straight line B extending along the platen roller engagement portion of the lock arm and pivoting-direction tangential lines Al and A 2  of a center axis P of the platen roller. 
         FIG. 10  is a view for illustrating a state in which the bearing of the platen roller is inserted into the roller insertion groove and locked by the lock arm, and illustrating a desirable relative positional relationship between the straight line B extending along the platen roller engagement portion of the lock arm and a pivoting-direction tangential line C at a contact point Pa between the platen roller engagement portion and the platen roller. 
         FIG. 11  is a partial enlarged view for illustrating an undesirable relative positional relationship between the straight line B extending along the platen roller engagement portion of the lock arm and pivoting-direction tangential lines C 1  and C 2  at the contact point Pa between the platen roller engagement portion and the bearing of the platen roller. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, a thermal printer including a thermal printer module according to one embodiment of the present invention is described with reference to the accompanying drawings.  FIG. 1  is a perspective view for illustrating a thermal printer when a paper cover is closed.  FIG. 2  is a perspective view for illustrating the thermal printer when the paper cover is opened. In the drawings, the arrow UP is defined as an upper side, the arrow FR is defined as a front side, and the arrow LH is defined as a left side. 
     As illustrated in  FIG. 1 , a thermal printer  1  is configured to be capable of performing printing on a recording sheet P 1 . The recording sheet P 1  is a heat-sensitive sheet that develops color through application of heat, and is suitably used for printing, for example, a variety of labels, receipts, and tickets. As illustrated in  FIG. 2 , the recording sheet P 1  is set in the thermal printer  1  under a state of a roll sheet PR having a hollow hole  5 , which is obtained by winding the recording sheet P 1 . Printing is performed on a part drawn out from the roll sheet PR. 
     The thermal printer  1  includes a casing  3  (housing) having an opening portion  3   a , and a paper cover  20  supported on the casing  3  in a pivotable manner and configured to open and close the opening portion  3   a  of the casing  3 . Further, the thermal printer  1  has a thermal printer module  30  mounted therein. 
     The casing  3  has a box shape, and is made of plastics such as polycarbonate or a metal material. An upper wall  10  is formed on a front part of the casing  3 . Ribs (not shown) or the like are formed on an inner surface of the casing  3  to enhance the mechanical strength of the casing  3 . On the upper wall  10  of the casing  3 , operation portions  14  configured to perform a variety of operations for the thermal printer  1  are arranged. As the operation portions  14 , a variety of function switches  15  such as a power switch and a FEED switch are arranged, and a variety of indicator lamps  16  are arranged, such as a POWER indicator lamp provided adjacent to the function switches  15  and configured to indicate ON/OFF information of the power switch, and an ERROR indicator lamp  16  configured to indicate an error of the thermal printer  1 . Further, an open button  18  configured to perform opening and closing operations for the paper cover  20  is provided between the upper wall  10  and a side wall  12 . Moreover, a first cutting blade  26  configured to cut the recording sheet P 1  is formed at a rear end edge of the front wall  10  of the casing  3 . 
     The paper cover  20  is made of plastic such as polycarbonate. A rear end of the paper cover  20  is supported by a hinge portion  32   a  so that the paper cover  20  is rotatable with respect to a main body frame  31  (support frame, see  FIG. 3 ) of the thermal printer module  30 . Further, a front end of the paper cover  20  is configured to be capable of being locked to the main body frame  31  by a platen roller  51  mounted to the front end of the paper cover  20 . Through pressing of the open button  18  of the casing  3 , locking between the paper cover  20  and the casing  3 , in which the main body frame  31  is mounted, is cancelled so that the paper cover  20  can be changed from the closed position (see  FIG. 1 ) to the open position (see  FIG. 2 ). Further, as illustrated in  FIG. 1 , when the paper cover  20  is positioned at the closed position, a gap is defined along the width direction of the recording sheet P 1  between a front edge of the paper cover  20  and the rear end edge of the front wall  10  of the casing  3 . The gap forms a delivery slot  19  through which the recording sheet P 1  subjected to printing is delivered. Moreover, a second cutting blade  27  (see  FIG. 2 ) configured to cut the recording sheet P 1  is formed at the front edge of the paper cover  20 . The recording sheet P 1  delivered through the delivery slot  19  is cut by being pulled down in a contact state with the first cutting blade  26  or the second cutting blade  27 . 
       FIG. 3  is a perspective view of the thermal printer module  30 . Further,  FIG. 4  is a perspective view of the thermal printer module  30  when viewed in a direction different from that in  FIG. 3 . For description, some components are removed in illustration of  FIG. 4 . Further,  FIG. 5  is an explanatory view for illustrating an operation of a lock arm  61  of the thermal printer module  30 , and is a partial enlarged view of the thermal printer module  30  when viewed in a direction indicated by the line X-X of  FIG. 3 . As illustrated in  FIG. 3 , the thermal printer module  30  includes the main body frame  31 , a thermal head  41  (printing head), the platen roller  51 , and the lock arm  61 . 
     The main body frame  31  includes a sheet receiving portion  32  (roll receiving portion) formed at a bottom part thereof to extend in a right-and-left direction, a pair of side wall portions  33  formed upright from both sides of the sheet receiving portion  32  in the right-and-left direction toward the upper side, and a front wall portion  34  formed upright from a front side of the sheet receiving portion  32  toward the upper side. The sheet receiving portion  32  is herein described as being included in the thermal printer module  30  (main body frame  31 ), but the sheet receiving portion  32  may be provided separately from the thermal printer module  30 . The sheet receiving portion  32  holds the roll sheet PR. The sheet receiving portion  32  is a member having an arc shape in cross section, and a rear end of the sheet receiving portion  32  extends to the rear end side of the paper cover  20  (see  FIG. 2 ), whereas a front end of the sheet receiving portion  32  extends to a lower side of the platen roller  51 . The hinge portion  32   a  configured to support the paper cover  20  in a pivotable manner is formed at a rear end edge of the sheet receiving portion  32 . Further, a plurality of guide members  37  are arranged above the front end of the sheet receiving portion  32  so as to be in conformity with a curved surface of the sheet receiving portion  32 . With the guide member  37 , the recording sheet P 1  is smoothly introduced toward the thermal head  41 . 
     As illustrated in  FIG. 3  and  FIG. 5 , roller insertion grooves  35  cut downward are formed in upper edges of the respective side wall portions  33 . Relative positions of the lock arm  61  and the roller insertion grooves  35  differ from each other along an axis of the platen roller  51 , and hence the roller insertion groove  35  is indicated by the broken line in  FIG. 5 . As illustrated in  FIG. 5 , the platen roller  51  is inserted into the roller insertion grooves  35  in a state of being removable from the roller insertion grooves  35  along an up-and-down direction. Each roller insertion groove  35  is defined by a groove bottom surface  35   a  extending along a front-and-rear direction, a groove front surface  35   b  extending from a front end of the groove bottom surface  35   a  toward the upper side, a groove rear surface  35   c  extending from a rear end of the groove bottom surface  35   a  toward the upper side, a groove inclined surface  35   d  extending from an upper end of the groove rear surface  35   c  obliquely toward an upper rear side, and a groove inclined surface  35   e  extending from an upper end of the groove front surface  35   b  obliquely toward an upper front side. 
     The length of the groove bottom surface  35   a  in the front-and-rear direction is slightly larger than the outer diameter of each bearing  54  of the platen roller  51 . Each of the groove front surface  35   b  and the groove rear surface  35   c  is orthogonal to the groove bottom surface  35   a . The length of the groove front surface  35   b  in the up-and-down direction is smaller than the outer diameter of the bearing  54 . The length of the groove rear surface  35   c  in the up-and-down direction is larger than the outer diameter of the bearing  54 . The groove front surface  35   b  and the groove rear surface  35   c  are parallel to each other, whereas the groove inclined surfaces  35   d  and  35   e  are formed so as to be gradually away from each other as extending toward the upper side. The roller insertion grooves  35  are formed at the same position when viewed in the right-and-left direction, and the platen roller  51  is inserted into the roller insertion grooves  35  in a state of extending along the right-and-left direction and being removable from the roller insertion grooves  35  along the up-and-down direction. Due to the groove inclined surfaces  35   d  and  35   e , the opening width of each roller insertion groove  35  is increased as extending toward the upper side. Thus, the bearings  54  of the platen roller  51  are smoothly insertable into the roller insertion grooves  35 . 
     The thermal head  41  illustrated in  FIG. 5  is configured to perform printing on the recording sheet P 1 , and is formed into a rectangular shape when viewed in the front-and-rear direction. The thermal head  41  is arranged on an inner side of the upper wall  10  (see  FIG. 2 ) of the casing  3  so as to be exposed into the opening portion  3   a , and is arranged under a state in which a longitudinal direction of the thermal head  41  matches with the width direction of the recording sheet P 1 . On a head surface of the thermal head  41 , a large number of heating elements (not shown) are arrayed in line and in parallel to the right-and-left direction. The head surface is opposed to a printing surface of the recording sheet P 1 , and the recording sheet P 1  may be nipped between the head surface and an outer peripheral surface of the platen roller  51 . The heating elements of the thermal head  41  are each controlled to generate heat based on a signal from a control unit (not shown). Through the control of heat generation of the heating elements, the thermal head  41  prints various kinds of letters and figures on the printing surface of the recording sheet P 1 . 
     As illustrated in  FIG. 4  and  FIG. 5 , the thermal head  41  is bonded and fixed onto the head support member  45  supported on the main body frame  31 . The head support member  45  is a plate-like member having its longitudinal direction defined as the right-and-left direction. The head support member  45  is arranged between the pair of side wall portions  33 , and the thermal head  41  is bonded onto a rear surface of the head support member  45 . The head support member  45  is arranged behind the front wall portion  34  of the main body frame  31 , and a lower end portion of the head support member  45  is supported on a shaft  38  in a pivotable manner. The shaft  38  is arranged so that a center axis of the shaft  38  is defined along the right-and-left direction, and both end portions of the shaft  38  are fixed to the pair of side wall portions  33 , respectively. As illustrated in  FIG. 4 , a plurality of (in this embodiment, five) elastic members  46  (biasing members) are interposed between the head support member  45  and the front wall portion  34  along the right-and-left direction. The elastic members  46  are coil springs configured to bias the head support member  45  and the front wall portion  34  in directions away from each other. The elastic members  46  are configured to press the head support member  45  constantly rearward. 
     Stoppers  45   a  configured to regulate a pivot range of the head support member  45  are formed at upper end portions of the head support member  45 . Each stopper  45   a  extends outward in a right-and-left direction of the head support member  45 , and is formed so as to face an inside of a recessed portion  33   a  formed in an upper part of the side wall portion  33  of the main body frame  31 . The stopper  45   a  is configured to move inside the recessed portion  33   a  along with the pivot of the head support member  45 , and may be brought into contact with both end surfaces of the recessed portion  33   a . Through the contact of the stopper  45   a  with the end surfaces of the recessed portion  33   a , the pivot amount of the head support member  45  is regulated. 
     The platen roller  51  is arranged so as to be opposed to the thermal head  41 , and is rotated about an axis extending along the right-and-left direction under a state in which the recording sheet P 1  is nipped between the platen roller  51  and the thermal head  41 , to thereby convey the recording sheet P 1 . As illustrated in  FIG. 4 , the platen roller  51  includes a roller shaft  52 , a roller main body  53  externally mounted on the roller shaft  52 , and a pair of the bearings  54  mounted at both ends of the roller shaft  52 . The roller shaft  52  is formed slightly longer than the separation distance between the pair of side wall portions  33  of the main body frame  31 . The roller main body  53  is made of, for example, rubber, and is arranged along an axial direction of the roller shaft  52  uniformly over the entire region excluding portions corresponding to both the ends of the roller shaft  52 . 
     As illustrated in  FIG. 2 , the platen roller  51  is mounted in a freely pivotable manner at the front end edge of the paper cover  20  through intermediation of a platen frame  55 , and is removable from the main body frame  31  along with the opening and closing operation of the paper cover  20 . As illustrated in  FIG. 1  and  FIG. 3 , when the paper cover  20  is closed, the pair of bearings  54 , which are mounted at both ends of the platen roller  51 , are inserted into the roller insertion grooves  35  of the main body frame  31 , respectively. Thus, the platen roller  51  is held so as to be rotatable about a center axis P (see  FIG. 5 ) relative to the main body frame  31  and removable from the main body frame  31 . The platen roller  51  is arranged so that the roller main body  53  is brought into contact with the thermal head  41  under the state in which the platen roller  51  is inserted into the roller insertion grooves  35  and the recording sheet P 1  drawn out from the roll sheet PR is nipped between the platen roller  51  and the thermal head  41 . 
     As illustrated in  FIG. 4 , a driven gear  56  is fixed to one axial end of the platen roller  51 . The driven gear  56  meshes with a gear transmission mechanism  57  mounted on the main body frame  31  when the platen roller  51  is held on the pair of side wall portions  33 . The gear transmission mechanism  57  is connected to driving means  58  such as a motor to transmit a rotational driving force from the driving means  58  to the driven gear  56 . Thus, the platen roller  51  is rotated in a state of being held on the pair of side wall portions  33 , thereby being capable of conveying the recording sheet P 1 . 
       FIG. 6  is a perspective view of the lock arm  61 .  FIG. 7  is an explanatory view for illustration the operation of the lock arm  61 , and is a partial enlarged view for illustrating a state in which a locked state illustrated in  FIG. 5  is cancelled. As illustrated in  FIG. 5  and  FIG. 6 , the lock arm  61  is supported so as to be pivotable about a pivot axis (center axis Q) extending along the right-and-left direction, and is configured to hold the platen roller  51  inserted into the roller insertion grooves  35 . The lock arm  61  includes a pair of side plate portions  62  extending along the pair of side wall portions  33  of the main body frame  31 , respectively, and a rear plate portion  66  connecting the pair of side plate portions  62 . The pair of side plate portions  62  are formed into the same shape, and hence only one of the side plate portions  62  is described below. Unless otherwise noted, the following description of the structure of the lock arm  61  is directed to a state in which the lock arm  61  holds the platen roller  51  inserted into the roller insertion grooves  35  (the state illustrated in  FIG. 5 , and hereinafter referred to as “holding state”). 
     As illustrated in  FIG. 5  and  FIG. 6 , the side plate portion  62  is a flat plate-like portion, and a through hole  81  is formed in a lower part of the side plate portion  62 . The through hole  81  is formed into a circular shape having an axis perpendicular to a plate surface of the side plate portion  62 , and the shaft  38  is coaxially fitted into the through hole  81 . Accordingly, the shaft  38  supports the lock arm  61  as well as the head support member  45 . The shaft  38  is arranged so that the center axis Q is defined along the right-and-left direction, and both axial end portions of the shaft  38  are fixed to the pair of side wall portions  33 , respectively. Through the through holes  81  and the shaft  38 , the lock arm  61  pivots along an arc path about the center axis Q of the shaft  38  in the front-and-rear direction. 
     As illustrated in  FIG. 6 , at an upper edge of the side plate portion  62 , there are formed an inclined surface  63   a , a horizontal surface  63   b , an arc surface  63   c , an inclined surface  63   d , an inclined surface  63   e . The inclined surface  63   a  extends linearly and obliquely toward a lower rear side. The horizontal surface  63   b  extends from a lower end of the inclined surface  63   a  toward the rear side. The arc surface  63   c  extends in a substantially arc shape from a rear end of the horizontal surface  63   b  toward the upper side. The inclined surface  63   d  extends from an upper end of the arc surface  63   c  linearly and obliquely toward the upper front side. The inclined surface  63   e  extends from an upper end of the inclined surface  63   d  toward the rear side. A distal end portion  63   f  formed between the inclined surface  63   d  and the inclined surface  63   e  has a small arc shape when viewed in the right-and-left direction. The arc surface  63   c  is formed into an arc shape conforming to the outer peripheral surface of the bearing  54  of the platen roller  51  when viewed in the right-and-left direction, and has a curvature radius slightly larger than a radius of the bearing  54 . Further, when the side plate portion  62  is viewed from the front side, the inclined surface  63   d  functions as a platen roller engagement portion configured to press the bearing  54  of the platen roller  51  from the upper side of the bearing  54 . 
     As illustrated in  FIG. 5 , the lock arm  61 , which includes the side plate portion  62  having the above-mentioned shape, holds down the bearing  54  in the roller insertion groove  35  through abutment of the inclined surface  63   d  (platen roller engagement portion) against a peripheral surface of the bearing  54 . On this occasion, the distal end portion  63   f  (front end of the inclined surface  63   d ) is located in a region close to the thermal head  41  side (front side) with respect to an imaginary plane including the center axis P of the platen roller  51  and the center axis Q of the shaft  38 . Further, the distal end portion  63   f  is formed so that a shortest distance between the distal end portion  63   f  and the horizontal surface  63   b  is larger than the outer diameter of the bearing  54 . 
     As illustrated in  FIG. 6 , the rear plate portion  66  is a substantially U-shaped flat plate-like portion having its longitudinal direction defined as the right-and-left direction, and is arranged between the pair of side plate portions  62 . The rear plate portion  66  is formed integrally with the pair of side plate portions  62  to connect front edges of the pair of side plate portions  62 . Further, as illustrated in  FIG. 5 , a reinforcement rib  66   a  is formed at a lower edge of the rear plate portion  66  to extend toward the rear side. As illustrated in  FIG. 6 , the reinforcement rib  66   a  is formed over a substantially entire length of the rear plate portion  66 , and is configured to prevent torsion and flexure of the rear plate portion  66 . 
     Further, two projecting portions  66   b  are formed on both upper portions of the rear plate portion  66 . The elastic members  46  are externally fitted to the projecting portions  66   b  so that the elastic members  46  are positioned. As illustrated in  FIG. 4 , the pair of elastic members  46  positioned by the projecting portions  66   b  are interposed between the rear plate portion  66  and the head support member  45 , and thus bias the lock arm  61  so as to cause the lock arm  61  to pivot about the center axis Q toward the front side. As a result, as illustrated in  FIG. 5 , the lock arm  61  can continuously lock the bearing  54  of the platen roller  51  in the roller insertion groove  35 . 
     As illustrated in  FIG. 4 , the thermal printer module  30  includes a lever  90 . The lever  90  is mounted on the left side wall portion  33  in a freely pivotable manner. One end portion  90   a  of the lever  90  is arranged on an outer side of the side wall portion  33 , whereas another end portion  90   b  of the lever  90  located opposite to the one end portion  90   a  across a pivot axis of the lever  90  is arranged on an inner side of the side wall portion  33 . The another end portion  90   b  includes an arm portion  90   b   1  that pivots together with the one end portion  90   a . A proximal end side of the arm portion  90   b   1  is rotated coaxially with the one end portion  90   a , and a distal end side of the arm portion  90   b   1  is held in abutment against a front surface of the rear plate portion  66  in a freely slide-contact manner. A lower end of the open button  18  is held in abutment against an upper surface of a rear end portion of the one end portion  90   a . With the configuration described above, when a user depresses the open button  18 , the one end portion  90   a  and the another end portion  90   b  of the lever  90  pivot. Then, as illustrated in  FIG. 7 , the distal end of the arm portion  90   b   1  pushes the lock arm  61  at the rear plate portion  66 , to thereby cause the lock arm  61  to pivot about the center axis Q in a direction indicated by the arrow RL. As a result, the inclined surface  63   d  is disengaged from the bearing  54 , and hence the platen roller  51  is disengaged from the roller insertion groove  35 , thereby being capable of opening the paper cover  20 . 
     From the viewpoint of ease of operation, it is preferred that unlocking of the bearing  54  by the lock arm  61  be smoothly performed when a user depresses the open button  18 . However, when only the ease of unlocking is taken into consideration, in a case in which an external force caused by dropping or the like is applied to the thermal printer  1 , there is a fear in that unlocking is unintentionally performed so that the paper cover  20  is opened. In order to prevent such unintentional unlocking, the thermal printer  1  according to this embodiment adopts a configuration described below. 
       FIG. 8  is a view for illustrating a state in which the bearing  54  is inserted into the roller insertion groove  35  and locked by the lock arm  61 , and illustrating a desirable positional relationship between a straight line B extending along the inclined surface  63   d  of the lock arm  61  and a pivoting-direction tangential line A of the platen roller  51 . A center axis R illustrated in  FIG. 8  corresponds to a pivot center line of the hinge portion  32   a , and is parallel to the center axes P and Q. Further, the broken line denoted by the reference symbol TR 1  shows a pivot track of the center axis P pivoting about the center axis R. As illustrated in  FIG. 8 , when viewed in the right-and-left direction extending along the center axis P, a relation of 90°≤θ AB ≤110° is satisfied in a case in which θ AB  represents an intersection angle in a closing direction of the lock arm  61 . The intersection angle θ AB  is formed between the straight line B extending along the inclined surface  63   d  and the tangential line A of the pivot track TR 1  obtained when the center axis P of the bearing  54  held in the roller insertion groove  35  moves along with the opening operation of the paper cover  20 . 
     In this case, the tangential line A corresponds to a tangential line at a position of the center axis P when the center axis P of the bearing  54  is fixed at one point on the pivot track TR 1  by holding down an upper surface of the bearing  54  inserted into the roller insertion groove  35  by the inclined surface  63   d  of the lock arm  61 . Accordingly, the tangential line A is orthogonal to a straight line L 1  that passes the center axes P and R. Further, the straight line B corresponds to a straight line extending along the inclined surface  63   d  when the inclined surface  63   d  is viewed in the right-and-left direction. In  FIG. 8 , for clear illustration, the straight line B includes an extension line of the inclined surface  63   d . The above-mentioned intersection angle θ AB  in the closing direction of the lock arm  61  refers to an angle formed between a vector VA at the position of the center axis P at the time when the center axis P starts pivoting along the tangential line A in an unlocking direction, and a vector VB having a direction of separating away from the center axis R along the straight line B. 
     The intersection angle θ AB  is now described with reference to  FIG. 9 .  FIG. 9  is a partial enlarged view for illustrating a state in which the bearing  54  of the platen roller  51  is inserted into the roller insertion groove  35  (not shown) and locked by the lock arm  61 , and illustrating an undesirable relative positional relationship between the straight line B extending along the inclined surface  63   d  of the lock arm  61  and pivoting-direction tangential lines A 1  and A 2  of the center axis P of the platen roller  51 . That is,  FIG. 9  is a partial enlarged view for illustrating a case in which the center axis R of the hinge portion  32   a  is located at an undesirable position (R 1 , R 2 ) with respect to the inclined surface  63   d  of the lock arm  61 . The reference symbol R 1  represents the center axis of the hinge portion  32   a  when the intersection angle θ AB  is smaller than 90°, whereas the reference symbol R 2  represents the center axis of the hinge portion  32   a  when the intersection angle θ AB  is larger than 110°. 
     First, when the intersection angle θ AB  is smaller than 90°, the inclined surface  63   d  is brought into abutment against the peripheral surface of the bearing  54  at a contact point Pa. The contact point Pa is closer to the center axis R 1  than the pivoting-direction tangential line A 1 . For example, when the external force received by the casing  3  causes a force F in a direction of disengaging the bearing  54  from the roller insertion groove  35 , the force F parallel to the pivoting-direction tangential line A 1  is applied to the inclined surface  63   d  at the contact point Pa. The force F generates a force component F·sin θ AB , which is applied to the inclined surface  63   d  in the vertical direction, and a force component F·cos θ AB , which is applied along the inclined surface  63   d  and pushes away the lock arm  61  in the unlocking direction. An angle denoted by the reference symbol α in  FIG. 9  is equal to the intersection angle θ AB . Even when a frictional force between the bearing  54  and the inclined surface  63   d  is increased by intensifying the elastic members  46  configured to bias the lock arm  61  in the closing direction, it is difficult to reliably maintain the locked state against the force component F·cos θ AB . Therefore, in a case in which the intersection angle θ AB  is smaller than 90°, when the external force is applied to the thermal printer  1 , there is a fear in that the locking of the bearing  54  by the lock arm  61  is cancelled so that the paper cover  20  is opened. 
     Meanwhile, in a case in which the intersection angle θ AB  is equal to 90° as illustrated in  FIG. 8 , even when the above-mentioned external force F is applied, the force component F·cos θ AB  satisfies a relation of F·cos 90°=0. Therefore, the force component of pushing away the lock arm  61  along the inclined surface  63   d  in the unlocking direction becomes zero. Thus, even when there is no pressurization by the elastic members  46  at the time of application of the external force to the thermal printer  1 , the locked state of the bearing  54  by the lock arm  61  is maintained so that the paper cover  20  is not opened. Further, when a user depresses the open button  18 , the intersection angle θ AB  is equal to 90°, and hence the inclined surface  63   d  can slide on the peripheral surface of the bearing  54  without being caught thereon. Therefore, the locking of the bearing  54  by the lock arm  61  can be smoothly cancelled. 
     Referring back to  FIG. 9 , in a case in which the intersection angle θ AB  is larger than 90°, the center axis of the hinge portion  32   a  matches with a position denoted by the reference symbol R 2 , and the pivoting-direction tangential line of the center axis P of the platen roller  51  matches with a line denoted by the reference symbol A 2 . However, the inclined surface  63   d  is brought into abutment against the peripheral surface of the bearing  54  at the same contact point Pa. On this occasion, the contact point Pa is farther from the center axis R 2  than the pivoting-direction tangential line A 2 . This point is contrary to that in the former case in which the intersection angle θ AB  is smaller than 90°. In the case of such a relative positional relationship, when the external force is applied to the thermal printer  1 , the locking of the bearing  54  by the lock arm  61  is maintained so that the paper cover  20  is not opened. However, in a case in which the intersection angle θ AB  is larger than 110°, a point Pb at the distal end position of the inclined surface  63   d  is excessively closer to the center axis Q than the contact point Pa. Therefore, even when a user depresses the open button  18 , the inclined surface  63   d  is caught on the bearing  54  at the point Pb, and thus cannot climb over the bearing  54 . Therefore, the locking of the bearing  54  by the lock arm  61  cannot be cancelled. For the above-mentioned reason, it is required that the intersection angle θ AB  be equal to or larger than 90° and equal to or smaller than 110°. It is more preferred that an upper limit value of the intersection angle θ AB  be 100° in consideration of manufacturing tolerance. In this case, the intersection angle θ AB  falls within a range of from 90° to 100°. Moreover, within this range, it is most preferred that a relation of the intersection angle θ AB =90° be satisfied. 
     As illustrated in  FIG. 10 , the inclined surface  63   d  (platen roller engagement portion) and the platen roller  51  not only move relatively about the center axis R (R 1 , R 2 ) of the platen roller  51  as described above, but also move relatively about the center axis Q of the inclined surface  63   d . Therefore, it is required that not only the above-mentioned intersection angle θ AB  but also an intersection angle θ BC  be specified.  FIG. 10  is a view for illustrating a state in which the bearing  54  is inserted into the roller insertion groove  35  (not shown) and locked by the lock arm  61 , and illustrating a desirable relative positional relationship between the straight line B extending along the inclined surface  63   d  of the lock arm  61  and a pivoting-direction tangential line C at the contact point Pa between the inclined surface  63   d  and the platen roller  51 . The broken line denoted by the reference symbol TR 2  in  FIG. 10  shows a track of the contact point Pa on the inclined surface  63   d  pivoting about the center axis Q. 
     As illustrated in  FIG. 10 , when viewed in the right-and-left direction extending along the center axis P, the intersection angle θ BC  in the closing direction of the lock arm  61  satisfies a relation of 0°≤θ BC ≤10°. The intersection angle θ BC  is formed between the straight line B extending along the inclined surface  63   d  and the tangential line C of the track TR 2  obtained when the contact point Pa between the bearing  54  held in the roller insertion groove  35  and the inclined surface  63   d  (platen roller engagement portion) pivots in the unlocking direction of the lock arm  61 . 
     In this case, the tangential line C corresponds to a straight line orthogonal at a position of the contact point Pa to a straight line L 2  that passes the contact point Pa and the center axis Q when the upper surface of the bearing  54  inserted into the roller insertion groove  35  is held down by the inclined surface  63   d  of the lock arm  61 . The above-mentioned intersection angle θ BC  in the closing direction of the lock arm  61  refers to an angle formed between a vector VC having a direction of separating away from the center axis R along the tangential line C, and the vector VB having the direction of separating away from the center axis R along the straight line B. 
     The intersection angle θ BC  is now described with reference to  FIG. 11 .  FIG. 11  is a partial enlarged view for illustrating a state in which the bearing  54  of the platen roller  51  is inserted into the roller insertion groove  35  (not shown) and locked by the lock arm  61 , and illustrating an undesirable relative positional relationship between the straight line B extending along the inclined surface  63   a  of the lock arm  61  and pivoting-direction tangential lines C 1  and C 2  of the center axis P of the platen roller  51 . As the pivoting-direction tangential line C 1 , a straight line that passes the position of the contact point Pa and is parallel to the pivoting-direction tangential line C 1  is indicated by the broken line. Accordingly, an intersection angle formed between the pivoting-direction tangential line C 1  and the straight line B is equal to the intersection angle θ BC  formed between the pivoting-direction tangential line C 1  indicated by the broken line and the straight line B, and represents a case of θ BC &gt;10°. Meanwhile, as the pivoting-direction tangential line C 2 , a straight line that passes the position of the contact point Pa and is parallel to the pivoting-direction tangential line C 2  is indicated by the broken line. Accordingly, an intersection angle formed between the pivoting-direction tangential line C 2  and the straight line B corresponds to the intersection angle θ BC  formed between the pivoting-direction tangential line C 2  indicated by the broken line and the straight line B, and is a negative angle, that is, satisfies a relation of θ BC &lt;0°. 
     First, in a case in which the intersection angle θ BC  is the negative angle smaller than 0°, the center axis Q matches with a position denoted by the reference symbol Q 2 , and a straight line passing the center axis Q 2  and the center axis P matches with a line denoted by the reference symbol L 4 . Further, a pivoting-direction tangential line matches with a line denoted by the reference symbol C 2 . Moreover, the point Pb at the distal end position of the inclined surface  63   d  is located on a locking direction side of the lock arm  61  with respect to the straight line L 4  and the contact point Pa. In this case, for the same reason as that described for the case in which the intersection angle θ AB &gt;110° in  FIG. 9 , the point Pb at the distal end position of the inclined surface  63   d  is excessively closer to the center axis Q 2  than the contact point Pa. Therefore, even when a user depresses the open button  18 , the inclined surface  63   d  is caught on the bearing  54  at the point Pb at the distal end position, and thus cannot climb over the bearing  54 . Therefore, the locking of the bearing  54  by the lock arm  61  cannot be cancelled. 
     Further, in a case in which the intersection angle θ BC  exceeds 0°, the center axis Q matches with a position denoted by the reference symbol Q 1 , and a straight line passing the center axis Q 1  and the center axis P matches with a line denoted by the reference symbol L 3 . Further, a pivoting-direction tangential line matches with a line denoted by the reference symbol C 1 . Moreover, the point Pb at the distal end position of the inclined surface  63   d  is located on the locking direction side of the lock arm  61  with respect to the straight line L 3 . Meanwhile, the contact point Pa is located on the unlocking direction side of the lock arm  61  with respect to the straight line L 3 . The position of the contact point Pa on the inclined surface  63   d  is the same as that in the case in which the intersection angle θ BC  is the negative angle smaller than 0°. Further, the angle β illustrated in  FIG. 11  is equal to the intersection angle θ BC . For example, when the external force received by the casing  3  causes the force F in the direction of disengaging the bearing  54  from the roller insertion groove  35 , as illustrated in  FIG. 9 , the force component F·sin θ AB  is applied to the inclined surface  63   d  in the vertical direction. At the contact point Pa on the inclined surface  63   d , the force component F·sin θ AB  generates a force component F·sin θ AB ·sin θ BC  of pivoting the lock arm  61  about the center axis Q 1  in an opening direction. Even when the frictional force between the bearing  54  and the inclined surface  63   d  is increased by intensifying the elastic members  46  configured to bias the lock arm  61  in the closing direction, the lock arm  61  is liable to pivot, and hence it is difficult to reliably maintain the locked state against the force component F·sin θ AB ·sin θ BC . Therefore, in a case in which the intersection angle θ BC  is larger than 10°, when the external force is applied to the thermal printer  1 , there is a fear in that the locking of the bearing  54  by the lock arm  61  is cancelled so that the paper cover  20  is opened. 
     Meanwhile, in a case in which the intersection angle θ BC =0°, specifically, the tangential line C and the straight line B are parallel to each other, a relation of sin θ BC =0 is satisfied. Therefore, the force component F·sin θ AB ·sin θ BC  in the direction of causing the lock arm  61  to pivot becomes zero. Accordingly, even when there is no pressurization by the elastic members  46  at the time of application of the external force to the thermal printer  1 , the locking of the bearing  54  by the lock arm  61  is maintained so that the paper cover  20  is not opened. Further, when a user depresses the open button  18 , the tangential line C and the straight line B are parallel to each other, and hence the inclined surface  63   d  can slide on the peripheral surface of the bearing  54  without being caught thereon. Therefore, the locking of the bearing  54  by the lock arm  61  can be smoothly cancelled. For the above-mentioned reason, it is required that the intersection angle θ BC  be equal to or larger than 0° and equal to or smaller than 10°. It is more preferred that an upper limit value of the intersection angle θ BC  be 5° in consideration of manufacturing tolerance. In this case, the intersection angle θ BC  falls within a range of from 0° to 5°. Moreover, within this range, it is most preferred that a relation of the intersection angle θ BC =0° be satisfied. 
     The main points of the above-mentioned embodiment of the present invention are summarized below. The thermal printer module  30  according to the embodiment of the present invention includes: the casing  3  including the sheet receiving portion  32 ; the paper cover  20 , which is mounted to the casing  3  so as to be pivotable, and is configured to open and close the sheet receiving portion  32 ; the platen roller  51  provided to the paper cover  20  so as to be freely rotatable; the main body frame  31 , which is provided in the casing  3 , and has the roller insertion groove  35  configured to receive the bearing  54  of the platen roller  51  to be inserted into the roller insertion groove  35  when the paper cover  20  is closed; the lock arm  61 , which is provided to the main body frame  31  so as to be pivotable, and is configured to hold down, by the inclined surface  63   d , the bearing  54  of the platen roller  51  inserted into the roller insertion groove  35 ; the elastic member  46  configured to apply a biasing force to the lock arm  61  in a direction of causing the lock arm  61  to pivot so as to maintain holding of the bearing  54  by the inclined surface  63   d ; and the thermal head  41  provided at a position opposed to the platen roller  51  having the bearing  54  held in the roller insertion groove  35 . When viewed in the direction extending along the center axis P of the bearing  54 , the inclined surface  63   d  and the tangential line A of the track TR 1  obtained when the axis P of the bearing  54  held in the roller insertion groove  35  moves along with the opening operation of the paper cover  20  form the intersection angle θ AB  in the closing direction of the lock arm  61 , and the intersection angle θ AB  satisfies the relation of 90°≤θ AB ≤110°. In addition, when viewed in the direction extending along the center axis P of the bearing  54 , the inclined surface  63   d  and the tangential line C of the track TR 2  obtained when the contact point Pa between the bearing  54  held in the roller insertion groove  35  and the inclined surface  63   d  pivots in the unlocking direction of the lock arm  61  form the intersection angle θ BC  in the closing direction of the lock arm  61 , and the intersection angle θ BC  satisfies the relation of 0°≤θ BC ≤10°. According to the thermal printer module  30 , through satisfaction of both of two conditions relating to the intersection angle θ AB  and the intersection angle θ BC , even when the external force is applied to the paper cover  20  in a direction of opening the paper cover  20 , the platen roller  51  can be more stably held by the lock arm  61 . 
     Moreover, in the thermal printer module  30  described above, when viewed in the direction extending along the center axis P of the bearing  54 , it is preferred that the inclined surface  63   d  and the tangential line C of the track TR 2  obtained when the contact point Pa between the bearing  54  held in the roller insertion groove  35  and the inclined surface  63   d  pivots in the unlocking direction of the lock arm  61  be parallel to each other. In this case, at the inclined surface  63   d , the platen roller  51  does not generate a force component in a direction of causing the lock arm  61  to pivot in the unlocking direction. Thus, the platen roller  51  is more reliably held by the lock arm  61 . 
     In addition, in the thermal printer module  30  described above, when viewed in the direction extending along the center axis P of the bearing  54 , the inclined surface  63   d  and the tangential line A of the track TR 1  obtained when the center axis P of the bearing  54  held in the roller insertion groove  35  moves along with the opening operation of the paper cover  20  may be orthogonal to each other. In this case, when the external force is applied to the platen roller  51  in the direction of opening the paper cover  20 , the force in the direction of disengaging the platen roller  51  from the roller insertion groove  35  does not include the force component exerted by the platen roller  51  on the inclined surface  63   d  to push away the lock arm  61  in the unlocking direction. Accordingly, the platen roller  51  is further reliably held by the lock arm  61 . 
     The thermal printer  1  according to the embodiment of the present invention includes the thermal printer module  30  having the above-mentioned configuration. The thermal printer  1  stably holds the platen roller  51 , and is resistant to the external force such as drop impact. 
     The present invention is not limited to the above-mentioned embodiment, and various modification examples may be employed within the technical scope of the present invention. For example, in the above-mentioned embodiment, the lever  90  is used for the operation of disengaging the platen roller  51 , but the present invention is not limited to this configuration. Other link mechanisms and cam mechanisms may be used instead. Further, in the above-mentioned embodiment, a cushioning member may be arranged around the casing  3  or between the main body frame  31  and the casing  3 . With this configuration, an influence such as drop impact can be further reduced. Further, the thermal printer module  30  may be formed integrally with the casing  3 . Alternatively, the thermal printer module  30  may be formed separately from the casing  3 , and fixed in the casing  3 . In a case of adopting the thermal printer module  30  formed integrally with the casing  3 , there is an advantage in that the number of components and manufacturing cost can be reduced. 
     Besides the above, the components in the above-mentioned embodiments may be replaced by well-known components as appropriate without departing from the gist of the present invention.