Abstract:
A printer containing a printhead that is detachably mounted on a support plate and is held thereon at two rigid support points located on a first side of the printhead and one elastic support point located on a second side of the printhead opposite to said first side, each of the support points being defined by a first portion on the side of the printhead and a second portion on the side of the support plate, and one of the first and second portions is a spherical surface engaged in a recess of the other of the first and second portions in a self-centering manner wherein the second part of the elastic support point has its spherical surface formed by a bead or ball fixed on a wire, said wire being elastically biased toward the recess, so that the elastic support point releasably biases the printhead towards said first side of the printhead and released by lifting the wire, for detaching the printhead.

Description:
This application claims priority to European Patent Application No. 04107045.9 filed on Dec. 29, 2004 in Europe, the entire contents of which is hereby incorporated by reference in its entirety. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a printer having a printhead which is detachably mounted on a support plate and is held thereon at two rigid support points located on a first side of the printhead, and one elastic support point located on a second side of the printhead, opposite to said first side, wherein each of the support points is formed by a first portion on the side of the printhead and a second portion on the side of the support plate, and one of the first and second portions is a spherical surface engaged in a recess of the other of the first and second portions in a self-centering manner. 
   The present invention is applicable to a scanning-type printer, e.g. an ink jet printer, wherein the support plate is provided on a carriage so that it may scan a recording medium in a main scanning direction, while the printhead faces the recording medium and prints individual pixels or sets of pixels. In order to obtain a high print quality, it is essential, that the printhead is stably positioned on the support plate with very high accuracy in all six degrees of freedom of the printhead. The accuracy requirements increase with increasing resolution of the printer and may, for example, only allow tolerances of up to ±3 mm. 
   In an ink jet printer, for example, the printhead typically has a plurality of nozzles that are arranged in a linear array forming an angle of exactly or approximately 90° with respect to the main scanning direction. The angle between the nozzle array and the main scanning direction must be defined with high precision. Frequently, the printer has a plurality of printheads mounted on the same support plate, e.g. printheads for different colors in case of a color printer. Thus, the angular positions of the nozzle arrays must be exactly the same for all printheads. Likewise, the printheads must be arranged with well defined spacings in the main scanning direction and must also be exactly aligned in that direction. Since a small gap is formed between the nozzle surface of the printhead and the surface of the recording medium, the ink droplets expelled from the nozzles must fly a certain distance through the air before they hit the recording medium. Since the carriage is moving, the flight distance of the ink droplets has an influence on the positions of the pixels formed on the recording medium, and, as a consequence, the gap between the printheads and the recording medium must also be defined with high accuracy. 
   In a conventional printer, the position and posture of the printhead in all six degrees of freedom is defined by six contact surfaces where the printhead engages the support plate, and a spring assembly is used for biasing the printhead against each contact surface. However, if, for any reason, an external force tends to deviate the printhead from the intended position, frictional forces at the contact surfaces may prevent the printhead from returning exactly to its original position. Moreover, if the printhead is detached and is then mounted again on the support plate, cumbersome mounting and adjustment operations are necessary, and these operations, in most cases, cannot be left to the user but require the intervention of a service engineer. 
   EP-A-0 791 461 discloses an ink jet printer wherein four support points are formed by two pairs of recesses on opposite sides of the printhead, and two spherical surfaces are formed directly on the mounting plate so as to engage the recesses of the first pair, and two spherical surfaces are formed on a detachable clamping plate so as to engage in the recesses of the second pair. The clamping plate can be biased against the mounting plate and is positioned relative thereto by another sphere-and-recess-type support point. 
   SUMMARY OF THE INVENTION 
   The present invention provides a printer in which the printhead can easily be detached and remounted and can be stably and reproducibly positioned with high accuracy. 
   The second part of the elastic support has its spherical surface formed by a bead fixed on a liftable wire for detaching the printhead, said wire being elastically biased toward the recess so that the elastic support point releasably biases the printhead towards said first side of the printhead. 
   When a printhead is mounted, it is at first placed onto the two rigid support points, and then an elastic force is applied at the elastic support point. This releasable support point is formed by a recess in the body of the printhead and a spherical surface in the form of a bead that is fixed on a piece of wire. Since this support point will be located on the side of the printhead that is remote from the support surface, the necessary biasing force can simply be provided by means of tension springs which draw the wire towards the support plate. 
   The elastic force is transmitted through the body of the printhead and also causes the spherical surfaces at the two rigid support points to center themselves in their respective recesses. Thus, one simple operation is sufficient for adjusting the printhead exactly in the desired position and posture and fixing it in all six degrees of freedom. Moreover, since the frictional forces involved in the self-centering operations are extremely small, the exact positioning of the printhead is highly reproducible and reliable. 
   Since the operation of a printhead is frequently accompanied by the generation of heat, e.g. in a hot melt ink jet printer, in which the ink is solid at room temperature and must be heated to 100° C. or more in order to become liquid, the mounting structure for the printhead should allow for a certain amount of thermal expansion and contraction of the printhead. This can be achieved by arranging the two rigid support points on one of the longer sides of the printhead, in the vicinity of the ends thereof, and by using a conical shape of the recess only for one support point but an elongated recess, for example in the form of a V-shaped groove, for the other support point. Thus, the spherical surface engaging in the conical recess will fix the position of the printhead in all three translational degrees of freedom, and the engagement of the other spherical surface in the V-shaped groove will fix two rotational degrees of freedom, but will allow this spherical surface to slide in the groove in order to compensate for thermal expansion. 
   Since thermal expansion not only of the printheads but also of the support plate may present a serious problem, it is one of the advantages of the present invention that the limited mechanical contact between the spherical surfaces and their recesses at only three support points helps to thermally insulate the support plate from the printheads. The elastic support point, which, in the above construction, only has to fix the printhead in the remaining third rotational degree of freedom, may also include a groove-shaped recess. 
   If the wire extends in the direction normal to the axis of rotation that is defined by the two rigid support points, the angular position of the printhead (third rotational degree of freedom) can easily and precisely be controlled by adjusting the longitudinal position of the wire, since any possible bending of the wire will have no influence on the angular position of the printhead. This construction is particularly advantageous for a printer in which a plurality of printheads are arranged side-by-side on the support plate. Then, the elastic support point for all the printheads may be formed by a single wire on which the beads are arranged with well defined spacings. The angular position of all the printheads may thus be adjusted by simply adjusting the longitudinal position of the single wire. Likewise, the elastic support points for all the printheads may easily be released simply be lifting and removing the wire. 
   It is possible that the elastic support points and the wire are located in a central position of the printheads, so that both rigid support points of each printhead would be subject to essentially equal biasing forces. However, for simplifying the operation of replacing one or more printheads, it is preferable that the elastic support points and the wire are located near the end portion of the printheads, so that it is sufficient to move the wire only a short distance in order to be able to remove the printheads. Advantageously, an additional spring mechanism may be used for exerting a biasing force on the other ends of the printheads and on the corresponding rigid support points. However, it should be realized that this additional spring mechanism is not involved in positioning the printheads but only serves to make the engagement at the rigid support points more reliable. When the printheads are to be removed, the additional spring mechanism may be lifted and shifted aside, similarly as the wire described above. Preferably, the lift and shift movements of the wire and the additional spring mechanism are mechanically coupled to one another so that the replacement operation becomes particularly simple. 
   The spherical surfaces for all support points are preferably formed by metal balls which have a sufficient hardness and are readily available with exactly defined diameters and exact spherical surfaces, so that a high positioning accuracy may be achieved. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the present invention will now be described in conjunction with the following drawings, wherein: 
       FIG. 1  is a side view, partially in section, of a printhead supported on a support plate, 
       FIG. 2  is a section taken along line II-II in  FIG. 1 ; 
       FIG. 3  is a section taken along line III-III in  FIG. 1 ; 
       FIG. 4  is a perspective view of the printhead; 
       FIG. 5  is a perspective view of the support plate with mounting positions for eight printheads, with two printheads being inserted; 
       FIG. 6  shows the support plate and the printheads of  FIG. 5  as well as spring mechanisms for biasing the printheads against the support plate; and 
       FIGS. 7 and 8  show a release mechanism for the spring mechanisms. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In  FIG. 1 , a hot melt ink jet printhead  10  is mounted in an upright position on a metal support plate  12 . The support plate  12  is attached to a carriage (not shown) of the printer, which reciprocates in a direction normal to the drawing in  FIG. 1 , so that the printhead  10  scans a recording medium that would be disposed right underneath the support plate. A lower portion of the printhead  10  is inserted with play in a recess  14  formed in the top surface of the support plate  12  and has a nozzle section  16  which projects through an opening of the support plate and forms a downwardly oriented nozzle face  18 . 
   The support plate  12  is rigidly held by the carriage and is so adjusted that it is exactly parallel to the surface of the recording medium. The printhead  10  has to be positioned on the support plate  12  with high accuracy, so that its position in all three directions in space is well defined and known, the nozzle face  18  is also exactly parallel with the surface of the recording medium, and the lengthwise direction of the printhead (from left to right in  FIG. 1 ) forms exactly a right angle with the direction of movement of the carriage. This is achieved by holding and positioning the printhead  10  on the support plate  12  at three support points  20 ,  22  and  24 . 
   The two support points  20  and  22  rigidly support the printhead  10  on the bottom of the recess  14  and are located at opposite ends of the printhead. Each of the support points comprises a spherical surface that is formed by a metal ball  26  which is adapted to engage recesses  28 ,  30  of the support plate  12 . The balls  26  are partially embedded in the body of the printhead  10  which may, for example, be made of carbon, and the extent to which the balls project from the printhead is adjusted with high accuracy. The recess  28  has the shape of an upwardly open cone that has been precisely machined in the support plate  14 . Thus, when the ball  26  is slightly pressed into the recess  28 , it will automatically be centered on the center of the recess and will then be fixed in all three directions in space. 
   In contrast, the recess  30  of the support point  22  is a V-shaped groove which extends in longitudinal direction of the printhead. The cross-sections of the recesses  28  and  30 , in the plane normal to the lengthwise direction of the printhead, are visible in  FIGS. 2 and 3 , respectively. 
   Thus, the support point  22  prevents the printhead  10  from rotating about a vertical axis passing through the support point  20 , and also from rotating about an axis passing through the support point  20  and being normal to the plane of the drawing in  FIG. 1 . The support point  22  permits, however, a slight thermal expansion or contraction of the printhead in its lengthwise direction. 
   The third support point  24  is located on the top side of the printhead  10  approximately above the support point  20  and is formed by a ball  32  engaging in a recess  34 . The bead or ball  32  is fixed on a wire  36  which is shown in cross-section in  FIG. 1 . The recess  34  is a V-shaped groove which extends in longitudinal direction of the printhead and passes over a roof-shaped structure on the top side of the printhead. Thus, the support point  24  prevents the printhead  10  from tilting about an axis that is defined by the two rigid support points  20  and  22 . 
   As is further shown in  FIG. 1 , the top portion of the printhead  10  forms a ledge  38  at the end opposite to that of the recess  34 , and a wire  40 , shown in cross-section, rests on this ledge. As will be described below, the wires  36  and  40  are spring-biased downwardly, so that the bead  32  is forced into the recess  34  and is centered therein, and the balls  26  are forced into the recesses  28  and  30 . The wire  40  does in no way define the position of the printhead and only serves to supplement the biasing force of the wire  36  and to prevent the ball of the support point  22  from upwardly escaping from the recess  30 . 
     FIG. 4  is a perspective view of the printhead  10  and shows a linear array of nozzles  42  in the nozzle face  18 . 
   In  FIG. 5 , two printheads  10  are shown in their position on the support plate  12 , which has recesses  14  for six further printheads. The support plate  12  has lugs  44  for attachment at the carriage. 
   As is shown in  FIG. 6 , the wire  36  extends over all the mounting positions for the printheads  10  and carries a bead  32  for each printhead. The spacings between the balls  32  correspond exactly to the spacing between the printheads  10 . A number of tension springs  46  are anchored at the wire  36  in the intervals between the balls  32  and bias the wire  36  towards the support plate  12 . Thanks to the distribution of the tension springs  46 , the biasing force is evenly distributed over the length of the wire  36 . The tension springs are anchored at the support plate  12  at positions laterally offset from the vertical projection of the wire  36 , so that the springs are slightly inclined and urge the wire  36  and the beads  32  against the slopes of the roof structures of the printheads  10 . These roof structures prevent the wire from slipping off from the printheads. The wire  40  at the other end of the printheads forms a number of regularly spaced loops in which tension springs  48  are anchored which bias the wire  40  towards the support plate  12 . 
   When the printheads  10  are to be removed from the support plate, the wires  36  and  40  are lifted against the forces of the tension springs and are moved aside. A useful mechanism for this will now be explained in conjunction with  FIGS. 7 and 8 . 
   As is shown in  FIG. 7 , the whole assembly of printheads  10  is accommodated in a case-like frame  50  which is mounted on the support plate  12  and has end walls  52  supporting the opposite ends of the wires  36  and  40 . In this way, the position of the wires in their longitudinal direction is precisely defined. Outwardly projecting top portions of the end walls  52  serve for rotatably supporting two shafts  54 ,  56  which each carry a set of hooks  58 ,  60 . In the positions shown in continuous lines in  FIG. 7 , the hooks  58 ,  60 , which are located in the gaps between the adjacent printheads  10 , are positioned slightly below the wires  36 ,  40  without contacting them. Thus, the hooks are inoperative in this position. By rotating the shafts  54 ,  56 , the hooks  58 ,  60  may be pivoted into the positions shown in phantom lines, thereby lifting the wires  36 ,  40  and moving them outwardly, so that the printheads  10  can be withdrawn out of the recesses of the support plate  12 . 
   In  FIG. 8 , the inoperative position of the hooks  58 ,  60  is shown in phantom lines, and the continuous lines show the lifted position of the hooks and the wires. The opposite ends of the shaft  56  each carry a lever  62  having a handle  64  which may be gripped for pivoting the lever and hence the hooks  60 . The levers  62  are arranged adjacent to and in parallel with the end walls  52  of the frame  50 . Similarly, the shaft  54  carries two arcuate levers  66  for pivoting the hooks  58 . The levers  66  have an arcuate slot  68  with an upwardly angled end portion  70 . A pin  72  projects from the lever  62  and into the slot  68 . When the hooks  58 ,  60  are in the inoperative position, the levers  62  and  66  have the position shown in phantom lines in  FIG. 8 . The pin  72  is then located at the left end of the arcuate slot  68 . 
   When the user wants to detach one or more printheads, he grips the handle  64  and tilts the lever  62  into the position shown by the continuous lines in  FIG. 8 . The pin  72  pushes the lever  66  upwardly, thereby tilting the hooks  58 , and, finally, the pin  72  will be positioned in the angled portion  70  of the slot, so that the assembly is arrested in an open position permitting the removal of the printheads. The handle  64  is a bow which connects the two levers  62  and is so arranged that it permits easy access to the printheads. It will be understood that the tension springs  46  and  48  ( FIG. 6 ) are expanded when the hooks are lifted, and hold the wires  36  and  40  in engagement in the hooks. 
   When the printheads have been exchanged and are to be positioned and secured again, the levers  66  are manually drawn upwardly so that the pin  72  will enter into the arcuate slot  68 . For this purpose, the levers  66  are provided with outwardly projecting tabs  74 . The pin  72  being at the right end of the arcuate portion of the slot  68 , the levers  62  may be tilted back into the position shown in phantom lines, and the levers  66  will be gently returned into their lowered position while the pin  72  moves to the right end of the slot  68 . By the end of this movement, the wires  36 ,  40  will again be placed on top of the printheads and on the ledges  38 , respectively, and the biasing forces of the springs will act upon the balls  32  and on the printheads, so that the balls  32 ,  26  are centered in their positions in the recesses  34 ,  28  and  30 . The design of the levers  62 ,  66  and the dimensions of the springs  46 ,  48  are such that, at first, the springs  46  will bias the wire  36 , so that the printheads will be positioned by the action of the three support points without being subject to any substantial frictional forces. Only then will the wire  40  engage the ledge  38  to secure the printheads in their position. 
   When the lever  62  reaches its lower end position, a fixture  76  provided at the free end of this lever will come to rest upon a co-operating fixture  78  on the lever  66 , and a stop (not shown) provided on the end walls  52  will terminate the pivotal movement of the levers. With the fixtures  76 ,  78 , the levers may then be locked to each other so that the whole assembly will be stably held in a position where the printheads are ready to operate. 
   As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its spirit and scope as defined in the appended claims, and therefore all changes and modifications that fall within the metes and bounds of the claims, or equivalence of such metes and bounds are therefore intended to be embraced by the appended claims.