Abstract:
There is disclosed an ink jet printer for printing on both sides of a label web. The printer has selectively movable guides which enable the label web to be easily threaded through the printer and which enable the label web to be brought into an operating position for printing and advancing the label web. The printer has individually and selectively removably mounted ink jet print modules to facilitate changing of ink colors, replacing or servicing a malfunctioning print module, and so forth. The ink jet print heads of the modules can be individually selectively moved between printing and stored positions. The unwinding of the label web from the supply roll is facilitated by an unwind mechanism to prevent stretching of the label web. The system for delivering ink and recovering waste ink includes needles for penetrating a cartridge. A guard shields the needles when the module(s) are out of the printer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation-in-part of patent application Ser. No. 08/932,483, now U.S. Pat. No. 6,142,622 filed Sep. 18, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to the printing art. 
     2. Brief Description of the Prior Art 
     The following prior art is made of record: PAXAR 7500 Operation/Maintenance and Parts List January 1995 and PAXAR 8500 Operation/Maintenance and Parts List November 1996. 
     SUMMARY OF THE INVENTION 
     The invention relates to an improved printer for printing on webs of labels. 
     It is a feature of the invention to provide an improved arrangement to facilitate threading a label web through a printer. The label web is loosely threaded along a path with guides in a loading position and thereafter the guides are repositioned to bring the label web to its operating position. In accordance with a specific embodiment of the invention, the label web passes from a roll and partially about first and second guides. Thereafter, the first and second guides are repositioned to bring the label web to an operating position. There is also a third guide and a back-up roll which are selectively operable between loading positions and operative positions. In the loading position the label web is passed partially around the third guide and a driven feed roll and in the operating position the label web passes partially around the guide and is in the nip between the feed roll and the back-up roll and is wrapped partially about the feed roll. 
     It is another feature of the invention to provide an improved ink jet printer which can print selectively either on one side or face of the label web or on both sides or faces of the label web in a single pass through the printer. According to the one specific embodiment, the label web can be threaded through the printer along either a first path for printing on one side of the label web or along a second path for printing on both sides of the web. The printer preferably has two print heads which are selectively movable between printing and non-printing positions with respect to the label web. 
     It is another feature of the invention to provide an ink jet printer for printing on both sides of a label web wherein one or more print modules are selectively removable from the printer. Each print module preferably includes all the structure for printing on the label web in response to data received. It is a simple matter to replace a print module having ink of one color with a like print module having ink of a different color. The removability of a print module also facilitates replacement of a malfunctioning print module, or removal of an unneeded print module while the remaining module is operational, or the servicing of the printer or its print modules. 
     It is also a feature of the invention to provide an ink jet printer for printing on both sides of a label web wherein one or more ink jet print heads are individually selectively positionable either in printing positions or in non-printing positions. A non-printing position is useful for example when threading the label web through the printer to prevent ink from the ink jet print head or heads from contacting the label web. The print heads can be moved to a non-operating or stored position whenever a the print head is not used. A wiper can wipe the print head clean of ink as the print head is being moved to its stored position and in the stored position a seal which seals off the orifices of the print head. 
     It is another feature of the invention to provide an improved printer having an ink jet print head connected to a source of ink by a pointed needle, wherein there is a guard for guarding off the pointed end of the needle. More particularly, the print head and the needle are part of a print head module which is removable from the printer, and the guard is effective until the print head module has been inserted into the printer. Upon insertion of the module into the printer the guard is deactivated. 
     It is another feature of the invention to provide an improved printer for printing on both sides of a label web, wherein an unwind mechanism helps to unwind a label web from a supply roll, to avoid excessive stretching of the label web wherein the label web can be threaded through the printer along one path in the event the label web is to be printed on both sides and along another path in the event the label web is to be printed on only one side, and wherein the label web is drawn through the printer by an advancing mechanism over either one of the paths. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary front elevational view of the printer of the invention in its label web loading position; 
     FIG. 2 is a view similar to FIG. 1, but showing the printer operating in a two-sided one mode in its printing or operating position; 
     FIG. 3 is a side elevational view showing a portion of the printer, but operating in a one-sided mode; 
     FIG. 4 is a view similar to FIG. 3, but showing one of the print modules partially in phantom lines and partially in solid lines; 
     FIG. 5 is a fragmentary front elevational view of the printer with additional structural details of the print modules; 
     FIG. 6 is a perspective view of one of the print modules; 
     FIG. 7 is an exploded perspective view of supply-side guide structure for the label web; 
     FIG. 8 is an exploded perspective view of feed guide structure for the label web; 
     FIG. 9 is a front view of one of the print modules; 
     FIG. 10 is a perspective exploded view of a print head and support structure; 
     FIG. 11 is an assembled perspective view of the print head and support structure; 
     FIG. 12 is a diagrammatic view of the ink delivery system; 
     FIG. 13 is a side elevational view of the ink jet printer showing an alternative arrangement for threading a web wherein both sides of the web can be printed; 
     FIG. 14 is a fragmentary side elevational view showing an alternative arrangement for threading a web wherein only the top side of the web is to be printed; 
     FIG. 15 is a perspective view showing an arrangement for unwinding the web from a supply roll and for maintaining tension in the web; 
     FIG. 16 is a side elevational view of an alternative embodiment of one print head module for the printer; 
     FIG. 17 is a perspective view of the print head module shown in FIG. 16; 
     FIG. 18 is a fragmentary perspective view showing the module frame, the needles, the guard for the needles, and the interposer in its interposing or blocking position; 
     FIG. 19 is a fragmentary elevational view of the fragment of the module shown in FIG. 18; and 
     FIG. 20 is a fragmentary perspective view similar to FIG. 18, but showing the interposer in its ineffective position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1, there is shown an ink jet printer generally indicated at  20  for printing on a web W of labels. The web W can be comprised of fabric such as is used in the garment industry, or the web can be comprised of paper, plastics or other printable materials. A supply roll R of the web W is shown to be rotatably supported on a pair of parallel rollers  21  and  22 . There is a sensor  22 ′ for reading a bar code on the core C of the roll R. The sensor  22 ′ is part of a system for controlling a hysteresis brake  23   a  which is coupled to the roll or spindle  23  by meshing gears  23   b  and  23   c . The web W is paid out of the roll R as it passes downwardly and partially about a direction-changing guide in the form of a rotatable roller  23 , and from there the web W passes upwardly and over and partially around a guide  24  preferably in the form of a crowned roller which is rotatably supported on a frame plate  26 . The web W passes from the guide  24  over and in contact with the guide  25  and over and partially about a guide  27  preferably in the form of a roller. From there the web W passes downwardly and partially around and under a guide  28  preferably in the form of a roller. From there the web W passes in front of the right-hand print head generally indicated at  29  and above guides  39  and  40 . In this position the print head  29  as well as a print head or print head pair  30  are in their respective stored position. The web W passes under a guide in the form of a roller  31  and over a feed roll  32 . From there the web W passes between an auxiliary feed roll  33  and a back-up roll  34 . From there the web W passes to a knife  35  having a rotary blade  35 ′ and a fixed blade  35 ″ by which the web W can be severed into labels. The foregoing describes the manner in which the web W is loaded or threaded into the printer  20 . The guides  27 ,  28  and  31  and a back-up roll  36  are shown in FIG. 1 to be in the loading position. It is seen that the path along which the web is loaded or threaded in essentially unobstructed in the loading position. 
     The print heads  29  and  30  are part of respective identical print modules  37  and  38 . The guides  39  and  40 , in the form of rollers, support the web W at the proper height with respect to the print heads  29  and  30  (FIG.  2 ). FIG. 2 shows the web W guided along a path in the printing position according to the mode in which the web W was loaded in the position shown in FIG.  1 . However, it is to be noted that the guides  27  and  28  have moved to the operating or printing position. It is seen that in FIG. 2, the web W passes partially around the guide  25 , and downwardly and partially around and under the guide  27  and over and partially around the guide  28 . As the web W passes between the guide  27  and  28  the underside of the web W is printed by the print head  30 . From there the web W passes over and in contact with the guides  39  and  40 . The print head  29  can now print on the upper or top side of the web W. After the web W passes over and partially around the roll or guide  40 , the web W is trained partially about the roll  31  and is wrapped partially about the feed roll  32 . The feed roll  32  has a frictional outer surface formed of a metallic grit surface. The feed roll  32  which cooperates with the backup roll  36 , has contact with the web W over a substantial portion of the periphery of the roll  32 . It is seen that in this mode both sides of the web W can be printed upon. 
     FIG. 3 illustrates how the web is threaded in a different mode in which only one side of the web W is to be printed. The web W is paid out from the roll R and passes partly around the guide  28  and over the guides  39  and  40 . It is seen that the guide  27  is slightly elevated from the position shown in FIG. 2 so that the upper side of the web W does not touch the guide  27 . This obviates the possibility that ink which has just been printed on the web W by the print head  30  might be smeared by the guide  27 . As the web W passes to the right (FIG. 3) the print head  29  can print on the upper side of the web  29 . 
     With reference to FIG. 4 the location of the print modules  37  and  38  are shown in greater detail. Except for the print heads  29  and  30  the components of the respective print modules  37  and  38  are designated by the same reference characters. The print modules  37  and  38  are identical except for the color of the ink which they respectively contain. Each print module  37  and  38  has a housing  41  with a handle  42  (FIG. 5) at its upper end. An ink cartridge  43  plugs into a pair of needles  43 ′ in the housing  41 . The print modules  37  and  38  include the print heads  29  and  30 , which are mounted for pivotal movement between printing and stored positions. 
     FIG. 5 shows one of the print modules in greater detail than in FIG.  4 . FIG. 6 shows one of the print modules, namely the module  37  with its print head  29  in the printing position. As shown, the housing  41  has parallel V-shaped vertically extending grooves  45  and  46  in opposite sides  47  and  48 . A rigid elongate jaw  49  extending the length of the groove  45  is received in the groove  45 . A spring-biased jaw  50  extending the length of the groove  46  is received in the groove  46 . 
     With reference to FIG. 7, there is shown a vertically extending frame plate  51  and plates  52  and  53 . The plates  51 ,  52  and  53  are parallel to each other. The plate  52  is rigidly secured to the plate  51  by standoffs or spacers  54  and  55  and to the plate  51  and a plate  56  by a standoff or spacer  57 . The plate  53  is rigidly secured to the plate  52  by standoffs or spacers  58 ,  59  and  60 . The guide  25  is rotatably mounted to both plates  52  and  53 . 
     A bearing  61  is received in a hole  62  in the plate  52 . A shaft  63  is rotatably received in the bearing  61 . A plate  64  with an axial collar  65  is secured to an L-shaped actuating member generally indicated at  66 . The collar  65  is received in a hole  67  in the member  66 . The shaft  63  passes through the plate  64  and its collar  65 , and receives a needle bearing  68 , C-rings  69 , and a cone screw  70 . A nut  71  bears against the front of the plate  53  (FIG. 7) and is threadably received by the cone screw  70 . 
     The plate  64  is secured to the plate  66  by three screws  64 ′, only one of which is shown. As shown, the guide  24  is crowned to help keep the web W centered. A cam follower stud  72  is secured to a vertical leg  66 ′ of the member  66  and cooperates with an adjustable stop generally indicated at  73  in FIGS. 2 AND 3. The guides  27  and  28  are rotatably secured to a horizontally extending leg  66 ″ of the member  66 . A handle  74  is secured to a tang or handle member  75  of the member  66 . 
     The stop  73  is movable between two positions as indicated respectively in FIG. 2 (which illustrates the two-side printing mode) and in FIG. 3 (which illustrate the one-side printing mode). The stop  73  includes a stop plate  76 , a handle  77  secured to the stop plate  76  and a cantilevered leaf spring  78  (shown in FIG. 2 only). FIG. 2 shows the stop plate  76  and the handle  77  oriented in the position corresponding to the two-side mode in which the follower  72  is received in a recess  79  close to axis  80 , and the handle  77  is shown to extend in an upward direction at an angle with respect to the vertical. Thus, the member  66  is in its most clockwise position as seen in FIG.  2 . This establishes a horizontal pass W′ of the web W immediately below the print head  30 . The spring  78  releasably holds the stop plate  74  in the FIG. 2 position when its flat lower surface bears against flat surface  74 ′ on the stop plate  74  and in the FIG. 3 position when its flat lower surface bears against flat surface  74 ″ on the stop plate  74 . 
     When the handle  77  is moved to the generally horizontal position shown in FIG. 3, the follower  72  is in a recess  81  further from the axis  80  than the recess  79 . Thus, the member  66  is slightly counterclockwise from the position shown in FIG.  2 . Accordingly, the guide  27  is in a position spaced above horizontal pass W″ of the web between guides  28  and  39 . This obviates the possibility of the guide  27  contacting the freshly printed pass W″ of the web W which could cause smearing of ink that had not dried sufficiently. 
     The stop  73  is also shown to include a fixed stop pin  82  which limits the travel of the stop plate  76 . The stop pin  82  cooperates with either a stop face  83  (FIG. 3) or with a stop face  84  (FIG.  2 ). The spring  78  releasably holds the stop plate  76  in either of these positions. 
     An overcenter mechanism generally indicated at  85  releasably holds the member  66  either in the position shown in FIG. 1 or in the position shown in FIG. 2 in response to movement of the handle  74  to either of these positions. The overcenter mechanism  85  includes a generally U-shaped link  86  pivoted at one end to a pivot  87  on an arm  88  of the member  66 . The other end of the link  86  is connected at  89 ′ to a tension spring  89  to a tension spring  89  which is connected to a post  90  on the plate  51 . In the FIG. 1 position, the spring  89  exerts a force on the pivot  87  along a line  92  which is above the axis of the shaft  63 . Thus, the member  66  is biased counterclockwise against a stop  66   s . When the handle  74  is moved to the FIG. 2 position, the spring  89  exerts a force along a line  93  which is below the axis of the shaft  63 , and thus the member  66  is biased clockwise as limited by the stop  73 . 
     With reference to FIG. 8, there is shown a bracket  94  secured to the plate  51 . Bearings  95  located in a bore  96  in the bracket  94  rotatably mount a shaft  97 . A plate  98  secured to the shaft  97  by a pin  99  is secured to an actuating member generally indicated at  100 . The member  100  has an elongate portion  101  with a tab  102  and a tang  102  extending in one direction and an arm  103  extending in the opposite direction. A handle  104  is secured to the tang  102 . The arm  103  mounts the guide  31  which is shown to be crowned to help keep the web W centered in its intended path. The arm  103  mounts a shaft  105  having a threaded end portion  106  which receives a nut  107 . The shaft  105  receives bearings  108  and  109 , washers  110 ,  111  and  112  and a C-ring  113 . 
     A pivot  114  is secured to the tab  101  by a screw  115 . An H-shaped mounting member generally indicated at  116  mounts bearings  117  and  117 ′ and  118  and  118 ′. Retaining rings  119  and  120  fixedly mount bearings  117  and  118  to the member  116 . The pivot  114  passes through the bearings and an E-ring retains the mounting member  116  in place. A pin  121  received by the member  116  rotatably mounts the back-up roll  36 . A horizontal portion  116 ′ of the H-shaped member  116  is relatively narrow and can twist to conform to the feed roll  32 . The portion  116 ′ is relatively flexible in the torsional direction and functions like a torsional spring. 
     The member  100  also mounts a post  122 . A tension spring  123  is connected at one end to the post  122  and at its other end to the member  116  to bias the member  116  clockwise (FIGS.  1  and  2 ). There is a stop  102 ′ for the member  100 . There is also a stop  116 ′ for the member  116 . 
     A pivot pin  124  secured to the elongate portion  101  pivotally mounts a generally U-shaped link  125 . A tension spring  126  is connected to the link  125  at  126 ′ and to a post  127  secured to the frame plate  51 . In the position shown in FIG. 1, the spring  126  exerts a force along line  128  below the axis of the shaft  93  to urge the actuator member  100  clockwise, thereby urging the guide  31  and the back-up roll in the loading or threading position. When the handle  104  is moved to the position shown in FIG. 2, the spring  126  exerts a biasing force along a line  129  which is above the axis of the shaft  97  to urge the member  100  into the position shown. It is apparent that as the member  100  is brought into the FIG. 2 position, the spring  126  stretches and the back-up roll  136  for the driven feed roll  32  urges the web W against the feed roll  32 . Because the guide  31  is now in the operating position, the web W is trained about a path which in turn causes the web W to be wrapped about a substantial portion of the periphery of the feed roll  32 . This contributes to excellent gripping of the web W by the feed roll  32 . The feed roll  32  is driven by an electric motor (not shown) disposed behind the plate  51  as seen in FIG.  8 . 
     With reference to FIGS. 6 and 9, there is shown one of the print modules, e.g. the module  37 . The module  37  includes the print head  29  (not shown in FIG. 9) which is mounted on a print head support  140 . The support  140  is a plate having a contoured surface also shown in FIGS. 10 and 11. Print head units  141  and  142  having nozzles (not shown) at their lower surfaces  143  and  144 . The support  140  is shown to be movably mounted, and preferably pivotally mounted on a shaft  145 . The shaft  145  is supported by spaced fixed members  146  and  147 . A bracket  148  is secured to arm  149  of the support  140 . A cap screw  150  is secured to the member  146 . A tension spring  151  is connected to the bracket  148  and to the cap screw  150 . In the printing position shown, the spring  151  is behind axis  152  of the shaft  145  and urges the support  140  overcenter in a clockwise direction (FIG. 6) against an adjustable stop screw  153  (FIGS. 1 and 9) in a fixed plate  153 ′. The adjusted position of the stop screw  153  precisely determines the printing position. When the support  190  is overcenter in the other direction, as when the support  140  is at or near the home position, the position of the support is determined by an adjustable stop screw  154 . 
     A bevel gear  155  secured to the vertical shaft  145  meshes with a bevel gear  156  secured to a horizontal shaft  157 . Also secured to the shaft  157  is a section  158  of a Geneva wheel of a Geneva mechanism  159 . The Geneva section  158  has a slot  160  for receiving a pin  161  on a plate  162 . The plate  162 , a gear  163  and a gear  164  are secured to a shaft  165 . The gear  163  is behind the gear  164  and the plate  162  is behind the gear  163 . The teeth on the gear  163  extend for 360 degrees, however, the gear  164  is considered to be a mutilated gear in that its teeth extend for only a portion of a circle as best shown in FIG.  5 . The gear  163  is not shown in FIG. 5 for clarity. 
     A stepping motor  166  secured to the plate  51  drives a gear  167 . A driven gear  168  meshes with the gear  167  (which is not a part of the module  37 ) when the module  37  is in place as shown in FIG.  9 . As the module  37  is moved into its operating or printing portion of FIG. 7, the gear  168  goes into mesh with the drive gear  67  and when the module  37  is moved away from its operating position the gear  168  moves out of mesh with the drive gear  167 . The gear  168  is fixed secured to a pinion  169  which in turn meshes with a gear  170 . The gear  170  meshes with the gear  163 . 
     A gear  171  can mesh with the mutilated gear  164  when the mutilated gear  164  is in the right position. The gear  171  meshes with a gear  172  which drives a one-way clutch  173 . The one-way clutch  173  drives a peristaltic pump generally indicated at  174 . The pump  174  has a rotor  175  which mounts a plurality, e.g. six, rollers  176 . As the rotor  175  rotates (clockwise in FIG.  9 ), the rollers  176  compress a flexible tube  177 . The tube  177  is part of the ink delivering system  190  shown diagrammatically in FIG.  12 . The tube  177  is locally compressed by the rollers  176  against an arcuate surface  178  of an anvil or block  179 . The block  179  is held in the operating position shown in FIG. 9 by a flat  180  on a cam  181 . The cam  181  has an eccentric axis  182 . A crank or handle  183  is secured to the cam  181  which enables the cam to be rotated to lower the block  179  for servicing the tube  177  or after components. 
     The motor  166  can be controlled to rotate the gear  167  in either direction. By rotating the gear  167  in one direction, the plate  162  is rotated counterclockwise (FIG. 9) to cause the pin  161  to enter the slot  160  and move the print head from the stored or non-operating position (FIG. 9) to the printing position (FIG.  6 ). There is enough clearance between the Geneva section  158  and the wheel  162  to provide a certain amount of play or lost motion in the shaft  145  to enable the stop screws  153  and  154  to determine the respective printing and stored positions of the print head  29 . As the pin  161  enters the slot  160  as shown in FIG. 9 upon counterclockwise rotation of the plate  162 , the Geneva plate  158  is rotated clockwise by 90 degrees. To return the print head  29  from its printing position to its stored position, the gear  167  is rotated in the opposite direction to bring the Geneva section from the FIG. 5 position to the FIG. 9 position. 
     To operate the pump  164 , the gear must be rotated to bring the mutilated gear  164  into mesh with the gear  171 . Upon rotation of the gear  164  in one direction the rotor  175  rotates clockwise (FIG. 9) to draw ink from the cartridge  43  and deliver it to the print heads  29  and  30 . When the gear  164  rotates in the opposite direction, the one-way clutch  173  does not transmit any motion to the rotor  175 . To advance the rotor  175  again, the gear  167  must be driven in the direction that causes the rotor  175  to be driven clockwise (FIG.  9 ). The motor  167  is a stepping motor that can be driven in precise increments in either direction alternately. Because the gear  164  is incomplete or mutilated, the pump  174  is never operated when the print head  29  is in the printing position (FIG.  6 ). In fact, the pump  174  is only operated when the print head  29  is in the stored position (FIG.  9 ). 
     FIG. 12 shows the entire ink delivery system generally indicated at  190 . The entire system  190  is part of the removable print module  29 . The system  190  includes tubing  191  of which the tube  177  forms a part. The cartridge  43  includes a supply container  192  and waste container  193 . A check valve  194  prevents ink in the tubing  191  from flowing back into the supply container  194 . Ink is pumped to both print heads  29  and  30 . Collectors  195  and  196  collect excess ink from the print heads  139  and  130  in their stored or non-operating positions. The pump  174  draws ink from the supply container  192  to the print heads  29  and  30  and removes excess or waste-ink from the collectors  195  and  196 . The excess ink is passed to the waste container  193  by the pump  174 . 
     With reference to FIGS. 10 and 11, the support  140  is shown to have a pair of holes  200  and  201  for mounting locating pins  202  and  203 . Similarly, holes  204  and  205  mount locating pins  206  and  207 . The locating pins  202 ,  203  and  206  and  207  locate respective print heads  142  and  141  vertically. The print heads  141  and  142  are mounted on respective plates,  208  and  209  which bear against the locating pins  206  and  207 , and  202  and  203 . A pair of cone screws or cams  210  and  211  enable two-direction lateral or horizontal adjustment of the print head unit  141 . The cone screws  210  and  211  are threadably received in holes  212  and  213 . Cam surfaces  210 ′ and  211  ′ act against respective edges  214  and  215  of the plate  208 . By threading the cone screw  218  further into the hole  212 , the plate  208  and the print head unit  141  which it mounts are moved to the right (FIG.  10 ); this assumes that the cone screw  211  is threaded out far enough to allow the plate  208  to move to the right (FIG.  10 ). To move the plate  208  to the left (FIG.  10 ), the cone screw  210  is loosened and the cone screw  211  is threaded into the hole  213 . In this way the horizontal or lateral position of the print head unit  141  can be precisely positioned relative to the print head unit  142 . The use of the two print head units  141  and  142  permits double wide printing to occur in the label web W, because one print head unit prints where the other cone leaves off. The same result could be achieved by using a print head unit that is twice as wide laterally as one of the print head units  141  or  142 . 
     The print head  142  unit is not laterally adjustable. However, it is clamped in place by screws  216  received in holes  217  and by screws  218  received in holes  219  cooperating with respective clamps  220  and  221 . The clamps  220  and  221  bear against surfaces  222  and  223 . Likewise screws  224  and  225  received in holes  226  and  227  urge clamps  228  and  229  against surfaces  230  and  231 . Connectors  232  and  233  for the print head units  141  and  142  are connected to the printed circuit board  44  (FIG. 5) via ribbon connectors  234  and  235  (FIG.  6 ). 
     Although details of the print module  29  have been shown and described, the print module  30  is identical thereto. 
     The print head units  141  and  142  are identical. By way of example, not limitation, a print head unit useful in carrying out the invention is Nu-Kote model  128 B with  128  nozzles. 
     With reference to the embodiment of FIGS. 13 through 20, the same reference characters are used as in the embodiment of FIGS. 1 through 12 for components having the same construction, function and relative location. The roll R comprised of the label web W is suitably rotatably supported. The web W passes from the roll R to an unwind mechanism U. The unwind mechanism U helps to prevent the web W from stretching excessively as the web is advanced by cooperating rolls  259  and  260 . Absent the unwind mechanism U, the inertia of the roll R can cause the web W to stretch excessively which can degrade print quality. The web W first passes partially around a freely rotatable roll  250  and from there more than 180° about a feed roll  251 . The web W passes between the nip of the feed roll  251  and a freely rotatable back-up roll  252 . From there the web W is shown to loop as indicated at  253  and then passes between a stationary curved surface  254  and a curved surface  255  on an arm  256  which is pivotally mounted on a pivot  257 . From there the web W passes upwardly and partially around a guide  24  and then partially around a guide  25 . From there the web W passes downwardly and partially around a guide  27  and thereafter partially around the guide  28 . From there the web W passes over and in contact with a guide  39  and then passes under and in contact with a roll  258  and into the nip of feed roll  259  and back-up roll  260 . From there the web can be cut into labels, rewound, or further treated. As the web W passes between the guides  27  and  28 , the underside of the web W can be printed by the print head  30 . The print head  29  located downstream of the roll  39  can print on the top side of the web. 
     FIG. 14 shows an alternative path for the web W from the path shown in FIG.  13 . After the web W passes around the curved surface  254  the web W passes under and partially around the roll  28  and over and partially around the roll  39 . In other respects the web path is the same as illustrated in FIG.  13 . Each of the guides  24 ,  25 ,  27 ,  28  and  29  preferably comprises a roll rotatable about a fixed axis. In particular, the rolls  24  and  25  can be crown rolls. It is to be noted that even though the web W passes under the guide  27 , the guide  28  prevents the web W from contacting the guide  27 . Except for rotation of the rolls which comprise the guides  24 ,  25 ,  27 ,  28  and  39  being rotatable about fixed axes, these guides are stationary. 
     With reference to FIG. 15, there is shown a stepping motor  261  for driving a toothed pulley wheel  262 . A toothed belt  263  engaged with the pulley wheel  262  also engages a toothed pulley wheel  264 . The pulley wheel  264  is secured to a drive shaft  265  which mounts the drive roll  251 . The roll  250  has equally spaced teeth  266  sensed by a sensor  267  when the roll  250  is rotated by the advancing web W. The arm  256  pivots clockwise (FIGS. 13 and 15) in response to tautness in the web W. The condition wherein the web W is taut is sensed by a sensor (not shown) responsive to the position of the arm  256  which in turn increases the speed of the motor  261  and the rotational speed of the roll  251 . The purpose of the sensor  267  is to detect the rotation of the roll  250 . The absence of rotation of the roll  250  indicates that the web W is no longer available from the roll R or that the roll R is no longer free to rotate. With reference to FIGS. 16 and 17, there is shown a print head module generally indicated at  268 . There are two identical such modules  268  which are functionally and structurally similar to identical print head modules  37  and  38 . Two such modules  268  are used in the illustrated printer. A peristaltic pump  269  (similar to pump  174 ) pumps ink from a source of supply (such as the supply container  192  of the cartridge  43 ) through tubing (such as the tubing  191 ) to the print head  29 . Waste ink is returned through the tubing  191  to a waste container (such as the waste container  193 ). 
     The module  268  has a frame  271  to which forwardly projecting needles  272  and  273  are rigidly mounted. The needles  272  and  273  (like the needles  43 ′) have pointed ends. The pointed needles  272  and  273  communicate respectively with the ink in the ink container  192  and with the waste container  193  by piercing respective diaphragms. With reference to FIG. 16, the needle  273  is connected through a fitting  275  which in turn is connected to the ink container via tubing  191 . A fitting  274  connected to the needle  272  is connected via the tubing  269 ′ to the pump  269 . 
     A stepping motor (not shown in FIG. 16 but corresponding to the motor  166 ) drives the pump  269  and a Geneva mechanism  276  through gearing  277 . A bevel gear  278  driven by the Geneva mechanism  276  drives a bevel gear  279  to swing the print head between its operating position as shown in FIGS. 16 and 17 and its stored position, as in the embodiment of FIGS. 1 through 12. 
     With reference to FIGS. 18 through 20, there is shown the frame  271  and a plate  280  connected by stand-offs  281 , only one of which is shown. A needle block or holder  282  which mounts the needles  272  and  273  is secured to the plate  280 . A pair of posts  283  project from the needle block  282 . A guard  284  includes a needle guarding portion  285  and a pair of arms  286 . The arms  286  have through holes  287  for receiving the posts  283 . The guard  284  is slidable on the posts  283 . A compression spring  288  nested in a pocket  289  in the needle block  282  urges the guard  284  away from the needle block  282 . The needle guarding portion  285  has a pair of respective round through holes  290  and  291  aligned with the needles  272  and  273 . In the position shown in FIG. 18 the pointed ends of the needles  272  and  273  are completely within the holes  290  and  291 . Accordingly, the guard  284  is received about the ends of the needles  272  and  273 . Thus, when the module  268  is out of the printer, the pointed needles  272  and  273  cannot be damaged and the user cannot contact the printed ends of the needles  272  and  273 . The guard  285  is incapable of separating from the pins  283  except by removal of screws  292  the heads of which are larger than the holes  287  and provide stops. An interposer generally indicated at  293  is shown to comprise an arm  294  pivotally mounted on a pin  295  and an arm or lever  296  pivotally mounted on a pivot  297 . The arm  294  has an elongate slot  298  for receiving a pin  299  carried by arm  296 . The lever  296  is pivotally mounted on a pivot  300 . The pivot  300  mounted on the frame  271  is located between the pin  299  and a pin  301  on the lever  296 . In the guarding position shown in FIGS. 18 and 19, the interposer  293  is shown in its effective or interposing position, whereas in FIG. 20 the interposer  293  is shown in its ineffective position. With the interposer  293  in the effective or guarding position, interposer element  302  is positioned between the guard  285  and the needle block  282 . Any attempt to move the guard sufficiently to cause the pointed ends of the needles  272  and  273  to project through the holes  290  and  291  is prevented when the needle guarding portion  284  abuts the interposer element  302 . When the needle guarding portion  284  contacts and is prevented from further movement relative to the needles  272  and  273  the pointed ends of the needles  272  and  273  are still completely within the holes  290  and  291 . 
     When the module  268  is slid vertically into its operating position in the printer, a position like that shown in FIGS. 5,  6  and  17 , the pin  301  contacts a vertically extending plate or projection  302 ′ on the printer frame, which causes the lever  296  to pivot clockwise (FIG.  20 ), which in turn causes the link  294  to pivot counterclockwise, thereby enabling the guard  285  to be shifted against the action of the spring  288  to expose the pointed ends of the needles  272  and  273  as the needles  272  and  273  penetrate the respective ink and waste containers  192  and  193 . When the cartridge, e.g. the cartridge  38  is withdrawn, the spring  288  is again effective to return the guard to its position as shown in FIG.  18 . In this position the pointed ends of the needles  272  and  273  are guarded, although depression of the guarding portion  285  and compression of the spring  288 , such as by reinsertion of a cartridge  43 , will enable the needles  272  and  273  to again penetrate the cartridge  43 .