Abstract:
A screen printing machine has article supports for articles to be printed, a displacement mechanism for displacing the article supports in succession around an endless path of travel and printing stations distributed along the path of travel and each having a printing head. The displacement mechanism has drive members engageable with the article supports for displacing the article supports, a reciprocating drive operable to reciprocate the drive members to and fro along the endless path to advance the articles in succession to the printing stations, and actuating devices for displacing the drive members into and out of engagement with the article supports. The article supports are vacuum tables communicating with a vacuum duct through a member sliding between flexible members extending along the duct.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of U.S. patent application Ser. No. 09/076,821, entitled “Screen Printing Machines”, filed May 13, 1998, now U.S. Pat. No. 6,089,149. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to screen printing machines and, more particularly, to screen printing machines of the type which employ a plurality of article supports, e.g pallets or vacuum tables, for carrying articles to be printed, the article supports being displaceable around an endless path in succession through a plurality of printing stations each provided with a printing head. 
     2. Description of the Related Art 
     It has previously been known to provide a screen printing machine of the above-mentioned type having an endless chain for pulling the article supports around the endless path. Since it is important to ensure that the articles to be printed, which are carried on the article supports, are accurately located in position at the printing stations beneath the printing heads, locating devices have been provided at the printing stations for engaging and accurately positioning the vacuum tables. One such screen printing machine is disclosed in my co-pending U.S. patent application Ser. No. 08/939,497, filed Sep. 29, 1997, the disclosure of which is incorporated herein by reference. 
     However, it is a substantial disadvantage of chain-driven printing machines that the drive chain, over the course of time, tends to stretch, causing inaccuracy and possible disruption of the printing process. 
     It is also well known in the art to employ vacuum tables for supporting paper, plastic and other articles, known as flat stock, as the vacuum tables are advanced from station to station around an endless path. 
     It is an object of the present invention to provide a printing machine having a novel and improved mechanism for displacing the article supports for the articles to be printed to successive printing stations. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to the present invention, a screen printing machine has a displacement mechanism which is provided for displacing a plurality of article supports in succession around an endless path of travel, with printing stations distributed along the path and each having a printing head. The displacement mechanism comprises drive members engageable with the article supports for displacing the article supports along the path, a reciprocating drive operable to reciprocate the drive members to and fro along the path and actuating devices for moving the drive members into and out of engagement with the article supports. 
     By the to-and-fro movement of the drive members, the article supports are advanced in succession to the printing stations, at which printing on the article supports is performed. 
     By using the reciprocating drive, the present invention avoids the disadvantages of prior art endless chain drives from displacing articles to successive printing stations. 
     The reciprocating drive preferably comprises elongate members and pivots connecting the elongate drive members, the elongate drive members and the pivots forming a drive connection between the prime mover and the drive members. 
     In a preferred embodiment of the invention, the endless path includes a pair of parallel elongate straight path sections, tracks extending along the straight path sections and a pair of carriages carried on the tracks, the drive members being mounted on the tracks and the reciprocating drive being connected to the carriages for displacing the carriages to and fro, in opposite directions, to one another, along the tracks. 
     The drive members are arranged in pairs spaced apart transversely of the path and each of the supports has leading and trailing pairs of projections to facilitate transfer of the article supports between the straight path sections. 
     In an alternative embodiment of this invention, an improved vacuum delivery system provides the vacuum tables with continuous access to an endless vacuum duct. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more readily understood from the following description thereof when taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 shows a plan view of parts of a screen printing machine embodying the present invention; 
     FIG. 2 shows a plan view corresponding to FIG. 1, but with some of the parts of the machine omitted; 
     FIG. 3 shows a view corresponding to FIG. 2, but with parts of the machine moved into different positions; 
     FIG. 4 shows a view corresponding to FIG. 2, but with further parts of the machine omitted to show components of an article support displacement mechanism; 
     FIG. 5 shows an exploded view, in perspective, of components of the article support displacement mechanism of FIG.  4  and of article supports which are displaced by that mechanism; 
     FIG. 6 shows a view, partly in cross-section, through an article support used in the machine of FIGS. 1 through 5; 
     FIG. 7 shows a broken-away view taken partly in cross-section through a printing station of the machine of FIGS. 1 through 5; 
     FIGS. 8 and 9 show broken-away views, in side elevation, of respective halves of the screen printing machine of FIG. 1; and 
     FIG. 10 shows a broken-away plan view of parts of the screen printing machine of FIG.  1 . 
     FIG. 11 shows a broken-away side view, partly in vertical cross-section of a table transfer device forming part of the machine of FIGS. 1 through 10; 
     FIG. 12 shows a partly-exploded, broken-away view in perspective of parts of a carriage and a vacuum ducting system of the machine of FIGS. 1 through 11; 
     FIG. 13 shows a view in side elevation of parts of a printing station in the machine of FIGS. 1 through 12 in an inoperative condition; 
     FIG. 14 shows a broken-away view, in side elevation, of one of the printing stations of the machine of FIGS. 1 through 13; 
     FIG. 15 shows a view corresponding to FIG. 13 but with the printing station in an operative condition; 
     FIG. 16 shows a broken-away view, partly in vertical cross-section, through a locking mechanism forming part of the printing station of FIGS. 13 through 15; 
     FIG. 17 shows a broken-away view, in vertical cross-section, corresponding to FIG. 7, but showing a modified vacuum ducting system; 
     FIG. 18 shows a plan view of the machine, corresponding to FIG. 1, but showing parts of the modified ducting system of FIG. 17; 
     FIG. 19 shows a view taken in vertical cross-section through parts of the modified vacuum ducting system of FIG. 17; 
     FIG. 20 shows a view taken in cross-section along the line  20 — 20  of FIG. 19; 
     FIG. 21 shows an exploded view in perspective, of parts of the vacuum ducting system of FIGS. 17 through 20; 
     FIG. 22 shows a section of an alternative embodiment of an endless horizontally extending vacuum duct, which is used in conjunction with an alternative embodiment of a vacuum port as shown in FIGS. 23 and 24; 
     FIG. 23 shows a side elevation view taken in cross-section of an alternative embodiment of a vacuum duct and vacuum port; and 
     FIG. 24 shows a plan view of the alternative embodiment of the vacuum duct and vacuum port shown in FIG.  23 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring firstly to FIG. 1 of the accompanying drawings, there is shown a screen printing machine indicated generally by reference numeral  10 , which is intended for printing flat stock and which has a machine frame indicated generally by reference numeral  12 . The machine frame  12  has opposite end sections indicated generally by reference numerals  14  and intermediate sections indicated generally by reference numerals  16   a - 16   e . Each of the intermediate sections  16   a - 16   e  has two printing stations, as described in more detail below. The number of these intermediate sections  16   a - 16   e  can be varied in order to correspondingly vary the number of printing stations in the screen printing machine 
     FIG. 1 also shows seven article supports in the form of vacuum tables  18   a - 18   g  which, in operation of the machine, are advanced stepwise around a horizontal endless path, as will also be apparent from the following description, by means of a support displacement mechanism, components of which will now be described to reference FIGS. 2 through 4. The machine  10  is normally equipped with a further five similar vacuum tables, which have been omitted from the drawings in order to facilitate the illustration of the machine  10 . 
     The endless path of travel of the article supports  18   a - 18   g  through the screen printing machine  10  includes two parallel elongate straight path sections, each defined by a pair of rails  20   a ,  20   b  (FIG.  2 ). A pair of reciprocatable carriages  22   a ,  22   b  are mounted on the rails  20   a ,  20   b  for movement to and fro along the rails  20   a ,  20   b . For this purpose, a reciprocating displacement mechanism is provided, which includes a prime mover in the form of an electric motor  24  (FIG. 4) which is mounted in the machine frame  12  and, more particularly, in the central intermediate section  16   c . The electric motor  24  drives a gear box  26 , containing a reduction gearing and the gear box  26 , in turn, pivots a crank arm  28  to and fro. The crank arms  28  are connected by a first pair of connection rods  30  to a second pair of connection rods  32 , with pivot connections  27  between the crank arms  28  and the connecting rods  30 . Each connecting rod  30  is connected to its respective connecting rod  32  by means of a pivot connection  34 , which is slidable along a rail  37  in a respective guide  38  fixedly secured to the machine frame  12 . 
     Each connection rod  32  is connected, in turn, by a pivot connection  33  to a lever arm  35 , which projects from a horizontally pivotable drive connector  36 . 
     The crank arms  28 , the connecting rods  30  and  32  and the lever arms  35  are thus elongate members which, with the pivot connections  27 ,  33  and  34 , form a drive connection or linkage between the motor  34  and the drive connectors  36 . The drive connectors  36  are each formed of a plate of generally triangular shape, which is pivotable to and fro in a horizontal plane about a vertical pivot axis of a respective pivot shaft  40  mounted on the machine frame  12 . When the electric motor  24  is energized to cause the crank arm  28  to pivot to and fro, the drive connectors  36  are each pivoted horizontally through 90° to and fro about their respective pivot shafts  40  between the positions in which they are shown in FIG.  2  and the positions in which they are shown in FIG.  3 . 
     As shown in FIGS. 2 and 3, the drive connectors  36  are connected by connecting rods or links  42  to the proximal ends of the reciprocatable carriages  22   a ,  22   b . More particularly, the connecting rods  42  are connected by pivot connections  44  to the drive connectors  36  and by pivot connections  45  to the reciprocatable carriages  22   a ,  22   b . Consequently, as the drive connectors  36  are pivoted to and fro about their pivot axis  40 , the carriages  22   a ,  22   b  are correspondingly displaced to and fro, in opposite directions to one another, along the rails  20   a ,  20   b . Thus, the arrangement is such that, as will be apparent from FIGS. 2 and 3, the carriage  22   a  is moved to the right, as viewed in these figures, as the table  22   b  is moved to the left, and vice versa. These components drivingly connecting the electric motor  24  to the carriages  22   a ,  22   b  are all parts of the above-mentioned reciprocating displacement mechanism. 
     It will be noted that the left-hand drive connector  36 , as viewed in FIGS. 2 and 3, has only one connecting rod  42 , by which it is connected to the carriage  22   b . The omission of any connecting rod  42  between this drive connector  36  and the carriage  22   a  facilitates the reciprocation of the two carriages  22   a  and  22   b.    
     Each of these pivotations of the drive connectors  36  to and fro through 90° causes a corresponding reciprocation of the carriages  22   a ,  22   b  through a predetermined distance corresponding to the distance from one to the next of the printing stations at each side of the machine. In this way, the article supports are advanced in steps to successive ones of the printing stations, as described below. 
     FIG. 5 shows a broken-way exploded view of parts of the displacement mechanism and of the vacuum tables themselves. 
     More particularly, in FIG. 5, reference numerals  44  indicate three perforated table tops, the constructional details of which are described below. The table tops  44  are each fixedly secured to a respective generally H-shaped sub-structure, of which six are shown in FIG.  5  and indicated generally by reference numerals  46   a - 46   f , and which are each provided with a pair of leading projections  48  and a pair of trailing projections  50 . The carriages  22   a ,  22   b  are provided at opposite sides thereof with pairs of drive members, four pairs of which are indicated by reference numerals  52   a - 52   d . The drive members  52   a - 52   d  can be raised into operative position, for driving engagement with the projections  48  and  50  or retracted downwardly, into inoperative position for disengaging the projections  48 ,  50 , as described in greater detail below. The table tops  44  and the table sub-structures  46   a - 46   f  are supported by sliding contact of the table tops with track  56  (FIG.  6 ), which extend along opposite sides of the path of travel of the table tops  44  and are in turn supported on the machine frame  12 . 
     Referring again to FIG. 5, the drive connector  36  forms part of a table transfer mechanism indicated generally by reference numeral  55 , which includes two table transfer devices carried by the drive connector  36  and indicated, respectively, by reference numerals  58   a ,  58   b . These table transfer devices  58   a ,  58   b  form pivotable supports and are disposed at right angles to one another. 
     To facilitate understanding of the manner in which the table sub-structures  46   a - 46   f  are engaged and advanced, chain-dot lines have been employed in FIG. 5 to illustrate the relationship between the drive members  52   a - 52   d  and the leading and trailing projections  48 , 50  of the table sub-structures  46   a - 46   f.    
     The table transfer device  58   a , as shown in FIG. 5, is in a first position in which it can engage the leading projections  48  of the table sub-structure  46   c  located at an output end  59  of the rails  20   b . From this first position, the table transfer device  58   a , carrying with it the table sub-structure  46   c , is moved, on pivotation of the drive connector  36  through 90° about its pivot shaft  40 , into an intermediate position, in which the table transfer device  58   b  is shown in FIG.  5 . At the same time, the table transfer device  58   b  is pivoted through 90 degrees from the intermediate position, in which it is shown in FIG. 5, to a third position at an input end  61  of the rails  20   a , above which the table sub-structure  46   e  is shown in FIG.  5 . 
     More particularly, the drive members  52   a  and  52   b  are spaced apart along their carriages  22   a ,  22   b  so that the drive members  52   a  engage the leading projections  48  of the table sub-structure  46   a  and the drive members  52   b  engage the trailing projections  50  of the table sub-structure  46   b . As the drive connector  36  is pivoted from the position in which it is shown in FIG. 5, through 90 degrees, the table sub-structure  46   b  will be advanced through a distance equal to the distance from one to the next of the printing stations, by driving engagement of the drive members  52   b  with the trailing projections  50  of the table sub-structure  46   b , into the position in which the table sub-structure  46   c  is shown in FIG.  5 . In this position, the drive members  52   b  are retracted downwardly out of engagement with the trailing projections  50  of the table sub-structure  46   b , and drive members  60  on the table transfer device  58   a  are extended upwardly into engagement with the leading projections  48  of the table sub-structure. When the drive connector  36  is then pivoted through 90 degrees from the position in which it is shown in FIG. 5, the table transfer device  58   a  drives the table sub-structure from the first position to the intermediate position, and the drive members  60  are then retracted downwardly to release the table sub-structure in the intermediate position. 
     Prior to this pivotation of the drive connector  36 , drive members  62  on the table transfer device  58   b  are extended upwardly into engagement with the trailing projections  50  of the table sub-structure in this intermediate position. Consequently, when the drive connector  36  pivots from its position shown in FIG. 5, this table sub-structure is advanced from the intermediate position to the third position, at the input end  61  of the rails  20   a.    
     As this occurs, the preceding table sub-structure is moved along the rails  20   a  from the third position by the drive members  52   c  in engagement with the trailing projections  50  of that table sub-structure and, simultaneously, the drive members  52   d , which have been raised into engagement with the trailing projections  50  of the table sub-structure  46   f , displace the latter along the rails  20   a.    
     FIG. 6 shows in greater detail the construction of one of the vacuum tables, which is indicated generally by reference numeral  18 , and it is to be understood that the vacuum table  18  is similar in construction to vacuum tables  18   a - 18   g  and that, in practice, the machine  10  is normally provided with twelve vacuum tables as indicated above, and that they are all similar to one another. 
     As shown in FIG. 6, the vacuum table  18  includes one of the table tops  44 , which has a hollow interior defined by a top  66 , formed with perforations  68 , a bottom  70 , formed with an opening  72 , opposite side walls  74 , an end wall  76  and an opposite end wall  78 . For reinforcement, the interior of the table top  44  is provided with an internal honeycomb structure formed by partitions  80 , and the partitions  80  are formed with openings  82 . 
     The table sub-structure, which in this Figure is indicated generally by reference numeral  46  and which is similar to the table sub-structures  46   a - 46   f  of FIG. 5, has a hollow interior  84  and vacuum ports indicated generally by reference numerals  86  and  88 , which are normally closed by slidable closure members  90 . 
     The vacuum ports  86  are stationary and the vacuum ports  88  are mounted on the carriages  22   a  and  22   b  for reciprocation therewith. When one of these vacuum ports  86  and  88  is opened and a vacuum is applied to the opened port, as described below, the vacuum is communicated through the hollow interior  84  of the table sub-structure  46 , through an opening  92  in the top of the latter and through an opening  72  in the bottom  70  of the table top  44  to the perforations  68 . In this way, an article of flat stock which is to be printed is held firmly by vacuum on the top of the table top  44 . 
     The end sections  14  are provided with stationary vacuum ports  87  and movable vacuum ports  89  are mounted on the drive connectors  36  for to-and-fro pivotation therewith. 
     Referring again to FIG. 1, it will be seen that the machine  10  includes a stationary vacuum duct  96  extending along each pair of rails  20   a ,  20   b  and also a movable vacuum duct  98 , which is secured to the respective carriage  22   a ,  22   b . The vacuum ducts  96  and  98  are provided with vacuum connectors  100  and  102 , one each of which is shown in FIG.  7 . 
     The vacuum connector  100  has an open-top tube  104  which can be lowered into an inoperative position against the action of a compression spring  106  and raised, into engagement with an overlying one of the vacuum ports  86  for applying a vacuum from the vacuum duct  96  to the overlying table sub-structure  46 . 
     The tube  104  is secured for movement with a vertically elongate, vertically displaceable alignment member  106  (FIG.  14 ), which is guided by means of rollers  108  mounted on the machine frame and which, when raised, engages between rollers  110  on the table sub-structure  46  for locating the latter in position at the printing station for correct alignment during printing. The vertical displacement of the alignment member  106  and, therewith, the tube  104  is effected by means of a lever linkage comprising levers  112 ,  114  and  116  connected to a rod  118 , which is horizontally reciprocatable, by means of a pneumatic piston and cylinder  120  (FIG. 8) connected to the machine frame. 
     As shown in FIG. 12, the vacuum connector  102  is provided in a duct section  122 , which is mounted on a support bracket  124  carried by the carriage  22   a . A similar arrangement is mounted on the carriage  22   b . The bracket  124  is mounted on the drive member  52   a . A bar  126  can be displaced to and fro, in a horizontal direction, relative to the carriage  22   a  by means of a pneumatic piston-and-cylinder device  128 . The bar  126  is provided with inclined slots, of which only one is shown and is indicated by reference numeral  130 , into which engage pins  132  on the drive members  52   a  and  52   b , which are mounted in guide rollers  134  so as to be vertically movable, to and fro, relative to the carriage  22   a  into and out of driving engagement with the projections  48  and  50  in accordance with the displacement of the bar  126  relative to the carriage  22   a . The bracket  124  is secured to one of the drive members  52   a  for vertical movement therewith, and this vertical movement serves to move the vacuum connector  102  into and out of engagement with the vacuum port  88 . During this vertical upward movement of the vacuum connector  102 , a pneumatic piston-and-cylinder device  138  on the vacuum duct section  122  is employed to effect horizontal displacement of a bar  140 , carrying a drive member  142 , which engages a roller  144  on the closure member  90  of the port  88  in order to open this port  88 . 
     Each of the table sub-structures  46 , as shown in FIG. 10, is provided with two of the vacuum connectors  102 , so that one of these vacuum connectors can be connected to the vacuum duct  98  during movement of the table sub-structure to the transfer mechanism  55 , while the other port  105  is then available for connection  142  for use during the transfer of this table sub-structure  46  from the first position to the intermediate position, as described above. The vacuum ducts  96  and  98  are connected to a vacuum pump  148  (FIG.  8 ). 
     FIG. 11 shows the mechanisms for raising and lowering the drive members  60  and  62  into and out of engagement with the leading and trailing projections  48  and  50  of the table sub-structures  46  at the transfer mechanisms  55 . 
     For this purpose, a pneumatic piston-and-cylinder device  154  at the underside of the respective drive connector  36  is provided for reciprocating a rod  156  and, thereby, through levers  158 , shafts  161 , and levers  162  and  164  correspondingly raising and lowering support frames  166 , on which the drive members  60  and  62  are mounted. 
     At each printing station there is provided a printing head indicated generally by reference numeral  160 , which bridges the path of travel of the vacuum tables  18 , and which is provided with a squeegee and squeegee holder and the flat bar and flat bar holder which are similar to those described in my above-mentioned co-pending U.S. patent application Ser. No. 08/939,407, and which, therefore, are not described in greater detail herein. 
     In the present machine, however, each printing head  160  is horizontally displaceable into an inoperative position, in which the printing head  160  is shown in FIG. 13, from an operative position, in which the printing head  160  is shown in FIG. 15, in order, thus, to provide ready access to a printing screen carried by the printing head and indicated generally by reference numeral  163  in FIG. 13, for the purpose of maintaining or replacing the printing screen  163 . For that purpose, rails  165  (FIG. 13) at the underside of the printing head  160  are slidable in guides  166  mounted on the machine frame, and guides  168  on the printing head are slidably engaged with rails  170  mounted on the machine frame. 
     For locking the printing head  160  in its operative position, as shown in FIG. 15, a pair of locking cylinders  172  (FIG. 14) are operable to raise and lower a locking cam  174  (FIG. 16) which, in its raised position, as shown in FIG. 16, engages between rollers  176  and  178  which are mounted, respectively, on the machine frame  12  and on the printing head  160 . By engagement with the roller  178 , the locking member  174  urges the printing head  160  into firm abutment with a nylon pad  180  on a wall  182  on the machine frame  12 , thus ensuring that the printing head  160  is securely and accurately locked in position. 
     FIGS. 17 through 21 illustrate a modified vacuum ducting arrangement for supplying vacuum to the vacuum tables  18  as the vacuum tables  18  are displaced around their endless path of travel. 
     Referring firstly to FIG. 17, reference numeral  200  indicates generally a vacuum port at the underside of one of the vacuum table sub-structures  46 , this vacuum port  200  being urged resiliently in a horizontal direction, as described in greater detail below, towards an endless, horizontally extending vacuum duct, indicated generally by reference numeral  202 , which, as will be more readily apparent from FIG. 18, extends around the endless path of travel of the vacuum tables  18 . The vacuum duct  202  is connected by a duct  204  to the vacuum pump  148 . 
     As shown in FIGS. 19 through 21, the vacuum port  200  comprises a horizontal cylinder  206  which, at its upper side, is formed with openings  208 , the cylinder  206  being closed at one end by a circular plate  210 . The cylinder  206  is slidably supported, for horizontal sliding movement, in a bronze bushing  212  and a bronze sleeve  214 . A retaining ring  216 , which is sprung into engagement with the exterior of the cylinder  206 , forms an abutment for one end of a helical compression spring  218 , the opposite end of which abuts the bushing  212 , so that the cylinder  206  is thereby resiliently biased to the right as viewed in FIG.  19 . The bushing  212  and the sleeve  214  are fitted into circular openings in rectangular plates  220  and  222  which, together with a housing member  224  (FIG. 21) and an apertured plate  226  at the underside of the table support, form a housing from which opposite ends of the cylinder  206  extend. 
     The end of the cylinder  206  remote from the end wall plate  210  is welded to a rectangular plate  228 , and a cushion plate  230  of plastic material sold under the trade mark DELRIN is fitted onto the plate  228 . More particularly, the cushion plate  230  has a circular opening formed with a flange  232 , which mates with a circular opening  234  in the plate  228  and abuts the cylinder  206 . 
     The cushion plate  230  is urged, by the compression spring  218 , into sliding contact with a generally rectangular component in the form of an apertured rectangular frame  235  which has, at opposite lateral sides thereof, parallel vertical elongate projections  236 , between which the cushion plate  230  is received, as apparent from FIG.  20 . Consequently, as the table is displaced around the endless path of travel, as described above, the cushion plate  230  and the plate  228 , in driving engagement with one of these projections  236 , drives the frame  235  around the duct  202 . The cushion plate  230  is, however, vertically slidable relative to the frame  235  to allow corresponding vertical movement of the vacuum table  18  at opposite ends of the straight elongate path sections, to allow the vacuum table to be transferred between the carriages  22   a  and  22   b , as described above. 
     The vacuum duct  202  is formed by a pair of vertically spaced, horizontal plates  240 , 242 , and a vertical rear wall  244 , which thus form a duct which is open at one horizontal side. This open side forms a horizontally elongate opening extending along the vacuum duct  202  and defined by brass strips  246  secured, respectively, to the top and bottom plates  240  and  242  of the duct  202  and projecting laterally from the top and bottom plates  240  and  242 , as shown in FIG.  19 . 
     This opening is partially closed by a closure which comprises a plurality of strip metal sections  248 , and blocks  250 , of DELRIN, which are secured to the strip metal sections  248  and which are engaged between and in sliding contact with the brass strips  246 . 
     Successive strips  248  are separated from one another by gaps, one of which is indicated generally by reference numeral  252  in FIG.  21 . 
     At one side of each gap  252 , an end of one of the strip metal sections  248  and one of the blocks  250  are secured to one side of the frame  235  by a pair of screws, of which only one is shown and is indicated by reference numeral  254 . However, the opposite side of the frame  235  is not connected to the adjacent end of the next section  248 , so that the latter is slidable relative to the frame  235 . The ends of the strips  248  at opposite sides of the gap  252  are resiliently connected to one another by means of a helical tension spring  256 , as shown in FIG. 20, the spring  256  being one of a plurality of tension springs connecting the strips  248  in this way. The strip metal sections  248  are thus connected to one another to form an endless closure in the form of a belt which travels with the vacuum tables  18  and, by means of these helical compression springs, is tensioned into contact with the vacuum duct  202  so that, even when vacuum within the vacuum duct  202  is interrupted, the closure will be held against the vacuum duct  202  and, more particularly, will be held against the brass strips  246 . 
     Turning to FIGS. 22,  23 , and  24 , there is illustrated an alternative embodiment of the vacuum delivery system wherein the vacuum tables are in continuous communication with an endless vacuum duct indicated generally by reference numeral  302 , a section of which is shown in FIG.  22 . The vacuum duct  302  is formed by a pair of horizontally spaced plates  304  and  306  and a vertically disposed wall  308  extending thereinbetween. A partial vertically disposed wall  310  extends upwardly from the plate  306  and defines, in combination with the plate  304 , a continuous or substantially continuous open channel  312 . First and second flexible members  314  and  316  are positioned to extend over the open channel  312  and provide a means by which the endless vacuum duct  302  is accessed by a vacuum port indicated generally by reference numeral  320  which will be described in conjunction with FIGS. 23 and 24. The flexible members  314  and  316  are manufactured from a material which is resilient and can be positioned so as to maintain a biased relationship at their juncture  318  where the members  314  and  316  are in separable contact, one with the other. The flexible members are thus positioned so as to define a means for continuous access to the vacuum duct  302 , but that continuous access defined by the juncture  318  is biased in a normally closed position. As will be appreciated by those skilled in the art, the physical layout of the vacuum ducts  202  and  302  is consistent with the endless path of travel of the vacuum tables  18  as shown in FIG.  18 . In another advantage of this alternative embodiment, sections of endless vacuum duct  302  can be manufactured individually and joined together by suitable fastening means. Moreover, the substantial reduction of moving and movable parts in this alternative embodiment simplifies manufacture and maintenance of this vacuum system. 
     Turning to FIGS. 23 and 24, the operation of this alternative vacuum duct system can be appreciated in side elevation and plan view. A description of the structure and general operation of a vacuum table  18  is described in detail above in connection with FIG.  6  and reference is made thereto. Generally, vacuum is communicated through the hollow interior of the table sub-structure  46 . In the alternative embodiment, a movable vacuum duct as at  98  in FIG. 1 is secured to the respective carriage  22   a ,  22   b . The alternative vacuum port  320  as shown in FIGS. 23 and 24 comprises a member  322  with a leading edge  324  and a trailing edge  326 . The leading and trailing edges  324  and  326  define a knife-edge like structure which engages the juncture  318  where the flexible members  314  and  316  are in separable contact, one with the other. As the vacuum port  320  travels along the length of the vacuum duct  302 , the leading edge  324  of the vacuum port continuously opens the juncture while the trailing edge  326  facilitates the smooth closure of the temporarily separated flexible members  314  and  316 . 
     The center portion  328  of the member  322  is disposed between the leading and trailing edges  324  and  326  and defines port means  330  by which a vacuum is communicated between the vacuum duct  302  and the hollow interior of the table sub-structure  46  by means of the parted juncture of the flexible members  314  and  316 . The vacuum port  320  is supported by a housing  332  mounted onto the table sub-structure  46 . Thus, as the tables are conveyed about the endless track, the table sub-structure remains in fluid communication with the vacuum system by means of the vacuum port  320  which by its continuous movement establishes a continuously moving access point to the vacuum duct  302 . The vacuum duct  302  is connected to a connecting duct  334  and to a vacuum pump such as shown in connection with the vacuum system illustrated in FIG.  17 . The aforedescribed system minimizes loss of vacuum while at the same time continuously maintaining communication between the vacuum duct  302  and the table sub-structure  46 .