Abstract:
A thickness sensor for layered sheet material entering a lamination machine includes an arm movable upon interaction with the layered sheet material. A variable resistance output dependent upon the position of the arm controls the speed of rotation of heated rollers in the lamination machine so that the layered sheet material will be drawn through the machine with variable speed to better infuse thermal adhesive into the material should such material be detected.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to laminators of the type used to laminate a sheet such as a photograph, certificate or chart for example between a pair of transparent plastics sheets. More particularly, although not exclusively, the invention relates to a laminator having a mechanism for detecting the thickness of the layers to be laminated and means for adjusting the through-speed of the sheets past the laminator&#39;s heating rollers depending upon the detected thickness. 
     Known laminators comprise a pair of internally heated rollers that are biased toward one another or at a fixed spacing and in between which the layers to be laminated are pressed. The heat from the rollers melts an adhesive surface on each of the plastics sheets and the melted laminate infuses to some extent into the sheet that is being laminated. Cards and thin paper are relatively straightforward to laminate because a pre-set temperature and speed setting of the rollers achieves the necessary adhesion between the layers. However, when a thick sheet of cardboard for example is to be laminated, the pre-set temperature and speed setting can result in insufficient adhesive penetration to the cardboard, and consequent ease of delamination of the finished product. For this reason, the laminated finished product might have to be passed a second time through the machine and perhaps even a third. This can result in loss of productivity or even damage to the finished product. 
     OBJECTS OF THE INVENTION 
     It is an object of the present invention to overcome or substantially ameliorate the above disadvantage and/or more generally to provide a thickness sensor for a lamination machine and a lamination machine incorporating a thickness sensor and speed-controlled lamination/heating rollers. 
     DISCLOSURE OF THE INVENTION 
     There is disclosed herein a thickness sensor for layered sheet material entering a lamination machine, comprising: 
     an arm movable upon interaction with the layered sheet material, and 
     means providing variable resistance output dependent upon the position of the arm. 
     Preferably the means providing variable resistance comprises an electrical conductor mounted to the arm and providing a pair of exposed electrical contacts, and a printed circuit board (PCB) having exposed tracks engaged by respective ones of the electrical contacts, and resistors electrically connecting adjacent portions of at least one of the tracks. 
     Preferably the tracks each comprise an end portion electrically isolated from one another and engaged by respective ones of the electrical contacts for indicating that the lamination is too thick. 
     Preferably one of said end portions comprises a sub portion electrically isolated therefrom and engaged by one of the electrical contacts for providing a warning signal that the lamination is of maximum allowable thickness. 
     Preferably the arm is pivotally mounted upon a pivot pin and the tracks are curved. 
     Preferably the arm comprises a roller at one end engagable with the layered sheet material and the pivot pin is closer to the roller than it is to the electrical contacts. 
     There is further disclosed herein a lamination machine comprising: 
     a pair of heated rollers between which layers of sheet material are pressed for lamination, and 
     the above-disclosed thickness sensor attached to the machine at a location upstream of the rollers and cooperating with the machine to vary the speed of rotation of the rollers. 
     Preferably at least one of said rollers is driven by an electric motor, the operational speed of which is affected by the thickness sensor. 
     Preferably the machine further comprises a pair of withdrawal rollers downstream of the heated rollers. 
     Preferably the withdrawal rollers rotate in unison with the heated rollers via a gear train therebetween. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred form of the present invention will now be described by way of example with reference to the accompanying drawings, wherein: 
     FIG. 1 is a schematic perspective illustration of a thickness sensor for layered sheet material, 
     FIG. 2 is a schematic perspective illustration of the thickness sensor of FIG. 1 when viewed from another angle, 
     FIG. 3 is a schematic cross-sectional elevational view of a lamination machine having attached thereto the thickness sensor of FIGS. 1 and 2, and 
     FIG. 4 is a schematic perspective illustration of the rollers and gear train ensuring rotation of the rollers in unison. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIGS. 1 and 2 of the accompanying drawings there is depicted schematically a thickness sensor  10 . Sensor  10  comprises a casing within which there is mounted a printed circuit board (PCB)  16 . There is an arm  18  mounted upon a pivot pin  17  about which a coil spring  24  is wound. The coil spring lightly biases the arm into the orientation depicted in the Figures. One end of the arm extends out of a casing and supports rollers  11  to bear against the downward-facing surface of a stack of three sheets to be laminated. This would usually be the bottom transparent plastics sheet, between two of which a photograph for example is to be laminated. 
     There is an idler  12  positioned above the roller  11  mounted at each end by an idler support  13 . Each idler support  13  projects telescopically and downwardly from a spring casing  15  within which a coil spring  14  is located. The coil spring  14  maintains the idler  12  at a datum position, but allows for upward movement of the idler  12 , should the overall height of the layered sheets of material be too high. Should such be the case, the lamination machine will shut down by virtue of a feature to be described later. 
     The arm  18  has an electrically conductive bus  21  therein. There are two springs  19  and  20  that extend from the bus to a position between the arm  18  and the PCB  16 . There might be electrical brushes attached to the springs  19  and  20  for engagement with tracks  22  and  23  formed upon the PCB  16 . Alternatively, the springs might bear directly upon the tracks. 
     As shown in FIG. 1, the distance between the pivot pin  17  and the rollers  11  is significantly less than the distance between the pivot pin  17  and the electrical contacts and tracks  22  and  23 . This ensures that small pivotal movements of the roller-end of the arm  18  result in large movements of the electrical contacts and thus a relatively large-scale PCB design can be adopted. 
     One of the tracks  22  is substantially continuous, but includes an electrically isolated end portion  22 A. The other track  23  is divided into a plurality of short segments attached electrically and in sequence with surface-mounted resistors  24 . Alongside the track portion  22 A, the track  23  comprises electrically isolated portions  23 A and  23 B. 
     As the layers of sheet material engaged the roller  11 , the arm  18  pivot about pivot pin  17  against spring  24 . The contact brushes attached to the springs  19  and  20  then ride along the tracks  22  and  23 . As contact is made with adjacent ones of the short segments of track  23 , resistance of the sensor  10  changes. This provides a near-continuous variable resistance that can be used to control the speed at which the rollers (to be described later) are driven. 
     When the arm  18  is aligned across portions  22 A and  23 A, a warning signal such as “beep” sound is made or a warning light is illuminated to indicate that the material being laminated is at or very close to the maximum allowable limit. If the arm  16  aligns across track portions  22 A and  23 B, a shut-off condition results, as a jam would be inevitable. 
     The thickness sensor  10  is attached to a lamination machine by a mounting  25  from which there extends a mounting spring  26  that passes through an eye  34  to retain the sensor. 
     The lamination machine itself comprises a pair of heated feed rollers comprising a resilient rolling surface  26  surrounding a heater  28 . At least one of these would be driven by an electric motor (not shown). There is also a pair of downstream withdrawal rollers  30 , at least one of which is driven to rotate in unison with the feed rollers  26 . A pair of gear trains  32  achieves this, one at each end of the rollers. In order to cause rotation of the rollers, an electric motor can be coupled to any one of the gears of one or both gear trains  32 , or one of the rollers directly. 
     Situated in between the feed rollers and the withdrawal rollers is an additional heater  29 . 
     In use, sheets to be laminated are presented at the upstream end of the lamination machine, perhaps upon a support tray  33 . The sheets are fed by hand into the machine whereupon the bottom sheet engages the roller  11 . In turn, the arm  18  takes on a pivotal orientation indicative of the thickness of the stack of sheets thereby inducing an appropriate resistance in the sensor  10 . This resistance is read and relayed to the motor to affect the throughput speed of the material. The feed rollers draw the sheets in at a speed appropriate for the material thickness and heat is applied by the heaters  28  to soften the adhesive on the inwardly facing surfaces of the outer plastics layers. The molten adhesive is then at least partially infused into the middle layer. The supplementary heaters  29  then apply further heat to the sheet en route to the withdrawal rollers  30  that are rotating in unison with the feed rollers. The withdrawal rollers also have a resilient surface to press the sheets together and then present the sheets at a dispensing tray  31  for collection. 
     It should be appreciated that modifications and alterations obvious to those skilled in the art are not to be considered as beyond the scope of the present invention. For example, the arm might move linearly instead of pivotally and the rollers might be caused to rotate in unison by means other than a gear train. Furthermore, instead of and/or in addition to controlling the draw speed, the thickness sensor&#39;s resistance reading might be used to control the heat produced by the heaters.