Abstract:
A blade assembly particularly for preforming a tab cut in a label. The blade assembly comprises a blade received in an injection molded plastics holder. The holder has shoulders on either side of the blade to assist in locating it and spacing it from a label to be cut. During manufacture, the blade is located within the mold for accurate injection molding.

Description:
This is a division of application Ser. No. 08/703,841, filed Aug. 27, 1996, now U.S. Pat. No. 5,695,705 which is a continuation of abandoned application 08/180,242 filed Jan, 11, 1994. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a tape cutting apparatus and is particularly concerned with cutting tape in printing devices. 
     BACKGROUND OF THE INVENTION 
     Thermal printing devices of the general type with which the present invention is particularly but not exclusively concerned are known. They operate with a supply of tape arranged to receive an image and a means for transferring an image onto the tape. In one known device, a tape holding case holds a supply of image receiving tape and a supply of an image transfer ribbon, the image receiving tape and the transfer ribbon being passed in overlap through a printing zone of the printing device. At the print zone, a thermal print head cooperates with a platen to transfer an image from the transfer ribbon to the tape. A printing device operating with a tape holding case of this type is described for example in EP-A-0267890 (Varitronics, Inc.). Other printing devices have been made in which letters are transferred to an image receiving tape by a dry lettering or dry film impression process. In all of these printing devices, the construction of the image receiving tape is substantially the same. That is, it comprises an upper layer for receiving an image which is secured to a releaseable backing layer by a layer of adhesive. 
     Once an image or message has been printed on the tape, it is desired to cut off that portion of the tape to enable it to be used as a label. For this purpose, it is necessary to remove the releaseable backing layer from the upper layer to enable the upper layer to be secured to a surface by means of the adhesive layer. With existing printing devices, it is difficult to remove the releaseable backing layer from the upper layer: it is necessary first to separate the closely adhered end portions of the releaseable backing layer and the upper layer, for example using a fingernail or tweezers so that the separated end portion of the releaseable backing layer can be finger gripped to peel it off the adhesive layer. This is a relatively difficult procedure and furthermore can result in the ends of the label being damaged in the process. 
     There have been several attempts to solve this problem. One approach is to provide a so-called tab cut. In these devices, a first cut is made completely through all the layers of the tape to cut off a portion of the tape and at the same time a cut is made through only one layer of the tape. This provides a “tab” which, in theory, can be peeled away reasonably easily. In embossing label makers a system is known whereby a sharp steel blade is used to cut the plastic label against a soft, serrated anvil, such that the soft plastic backing layer remains substantially intact. Such a system does not work satisfactorily with electronic label makers as the backing layer used is normally paper and the plastic label tape is thinner than normal embossing tape. Any attempt to use a similar approach in an electronic label printer would require high cutting forces and frequent replacement of the soft cutting anvil. Although there have been several proposals, no such tab cut has successfully been implemented in a thermal printing device. By way of example, reference is made to EP-A-0319209 which describes one attempt to form a tab cut system. In EP-A-319209, the tab cut is made only through the thickness of the backing layer which is applied as a release layer of double-sided adhesive tape. 
     In that system, two blades are provided on a cutter support, the blades having different heights so that they penetrate the backing layer to different extents. In this way, one blade cuts through all the layers of the tape at one location while the other blade cuts only through the releaseable backing layer. 
     One problem which arises with the tab cutting apparatus described in EP-A-0319209 is the control of the height of the blades to ensure that there is reliability in that one blade always cuts through the whole tape and the other blade only cuts through the backing layer. This is difficult to achieve where tapes of differing thicknesses are provided for use with the cutting apparatus. A variation in thickness such that could arise due to normal manufacturing tolerances could even give rise to problems in this respect. 
     Another difficulty is that the tab cut depends on making two cuts simultaneously from the common cutter support, requiring increased force to be applied by the user. The force is applied manually and the force applied by some users may be insufficient to provide a proper tab cut, causing the label to be damaged when the backing is removed. Conversely too great a force may cause both tapes to be fully cut in both positions, leaving a portion of material within the cutting mechanism. 
     These problems have meant that to date the above described system has not been successfully implemented. 
     In this regard reference is made to British Patent Application No. 9212423.9 in the name of the present applicant which describes a tape cutting apparatus in which a tab cut is implemented using a drive means to provide the force for the tab cutting blade. The contents of that Application are incorporated herein by reference. 
     A particularly important aspect to providing a successful implementation of a tab cut is to enable the depth of cut to be controlled carefully. Only in this way can a reliable tab cut be produced which cuts only through one layer of the multilayer tape leaving the remaining layers intact. 
     SUMMARY OF THE INVENTION 
     According to the present invention in one aspect there is provided a blade assembly which comprises an injection molded plastics holder receiving a cutting blade, the holder having supporting surfaces which extend beyond a surface of the holder from which the blade protrudes and which hold the tape and prevent the blade from piercing all the layers of a multilayer tape. 
     The tab cutting assembly is made by providing a mold, locating the cutting blade within the mold, injecting plastic material into the mold, and allowing the plastics material to set. The location of the cutting blade is done so that a very close tolerance can be achieved on the blade height of the order of 20 to 25 μm. Typically 100 to 120 microns of blade height is required for cutting through a 75 micron polyester tape with a total thickness of 135 microns including adhesive backing paper. The lower limit would be 95 μm instead of 100 μm. For polyester tape of a different thickness a different blade height would be required. Preferably the blade height is 20 to 45 μm greater than the thickness of the polyester tape. 
     According to another aspect of the invention there is provided a tape cutting apparatus comprising: 
     locating a cutting blade within a mold using at least one biased pin member, a cutting portion of the blade protruding from said mold; 
     injecting plastics material into the mold, and allowing the plastics material to set to form an injection molded plastics holder around said blade with an amount of the blade protruding from the holder which is less than the height of the holder; and 
     removing said injection molded plastics holder from said mold. 
     This method aids in achieving the required close tolerance for the protruding blade height. 
     Preferably the cutting blade is made from a ceramic material since this typically requires as low as a half to one third the force of a steel blade for the same cut. In fact, it has been discovered that with the thin polyester material used as the image receiving tape, the blade does not actually cut the material in the normal sense of the word but rather cracks it. A ceramic blade is ideal for this. 
     Preferably, the tab cutting assembly forms part of a tape cutting apparatus which further includes a second cutting means which is operable to cut through all of the layers of said multilayer tape. In the described embodiment, the second cutting means is actuated by the same drive means as the tab cut blade so that a portion of tape can be cut off while a cut is also made through only one layer of the tape at a position spaced apart from the cut off edge. This enables labels with tabs to be produced. 
     The second cutting means can comprise two blades cooperable to form a scissor cut, for example in which one of the blades is fixed while the other of the blades is arranged to move towards the fixed blade when actuated by the drive means. 
     The holder of the tab cut assembly is preferably resiliently mounted and acts against an anvil which can form part of the tape cutting apparatus or can be provided as part of a thermal printing device with which the tape cutting apparatus is to cooperate. 
     Where both the tab cut assembly and the second cutting means are arranged to be driven from the common drive means, means can be provided for disconnecting the second cutting means while the tab cut assembly remains driven thereby. In this way, it is possible for a thermal printing device to produce a continuous strip of labels, separated one from another by a cut but being secured to a common backing layer. 
     For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made by way of example, to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view showing two cassettes inserted in a printing device; 
     FIG. 2 is a plan view seen from the top of a cassette bay of the printing device; 
     FIG. 3 is a view taken from the underneath of the bay of a printing device; 
     FIG. 4 a  is an end view of a tab cut assembly; 
     FIG. 4 b  is a side view of the tab cut assembly; 
     FIG. 4 c  is a view from the other side of the tab cut assembly; 
     FIG. 4 d  is a view onto the cutting surface of the assembly; 
     FIG. 4 e  is a transverse section through the assembly; 
     FIG. 5 shows a label with a tab cut; 
     FIG. 6 is a block diagram of motor control circuitry; 
     FIG. 7 a  is an end view of an alternative embodiment of the blade holder; 
     FIG. 7 b  is a view onto the cutting surface of the assembly of FIG. 7 a;    
     FIG. 8 shows an end view of the tab cut assembly of FIG. 4 a  in more detail; 
     FIG. 9 is a side view of two mold halves of an insert molding assembly; and 
     FIG. 10 shows the blade location in more detail. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows in plan view two cassettes arranged in a printing device. The upper cassette  2  contains a supply of image receiving tape  4  which passes through a print zone  3  of the printer to an outlet  5  of the printer. The image receiving tape  4  comprises an upper layer  4   a  for receiving a printed image on one of its surfaces and having its other surface coated with an adhesive layer to which is secured a releaseable backing layer  4   b  (see FIG.  5 ). The cassette  2  has a recess  6  for accommodating a platen  8  of the printer. The platen  8  is mounted for rotation within a cage molding  10 . The platen  8  could as an alternative be mounted for rotation on a pin. 
     The lower cassette  7  contains a thermal transfer ribbon  12  which extends from a supply spool to a take-up spool within the cassette  7 . The thermal transfer ribbon  12  extends through the print zone  3  in overlap with the image receiving tape  4 . The cassette  7  has a recess  14  for receiving a print head  16  of the printer. The print head  16  is movable between an operative position, shown in FIG. 1, in which it is in contact with the platen  8  and holds the thermal transfer ribbon  12  and the image receiving tape  4  in overlap between the print head and the platen and an inoperative position in which it is moved away from the platen  8  to release the thermal transfer ribbon  12  and image receiving tape  4 . In the operative position, the platen is rotated to cause image receiving tape to be driven past the print head and the print head is controlled to print an image onto the image receiving tape by thermal transfer of ink from the ribbon  12 . The print head is a conventional thermal print head having an array of pixels each of which can be thermally activated in accordance with the desired image to be printed. 
     FIG. 2 shows the drive train of the printing device. The printing device carries a stepper motor  18  secured to the base of the printing device by a bracket  20  (see FIG.  3 ). The motor drives a double radius gear  22  on its larger diameter  24  while its smaller diameter  26  drives a second gear wheel  28 . 
     The second gear wheel  28  drives a gear  29  which causes the platen  8  to rotate and which also drives through an intermediate gear  30  a third gear  32  which drives the take-up spool for the ink ribbon in the cassette  4 . 
     The stepper motor  18  drives the platen  8  in steps so that for each position of the platen a column of pixels is printed on the image receiving tape  4 . The platen  8  drives the image receiving tape through the print zone under the action of its own rotation. The rotation of the platen and the energisation of the print head  16  are controlled by a microprocessor as described in our British Application No. 9212423.9, the contents of which are incorporated herein by reference. 
     FIG. 2 also shows a tab cutting assembly comprising a spring loaded blade holder designated generally by reference numeral  56  holding a blade  58  which can be forced against an anvil  60 . The blade holder  56  is biased by a spring  57 . In an alternative arrangement, the anvil  60  could be biased instead of the blade holder  56 . The blade  58  is not designed to cut entirely through the tape but is designed to cut only through the image receiving layer  4   a  of the image receiving tape  4  and not through the releaseable backing layer  4   b.    
     FIG. 3 is an underside view showing a cutting mechanism of the printing device. A cutter motor  36  drives a worm gear  38 . This drives a gear train comprising three gears  40 , 42 , 44 , the last gear  44  then driving a cam  46 . The cam  46  has in its surface a cam track  48  extending circumferentially and asymmetrically. A tab cut lever arm  50  runs in the cam track  48  via a pin  52 . The tab cut lever arm is pivotably mounted about a pivot point  54  and is arranged so that, as the cam rotates, it is brought into contact with the blade holder  56  to bring the blade  58  against the anvil. 
     When the image receiving tape  4  lies between the blade  58  and the anvil  60 , the blade  58  cracks the upper polyester layer  4   a  while leaving the backing layer  4   b  intact to make a tab cut as designated by reference numeral  200  in FIG.  5 . In the preferred embodiment and as described in our copending Application No. 9212423.9 a cut is simultaneously made through all of the layers of the image receiving tape to cut off a portion of tape once printed by two cooperating blades operating as scissors. The scissors can be driven by the motor  36  to cut off a portion of tape while the blade  58  makes the “tab-cut”. 
     The cutting mechanism operates as follows. As the cam  46  rotates, the tab cut lever arm  50  is caused to move in the track  48  to bring the blade holder  56  from an inactive position spaced from the tape  4  into a cutting position where it brings the blade  58  against the anvil  60  with the tape in between. At the same time, the first scissor blade is brought into contact with the second scissor blade to perform a scissor cut. Thus, a portion of a printed tape is cut off while a tab cut  200  (see FIG. 5) is made at a short distance from the main cut. 
     The cutting mechanism operates with a dynamic braking system which will now be described. The cam  46  carries on its outer surface a protrusion  64 . A microswitch  62  is mounted underneath the cassette receiving bay as shown in FIG.  3 . FIG. 6 is a block diagram showing the main elements of the motor control circuitry. The microswitch  62  provides a signal to a microprocessor  100  for controlling the motor  36  and a brake circuit  102 . The microswitch is normally open during rotation of the cam, except when the protrusion  64  is brought into contact with it, whereupon it is closed. The protrusion  64  is located so that it contacts the microswitch  62  in a position that the blade  58  is at its furthest location from the tape  4 . The motor is an ordinary electric motor having a stator comprising permanent magnets which provide a magnetic field within which a wound rotor rotates. On receipt of a cut instruction, current is supplied to the rotor windings of the motor and the rotor rotates thus driving the worm gear  38 , the gear train  40 , 42 , 44  and thence the cam  46 . The cam  46  completes a full rotation bringing the blade  58  from its position furthest away from the tape  4  into contact with the tape to make a tab cut and then returning the blade  58  to its furthest position. In the basic implementation as described in our earlier Application, at this point the current supplied to the rotor windings of the motor is ceased. However, there is still a significant amount of inertial energy in the drive mechanism which means that the rotor of the motor continues to rotate for a short time even after current supply to the rotor windings has been cut off. This is prevented by the protrusion  64  coming into contact with the microswitch  62  with the blade  58  in its position furthest away from the tape. The microswitch passes a signal to the microprocessor  100  which responds by issuing a signal to the brake circuit  102  which acts to short circuit the terminals of the motor directly or through a low resistance thereby applying a braking torque and bringing the rotor rapidly to a standstill. 
     Reference is now made to FIGS. 4 a  to  4   e  to describe an accurately constructed tab cut assembly. FIG. 4 a  is a view of the tab cut assembly from one end, comprising the blade holder  56  with a blade  58 . The blade holder  56  comprises an insert molded plastics body  57  having two supporting surfaces  57   a , 57   b  which extend on either side of the blade  58 . It also has a lower protrusion  59  with angled surfaces by which the holder can be resiliently mounted in a support body  61  (see FIG.  2 ). 
     FIGS. 4 b  and  4   c  are side views of the plastics body  57 , showing that it has a plurality of vertical ribs  104  on each side. FIG. 4 d  is a view from above of the body  57  and FIG. 4 e  is a section taken through the body  57  and showing the ribs and the blade in more detail. The ribs provide the body  57  with additional strength. 
     The assembly is such that the tip of the blade  58  protrudes beyond the support surfaces  57   a , 57   b  by a small but controllable amount, for example 100 to 120 microns, with a tolerance of the order of 20 um. 
     FIG. 8 shows an end view of the molded tab cut assembly in more detail. 
     Reference numerals  210 , 220  show regions into which plastic can be injected into the mold in two balanced positions such that the blade  58  is not shattered, or the cutting edge damaged during injection of the plastic. 
     Grooves  222 , 224  are provided either side of the blade  58  to ensure that adhesive build-up in use does not stop the blade  58  from cutting through the upper layer of the tape by the correct distance. 
     The surfaces  226 , 228  of the support portions  57   a , 57   b  which define the controlled cutting depth can be molded accurately and are close enough to the blade that a flat, hard, support can be produced to allow reliable cutting to be achieved. 
     Features are included in the blade shape to ensure it is retained securely in the finished molding. Furthermore, the molded wall thicknesses are such that the component shrinks after cooling whilst maintaining the blade protrusion dimensions and tolerances. 
     FIGS. 7 a  and  7   b  are an end view and a top view of an alternative tab cut assembly in which the surface of the holder which faces the tape in use has four “pips” or protrusions  300  which help to stop the blade  58  damaging the anvil when no tape is present and also help to prevent the blade cutting through all the layers of a narrow (6 mm) tape. The surface also has central raised portions  302  which help to overcome tolerance problems when the blade assembly is made. 
     It is easier to control the smaller area of the raised portions  302  to overcome the problem of plastic shrinkage during molding. 
     FIG. 9 is a side view showing two halves of an insert molding assembly for manufacturing the insert molded tab cut assembly described herein. The insert mold assembly comprises a fixed mold half  230  and a moving mold half  232 . The mold halves  230 , 232  define, when mated, an inner mold cavity having a mold surface conforming to the desired shape of the blade holder  56 . The fixed mold half  230  is provided with positioning pins  234  which serve to locate the blade  58  in a manner which will be described in more detail hereinafter with reference to FIG.  10 . For a similar purpose, the moving mold half  232  is provided with spring-loaded pins  236 , reference numeral  238  denoting the springs. The arrow A in FIG. 9 illustrates the direction of movement of the moving mold half  232  to the fixed mold half  230 . The arrow B denotes the movement of the blade  58  towards the positioning pins, cutting side to the right in FIG.  9 . 
     FIG. 10 shows how the blade  58  is located against the fixed mold half  230 . The spring-loaded pins of the moving mold half  232  cause the blade  58  to be sprung towards the positioning pins of the fixed mold half  230 , thereby accurately guiding and locating it. The positioning pins  234  include upper and lower pins  234   a , 234   b  for guiding opposed longitudinal edges of the blade  58  and intermediate positioning pins  234   c , 234   d  which are split to receive and locate the blade on opposed surfaces thereof. The fixed mold half  230  is provided with a recess  240  which has a depth d defining the protruding height of the blade  58 . 
     The blade  58  has an aperture  242  through which plastic material flows and thus causes the blade to be held firmly in the final molded holder. Another possibility is to provide vertical ribs which extend along opposite blade surfaces. 
     Thus, the present invention provides a system which reliably cuts a plastic label through from the front whilst leaving the peelable release layer substantially intact. 
     In addition it provides a system which can cut reliably tapes of differing widths, for example 6 mm, 12 mm and 19 mm in a fully automatic cutting system. The described cutter can cut the label tape reliably from the front to a very tightly controlled depth, typically 100 microns, for a 74 micron plastic tape having a total thickness of 135 microns including adhesive and peelable release layer. Further, the cutter anvil is not damaged and the blade has a long life. 
     These attributes are provided in one aspect by the ceramic blade which is insert molded into the plastic body to provide a very accurate blade protrusion distance. Insert molding and ceramic blades are of course known per se but the special combination required to achieve the desired performance for this application are new. In particular, by selecting the correct ceramic material and grinding conditions it is possible to produce a blade which can cut through the tape with forces of less than half that required with steel blades. Further, by selecting the correct plastic material for molding it is possible to achieve a blade protrusion tolerance of less than 20 microns. This tolerance is for example significantly less than the material shrinkage encountered during cooling after the molded process. Also, by correct design of the molding tool it is possible to ensure that the desired tolerances are achieved without damaging the ceramic blade, or its cutting edge, during the molding process.