GB2349605A - Thermal tape transfer mechanism wherein the tape is driven in a reverse direction following a print action for reuse thereof - Google Patents

Abstract

The mechanism includes two spools 46,49 between which a transfer tape 43 is fed via guide rollers 42. The spools, which are driven in opposite directions by a roller 44 and belt 45, are arranged such that only one spool is driven at any one time. In use, when the feed on spool 49 is driven, the tape is fed in a forward direction, and when the take up spool 46 is driven, the tape is fed in a reverse direction. Before a print action, the tape is accelerated to the same velocity as a moving substrate A and a print action occurs thereon via a printhead 41. The tape is then decelerated and re-wound during a non-print period of the printhead before it is accelerated again to the same velocity as the substrate for the next print action over the used tape. The arrangement reduces wasted tape.

Description

TAPE FEED MECHANISM Field, of the Invention This invention relates to a tape feed mechanism of the type which allows a tape to be fed in a forward and a reverse direction. The invention also relates to contact printers incorporating this type of tape feed mechanism and to a method of printing using such a contact printer.

Description of the prior art Thermal printers are known in which a print head is moved into contact with an ink foil or ribbon that is interposed between the print head and a reading medium. The print head inclues a plurality of heat-generating elements which are selectively energisable. These heat-generating elements contact the ink foil and cause ink to be released from the foil in the regions of the heat elements.

This type of thermal transfer printer is often used for over printing product codes on packaging used in the food and drink industry and on pharmaceutical products. Ink-jet printing typically cannot be used because the resolution provided is insufficient for the accurate production of bar codes and other product codes to be clear. Also, ink-jet printers are often not suitable for use in food packaging environments because of the danger of contamination from the ink jet process.

The foil used with thermal transfer printers has a carbon deposit on one side of the foil. The application of heat by the energised elements of the print head causes a transfer of carbon from the film onto the substrate to be printed. The ink foil itself is expensive and is typically provided in a cassette or cartridge. Once the cassette of foil is used up it is replace with a new cassette.

A problem arises when contact printers are used to print onto a moving substrate. The print head is generally arranged to be static and the substrate along with the foil or ribbon move past the print head at substantially the same speed. The speed can vary depending on the processing time but can be up to 600 mmlsec.

The fact that the foil must move at the same speed as the substrate during printing presents a number of problems. Firstly, if the foil moves at a constant speed then most of the foil will pass through the print head unused. This is wasteful of foil, which is an expensive consumable. There is also a practical disadvantage in that the foil cassette would empty relatively rapidly. Every time the cassette is empty the line must be stopped, the cassette changed and the line restarted. This is wasteful in time and manpower as well as frustrating for the operator. Production levels from such a line would also drop significantly.

A more normal arrangement is for the foil to stop moving between imprints.

Thus, as a region on the substrate where the head must print approaches, the foil is accelerated from a standing start to substantially the same speed as the line. Once printing has finished in that particular region then the foil is braked to a halt. This process is repeated as every printing region approaches and passes under the print head.

The acceleration/deceleration process inevitably means that there is a region of unused foil between each imprint. In the prior art printer manufacturers have used more and more sophisticated equipment to increase the rate acceleration of the foil.

However, the mechanical stability of the foil puts constraints on the improvement possible by this route. in any event, a certain spacing between impressions is inevitable. Typically the area of unprinted foil equals or exceeds the area of printed foil. Accordingly, it is an object of the present invention to provide a tape feed mechanism which overcomes or mitigates some or all of the above disadvantages.

Summarv of the Invention According to a first aspect of the present invention a tape feed mechanism suitable for use in a contact printer incorporating a print head and adapted for printing onto a moving substrate said tape feed mechanism comprising: (i) A first spool mechanism for containing substantially unused tape; (ii) A second spool mechanism for containing substantially used tape; (iii) Drive means adapted to drive each of said spool mechanisms such that after printing has taken place onto a moving substrate used tape can be rewound onto the first spool mechanism during a period when printing is not taking place such that subsequent acceleration of the tape past the print head takes place over substantially used as opposed to unused tape.

This provides the advantage that the tape is quickly and easily brought up to the speed of a moving substrate such that contact printing can take place effectively without creasing or tearing of the tape. This is achieved without wasting tape and so that cassettes of tape do not have to be replaced in the printer more often than is necessary.

Preferably said drive means comprises a single drive belt and drive roller. This provides the advantage that only one drive motor is required to drive both spool mechanisms and this saves costs as well as space.

It is also preferred that said spool mechanisms each incorporate a one-way clutch mechanism such that each spool mechanism can only be driven in one direction. In this way the spool mechanisms are similar to one another and simplified in their construction.

Advantageously said spool mechanisms are substantially identical except for the orientation of the one-way clutch mechanisms. This enables the spools mechanisms to be swapped over in the event of a malfunction and for manufacturing and maintenance costs to be reduced.

Preferably each spool mechanism comprises a drive clutch adapted to impart drive from the drive means to the spool mechanism except when the tension in the tape is greater than a threshold level. This provides the advantage that the same amount of tape can be indexed to or from each spool mechanism no matter what effective radius each spool has as a result of foil or tape stored on that spool.

Advantageously the tape feed mechanism as described immediately above further comprises a drive control means adapted to control the drive means on the basis of a plurality of stored values and also on the basis of information derived from the motion of the substrate. This enables the spool mechanisms and hence the movement of the tape to be controlled, for example to ensure that the tape is moving at a certain speed.

Preferably the drive control means comprises an encoder means adapted to convert movement of the substrate into a series of pulses and a look up table of predetermined optimum values of motor drive speed compared to pulse rate, the control means being adapted to bring the tape up to substantially the same speed as the substrate in optimum time and to maintain it at that speed until a print region appears beneath the print head. This provides the advantage that the drive means may be controlled in such a way as to ensure that the tape is moving at the same velocity as the substrate before a print action occurs.

Accordingly to a second aspect of the present invention there is provided a method of contact printing from a tape and print head onto a moving substrate comprising the steps of :- (a) accelerating the tape up to substantially the same speed as the moving substrate; (b) carrying out a contact printing action to the substrate; (c) decelerating the tape ; (d) rewinding the tape by a pre-determined amount; (e) repeating steps (a) to (d) inclusive as necessary.

This provides the advantage that unused tape is not wasted by being used during an acceleration process to accelerate the tape up to the same speed as a moving substrate. Any index-gap is substantially reduced.

Accordingly to a third aspect of the present invention there is provided a contact printer incorporating a tape feed mechanism as described immediately above. This provides the advantage that the contact printer is able to operate without wasting tape because any index-gap between spent regions of tape is substantially reduced.

Brief description of the drawings Figure 1 is a schematic diagram of a contact printing process for printing onto a moving substrate; Figures 2 and 3 are plan views of a piece of foil showing the spacing between adjacent print impressions; Figure 4 is a plan view of a foil indexing mechanism according to the present invention; Figure 5 is a cross-section through a take up spool mechanism; Figure 6 is a graph of foil speed against time as the foil accelerates and decelerates.

Detailed description of the invention Embodiments of the present invention are described below by way of example only. These examples represent the best ways of putting the invention into practice that are currently known to the Applicant although they are not the only ways in which this could be achieved.

By way of background information, Figure 1 illustrates schematically a contact printer 12 that is arranged to print product codes 15 onto a substrate 14. The substrate 14 can be a tube of plastics material for packing products 11 such as loaves of bread, as shown in this example. The substrate 14 is stored on a roll 10 and drawn in the direction of the arrow in figure 1. The print head 12 moves down into contact with the substrate 14 and presses foil 13 against the substrate to print a product code onto the empty substrate. The tube of substrate 14 is then drawn over loaves of bread 11 and the sides of the tube sealed together between each loaf of bread using heat sealers or other known apparatus (not shown). Because the substrate 14 is continually moving then the foil 13 must be arranged to also move at substantially the same speed and in the same direction as the substrate during the printing process, otherwise the foil 13 simply creases up or tears. The printing mechanism therefore needs to include a foil indexing apparatus that can arrange for the foil to move parallel to the substrate and at the required speed. However, if the foil 13 is arranged to continually move at this required velocity then large regions of the foil remain unused. That is, the distance between each spent region of foil is the same as the distance between two printed product codes 15 on the substrate 14. This is extremely wasteful of foil and very expensive.

Figure 2 illustrates a piece of foil 13. Regions 16 on the foil 13 indicate spent areas which have been used for printing and where the ink has been used. The distance 17 between the spent regions 16 represents wasted foil and is often referred to as an index gap.

In order to minimise the wasted foil between spent regions 16 known printing machines arrange for the foil 13 to stop moving in-between each print action. The foil 13 is accelerated up to the same speed as the substrate at which point the printing action takes place and then the foil is stopped. However, this still produces wasted foil that is moved on during the acceleration and deceleration of the foil.

* Figure 6 shows a typical graph of foil speed against time. Curve 61 represents the speed of the foil during one acceleration, print action and stopping of the foil.

During time t, the foil is accelerated up to speed A which is the speed of the substrate.

The foil is then maintained at this constant velocity A during the contact printing action which can involve up to about 120 printed lines. Then in time t2 the foil is decelerated until it is effectively stationary.

Known indexing and tape feed mechanisms have tried to reduce the time t1, by using various mechanical arrangements, usually of increasing complexity. There is obviously a limit to the improvement in spacing between adjacent print impressions that can be achieved by this type of technology. Clearly a different approach is required.

Figure 4 is a plan view of a foil indexing or tape feed mechanism according to the present invention. This indexing mechanism allows fol, tape or ribbon to be accelerated up to the required substrate speed over any required length of foil, the socalled index gap. However, the foil or tape feed mechanism is able to re-wind or reverse the foil in-between print actions so that acceleration takes place over used foil.

In this way only a minimal amount of foil is wasted. The tenm utapen is used to refer to any type of foil, ribbon or similar medium that is used to carry ink or other printing material for contact printing.

The term"spool mechanism"is used to refer to any support, such as a guide post or roller, about which tape can be wound such that that tape can be spooled to and from the spool mechanism.

In figure 4, unused foil is stored on a first, feed on spool 49 and passes around rollers or guides 42 and past a print head 41. Packages or goods A are positioned below the print head on a platform which moves in the direction arrowed M. The print head 41 is movable up and down onto the foil 43 so that the foil is pressed into contact with a package A and a product code can be printed onto the package. The foil passes over another roller or guide 42 and onto drive roller 45. From drive roller 45 the foil passes around another guide 42 and onto a second, take up spool 46.

A drive belt 44, which may be a toothed belt, passes around the drive roller 45 and the take up spool 46. This drive belt 44 is also arranged to pass around feed on spool 49. However, this is not essential. Two separate drive belts could be used, one linking the drive roller 45 and the take up spool 46 and the other linking the drive roller 45 and the feed on spool 49. In this example the drive belt 44 passes around the core of the take up and feed on spools 46,49 so that the drive belt 44 is not affected by changes in the radii of these spools as foi travels from the"unused"spool 49 to the "used"spool46.

The take up spool 46 and the feed on spool 49 both incorporate a one-way sprag clutch so that these spools may only be driven in one direction and in the other direction they simply slip. It is not essential to use sprag clutches. Any mechanism suitable for allowing rotation of the spool in only one direction can be used. These sprag clutches are arranged to operate in opposite directions on each spool 46,49.

That is, when the drive roller 45 drives the belt 44 in an anticlockwise direction in the arrangement illustrated in Figure 4 then, the take up spool 46 is driven in an anticlockwise direction to take up foil. The feed on spool 49 slips against the drive belt 44 to dispense foil and is not positively driven in an anticlockwise direction. Similarly, if the drive roller 45 drives belt 44 in a clockwise direction, the feed on spool is driven in a clockwise direction to draw foil back onto the feed on spool 49. In this case the take up spool 46 slips against the drive belt 44 to dispense used foil and is not positively driven in a clockwise direction.

Both the take on spool 46 and the feed on spool 49 are each provided with a brake spring 47,48. Each break spring 47,48 passes over a pulley attached to its respective spool and is secured under tension to a fixed part of the tape feed mechanism. The break springs 47,48 prevent each of the take up and feed on spools 46, 49 from over-running after that spool has been driven by the drive roller 45 such that the spools have a natural inertia.

Figure 5 is a cross-section through a take up spool 46 mechanism. As already mentioned the take up spool 46 has a one way sprag clutch 55. The feed on spool mechanism is identical to the take up spool mechanism except that the direction of the sprag clutch 55 is reversed. This is particularly advantageous because the manufacturing process is simplified.

Figure 5 shows a central shaft 59 of the take up spool 46 which is fixed to a magazine plate upon which the tape feed mechanism is supported. A sleeve 57 is threaded over the central shaft and is able to rotate about the central shaft 59 but not to move up and down the shaft 59. A one way sprag clutch 55 passes around the sleeve 57 and acts to allow the sleeve 57 to rotate about the shaft 59 in one direction only. Drive belt 58 acts on the sleeve 57 via the sprag clutch so that the drive belt 58 is only able to rotate the sleeve 57 in one direction. A break spring 56 also acts on the sleeve 57 in order to prevent the sleeve 57 from continuing to rotate after being driven by the drive belt 58. Any suitable damping or inertia means for damping the motion of the sleeve 57 can be used instead of the break spring 56.

Foil 52 is held on the take up spool using a foil sleeve which is also threaded over the central shaft 59 and able to rotate about the central shaft. The foil sleeve 50 is positioned above the sleeve 57 with a clutch pad 54 located between the foil sleeve 50 and the sleeve 57. The foil sleeve 50 is held down against the clutch pad 54 by a coil spring 53 which is threaded onto the central shaft 59 above the foil sleeve 50 and held in place by a head 51 at the top of the central shaft 59.

When the drive belt 58 drives the take up spool 46 anticlockwise the clutch 54 transfers the rotational force from the sleeve 57 to the foil sleeve 50. This is because the downward force from the coil spring 53 is sufficient to engage the clutch pad 54 against the two sleeves 57 and 50. Thus as the sleeve 57 rotates, the foil sleeve 50 also rotates and foil 52 is wound onto the take up spool 46. However, as the foil continues to be driven, the tension in the foil 52 gradually increases and eventually reaches a certain threshold level. At this point the tension in the foil is great enough that an upward component of this force is greater than or equal to the downward force from spring 53. The clutch pad 54 is then no longer engaged between the two sleeves and rotation from sleeve 57 is not transferred to foil sleeve 50. This means that foil sleeve 50 slips despite drive from drive belt 58 being applied. This allows the same amount of foil to be drawn onto the take up spool no matter what effective radius the take up spool has.

The operation of the indexing and control mechanism will now be described with reference to Figures 1, 2,3 and 4. At first the foil 13 is stationary and needs to be accelerated to substantially the same velocity as the substrate 14. Referring to figure 4, the take up spool 46 is driven in an anticlockwise direction in order to take up foil 43.

This is done by driving the drive belt 44 in an anticlockwise direction. As described above, when the drive belt 44 drives the take up spool 46 the feed on spool 49 is not driven but slips because of the sprag clutch 55. The take up spool 46 draws foil 43 onto itself and off the feed on spool 49. A system of markers is used to determine when the foil is being drawn at the required velocity and this will be described in more detail below. At the required foil velocity the print head 41 is moved into contact with the foil 43 and printing takes place onto a pre-determined printing zone. After the print action, the print head 41 is moved away from the foil 43 to its rest position and the drive roller 45 stops. The break spring 48 acts to prevent the feed on spool 49 from "free wheeling"and the foil velocity retums to zero.

The next stage involves re-winding foil back onto the feed on spool. The drive roller 45 is driven in the opposite direction (clockwise) so that the feed on spool 49 is driven in a clockwise direction and the take on spool 46 slips. Any slack in the foil 43 is taken up. When the required amount of foil 43 has been rewound the drive roller 45 is stopped and the foil velocity again returns to zero. At this point, break spring 47 acts to prevent the take up spool 46 from"free wheeling"and continuing to unwind foil after the drive belt 44 has stopped.

As the next printing zone approaches the print head the drive roller 45 is then driven again in an anticlockwise direction in order to bring the foil velocity up to the required level for the next print action. Foil 43 is drawn off the feed on spool 49 in order to do this. The foil 43 that is drawn off the feed on spool mostly comprises foil 43 that was rewound onto the feed on spool in the previous re-wind stage. The first print action involves accelerating the foil 13 over a distance x and then creating spent region of foil 16A. The foil is then decelerated using foil 13 over a distance y from spent region 16A. The foil is then rewound by an amount z before being accelerated over a distance x again and creating spent region 16B. In this way the spent regions of foil 16 are located close together and wastage is reduced. The rewind distance z is approximately equal to the acceleration distance x plus the size of the print regions 16A, 16B, index gap 17, and deceleration distance y.

Another problem involves ensuring that the product codes are printed at the correct intervals on the substrate 14. This problem is associated with determining when the foil is being drawn at the required velocity for printing. Prior art systems have addressed this problem by directly monitoring the speed of the substrate 14.

However, this is complex and expensive to implement.

In the present invention, an encoder wheel that is driven as the substrate 14 moves is provided. The encoder wheel creates a fixed number of pulses per unit displacement of the substrate 14, for example, 12 pulses per mm. This means that the required interval or distance between product codes on the substrate 14 is equivalent to a fixed number of pulses, for example 1000 pulses. That is, after 1000 pulses have occurred a new print action should take place. Using pulses in this way is advantageous because the velocity of the substrate 14 often fluctuates during the printing process. The pulses are used to monitor the required distance in a way that is independent of the velocity of the substrate.

In order to ensure that the foil is moving at the same velocity as the substrate 14 when a print action occurs, the motor which drives drive roller 45 is controlled on the basis of the pulses that have been recorde since the first pulse in that cycle occurred. A look-up table is predetermined and stored in a microprocessor. For pulses 1 to 1000 the look up table indicates the appropriate motor level or drive speed for the particular time that has elapsed since the first pulse occurred. The pulses from the encoder wheel are monitored and the motor level adjusted according to the entries in the look up table. In this way the motor drive increases as-the number of pulses increases and the foil is accelerated until at the 1000 pulse the foil velocity is at the required level i. e. approximately the same velocity as the substrate 14. The entries in the look up table are predetermined so that the motor will be controlled in such a way as to achieve acceleration to the required velocity in a short time. A phase lock loop is used to create 32 pulses between each pulse from the encoder wheel. This enables more entries in the look up table to be made to give finer control of the motor.

It is also possible to use a second encoder wheel that is driven as the foil or tape 13 moves. This second encoder wheel creates a fixed number of pulses per unit displacement of the tape. In order to determine when the tape 13 and substrate 14 are moving at the same speed the pulses from the first and second encoder wheels may simply be compare. When these pulses correspond and move at the same rate the tape and substrate are moving at substantially the same speed.

Using this method it is not necessary to monitor the time that has elapsed since the first pulse occurred and the look-up table is used in a similar way as described above.

Claims (12)

1. CLAIMS 1. A tape feed mechanism suitable for use in a contact printer incorporating a print 'head and adapted for printing onto a moving substrate said tape feed mechanism comprising: (iv) A first spool mechanism for containing substantially unused tape; (v) A second spool mechanism for containing substantially used tape; (vi) drive means adapted to drive each of said spool mechanisms such that after printing has taken place onto a moving substrate used tape can be rewound onto the first spool mechanism during a period when printing is not taking place such that subsequent acceleration of the tape past the print head takes place over substantially used as opposed to unused tape.
2. 2. A tape feed mechanism as claimed in claim 1 wherein said drive means comprises a single drive belt and drive roller.
3. 3. A tape feed mechanism as claimed in claim 1 or claim 2 wherein said spool mechanisms each incorporate a one-way clutch mechanism such that each spool mechanism can only be driven in one direction.
4. 4. A tape feed mechanism as claimed in claim 3 wherein said spool mechanisms are substantially identical except for the orientation of the one-way clutch mechanisms.
5. 5. A tape feed mechanism as claimed in any preceding claim wherein each spool mechanism comprises a drive clutch adapted to impart drive from the drive means to the spool mechanism except when the tension in the tape is greater than a threshold level.
6. 6. A tape feed mechanism as claimed in any preceding claim which further comprises a drive control means adapted to control the drive means on the basis of a plurality of stored values and also on the basis of information derived from the motion of the substrate.
7. 7. A tape feed mechanism as claimed in Claim 6 wherein the drive control means comprises an encoder means adapted to convert movement of the substrate into a series of pulses and a look up table of pre-determined optimum values of motor drive speed compare to pulse rate, the control means being adapted to bring the tape up to substantially the same speed as the substrate in optimum time and to maintain it at that speed until a print region appears beneath the print head.
8. 8. A tape feed mechanism substantially as described herein with reference to and as illustrated in the drawings.
9. 9. A method of contact printing from a tape and print head onto a moving substrate comprising the steps of :- (a) accelerating the tape up to substantially the same speed as the moving substrate; (b) carrying out a contact printing action onto the substrate; (c) decelerating the tape; (d) rewinding the tape by a pre-determined amount; (e) repeating steps (a) to (d) inclusive as necessary.
10. 10. A method of contact printing substantially as described herein with reference to and as illustrated in the drawings.
11. 11. A contact printer incorporating a tape feed mechanism as claimed in any of Claims 1 to 8 inclusive.
12. 12. A contact printer substantially as herein before described with reference to and as illustrated in any suitable combination of the accompanying drawings.