EP1090769A2

EP1090769A2 - Temperature control system for a contact printer

Info

Publication number: EP1090769A2
Application number: EP00308790A
Authority: EP
Inventors: Richard Lee Campbell
Original assignee: Allen Coding Systems Ltd
Current assignee: Thermo Allen Coding Ltd
Priority date: 1999-10-05
Filing date: 2000-10-05
Publication date: 2001-04-11
Also published as: EP1090769A3; GB9923508D0; GB2354974A

Abstract

A temperature control system is described that is suitable for use in a contact printer that is adapted for printing onto a moving substrate. A heater such as a high wattage resistor is attached to a metal plate extending from a print head assembly in a contact printer. A heater control unit (87) is arranged to control the heater (82) on the basis of the output of a thermistor (83) in the print head assembly together with a pre-specified voltage level (84). Using the heater, the temperature around the print head and of foil in the printer is increased relative to the room temperature and this improves print quality and efficiency in cold environments such as cold rooms where food is packed. The tendency of foil in the printer to break and crease in cold environments is also reduced.

Description

Field of the Invention

This invention relates to a temperature control system suitable for use in a contact printer. The invention also relates to contact printers incorporating this type of temperature control system 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 includes a plurality of heat-generating elements which are selectively energisable via control lines. These heat-generating elements or pixels 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 replaced with a new cassette.
A problem arises when contact printers are used to print onto products in a cold environment. For example, contact printers are often used to print codes onto food packaging and this needs to take place in cold rooms where the food is packed. In these conditions the room temperature is as low as 6° C and this leads to the foil used in contact printers becoming brittle and snapping easily. When the foil breaks in this way, the printer must be stopped and the cassette of foil replaced. This is time consuming and means that valuable printing time is lost. Also, the foil itself is expensive and because new cassettes are required each time the foil breaks costs are increased. Another problem is that at low room temperatures the ink in the foil is not released properly and in the worst cases is not released at all. This reduces the quality of the resulting print and means that some products must be re-printed. In order to meet regulations, food and other products must be correctly coded and because of this, it is essential that the printing process is effective.
The problems mentioned above which arise at low room temperatures are particularly acute for contact printers that are used to print onto a moving substrate. In this type of contact printer the print head is generally arranged to be static and the substrate along with the foil or ribbon is arranged to move past the print head at substantially the same speed. The speed varies depending on the processing time but can be up to 600 mm/sec. This type of contact printer is described in detail in the Applicant's pending UK patent application number 9910255.0.
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.
Accordingly, it is an object of the present invention to provide a print head temperature control system suitable for use in a contact printer which overcomes or mitigates some or all of the above disadvantages.

Summary of the Invention

According to a first aspect of the present invention there is provided a temperature control system suitable for use in a contact printer comprising a print head, said temperature control system comprising:-
(i) a heater associated with said print head but disassociated with any control lines for directly heating print elements of said print head;
(ii) a monitor arranged to monitor a parameter related to a temperature associated with said print head; and
(iii) heater control means adapted to control said heater on the basis of said monitored parameter; such that in use, if said parameter falls below a prespecified level the heater is activated.
According to another aspect of the present invention there is provided a corresponding print head assembly suitable for use in a contact printer said print head assembly comprising:-
(i) a print head;
(ii) a heater associated with said print head but disassociated with any control lines for directly heating said print head;
(iii) a monitor arranged to monitor a parameter related to a temperature associated with said print head; and
(iv) an output arranged to provide an output from said temperature monitor, such that in use, said output may be provided to a heater control means adapted to control said heater on the basis of said monitored parameter.
According to another aspect of the present invention there is provided a method of contact printing onto a substrate using a contact printer comprising a print head said method comprising the steps of:-
(i) providing heat around said print head using a heater that is disassociated with any control lines for directly heating said print head;
(ii) monitoring a parameter related to a temperature associated with said print head;
(iii) controlling said heater on the basis of said monitored parameter; and
(iv) carrying out a contact printing action onto the substrate.
This provides the advantage that the print head temperature is controlled such that printing is effective even at low room temperatures. Also, the efficiency of the printer is improved and the "life-time" of the print head is not detrimented and can even be increased. When the temperature control system is used in a contact printer, foil in the printer is prevented from becoming brittle and snapping easily. This means that cassettes of tape do not have to be replaced in the printer more often than is necessary.
Accordingly to another aspect of the present invention there is provided a contact printer incorporating a print head temperature control system as described above. This provides the advantage that the contact printer is able to operate without wasting tape because the tape is prevented from becoming brittle and snapping easily. Also, the printer is able to operate effectively at low room temperatures such as 6°C and the efficiency of the printer and life-span of the print head are improved.

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.
Figures 7 is a schematic graph of pixel temperature in a print head of a contact printer against time.
Figure 8 is a schematic block diagram of a print head temperature control system.
Figure 9 is a cross-section through a contact printer incorporating a print head and a print head heater assembly.
Figure 10 is an exploded view of a print head and a print head assembly for use in the contact printer of Figure 9.

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.
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.
In addition to the process of acceleration and deceleration of the foil between print actions, heating elements or pixels in the print head must be heated to the required ink-release temperature. For example, starting from a room temperature of about 20° C the pixels in the print head need to reach about 70° C in order for ink to be released from the foil. In between print actions, the print head is moved away from the foil and the temperature of the pixels is reduced in order to prolong the life of the print head. This has meant that an important factor in print head assembly design has been arranging for heat from the pixels to be dissipated quickly in-between print actions. For example, this has been achieved by provision of fins and other heat radiating elements around the print head.
Figure 7 is a schematic graph of pixel temperature, in a typical contact printer print head, in °C against time in microseconds. Line 71 in Figure 7 illustrates the case when the contact printer is in a room with a room temperature of 20° C. The heater elements in the print head are energised and this increases the pixel temperature to about 70°C after about 140 microseconds. At this point a print action occurs after which the power to the heater elements is stopped and the pixel temperature falls to between 20°C and 30°C after about another 30 microseconds. The elements in the print heat are then re-energised in order to heat the pixels to 70°C again for the next print action. As shown in Figure 7 this takes about a further 140 microseconds. Thus for the second print action, the "starting" pixel temperature is between 20°C and 30°C and is slightly higher than for the first print action. This is due to heat generated during the first print action.
Line 72 in Figure 7 illustrates the pixel temperature change in the situation that the contact printer is located in a cold room where the room temperature is about 6°C. Again the heater elements are energised but after about 140 microseconds the pixel temperature is only about 60°C which is not adequate for optimum ink release. The heater elements are next de-energised and the pixel temperature falls to between 10°C and 20°C. However, when the heater elements are again energised to heat the pixel elements ready for the next print action, the pixel elements only reach about 60°C at the time of the next print action. Over several repeated print actions, the "starting" temperature of the pixel elements gradually rises but this never provides a temperature change performance as for line 71 in Figure 7.
Line 73 in figure 7 shows the temperature change of pixel elements when using a print head temperature control system of the present invention in a contact printer that is located in a cold room with a room temperature of about 6°C. It can be seen that by using the print head temperature control system, the pixel temperature reaches the target of 70°C after 140 microseconds. Also the "starting" pixel temperature is around 40°C which is considerably higher than for the situations illustrated by lines 71 and 72 in Figure 7. This is unexpected, in that the print head temperature control system does not simply produce a step change in the temperature curve as is effectively the case between the 20°C room temperature and 6°C room temperature situations which do not use the print head temperature control system. This is advantageous because if the pixels in the print head reach a temperature greater than the target printing temperature this can cause damage to the print head. Each print head is expensive and damage to these needs to be prevented as far as possible. Also, if a print head fails and needs to be replaced, valuable printing time is lost and the operator is inconvenienced.
Figure 8 is a schematic block diagram of the print head temperature control system 80. This comprises a heater 82 which is attached, either directly or indirectly to a print head 81. The heater 82 may be a high wattage resistor or any other suitable means for providing heat. The print head temperature control system 80 also comprises a monitor for monitoring a parameter that is related to a temperature. In one example, this is a thermistor 83 which provides a voltage that is dependant on the temperature of the thermistor. However, it is not essential to use a thermistor, any suitable device for measuring temperature may be used.
In one example, the thermistor is incorporated into the print head and the output of the thermistor is used for other purposes besides the temperature control system described here. However, this is not essential. The thermistor can be located in any suitable place such that it effectively monitors the "temperature" around the print head.
A pre-specified voltage is recorded in the temperature control system and is represented by 84 in Figure 8. This pre-specified voltage may be set by an operator or may be set during manufacture of the print head assembly by the printer manufacturer. For example, the pre-specified voltage may be set at a value corresponding to between 35°C and 45°C. This pre-specified voltage is related to the required temperature in order for successful and efficient printing to take place.
The thermistor output is provided to a heater control unit 87 which also has access to the stored pre-specified voltage. The heater control unit 87 is arranged to control the heater 82 on the basis of a comparison between the thermistor output and the pre-specified voltage. For example, if the thermistor and pre-specified voltage comparison indicates that the temperature around the print head is less than required for efficient and successful printing then the heater 82 is activated by passing a current through the resistor. The comparison process then repeats and depending on the outcome of the comparison the heater is either switched off or left on until the thermistor output reaches the pre-specified voltage. Once the thermistor output does reach the pre-specified voltage the heater is switched off by the heater control unit 87. The temperature around the print head 81 then gradually drops because the room temperature is lower and as a result the thermistor output is again lower than the pre-specified voltage. At this point the cycle repeats.
The print head 81 should not be heated to a temperature about about 60°C, for example, in case of damage to the print head. Therefore, in one example of the temperature control system, a temperature control unit 85 is included. This receives the output from the thermistor 83 and from this is able to determine whether the temperature around the print head 81 is reaching a dangerous level. A second pre-specified voltage level is recorded in the temperature control unit 85 and if the thermistor output exceeds this second pre-specified level then the whole printer is shut down. If an output from the temperature control unit 85 indicates this situation then a processor 88 shuts down the printer. In this way damage to the print head, which is expensive and time consuming to replace, is avoided.
When current is passed through the high wattage resistor heater 82, the ambient temperature around the print head 81 rises and the temperature of foil in the printer also rises relative to the temperature in the rest of the room in which the printer is situated. It has been found that this prevents foil in contact printers from becoming brittle at low room temperatures such as 6°C.
When a operator first begins to use a contact printer incorporating the temperature control system he or she switches the contact printer on and leaves the printer for a few minutes to "warm up". During this time the heater control unit 87 activates the heater 82 as required inorder to pre-heat the print head assembly, foil and surrounding area. It has been found that this pre-heating and subsequent use of the temperature control system during printing significantly improves the performance and efficiency of the contact printer both when used in cold rooms of about 6°C and in environments with room temperatures between about 15°C and 25°C.
The high wattage resistor heater 82 is attached to the print head 81 via a metal plate or heat sink. This enables heat from the heater 82 to permeate the print head 81 and causes pixels in the print head to be warmed relative to the ambient room temperature. However, the heater 82 is not associated in any way with control lines within the print head which are selectively energisable in order to bring specified pixels in the print head to the release temperature for a print action. The print head has an expected "life time" which can be specified as a certain number of times that the pixels are brought to the release temperature. By using the heater 82 to heat the area around the print head and the body of the print head, rather than heating the individual pixels selectively, the life of the print head is not detrimented.
Figure 9 shows an example of a contact printer incorporating a heater 82. The heater is a resistor 82 that is attached to a metal plate 91 extending from the back of a print head assembly 92. No fins or other heat dissipating elements are incorporated into the print head assembly 92. Also, the resistor 82 is not located adjacent pixel elements within the print head. However, the metal plate 91 to which the resistor is attached extends along the back of the print head assembly.
Figure 10 is an exploded view of the print head assembly showing two main portions, a main body 100 which is designed to remain within the contact printer and a removable portion 101. The removable portion comprises the print head in order that this can easily be replaced if required. As well as this the removable portion comprises the resistor 82 together with connecting wires 103 to power the resistor 82 which plug into corresponding wires 104 in the main body 101. As shown, the resistor is located on a metal plate 91 which extends away from and is angled away from the print head 92 itself.
In one example, the temperature control system is incorporated into a contact printer that is designed to print onto moving substrates without wasting foil. An example of such a contact printer is described in detail below.
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 t₁ 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 foil, tape or ribbon to be accelerated up to the required substrate speed over any required length of foil, the so-called 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 term "tape" 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 foil travels from the "unused" spool 49 to the "used" spool 46.
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 returns 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 recorded 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^th 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 compared. 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

A temperature control system suitable for use in a contact printer comprising a print head, said temperature control system comprising:-

(i) a heater associated with said print head but disassociated with any control lines for directly heating print elements of said print head;

(ii) a monitor arranged to monitor a parameter related to a temperature associated with said print head; and

(iii) heater control means adapted to control said heater on the basis of said monitored parameter; such that in use, if said parameter falls below a prespecified level the heater is activated.
A temperature control system as claimed in claim 1 wherein said monitor comprises a thermistor arranged to provide an output voltage that is temperature dependent.
A temperature control system as claimed in claim 2 which further comprises comparison means adapted to compare said thermistor output voltage with a prespecified voltage.
A temperature control system as claimed in any preceding claim wherein said heater comprises a resistor.
A temperature control system as claimed in any preceding claim which further comprises means for deactivating said heater in the event that said monitored parameter reaches a second prespecified level.
A print head assembly suitable for use in a contact printer said print head assembly comprising:-

(i) a print head;

(ii) a heater associated with said print head but disassociated with any control lines for directly heating said print head;

(iii) a monitor arranged to monitor a parameter related to a temperature associated with said print head; and

(iv) an output arranged to provide an output from said temperature monitor, such that in use, said output may be provided to a heater control means adapted to control said heater on the basis of said monitored parameter.
A method of contact printing onto a substrate using a contact printer comprising a print head said method comprising the steps of:-

(i) providing heat around said print head using a heater that is disassociated with any control lines for directly heating said print head;

(ii) monitoring a parameter related to a temperature associated with said print head;

(iii) controlling said heater on the basis of said monitored parameter; and

(iv) carrying out a contact printing action onto the substrate.
A contact printer comprising a temperature control system as claimed in any of claims 1 to 5.
A contact printer as claimed in claim 8 and suitable for printing onto a moving substrate, said contact printer further comprising a tape feed mechanism, 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.