GB2204834A

GB2204834A - Improvements in or relating to thermal label printers

Info

Publication number: GB2204834A
Application number: GB08811554A
Authority: GB
Inventors: Christopher Raymond Eagle; Paul Griffith
Original assignee: Portals Engineering Ltd
Current assignee: Portals Engineering Ltd
Priority date: 1987-05-14
Filing date: 1988-05-16
Publication date: 1988-11-23
Anticipated expiration: 2008-05-16
Also published as: GB8711404D0; GB8811554D0; GB2204834B

Abstract

In a thermal printer the printing element array (23) is supported on a carrier 22 which is accepted in a holder 20 and retained therein by axial insertion of a pin 26. When so mounted the carrier is able to pivot through a limited angular range about the pin, and is biassed by a pneumatic arrangement (28)(30) towards the substrate to be printed upon. The whole printhead assembly (20, 22, 23,) is axially slidable along rods (19) from the head changing position (shown in Fig 4) to the operating position. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO THERMAL LABEL PRINTERS This invention relates to thermal label printers and is particularly, although not exclusively applicable to printers for use in weigh/price labelling machines.

The invention provides a thermal label printer for a labelling apparatus comprising a thermal print cartridge having a thermally actuated printing face for printing on a thermal label, a holder for the cartridge and means to mount the holder in the cartridge with the printing face exposed and to permit the cartridge to float over the limited range of movement with respect to the holder to project the printing face from the holder and biasing means to bias the cartridge outwardly of the holder.

In one specific construction a spindle is mounted in the holder extending across the holder and the cartridge is pivotally mounted on the spindle to move over a limited range towards and away from the holder.

Preferably, the spindle is removable from the holder to allow the cartridge to be extracted from the holder.

In any of the above arrangements the means to bias the cartridge outwardly of the holder may comrpise pneumatically operated rams.

The following is a description of a specific embodiment of the invention, reference being made to the accompany drawings in which: Figure 1 is a general view of a thermal label printer installed in a weight/price label machine; Figure 2 is a similar view to Figure 1 showing a thermal printing device of the printer partially extracted from the apparatus seen from below; Figure 3 is a similar view to that of Figure 2 showing the device as seen from above; Figure 4 shows a release of the cartridge of the thermal printer; and Figures 5 and 6 show details of the power supply circutry to the cartridge.

Referring firstly to Figure 1 of the drawings, there is shown a thermal label printer installed in a weigh/price labelling machine comprising a supply 10 of web carrying thermally sensitive labels 11. The web is fed through a tracking roller 12 underneath a thermal printer indicated generally at 13 around a stripping beak 14 where the backing from the web releases a label to a temporary label holding device 15 from whence the labels are transferred by a pick-up and suction head assembly 16 to be applied to articles. The stripped web then passes back through a nip roller 17 to a take-up roll 18.

The thermal printing head 13 is illustrated in greater detail in Figures 2 and 3 to which reference will now be made. The head is mounted on a pair of sliding rods 19 to permit the head to be extracted from the machine. The head comprises a block form holder 20 having a recess 21 in its underside in which a plate 22 is mounted on the underside to which a thermal printing cartridge 23 is secured. The cartridge has printing elements on its substrate activated by an electrical power circuit embodied in the cartridge and having electrical connection 24 to a weigh/price head which determines the material to be printed on the label.The holder has end lugs 25 between which the plate 22 is supported and a spindle 26 extends through the lugs and through a bore in the plate to support the plate for limited rocking movement with respect to the holder to vary the extend to which the cartridge projects from the holder. The spindle has a knurled knob 26a at one end and is held by a spring loaded ball into the lug of the holder. Thus the plate can readily be extracted from the holder by withdrawing the spindle as shown in Figure 4.

The plate 22 is biased downwardly to hold the cartridge 23 downwardly against a label to be printed on by means of a pair of pneumatic cylinders 28 mounted on the base of the holder and having extendable pistons 29 as can be seen in Figure 3 and connected to an air supply through conduiting 30.

Air pressure supplied through the conduit causes the pistons 29 to extend and bear on the back of the plate 22 to bias the plate downwardly and therefore hold the thermal printing plate 23 against the label to be printed for the printing operation. The air pressur supplied may be adjusted to suit the head to be utilised and may be arranged to be relieved when not printing to reduce head wear.

It will be appreciated that since the holder is readily extractable from the machine and the cartridge with its supporting plate is readily releasable from the holder, the cartridge can easily be replaced as and when necessary.

In accordance with further features of the invention, the thermal printing cartridge 23 contains heating elements to maintain the print head surface at a constant working temperature. The heating elements comprise two power transistor packages TR1 and TR2 connected in an electrical power circuit 31 embodied in the cartridge. The temperature of the print cartridge substrate is monitored using a thermistor (not shown) mounted in the cartridge. The thermistor resistance is converted into a digital quantity by means of an analogue to digital convertor. A software control loop compares the digital quantity to another representing the desired temperature and energises the heating transistors whenever the current temperature is less than the desired temperature.

The circuit 30 also includes a calibration resistor R1 which is used to programme the power supply output voltage. It is a characteristic of thermal print cartridges that the resistance of the printing elements varies considerably in manufacture. In order to obtain consistant printing, it is necessary to maintain a constant power in the printing elements. This is normally achieved by adjusting the voltage of the print head power supply to suit the printing element resistance. In accordance of the feature of the present invention, the power supply output voltage is programmed by an external resistance. This programming resistor is split into a fixed part which is mounted on an electronic circuit board inside the label printer and a variable part R1 which is mounted inside the print head cartridge and wired to a connector. The value of the latter resistor is selected during the manufacture of the cartridge to suit the resistance of the print head. Thus all print head cartridges can be arranged to operate at the same printing power.

Claims

1. A thermal label printer for a labelling apparatus comprising a thermal print cartridge having a thermally actuated printing face for printing on a thermal label, a holder for the cartridge and means to mount the holder in the cartridge with the printing face exposed and to permit the cartridge to float over a limited range of movement with respect to the holder to project the printing face from the holder and biasing means to bias the cartridge outwardly of the holder.

2. A printer as claimed in Claim 1, wherein a spindle is mounted in the holder extending across the holder and the cartridge is pivotally mounted on the spindle to move over a limited range towards and away from the holder.

3. A printer as claimed in Claim 2, wherein the spindle is removable from the holder to allow the cartridge to be extracted from the holder.

4. A printer as claimed in any of the preceding claims wherein the means to bias the cartridge outwardly of the holder comprise pneumatically operated rams.

5. A thermal label printer as claimed in any of the preceding claims, wherein the cartridge has electrical heating means for heating the printing face of the cartridge and control means are provided for controlling electrical power supply to the heating means to maintain the printing surface at a substantially constant working temperature.

6. A thermal label printer as claimed in Claim 5, wherein the control means is adjustable to enable the temperature at the printing surface to be set to a required level.

7. A thermal label printer as claimed in any of the preceding claims, wherein the cartridge includes electrical circuit means for actuating the printing elements on the printing face and the circuit includes calibration means to suit the characteristics of the printing elements to enable cartridges to be interchanged without adjusting the power supply to the cartridge.

8. A thermal label printer substantially as described with reference to and as illustrated in Figures 1 to 4 of the accompanying drawings.

9. A thermal label printer substantially as described with reference to and as illustrated in Figures 1 to 4 of the accompanying drawings as modified by Figures 5 and 6 of the accompanying drawings.