WO2003051641A1

WO2003051641A1 - Numberplates

Info

Publication number: WO2003051641A1
Application number: PCT/GB2002/002300
Authority: WO
Inventors: Richard Ivor Taffinder
Original assignee: Hills Numberplates Limited
Priority date: 2001-06-04
Filing date: 2002-06-05
Publication date: 2003-06-26
Also published as: AU2002257925A1

Abstract

A method of applying marking liquid to a sheet for a numberplate or the like using a thermal pring head, the method including the preliminary steps of heating the surface of the sheet and reducing the operating print head intensity (PHI) of the print head whereby the operating life of the print head is increased.

Description

NUMBERPLATES

The invention relates to numberplates or licence plates and in particular to numberplates comprising a substrate having a reflective or retroreflective surface.

Indicia are applied to a numberplate using an ink which is transferred from a source, such as a printer ribbon by printing means. Thermal transfer printers have been used to print indicia onto a substrate to form a numberplate. Such printers have a print head provided with pixel heaters which, under computer control, selectively heat portions of a printer ribbon to transfer colourant from the ribbon to the surface to be printed on.

Reflective and retroreflective surfaces tend to be difficult to print on to. This is due to a number of factors, the most important of which are due to the surface of a retroreflective material comprising glass beads embedded within a plastics matrix. The glass beads typically protrude up to about 5 micrometres from the plastics matrix. Thus, the printing surface is rough or uneven and the thermal conductivity along that surface may vary.

In order to print on such surfaces using thermal transfer printers, it has been proposed to increase the thermal energy of the print head and to increase the pressure applied to the substrate during printing by a back-up roller. The ink is typically applied at a relatively high speed. This increase in temperature aggravates the overall cost and reduces the life of the printing head. Further, the print quality is often poor. In PCT/US99/22388 (published as WO-A1 -00/68022) there is disclosed a method of printing on to retroreflective sheet material wherein the surface of the sheet material to be printed on is heated prior to printing. The heating of the sheet surface is carried out by inter alia a hot can roll. The surface is heated to a pre-heat temperature of 76.7 to 93.4°C.

It is an object of this invention to provide a method of printing on a sheet material, for example a reflective or retroreflective sheet, which is more economical and efficient than prior art methods and which results is a better or at least identical print quality.

A first aspect of the invention provides a method of applying marking liquid to a sheet for a numberplate or the like using a thermal print head, the method including the preliminary steps of heating the surface of the sheet and reducing the print head intensity (PHI), as defined below, whereby the operating life of the print head is increased.

Preferably the sheet is a retroreflective sheet material.

A second aspect of the invention provides a method of printing onto a retroreflective substrate, the substrate having two opposed major surfaces, the method comprising heating one major surface of the substrate and subsequently applying marking material to the other major surface of the substrate with a print head operating at a reduced PHI.

The operating PHI of the print head is preferably within the range of about 34 to about 58% and, most preferably, between about 40 and about 51 % of full PHI. The actual operating print head intensity will depend on the nature of the form and characteristics of the marking material, say ink, to be applied to the sheet material.

Preferably the sheet material is heated to a temperature of from about 20°C to about 100°C depending on its physical and chemical nature. Not only is it easier then to anchor or key an ink to the surface, but a better quality print product results.

It is a much preferred feature of the invention that the heating of the sheet surface is done for a short time period only. A heating device such as an infra red light device or the like is suitable. It is preferred to actuate the device for a short time period. Preferably the device is associated with a switch mechanism arranged to trigger the heating device for example when the surface to be marked passes thereby, and switch it off once the surface has passed.

The sheet material may be drawn passed the heater and through the printer at a speed of from about 6 cm/s to about 15.5 cm/s, most preferably from about 9cm/s to about 12 cm/s, and in a most preferred embodiment at 10.2 cm/s.

When the lamp is arranged to operate intermittently, the switch means will be arranged to operate the lamp such that the entire surface of the sheet material is illuminated and, consequently, heated. Thus, actuation of the lamp by said switch means will be determined by the rate of feed of the sheet material through the printer. Clearly, the rate of feed will be determined by the rate at which the printer can effectively transfer marking liquid from, say, a ribbon, to the substrate and by the effectiveness of the heater in heating the substrate (i.e. if the sheet material is drawn passed the heater at too high a rate, the sheet material will not be exposed to the heat for a sufficient period to be heated). Using intermittent operation of the lamp damage to the sheet material to be printed is less likely to occur.

Usually it is necessary to print different indicia or to apply different colours to one or more indicia in line to the substrate and several print heads may be arranged in line. Because of the heat capacity of the reflective substrate it may be useful to heat the area of the plate to receive the first indicium only, as the heat retention of the substrate may enable the temperature of subsequent print heads to be reduced. In one alternate arrangement, heaters may be associated with each print head.

The numberplate will typically have a multi-layer structure, but basically will usually comprise a retroreflective layer and a supporting layer. In one form, the plate consists of a film face layer such as polyethylene; adhesive; a retroreflective layer carrying thermal printing indicia; adhesive; and a back supporting layer made of, for example, acrylonitrile butadiene styrene (ABS). The supporting layer may have a thickness of about 2 mm to about 6 mm.

In one embodiment, the numberplate comprises a rigid transparent front face layer; adhesive, a retroreflective sheeting carrying indicia and; adhesive and a polyethyleneterephthalate backing film. The front face layer may have a thickness of about 2 mm to about 3 mm to give strength.

In another embodiment the number plate comprises a relatively thin PMMA film face layer; adhesive; retroreflective sheeting carrying indicia; adhesive; adhesive layer; and relatively thicker backing and support layer made of ABS. The face layer has a thickness of no greater than about 0.1 mm, to provide improved retroreflectivity and luminance. A suitable retroreflective sheet material is Scotchlite Diamond Grade sheeting supplied by 3M of St. Paul Minnesota.

The printing may be done in a wide variety of ways. Most preferably, however, a thermal transfer printer is used. Preferably the thermal transfer printing apparatus is controlled using a computer, the computer having data corresponding to numbers, letters and symbols to make up the indicia of the numberplate. The data is transmitted to the printer to apply the indicia to the numberplates. (Typically two plates are prepared for each vehicle, they being the front and rear numberplates.)

The printing material may be an ink, paint or the like and may be applied via a ribbon or transfer foil. The ink or paint may be made up of a wax, resins, pigments or the like in known manner. The ribbon may be a dual layer ribbon, by which is meant a ribbon have a base layer and a colourant layer, the two layers being applied to the retroreflective sheet material at the same time with the base layer being sandwhiched between the colourant layer and the sheet material. Using such a dual layer ribbon one ensures that the desired colour of the colourant layer is seen in the completed plate. The base layer is typically a white ink.

A suitable thermal transfer printer is a QLS-4100, supplied by Astro-med Incorporated of Rhode Island. When printing using such a printer, a potential difference is applied across the print head. The PHI determines the time of contact between the pixel heaters and the ribbon to impart some marking material onto the substrate. The potential difference applied across the print head is about 4.0 V in a QLS-4100, and it has a PHI range of 0 to 35 arbitrary units. The higher the value of PHI, the greater amount of contact time, and consequently the hotter the marking material and substrate becomes.

The invention includes a numberplate carrying indicia applied by the method. It also embraces a printing system including a heater for the sheet material.

In order that the invention may be more fully understood, it will now be described by way of example only with reference to the following diagrammatic drawings, in which:

Figure 1 shows a schematic diagram for part of a printing system; and

Figure 2 is a vertical section of a numberplate.

A computer 1 having data suitable for numberplates is connected to a thermal printer 2, having a print ribbon 21 and a print head 22. A suitable printer being a QLS- 4100 of Astro-med Incorporated. The data may be letters, numbers, symbols. There may be a bank of printers 2, arranged to apply ink of different colours.

A continuous sheet 14 of retroreflective material 11 is conveyed in the direction of arrow A at a speed of 10.2 cm/s. The sheet 14 is passed over an infra red light bulb 15, activated by a switch device (not shown), to raise the temperature of the surface of the sheet 14 to be printed on to about 90°C. The switch is arranged to switch on the bulb on as a 'length' of the sheet approaches the bulb and to switch off the bulb when the 'length' has moved passed. The sheet 14 is then passed to the printer 2 and indicia are applied by transferring ink from the ribbon 21 by the print head 22. The print head 22 is operated at a PHI of 15. The sheet material 14 then passes on to other stations so that a numberplate may be fabricated. The sheet is cut into appropriately-sized lengths and is adhered to an ABS backing layer 13 using an adhesive 12. A polyethylene film face layer 17 may be applied to the retroreflective sheet 11 using an adhesive 16. The transparent face layer 17 acts as a lens for the retroreflective material 11.

Because the surface of the material to which the ink is applied is heated, it is not necessary to operate the print head 22 at the more usual PHI of 32 (i.e. 91%) which is required to print onto retroreflective sheet materials if no pre-heating of the sheet material is carried out. This reduces the cost and increases the useful life of the printing equipment. It has been found that by heating the retroreflective sheet material 11 and reducing the operating PHI of the print head 22, the operating life of the print head 22 can be extended from between above 25% to up to 35%. Such an improvement drastically reduces the costs associated with fabricating numberplates. Such a reduction in PHI does not led to a commensurate reduction of print quality.

The invention is not limited to the embodiment shown. The structure of the precursor and the plate may be different and the heating may be done such that the surface to be printed is illuminated by the lamp.

Claims

1. A method of applying marking liquid to a sheet for a numberplate or the like using a thermal print head, the method including the preliminary steps of heating the surface of the sheet and reducing the operating PHI of the print head whereby the operating life of the print head is increased.

2. A method according to Claim 1 , wherein the sheet is a retroreflective sheet material.

3. A method of printing onto a retroreflective substrate, the substrate having two opposed major surfaces, the method comprising heating one major surface of the substrate and subsequently applying marking liquid to the other major surface of the substrate with a print head operating at a reduced PHI.

4. A method according to any of Claims 1 to 3 comprising operating the print head at a PHI within the range of about 34 to about 58%.

5. A method according to Claim 4 comprising operating the print head at a PHI within the range of about 40 to about 51 %.

6. A method according to any preceding Claim, wherein the surface of the sheet material to be printed on is heated to a temperature of from about 20°C to about 100°C.

7. A method according to any preceding Claim comprising heating the sheet by exposing the sheet to infra-red radiation.

8. A method according to any preceding Claim comprising heating the sheet material by exposing it to an intermittently actuating heating means.

9. A method according to any preceding Claim comprising passing the sheet material passed the heater and through the printer at a speed of from about

6 cm/s to about 15.5 cm/s.

10. A method according to any preceding Claim comprising passing the sheet material passed successive print heads and printing further indicia onto the sheet material.

11. A method of forming a numberplate for a vehicle or the like comprising printing indicia on to a retroreflective sheet material in accordance with the method according to any of Claims 2 to 10, and adhering said printed sheet material to a supporting layer.

12. A numberplate made in accordance with the method of Claim 11.