Anodised aluminium is widely used for display purposes, e. g. for signs, labels, notices and the like. An image can be formed on open pored anodised aluminium by dying the aluminium oxide surface layer with one or more colours. A photolithographic technique is typically used in the formation of the image. According to the technique, the aluminium oxide layer is coated with photoresist, which is processed to form a negative image layer. Forming the negative image photoresist layer is a multi-step process, which includes selectively exposing areas of the photoresist, e. g. by means of a mask bearing the desired image, washing away the unexposed photoresist and drying the remaining exposed photoresist. Ink is then rubbed into the surface of the aluminium so as to fill the pores in the

uncovered aluminium oxide layer, with the remaining photoresist preventing the ink from filling pores beneath it, to thereby form the desired image.

The present applicant has realised that such a conventional technique tends to be time consuming and can require considerable operator skill. The present invention has been devised in the light of this realisation.

Therefore, according to an aspect of the present invention, there is provided a method of printing on aluminium, comprising: providing an aluminium member having an open pored surface; providing an intermediate image comprising at least one sublimation ink; positioning the intermediate image in proximity to the open pored surface; and transferring image information from the intermediate image into the open pores of the aluminium member by sublimation of the at least one sublimation ink.

The printing method according to the invention may involve fewer steps than the conventional~ technique and may require less skill on the part of the operator. It is believed that during sublimation gas vapour diffuses into the pores in the aluminium where it condenses.

The intermediate image may comprise ink particles of a diameter of less than about 10 microns, and perhaps between 5 and 10 microns. Providing particles of such dimensions may help with sublimation and diffusion of the gas vapour into the open pores. In addition, subsequent retention of ink in the open pores may be improved.

Subliming the at least one sublimation ink may

comprise heating the sublimation ink. Sublimation may occur at a temperature between 100°C and 300°C. With certain sublimation inks, sublimation may occur at a temperature between 160°C and 250°C.

The aluminium member and/or its open pored surface layer may be configured to prevent crazing of the open pored surface during the sublimation process. For example, the aluminium member and its open pored surface may have sufficient ductility to avoid crazing when heated to the requisite temperature.

Alternatively or in addition, subliming the at least one sublimation ink may comprise pressing the image against the open pored surface. The application of pressure may aid the sublimation process and/or aid the diffusion of the gas vapour into the open pores.

The step of providing an intermediate image comprising at least one sublimation ink may comprise depositing a liquid suspension of a sublimation ink on an image carrier and drying the deposited liquid suspension. The sublimation ink may be suspended in water. The ink particles may be substantially evenly dispersed in the liquid. A substantially even dispersion of the ink particles in the liquid suspension may aid the dry ink to sublime and/or aid the diffusion of the gas vapour into the open pores.

The image carrier may have an absorbency which resists penetration of the liquid into its fabric. When a liquid suspension of sublimation ink is deposited on such a low

absorbency image carrier a substantial amount of the suspension will remain on or near the surface of the image carrier while it dries. This can provide for an optimum amount of the dry ink being brought into close proximity to the open pored surface of the aluminium member, thereby providing for optimal diffusion of gas vapour into the open pores. Thus, a sharp image may be printed on the aluminium member.

The image carrier might, for example, be a sheet of paper.

The intermediate image may comprise differently coloured sublimation inks. Upon positioning the intermediate image in proximity to the open pored surface, the differently coloured sublimation inks sublime so as to fill the open pores to create a multi-coloured image on the aluminium member. Advantageously, the differently coloured inks can be printed on the aluminium at the same stage in the method. This is in contrast with the conventional photolithographic technique, which typically requires the creation of a fresh photoresist image layer for each colour.

The intermediate image may be created using one or more techniques selected from the group consisting of laser printing, screen printing, photocopying, digital printing, inkjet printing and offset litho printing.

The intermediate image may be created using a deposition technique which relies on pressure to deposit the sublimation ink where required, e. g. inkjet printing in

which the print head is actuated by the piezoelectric effect. Printing techniques that involve the application of heat, e. g. bubble jet printing, can cause the sublimation ink to lose its ability to sublime.

A specific embodiment of the present invention will now be described, by way of example, and with reference to the accompanying drawings in which: Figure 1 shows apparatus for creating an image for printing onto aluminium in accordance with the present invention; Figure 2 is a representation of an aluminium member having an open-pored surface; Figure 3 shows apparatus for printing an image onto aluminium in accordance with the present invention; and Figure 4 is a flow chart representation of a method according to the present invention.

Figures 1 to 3 show apparatus and components used in a method according to the present invention.

Figure 1 shows an inkjet printer 10 printing an image 11 onto a sheet of paper 12 (which constitutes an image carrier). The inkjet printer Figure 1 prints an image by controlled ejection of ink through its print head under pressure created by a piezoelectric actuator.

Figure 2 shows an aluminium member 14 having an open pored surface 16.

Figure 3 shows a heat press 18 which is used to heat a sheet of paper 12 whilst it is pressed against the open pored surface of an aluminium member 14.

A method according to the present invention will now be described with reference to Figure 4. An aluminium member 14 having an open pored surface 16 is provided 22.

The open pored surface 16 is typically formed by anodic oxidation of the aluminium member. The aluminium member and/or its open pored surface should have sufficient ductility to prevent crazing of the open pored surface when heat is applied during subsequent processing.

The inkjet printer 10 is used to form the desired image on the sheet of paper 12 (which constitutes providing an intermediate image comprising at least one sublimation ink). The image might, for example, be designed on a Personal Computer. The image can be a multi-colour image.

Formation of the intermediate image on the sheet of paper 12 involves printing a suspension in water of one or more differently coloured suspension inks onto the sheet of paper 24 (which constitutes depositing a liquid suspension of a sublimation ink on an image carrier to form an intermediate image). The sublimation ink particles have a diameter of between 5 and 10 microns and are substantially evenly dispersed in the water. The printed image is then dried 26. The sheet of paper 12 has an absorbency which resists penetration of liquid into its fabric while the ink dries.

The aluminium member 14 is positioned in the heat press 18 as shown in Figure 3 and with an open pored surface upper-most. The sheet of paper 12 bearing the image is positioned on the aluminium member so as to bring

the image into contact with the open pored surface 28 (which constitutes positioning the intermediate image in proximity to the open pored surface). The heat press 18 is then closed and pressure is applied to the sheet of paper 12 and aluminium member 14 to press the image evenly against the open pored surface. The heat press heats the sublimation inks in the image to a temperature of between 160°C and 250°C to cause sublimation of the dried ink. The gas vapour diffuses into the open pored surface 30 (which constitutes transferring image information from the intermediate image into the open pores) to form an image in the aluminium member. The heat press is typically closed for a period of 30 to 90 seconds, depending on the thickness of the aluminium.

When the heat press is opened, the aluminium member and sheet of paper are removed and allowed to cool. The paper is then removed from the aluminium member and the image sealed in the aluminium member 32, e. g. by immersing it for 40 minutes in a water bath operating at a temperature of between 90°C and 100°C.

It is to be appreciated that the method of printing on aluminium and other features described with reference to the embodiment discussed above can be combined in other embodiments of the present invention.