GB2535172B - Embossed vitreous enamel coating and method of forming the same - Google Patents

Description

EMBOSSED VITREOUS ENAMEL COATING

AND METHOD OF FORMING THE SAME

[001] The present invention concerns a method of forming a vitreous enamel (VE) coating having embossed features and, in particular, where the coating surface has a texture or design raised above the surface in low relief. The present invention also concerns the resultant embossed coating and an enamelled product including the embossed coating.

[002] A vitreous enamel (VE) coating is formed by applying an unfired enamel mixture, the frit, to a metal substrate such as sheet metal or cast iron. The glass particles are then fused to the substrate by firing it at a temperature in excess of 800°C. This results in a glossy uniform surface that is incredibly hardwearing; it is temperature resistant up to 600°C, corrosion resistant, and chemical resistant with exceptional colour stability. As such, the finished enamelled product is easy to maintain, clean and hygienic. Consequently, enamelled products are used for many applications both internally and externally, such as signage, cookware, architectural panelling, cast iron baths, and range cookers .

[003] It is known that various colours can be incorporated into vitreous enamel coatings in order to provide coloured panels or graphics. Industrially, this can be achieved by screen printing block colours through a screen mesh before firing, or by spraying a blended colour enamel mix directly onto the metal substrate.

[004] In addition to colouring, it has also been viewed as desirable to be able to produce enamel surfaces with a textured finish or three-dimensional surface features. Such embossed features can be used for aesthetic reasons or to provide functionality, such as grips, in the otherwise uniform glossy vitreous enamel finish. However, there are a number of issues with conventional methodologies that have prevented more widespread adoption of embossed enamel coatings .

[005] In this connection, one technigue for imparting surface texture to an enamelled coating has been to press features into the metal substrate. However, this technigue is expensive, not least because customised dies are required for each design.

[006] An alternative technique for forming embossed features into a coating has been proposed JP 2005 200726. In this document, a technique is disclosed in which a water-repellant base layer is applied to a substrate and is used to create island shaped mounds of outer enamel layer material due to surface tension. Consequently, when fired, the glaze forms a textured finish due to the island shaped mounds of enamel. However, this technique is limited in that it is difficult to control the textures that are formed as a consequence of surface tension, meaning that uniformity is difficult to achieve. Furthermore, the irregularities in the glazed finish would be more prone to thermal shock and consequently coating breakage.

[007] The present invention therefore seeks to provide a solution to the above identified problems.

[008] According to an aspect of the present invention there is provided a method of forming a vitreous enamel coating on a substrate, comprising: applying a first layer of ground coat frit to the substrate to form a first intermediate having a ground coat surface; firing the first intermediate by heating to above 800°C; selectively applying a second layer of ground coat frit to regions of the ground coat surface of the fired first intermediate to form a further intermediate, wherein the step of selectively applying the second layer comprises stencilling; firing the further intermediate by heating to above 800 °C to form a ground coat of varying thickness; applying a cover-coat frit layer uniformly over the ground coat surface of the further intermediate to form a coated intermediate; and firing the coated intermediate by heating to above 800 °C.

[009] In this way, layers of ground-coat material may be built up to form different thicknesses of ground-coat within the enamelled surface coating. The ground coat frit used to form the ground-coat layer may be a readily available borosilicate based material. The different thicknesses in the resultant ground coat produces surface features in the top cover coat which provides an 'embossed' finish which, depending on the stencil used, can take the form of a pattern, text or a company logo.

[0010] Preferably, the step of stencilling comprises removing the stencil once the second ground coat frit layer has dried. This helps to preserve the shape of the stencilled ground coat frit layer prior to firing.

[0011] Preferably, the step of stencilling selectively applies the second ground coat frit layer to unmasked areas of the ground coat surface of the fired first intermediate.

[0012] Preferably, applying the second ground coat frit layer increases the thickness of ground coat in the unmasked areas by 50-120 microns.

[0013] In embodiments, the method further comprises one or more further iterations of selectively applying ground coat frit layers and firing before applying the cover-coat frit layer. In this way, more pronounced surface features may be created by building up multiple layers of ground coat.

[0014] Preferably, the steps of applying frit layers comprise spraying. This minimises the quantity of frit required in the process and provides for the application of an even thickness of each layer.

[0015] Preferably, the steps of firing comprise heating at between 830-850°C for 4 minutes at temperature.

[0016] Illustrative embodiments of the present invention will now be described with reference to the accompanying drawings in which:

Figure 1 shows the steps for producing a vitreous enamel coating according to an illustrative embodiment of the present invention.

[0017] In modern two layer enamelling processes, a ground-coat is used to bond to the substrate, and a cover-coat is then used to provide the desired external properties. The present inventors have realised, however, that by using stencilling, layers of ground-coat material may be built up to form different thicknesses of ground-coat within the enamelled surface coating. Once the top cover coat is formed over this, these regions of different thicknesses of ground-coat provide three dimensional surface features in the top coat which produces a different appearance from the standard uniform vitreous enamel finish.

[0018] In this connection, although stencilling has been used within vitreous enamel before, this has been limited to use on the top cover-coat as a way of achieving multiple colours/graphic on a surface, rather than to produce a texture or surface features. Indeed, the ground coat layer has previously been merely considered to function as a binding layer. Accordingly, the present inventors' use of stencilling to develop regions of increased thickness within the ground coat layer for creating a texture/pattern/image represents a significant departure from conventional methods.

[0019] In view of the above, embodiments of the present invention therefore allow for a textured surface finish to be created by forming embossed features in an enamel surface coating using layering and multiple firing of ground coats. The effect is one of an 'embossed' finish which, depending on the stencil used, can take the form of a pattern, text or a company logo.

[0020] Figure 1 shows the steps of an illustrative embodiment of the present invention. Figure 1(a) shows a cross sectional view of a section of sheet metal substrate (1), in this embodiment being a steel plate. As shown in Figure 1(a), a first layer of ground coat (2) is firstly applied to the surface of the substrate by spraying (3) a liquid slurry of frit. As shown in Figure 1(b), the substrate (1) and first ground coat layer (2) are then fired in a furnace (4) at between 830-850°C for 4 minutes at temperature to form an intermediate having a continuous glass layer of ground coat.

[0021] As shown in Figure 1(c), a removable stencil (5) is then applied over the surface of the fired first ground coat layer (2) . The stencil may be magnetic, adhesive backed or a rigid material. A second ground coat layer (6) is then sprayed on and, with the stencil (5) in place will selectively apply to the unmasked areas of the fired first ground coat layer (2). The removable stencil (5) is removed once the second ground coat layer (5) has dried, prior to firing. This results in a further intermediate in which the unmasked areas of ground coat material are thicker and raised above the masked areas by thicknesses of between 50-120 microns. As shown in Figure 1(d), the further intermediate is then fired in a furnace (4) at between 830-850°C for 4 minutes at temperature. Further layers of ground coat may be applied by repeating the stencilling and firing steps for each additional layer of ground coat in successive iterations .

[0022] Once sufficient layers of ground coat have been applied, a top colour coat (7) is then applied by spraying as shown in Figure 1(e). As shown in Figure 1(f), the final intermediate is then fired at between 830-850°C for 4 minutes at temperature to produce the enamelled panel.

[0023] The above results in an enamelled surface coating that is uniform in colour due to the uniformity of the top coat (7) yet has an 'embossed' appearance formed by the thicker areas of ground coat built up in layers during the stencilling and firing iterations. The pattern of these raised areas can be any shape, image or graphic that can be achieved through stencilling.

[0024] It will be understood that the embodiment illustrated above shows applications of the invention only for the purposes of illustration. In practice the invention may be applied to many different configurations, the detailed embodiments being straightforward for those skilled in the art to implement.

[0025] In this connection, for example, although in the above example, the top cover coat has been provided with a uniform colour, it will be understood that different colours and graphics may also be incorporated into the top layer to provide combinations of surface colours and textures.

Claims (7)

1. A method of forming a vitreous enamel coating on a substrate, comprising: applying a first layer of ground coat frit to the substrate to form a first intermediate having a ground coat surface; firing the first intermediate by heating to above 800°C; selectively applying a second layer of ground coat frit to regions of the ground coat surface of the fired first intermediate to form a further intermediate, wherein the step of selectively applying the second layer comprises stencilling; firing the further intermediate by heating to above 800°C to form a ground coat of varying thickness; applying a cover-coat frit layer uniformly over the ground coat surface of the further intermediate to form a coated intermediate; and firing the coated intermediate by heating to above 800°C.

2. A method according to claim 1, wherein the step of stencilling comprises removing the stencil once the second layer of ground coat frit has dried.

3. A method according to claim 1 or 2, wherein the step of stencilling selectively applies the second ground coat frit layer to unmasked areas of the ground coat surface of the fired first intermediate.

4. A method according to claim 3, wherein applying the second ground coat frit layer increases the thickness of ground coat in the unmasked areas by 50-120 microns.

5. A method according to any preceding claim further comprising one or more further iterations of selectively applying layers of ground coat frit and firing before applying the cover-coat frit layer.

6. A method according to any preceding claim wherein the steps of applying frit layers comprise spraying.

7. A method according to any preceding claim, wherein the steps of firing comprise heating at between 830-850°C for 4 minutes at temperature.