Coating processes – Direct application of electrical – magnetic – wave – or... – Pretreatment of substrate or post-treatment of coated substrate
Patent
1993-01-29
1995-02-21
Pianalto, Bernard
Coating processes
Direct application of electrical, magnetic, wave, or...
Pretreatment of substrate or post-treatment of coated substrate
427197, 427201, 427287, 4273764, 427379, 427557, 427559, B05D 500
Patent
active
053914082
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for firing enamel on a metal article, especially an aluminum article. It also relates to a device for its implementation.
Current methods for firing enamel on aluminum, derivatives of the methods for firing enamel on sheet metal and previously on ceramic, employ a convective-environment firing. Electric- or gas-heating elements heat, to a set temperature, the ambient air of a furnace into which an article to be enamelled placed on a support is placed. This air is next conveyed, rendered uniform and then projected onto the article by turbines. The hot air yields part of its thermal energy to the article, part of the surface of which has been covered beforehand with an enamel frit slip which, after drying, constitutes a bisque according to a technique well known in the field of enamel firing. The hot air is then recovered under the support and is heated once again by the heating elements placed in the walls of the furnace.
In these methods, the entire article to be enamelled is heated since this article and the enamel layer reach an equilibrium temperature equal to the temperature of the ambient air in the part of the furnace where the article is located. Consequently, the power of the furnace and its response depend on the mass and surface of the article.
Furthermore, the construction of a furnace employing a convection technique is tricky and expensive on account of the difficulty of conveying hot air at temperatures in the vicinity of 560.degree. C., which correspond to the usual temperatures for firing the enamel on aluminum articles.
Moreover, the heating of such a furnace is lengthy,--1 to 2 hours in practice--, and therefore expensive in terms of energy. In addition, when the article has reached the set temperature, it is necessary to provide a temperature hold of at least 5 minutes in order for the enamel to reach its optimum properties.
In addition, the uniform heating of the article to be enamelled causes the expansion of the metal to take place in conjunction with the temperature rise of the enamel, which has the result of generating, on dished or pleated surfaces, breaks in the enamel bisque which does not have the same expansion coefficient as the metal.
The object of the present invention is to remedy these drawbacks by providing a method for firing enamel on a metal article, especially an aluminum article, subjected to a prior step for depositing an enamel frit slip onto part of the surface of the article, followed by a drying step leading to an enamel bisque being obtained on said article.
According to the invention, this firing method is distinguished in that it comprises, after the drying step, a step for exposing the article to infrared radiation of a predetermined maximum emission wavelength such that this radiation heats the enamel bisque in order to obtain the firing of the latter, without significantly heating the metal article.
The method according to the invention enables surprising results to be obtained in terms of operating flexibility, manufacturing simplicity, cost and quality of the enamel obtained.
Thus, a furnace implementing the method according to the invention is operational in a few seconds instead of one to two hours in the case of the prior methods, which results in rapid and cost-effective start-ups. Because of this, in the event of a momentary stop in production, the furnace may be stopped if it is empty.
With the method according to the invention, the ambient air has a temperature less than the vitrification temperature which is equal to 560.degree. C. in the case of the firing of enamel on an aluminum article and is very much less than that of the infrared emitter (1000.degree. to 2000.degree. C.). In addition, this air only contributes to a slight degree to the heat exchange between the infrared emitter and the article to be enamelled which is carried out almost integrally by radiation, the air being practically stationary in the firing chamber. The construction of a furnace implementing the method according to the inventio
Pianalto Bernard
Seb S.A.
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