The main objective of enamelling is to “protect” the workpiece with ceramic enamel whose melting temperature exceeds (in the case of steel) 800°C. The glazing can be done in with a single hand or with a base coat and just secondly with another glaze layer to guarantee a pleasant appearance in addition to giving colour and brilliance.
In order to achieve the main objective, the shape of the workpiece must be carefully studied since it must allow the enamel to cover it completely, and at the same time it must take into account some of the limitations and needs that the enamelling process requires.The support materials that are able to flawlessly "receive" and "maintain" enamel over time are:
In any case, the first three elements must have a specific chemical composition and characteristics in order to make the enamelling process possible.
The three main phases of the enamelling process are:
Pre-treatment of the support: Pre-treatment of the support can be done both through chemical or mechanical processes.During chemical pre-treatment, mainly used for steel and aluminium, it is essential to consider that the workpiece must be specifically designed to facilitate both the entry and outflow of liquid detergent and rinsing products; in fact, any stagnation would definitely compromise the result of the enamelling process.Chemical pre-treatments can be done by both immersion and spray. Mechanical pre-treatment, used mostly for cast iron products, is composed of burring followed by a sandblast that guarantees both the removal of the workpiece’s superficial layer as well as the correct degree of roughness; the thickness of the workpiece must therefore allow mechanical resistance to the stress caused by the sandblasting process. The sandblasting process is often used for the glazing of boilers made of steel.
Enamel application: During application, it is essential that the entire surface involved be easily reachable by the enamel itself, and enamel outflow must be just as easy.Depending on the type of technology used, enamelling can be done through:
Wet application enamelling includes:
Immersion (by immersion or electrodeposition), in which the workpiece is fully or partially immersed in an enamel pulp and then removed at a controlled speed in order to allow the excess enamel to flow off. The pulp must therefore have precise and controlled viscosity and specific weight characteristics.Application by immersion is still used for workpieces that do not have particular aesthetic requirements, since it is not always possible to reduce and homogenise the drippings; it is an easy and inexpensive technology.However, electrodeposition guarantees more consistent and homogeneous layers of enamel; it is more costly, but offers significantly better aesthetic results in all parts.
Flow coating in which the part is completely covered by the enamel pulp that is poured over the workpiece by means of pumps and/or gravity. The dripping lines cannot always be eliminated, though it is possible to minimise them by rotating the workpiece during the process. This process is successfully used for example for enamelling stovepipes.
Spraying in which the enamel pulp is sprayed by means of spray guns, both manually and electrostatically. The surface to be covered must be easily accessible by the spray. There are practically no problems of enamel stagnation or dripping.
Powder application enamelling includes:
Sprinkling (used for example for cast iron workpieces) in which the part is pre-heated and then “sprinkled” by means of a special vibrating sieve.
Electrostatic , in which particular care must be paid to how the workpiece is moved for the final drying phase. In fact, even though powder enamel adhesion to the workpiece is excellent, it is important to avoid stress or violent shocks that would cause detachments that compromise the final result.
Enamel Burning: This is the last phase of the enamelling process, in which enamel vitrification occurs.This operation is performed at extremely high temperatures, and it is therefore particularly important to avoid any deformation caused by tension in the support itself or in the enamel .
Thanks to the continuous collaboration with the Customer during the design and construction of these types of systems,Trasmetal is able to select the most appropriate technology based on the geometry of the support, on process constraints and on the type of enamel used.