The components produced during die casting are subjected to a series of post-casting operations such as painting, and application of adhesives and polymers. Mostly, these secondary coatings are applied to the castings in order to improve its key physical properties and shield it from future defects. At times, there occur numerous issues with these post-casting processes, the most common being problems with the adhesion. This can lead to the improper bonding of the polymer onto the metal substrate of the casting.
Chemical bonds and mechanical interlocking are the two factors that cause the adhesion of polymers or other substances to the casting substrate. In case of a chemical bond, the adhesion is because of the linking of the subatomic particles. On the other hand, mechanical interlinking takes place due to the surface irregularities and frictional force that takes place between the pores, and crevices of the casting.
The lack of good adhesion occurs due to the improper contact of the polymer with the metallic substrate. The presence of any materials such as dirt, wax, oils and other organic or inorganic substances can hinder proper adhesive bonding. The adhesion problem that occurs in casted components is assessed through various steps. Analytical chemical testing is used to find out the nature of the contaminants and the chemical nature of the casting surfaces.
The other important steps involve identifying the source of the contaminants and determining the pretreatment process. This latter process will eliminate any of the contaminants to improve the adhesion levels of the casting surface. Besides, subjecting the casting to the pretreatment process will also shield it from corrosion. Various pretreatment methods are used to ensure proper adhesion.
One of the most common pretreatment methods to resolve adhesion problems is to use cleaners. Mild alkaline cleaners can actually remove many of the contaminants from the casting surface. After the casting is cleaned properly, it is rinsed in an acid bath and a conversion coating is applied to prepare the polymer.
A sealer is also applied after the casting receives a conversion coating. Applying the sealer will improve the corrosion and humidity resistance of the casting. Apart from this, numerous other methods are used during the pretreatment methods. One of these methods is high-pressure cleaning using water, which is highly effective in removing contaminants when compared to immersion.