Jeners Druckgusstechnik - Processing and surface coating

Colourings and metallic effects, gloss and matt surfaces and other decorative effects can be achieved through the use of metallic or non-metallic coatings.

Surface treatments of die-cast products are used to, for example, to improve chemical resistance, to increase wear resistance, and for decorative purposes. Changes in the electrical resistance of the surface layer, improved solderability, tagging compounds, and added value are further reasons for choosing a surface treatment. The fundamental rules of surface treatments should be observed at the earliest possible stage e.g. during moulding.

With aluminium die-cast products one has a choice between mechanical, chemical, electrolytic or galvanic surface treatments;
Electrolytic oxidation (anodising) is, unfortunately, less attractive because of the more pronounced “flow lines” caused by the anodising pool, but it does offer advantages to the surface in terms of strength, corrosion resistance and is a good base for varnishing. A considerable increase in wear resistance is achieved through hard anodising. A good casting microstructure and a perfect surface are however pre-requisites for galvanising.

Processing and surface coating: Process and applications

Décor and protection - surface treatments and their properties

Decorative effect	Colourings and metallic effects, gloss and matt surfaces and other decorative effects can be achieved through the use of metallic or non-metallic coatings. Galvanisation is the most commonly used method for achieving a decorative and anti-corrosion surface on zinc die-cast products.
Surface protection	Zinc is corrosion-proof. In neutral or weak alkaline environs where thick and highly adhesive layers of oxide, hydroxide and alkaline carbonate form on the surface. These layers give zinc die-cast products a passive potential of - 0,3 to - 0.1 V as compared to the normal hydrogen electrode. The normal potential of metallic glossy die-cast products is - 0.76 V. Protective metallic or non-metallic coatings are necessary under harsher conditions particularly in acidic environs.
Surface treatments	Surface treatments of die-cast products are used to, for example, to improve chemical resistance, to increase wear resistance, and for decorative purposes. Changes in the electrical resistance of the surface layer, improved solderability, tagging compounds, and added value are further reasons for choosing a surface treatment. The fundamental rules of surface treatments should be observed at the earliest possible stage e.g. during moulding.
Aluminium alloys	With aluminium die-cast products one has a choice between mechanical, chemical, electrolytic or galvanic surface treatments; Electrolytic oxidation (anodising) is, unfortunately, less attractive because of the more pronounced “flow lines” caused by the anodising pool, but it does offer advantages to the surface in terms of strength, corrosion resistance and is a good base for varnishing. A considerable increase in wear resistance is achieved through hard anodising. A good casting microstructure and a perfect surface are however pre-requisites for galvanising.
Zinc alloys	Chemical coatings on zinc die-cast products are produced by using phosphate or chromate. Transparent or blue chrome layers, as well as yellow, olive or black chrome layers can be achieved by chromating (according to DIN 50960, Part 1). A weak yellow chromating serves as an undercoat for powder coatings. Almost all metals can be galvanically precipitated onto zinc die-cast products, however a previous sub-coppering is required as a result of zinc’s high solubility in strongly acidic pools. A limiting layer of nickel is advisable for rapid diffusing metal coatings such as gold and silver. Double nickel layers and micro-crack or micro-pore chromating are employed for products exposed to harsher corrosive environs, e.g. in vehicles.
Magnesium alloys	Varnishing or powder coating is the most common surface treatments for magnesium. The products can be bichromatised first for difficult applications, increasing the adhesive properties for the subsequent surface treatment. However new surface treatments are currently being developed as the use of chrome compounds for surface protection is soon to be outlawed. Magnesium components are varnished either for optical reasons or, occasionally, to protect against environmental effects. The surface is usually coated before varnish is carried out. Cleaning of the product is of considerable importance. Surface treatments can involve a mechanical process, such as trowalisation with plastic or ceramic stones. However, other processes such as grinding, polishing, etc., can also be used. Magnesium can also be blasted. Aluminium oxide, aluminium grains, glass particles are suitable materials as long as they are free from iron, nickel and copper. After blasting, scouring with acids is recommended to remove any residual blasting material from the magnesium surface.