Prior to coating, components must be free from rust, scaling and oxides, dirt and dust, oil and grease, as well as being dry. Typical forms of pre-treatment include alkaline degreasing, blasting with various materials (e.g. stainless steel grains, steel grains etc.) or chemical pre-treatment such as phosphatising or pickling. In the case of zinc flake coating, alkaline degreasing plus blasting is recommended, with phosphating as an alternative treatment.

Coating may occur without the use of electricity, in the form of painting, or with electrical current used to deposit the material on a component.

Zinc flake technology (electroless coating)

In the case of zinc flake coating there are two coating processes, which are very similar. First the zinc-aluminium basecoat is applied and cured, then the topcoat is applied in the same way.
For the basecoat, after the pre-drying stage the zinc flake coating is cured in an oven at a specific temperature, depending on the product concerned (a temperature of from 180°C is common for basecoats). This enables the coating to achieve its protective function. 1-2 coats of basecoat are usually applied.
Application of the topcoat follows the same procedure. It is also cured (particularly solvent-based topcoats) or dried (typically water-based topcoats).
In addition, different surface coating units are also employed, depending on component. For small parts that are to be coated as bulk products the dip spin process is advisable. Depending on the quantity or volume of parts different sized baskets are used, with these dipped into liquid material in a coating tank before being spun at a defined speed and with a defined angle. This enables the required coating thickness to be set – including for difficult geometries.
In addition to the dip spin process for bulk products, the same technology also exists for racked goods. The baskets are laid out in a way that keeps larger components apart for the coating process, without these damaging one another during the spinning.
Spraying is also an alternative for large parts. This may occur manually or via robots. This procedure is particularly suitable where a partial coating is required. An e-static unit can be used to reduce overspray.
The dip drain procedure is a further option for large parts. Here the coating thickness is achieved via a defined withdrawal speed and the viscosity of the coating material.

KTL technology (with electricity)

The KTL is also applied to the component in liquid form. In the classic form, racked goods are moved through enormous tanks. The application of direct current deposits the required coating thickness to the component.
DELTA-eLACK® KTL takes a different approach: larger bulk products are not suspended from a rack, but instead coated directly in a drum. The drum unit is loaded and dipped into the liquid KTL medium, with rotation, electricity is applied and KTL is evenly deposited. The drum is then moved to an ultrafiltrate rinser to remove excess material.
The twin line specially developed by Dörken MKS functions with a similar principle, with the difference that two drums work in parallel, making a higher throughput possible.
The EC-device 2000+ is particularly well-suited to very small parts such as bolts <M6, including with self-tapping thread. The Archimedean form of the drum interior means that the small parts are coated in a throughput process.
At the end of each coating process the coated parts are transported directly to the oven (usually a continuous oven) via a conveyor belt, where they are cured at a defined temperature and duration.

Electroplating technology (with electricity)

In the DELTA-PROZINC® process application occurs using the drum or rack procedure, depending on the component. The components pass through various pre-treatment stages before the galvanisation process begins. Depending on system requirements, the components are coated with an acidic or alkaline zinc electrolyte. In the DELTA-PROZINC® process they undergo an initial passivation stage. This is followed by a second passivation stage with interim drying. Finally, a topcoat is applied. This is also dried. In the drum process drying occurs in centrifugal dryers or rack dryers.
The plant of our licensed coaters are fully tailored to the components and the system and are individually approved by Dörken MKS. Characteristic here are the high degree of automation and monitoring of the complete process.
 

Depending on type of coating and component, various types of ovens can be used. Both the zinc flake basecoats and the matching organic system topcoats need to be cured in order to form a film. Depending on product, the inorganic system topcoats are also cured or force dried. The following oven types are typically available: conveyor oven (continuous oven), rack oven or chamber oven, increasingly also inductive drying and infrared drying.

Each oven type has advantages and disadvantages, which need to be considered individually and assessed with regard to the components that are to be coated. Decisive for choice of oven is in particular the degree of automation of the coating process as a whole.

In the DELTA-PROZINC® process no oven is required: the topcoat is dried directly in the centrifuge.
 

After the curing or drying process the components should always be cooled back down to ambient temperature. The dew point should never be reached in this, particularly for zinc flake coatings.