Before a Paint finish can be successfully applied, the surface of the item to be painted should be clean and meticulously free from any type of oil or grease contamination. To remove this contamination from the product it is usual for metal products to be 'Pre-Treated' prior to either a Paint or a Powder coating being applied.

'Pre-treatment' has additional importance when Powder Coatings are to be applied to a product. Because Powder coatings exhibit higher cohesion properties and lower adhesion properties than Wet paints, the coating tends to be tougher than with Wet Paint systems but its adhesion to the product tends to be lower. Ideally therefore the metal surface should be prepared chemically so that it presents an ideal 'key' for the powder to successfully adhere.

The RDM Aqueous Combined Degrease/Phosphate Plants are ideally suited to 'Pre Treating' metal products, with the degrease chemicals removing any oils and the phosphate chemical providing the chemical reaction to provide an ideal surface key. The vast majority of contemporary RDM Pre-treatment systems utilise the design shown below. The product is hung from a motorised conveyer which conveys it through typically a large robust Tunnel arrangement. The Tunnel comprises of a series of zones or Stages each with a different function. The Inlet and Outlet Vestibule Zones ensure that as Products enter or leave the Plant, the Sprayed Aqueous solution does not escape. The Heated Zones comprise of a holding tank beneath the Plant. This tank is Heated by means of a fuel burner or steam. The Hot Solution or Water is drawn from the tank by a pump and fed to a precisely spaced set of spray nozzles around the product. As the product passes through the Fine Spray it is cleaned, phosphated or rinsed. The liquid runs back into the tank for recirculation and filtration. Drain zones between the Stages reduce the risk of cross contamination. Following Aqueous Pre treatment, products usually pass through a forced hot air recirculation dry off Oven. This system requires a minimum amount of operator input and so it is ideally suited to conveyorisation and automation.

Process Features

Totally free of Hazardous Chlorinated Solvents, C.f C.s or Flammable chemicals.

Both mechanical (spray) and chemical cleaning actions.

Easily conveyorised.

Minimal effluent Treatment requirement with Iron Phosphate.

Minimal Health & Safety hazards.
Minimal operator labour input required.

Choices of Dipping or Spraying application, Iron or Zinc Phosphating Solutions, Separated Cleaning and Phosphating Zones or Combined Chemical Zones.

Hot or cold final rinses.

Ideal for medium to high volume cleaning operations.

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THE CHEMISTRY

The chemical processes involved in the Phosphating process are quite complex but in brief terms the phosphating process converts the surface layer of certain metals into strong, adherent, insoluble metal phosphates. These can either be crystalline or amorphous in structure. Phosphating forms both a rust resisting layer in itself and also gives an excellent key to the subsequent coating. If scratched, the underlying metal surface does not corrode as rapidly as it would without the phosphate layer.

Phosphating solutions are usually based on iron or zinc acid phosphates, dissolved in dilute phosphoric acid. The products to be phosphated are either dipped or more usually sprayed with the phosphating solution, the concentration and ingredients of which are carefully balanced so that they are just held in the solution. The free acid present reacts with the metal surface liberating hydrogen and causing the pH of the solution immediately adjacent to the metal to rise. This unbalances the solution and the metal phosphate begins to precipitate. The careful balance of the solution confines this precipitation to the metal surface, so that the phosphate film becomes a chemically bonded, integral part of the surface.

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