CHEMICAL PRE-TREATMENT
DEGREASING
The term "degreasing" describes the cleaning of component parts from fats and oils.
Why is it necessary to degrease?
Fats and oils prevent bonding of coatings of all kinds. The chemical coatings cannot refine on the surface if this surface is coated with a layer of oil. Lacquer surfaces do not bond at all or only bond badly.
Why are the surfaces covered in fats and oils?
Many materials are greased in order to prevent corrosion. Often the lubricants are required for another, previously carried out step and are then not removed. Examples are the cooling lubricants on a rough turned component part or deep-drawing oils, which are needed to remove the deep-drawn component part from the deep-drawing forms. For the rolling process, too, component parts are coated with lubricants.
Which ways of degreasing are there?
Each manual degreasing, such as the wiping with acetone, dilution or brake cleaning fluids, is not acceptable regarding quality and reproducibility. If the health risks, which occur with such works, are not considered, nobody can judge if the surface is sufficiently free of oil or not.
Degreasing of equipment with CHC (e.g. trichlorethylene) leads to outstanding results, but due to environmental reasons a very limited procedure with enormous conditions, as chlorinated hydrocarbons (CHC) are suspected to destroy the ozone layer.
Therefore, as the only economic method of degreasing, the cleaning process in alkaline devices remains. There are two different ways: Spray degreasing and immersion degreasing. When degreasing with alkali cleaners the oil removed from the component part due to the chemical characteristic of the degreasing liquid to dissolve oils. The residence time varies depending on the level of staining of the surface.
In the case of the spray degreasing the degreasing liquid is sprayed onto the component parts with high-pressure nozzles so that even distant sections of the component part, such as undercuts are sufficiently moistened and thus are degreased. Generally the residence time is between 2 and 3 minutes. This time may vary depending on the component part.
In the case of immersion degreasing the component parts are immersed in the degreasing medium as long as necessary for the surface to be cleaned. The average residence time here is roughly 5 to 6 minutes.
Now the conclusion seems obvious that the spray method is better because the exposure times are shorter. This is not correct. The respective procedure must be adjusted to the geometry of the parts. Also, spray devices, which are loaded in passing, with residence times of only 2 minutes and a conveying speed of 4 m/minute, are already more than 8 meters long in the degreasing zone only.
After degreasing and the subsequent rinsing the component part must be processed further immediately, or else corrosion begins immediately. This process may be delayed with the help of passivation.
We have chosen the immersion device for the entire pre-treatment, as we are able to use the immersion procedure universally. We have got two degreasing baths available, which are used depending on the level of staining of the work pieces. After degreasing the rinsing is in two cascading rinsing baths with desalinated water.
To the chapters
pickling
zinc phosphating
chromalising
CHEMICAL PRE-TREATMENT
