The importance of Cascades at Vertical Powder Coating Lines

Publicado por Alsan Blog el 6 junio, 2023 en aluminio, Artículos, Lacado

The cascades on the vertical lines are basic to obtain a perfect pre-treatment on the surface of the aluminium.  It is not enough to wet the profiles, they must be adapted in height, length and flow.

Recently, we were solving a contamination problem in a conversion bath. The conductivity of the bath was constant while the product concentration was falling.  A clear symptom that the bath was receiving pollutants from previous baths.

Several options were discarded and finally reaching the conclusion that the contamination was coming from acid etching, due to an incorrect regulation and installation of the cascades.

This process sequence was the one found:

STAGE 1 – ACID ETCHING: Cascade stage. Originally this etching was by spray but it was modified to cascade.  The incorrect position of the drawers caused the profiles to get soaked both inside and outside.  Instead of hitting the outside of the profiles, the liquid went inside the profiles.

STAGE 2 – TAP WATER RINSE: Spray stage. The profiles were only cleaned from the outside.

STAGE 3 – DI WATER RINSE: Cascade stage. The cascade is uniform and the height where it impacts on the profile is good, a correct external rinsing is achieved.

STAGE 4 – CONVERSION: Stage with cascade which, partially, entered inside the profiles.  That is to say, they were rinsed both inside and outside.

Reducing the flow rate of the conversion cascade by preventing it from entering the inside of the profiles considerably reduced the contamination of the bath. But the final solution was given when the acid etching cascades were modified and adjusted.

The conclusions were:

1. When the acid etching entered through the inside of the profiles, the consequences could be the following:
   • Increased chemical consumption since the product etches both internally and externally.
   • Incorrect external etching can occur.
   • When not rinsing internally in the tap water stage, the following situation are possible:
     • Internal rinsing in DI water stage, that will increase demineralized water equipment regenerations and chemical consumption. 
     • If internal rinsing takes place in the conversion stage, the bath will be contaminated.
     • The absence internal rinse will leave sulfuric acid residue inside the profiles.
2. Etching and Conversion products should impact three to four centimeters below the top of the profile, and rinses should impact one to two centimeters below the top of the profile.
3. After the analysis of the conversion bath at our laboratory, the contamination of the bath by sulphates has been confirmed.  The concentration of sulphates found in the analysis can only come from acid etching.