Aluminium Window and Door manufacturers will often be involved in the specification of powder coated finishes. There are occasions when a customer may not be happy with the quality of the powder coating. Before embarking on the specification of a powder coat finish, it is important to understand some of the processes involved in polyester powder coating.
A typical alumnium sheet being transferred by a gantry into the powder coating booth
It is often the case that in the rare situations where there are quality issues involved in powder coating it will not be the polyester powder itself that will be at fault.
Pre-treatment
Pretreatment is very important in the overall polyester powder coating process. It is useful to know, that the aluminium bars most window and door manufacturers will receive from the Aluminium Systems Companies will be contaminated with oil, finger marks and other residues evident from handling and extruding the material as well as the onset of natural oxidisation. All of these contribute to the life of the polyester powder coating.
It is very important that the correct pretreatment process is carried out to ensure paint adheres correctly and results in a finished aluminium extrusion coated to the highest standard. Where there is an issue over the quality of the powder coating bad pre-treatment will often be the cause!
The pre-treatment of aluminium extrusions prior to coating not only provides water and air seal to the aluminium section, but the provision of a good surface for the powder to adhere to is very important.
What is the procedure involved in pre-treatment?
The pre treatment procedure, whether this is carried out in physical immersion tanks or any other method must carry out these basic functions:
- Clean
- Rinse
- Etch
- Rinse
- Desmut (if using alkaline etch)
- Rinse
- Chromate or Chrome Free Conversion
- Rinse
- Demin Rinse
- Dry
It is critical that where the above process of pre-treatment of aluminium are carried out they must confirm to recognised industry standards.
Polyester Powder Coating Process for pre-treated extrusions.
The powder coating process with take the pre-treated aluminium sections and these will be positioned in the gantry that will move them into the paint booth. The powder coating “guns” or other appliances relevant to the paint plant installed will coat the aluminium bars with a controlled thickness of polyester powder in the required colour.
It is inevitable that some powder will miss the extrusion and fall to the bottom of the booth. This can be re-used.
The aluminium sections are then moved to the oven where the temperature is up to 200 degrees Centigrade. In this baking process the powder will go though four stages of “curing”. Once complete and cooled the aluminium sections are ready for tape protection if used and ready for handling.
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