Anodizing Process of Aluminium: Every Profile Has its Cost

The cost of the anodic oxidation of aluminium profiles is affected by numerous technical factors, which in many cases the very clients who use the profiles ignore

by Mario Favaron

Anodic oxidation, as we know, is the most widespread among surface finishing for aluminium profiles. A lesser-known aspect, even by the best part of end users of profiles, is the relationship between production cost and selling price of the anodized profiles. Considering the characteristics of this process, productivity and therefore industrial cost are influenced by the technical parameters of each profile, such as:

a) absorption perimeter;
b) cleansing perimeter;
c) weight;
d) length of the bars;
e) footprint;
f) shape complexity.
In Germany, the most prominent user of this finishing in Europe, the selling price is generally calculated per square metre (measured according to the absorption perimeter), while in Italy, the second European user, price per kilo prevails, as a consequence of the traditional sale of extruded profiles by weight. Both methods are valid, as long as behind the calculation of selling prices there is a detailed and accurate evaluation of costs for every single process phase, each with its own measurement parameter (per square metre, per kg/m or per bar).

Phases and costs of anodizing
Anodic oxidation is a process with many phases, but not a complicated one; technical knowledge, supported by the experience of the company carrying out the treatment, guarantees practically certain results. This stability helps to determine beforehand and correctly the single costs. This article intends showing the influence of each elementary phase of the process on the composition of the final cost.
The process examined was anodizing with sulphuric acid (one of the most frequently used), segmented into its most significant phases (figure 1). For each phase a value has been indicated (in red) which approximately shows the “weight” of the cost of each phase with respect to the final cost.
The simplified hypothesis considers a typical-average profile with the following properties:

Profile: tubular:
Weight: 800g/m;
Absorption perimeter: 350 mm;
Weight to perimeter
conversion ratio: 2.28;
Footprint: ≤ 100 mm (confined);
Shape complexity: medium;
Length of bars: 6000 mm.

Looking at the weight indexes of the cast shown in the scheme (in red), the phases of the process which most affect the final cost immediately stand out: these are the preparation of the surfaces, the thickness class of the oxide and colouring. By summing all of the “indexes” for the chosen cycle and multiplying the value thus obtained by 0,04 €/kg a realistic estimate of the final cost of the anodic oxidation may be obtained.

The most significant phases Phase A) Storage and unpacking Storage
It sometimes happens that large-scale users find it more convenient to ask the oxidizer to store the product, giving him the advantage of also stockpiling a guaranteed amount of work; the storage and handling costs should however not be underestimated, especially as regards the small orders which need to be processed and rearranged.

Profiles may be protected in many ways, ranging from the single sheet placed between layers of profiles packed in metal crates, to the packaging of single rods placed in strapped packages. Considering these operations are often manual, costs may rise significantly, especially for light profiles, short bars and small lots of material.

Phase B) Mechanical preparation of surfaces
Once this was carried out on the single rod; currently cleansing centres are used which can deal with several rods at the same time. It is necessary to evaluate correctly the real productivity also considering difficulties in carrying out the process and the real footprint and shape of the profiles.

Phase C) Placement on the anodizing racks
This is a cost deriving from the time needed to prepare the anodizing bath correctly. The elements which will turn out to be more costly are:
Light profiles and short bars (larger number of operations);
Heavy profiles with long bars (greater number of persons in charge).
Phase F) Formation of the oxide thickness
The increase of the thickness class brings about an increase in costs. The increase is almost proportional, were it not that, for larger thicknesses, a component of solution of the oxide thickness caused by the electrolyte should not be neglected
Phase G) Colouring
The oxide layer formed (as shown in the previous paragraph) is porous and therefore highly suitable for the different possibilities of colouring. The cost is connected to the thickness of the oxide and to the shade (light colours may be obtained in a shorter time with respect to darker ones) as well as to the type of process used.

Phase I) Sealing and closing pores
This process is necessary in order to close the pores which are present on the oxide layer formed during the anodizing phase. Sealing is carried out mainly by hydrating the layer. The methods most widely used are:
a) hot sealing at 95÷98 °C in deionised water;
b) cold sealing at 25÷30°C in water with specific salts added;
c) mixed cold and hot sealing.
The length of these processes is linked to the class of the thickness previously obtained: the greater the thickness of the oxide, the longer will the hydration/sealing phase be. It should be stressed that if sealing is poorly carried out this may result in a non-compliant product thereby jeopardizing the entire process.

Phase L) Final testing
The current trend is, verifying the desired quality by means of tests on the line during the process. It is however necessary not to underestimate the costs deriving from specific contract requirements previously agreed upon with the client and/or from applicable norms.

Phase M) Disconnection from the anodizing racks
The same considerations apply as in Phase C.

Phase N) Packaging
The same considerations reported for Phase A apply. It is recommended to evaluate it carefully, since clients often demand specifications which derive from their qualitative requirements and/or their stockpiling system, especially as regards maximum weight, size of the packages, positioning of the packing elements and such like.

This paper aimed at highlighting the opportunity of an analytical approach in terms of process phases which everyone will be able to adapt to their situation. By all means, more efficient training at all levels, the insertion of professionals capable of enacting a more modern corporate management (quality/process managers, management control and so on), a greater attention to costs today allow to trust that it will soon be possible to provide more rational price quotes. The market is the way it is, and sometimes other motivations prevail, but a deeper knowledge might be the basis of more informed choices.