Products Finishing

SEP 2018

Products Finishing magazine is the No. 1 industrial finishing publication in the world. We keep our readers informed about the latest news and trends in plating, painting, powder coating, anodizing, electrocoating, parts cleaning, and pretreatment.

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Page 24 of 59

PRODUCTS FINISHING — 23 Table 1. Acid Etch and Low Caustic Process Conditions for the First Set of Samples ACID ETCH CAUSTIC ETCH SAMPLE CODE Temp. [°C] Time [min.] Temp. [°C] Al Cons. [g/L] Time [min.] 40 4 55 10 2 A 40 4 50 2 B 40 4 100 2 C 45 4 100 0.25 D — — 65 100 12 E anodizing is applied. The problems caused by the base metal, such as grain growth, segregation and streak marks, originate mainly from the production parameters (extrusion speed, die shape, cooling speed and even heat-treatment conditions) in the steps prior to anodizing. The need for more aluminum profiles with the desired esthetic properties has increased the demand for a replacement to conventional long-life caustic etching processes, the main cause of preferential etching and non-uniform surface appearance. Acid etch processes have been developed that create an even surface that masks these surface defects, however, the post-treatment caustic step (called the "low caustic" step) necessary to remove the aluminum fluoride residue from the surface can also lead to preferential etching and require resurfacing due to the lack of dissolved aluminum to inhibit the aggressiveness of the caustic. Increasing the dissolved aluminum amount would be the ideal option, but this solution can have two main drawbacks: increasing the process time and increasing the viscosity of the bath. Although the process time can be shortened by raising the temperature, elevated viscosity can cause many other problems, including increased dragout, uneven etching, "tiger stripes" and problems in the desmuting stage. The increase in demand for aluminum is correspondingly pushing the capacity of existing facilities, inclining them to sacrifice some of the surface properties required for anod- izing. Profiles with relatively poorer attributes require more masking than standard profiles, therefore products with complex shapes and increased extrusion speed are treated with acid etching prior to anodizing. But nowadays even the concealing capability of acid etching can fail to meet the standards, due to the process parameters of the low caustic post-treatment. Experimentation Two main sets of samples were prepared and treated in low caustic baths with different dissolved aluminum concentrations at various process parameters. The first set of tests studied the effect of dissolved aluminum on the aluminum surface; only the amount of dissolved aluminum was changed, and all the other parameters of the acid etch and low caustic step were the same. In the second set of tests, the surface gloss of the samples was adjusted to relatively similar values to study the effect of dissolved aluminum on the defect-concealing property of the acid etch process. The surface gloss was adjusted by changing the process time in various low caustic baths with different levels of dissolved aluminum. By comparing the second set samples using SEM and 3D profiling analyses, we were able to obtain the best possible flaw-masking properties for the combi- nation of acid etch and low caustic processes. The study used 6061 aluminum profiles. The chemical composition of the aluminum was investigated by using optical emission spectroscopy. Figure 1. The first set of samples, based on the specifications in Table 1 below. LOW CAUSTIC ETCH BATHS

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