This article is based on the technical data of the 3rd National Conference on Surface Finishing Technology of Metal Products and the exchange of discussions. The actual process of light industrial aluminum products can be used mainly for production scale, and the new processes and technologies are briefly introduced and summarized. Content is divided into the following sections:
(a) Surface treatment before oxidation.
(b) Oxidation process.
(c) Oxidized finishes.
(d) Other surface treatment processes.
(five) some superficial views.
Overview
With the development of aluminum smelting and processing technology, aluminum products are now widely used in aviation, building materials, vehicles, ships, light industry and other departments. In the advanced industrialized countries abroad, such as the United States, Britain, and Japan, since the development of industrial production of anodized aluminum films for aluminum products in the 1920s, the surface treatment technology for aluminum products has developed extremely rapidly over the past 60 years. In terms of light industrial products, the use of automated equipment, rigorous process oxidation products, through offset printing, thermal transfer printing coloring, electrophoretic painting and other decorative treatment will give the product a very beautiful, delicate appearance. China's aluminum product industry started slowly, but its development is also fast. From the beginning of the liberation, only a few small companies have now owned more than one hundred large enterprises across the country, and the annual output has exceeded 50,000 tons. However, with the continuous improvement of people’s living standards, aluminum products are still far from meeting the needs of people’s lives. In particular, compared with foreign products of the same type, there is a large gap between quality and fancy varieties, which shows that China’s aluminum products processing and surface treatment The technology is relatively backward. In recent years, through frequent domestic and foreign scientific and technological exchanges and the efforts of scientific and technological workers, new technologies and new technologies have been tested and adopted. However, in general, the surface treatment technology of aluminum products in China is still in a period of development to be developed. .
First, the surface treatment before oxidation
The effect of purity of the material composition of aluminum products on the quality of the oxidation film of aluminum oxide has long been understood by scientists and technologists. However, in the industrial production, the quality of the surface treatment of domestic aluminum products has seriously affected the quality of anodized products. This contradiction has become increasingly prominent. The aluminum sheet of foreign countries has undergone milling processing, smooth as a mirror, excellent quality after oxidation. In China, aluminum materials are often not very glossy, and bubbles, scratches, and rough skins are very serious. After anodizing, these defects still appear. The strong decorative process such as a variety of pattern printing, thermal transfer printing, etc. due to poor quality of the surface quality of the plate caused by poor results is particularly serious. Therefore, improving the appearance quality of the plate becomes an important part of ensuring the best quality of the oxidation of aluminum products.
In the degreasing treatment of aluminum products before oxidation, most factories still use weak alkali chemical degreasing or caustic soda degreasing and whitening, but in recent years, more and more factories have used the method of adding surfactants to degreasing. . Utilizing the unique physical and chemical properties of surfactants, reducing surface tension, emulsifying, solubilizing foaming, and its own excellent chemical stability to alkalis, hard waters, etc., to improve the degreasing effect, and to form a layer of alkaline surface The foam layer inhibits the evolution of alkali mist, reduces pollution, and improves operating conditions. For example, Tianjin Aluminium Products Factory introduced the use of AS anionic surfactant produced by Tianjin or Shanghai Synthetic Detergent Factory to wash white aluminum pots, which has received good economic results. The specific process is: caustic soda (NaOH) 10%; temperature 97 ~ 100 °C; time 5 ~ 7 seconds. Surfactant AS is added in an amount of about 0.2 g/l, depending on the situation.
In order to make the surface of aluminum products bright and smooth, the molded aluminum products are generally mechanically polished. However, oxidation is performed after mechanical polishing, and only a smooth oxide film can be obtained, and glossiness is often insufficient. For products requiring a high gloss decorative oxide film, chemical polishing or electrolytic polishing is also performed after mechanical polishing. Pure aluminum or high-purity aluminum-magnesium alloys can obtain a glossy surface with a high reflection coefficient after chemical polishing or electrolytic polishing, and particularly electrolytic polishing can achieve satisfactory results.
Unfortunately, the general tri-acid chemical polishing solution contains HNO 3 and heats to decompose nitrogen dioxide (NO 2 ) gas to produce the “Huanglong†hazard. The quality and stability of the electrolytic polishing solution contain chromic anhydride, resulting in chromium-containing wastewater, polluting the environment, resulting in three waste treatment problems. Naturally, people want to study chemical polishing that does not produce “Huanglong†and electrolytic polishing process that does not contain chromium. The "Wipe Additive for Aluminum Chemical Additives without Yellow Smoke, WXP" produced by Zhejiang Huangyan Fluorescence Chemical Factory passed the appraisal with the assistance of Beijing Environmental Protection Department. The additive WXP is used as a brightener for phosphoric acid and sulfuric acid-based polishing solutions, and has the function of inhibiting the acid mist. The process recipes used were: phosphoric acid (specific gravity d=1.7), industrial pure 800 ml/l; sulfuric acid (specific gravity d=1.84) industrial grade 200 ml/l; WXP 2 ml/l; temperature 95 to 120°C. Three grams of aluminum should be added per liter of new solution. Because the formula does not contain nitric acid, it does not produce "Huanglong" pollution during polishing; the effect of inhibiting the acid mist is also good.
As for electropolishing without chromic anhydride, many articles have listed many formulations, mostly adding some organic acids (such as citric acid, tartaric acid, oxalic acid) or alcohols (such as ethanol, butadiene) to phosphoric acid and sulfuric acid-based polishing solutions. Alcohols, glycerol, etc., in order to replace the chromic anhydride by these additives, acts as a corrosion-inhibiting polishing on the aluminum surface. However, after testing, such formulations have the disadvantages of high cost and poor electrolyte stability, which are difficult to promote in large industrial production. According to reports, many factories in Shanghai have been working on electroless polishing tests for chromium-free anhydride. Some plants have also passed the appraisal recently. We are looking forward to breakthrough test results in this area.
Second, oxidation process
The sulfuric acid method, the oxalic acid method, and the chromic acid oxidation method, particularly the sulfuric acid anodic oxidation method, are almost unknown to anyone engaged in the surface treatment of aluminum products. Each of these oxidation methods has its own characteristics and application range. For example, the oxalic acid method can obtain a thick oxide film layer. The oxide film itself has some decorative colors, but this method has high costs and consumes large amounts of electric energy. Sulfuric acid oxide film is transparent, colorless, and has good color-absorption properties. In addition, the electrolyte composition is simple and stable, the process is easy to operate, and the cost is low, so it is widely used. However, the sulfuric acid tank temperature control range is small, rapid heating, often need to install refrigeration equipment, which has become a difficult one. In Japan, a sulfuric acid-oxalic acid mixed-acid oxidation method was developed for a long time and both were eliminated, and it became Japan's main oxidation bath. Shenyang Aluminum Products Factory in China introduced the use of mixed acid oxidation technology. Their recommended formula is: sulfuric acid 10 to 20%; oxalic acid 1 to 2%; DC voltage 10 to 20 volts; anodizing.
For the oxidizing solution heating and cooling method, Harbin Aluminum Products No. 1 Factory introduced the experience of adopting BXO pole heat exchanger produced by Siping General Machinery Plant in Jilin Province. The heat exchanger is considered to be effective. It has the advantages of less investment, less land occupation, quick installation, high heat exchange efficiency, strong corrosion resistance, and good sealing performance. The heat exchanger is more suitable for oxalic acid oxidation solution. As for the choice of model size, it can be calculated based on the amount of bath fluid, referring to the description of “Polar Heat Exchanger Selection Guide†edited by Siping General Machinery.
III. Finished after oxidation
After oxidation, many aluminum products have to be dyed or printed and sprayed to obtain a variety of bright colors or beautiful patterns, increasing the number of fancy varieties. In 1980, the aluminum product transfer printing method identified by the Ministry of Light Industry was able to obtain a variety of color patterns with clear patterns, bright colors and rich levels. The aluminum product transfer printing method is also called sublimation transfer color technology. Its basic principle is to use a disperse dye specially made to transfer ink, and then print on the paper according to the pattern requirements to make a color printing paper, and then attach the printing paper to the aluminum products. On the oxide film, by heating and hot pressing, the disperse dyes on the printing paper are transferred into the pores of the oxide film in the vapor phase to form a color pattern. The transfer printing method obtains a color image on the surface of the aluminum product by its technology that is detached from the conventional oxidative dyeing method, and thus attracts many aluminum product companies to pay attention to trial and production. Aluminum products factories in Shanghai, Wuhan and Qingdao have all been put into industrial production. At this stage, the cost of the process is still high. To produce the transfer printing paper, the printing industry must rely on the printing industry. Most of the high-temperature pressurized transfer color patterns are still in the manual operation stage. Theoretically, the mechanism of the transfer of printing seems to have not been thoroughly studied. More people think that mainly in the high temperature heating conditions, the disperse dyes in the transfer ink sublimation to the oxide film. However, some experimental results show that the thickness and the adsorption performance of the oxide film also play a very important role, so it is not simply a phenomenon of sublimation of a disperse dye. It is believed that with the continuous exploration of the transfer mechanism, and continuing to improve the application process and accumulate experience, the transfer printing method will open up a vast new field for the beautification of aluminum products (see the materials 2 and 3 for the aluminum oxide transfer printing process).