Introduction to the work experience of aluminum anodic oxidation dyeing black

1 Design and production of fixtures

1.1 Features of the fixture

The fixtures used for anodizing are completely different from the hangers used for electroplating. It is not suitable to use hangers similar to the hooks used for electroplating as the anodizing fixtures, because the fixture and the workpiece surface will quickly form an oxide film during anodizing. In this process, a slight looseness of the clamp and the workpiece will change the contact position and hinder the flow of current. For this reason, a flexible clamp must be used to clamp the workpiece. Only in this way can the anodizing process proceed normally.

1.2 The structure of the fixture

The fixture structure is suitable for individual type. If the assembled type is used, the riveted or welded joints will loosen due to corrosion after several times of use, which will hinder the normal flow of current during the anodizing process. At the same time, the fixture must have a certain cross-sectional area. A fixture with a certain cross-section also has sufficient elasticity and clamping force to keep the workpiece in good contact with the fixture, and to ensure the normal flow of current required when the clamped workpiece is anodized. Avoid burning the workpiece due to heat generated by poor contact.

1.3 Selection of fixture materials

It is better to choose hard aluminum materials for making fixtures. Hard aluminum materials have good elasticity and are fastened and durable.

2 Clamping of the workpiece

2.1 Clean and remove the film from the fixture

The fixture will also produce oxide film during the anodizing process. For this reason, the oxide film must be removed before using the used fixture again. The film removal can be carried out in the degreasing solution of aluminum. The oxide film on the contact part of the fixture and the workpiece can also be filed with a file. This method can also extend the service life of the fixture for some fixtures.

2.2 Selection of clamping position

The position for clamping the workpiece should be selected properly, generally it should be clamped on the secondary surface of the workpiece (that is, the non-decorative part). Otherwise, the contact part of the workpiece and the fixture will not be able to form an oxide film because it is covered by the fixture, and of course it will not be dyed. There will be obvious white spots, which will affect the appearance quality. In addition, whether the recessed part of the workpiece suspended in the solution will generate air pockets after the workpiece is clamped should also be considered.

2.3 Prevent deformation of workpiece during clamping

The clamp is not the same as the hanger, and the clamp has a certain degree of flexibility. Particular attention should be paid when clamping deformed workpieces, and excessive force should be avoided to cause deformation of the workpiece.

2.4 Prevent too loose clamping

When the workpiece is clamped too loosely, the current between the fixture and the workpiece will be on and off from time to time. In this case, it is likely to burn the workpiece.

2.5 Clamp the anodized parts to be dyed one by one

Some units use screen cloth to wrap some small pieces or use other methods for anodizing. Although this method can save man-hours and improve efficiency in certain occasions, it can only be used for some natural anodizing with low quality requirements. Even if a small number of workpieces cannot form an oxide film where they cover each other, it is not easy to be identified. But for workpieces that need to be dyed black, this clamping method is obviously not advisable. It must be clamped one by one to ensure the quality of anodizing.

3 Control of anodic oxidation process conditions

3.1 The relationship between the temperature of the solution and the voltage

In the rated range, the lower the temperature of the solution, the higher the voltage required, because when the solution temperature is lower, the oxide film formation speed is slower and the film layer is denser. In order to obtain a certain thickness of oxide film, the anodic oxidation process needs to be increased. high voltage. When the temperature of the solution is high, the dissolution rate of the oxide film increases, and the resulting oxide film is loose. At this time, reducing the voltage can appropriately improve the quality of the oxide film.

3.2 The relationship between temperature and time of anodizing solution

The lower the temperature of the solution, the longer the anodizing time required. This is because when the solution temperature is low, the formation rate of the oxide film is slow. When the temperature of the solution increases, the rate of formation of the oxide film increases. At this time, the anodic oxidation time should be shortened, otherwise the film will dissolve due to the increase of the outer resistance of the oxide film, and the workpiece size will change, and the surface will be rough and the film will drop.

The above measures are only emergency measures taken under the conditions of neither cooling equipment nor heating devices.

4 The formula and preparation process of the black dyeing solution

4.1 Formula and process conditions

Acid wool ATT 20～30g/L Acid lake blue 2～3g/L Temperature 50～70℃ Time 10～15min pH 3～3.5 (or 4.5～5.5)

4.2 Boil the solution to promote the complete dissolution of the dye and ensure no lumps. After the solution is cooled, filter with filter paper to filter out insoluble particles and oily substances floating on the liquid surface. Afterwards, measure the pH value and adjust it to the process requirements with cold acetic acid or ammonia water.

5 Matters needing attention in the dyeing process

5.1 Strengthen the washing before dyeing

The workpiece should be fully rinsed after being taken out of the anodizing tank, especially the slits and blind holes of the workpiece, otherwise the residual acid and alkali will flow out slowly during the dyeing process, causing the pH value of the dyeing solution to deviate from the normal range, and The color of the residual acid-base parts is obviously different from that of the clean parts, and even the oxide film is corroded to show white.

5.2 Dyeing after anodizing

The workpiece should be dyed immediately after anodizing. If the workpiece is exposed to the air for too long after anodizing, the pores of the film will shrink and may be stained with dirt, which will cause difficulty in dyeing. If the dyeing tank is too small, when dyeing in batches is required, the parts to be dyed should be soaked in clean water.

5.3 Workpieces cannot overlap during dyeing

During dyeing, the workpieces must not overlap, especially the flat parts, otherwise the overlapped parts will be covered and form a yin and yang surface.

5.4 Strengthen the washing after dyeing

If the surface of the workpiece is not rinsed, residual pigments will contaminate the assembly.

6 Precautions for cleaning up

6.1 Carefully disassemble the workpiece

Strictly prevent the workpiece from being scratched when disassembling the fixture, otherwise white marks will appear. Loosen the fixture to allow the workpiece to fall off freely. Never pull the workpiece before the fixture is loosened.

6.2 Workpiece drying method

Shake the water in the hole of the workpiece first to prevent the residual water from polluting the surface of the workpiece. The drying method is to wipe with a towel. During the wiping process, the surface frost caused by the aluminum material or the operation process can also be wiped clean.

6.3 Packaging requirements

Use soft, clean paper for packaging, and pack them one by one to prevent each other from wiping.

7 Maintenance and maintenance of black dyeing solution

The maintenance and maintenance work of the black dyeing solution is mainly to maintain the pH value of the solution. According to relevant information, acid ATT dyes are composed of acid blue black 10B and acid orange II with different diffusion properties. When the pH value of the dyeing solution is in the range of 5-6 or 3-3.5, the absorption of Acid Orange II and Acid Blue Black 10B is better, and the amount absorbed by the oxide film is also larger. There are two dyes in the film. The amount meets the requirements of the ratio, and the obtained color is normal black. When the pH value is around 4, the oxidation film layer has an increased effect of absorbing acid orange II, and the absorption capacity will naturally increase. The acid orange II in the film layer is greater than the normal ratio, so the color appears reddish. When the pH value returns to the range of 3 to 3.5 or 5 to 5.5, the absorption of acid orange II by the oxide film layer will fall again, and the dyed workpiece will appear normal black again. Based on this, the pH of the black dyeing solution Accurate control of the value is very important.