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The anodizing process at the plant is similar in appearance to electroplating. The key difference with anodizing is that it does not involve coating one metal over another. Rather, it is a method of adding an additional layer of oxide to an existing metal. Anodizing is applied to many different metals, but is most often associated with aluminum, which is what this plant specializes in. Aluminum, when exposed to air, forms an oxide layer that naturally protects the metal and prevents further oxidation of the metal underneath, but this natural layer is very thin—anodizing creates a thicker protective layer that is more susceptible to staining.

The first step in anodizing is to secure the aluminum part to a conductive metal support. There are two different types of brackets available, depending on the part being coated: Titanium brackets are stronger and can hold more parts, but do not have the best conductivity, so these brackets are only used for anodizing for protection, not for aesthetically functional parts. Aluminum brackets conduct electricity better, but hold fewer components, so they are used for visible components, such as the outer panels of the Lian Li case.

Any point where the hook contacts the workpiece is not anodized, so aluminum hooks are custom-made for each new type of part to minimize contact, while titanium hooks are a more standardized design. Factory technicians determine the best place to attach each product to the hook, often choosing a spot hidden from view, such as inside a screw hole. The hooks themselves are anodized during production, so they do not last forever, but they can be reused. Factory technicians bring empty titanium racks from the nearby packing room to begin the cycle anew.

There are many types of aluminum, but the most commonly used alloys in factories are the 5000 and 6000 series. When asked if some alloys are easier to machine than others, we were told that harder alloys tend to be gray in color, while softer alloys produce better colors due to changes in conductivity.

Once the part is secured to the conductive support, cleaning is necessary. It is essential for the subsequent work that the surface is clean and free of fingerprints and other contaminants, so it is first cleaned with oil and then soaked in a degreaser for 35 minutes, then the degreaser is rinsed with water and the contaminants removed. These tanks are located in the middle of the first room and are surrounded by a short concrete curb to prevent splashing and overflow from the flush tank, which is constantly filled with fresh water. The insulated bumpers above these tanks are simply metal rods that hold the hanging brackets in place – no electricity is used until later in the process.

Chemical preparation for the anodizing process begins after the initial cleaning. At this stage, 7 slots are located along the back wall of the factory, away from the entrance. The first step is to clean with clean water, and then the first chemical step is to soak the parts in an alkaline mixture of sodium hydroxide, also known as lye or caustic soda, for about ten seconds. The temperature of the mixture was maintained between 60 and 70 degrees Celsius, and steam was visible in the tank, which had to be pumped out through a fume hood specially installed at the end of the pipeline. Lye can cause severe chemical burns, so care must be taken.

Alkaline baths are necessary to dissolve the natural aluminum oxide layer, which lacks the pores needed for oxygen to penetrate and create an oxide layer. Aluminum oxide is amphoteric, meaning it can react with both bases and acids. Sodium hydroxide reacts with it to form water, aluminate, and heats up. Sodium hydroxide also reacts with the underlying raw aluminum to form aluminate and hydrogen gas, so limit the bath time to 10 seconds. The surface finish is determined at this stage – special chemicals can be used here to make the final result lighter or darker.

After a ten second soak, five separate water baths are needed to rinse away any remaining sodium hydroxide. A mixture containing nitric acid is then used. Lye is very effective at removing aluminum oxide, but aluminum is often used as an alloy and may not be as effective when alloying metals. Dilute nitric acid is used to rinse away any remaining metals, a process called decontamination, followed by another series of water baths. Before dilution, nitric acid is supplied in cans at a concentration of 68% – any higher concentration would be considered potentially fuming nitric acid, which can produce smoke when exposed to air, which is undesirable and a safety hazard. The time between placing the parts in the lye bath and the final rinse with water is approximately ten minutes.

At this point, the aluminum oxide coating on the part has completely peeled off, so it is necessary to exclude as much contact with air as possible. There are several “spare tanks” in which the aluminum parts remain submerged in water in case of a blockage of the production line.

Finally, the anodizing process can begin. As in electroplating, the aluminum is immersed in a liquid, in this case a solution of 20% sulfuric acid and 80% water, the temperature is maintained at 20-25°C, and an electric current is passed through it. The brackets containing the aluminum components are suspended from a metal frame connected by large copper rods through which the electric current passes. Copper wedge-shaped blocks at each end form the contacts. A crane, manually operated from a nearby control station, moves back and forth over the tanks, lifting the frames into and out of their respective tanks.

There are only four anodizing tanks on this production line, the rest are used for rinsing and painting. Unlike galvanizing, the anodized metal is the anode of the circuit, hence the name. The current splits water molecules into hydrogen at the cathode and oxygen at the aluminum anode, creating visible bubbles and causing the aluminum to oxidize. The acidic electrolyte eats away at the oxide layer without completely dissolving it, allowing oxygen to penetrate deeper and oxidize more of the metal, creating a thicker, more porous structure than occurs in nature. The end result is an inflexible oxide shell that does not conduct electricity and has lower thermal conductivity than the aluminum underneath.

In terms of power, this series of water tanks can handle up to 25kW. We were told that the actual duration of the anodizing process is 20-40 minutes, the longer the darker the color. Black takes about 40 minutes, white or translucent coating takes about 20 minutes, and the rest is somewhere in the middle. The process can start from either side of the production line, depending on the daily logistics. The parts were washed 6 times in water and then painted.

Anodizing does not directly color the aluminum, but creates a beautiful open porous surface to which the dye can adhere: the darker the color, the thicker the oxide layer, and the longer it takes to anodize. The main production line processes only these two colors: black and white, as they are the most popular. Parts of other colors or small batches are processed by hand in small grooves located next to the larger parts. The factory offers thirty different standard colors. Custom colors are also available, but they require more labor. The dye tank is maintained at 110-160 ° C, but only requires about 15 minutes of preheating in the morning. Parts must be immersed in it for 5-10 minutes before painting. After painting, the parts are rinsed three times in water.

 

Each step of the anodizing process requires large amounts of water, which must be replenished throughout the day. Anodizing produces relatively little waste compared to spray painting, as aluminum-containing byproducts (such as aluminum hydroxide) filtered from the water can be used in other industries and resold. The water runs off from drainpipes in front of the plant, and the government often conducts surprise water quality checks. We were told that these requirements are easy to meet, as the chemicals used are not very toxic.

After painting, a quality check is performed. Compare the color to the standard and make sure it is within acceptable limits. The most common defects are caused by parts moving or hitting objects and damaging the surface. If a rack is moved carelessly, its parts can scratch each other. Smaller parts are easier to handle, while larger parts can cause more problems. The factory can be picky about defects, as almost everything can be repaired by re-sanding the oxide coating and repeating the process from scratch, but if there is serious damage, the part will be sent back. The customer sends an exact number of parts for anodizing and expects them all to be returned, so the factory must do everything possible to rework every defective part.

The final step is packaging. The factory itself doesn’t sell the parts, so that means safely packing the parts in bulk and shipping them back to customers.

As mentioned above, this is just one of several processes that case manufacturers use to coat surfaces on demand. Anodizing is the preferred process for visible aluminum parts, but electroplating is used for a smoother, less expensive finish, and paint is used for bright, varied colors. Each of these options has interesting differences.