1 Overview
Barrel plating is suitable for electroplating of small parts that cannot be affected or affected by shape, size and other factors. Compared with the method of hanging plating or basket plating in early small parts, it saves labor and improves labor productivity. Moreover, the surface quality of the plated parts is greatly improved. Therefore, the invention and application of barrel plating undoubtedly have a very positive significance in the field of small parts plating. Barrel plating has been used industrially since the 1920s. Domestic barrel plating first appeared in Shanghai in the mid-to-late 1950s. Mechanized continuous barrel plating equipment was used around the 1960s, but the equipment at that time could only be manually controlled, while the large-scale automatic barrel plating production line was probably from the 20th century. It has only been widely used since the beginning of the decade. At present, the production of barrel plating accounts for about 50% of the entire electroplating process, and involves dozens of plating types such as galvanizing, copper, nickel, tin, chromium, gold, silver and alloy. Barrel plating has become an electroplating process that is very common in applications and is almost in parallel with the plating.
2 concept of barrel plating
Barrel plating is strictly called roller plating. It is a kind of electroplating in which a certain number of small parts are placed in a special roller, and various metal or alloy plating layers are deposited on the surface of the part in an indirect conductive manner in a rolling state to achieve surface protection decoration and various functional purposes. Processing methods. The typical barrel plating process is as follows: the small parts that have been pre-plated are loaded into the drum, and the parts are pressed against the cathode conductive device in the drum by its own gravity to ensure the current required for the parts to be plated. Can be transmitted smoothly. Then, the drum rotates in a certain direction at a certain speed, and the parts are continuously rolled and dropped after being rotated in the drum. At the same time, the main metal ions are reduced to metal plating on the surface of the part after being subjected to an electric field, and the fresh solution outside the drum is continuously replenished into the drum through numerous small holes in the drum wall, and the old liquid in the drum and the electroplating process are generated. Hydrogen also exits the cylinder through these small holes.
Barrel plating should generally have the following basic characteristics:
2.1 Barrel plating is carried out in the drum
The biggest difference between barrel plating and small part hanging plating is that it uses a roller, which is a feeding device that carries small parts and is plated during the continuous rolling process. A typical roller has a hexagonal prism and is placed horizontally horizontally. One side of the drum wall is open, and a certain number of small parts are loaded into the drum from the opening when plating, and then the drum door is closed to close the opening. The wall of the drum is covered with a large number of small holes, and the conduction of current between the parts and the anode during plating, the renewal of the solution inside and outside the cylinder, and the discharge of exhaust gas need to pass through these small holes. The cathode conductive devices in the drum are pierced from the central shaft holes on both sides of the drum by copper wires or rods, and then respectively fixed on the conductive legs of the left and right wall panels of the drum. The part is naturally connected to the cathode conductive device by its own gravity in the drum. The barrel plating of small parts is carried out in such a device. The structure, size, size, rotation speed, conductivity and opening ratio of the roller are related to the production efficiency of the barrel plating and the quality of the coating. Therefore, the roller is one of the focuses of the entire barrel plating technology research.
2.2 Barrel plating is performed during the process of small parts rolling over and over.
When barrel plating, the small parts are not stationary in the drum, but are constantly rolling as the drum rotates. This type of tumbling is specific to a part: it is buried inside the entire stacked part for a while, and then turned to the outer surface. This is repeated until the end of the entire barrel plating process.
So why do you keep small parts rolling in the drum?
(1) Ensure that each part can be uniformly plated. Small parts are stacked together in the drum. Some parts are distributed inside the pile, called inner parts. The other parts are distributed on the outer surface of the pile, called surface parts (as shown in Figure 1). . In the case of barrel plating, the main metal ions are actually reduced only on the surface of the surface parts to form a metal plating layer, and the inner layer parts are affected by the shielding and shielding of the surface parts, and only the current passes, but almost no electrochemical reaction occurs. Therefore, in order to have the opportunity to be plated, the inner parts need to be turned from the inside of the pile to the surface parts. The surface parts can not stay for a long time, and after plating for a while, they are turned into inner parts by the rotation of the drum. In this way, small parts can only be tumbling continuously, so that the inner and surface parts can be continuously changed and converted, and finally, each part has a chance to be uniformly plated.
1-Inner parts; 2-surface parts Figure 1 Schematic diagram of inner and surface parts
(2) Avoid surface parts “burn black” or “scorch”. If the small part is in a stationary state in the drum without rolling, the use of a small current density may cause a lack of metal ions in the vicinity of the surface part to cause a "burning" phenomenon. Especially close to the surface parts of the drum wall, the current entering the drum from the hole is hindered, so that it stays on the narrow surface of the part close to the hole, causing the coating to burn and leave black spots, so-called “ Roller eye prints & rdquo;. At this time, the effect of the small part rolling in the drum is similar to the stirring of the solution or the movement of the cathode. If there is no solution stirring or cathode movement during the plating, the upper limit of the current density is not easy to be improved, and the deposition speed of the plating layer is also difficult to accelerate.
2.3 The current required for small parts during barrel plating is transmitted in an indirect manner
When hanging plating, the current required by the part is directly transmitted by the hanger, and the part is in close contact with the hanger without any medium in between. Therefore, the current of the hanging plating is smooth, the contact resistance is small, and the current obtained by each part is basically not different due to the transmission problem. However, during barrel plating, the part is integrally pressed against the cathode conductive device in the drum, and the parts directly connected to the cathode conductive device have only a very small part, and most of them can only be electrically connected to the cathode by stacking overlapping parts. Therefore, the cathode conductive device in the drum can only deliver the current to the parts that are in direct contact with itself, and then can be transported to other parts by these parts and transferred one by one between the other parts and parts. This is barrel plating. Indirect conduction. This indirect conduction is undoubtedly another important feature of barrel plating. Because it mainly relies on indirect conduction between parts and parts, rather than direct contact of the parts with the cathode, the contact resistance of the parts during barrel plating increases correspondingly to the plating.