Roll plating is suitable for the plating of small parts which cannot or cannot be mounted due to the influence of shape, size and other factors. Therefore, the invention and application of rolling plating in the field of small parts plating undoubtedly has a very positive significance. Rolling plating has been used in industry since the 1920s. Domestic rolling plating first appeared in Shanghai in the middle and late 1950s. Mechanized continuous rolling plating equipment began to be used in the 1960s or so, but the equipment at that time could only be controlled manually, while large automatic rolling plating production line was widely used from the 1990s. At present, the output of rolling plating accounts for about 50% of the whole electroplating process, and involves dozens of plating species such as zinc, copper, nickel, tin, chromium, gold, silver and alloy. Rolling plating has become a kind of electroplating processing method which is widely used and almost parallel with hanging plating.
Concept of roll plating
Roll plating is technically called roller plating. It is to put a certain number of small parts in a special roller, in the rolling state of indirect conductive way to make the parts on the surface of the deposition of various metals or alloy coating, in order to achieve the surface protection decoration and a variety of functional purposes of electroplating processing. The typical rolling plating process is as follows: the small parts are loaded into the drum after the pre-plating treatment, and the parts are pressed tightly by their own gravity on the cathode conductive device in the drum, so as to ensure the smooth transmission of the current required by the parts during plating. Then, the drum with a certain speed according to a certain direction of rotation, parts in the drum by the rotation action after continuous rolling, falling. At the same time, the main metal ion is reduced to metal coating on the surface of the parts after being affected by the electric field, and the fresh solution outside the drum is continuously replenishes to the drum through numerous holes in the drum panel, and the old liquid inside the drum and the hydrogen produced in the electroplating process are also discharged out of the barrel through these holes.
Roll plating generally should have the following basic characteristics.
2.1 roll plating is carried out in the drum
The biggest difference between roll plating and small parts hanging plating is that it USES a drum, the drum is carrying small parts in the process of continuous rolling by a plating device. The typical roller is six prismatic and is placed horizontally. One side opening of the drum panel, when electroplating, a certain number of small parts from the opening into the drum, and then cover the drum door opening closed. The drum panel is full of many holes, which are needed to conduct the current between the parts and the anode, update the solution inside and outside the cylinder, and discharge the waste gas during electroplating. The cathode conducting device in the drum is passed through the central shaft hole on both sides of the drum by copper wire or rod, and then fixed on the conductive feet of the left and right wall panels of the drum respectively. The parts are naturally connected with the cathode conductive device by their own gravity in the drum. The rolling plating of small parts is carried out in such a device. The structure, size, size, rotating speed, conductive mode and hole ratio of the roller are all related to the production efficiency and quality of the coating. Therefore, the drum is one of the key points in the research of the whole rolling plating technology.
2.2 rolling plating is carried out in the continuous rolling process of small parts
Roll plating, small parts in the drum is not stationary, but to keep rolling with the rotation of the drum. This roll is specific to a particular part: one moment it is buried in the interior of the entire stack of parts, another time it is flipped over to the outside surface. This cycle continues until the entire rolling process is complete.
So why keep the little parts rolling around inside the drum?
(1) to ensure that each part can be evenly plated. Small parts are piled together in the drum, part of the parts are distributed in the accumulation of the body, called the inner parts; The other part of the parts are distributed on the outside surface of the accumulation body, known as surface parts. During rolling plating, the main metal ion is actually reduced to form a metal coating on the surface of the surface parts, and the inner parts are only through the current due to the influence of the surface parts shielding, shielding, and so on, but there is almost no electrochemical reaction. Therefore, in order to have a chance to be plated, the inner parts need to be turned out from the inside of the accumulation body into the surface parts. And surface parts can not stay for a long time, after electroplating for a while, by the rotation of the roller into the inner parts. In this way, small parts only keep rolling, to make the inner parts and surface parts constantly changing, conversion, and finally ensure that each part has a uniform plating opportunity.
(2) avoid "burning black" or "charring" of surface parts. Small parts in the drum if not rolling and in a state of rest, then the use of very small current density, may make the surface parts near the metal ion shortage and produce "charring" phenomenon. Especially close to the surface parts of the drum panel, will make the current from the hole into the drum is blocked, so as to concentrate on the parts close to the hole part of the narrow surface, resulting in the coating charred left black eye, that is, the so-called "cylinder eye print". At this time, the role of small parts in the roller tumbling, similar to hanging plating solution stirring or cathode movement. If there is no effect of solution stirring or cathode moving during hanging plating, the upper limit of current density is not easy to be raised, and the deposition rate of coating is difficult to be accelerated.
2.3 the current required for small parts during rolling plating is transmitted in an indirect manner.
During hanging plating, the current required by the part is directly transmitted by the hanging tool, and the part is in close contact with the hanging tool without any medium in the middle. Therefore, the current transmission of hanging plating is stable, the contact resistance is small, the current obtained by each part is basically not different because of the transmission problem. But when rolling plating, the part is the overall pressure in the drum on the cathode conductive device, and the cathode conductive device directly connected to the parts only a few parts, and the vast majority of only through the accumulation of overlapping parts and cathode conduction. Therefore, the cathode conductive device in the drum can only first send current to their direct contact parts, and then by these parts to other parts, and in other parts and parts one by one transmission, this is the indirect conductive way of rolling plating. This indirect conductive mode is undoubtedly another important feature of roll plating. Because it mainly depends on the parts and parts between the indirect conductive, rather than the parts directly contact with the cathode conductive, so, roll plating parts contact resistance than the corresponding increase in hanging plating.
Classification of roll plating
According to the shape and axial direction of the roller used in rolling plating, the types of rolling plating should be divided scientifically. Drum shape refers to the appearance of the drum is similar to what kind of objects, drum axial direction refers to the drum rotation axis and the horizontal surface of the relationship. According to the differences of the two aspects of the drum, the common rolling plating methods in electroplating production are divided into three categories: horizontal rolling plating, inclined rolling plating and vibration plating.
2. Horizontal roll plating
The roller shape of horizontal roll plating is "bamboo tube" or "column", which is placed horizontally when used. The drum axial direction is horizontal, so horizontal roll plating is also called horizontal roll plating. In the production of common hexagonal drum, chrome drum, rod (or spokes) drum, needle drum, etc., belong to the category of horizontal rolling plating. Among them, the hexagonal drum is the most widely used.
2.1 cross section shape of drum
The cross - section shapes of horizontal drum are hexagonal, octagonal and circular. The use of hexagonal drum, parts in the rolling fall of large amplitude, the mixing of parts is sufficient, so the coating thickness volatility is better than other shapes of the drum. This advantage is more obvious when the load does not exceed one half of the drum volume. In addition, the effect of mutual polishing between the hexagonal drum parts is strong, more conducive to improve the brightness of the coating.
2.2 drum axial direction
The axial direction of the horizontal drum is horizontal. Therefore, when the horizontal drum to drive parts rolling, parts running direction and horizontal plane vertical, which is conducive to the full mixing between parts and improve the brightness of the coating. Moreover, the vertical operation of the parts also wins the advantage for the load capacity of the horizontal drum.
For example, it is not uncommon to see a horizontal drum in production with about 150kg of parts, which is unthinkable for other types of rolling plating. Especially in recent years, the length and diameter of the horizontal drum has a greater development, suitable for rolling parts size and weight has increased, many of the original hanging parts can also be rolled. All these make the advantages of high production efficiency of rolling plating labor better reflected.
Horizontal rolling plating is most widely used in rolling plating because of its advantages such as high production efficiency, good surface quality and wide range of applicable parts. The application scope of horizontal rolling plating covers the hardware, home appliances, automobile and motorcycle, bicycle, electronics, instruments, watches, pens, magnetic materials and other industries of small parts electroplating processing of the vast majority of the small parts is worthy of the name of the main force of small parts electroplating processing. Therefore, for many years the research focus of roll plating technology is always around the horizontal roll plating. However, due to the closed structure of the horizontal roller, the defects such as long time, uneven coating thickness, poor coating quality in the low current area of parts and so on have affected its application in production.
3. Inclined roll plating
The shape of the inclined drum is "bell" or "bowl", so the inclined drum is also known as the bell drum. The axial direction of the drum and the horizontal plane approximately 40 ~ 45 Angle, then the running direction of the parts inclined to the horizontal plane, the name of the inclined rolling plating is from this.
The tilting roller plating equipment currently in use is called tilting submersible roller plating machine. Tilting submersible roller plating machine has been used in Shanghai since 1960s. It is popular because of its light and flexible operation and easy maintenance. In addition, the use of inclined roller plating machine plating parts are less damaged, more suitable for vulnerable or high dimensional precision parts. However, tilting roller plating machine is inferior to horizontal roller plating machine in terms of labor production efficiency and surface quality due to its small drum load and insufficient rolling strength of parts. Therefore, the application and development of inclined roll plating always lag behind that of horizontal roll plating.
4 vibration plating
Vibration electroplating is a new electroplating technology for small parts developed in the late 1970s and widely used in the early 1980s. It has more outstanding advantages than conventional rolling plating technology, so it has been rapidly applied and developed. Vibration electroplating appeared in China in the late 1980s and has been widely used in the field of small parts electroplating since the late 1990s.
The drum shape of vibrating electroplating is "round sieve" or "disk", the movement of the drum parts is realized by the vibration force from the oscillator. Therefore, the drum of vibration electroplating is generally called "vibrating screen". The vibration axial direction of the vibrating screen is perpendicular to the horizontal plane, then the movement direction of the parts in the vibrating screen is horizontal.
The structure and axial vibration of vibrating electroplating sieve are essentially different from the traditional horizontal roller, so it will produce a completely different effect from the traditional horizontal roller plating:
After the upper part of the material basket of the vibrating sieve is opened, the closed structure of the traditional horizontal drum is completely broken, and the ion concentration difference between the inside and outside of the drum is eliminated. Therefore, the defects of rolling plating caused by the roller enclosed structure are improved to the greatest extent. For example, coating deposition rate is fast, thickness is uniform and parts of low current area coating quality is good.
(2) by controlling the vibration frequency or amplitude of the vibrating screen, the purpose of controlling the mixing condition of parts in the vibrating screen can be achieved, so as to minimize the fluctuation of the coating thickness of parts.
(3) electroplating when the use of large current density and at the same time a mechanical optical integration, coating crystallization, surface brightness is high. (4) the parts of the abrasion, wear are less than other rolling plating.
In addition, the cathode conduction is stable during the vibration electroplating, the phenomenon of clamped and stuck parts is lighter, and the quality of parts can be inspected at any time.
However, due to the limitation of vibrating screen structure and vibration axial direction, the loading capacity of vibrating screen is relatively small, and the cost of vibrating electroplating equipment is also relatively high. However, it is not suitable or can not use conventional rolling plating or high quality requirements of small parts, such as needle, small, thin wall, easy to scratch, easy to deformation, high precision parts, vibration plating has other rolling plating method incomparable advantages. Therefore, vibration plating is a powerful supplement to conventional rolling plating.
The three methods of rolling plating have their different characteristics, advantages and disadvantages and applicable scope, etc., according to the shape, size, batch and quality requirements of the plating, the production should choose an accurate and reasonable rolling plating method, in order to achieve the purpose of saving, increasing efficiency and improving product quality for the enterprise. For example, for conventional small parts, horizontal rolling plating should be preferred. And to unfavorable or cannot use horizontal type roll plating or the small part with high quality requirement, the way that considers vibration electroplating commonly. But vibration electroplating is not a kind of "panacea", sometimes for vibration electroplating can not solve the small parts, can use some more special electroplating, such as basket plating, screen plating or cloth bag plating.