During barrel plating, the solution temperature rises much faster than in rack plating. This is quite harmful to production using room-temperature barrel plating processes (such as potassium chloride barrel zinc plating and sulfate barrel tin plating). It can lead to deterioration of plating bath performance, reduced coating quality, high brightener consumption, and in severe cases, production shutdowns, making continuous summer production impossible. Therefore, it is necessary to take measures to control the temperature of the barrel plating solution (mainly the solution of room-temperature barrel plating processes) within an ideal range. The measures are summarized as follows.
1. Reduce volume current density to slow down the temperature rise of the barrel plating solution.
Current density refers to the current intensity passing through a unit volume of solution. For example, if the total bath volume is 500 L and the plating current is 100 A, then the volume current density is 100 A/500 L = 0.2 A/L.
A high volume current density is one of the important causes of the rapid temperature rise of barrel plating solutions. Therefore, reducing the volume current density is an effective measure to slow down the temperature rise.
The common phenomenon of "small barrel, large plating tank" in production is based on this principle: a small barrel uses less current, and a large tank has a larger solution volume, resulting in a lower volume current density.
Practice has shown that controlling the volume current density of barrel plating within an appropriate range can basically keep the solution temperature stable during continuous summer production.
As for what value is appropriate, it varies with the plating bath, and different operators and equipment manufacturers have different empirical data, so it will not be elaborated here.
2. Reduce solution resistance.
The relatively high resistance of the barrel plating system is another important cause of the rapid temperature rise of the solution.
The high resistance of the barrel plating system is manifested as a large (R_solution + R_electrode), where R_solution is the solution resistance.
Reducing the solution resistance R_solution can reduce the heat generated during barrel plating, effectively slowing down the temperature rise.
(1) From the solution perspective: under the premise of causing no other effects, the conductive salt content should be kept as high as possible, especially ensuring that the conductive salt is not insufficient or even deficient.
A higher conductive salt content leads to lower internal resistance and less heat generation. In addition, a higher conductive salt content is also beneficial to improving the throwing power of the solution.
(2) From the equipment perspective: this mainly refers to the barrel, specifically its enclosed structure. The enclosed structure of the barrel is an important cause of the high internal resistance of the barrel plating solution, and its influence is far greater than the appropriate fluctuation of the conductive salt content in the solution.
Therefore, improving or breaking the enclosed structure of the barrel is the key to reducing the internal resistance. Common measures include improving barrel perforations, circulating spray flow inside and outside the barrel, and adopting vibratory plating.
3. Reduce electrode resistance.
Another manifestation of the high resistance of the barrel plating system is a large R_electrode. In rack plating, R_electrode is small and can generally be ignored. However, due to the indirect conduction method in barrel plating, R_electrode is large and cannot be ignored, so the heat generated by R_electrode also cannot be ignored.
Therefore, reducing the electrode resistance R_electrode can slow down the temperature rise of the barrel plating solution to a certain extent.
For example, the cathode wire of the barrel should not be too thin, the cathode conductive nails should not be too small, and poorly conductive parts should be plated with the addition of metal balls. These are common measures to reduce electrode resistance.
4. Use wide-temperature additives.
Using wide-temperature additives is a common measure to achieve continuous summer production in barrel plating.
There is no denying that wide-temperature additives bring great convenience to the summer production of room-temperature barrel plating processes, but it is unscientific to unilaterally exaggerate or promote their effects.
The use of wide-temperature additives also needs to be combined with other measures (such as reducing volume current density, reducing solution resistance, and adopting solution cooling measures). Otherwise, if the solution temperature is high, production can only continue, but many problems will arise, such as poor coating quality (e.g., brightness, low-current-density plating performance, coating purity), high additive consumption, increased tank sediment, fast anode dissolution, and rapid changes in bath composition and pH value, which is not worth the loss.
In addition, high-quality wide-temperature additives should be selected. The anionic surfactants in such additives are not easily decomposed at high solution temperatures, causing relatively little impact on coating quality (mainly coating purity).
5. Adopt solution cooling measures.
Adopting solution cooling measures is an effective way to control the temperature rise of the barrel plating solution.
Common cooling measures for barrel plating solutions can be summarized into two types: in-tank cooling coil cooling and external heat exchanger cooling.
(1) In-tank cooling coil cooling: Cooling pipes of a certain surface area are laid in the solution inside the plating tank, and a circulating cooling medium with a calculated flow rate and required temperature is passed through the pipes, thereby lowering or controlling the solution temperature.
The cooling pipes can be metal tubes such as lead-lined steel pipes, lead-antimony alloy, stainless steel, titanium tubes, or plastic tube bundles such as polytetrafluoroethylene (PTFE), polyethylene (PE), and polypropylene (PP).
Figure 1 Schematic diagram of a double-sided tank-side frame tube-bundle heat exchanger: 1- chilled water inlet; 2- chilled water outlet; 3- plastic tube bundle; 4- frame.
The cooling medium can be tap water, groundwater, chilled water, etc., among which chilled water has the best effect. When chilled water is used, a suitable chiller needs to be matched, and the chiller size can be determined through consultation with the manufacturer.
(2) External heat exchanger cooling: The bath solution to be cooled is pumped into a heat exchanger installed outside the plating tank with a corrosion-resistant pump. The solution continuously circulates and exchanges heat with the cooling medium in the heat exchanger to lower its temperature, and the cooled solution finally returns to the plating tank. Through this repeated cycle, the bath temperature can be controlled within the process-required range.
The advantages of this method are that it does not occupy any space inside the plating tank, the heat transfer conditions between the bath solution and the cooling medium are good, the heat exchange efficiency is high, and the bath temperature is relatively uniform.
Figure 2 Schematic diagram of a shell-and-tube heat exchanger: 1- heat exchanger core; 2- shell; 3- chilled water inlet; 4- chilled water outlet; 5- solution inlet to be cooled; 6- solution outlet to be cooled.
The rapid temperature rise of barrel plating solution brings great harm to barrel plating production. By controlling volume current density, reducing solution and electrode resistance, adopting wide-temperature additives, and taking solution cooling measures, this problem can be solved to achieve continuous summer production.