During barrel plating, the solution temperature rises much faster than in rack plating. This is particularly harmful to normal-temperature barrel plating processes (such as potassium chloride barrel zinc plating and sulfate barrel tin plating), causing deterioration of plating solution performance, decline in coating quality, high brightener consumption, and in severe cases, shutdown. As a result, continuous summer production becomes impossible. Therefore, it is necessary to control the temperature of barrel plating solutions (mainly those of normal-temperature barrel plating processes) within a desirable range. The measures are summarized as follows.
1. Reduce the volume current density to slow down the temperature rise of the barrel plating solution.
Volume current density refers to the current intensity passing through a unit volume of solution. For example, if the total plating solution volume is 500 L and the current used is 100 A, the volume current density is 100 A/500 L = 0.2 A/L.
High volume current density is one of the important reasons for the rapid temperature rise of barrel plating solutions. Therefore, reducing the volume current density is an effective measure to slow down the temperature rise.
This is the principle behind the common industrial phenomenon of "small barrel, large tank". A small barrel uses less current, and a large tank contains a larger solution volume, resulting in a lower volume current density.
Practice has proven 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 the appropriate volume current density, it varies with different plating solutions, and different operators and equipment manufacturers have different empirical data. This will not be discussed in detail here.
2. Reduce solution resistance. The high resistance of the barrel plating system is another important cause of the rapid temperature rise of barrel plating solutions.
High resistance in the barrel plating system is reflected as (R solution + R electrode) being large, where R solution is the solution resistance.
Reducing the solution resistance R solution can reduce the heat generated during barrel plating and effectively slow down the temperature rise of the solution.
(1) From the solution perspective: As far as the solution is concerned, the content of conducting salt should be made as high as possible without causing other effects, and in particular, it must not be insufficient or lacking.
A higher conducting salt content means lower internal resistance of the solution, thus less heat generation. In addition, a higher conducting salt content also helps improve the throwing power of the solution.
(2) From the equipment perspective: This mainly refers to the barrel, specifically the closed structure of the barrel. The closed structure of the barrel is an important cause of the high internal resistance of the barrel plating solution, and its impact is far greater than the moderate fluctuations of the conducting salt content in the solution.
Therefore, improving or breaking the closed structure of the barrel is the key to reducing the internal resistance of the barrel plating solution. Common measures include improving the barrel openings, circulating jet flow inside and outside the barrel, and adopting vibratory plating.
3. Reduce electrode resistance. Another manifestation of high resistance in the barrel plating system is that R electrode is large. In rack plating, R electrode is small and generally negligible, but in barrel plating, due to the indirect conductive method, R electrode is relatively large and cannot be ignored. Therefore, the heat generated by R electrode also cannot be ignored.
Thus, 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 contact nails should not be too small, and parts with poor conductivity should be plated together with metal balls. These are common measures to reduce electrode resistance.
4. Use wide-temperature additives. Using wide-temperature additives is a common measure for barrel plating to achieve continuous summer production.
Admittedly, wide-temperature additives bring great convenience to the summer production of normal-temperature barrel plating processes. However, it is unscientific to one-sidedly exaggerate or pursue the effect of wide-temperature additives.
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 too high, production can only barely continue, and many problems will arise, such as poor coating quality (e.g., brightness, low-current-density plating performance, coating purity, etc.), high additive consumption, more sludge at the tank bottom, faster anode dissolution, and rapid changes in bath composition and pH. The losses will outweigh the gains.
In addition, high-quality wide-temperature additives should be selected. The anionic surfactants in such additives are not easily decomposed at high solution temperatures, so their impact on coating quality (mainly coating purity) is relatively small.
5. Adopt solution cooling measures. Taking solution cooling measures is an effective way to control the temperature rise of barrel plating solutions.
Common cooling measures for barrel plating solutions can be summarized into two categories: in-tank cooling coil cooling and external heat exchanger cooling.
(1) In-tank cooling coil cooling: Lay cooling pipes with a certain surface area in the solution inside the plating tank, and then pass a circulating cooling medium with the calculated flow rate and required temperature through the cooling pipes, thereby lowering or controlling the solution temperature.
Cooling pipes can be made of steel pipes lined with lead, lead-antimony alloy, stainless steel, titanium tubes, or other metal tubes, or plastic tube bundles such as polytetrafluoroethylene (PTFE), polyethylene (PE), and polypropylene (PP).
Figure 1 Schematic diagram of a frame-type tube bundle heat exchanger with side channels in both directions: 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 is the most effective. The use of chilled water requires a matching chiller, and the size of the chiller can be determined in consultation with the equipment manufacturer.
(2) External heat exchanger cooling: The bath solution to be cooled is pumped into a heat exchanger installed outside the plating tank using a corrosion-resistant pump. The bath solution and the cooling medium in the heat exchanger continuously exchange heat in a circulating manner to cool down. The cooled solution finally returns to the plating tank. This cycle is repeated to control the bath temperature within the required process range.
The advantages of this method are that it does not take up any space inside the plating tank, the heat transfer conditions between the bath solution and the cooling medium are good, the heat transfer 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 solutions causes considerable harm to barrel plating production. It can be solved by controlling the volume current density, reducing solution and electrode resistance, using wide-temperature additives, and adopting solution cooling measures, so as to achieve continuous summer production.
