Bicycle dusting process

In the early years, bicycle workpieces were mainly steel parts, which had to undergo processes such as degreasing, rust removal, phosphating, and passivation. Oil removal and phosphating are mainly phosphorus-containing substances, and rust removal is mainly pickling, using hydrochloric acid or sulfuric acid process. Giant's environmental protection process in pre-treatment has gone through several stages. First, the use of corrosion-inhibiting acid mist inhibitors during pickling reduces the discharge of acid mist. For the discharged acid gas, it is neutralized by alkaline spray water washing tower. rear discharge. Secondly, for the passivation treatment, the hexavalent chromium passivation agent is replaced by weakly alkaline phosphorus-containing substances, which eliminates the heavy metal pollution of hexavalent chromium. Thirdly, for phosphating, low-temperature and normal-temperature phosphating is adopted, and complexing agent is added in the formula to reduce the formation of sediment, and the phosphating tank is not discharged as a whole. Then, the supernatant liquid is pumped out at regular intervals, the sediment in the phosphating tank is cleaned up, and then the supernatant liquid is pumped back to add chemicals to reduce the water pollution caused by the discharge of the entire tank. The degreasing agent adopts a phosphorus-free degreasing agent, which reduces the discharge of phosphorus-containing wastewater. In recent years, with the rise of zirconium series and silane agents, Giant has also actively explored and researched this, and adopted the silane-zirconium salt composite coating process to solve the problems of poor corrosion resistance, easy rust and workpiece contact points after sulfuric acid pickling. For the problem of rust, the color of the silane-zirconium salt composite film is also gray-white similar to that of normal temperature phosphating, and the composite anti-rust effect after painting is basically the same. The pretreatment of iron and steel workpieces adopts phosphorus-free degreasing, sulfuric acid + corrosion inhibitor and fog inhibitor + spray water tower, silane zirconium salt composite film process, which is an environmental protection process with no phosphorus, no nitrogen, no briquettes and less acid mist. The pretreatment process of steel parts is shown in Figure 1.

Figure 1 Pretreatment process of steel parts

(1) For aluminum alloy workpieces, the initial Alodine hexavalent chromium treatment process was also changed to trivalent chromium process, and in recent years, it was changed to silane zirconium salt coating agent, which eliminated the use of heavy metal chromium;

(2) Environmental protection coating process, the main methods are powder coating, light curing coating, high solid coating, water-based coating and so on.

1.1 Powder coating

The advantages of powder coatings are that they have no VOC emissions, good physical and chemical properties, can be recycled, and have high paint utilization; the disadvantage is that the coating equipment takes a long time to change colors. In order to take advantage of the advantages of powder coatings and avoid disadvantages, first of all, from varnishes that do not require color change to transparent powders. In 2002, Giant took the lead in introducing low-temperature (160°C curing) acrylic transparent powder used in the automotive industry in Europe to replace solvent-based varnish, which made great contributions to leveling and fullness. Restricting its large-scale use is mainly because most of the label paper of bicycles is internal labeling. It is necessary to spray transparent powder after labeling. The general ink has insufficient baking resistance and will change color. It is necessary to change the ink system of the entire sticker. Another one The factor is the high price of acrylic transparent powder coatings, which are more than 5 times the cost of ordinary coatings.

In 2004, the combination of base color powder coating and oily gold oil was adopted, avoiding the characteristics of stickers that are not resistant to baking, and jointly developed metal powders suitable for the bicycle industry with international famous brands, and used them for mass production of basic vehicles. However, the decorative effect of metal powder coatings, especially the arrangement effect of metal powder, is always far from the metal texture of the paint, even if the bonding process is used, there is still a gap. In addition, the long color change time of powder coatings still restricts its application in the bicycle industry. It takes about 40 minutes to change a color, while the color change of paint only takes 1 to 2 minutes. ), and also ensure that one spraying batch ≥ the color changing time of another spray booth is 40 minutes. (No more than 40min, another spray booth has not finished color change, still have to wait).

Figure 2. Powder coating equipment for alternating colors in series

The powder coating process is suitable for the production of large batches with few color changes, and the products to be applied in the bicycle industry should also have large enough orders (one color order is small, and the color changes are too frequent). There is also low temperature curing, which can not only reduce carbon emissions, but also be suitable for matching with more paints and stickers. In the past two years, the metal decoration effect of powder coatings, low temperature and rapid color changing process of powder equipment have been improved, and there have been great breakthroughs. Some manufacturers have developed powders cured at 130°C/20min, powder coatings with a film thickness of 30-40μm, and To achieve 10min fast color change, the application of powder coatings in the bicycle industry will be less and less restricted.

1.2 UV Curable Coatings

UV curing is the earliest coating process used in the wood board industry. It is characterized by extremely fast curing speed but poor impact resistance. Traditional coatings are designed as indoor products, and the selected resins have poor weather resistance. Some are completely solvent-free and some are added with a solvent to adjust the workability. Curing requires UV light irradiation, which has certain requirements on the shape of the workpiece, or high requirements on the placement of the lamp tube. It must be able to ensure that the sprayed paint is irradiated to ensure complete curing. In recent years, some paint manufacturers have also begun to develop outdoor UV-curable paints. The bicycle industry has not reported using them in China, and some companies in Taiwan are using them.

1.3 High solid paint

The oil-based paint commonly used in bicycles generally has a solid content of about 50%, and the pearl silver powder paint with high metal effect requirements is about 40%. After dilution, the solid content is mostly between 20% and 30%. For primers and gold oils, develop high-solid coatings with a solid content of more than 65% to reduce solvent content, thereby reducing VOC emissions. For high-solid coatings with a solid content of more than 80%, the research process found that the curing degree of coatings that meet the requirements of construction viscosity is not enough, and the performance does not meet the requirements, and further research is needed.

1.4 Water-based paint and coating

Giant started the application research of water-based coatings in the bicycle industry in 2006. At the beginning, a primer and a topcoat were developed to explore its performance and formulation characteristics. The proposed development goal is that all properties meet the standards of paint, and there should be no obvious difference with paint. First, confirm its physical and chemical properties by manual air spraying, and confirm its storage stability, including the storage stability of the original paint and the storage stability after dilution. Bicycle painting is generally a process of 4 sprays and 5 bakes, as shown in Figure 3. When using paint spraying, most of them do not have forced air supply, they are natural air intake and forced exhaust air. They do not adjust the temperature and relative humidity of the air, but adjust the volatilization rate of the paint in different seasons to adapt to changes in ambient temperature and humidity; the leveling room has no Pre-baking, the temperature in the oven is not increased in stages, but the air temperature in the entire oven is basically the same. The advantage of this process is that the operating cost is relatively low, but the application window of the coating is wide. The water-based paint coating conditions mentioned in the literature require that the equipment be made of stainless steel, the air temperature should be (23±2) °C, the relative humidity should be (65±5)%, the leveling room should be about 60 °C, and the oven should be divided into two parts. Sections are heated up and the construction conditions are demanding, all of which will greatly increase the cost of bicycle painting. To this end, we adopted the paint spraying process, reserved room for improvement, first adjusted the construction adaptability of water-based coatings, and then adjusted the process and hardware equipment as a last resort for verification. Through testing and adjusting the coating formula in time, the problems of easy precipitation after dilution, easy foaming and sagging when spraying, poor water resistance, low gloss, low film thickness, and poor impact resistance of water-based coatings have been improved. Paint quite.

Figure 3 The process flow of bicycle 4 spray and 5 bake coating

For bicycle spraying, the whole industry uses Ω electrostatic rotary disc spraying machine for spraying, and the dead corners that cannot be sprayed by electrostatic spray are manually corrected before and after the electrostatic machine. The relatively economical electrostatic high voltage is controlled at 90kV, but the first problem encountered by the traditional electrostatic machine spraying water-based paint is that the electrostatic high voltage cannot be applied (leakage current protection). It cannot be sprayed. To spray water-based paint with an electrostatic machine, the problem of electrostatic high voltage must first be solved. In principle, if the electrostatic high voltage cannot be applied, it is the problem of electrostatic leakage. The electrostatic leakage should be modified in the original coating laboratory test line by solving the insulation of the entire paint spraying system. By changing the length of the insulating rod of the paint supply system, the insulating table on which the paint bucket is placed, the material of the lifting rod and the paint pipe passing through it, and the specially isolated paint supply system for testing, after many improvements, the Ω electrostatic machine was finally made. Water-based coatings can be sprayed normally, and two primer lines were officially refitted for pilot testing in 2014, see Figure 4.

Figure 4 Bicycle single-layer coating process

This pilot line has no special frame preheating process, pre-drying process, and no temperature and humidity adjustment. All conditions remain the same as solvent-based coatings. The baking conditions are (115-120℃×20～30min). After 2a's test and many improvements to the paint, the water-based paint primer can be consistent with the solvent-based paint in various properties, and can also be applied in a wide range of temperature and humidity. Add water about 20% in winter, Iwata NK-2 viscosity cup measurement viscosity is 40~60s, add water about 30% in summer, viscosity is between 25~35s, the relative humidity of air only needs to be below 80%, and the spit volume is based on the relative humidity of the air. For fine-tuning, the output volume is slightly reduced when the relative humidity is large, and the output volume is appropriately increased when the relative humidity is small. The biggest influence is the air relative humidity, followed by the temperature, but the temperature and humidity range can be much wider than that in the literature. In the absence of residual heat and leveling pre-baking, it can also be sprayed.

In 2016, Giant carried out technical transformation on the coating production line according to the experience of the previous pilot test, and designed 7 complete production lines with 4 sprays and 5 baking water and oil. Compared with the original, the temperature and relative humidity control between the coatings are increased. The temperature is between 15 and 30 °C, the relative humidity is below 80%, the line speed is 2 to 3 m/min, the leveling time is 10 to 15 minutes, and the bottom, middle and top are coated and baked. The curing condition is 115°C/20min, and the baking curing condition of the varnish is 125°C/20min. The process of sharing water and oil has also won valuable development time for the oil to water conversion of coatings. After 2a of development, 15 primers, 211 topcoat colors, and 3 varnishes have been developed. The amount of water-based paint exceeds 80%. It is expected to achieve the all-water-based target in 2019.

Figure 5 Process flow chart of water-based coating 4 sprays and 5 bakes

2 Conclusion

For the environmental protection process of bicycle coating, the pretreatment adopts processes such as phosphorus-free degreasing, acid mist inhibitor, silane-zirconium salt film, and chromium-free passivation to reduce air pollution and water pollution. The spraying process part is mainly developed into high-solid coatings, water-based coatings, low-temperature curing powder coatings + liquid gold oil. Different product positioning and different order modes can adopt different environmental protection coatings and coating processes. These explorations and practices have reference significance for the relevant technological transformation of the industry