Taking the high-quality mirror burnishing roller body as an example, use a wet belt polishing machine to focus on polishing the roller surface before electroplating to reduce shading and ensure the surface roughness requirements. After electroplating, the roller body with stable coating thickness and mirror effect can be obtained by grinding with abrasive belts of different meshes.
1 preamble
The ironing roller is one of the important parts of the ironing machine. The roller body is usually made of low carbon steel seamless tube. In order to make the surface of the fabric have a high gloss, the surface of the roller body requires hard chrome plating treatment, and the surface roughness value Ra=0.01～0.025μm is required, and the mirror effect can be achieved after polishing[1 ]. General processing methods include mechanical polishing, electrolytic polishing, etc. For the roller body with an outer diameter of 415mm, the company adopts the mechanical abrasive belt polishing method to obtain a mirror surface roller body with a surface roughness value R a=0.02μm. During the processing, it was found that the rough polishing quality before electroplating has an important influence on the fine polishing after electroplating. By controlling the rough polishing quality, a high-quality hard chrome layer mirror polishing effect can be obtained.
2 Analysis of roller body processing problems
As shown in Figure 1, the roller body is made of 20# seamless steel pipe. Since there is an edge on the surface A of the roller body, its function is to beat and comb the rough surface of the fabric, so the edge cannot be squeezed during processing, and it must be ensured that no rounded edge appears during grinding. At the same time, in the process of abrasive belt polishing, due to the existence of grooves, the polishing head is in a state of intermittent polishing, so abrasive belt shading often appears in the process of changing the surface, especially during rough polishing, it will appear on the surface of the roller body. Continuous shades of different shades. Because if the surface roughness cannot be guaranteed before electroplating, it will be difficult to process the high-hardness chrome plating layer to achieve a mirror effect, so the shading process before electroplating is very important.
3 process analysis
The process route after grinding the roller body is: grinding→rough polishing→electroplating→finish polishing→subsequent processing.
After the roller body is ground, the surface roughness value reaches Ra=0.8μm, and the surface is slightly recognizable. The surface of the roller body after polishing is shown in Figure 2.
Figure 2 The surface of the roller body after polishing
The polishing machine is transformed from CW6180E horizontal lathe of Dalian Machine Tool Company. Polishing is shown in Figure 3.
Picture Figure 3 Polishing schematic
After the roller body is fixed and debugged on the polishing machine, the roller surface should be cleaned to prevent the grinding wheel particles from adhering to the roller surface, otherwise the final mirror effect may be affected. The processing parameters of the polishing machine tool are: the speed of the roller body is 126r/min, the feed rate is 0.26mm/r, and the frequency of the swing head is 12-18Hz.
Adjust the frequency of the swing head according to the number of meshes of the abrasive belt. The specific rough polishing → electroplating → fine polishing process is shown in Table 1. When polishing, the surface of the abrasive belt reciprocates in the axial direction perpendicular to the roller surface. After polishing twice with the 250-mesh abrasive belt, the roller surface will have continuous shading along the radial direction, and the roller surface will be dark glossy at the same time. There are obvious differences in the ground roller body. This kind of shading is mainly caused by the different sizes of sand grains on the surface of the abrasive belt [2]. The lower the mesh number of the abrasive belt, the higher the particle size, and the more obvious the traces left on the surface of the roller body during polishing; at the same time, the roller surface beats It also has a certain impact on polishing. The beating affects the pressure of the abrasive belt on the roller surface, resulting in different depths of shading. For the special roller body of the ironing roller, the shading mainly exists near the edge, and the intermittent contact between the abrasive belt and the roller surface will make the grinding degree of the sand grains at the notch higher than that at the roller surface, and at the same time, the roller body rotates It is easy to make the edge into a circular arc edge, so the control of the contact pressure between the abrasive belt and the roller surface is very important. After polishing with 500-mesh and 1000-mesh abrasive belts, the shading is refined. When the light shines on the roller surface, it is diffusely reflected by the shading. It needs to be observed along the direction of the light, and compared with the surface roughness standard sample to judge whether it is Polishing with a 1500 grit abrasive belt is required.
Table 1 Rough polishing → electroplating → fine polishing process pictures
After rough polishing, the surface roughness value is guaranteed to be Ra≤0.1μm, and there are basically no visible processing traces on the surface, and 1500-mesh abrasive belt shading is allowed in the light-facing direction. The real shot surface of the roller body after rough polishing is shown in Figure 4.
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Figure 4 The surface of the roller after rough polishing
The actual surface of the roller body after electroplating is shown in Figure 5. The surface is bright and glossy, but the shading still exists. After on-site inspection and statistics, it is found that the surface roughness value before electroplating is proportional to the surface roughness value after electroplating, and the thickness of the coating also affects the surface roughness of the roller surface after electroplating.
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Figure 5 The surface of the roller body after electroplating
The relationship between the surface roughness of the roller before and after electroplating and the thickness of the coating is shown in Table 2. It can be found that the increase of the thickness of the coating makes the surface of the roller rougher. According to the current spike effect, the current density is high at the peak of the microscopic unevenness of the workpiece substrate, and the electroplating metal ions are deposited there first, making the coating thicken faster at the peak, and as the electroplating time increases, the coating becomes thicker and the surface It becomes rougher [3], considering the economic cost and subsequent processing, the thickness of the coating is required to be ≥ 80 μm.
Table 2 Statistical statistics of the relationship between the surface roughness of the roller body and the thickness of the coating before and after electroplating (unit: μm)
The electroplated roller body is fixed on the polishing machine for final processing. The difference from the previous rough polishing is that the lower limit of the swing frequency is selected for fine polishing, so that the new abrasive belt is in contact with the roller surface as much as possible to improve the shading removal rate. After polishing with abrasive belts of different meshes, confirm the surface roughness of the roller surface (surface roughness value Ra≤0.03μm) before the final glazing. If not, continue to polish with 1500 mesh abrasive belts. The 3000-mesh glazing abrasive belt can basically remove the residual shading. The finished roll body is shown in Figure 6.
Picture Figure 6 The real shot of the finished surface of the roller body
4 Conclusion
In this paper, a wet belt polishing machine is used to process the roller surface before electroplating. By controlling the rough polishing quality, a high-quality mirror polishing effect of the hard chromium layer is obtained, and the following conclusions are drawn.
1) The lower the surface roughness value of the roller surface before electroplating, the lower the surface roughness value of the electroplating layer, the less shading in the subsequent fine polishing process, and the easier it is to obtain a high-quality mirror roller body. Abrasive belts of different meshes can gradually refine the shading until it is basically removed.
2) The roller body and the polishing belt rotate in the opposite direction, which can improve the grinding efficiency while protecting the edge notch
