Major limitations of chemical etching

Chemical etching can only be carried out progressively based on the original surface condition of the part. Therefore, the shape and surface state of the part after chemical etching are directly related to the original shape and surface state of the part. In more cases, the processed surface after chemical metal etching remains completely parallel to the original initial reference surface state. The geometry of the etched edge is mainly related to the material thickness.

It can be seen from these limitations that chemical etching cannot be used for rough-surfaced plates, bars, etc. to process parts with complex shapes. If it is necessary to process very thin flags or shallow flanges in certain parts of complex parts. When the process is completed, the entire geometric shape must be processed mechanically to a certain extent first, and the subsequent chemical etching is only to uniformly remove a layer of metal parallel to the processed surface to achieve the required thickness and shape. . At the same time, chemical etching cannot process the vertical side of the processed edge, and can only process an arc shape with a radius similar to the corrosion depth, as shown in Figure 1-19(b). Using some special etching agents and under good control, a beveled edge shape can be etched as shown in Figure 1-19(C).

The adhesion of the patterned anti-corrosion layer and changes in the corrosive composition during the processing will cause changes in the corrosion coefficient. The geometric shape of the chemical etching edge will also have a certain deviation, and sometimes the partial cover will be relatively large. Therefore, Chemical etching cannot be used to machine structural parts with tight tolerances.

Pure dimensional processing, also known as chemical blanking, is usually used in only two situations.

  • For the processing of delicate parts with small material thickness, such as the processing of various spring sheets or other delicate structural parts;
  • For those materials that are very hard and difficult to machine, it is often impossible to use mechanical methods to process the shape of metal materials when cutting large-size precision blanks. With the continuous improvement and popularization of photographic chemical etching technology, very high geometric shape fidelity and chemical etching accuracy can be achieved for dimensional processing.

Another issue that must be noted is that chemical etching should not be used to add narrow and deep grooves. This is because the bubbles generated during the chemical etching reaction will gather under the edge of the anti-corrosion layer, and these will be blocked under the anti-corrosion layer. The bubbles under the corrosion layer actually serve to separate the metal surface from the corrosive agent. As a result, it causes a very irregular corrosion and forms very uneven edges, which is very troublesome for deep processing. Although some anti-corrosion materials with good performance are soft and can easily discharge bubbles, why should After processing to a certain depth, even mechanical stirring is not enough to completely discharge the bubbles at the edge of the anti-corrosion layer. An effective method for this kind of processing is to use a more time-consuming manual method to smooth the anti-corrosion layer on the edge of the pavilion. Another possible reason is the effect of the surface tension of the corrosive liquid, which will also cause corrosion to fail to proceed on narrow or small radius surfaces. For groove processing with a larger depth, the width is required to be no less than 4mm. For grooves or round holes with a small depth, the width or radius must be no less than 5 times the depth.

The appropriate chemical groove corrosion width should be 4mm, plus 2 times or slightly deeper corrosion depth. The maximum depth of the chemical corrosion groove should generally be about 12mm, because in this case, even if the groove area is larger , but because the anti-corrosion film with a width of about Umm will overhang like a skirt, it will inevitably cause the accumulation of bubbles and cause uneven quality around the edge of the groove. defect. When the corrosion depth reaches a certain depth, even if the parts are moved significantly or the solution is stirred, it is impossible to completely eliminate the impact caused by bubble accumulation. Although some new anti-corrosion layers can maintain sufficient softness, It can make the formed bubbles escape immediately. Figure 1-20 Quality problems on the side of the cut surface caused by the accumulation of bubbles under the anti-corrosion layer U) The cross-sectional geometry required by the design; if < (2D+4) is taken, the quality of the groove edge is likely to be damaged Night; a narrow and deep cut n, its edge will be affected by the phenomenon of bubble accumulation;

A very time-consuming method is to take the parts out of the corrosive agent after etching every 2mm, and carefully use a knife to remove the overhanging anti-corrosion film along the edges of each etching process.

Chemical etching is also widely used to reduce the wall thickness of pipes. When adding T, the method of immersing the entire pipe in the corrosive agent is usually used to remove the metal from the inner and outer diameters of the pipe wall at the same time. However, if metal is only allowed to be removed from the inner surface of the pipe, in order to achieve satisfactory results, the inner diameter of the pipe must not be smaller than a certain limit size. For example, when the inner diameter of the pipe is less than 12mm, irregular shapes will be processed due to the influence of bubbles, corrosive eddy currents and other factors. Therefore, for pipes with a diameter less than 12mm, both ends of the pipe can only be plugged and excess metal removed from the outside of the pipe.

Limitation: The chemical etching method for drilling holes is different from the mechanical method and the electrolytic method. It cannot produce the hole shape that can be processed by the latter two. Since electrolytic drilling does not use corrosive agents, it supplies electrolyte through a straight pipe equivalent to a drill bit to drill through very hard materials. Choosing the appropriate process method can drill holes with straight walls. Chemical etching drilling can only drill irregular tapered holes. For deep chemical etching drilling, the tolerance increases due to the long etching time, so chemical drilling is rarely used except in special circumstances.