What is chemical etching?
(1) Principles and Characteristics of Chemical Etching
Chemical etching is a process method that exposes the parts requiring processing to a chemical medium, where a chemical reaction occurs, causing the material of the part to corrode and dissolve, thereby obtaining the required shape and size. During chemical etching, the parts of the workpiece surface that are not to be processed should first be covered with a corrosion-resistant coating. Then, the workpiece is immersed in the etching solution or the etching solution is applied to the workpiece surface, removing the allowance from the exposed areas to achieve the processing objective.
Chemical etching can process both metallic and non-metallic (such as glass, slate, etc.) materials, is not affected by the hardness of the material being processed, and does not cause physical changes. After processing, the surface is free of burrs, does not deform, and does not exhibit work hardening. Any surface that the etching solution can penetrate can be processed, making it suitable for surfaces that are difficult to machine mechanically. No clamps are required during processing. Since the etching solution and vapor pollute the environment and are harmful to equipment and human health, appropriate protective measures must be taken during processing.
Chemical etching is mainly used in mold manufacturing to process the patterns, designs, and text on the surface of plastic mold cavities, with photo-etching being the most widely used.
Photo-etching involves photographing the required image onto a photographic negative, and then the image on the negative is duplicated onto the working surface of the mold cavity coated with a photosensitive emulsion (emulsion) through a photochemical reaction. The exposed film layer becomes insoluble in water and its corrosion resistance is enhanced. The unexposed film layer is water-soluble; after washing with water to remove the unexposed film layer, part of the metal is exposed. Through immersion in the etching solution, the required patterns and designs are obtained.
(2) Chemical Etching Process
The process of photo-etching is as follows:
Original Drawing -> Photography -> Reverse Mounting of Negative -> Hardening of Film -> De-coating -> Etching -> Trimming.
① Original Drawing and Photography The required graphic and text are drawn on the paper according to a certain proportion, which constitutes the original drawing. Then, using specialized photographic equipment, the original drawing is photographed to reduce it to the required size on the photographic negative. An example of the film made for the etching of text on a soap box cavity is shown in Figure 6-3.
② Photosensitive Emulsion There are many formulas for photosensitive emulsions, with polyvinyl alcohol (PVA) photosensitive emulsion being the most commonly used. The operating principle of the photosensitive emulsion is as follows:
Polyvinyl alcohol and ammonium dichromate do not undergo a chemical reaction with each other. Polyvinyl alcohol is characterized by being easily soluble in water, colorless, transparent, and possessing adhesive properties. After water evaporates, it forms a thin film, but it can be removed by rinsing or wiping with water. Ammonium dichromate is a photosensitive material, which, after light exposure, sensitization, and development, becomes difficult to dissolve in water. It combines with the polyvinyl alcohol mixture to form a thin film that is relatively firmly adhered to the mold surface. The unexposed parts, which are still predominantly polyvinyl alcohol, can be removed by rinsing with water and wiping with absorbent cotton. The photosensitive emulsion film adhered to the mold surface, after curing, possesses a certain level of corrosion resistance, protecting the metal from corrosion.
③ Cleaning of the Etched Surface The mold surface must be cleaned before applying the emulsion. For small molds, oil can be removed by heating them in a 10% NaOH solution, followed by rinsing with clean water.
For larger molds, they are first boiled in a 10% NaOH solution and rinsed, and then rinsed again with boiling water. After cleaning, the mold should be baked in an electric furnace to about 50°C before the emulsion is applied; otherwise, the photosensitive emulsion may easily peel off. The cleaned mold cavity is shown in Figure 6-4.
④ Applying the Emulsion The emulsion can be applied by spraying under a red light in a darkroom. The areas of the mold requiring photosensitive imaging should be sprayed repeatedly, with the interval between coats depending on the room temperature-shorter for higher temperatures and longer for lower temperatures. Care must be taken to ensure uniform coating. The mold cavity after emulsion application is shown in Figure 6-5.
⑤ Attaching the Photographic Negative Lay the prepared photographic negative flat on the surface to be etched, level the surface, press the negative down with a piece of glass, making it perpendicular to the surface, and secure the negative with transparent adhesive tape. For rounded or curved areas, white petroleum jelly can be used to adhere the negative. The design of the mold cavity should consider the convenience of attaching the film beforehand; if necessary, the cavity can be designed as an insert structure. The mold cavity after attaching the film is shown in Figure 6-6.
⑥ Exposure The workpiece area that has been coated and had the negative attached is exposed to an ultraviolet light source (such as a mercury lamp) to sensitize the photosensitive emulsion film on the workpiece surface according to the image. During this process, the position of the light source should be adjusted to ensure uniform exposure of the photosensitive area. The duration of exposure is determined based on practical experience.
⑦ Development and Rinsing The exposed workpiece is immersed in hot water at 40 to 50°C for about 30 seconds, allowing the unexposed part of the emulsion film to dissolve in the water. After removal, alkaline violet dye is dropped onto the surface and spread evenly for development. Once a clear pattern appears, it is rinsed with clean water, and the unexposed parts are wiped off with absorbent cotton. Finally, it is dried with hot air, as shown in Figure 6-7.
⑧ Film Hardening and Repair The developed mold cavity is placed in an electric constant temperature drying oven at 150 to 200°C and baked for 5 to 20 minutes to improve the adhesion strength and corrosion resistance of the film. If there is any emulsion film remaining on the cavity surface that has not been completely removed, it can be trimmed clean with the tip of a knife. Missing film areas should be repaired using printing ink.
⑨ Protection of Non-Etched Areas Areas that do not require etching should be protected with a film, as shown in Figure 6-8.
⑩ Etching Different etching solutions should be selected for different materials. For steel mold cavities, ferric chloride aqueous solution is commonly used and can be applied by immersion or spraying. If an appropriate amount of copper sulfate powder is added to the ferric chloride aqueous solution to form a paste, and then applied to the cavity surface (coating thickness of 0.2 to 0.4mm), side penetration can be reduced. To prevent undercutting (side etching), a protective agent can also be added to the etchant, or rosin powder can be brushed and embedded into the side walls of the exposed pattern. The etching temperature is 50 to 60°C. Depending on the density and depth of the patterns and designs, etching generally needs to be performed 1 to 3 times, with a typical etching depth of 0.3mm.
⑪ De-coating and Finishing The etched mold cavity is wiped clean with lacquer solvent and industrial alcohol. Plasticine can be used to take an impression of the text to check the etching effect, as shown in Figure 6-9. Any defective areas are locally retouched and then subjected to secondary etching or mechanical repair. After etching is complete, the photosensitive emulsion adhered to the surface should be washed off with a NaOH solution to burn away the protective layer, followed by rinsing several times with water. It is then dried with a fan and coated with an oil film, thus completing the entire process. The text on the finished mold cavity section is shown in Figure 6-10. A production site processing video can be found in the "Case Videos" folder under the "Drying Text.mp4" file in the resource library of the China University MOOC "Mold Manufacturing Technology" (Course Number: 0802SUST006).
