Chemical Machining Essay Sample

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Introduction of TOPIC

Nontraditional machining processes are extensively employed to produce geometrically complex and precision parts from engineering materials in industries as diverse as aerospace, electronics and automotive manufacturing. There are many multiple geometrically designed precision parts, such as deep internal cavities, miniaturized microelectronics and fine quality components may only be manufactured by nontraditional machining processes. Special coatings called maskants protect areas from which the metal is not to be removed. The process is used to produce pockets and contours and to remove materials from parts having a high strength-to-weight ratio. Moreover, the machining method is widely used to produce micro-components for various industrial applications such as micro-electromechanical systems (MEMS) and semiconductor industries.

Chemical machining method may be the oldest nontraditional machining method which is used to shape copper with citric acid in the Ancient Egypt in 2300 BC. Until the 19th century this process was widely used for decorative etching. The development of photography provided a new dimension to chemical machining and in 1826 J.N. Niepce was the first to use a photo-resist made from bitumen of Judea asphalt for etching pewter (an alloy of 80-90% of tin and 10-20% of lead). William Fox Talbot (1852) patented a process for etching copper with ferric chloride, using a photo-resist made from bichromated gelatin (GB Patent No:565). John Baynes, in 1888, described a process for etching material on two sides using a photo-resist which was patented in the USA (US Patent No: 378423).

The main industrial application of chemical machining developed after the war. In 1953, North American Aviation Inc. (California USA) used the process to etch aluminium components or rockets. Manuel C. Sanz, an engineer with the North American Aviation Company, is credited with solving a critical weight problem on a missile casing by utilizing the process. The company named the process “chemical milling” and patented it (US Patent No: 2739047) in 1956. The machining method is called in different names such as etching, chemical etching, wet etching, etc.

There are several factors contributing to the popularity of chemical machining processes as follow:
a. Chemical machining process is mature and well established.
b. It is simple to implement.
c. There is no additional cleaning step needed.
d. Cheaper machining process.

CHEMICAL MACHINING (CHM)

Chemical machining is a well known nontraditional machining process; the controlled chemical dissolution of the machined workpiece material by contact with a strong acidic or alkaline chemical reagent. It is used to remove large amounts of metal to obtain parts that cannot be machined easily through traditional machining methods. Parts that require precision engineering, like miniaturized micro-components or those containing deep internal cavities are just some of the components produced with chemical milling. While it has numerous applications in automotive manufacturing and electronics, it is very extensively used in the aerospace industry.

The entire process is quite simple and involves cleaning and masking the parts that don’t require etching. The metal is then submerged in large tanks of etching solution. The amount of material removed is controlled by the concentration of the chemical solution, the type of etchant used, the time spent in the tank, and the temperature. An ultrasonic thickness tester helps the operator to regularly assess the thickness of the part until it meets the thickness specified in the blueprint. The etched parts are cleaned and inspected for quality control.

Chemical Machining Process

TYPES OF CHEMICAL MACHINING

1.) CHEMICAL BLANKING

Chemical blanking is similar to the blanking of sheet metals and it is applied to produce features, which penetrate through the thickness of the material, with the exception that the material is removed by chemical dissolution rather than by shearing. Typical applications for chemical blanking are the burr-free etching of printed-circuit boards, decorative panels, and thin sheet metal stampings, as well as the production of complex or small shapes. It is otherwise called as Chem-blanking, Photo forming, Photo fabrication, or Photo etching. In this process, the metal is totally removed from certain areas by chemical action. The process is used chiefly on the sheets and foils. This process can work almost any metal, however, it is not recommended for material thinner than 2 mm. A Schematic sketch of the chemical blanking process is shown in Figure-2.

Figure-2. Chemical Blanking Process

2.) PHOTOCHEMICAL BLANKING

This process is effective in blanking fragile work pieces and materials. Material is removed using photographic techniques. Applications are electric motor lamination, flat springs, masks for color television, printed circuit cards etc. is otherwise called as photo etching.

THE PROCESS

Engineers typically create the desired image using computer aided design software, commonly called CAD drawing software. When used for cutting particular parts, technicians replicate this pattern in columns and rows forming multiple images on one screen. The computer transfers the image to laminate film that has a Mylar® base and silver emulsion coating. Before photochemical machining, the metal of choice undergoes a thorough cleaning process, which ensures adhesion to the photographic film. After being cleaned with a diluted solution, the sheet undergoes a water rinse and a heated drying process.

When machining a complete metal cutout, technicians laminate, or sandwich, the piece of metal between two pieces of photo tool film. One side of the metal only needs to be covered with fi

lm during engraving or etching. Technicians laminate the metal using a dry roller or wet dip method.

They use cameras during this process to ensure proper metal and film alignment. The laminate and photo tool match each other identically in dimensions.

The roller method involves passing the metal through a roller where the machine inserts the sheet between two pieces of laminate. Lamination requires contamination free environments and elimination of possible air bubbles. The wet dip method entails dipping the metal into a liquid film and baking the sheet in an oven to harden the film. The photochemical process continues by exposing the laminated metal to high intensity ultraviolet light, hardening the image on the photo tool.

After exposure, technicians expose the laminated metal to developing solution, which removes any undeveloped laminate. Via a conveyor belt, the laminated metal enters a chamber lined with spray nozzles positioned above and below the conveyor. Hot etching acid sprays the metal from one or both sides, depending on the desired design. The acid dissolves the metal not covered with laminate without leaving rough edges or altering the metal quality. This step in the photochemical machining process creates the finished image formed on the CAD drawing.

The part now undergoes a water rinse and exposure to a stripping solution that removes any remaining laminate. The sheet goes through one final water rinse followed by hot air drying. Technicians may use microscopes for a final inspection as a means of quality control.

3. CHEMICAL ENGRAVING

Is a chemical machining process for making name plates and other flat panels that have lettering and/or artwork on one side. These plates and panels would otherwise be made using a conventional engraving machine or similar process. Chemical engraving can be used to make panels with either recessed lettering or raised lettering, simply by reversing the portions of the panel to be etched. Masking is done either the photo-resist or screen resist methods. The sequence in chemical engraving is similar to the other CHM processes, except that a filling operation follows etching. The purpose of filling is to apply paint or other coating into the recessed areas that have been created by etching. Then, the panel is immersed in a solution that dissolves the resist but does not attack the coating material. Thus, when the resist is removed, the coating remains in the etched areas but not in the areas that were masked. The effect is to highlight the pattern.

PROCESS OF CHEMICAL MACHINING

Chemical machining process has several steps for producing machine parts. These are given below; 1.) Workpiece Preparation (Cleaning)

The workpiece material has to be cleaned in the beginning of chemical machining process. The cleaning operation is carried out to remove the oil, grease, dust, rust or any substance from the surface of material. A good cleaning process produces a good adhesion of the masking material. There are two cleaning methods; mechanical and chemical methods. The most widely used cleaning process is chemical method due to less damages occurred comparing to mechanical one. Ultrasonic cleaning machine is applied with using special cleaning solution and heating is beneficial during the cleaning process.

2.) Coating with masking material (Masking)

The next step is the coating cleaned workpiece material with masking material. The selected masking material should be readily strippable mask, which is chemically impregnable and adherent enough to stand chemical abrasion during etching.

3.) Scribing of the mask

This step is guided by templates to expose the areas that receive chemical machining process. The selection of mask depends on the size of the workpiece material, the number of parts to be produced, and the desired detail geometry. Silk-screen masks are preferred for shallow cuts requiring close dimensional tolerances.

4.) Etching

This step is the most important stage to produce the required component from the sheet material. This stage is carried out by immerse type etching machine (Fig 9.30a). The workpiece material is immersed into selected etchant and the uncovered areas were machined. This process is generally carried out in elevated temperatures which are depended on the etched material. Then the etched workpiece is rinsed to clean etchant from machined surface.

5.) Cleaning masking material (De-masking)

Final step is to remove masking material from etched part. The inspections of the dimensions and surface quality are completed before packaging the finished part.

MASKANTS

Masking material which is called maskant is used to protect workpiece surface from chemical etchant. Polymer or rubber based materials are generally used for masking procedure. The selected maskant material should have following properties. ← Tough enough to withstand handling

← Well adhering to the workpiece surface
← Easy scribing
← Inert to the chemical reagent used
← Able to withstand the heat used during chemical machining
← Easy and inexpensive removal after chemical machining etching

Multiple maskant coatings are used to provide a higher etchant resistance. Long exposure time is needed when thicker and rougher dip or spray coatings are used. Various maskant application methods can be used such as dip, brush, spray, roller, and electro-coating as well as adhesive tapes. When higher machined part dimensional accuracy is needed, spraying the mask on the workpiece through silk screen would provide a better result. Thin maskant coating would cause severe problems such as not withstanding rough handling or long exposure times to the etchant. The application of photo resist masks which are generally used in photochemical machining operation, produce high accuracy, ease of repetition for multiple part etching, and ease of modification.

ETCHANTS

Etchants are the most influential factor in the chemical machining of any material. Various etchant are available due to workpiece material. The best possible etchant should have properties as follow:

← High etch rate
← Good surface finish
← Minimum undercut
← Compatibility with commonly used maskants,
← High dissolved-material capacity
← Economic regeneration
← Etched material recovery
← Easy control of process.
← Personal safety maintenance

Different etchants are commercially available or the required etchant can be prepared in shop. Alkaline etchants are introduced to the fabrication of electronic components such as printed circuit board. There are some other etchants can be named, but the industrial application of chemical machining is generally used these three etchants, even most of the companies use only ferric chloride due to economical considerations.

ADVANTAGES AND DISADVANTAGES OF THE PROCESS

The application of chemical machining provides several advantages as follow:
← Easy weight reduction
← No effect of workpiece materials properties such as hardness
← Simultaneous material removal operation
← No burr formation
← No stress introduction to the workpiece
← Low capital cost of equipment
← Easy and quick design changes
← Requirement of less skilled worker
← Low tooling costs
← The good surface quality
← Using decorative part production
← Low scraps rates.

However, chemical machining has also some disadvantages:
• Difficult to get sharp corner
← Difficult to chemically machine thick material (limit is depended on workpiece material, but the thickness should be around maximum 10 mm) ← Scribing accuracy is very limited, causes less dimensional accuracy ← Etchants are very dangerous for workers

← Etchant disposals are very expensive

SUMMARY

Chemical machining is a process of removing a material by chemical means. There are types of chemical machining, the chemical blanking, Chemical engraving and the photochemical blanking or photo etching process. Chemical machining process has several steps for producing machine parts, first is cleaning, second is masking, then scribing of the mask, etching and the last is cleaning masking material or de-masking. Chemical machining is widely used to produce complex machine parts for various application as well as decorative parts. The machining operation should be carried out carefully to produce a desired geometry. Environmental issues in chemical machining operations may be the most important factor affects the machining process should be used or not. Most of the chemicals such as cleaning solutions, etchants, strippers etc. are very hazardous liquids. Therefore, the handling and disposal of them are very costly and so advantage and disadvantages of the process is much to be understand.

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