The Chemical Machining Process
In the chemical machining process, chemical reagents are used to remove material from the appropriate sections of the work piece.
What Is Chemical Machining and How Does It Work?
Chemical Machining is the clean removal of metal from defined locations without affecting the integrity or characteristics of the metal through the use of a photochemical process. Small thin metal components with complicated designs are created with this method, and there are no burns or pressures on the pieces as a result of the procedure. This is known as “chemical machining processes.”
Step-by-Step Process of Chemical Machining Process
The chemical machining process is done in the following phases.
It is necessary to clean the workpiece surface with care. Cleaning is done on the work pieces to remove any oil, grease, dust, or other material that may have accumulated on them throughout the manufacturing process. This allows for effective completion of the subsequent processing steps.
Cleanings are required to guarantee that the masking material adheres well to the work piece after it has dried. Stray etching happens in the context of masking debugging.
Depending on the work material and the desired machining depth, many kinds of cleaning operations, such as vapor degradation, alkaline etching, and maskants, are carried out, for instance. This material is quite difficult to clean since it is porous.
Mechanical and chemical cleaning procedures are the most common types of cleaning. Because it produces far less harm than the mechanical technique, the chemical method is extensively used.
The thin or cleaned components of the masked sheets are refined if the masked is thicker, thinner, and chemically cleansed. It is beneficial to heat the cleaning procedure.
The areas of the work piece which do not need chemical machining are wrapped with masking sheets. Masking sheets (Maskant) are harmless in nature and do not react with the chemicals that are used in the machining process, which makes them ideal for masking applications. The masking sheet is trimmed and removed from the region that requires chemical machining.
Strippable masking sheets should be utilized, and they should be easy to take off. This is accomplished via the usage of templates. If the full surface of the work item is to be machined, masking isn’t required. Mask sheets are often made of vinyl, neoprene, or rubber-based materials. Using one of the three procedures listed below, the maskant is adhered to the work piece:
- The cut-and-peel approach.
- Screen method
- The photo resist method.
Scribing is performed after the masking process in order to remove the masking paper from the region of a work piece that is to be machined in order for the chemical reaction to take place on that section of the work piece. After the scribing process, only those regions that are exposed to the chemical machining are machined.
An immersion in a hot chemical solution follows the masking and scribing of the work piece. Etchant is the name given to this solution. Caustic soda is used as an etchant for aluminum. Steel, magnesium, and titanium alloys are all processed using acids. Etchants are chemical agents that are used to remove metal from work pieces.
If you dip a work piece into a chemical, the region that is masked does not have any chemical reactions, while the area that is not masked undergoes a chemical reaction with the chemical, and the material will begin to be taken from the unmasked part of the work piece as a result.
On average, 0.025 millimetres of metal are removed per minute throughout this process. Etchant concentration and temperature both influence how quickly metal is removed from solution. Increased metal removal rates are seen at higher concentrations and temperatures. The length of time that the work piece is submerged in the etchant has an effect on the quantity of metal that is removed. Allowing gas bubbles to get caught during the machining process will help to prevent non-uniform machining.
Etching may be accomplished using a formula:
‘E’ is the etching rate in meters per second; Depth of cut is represented by the letter ‘s’; and ‘t’ denotes the amount of time spent immersed in the pool.
Demasking and washing
The work piece is removed from the etchant after it has been etched. A treatment of water is used to clean the work piece, and the oxide layer on the surface of the work piece that is being machined is removed. The masking sheet is then removed from the frame at this point.
Parameters impacting the effectiveness of the chemical machining process:
The performance of the chemical machining process is governed by various processing parameters, which are:
- Types of etchant.
- Etchant temperature (in degrees Celsius).
- Types of maskants
- Maskant application techniques
- Etchant circulation method.
Methods of Applying Maskant:
There are 3 methods of applying maskant:
- Cut and Peel Method
- Screen Printing
- Photoresist Maskant
i) Cut and Peel Method
In the cut and peel method, maskant materials such as neoprene, butyl, or vinyl based materials are used as maskants. Depending on the process, the maskant material may be applied by dipping, spraying, or flow coating. When using this process, the thickness of the coating ranges from 25 microns to 130 microns.
The maskant is first applied to the entire surface of the work piece, and then it is cut and peeled away from the area of the work piece that will be exposed to the etchant in order to be machined later. With the help of a template, the scribing and peeling off of maskant are accomplished by hand. Precision achieved through this process ranges from 130 microns to 750 microns, depending on the size and type of component being manufactured.
ii) Screen Printing
Using stencils and a fine polyester or stainless steel mesh screen, the mask material is transferred to the work piece surface by screen printing. When it comes to high volume production, low accuracy, and etching depths of less than 1.5 mm, screen printing is the best option. Because of the thin body of the coating, the etching depth is limited in this process. Stainless steel screens are frequently used in this method.
An etched area is prevented from being seen by using a screening. Following that, the screen is forced against the surface of the part. The maskant is then folded up. After that, the screen is removed and the part is baked to dry it out completely.
iii) Photoresist Maskant:
This technology has become quite popular these days, and this process is also called photochemical machining (PCM). Shapes that are complex but accurate can be created using this technique. Using a light-activated resist material, this method creates intricate and finely detailed shapes. An iterative process is used to carry out this method.
Photo resistant maskant is made up of several steps.
1. Photoresist material is applied to the work piece, and a master transparency is placed against the work piece and exposed to ultraviolet rays.
2. In the following step, the light activates the photo resist material in the areas that correspond to the opaque portions of the photograph.
3. A tolerance of 0.025 to 0.005 mm can be achieved by employing this technique.
Chemical machining can be classified into the following types.
The three most popular forms of chemical machining are chemical milling, chemical blanking, and chemical engraving.
Through the process of chemical milling, material is removed in order to create blind details such as pockets and channels or to reduce the overall weight of the component.
It is possible to create cavities, such as holes and slots, by using a chemical blanking process. The process can also be used to blank complete parts from sheets by chemically etching the visual field of the desired shape into the sheet surface.
History of the Chemical Machining Process
In ancient days, this procedure was used by craftsmen for engraving metals. However, the designers were totally unclear about the functioning of the method or how the procedure worked. This machining procedure was used to form copper with citric acid in Ancient Egypt about 2300 BC. Before the 19th century, this procedure was commonly used for ornamental etching. In 1852, William Fox Talbot invented a procedure for etching copper with ferric chloride, utilizing a photo resist. In 1888, John Baynes invented a method for etching the material on both sides using a photo resist.
What are maskant and etchant?
There are two chemicals that are usually employed in the machining process: maskant and etchant.
Maskants comprise chemical resistant coatings which are used to protect the areas that are not to be machined. Maskant does not allow etchant to permeate through it and reach the substance and is not to be dissolved. This approach is highly beneficial for generating intricate configurations in delicate items that cannot be done by the traditional machining procedures.
Choice Of Maskant:
The maskant is selected by keeping the following criteria in mind :
- The maskant must be resistant to the etchant.
- It should be readily removed when the machining is done.
- The maskant might not have any chemical influence on the work item.
- It should be entirely stable at the high temperature of the etchant bath.
The etchant is a chemical that is used in the chemical machining process to dissolve the work piece and remove material via a chemical reaction. To minimise uneven material removal from the work piece, new etchant is continually sprayed or the work piece is immersed in an etchant tank. To improve the material removal rate (MRR), the etchant is stirred, and if required etchant is heated. The strength of etchant may be maintained by good filtration, the addition of new chemicals, replace some proportion of old etchant by fresh one frequently.
Different etchants are used for machining different materials. Some of the etchants include FeCl3 (It is utilised for Al, Cu, Ni, and related alloys), FeNO3 ( for Ag ), Hydrofluoric acid ( for Ti ), andNital (for tool steel). Apart from these, additional etchants employed include chromic acid and ammonium sulphate.
Points to consider when picking an etchant:
- It should produce an excellent surface finish.
- It should have a good material removal rate.
- It should have a great depth of penetration.
- It should not harm the workpiece.
- It should be readily accessible.
- It should not be excessively pricey.
Advantages of the chemical machining process:
- Material is removed in a uniform manner during this machining process.
- It is possible to produce tapered sheets and structural members with tight tolerances and a high level of surface finish using this machining procedure.
- High-skilled labor is not needed for the chemical machining process.
- It is appropriate for small batch sizes.
Disadvantages of the chemical machining process:
- Using this machining process, only a small number of metals can be machined.
- It is possible that evolved gas will collect under the maskant, causing uneven etching of the material.
- The material removal rate is extremely slow in this application.
- In addition to being corrosive in nature, the chemicals used in the process are also potentially toxic.
- When machining alloys, the surface finish is degraded due to the use of different machining rates. As the etchant becomes contaminated with the reaction product, the rate of machining decreases.
Applications of the Chemical Machining process:
- It is utilised for generating intricate configurations in delicate items.
- However, apart from that, it is used in the aviation industry to manufacture aircraft wing panels.
- It is employed in the production of very thin laminations that are free of burrs and imperfections.
- In addition, it is employed in the production of printed circuit boards (PCB).
- Certain sheet metals are also prepared by Chemicchemical machining utilised in welding or riveting.
- Fine screens and meshes are used in industrial applications.
Electro Chemical Machining Process (ECM)
The electrochemical machining process is the reverse of the electroplating process. The workpiece becomes an anode, and the tool becomes the cathode. Therefore, the workpiece loses metal. Normally, the metal will flow through the electrolyte and get deposited on the cathode. Here the tool is the cathode. Therefore, the metal gets deposited on the cathode. The dissolved metal is forced away with the electrolyte.
The workpiece is held in a suitable fixture inside a tank. The workpiece is connected to the +ve terminal (anode) of a 20 V D.C. supply. The tool is held in position over the workpiece. The tool is hollow one. It is connected to the –ve terminal (cathode) of the supply.
The shape of the tool depends on the shape to be produced on the workpiece. A small gap of about 0.2mm is maintained between the workpiece and the tool. The sides of the tool are insulated. So the sides of the tool will not machine the workpiece, this avoids taper in the hole machined.
An electrolyte, usually sodium chloride, sodium nitrate or sodium chlorate is passed through the hollow tool.
When the D.C. supply is given, the current flows through the circuit. Electrons are removed from the surface of the workpiece (anode). These ions will attempt to reach the cutting tool (cathode). But these ions are carried away by the fast flowing electrolyte. The tool is fed towards the workpiece the workpiece automatically to maintain the gap between the workpiece and tool surface. The machining rate and surface finish are directly proportional to the current. The electrolyte is filtered and recirculated using a pump. The temperature of the electrolyte is maintained between 25 to 60°C.
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