What Is Chemical Machining?
The metal is slowly converted into a metal salt by a chemical reaction and eventually removed as such. Areas from which the material is not to be removed are protected by an etching-resistant material, known as a ‘maskant or’ resistance. ‘
Almost all materials from metals to ceramics can be chemically machined. The component to be mechanized is first cleaned in a solution of 80 to 90 ° C in trichloroethylene vapor or mild alkaline solution, followed by rinsing in clean water.
One of the thickest methods is to coat all by spraying or dipping the component. It removes dust and oil. Cleanliness ensures good adhesion of the coating or masking agent.
After cleaning, the components are dried and coated with mask material which can be cut and peeled, photorealistic or screen-print, type. Finally, the metal is removed by Etching.
Chemical machining is a process that is used for the purpose of metal removal by the dissolution of the workpiece in a controlled manner by the application of an acidic or alkaline solution, and this solution is called an etchant.
The chemical machinings process is widely used to produce micro components for various industrial applications such as micro electrochemical systems (MEMS) and semiconductor industries.
Working of Chemical Machining:
Differents processes of chemical machining are givens below:
In the cleaning process, the workpieces are cleaned to remove oil, grease, dust, or any substance from the workpiece so that further processing can be completed properly.
Cleanings are necessary to ensure proper adhesion of masking material to the workpiece. In the case of masking debugging, stray Etching occurs.
Various types of cleaning processes such as vapor degradation, alkaline Etching, etc., like maskants, are carried out depending on the work material and the required machining depth. Cleaning porous workpiece material is difficult.
There are mainly two methods of cleanings, mechanical & chemical methods. The chemical method is widely used because it causes much less damage than the mechanical method.
If the masked is thicker and thinner and chemically cleaned, then the thinner or cleaned parts of the masked are refined. Heating the cleaning process is helpful.
Masking is done using masking. These masks are inert in nature and do not react with the chemicals used in the machining process. The masked to be used should be an easy stripe mask. In this masking process, the parts of the workpiece that are not machined are masked using a mask so that the machined part is exposed to the chemical used in the machining process. But when the masking process is performed, the entire workpiece is masked.
Masks are applied to any of the three methods below:
- Cut and peel method.
- Screen method.
- Photoresist method.
After the masking process, masking is performed to remove from the area of the workpiece that is to be mechanized so that a chemical reaction can occur on that part of the workpiece. After the scribe process, only those areas that are to be mechanized are exposed to chemical machining.
After filtering the workpiece, it is immersed in a container containing a chemical that undergoes a chemical reaction with the workpiece.
When the workpiece is immersed inside the chemical, the area that is masked does not undergo any chemical reaction, and the area that is not masked undergoes a chemical reaction with the chemical and the material unmasked area of the workpiece. I will start moving away from it.
The etching process is usually performed at an elevated temperature.
Gas bubbles should not be allowed to become trapped during the process to avoid non-uniform machining.
There is a formula for Etching:
E = Rate of Etching.
s = depth of cut.
t = Immersion time
After the etching process, the masks are removed from the area of the workpiece, which is not mechanized, and the oxide layer is also removed from the area of the workpiece, which is mechanized.
After the demasking process, the workpiece is thoroughly rinsed under fresh water to completely remove any substances, etc., from the surface of the workpiece.
Apart from all these steps, one step is used optionally, which is the Etchant of heating and cooling:- According to the temperature, the temperature of the waiter is maintained in the container using a heating or cooling rod.
Chemical Machining Process:
The process can be applied to a wides variety of operations such as milling, blanking, and engraving. The various chemical machining processes can be classified as follows:
- Chemical Milling
- Chemical Blanking
- Chemical Engraving
Chemical machining for some specials purposes can also be achieved by using reactive gases, e.g., jets of chlorine, in the machining area. This is known as gaseous chemical machinings or hot chlorine machining and can be used for debugging metal parts.
#1. Chemical Milling
Chemical milling is sometimes called Chem milling or contour machining, or Etching. It is used to produces shapes by selective or overall removal of metal parts from relatively large surface areas.
The main purpose is to achieve shallows but complex profiles, reduce weight by removing unwanted materials from the surfaces, as in the skin of an aircraft. Components are cleaned & degreased by immersion in trichloroethylene vapor. Or some alternative chemicals cleaner followed by washing in clean water.
The components are then coated with a cut & peel maskant by brushing, dipping, or spraying (up to 0.2 mm). Thises can be a suitable fluid with a neoprene base. Or some alternative plastics solutions impervious to the action of the etchings agent (permitting Etching depths up to 10 mm).
When this has dried, by mild heatings otherwise, the desired shape to be processed on the work materials is cut on the maskant with ascribing knife, and the unmachined portions of the maskant are peeled away. Usually, templates are used to portray. The desired machinings shape within tolerance.
The part is then dipped completely into tanks of chemicals that will dissolve (etch) away from the exposed metal. After etching to the required depth and washings to remove all traces of the Etchant, the entire maskings are stripped from the component, and their surfaces are anodized or treated with a temporary protective agent as necessary.
#2. Chemical Blanking
Chemical blankets, chemical blanking, photo-making, photo vibration, or photo etching are variations of chemical milling. In this process, the materials are completely removed from many areas by chemical action. This process is mainly used on their sheets and foils. Almost any metals can be worked by this process; however, it is not recommended for thinning of materials more than 2 mm.
The workpiece is cleaned, reduced, and selected by acid or alkali. The cleaned metal is dried, and the photoresist material is applied to the workpiece by dipping, whirl coating, or spraying. It is then dried and cured. Photography materials have been used to produce degradation-resistant images in photoresist materials.
This type of mask is sensitive to light of a particular frequency, ultraviolet light in general, and not to room light. This surface is now exposed to light through a negative, namely, a photographic plate of the required design, as in developing drawings. After exposure, the image is developed. Unexpected parts break apart during the developing process showing bare metal.
The metal used is kept in a machine, which is sprayed with chemical substances or is added to the slurry. The etching solution can be hydrofluoric acid (for titanium) or one of many other chemicals. After 1 to 15 minutes, the unwanted metal has been ingested, and the finished portion is ready to remove the ashes immediately.
#3. Chemical Engraving
Printed circuit cards, other engraving operations, and cuttings of complex designs can be chemically blanked using photoresist maskants.
- Very thin metals (0.005 mm) can be dug well.
- High accuracy of the orders of +0.015 mm can be maintained.
- High production rates can be accomplished using an automated photographic technique.
History of Chemical Machining:
In ancient days, thises process was used by artists for engraving metals. However, the artists were unaware of the process or the process work. This machining process was used to shape coppers with citric acid in ancient Egypt in 2300 BC.
Prior to the 19th century, this process was widely used for ornamental Etching. In 1852, William Fox Talbot patented the copper digging process with ferric chloride, using photoresist. In 1888, John Baynes described the process for fabricating two-sided material using a photoresist, which was patented in the USA.
Two materials are mainly used in the machining process: mask and etc.
Maskants are chemical-resistant coatings that are used to cover surfaces that are not machined. It does not allow the maskant etc., to penetrate through it and access the material, which is not to be dissolved.
This technique is very useful for producing complex configurations in fragile parts that cannot be performed by traditional machining processes.
Choice of Maskant:- The masked is chosen to keep in mind the following points:
- Must be resistant to masked primitives.
- It should be easily removable after the machinings are over.
- The mask should not have any chemical effect on the workpiece.
- This captivity should be completely stable at the high temperature of the bath.
The Etchant is the chemicals used in the chemicals machining process, which dissolves the workpiece & removes material by chemical reactions. To avoid uneven material removal from the workpieces – a fresh etchant is continuously sprayed, or the workpieces are submerged in an etchant tank.
To increase the materials removal rate (MRR), the Etchant is agitated, and if necessary, the Etchant is heated. The strength of etchants can be maintained by proper filtrations, the addition of new chemicals, replace some percentage of used etchants with fresh ones regularly.
Different etchants are used for different machinings materials. Some of the etchants are Fe Cl3 ( It is used for Al, Cu, Ni, & their alloys), FeNO3 ( for Ag ), HF ( for Ti ), & HMO (for tool steel). Apart from these, others etchants used are chromic acid & ammonium persulphate.
Points to consider while choosing an etchant:-
- It should give a good surface finish.
- It should have a good materials removal rate.
- It should have a high depths of penetration.
- It should not damage the workpiece.
- It should be easily available.
- It should not be very costly.
Important Measurements in Chemical Machining:
The material removals are measured by using two main values:
Depth of cut:- It is the downward depths up to which the material is removed.
Undercut:- It is the laterals distance up to which the materials are removed.
The extents of the undercut depend upon the depths of cut, type & strength of the Etchant, & the workpiece material. The total machined depth & extent of the undercut are controlled by immersion time.
Advantages of Chemical Machining:
- This machining process removes material uniformly.
- In this machinings process, it is possible to make tapered sheets & structural members.
- With close tolerances & a good surface finish.
- High-skill laborers are not required for the chemical machining process.
- It is suitable for low production runs.
Disadvantages of Chemical Machining:
- Very few metals can be machined using a thises machining process.
- Sometimes evolved gas gets collected under the maskant & result in uneven itching of the material.
- The material removal rate is very low.
- Chemicals used in the process are corrosives in nature & can also be toxic.
- In the case of machinings of alloys, different machinings rates result in poor surface finish. The machinings rate goes down as the Etchant gets contaminated with the reaction product.
Application of Chemical Machining:
- CHM has been applied in the number of usages where the depths of metal removal are crucial to a few microns, & the tolerances are close.
- The surface finishes obtained in the process are in the range of 0.5 to 2 microns.
- Besides, it removes metals from a portion of the entire surface of formed or irregularly shaped parts such as forgings, castings, extrusion, or formed wrought stocks.
- One of the major applications of chemical machinings is in the manufacture of burr-free, intricates stampings.