Chemical Machining – History, Working, Applications, Advantages, Disadvantages and Common Types

Chemical Machining is the process of removal of material from the workpiece using chemical reactions by immersing the workpiece into a chemical solution.
Chemical Machining is a process used for metal removal purpose by dissolution in a controlled manner from the workpiece by the application of acidic or alkaline solution and this solution is called etchant.
The chemical machining process is widely used to produce micro-components for various industrial applications like micro electrochemical systems (MEMS) and semiconductor industries.

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History Of Chemical Machining:

• In ancient days, this process was used by artists for engraving metals. However, the artists were unaware of the working of the process or how the process works.
• This machining process was used to shape copper with citric acid in Ancient Egypt in 2300 BC.
• Before the 19th century, this process was widely used for decorative etching.
• In 1852, William Fox Talbot patented a process for etching copper with ferric chloride, using a photoresist.
• In 1888, John Baynes described a process for etching the material on two sides using a photoresist which was patented in the USA.

There are two substances mainly used in the machining process: maskant and etchant.

1) Maskant:
Maskants are chemical resistant coatings that are used to cover the surfaces which are not to be machined. Maskant do not allow etchant to penetrate through it and reach the material which is not to be dissolved.
This technique is very useful for produce complex configuration in delicate parts which can not be done by the conventional machining processes.

Choice Of Maskant: The maskant is chosen by keeping the following points in mind :
1) The maskant should be resistant to the etchant.
2) It should be easily removable after the machining is over.
3) The maskant should not have any chemical effect on the workpiece.
4) It should be completely stable at a high temperature of the etchant bath.

2) Etchant:
The etchant is the chemical used in the chemical machining process which dissolves the workpiece and removes material by chemical reaction.
To avoid uneven material removal from the workpiece – fresh etchant is continuously sprayed or the workpiece is submerged in an etchant tank.
To increase the material removal rate (MRR), the etchant is agitated, and if necessary etchant is heated.
The strength of etchant can be maintained by proper filtration, the addition of new chemicals, replace some percentage of used etchant by fresh one regularly.
Different etchants are used for machining different materials. Some of the etchants are FeCl₃ ( It is used for Al, Cu, Ni, and their alloys), FeNO₃ ( for Ag ), HF ( for Ti ), and HMO (for tool steel). Apart from these, other etchants used are chromic acid and ammonium persulphate.

Points to consider while choosing an etchant:
i) It should give a good surface finish.
ii) It should have a good material removal rate.
iii) It should have a high depth of penetration.
iv) It should not damage the workpiece.
v) It should be easily available.
vi) It should not be very costly.

Working Of Chemical Machining:

Different processes of chemical machining are given below:

1) Cleaning:
>> In the cleaning process, the workpiece is cleaned to remove the oil, grease, dust, or any substance from the workpiece so that further process is accomplished properly.
>> Cleaning is necessary to ensure proper adhesion of the masking material on the workpiece. In the case of debonding of maskant, stray etching occurs.
>> Different types of cleaning processes like vapor degreasing, alkaline etching, etc. are done based on the kind of maskant, kind of work material, and required machined depth.
>> Cleaning of the porous workpiece material is difficult.
>> There are mainly two methods of cleaning, mechanical and chemical methods. The chemical method is most widely used because it causes very less damage as compared to the mechanical method.
>> The parts are cleaned mechanically if the maskant is thicker and more durable and cleaned chemically if the maskant is thin or the parts to be cleaned are sophisticated.
>> Heating aids the cleaning process.


2) Masking:
>>Masking is done using maskants. These maskants are inert in nature and do not react with chemicals used in the machining process.
>> The maskant used should be a readily strippable mask.
>>In this masking process, the part of the workpiece which is not to be machined is masked using the maskants so that only the part to be machined comes in contact with a chemical used in the machining process. But when the masking process is done, the whole workpiece is masked.
>>Maskant is applied to the workpiece by any of the three methods given below:
i) cut and peel method.
ii) screen method.
iii) photoresist method.

3) Scribing:
>>After the masking process, scribing is done to remove the maskant from the area of the workpiece which is to be machined so that the chemical reaction can take place on that part of the workpiece.
>> After the scribing process, only those areas are exposed to the chemical machining which are to be machined.

4) Etching:
>>After scribing the workpiece is immersed in a container containing the chemical which undergoes the chemical reaction with the workpiece.
>>When the workpiece is dipped inside the chemical, the area which is masked does not undergo any chemical reaction and the area which is not masked undergoes a chemical reaction with the chemical and the material will start getting removed from the unmasked area of the workpiece.
>> The process of etching is generally carried out at an elevated temperature.
>>

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To avoid non-uniform machining gas bubbles should not be allowed to get trapped during the process.

There is a formula for Etching:
E=s/t
E = Rate of etching.
s = depth of cut.
t = Immersion time

5) Demasking:
After the etching process, the masked is removed from the area of the workpiece which is not machined, and also the oxide layer is removed from the area of the workpiece which is machined.

6) Washing:
After the demasking process, the workpiece is washed thoroughly under fresh water to remove the etchant or any substance completely from the workpiece surface.

Apart from all these steps, a step is optionally used which is
Heating and Cooling Of Etchant:
According to temperature, the temperature of the etchant in the container is maintained using the heating or cooling rod.

Setup of Chemical Machining Process:

Important measurements in Chemical Machining:

The material removal is measured by using two main values:
1) Depth of cut: It is the downward depth up to which the material is removed.
2) Undercut: It is the lateral distance up to which the material is removed.

The extent of the undercut depends upon the depth of cut, type and strength of the etchant, and the workpiece material.
The total machined depth and extent of the undercut are controlled by immersion time.

Parameters affecting the efficiency Of Chemical Machining:

The performance of the chemical machining process is affected by many process parameters which are:
i) Type of etchant.
ii) Temperature of etchant.
iii) Type of maskant.
iv) Maskant applying method.
v) Etcant circulation method.

Methods Of Applying Maskant:

There are 3 methods of applying maskant:
i) Cut and Peel Method
ii) Screen Printing
iii) Photo resist Maskant

i) Cut and Peel Method:
>>In the cut and peel method, neoprene, butyl, or vinyl based materials are used as maskant.
>>In this process, dipping, spraying, or flow coating the maskant material can be used for masking.
>> The thickness of the coating in this process is generally from 25 μm to 130 μm.
>> The maskant is first applied to the entire surface of the workpiece and after that, the maskant is cut and peeled off from the area which is to be exposed to the etchant for machining.
>> Scribing and peeling off of maskant is done by hand using a template.
>> The accuracy obtained in this process varies from 130 μm to 750 μm depending upon the size and type of component being produced.

ii) Screen Printing:
>> In the screen printing method, the mask material is applied to the workpiece surface by printing, using stencils and a fine polyester or stainless steel mesh screen.
>> Screen Printing is good for high volume production, low accuracy, and low etching depth < 1.5 mm.
>> Etching depth is low in this process because of the thinness of the coating.
>> In this method usually screen of stainless steel is used.
>> The screen block the area which is to be etched.
>> After that screen is pressed against the surface of the part.
>> Then the maskant is rolled up.
>> Then the screen is removed and the part is dried by baking.

iii) Photoresist Maskant:
>> This technique has become very common nowadays and this technique is also called photochemical machining (PCM).
>> This technique is used to produce complicated but accurate shape.
>> This technique produces intricate and finely detailed shapes using a light-activated resists material.
>> This method is carried out in a step by step process.
Steps:
i) The workpiece is coated with photo resist material and a master transparency is held against the workpiece and it is exposed to UV rays.
ii) After that light activates the photo resist material in those areas corresponding to opaque parts.
iii) Using this technique, a tolerance of 0.025 to 0.005 mm can be produced.

Advantages of Chemical Machining:

1) This machining process removes material uniformly.
2) In this machining process, it is possible to make tapered sheets and structural members with close tolerances and a good surface finish.
3) High skill labours are not required for chemical machining process.
4) It is suitable for low production runs.

Disadvantages of Chemical Machining:

1) Very few metals can be machined using this machining process.
2) Sometimes evolved gas get collected under the maskant and result in uneven itching of the material.
3) The material removal rate is very low.
4) Chemicals used in the process are corrosive in nature and can also be toxic.
5) In the case of machining of alloys, different machining rates results in poor surface finish. The machining rate goes down as the etchant gets contaminated with the reaction product.

Applications of Chemical Machining:

1) It is used for producing complex configurations in delicate parts.
2) It is also used in aviation industries for making aircraft wing panels.
3) It is used to manufacture very thin laminations without burrs.
4) It is used for Printed Circuit Boards (PCB).
5) Some sheet metals are also produced by Chemical Machining to be used in welding or riveting.
6) Industrial applications include fine screens and meshes.

Common types of Chemical Machining:

The two most common type of chemical machining is chemical milling and chemical blanking.
In chemical milling, the material is removed to produce blind details like pockets, channels, etc, or to reduce the weight.
Chemical blanking is used for the production of cavities like holes, slots, etc. It is also used for blanking complete parts from sheets by chemically etching the periphery of the desired shape.