Saturday 22 May 2021

Maskants In Chemical Machining

Introduction :-

Hard and difficult to shape metals cannot be easily machined by conventional machining methods. Thin work parts with complex configuration or delicate parts fail to withstand the forces of cutting tool. 
Although some nontraditional process can be used in such conditions, they are not economical due to high capital and tooling costs involved. These disadvantages are overcome with the use of chemical machining process. 

Fig 1 :- 
a. Brief illustration of chemical machining process 
b. Stages of producing a profile of cavity by the help of machine called agitator


Types of Chemical Machining Process :-


1. Chemical blanking method :- 

Chemical blanking is a process of producing a part from thin sheet metal by chemically etching the periphery of the desired shape. The material is removed by chemical dissolution.

Fig 2 :- 
                                 

Chemical blanking employs the following four steps :- 

a) Pre-process - Material cleaning - Metal surface is cleaned by degreasing, pickling and grinding.

b) Masking. Metal portion not to be removed by etching is covered with chemical resistant coating.

c) Through-material etching. 

d) Post-process.


2. Chemical milling or contour machining method :- 


Fig 3 :- 
 


Chemical milling is the subtractive manufacturing process of using baths of temperature-regulated etching chemicals to remove material to create an object with the desired shape. It is mostly used on metals, though other materials are increasingly important.


Procedure for chemical machining 


Workpiece pre cleaning process :-

 The surface of workpiece metal is cleaned thoroughly, degreased and pickled by acid or alkalis. Pre cleaning is the most important method to remove oil, grease, dirt, rust, or any foreign substance from the work surface to produce a good adhesion of masking material.

 Masking and scribing mask:- 

Masking involves covering the portion of the workpiece metal where material is not to be removed by the chemical action .masking with adhesive types or paints is a common practice although rubber (elastomers) and plastics are also used. Since it is difficult to apply maskant on small surface, the maskant is initially applied on a large surface.

  Etching:-

The unmasked surface of the workpiece is machined chemically with selected etchant. Etching is carried out by immersing the work material in a tank of agitated etchant. The process is carried out at high temperature depending on the etched material. Temperature control and agitation during chemical machining. Erosion of the work material takes place from the exposed surface. The work piece is converted into metallic salt, which is then dissolved and carried away in the etchant solution.

 Damasking:-

 When etching is completed the mask is removed either through mechanical or chemical means. any etchant on the work material is also removed by cold water to clean. A deoxidizing bath may also be required in order to remove the oxide coating or films left on the surface of the work material. 


Maskants 

Masking material is known as Maskant which is used to protect workpiece surface from chemical etchant.
 In another words maskants protect the portion of workpiece metal where material is not to be removed by chemical action of the etchant. 
Polymer or rubber based materials are generally used as a maskant material. Various maskant application methods can be used such as dip, brush, spray, roller, and electro coating as well as adhesive tapes. 


The type of maskant to be selected for machining is based on the following factors :-

• Be inert to the chemical reagent used.

• Chemical resistance required.

• Be tough enough to withstand handling.

• Adhere well to the work piece.

• Allow itself to be scribed easily.

• Be removed easily after etching.

• Be inexpensive after etching.

• Be able to withstand the heat generation by etching.

 • Availability and low cost.

● Masking material for various work material :-

1. Work material = Aluminium & Alloys

Maskant = Polymer, butyl rubber & neoprene 


2. Work material = Copper & alloys 

Maskant = Polymer


3. Work material = Iron based alloys

Maskant = Polymer, poly vinyl chloride, polyetilien butyl rubber 


4. Work material = Nickel

Maskant = Neoprene


5. Work material = Magnesium

Maskant = Polymer


6. Work material = Titanium

Maskant = Polymer


Etchants :-

Etchants are acid or alkaline solutions maintained within a controlled range of chemical composition and temperature. 
The workpiece material to be removed is sprayed or immersed in a suitable etchant.
various etchant are available for machining different material as listed in table.



Conclusion :- 

Material  removal  rate,  accuracy,  surface  finish  and environmental  issues  are  the  major  challenges  what  chemical machining  face.
  Rate  of  removal  of  material  is  depended  on the  type  of  etchant  we  are  using  for  machining  process. Etchant  which  remove  metal  in  a  faster  manner  will  have many  side  effects  including  surface  finish,  higher  heating  etc. 
So  the  etchant  should  be  selected  in  such  a  way  that  it  should balance  among  all  these  problems.  Undercutting  is  the  issue which  reduces  the  accuracy  of  machining.  Hydrogen embrittlement  also  causes  alterations.  
The  surface  produced  by CHM  process  is  otherwise  stress  free  and  show  no  thermal effects.  Chemical  machining  greatly  affects  the  environment since  it  make  use  of  chemical  etchants.  
To  improve  the material  removal  rate  of  chemical  machining  is  to  combine both  laser  machining  and  chemical  machining.   

References :- 

1. Benedict,  G.  (2017).  Nontraditional  manufacturing processes.  Routledge. 

2  Çakir,  O.,  Yardimeden,  A.,  &  Ozben,  T.  (2007). Chemical  machining.  Archives  of  Materials  Science and  Engineering,  28(8),  499-502. 

3 Langworthy,  M.  (1994).  Chemical  Milling  By Nontraditional  machining  process.  Machining  ASM Handbook. 

4 Tehrani,  A.  F.,  &  Imanian,  E.  (2004).  A  new  etchant for  the  chemical  machining  of  St304.  Journal  of materials  processing  technology,  149(1-3),  404-408.

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