NTM - Chemical Machining_gurpreet Singh

7/29/2019 NTM - Chemical Machining_gurpreet Singh

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Presentation on

Chemical Machining

Gurpreet SinghM.tech. 4th Sem.


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Introduction

Chemical machining also known as chemical etching, wet

etching, chemical milling It is a non traditional machiningprocess in which material removal is carried out by using

strong chemical solution , called etchant. This is simply the

accelerated and controlled corrosion process. This method is

recentaly employed as a micromachining process in

production. The two key elements used in chemical machining

process are etchant and maskant. Etchants are acid or alikine

solutions maintained with controlled ranges of chemical

composition and temprature. Maskants are specially designed

elastomeric products that are hand strippable and chemically

resistant to the harsh etchants.


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Intro.........

Chemical machining has a long history back to BC 2500, used to

produce jewellery from copper by citric acid in the ancient Egypt(Harris 1976) It has been accepted one of the important NTM

method since 1950s. This machining process widely used to

machine thin and flat materials producing geometrically complex

and dimensionally accurate components. It also used to reduceweight of the workpiece materials such as aircraft wings


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Steps in chemical machining process

Preparing- degrease, clean, rinse, pickle and preclean of the

worpiece surface to provide a clean surface for good adhesion

of masking material

Masking- coating or protecting area not to be etcheed

Etching chemically dissolves by etchant.

Demasking- to strip or demask of the maskant and clean the

workpiece.

Post process- Post treatment and surface inspection( surface

topography test, scan electron microscopy, thickness and surface

finish test.)


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MASKS

The selection of masant for given application depends onfollowing considerations:-

1) Chemical resistance- thicker maskants can resist the erosive

action of the etchant for a longer period and hence larger depth of

cut is possible with them.2) Quantity of parts- For a large volume production the masking

process should be as simple as possible to reduce the cost.

3) Ease of removal- Delicate parts require that the maskant should

be easier to remove since the mask should be removed before the

part is used4) Required resolution- Thicker masks are generally less accurate

though the method of masking the mask also contributes to it.

h d f ki


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Methods for masking

1)Cut and peel- This is generally a neoprene, butyl or vinyl

based thick material , which is applied by dip, spraying or flowcoating, Afterwards the pattern is scribed on the mask and

peeled away exposing the area to be etched. The thickness of the

mask can range from 0.025 0.125 mm which allow it to be

used for large depth of cut . It also used for step etchingapplications.

2) screen printing- The maskant is transferred to the workpiece

using a fine mesh screen such as those used for silk screen

printing. This is best and economical for large volumeproduction with relatively less accuracy. The thickness of mash

is relatively small( less than 0.05 mm) and hence used for

shallower etching depths.


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3) Photo resist masks- This is mostversatile method compared to the other

masking technologies use of these maskesmakes the process to be called photochemical

machining(PCM) though the rest of the

process remains chemical machining..


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EtchantsThe function served by the etchant is to dissolve the metal from the

part by converting it to a salt, which then goes to the solution .

The choice of etchant depends upon number of factors; some are:-

1)Surface finish- some etchants promote the formation of surface

oxides, which detrimental to the surface finish.

2)Removal rate- active etchants remove material faster reducing

the machining time, but can also to attack the maskant, giving poor

surface finish and are generating more heat.

3) Material type- the etchant while removing the material should

not cause inter-granular attack, hydrogen embrittlement or stress

corrosion cracking.


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Some Chemical machining processes

a)chemical machining of alumnium[ journal Material processes technology 199(2008) 337-

340]

in study it was observed that ferric chloride is very usefulchemical etchant for aluminium.

Etching procedure- selected etchant ferric chloride was

prepared at 1.25 mol and poured into beaker that was put

into water jacket to control etching temperature.The selected etching temperature were 20,30,40 and 50

degree centigrade. Etching process was carried out for 20

min. In total


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process


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b) chemical machining of coppercopper etching is considerably important process in electronics

industry, particularly in the field of printed circuit board. Various

etchants can be used for this purpose, but now days cupric chlorideis more accepted etchant, because of it's high etch rate and easy

regeneration properties.

The ideal etching solution for copper should have some properties

as follows:a) high etch rate

b) Minimum under cut

c) high dissolved copper capacity

d) stable and easy control of copper etching process

e) not generate toxic flumes

f) environmentally acceptable

g) economic regeneration of waste etchant

h) economic etched copper recovery from waste etchant.


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Cupric chloride is mainly used because it can also etch copper

alloys lie brass, bronze and beryllium. It has easy regeneration

properties which is important factor because Environmental and

health restrictions have become an important parameter since1970s

Process Copper etching with cupric chloride -

it is simply expressed by following reaction:

the uncoated copper surface gets attacked by cupric chloride . The

one copper atom reacts with one cupric ion and forms two atoms ofcuprous ions as follows

Ph h i l hi i


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Photo chemical machiningit is a chemical milling process used to fabricate sheet metal

components using a photoresist and etchants to corrosively

machine away selected area. It can produce highly complex parts

with very fine detail.

Process- 1)the process starts by printing the shape of the part on to

optically clear and dimensionally stable photographic film. The

phototool consist of two sheets of this film showing negative

images of the parts (meaning that the area that will become the partis clear and all other area to be etched are black)

2)The metal sheets are cut to size, cleaned and then laminated on

both sides with a UV-sensitive photoresist.

3)The coated metal is placed between the two sheets of phototooland the metal plate.

4)The plate is then exposed in UV light that allows the area of

resist that are in clear section of the film to be hardened.

5)After exposure, the plate is developed, washing away the

unexposed resist and leaving the area to be etched un protected.


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CHM Advantages

Weight reduction is possible on complex contours that are

difficult to machine using conventional methods.

Simultaneous material removal, from all surfaces, improves

productivity and reduces wrapping.

No burrsNo stress is introduced to the workpiece, which minimizes the

part distortion and makes machining of delicate parts possible.

* A continuous taper on contoured sections is achievable.

The capital cost of equipment, used for machining large

components, is relatively low.

* Design changes can be implemented quickly.

* Decorative finishes and extensive thin-web areas are possible


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CHM Limitations and of disadvantage

*Handling and disposal of chemicals can be troublesome.

*Hand masking, scribing, and stripping can be time-consuming,

repetitive, and tedious.

*Surface imperfections are reproduced in the machined parts.

* Metallurgical homogeneous surfaces are required for best results.

*Deep narrow cuts are difficult to produce.

* Fillet radii are fixed by the depth of cut.

* Porous castings yield uneven etched surfaces.

*Welded areas frequently etch at rates that differ from the base metal.

*Material removal from one side of residually stressed material can

result in a considerable distortion.


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CHM Limitations and of disadvantage

The absence of residual stresses on the chemically machined surfaces

can produce unfavorable fatigue strength compared with the processes

that induce compressive residual stresses.

Hydrogen pickup and intergranular attack are a problem with some

materials..

.


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CHM Applications

1. Manufacturing of encoders

2. Masks

3. Filters (aerospace filters)

4. Lead frames flat springs

5. Strain gauges6. Chip carriers

7. Heat sinks

8. Shutter blades9. Laminations

10.Fuel cell plates

11.jewelry


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Thanks...

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NTM - Chemical Machining_gurpreet Singh