CHEMICAL MACHINING

In chemical machining reactive chemical solution is used to selectively dissolve and remove material
from the w/p. The process is not new. Around 2500 BC, Egyptians used to use citric acid to etch
copper jewellery. About 1000 years back in Arizona, USA, Hohokam Indians used to used fermented
cactus juice to etch snail shell jewellery. In 15th century, chemical etching was used to decorate suits
of armour. The armours are made by forging and they were as hard as chisels. Hence, it was not
possible to engrave patterns on them. One of the earliest etching solutions was made from common
salt, vinegar and charcoal. For selective removal of material, the etchant was used in conjunction
with a mask. The mask was hand scribed and made of linseed oil paint. For producing decorative
patters on swords, mask (resist) made of wax was used. By 17th century armours produced were a
great work of art.
 Armour of Emperor Ferdinand I(1503-1564) dated 1549

https://www.metmuseum.org/art/collection/search/23944,
https://www.metmuseum.org/toah/hd/dect/hd_dect.htm
Ref: Weapon- A Visual History of Arms and Armor ,pub:DK,2006,p.12
16th century – printing plates were produced using etching. These plates were superior to the ones
produced by engraving because these plates were burr free.

In mid 17th century – indelible calibration of measuring instruments and scales; ex. Artillery gunners
conversion table (1650)

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.

1925- large scale production of printing plates for newspaper industry. Nitric acid solution is used to
etch the plates. By 1927, chemical milling through rubberised paint mask became prevalent. The
open areas in the mask were cut manually using a template.

Both sides of Atlantic Ocean a number of patents were awarded in this area. Notably, in 1944 John
Snellman took a patent on photo chemical machining for producing flat metal components from
shim stock that is too hard for punching.

In 1936, Paul Eisler developed the first printed circuit board for use in a radio set. In 1943, Paul Eisler
patented a more advanced PCB design that involved etching the circuits onto copper foil on glass-
reinforced, non-conductive substrate. It is only in 1948, The USA Army released the PCB technology
to the public.

https://www.printedcircuits.com/blog/history-of-pcbs/

since 1960’s chemical machining became a standard method for industrial production. Two major
classes of chemical machining are: chemical milling (machining blind details, or used for weight
reduction) and chemical blanking (machining very complex through details)

There are three methods used for chemical machining: i) cut and peel method, ii) Screen and iii)
photochemical machining (PCM).

The common steps followed for chemical machining are:[1]

1. Preparing the work piece: cleaning

2. Masking the w/p: To apply etchant resistant material on the w/p. It is necessary if material
removal from the w/p is restricted to specific locations only.

3. Etching: To apply reactive chemical solutions on the w/p to remove the material from the areas
not protected by mask/ resist.

4. Mask/ resist removal: After material from the w/p has been removed to the desired depth, the
mask/ resist is removed by mechanical or chemical means.

5. Finishing: Inspection and post processing, if any.

The details:

Pre cleaning: The surface of the raw stock/ work piece needs to be cleaned to ensure proper
adhesion of the maskant/ resist on the w/p surface. If the maskant/ resist does not adhere to the
w/p surface effectively, then debonding may take place during etching. It will result in stray etching
i.e. removal of material from unintended areas of w/p.

Depending on the methods of chemical milling, the precleaning procedure may change from
simple mechanical methods like wire brushing, solvent wipe to more involved techniques like flash
etching, vapour degreasing, alkaline etching, etc.

Step 2

For material removal from specific locations, chemically resistant mask/ resist must be applied to the
areas that are to be protected from the action of etchant. So, maskant/resist is applied on the
surface of the w/p and then inspection is carried out to check for incomplete coverage and lack of
adhesion. Flow coating, spray coating, dipping, laminating may be used to apply the maskant.

Step 3

Next, the maskant is to be removed from those areas from where materials are to be removed.
Different methods mechanical / chemical are used to remove the maskant/ resist selectively.

Step 4

The w/p is exposed to etchant to remove material from the exposed surfaces. Once the required
depth of cut is achieved, then the w/p is washed and dried.

Step 5

Next, the maskant/ resist is removed either mechanically/ chemically.

Step 6

The w/p is ready. Inspection is carried out and in some case post processing may be necessary.

Selection of Maskant/Resist: following factors need to be considered.

1. Chemical resistance required

2. Number of parts to be produced.

3. Detail or resolution required.

4. Size and shape of parts.

5. Ease of resist removal

6. Economics
Chemical resistance required

The chemical resistance of maskant/resist depends on 1. Type of muskant/resist

2. Thickness of muskant/resist

3. Type of w/p material

4. Processing difficulty

5. Etchant used

6. Operating conditions

Thicker the maskant longer it will resist the chemical action of the etchant. As a result, deeper
etching (deeper depth) can be achieved. The chemical resistance of maskant/ resist varies widely.
Some maskant can resist concentrated acid/ alkali solutions for hours; while some modified photo
engraving resists may be having a useful life of a few minutes when exposed to an etchant.

Usually, thicker maskants (0.2 -0.38mm in dry film form) adhere much better to the w/p surface than
the thin resists. With thick maskants there is minimal attack at the interface of maskant- w/p. With
the maskants used in cut & peel method etch depth of 12.7 mm from one side is possible. Whereas,
with photoresists, which are very thin, the depth of etch is restricted to a maximum of 1.3 mm. The
photoresist does not adhere to the w/p well, may lead to debonding because of etchant attack at
the interface. Further, the photoresists are brittle in nature and are prone to chipping. Because of
undercutting during chemical milling, the overhanging photoresist may break leading to uneven
etching, leading to irregular etching and loss of tolerance. Because of larger thickness the maskants
used in cut &peel or screen methods are not brittle and can support overhanging sections.

Number of parts:

If number of parts are small then cut & peel method can be economical. Cut & peel method is
basically a manual method. A knife is used the scribe the maskant and peel it away from the area
that need to be etched. It is done with the help of a template.

However, as the number of parts increases, then the manual method won’t be economical. Next, is
screen method which is semi- automatic. If the number is large them PCM (photo chemical
machinery) is the best option as it is an amiable to high level of automation.

(Not all parts can be chemically milled by all the three methods. There are advantages and
limitations of every method. That basically translates to: some features can be made may be by only
one method)

Detail or resolution required.

Type of maskant/
resist
Minimum width of area to be accuracy
Minimum line width of
etched (minimum width of
etch resistant pattern
cut)
Cut & peel 3.18 mm 1.5 mm ±0.18 mm
Screen 0.3 mm 0.3 mm ±0.076
PCM 0.013 mm 0.013 mm ± 0.03-0.005mm
Size and shape of parts.

Cut & Peel method: No limitation on size and shape.

Screen: Only flat part or part curved in one dimension can be chemically milled in screen method.
The size of the screen limits the w/p size. A common limit is 1.5m2.

PCM: Initially when photographic films were used the size of the w/p was limited to 1.22m x 3.66m.
This is achieved by assembling a series of negative. But due to development in laser technology this
limitation has been removed.

Processing Difficulty:

Cut & Peel method: processing difficulty is relatively simple and can be handled under normal plant
conditions. Only proper ventilation in the plant is necessary. W/p surface preparation is relatively
simple. Solvent cleaning or light abrasive cleaning followed by flow, dip or spray is used for coating
w/p surface with maskant.

Screen: cleaner plant environment is necessary in the case. This is necessary so that dirt may not
clog the fine pores of the screen. Other requirements are same as that of cut & peel method.

PCM: Most difficult to use. Extreme cleanliness in plant environment is necessary. W/p surface
preparation is very critical. Any dust or dirt may cause defect in coating, printing and development of
image in PCM. As photoresist does not adhere to the w/p well, w/p surface preparation is extremely
important. Grease, oxide layer, dirt, dust have to be removed to enhance the adhesion of
photoresist to w/p. Vapour degreasing, flash etching, alkaline etching is used.

Photoresist are sensitive to normal fluorescent light so, gold fluorescent light or tungsten light of
relatively high illuminating power can be used safely without exposing the photoresist.
a
pebbled

toxicological
Ref: Tom Drozda, Tool and Manufacturing Engineers Handbook, vol.1-machining, 1983, SME, p.14-84
Ref: Tom Drozda, Tool and Manufacturing Engineers Handbook, vol.1-machining, 1983, SME, p.14-85
Ref: Tom Drozda, Tool and Manufacturing Engineers Handbook, vol.1-machining, 1983, SME, p.14-86,87,88.
The Baumé scale is actually two independent and mutually exclusive (non- overlapping) hydrometer scales that covers (a) liquid with specific gravity greater
than 1.0 (b) liquids with specific gravity less than 1.0

For liquids with specific gravity greater than 1.0

0o Bé = distance the hydrometer sinks in water.

15o Bé ==distance the hydrometer sinks in a solution of 15% sodium chloride by mass.

To convert from oBé to specific gravity at 60o F,

145
Specific gravity = (145− 0 Bé)

For liquids with specific gravity less than 1.0

0o Bé = distance the hydrometer sinks in a solution of 10% sodium chloride by mass.

10o Bé = distance the hydrometer sinks in pure water.

To convert from 0Bé to specific gravity at 60o F,

140
Specific gravity = (130+ 0 Bé)
General description of cut and peel method.

Cut & peel method is basically a manual method. Dipping, spray or flow method is used for masking the parts. Usually
very large parts having many irregularities are prime candidates for this process. The required depth of should be
0.25mm. The dry film thickness of the maskant varies between 0.2mm – 0.38mm and is highly resistant to etchant.
Etchant depth of 12.7mm from one side is possible in cut & peel method. Because of high stability of the maskant ( i.e.
the maskant that overhangs due to undercutting do not chip off easily) step etching is possible. By using step etching it
is possible to achieve a depth of 50mm. portion A knife is used the scribe the maskant and peel it away from the area
that need to be etched. It is done with the help of a template.
 Ref: Edmund L Van Deusen, Chemical Milling, Scientific American, 1956,p.105-112.

Ref: ASM International Handbook, vol.16 machining, 1989,p.582.

Ref: MDS, Machining Data Handbook, vol.2, 3rd Ed., p.13-10

Because of hand scribing the it is very difficult to hold the tolerance tighter than ±0.13 mm. But this tolerance is on
position not on depth of etch. The depth of etch can be controlled very accurately i.e. within ±0.05-0.076 mm.

Material used for maskant: vinyl for acid etchants only, styrene, butadiene, neoprene, styrene-ethylene, TP polymers
and butyl- based materials.

Ref: Tom Drozda, Tool and Manufacturing Engineers Handbook, vol.1-machining, 1983, SME, p.14-82
Ref: MDS, Machining Data Handbook, vol.2, 3rd Ed., p.13-9
Ref: MDS, Machining Data Handbook, vol.2, 3rd Ed., p.13-13
Ref: MDS, Machining Data Handbook, vol.2, 3rd Ed., p.13-6

Screen Printing: In this case a silk or stainless steel screen is used. A strencil made photographically, is placed on top of
the screen. That selectively blocked some areas i.e. those areas from where material are to be removed. The screen is
placed on the pre-cleaned w/p.Resist is rolled on the screen. The resist went through the open areas of the screen and
got deposited on the work piece. Then the screen is lifted off the w/p. Afer the resist got dry, the w/p is exposed to
etchant. Material got removed from the resist free areas on the w/p only. After etching is over, w/p is washed and then
the resist is removed.

As the thickness of the resist is more than that of PCM but much less than that of cut & peel method, hrnce, the
depth cut etch that can be achieved is limited to max. 1.5 mm. Tolerance is much better than that of cut & peel method
(±0.076mm).

It is a semi- automatic process, hence, the production rate is much higher than that off cut & peel method.

PCM (photochemical machining) though it is a part of chemical machining, but due to its importance it has become a
independent process in itself.
Ref: Ref: ASM International Handbook, vol.16 machining, 1989, p.587.

Artwork is 2X to 20X. The reduced art work is 1X plus compensations. Almost all the steps are critical in nature. With the development of Laser technology
and CAD, it became possible to reduce the steps substantially and hence, the production time.

The first development was, instead of making photo-master (reduce artwork photographically) from a drawing, a 5mW argon laser (488µm) is used to
generate the image from a CAD file directly on the photographic negative. The wavelength of the light reacts with the photographic film to expose and
create the photomaster. Accuracy of ±0.25 mm and repeatability of ±0.0005 mm is possible. A photomaster of size 0.45m X 0.6m can be generate by raster
scan method in less than 6 minutes. Not only processing steps and consequently, lead time have been reduced but accuracy of product is also increased.

Another development is elimination of multiple image masters also. A 3W argon laser is used to directly expose the photoresist using the data stored in
CAD file. This is known as Direct Laser Imaging. It takes only 2 minutes to expose the photo resist using raster scanning method to generate a pattern of size
0.45m X 0.6m.
Type of photoresists: There are four types of photoresists: wet film positive, wet film negative, dry positive film and dry
negative film.

Ref: D.M.Allen, Photochemical machining: from ‘manufacturer’s best kept secret’ to a $6 billion per annum, rapid
manufacturing process, Annals CIRP-manufacturing, vol.23, issue 1, 2004.

Wet film : this film is applied to the metal in liquid form. The cost is lower compared to dry film and if correctly applied it
allows finer etched lines i.e. better resolution can be achieved. The edges of the metal including any slot and hole are
coated. However, metal has to be rigorously cleaned and working atmosphere should be dust free. Applying resist by
dipping is not easy. It is difficult to maintain constant thickness of film. Viscosity of the liquid is also critical. Usually, the
thickness of the film gets thicker from top to bottom. The optimum thickness of the film is 5µm. Next, the film has to
baked which is time consuming and costly.

Dry film: Dry film is available in form of rolls. The thickness of the dry film layer varies between 25µm to 125µm. The
resist is protected on the roll by a polymer film. This film is not removed before the sheet is developed. The film is
applied to the metal sheet by applying both heat and pressure by using heated rollers (lamination process). The most
common thickness of the film is 38µm. Film thickness above 38µm is rarely used. But a double layer of film can be
applied, provided the polymer film may be removed from the first film before rolling.

Advantage: Dry film laminating is a faster process than the wet film process. But, as the thickness of film increases, the
thickness of the line that can be etched increases i.e. the resolution of cut- line decreases. The cost of dry film is higher
than the wet film. It is not possible to coat the edges of holes, slots or metal sheets with resist. A range of rolls having
different widths and thickness of film has to be stored for different types of work.

Ref:
 Half etch details

Ref: white paper document, white paper.pdf, ‘photo chemical machining (PCM)- an overview’ from qualitetch.com

Part produced by using half etch fold lines.

Ref: David M. Allan Photochemical Machining: From Manufacturer’s Best Kept Secret to a $6 billion per annum,
Rapid Manufacturing Process, Annals. CIRP journal of Manufacturing Systems, vol.23, 2004
https://www.boydcorp.com/engineered-materials/insulation-shielding/emi-rfi-management/emi-rfi-shielding.html

EMI (electromagnetic interference) /RFI (radio frequency interference) shield

EMI (electromagnetic interference) /RFI (radio frequency interference) shields

https://www.photofab.co.uk/products/rfi-shielding-products/
Chemical Blanking
Tabs:
http://www.etching86.com/skjsyy/
Ref: E J Weller & Matthew Haavisto, Nontraditional Machining Processes, 2nd ed., SME,1984, 258-261
APPLICATIONS
Ref: Figs & text taken from " E J Weller & M Haavisto, Nontraditional Machining Processes,2nd ed., SME,1984,p.230-234
More examples:
 Ref: text and figs. taken from,"Gary F Benedict, Nontraditional Manufacturing Processes, Taylor & Francis, 1987, p.203-204

Made from 0.25 gage 6Al-4V-Ti alloy.

Ref: text and fig. taken from ASM International Handbook, vol.16 machining, 1989,p.580.
Part made from single piece 0.25 in 2014 aluminium alloy. After forming aged to T6 condition before chemically milling.
Radial lands are 7.6 mm wide and remain as 6.3mm stock thickness to help stiffen the structure. Thinnest section is of
1.9mm.

Ref: text and fig. taken from ASM International Handbook, vol.16 machining, 1989,p.585.

Used for Boeing 747 & 767. The door is made by stretch forming (6.4mm) Alclad 7075 and then chemically milled to final
thicknesses of 1.0,2,0 and 3.2 ±0.1mm

Ref: text and fig. taken from ASM International Handbook, vol.16 machining, 1989, p.586.

http://www.tecometetch.com/etched-aerospace-parts.html

please go to the site. Examples of chemical machining in different sectors are given.

Improving surfaces:

1.. Removing of alpha case from titanium forging and superplastic formed parts.

2. Removal of decarburized layer from low alloy steel forgings.

3. Removing recast layers from surfaces machined by EDM.

4. Deep gouges, pits and scratches can be removed by chemical milling by first filling the defect with maskant and then
chemically milling the part to a depth greater than the depth of defect. Then the resulting island is mechanically.
removed

5. It is possible to remove the sharp burrs produced during contact machining or piercing by removing 0.03 to 0.13mm
of material from the w/p by etching.

Advantages:

Most of the metals can be machined effectively.

As material is removed simultaneously from all the surfaces exposed to etchant, hence, the material removal rate is
high (though etch rate is quite low). The productivity is high compared to most of the non- traditional processes.

Complex shapes and deeply recessed areas can the uniformly milled.

Hardness or brittleness of the material does not affect machinability of chemical machining process.

No burr formation.

No stress is induced. Machining of delicate parts are possible as little distortion takes place.

Mechanical properties of the parts mostly remain unchanged.

Capital cost of equipment required for machining large components is relatively low.

Highly skilled operator is not necessary.

Time required for developing tooling for a new job and the associated cost are low. That makes the process extremely
competitive for both prototype part fabrication and for accommodating design changes for products.
Hydrogen embrittlement and intergranular attack can take place and can have detrimental effect on mechanical
properties of component produces unless post-treatment corrective action is carried out.

The quality of chemically milled product depends on the homogeneity of the w/p material.

Surface texture of chemically milled parts are dependent of the grain structure of the metal.

To have a uniform and acceptable surface finish of the chemically milled surface, mechanical polishing may be
necessary.

Process control coupons are often required. It adds to the cost.

Safe disposal of etchants is a major issue.

Chemical Engraving
 material

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produced