1. Prior Art
Chemical milling is a process of metal removal by chemical etching where the
object is coated with a maskant in some specific areas, according to the design to be
made by chemical milling, and immersed into an etchant bath for chemical reaction. It
consists of the following essential steps1-4:
(a) Precleaning: Before applying the maskant, the object surface should be cleaned
by wiping with solvent wet cloth and/or by vapour degreasing method to remove the
visible oil or grease etc. Any invisible oil, surface dust etc. should be removed by
immersing the object into the alkaline solutions. Scale oxide layer etc. must also be
removed by using appropriate descalers to ensure uniform adhesion to the maskant.
Each of these cleaning step should be followed by thorough rinsing.
If the object surface is not cleaned prior to the maskant application, the
adhesion properties of the maskant may be seriously impaired. The most likely
problems are extremely high adhesion, making the maskant very difficult to strip, or
very low adhesion, resulting in poor line definition between etched and
nonetched areas.
(b) Masking: Maskant is most commonly applied by immersing the parts into dip
tanks or covering the parts with hot airless spray equipment. Conventional air
spraying, flow coating, brushing, and roller coating are less frequently used. Maskants
must provide impermeable, chemically resistant coatings that can be easily hand
stripped, yet that strongly adhere at cut edges during etching. An appropriate maskant
is selected depending upon the material of the object. Some typical maskants and type
of etchants used for various types of object material are listed in Table 1.

(c) Scribing: The pattern is normally scribed through the mask by guiding a sharp
knife along with edges of a template located precisely on the object. The mask is then
peeled off the areas to be etched. Computer-guided lasers are also used for
maskant cutting.

(d) Etching: The etching procedure consists of immersing the masked object in the
tank of etchant for a specific period depending upon the etching rate and the desired
depth of etching. The type of etchant used for different types of materials is indicated
in Table 1.
In summary, the art of chemical milling is largely dependent upon the quality of
masking which involves high degree of surface cleanness and an appropriate maskant
with good adhesiveness (even though it should be hand peelable) to the specific metal
surface. The innovative approaches in the field of chemical milling are mostly
directed towards the development of better maskants for different type of materials
and better milling solutions i.e. the etchants for obtaining better control in etching.
There are numbers of patents5-9 available on the several aspects of chemical milling.
Some standard practices may also be available in ref.10.
References:
1. Bryan, W. J., "Chemically Machined parts," American Society of Tool and
Manufacturing Engineers, ASTME, MR66-165,1966
2. Hans, W. T., Chemical Milling, Clarendon Press, Oxford, 1976
3. Dini, J. W., "Fundamental of Chemical milling". Machining Handbook,
Vol.-1 5, McGraw Hills Publishing co., 1984
4. Langworthy, E. M., "Chemical Milling", Proceedings of 1985 Conference on
Nontraditional machining, American Society for Metals, Dec 1986, p 579-593
5. "Etching Metals", Dow Chemical Co. ,USA, Patent No. 51878,12 May, 1954
6. "Removing Material Chemically from Metals", NOEL, L., Patent No. 59952,
20 March, 1957
7. "Etching Metallic Surface", Dow Chemical Co. ,USA, Patent No. 63495, 21
March, 1958
8. "Curved Metal Bodies with Apertures, Philips GLOEILAMPEN-
FABRIEKEN, N.V. (Netharlands), Patent No. 95803,26 September, 1964
9. "Engraving Dies and Plates", Yokes, L. G. (U.K.), Patent No. 63741,16 April,
1957
10. "Chemical Milling of Metals", MIL-C-81760
2. Drawbacks Associated with Known Art
The major drawbacks associated with the existing procedure of masking an
object for chemical milling are as follows:
(i) The object should be properly cleaned otherwise adhesion properties of
the maskant may be seriously impaired,
(ii) The choice of a maskant is largely dependent upon the object material.
Non-availability of an appropriate maskant in the local market or in the
country could also be a serious problem,
(iii) Scribing process is rather complicated considering the fixing of template
on the mask, cutting the mask by guiding the knife along the edges of the
template and stripping the mask from the areas to be etched,
(iv) Accuracy can be questionable if scribing is to be done on a curved or on
a complex shape surface.

3. Objective of tbe Invention
The main objective of the invention is to develop a procedure of masking an
object for chemical milling such that the following advantages can be visualized.
(i) High degree of surface cleanness is not essential.
(ii) Any maskant which is non-reactive to the etchant can be used irrespective of
the object materal.
(iii) Scribing and stripping the mask off the object surface are eliminated,
(iv) Objects having curved surfaces or moderately complex geometries can be
masked comfortably,
(v) Accuracy is improved over the conventional method.
4. A Summary of the Invention
The invention is related to a process for producing a masked object for
chemical milling. The desired pattern is first drawn on a plane 0.1 to 0.15 mm thick
white paper, on which a thin layer of a maskant is applied on both the sides. Using a
sharp knife or a surgical blade, some of the areas of the paper from the pattern
drawing are removed according to the design to be made by chemical milling. The
paper-mask thus obtained is pasted on the pre-cleaned object by using an etchant
resistant paint or adhesive and the object is ready for chemical milling.
5. Detailed Description of the Invention
The invented procedure for masking an object for chemical milling comprises
the following essential steps:
(i) The pattern, to be made on the object by chemical milling, is
accurately drawn on a 0.1 to 0.15 mm thick plane white paper. A
computer software e.g. Autocad may be used to draw the pattern in a
computer from which a print out is taken through a plotter or a printer.
(ii) A thin layer of a maskant is applied on both the sides of the paper
containing the pattern drawing. Any type of maskant which is
non-reactive to the etchant and which makes a transparent coating after
drying can be used. A simple hand brush or any conventional coating
mechanism can be used for applying the maskant. The coating should
be completely dried in the open air.
(iii) Using a sharp knife or a surgical blade, only those areas from the
pattern drawing where the corresponding areas of the object should be
exposed to the etchant for chemical milling, are removed. Also giving
appropriate margins to all sides of the drawing, the excess papers
outside the drawing area arc removed and thus a paper-mask is
obtained.
(iv) A thin layer of an etchant resistant paint or adhesive is applied
uniformly on one side, the side to be pasted on the object, of the
paper-mask and left for air drying for some time so that the paint or
adhesive becomes very sticky.

(v) Using the stickiness of the adhesive or paint coating, the paper-mask is
pasted over the pre-cleaned object by using a suitable fixture or an
attachment Unlike the conventional masking procedure, high degree
of surface cleanness is not an essential requirement.
(vi) Any amount of the adhesive or paint that squeezes out, while pressing
the paper-mask over the object, should be removed by means of a
suitable scraper. Thus the masking process is completed and the object
is ready for chemical milling.
The accuracy of the pattern to be obtained by chemical milling is by and large
related to the making of the paper-mask which, being done on a plane piece of paper,
can be controlled very precisely. Moreover, the steps like scribing and peeling the
mask off the object surface being eliminated, higer accuracy, compared to the
conventional method of masking, can be achievable in this invented method. The
making of a paper-mask for curved or moderately intricate surfaces should also not be
a difficult task.
6. Best Method of Performing the Invention
The best method of performing the invented procedure for masking an object
for chemical milling should consider the following aspects.
(i) The thickness and quality of the paper
The thickness of the paper can vary from 0.1 to 0.15 mm and a thickness of
0.1 mm was found to work very satisfactorily. The quality of paper should be such
that it not only strongly adheres with the maskant but also it absorbs the maskant. The
papers available for photocopy or xerox purpose was found to be very suitable.
(ii) Making of the pattern drawing on a paper
The best method could be drawing the desired pattern in a computer vising a
design software, e.g. Autocad, and then taking a print out by using a plotter or a
printer. The thickness of the lines in the pattern should be as thin as possible so that
paper cutting can be done accurately. It is to be noted that the drawing should not get
distorted when the maskant is applied over it. Print out taken using black ink,
BC-01/BC-02 , AMKETTE, in a Inkjet printer showed no sign of distortion when
PVC coating was applied over it
(iii) Selection of the maskant
The criteria for the selection of the maskant are that it should be etchant
resistant and the coating should be transparent after drying. Commonly available
coating purpose PVC powders may be used when acid is used as the etchant. A
solution is made by dissolving the PVC powders into a suitable solvent e.g. Tetra-
HydroFuran (THF) and applied by using a hand brush or by any other conventional
mechanism. The curing time of tills PVC coating is normally 10 to 15 minutes.
(iv) Selection of the paint or adhesive
Any paint or adhesive that is non-reactive to the etchant can be used. In case of
acidic etchant, acid resistant paints commonly available in the market were found to
be very effective. The paint coated paper-mask should be left for air drying for about
30 to 45 minutes to make the paint very sticky.

We claim:
A process for producing a masked object for chemical milling comprising of
drawing desired pattern on a plane 0.1to 0.15 mm thick white paper, on
which a thinlayer of a maskant is applied on both the sides of the said paper and
left for complete air drying; making a paper-mask from the said maskant coated
paper by cutting out, using a sharp knife or a surgical blade, the specific areas of
the said paper according to the pattern to be made by chemical milling, on which
a uniform and thin layer of an etchant resistant paint or adhesive is applied only
on the side to be pasted on an object for masking, and pasting the said paper-
mask on the said object using the stickiness of the said paint or adhesive coating,
wherein any squeezed out paint or adhesive from the said paper-mask is is
removed by means of a suitable scraper.

Chemical milling is a process of metal removal by chemical etching where the
object or workpiece is coated with a maskant in some specific areas, according to the
design to be made by chemical milling, and immersed into an etchant bath for
chemical reaction. The art of chemical milling is largely dependent upon the quality
of masking involving high degree of surface cleanness, an appropriate maskant and
scribing-peeling the mask off the object surface. A procedure for masking an object
for chemical milling is invented comprising drawing of the desired pattern on a plane
0.1 to 0.15 mm thick white paper, on which a thin layer of a maskant is applied on
both the sides. Using a sharp knife or surgical blade, some of the areas from the
pattern drawing are removed according to the design to be made by chemical milling.
The paper-mask thus obtained is pasted on the object by using an etchant resistant
paint or adhesive and the object is ready for chemical milling.