Photogravure

Photogravure

Postby Edgardo_Rosatti » Sun May 29, 2016 6:48 pm

Hello to all, I open this post to talk photogravure. Topic that with pleasure I got to see who was
treated here.
In particular I would like to talk about how to make a project and techniques required to get a quality
job trying to bring my experiences about.
So do not talk about commercial kits and how they assemble, but how to make their own photo-etched to
allow us to make a model.
I must say that with a handmade instruments, like the one that is used to make printed circuits for
electronics, the results are not very good, in fact, in my humble opinion are bad. But this is just a
personal opinion but :-)
I have tried in the past to do some piece, because I have a UV contact printer and development and acid
corrosion, but the best I could get were the portholes for a model of a motorboat.
To obtain pieces with a lot of details and well detailed in the forms, only specialized companies in
this field can solve.
The pattern will only draw the slab following precise rules to provide the specialist firm a drawing
with the correct specification.

Drawing:
The photoengraving always takes place on both faces of the plate, the upper one and the lower one, then
serve 2 drawings.
The drawings of the 2 sides of the sheet must always be inverted in the mirror, because the inked side
must be in contact with the sheet itself to avoid distorsoni image that may be caused by the thickness
of the transparent sheet of acetate.
An example:

fotoincisione.png
fotoincisione.png (8.54 KiB) Viewed 419 times


Prototipo_Rev1.pdf
Locomotor Badoni type VII, prototype
(1.92 MiB) Downloaded 212 times


Processing:
The specialist company, with figures provided by the modeler performs a press professional BW in
photolithography on acetate sheet where the black color has a saturation of 100% and a very high dpi
value in order to realize even the smallest eminently.
The next step is the cutting of the slab which can be generally of brass or nickel silver with a
thickness variable between 0.2 and 0.3 mm. This is a good thickness for the scale TT.
After the cutting is applied to a light-sensitive lacquer (photoresist) on both faces of the plate for
the photo transfer of the drawing. This varnish is sensitive to UV rays and longer exists in 2 types:
positive or negative.
Positive means that if the UV light illuminates the paint, it sensitizes and may be eliminated in the
next step of development.
Negative means that if the UV light illuminates the paint, it raises awareness, but will not be deleted
in the next step of development, will remove the paint not exposed to UV rays.
In my experiences, here in Italy the companies working the etching using photoresist of negative type.
Then I colored in the drawing in black all the parts you want to remove.
The specialist company, once he has received from pattern design, sends it to the processing area
(yellow room) that provides for the transfer of photographic plate.
The next step is the chemical corrosion also called chemical blanking, which consists in immersing the
plate thus treated in a chemical bath consisting of acids corrosive for the elimination of areas not
protected by the photoresist.
The result is a double corrosion on both faces that allows us to have the pieces for the assembly of a
model.
This is a summary of the procedure adopted by a specialized company for the production of photo-etched.
The processing costs are a bit 'high; about 50/80 euro for professional print design for
photolithography and about 20 euro for each plate required (A4 size reference).
But you can optimize machining by a design that includes many details, or use a smaller working paper
size.
GAS TT in our group we have made some work in photogravure among friends with the same interest and
that way we amortized costs by placing pieces created by more people.

Technique and size of the design:
The technique to draw a slab is that of vector graphics can be done with software such as Corel Draw,
Autocad or any open source program like LibreCAD for Linux.
bitmap graphics programs are not good for this job, as such software store images as pixels and do not allow an
accurate precision in the drawing.
The final work of the design must be saved as PDF files, as this format is the most widely used and
supported by all platforms (Win, Mac and Linux) and by specialized firms.

The design characteristics:
In this section we will see how to draw an object to the technique of etching and what are the basic
parameters for a good result.
Thickness of the plate:
The modeller can choose the thickness of the sheet according to the scale of the model that wants to
play.
For the scale TT, a thickness of 0.2 mm or 0.3 mm is more than sufficient to accomplish all the pieces
of a model.
For the H0 scale or 0, you can increase the sheet thickness from a minimum of 0.3 to a maximum of 0.5
mm.
For the N scale you are using a thickness between 0.2 and 0.15 mm.
However, the thickness of a piece also depends on its usage.
Let me give an example; a load-bearing part of a locomotive must have a thickness such as to allow its
constructive stability, while a precise as a simple plate may have a much thinner thickness.
This is a problem, because when we do a project we would like that all the pieces have the same
thickness iserirli in a single design.

The half-etching and the fold lines:
There in chemical corrosion process, the slab undergoes the metal removal on both sides at 50% and this
allows us to manage at will what and how we want to highlight the eminently on the pieces.
In this simple illustration shows a simple cube with the folding lines and in the relief of a playing
dice numbers;

cubo1.png
Top
cubo1.png (7.67 KiB) Viewed 419 times


cubo2.png
Bottom
cubo2.png (9.33 KiB) Viewed 419 times


We may note that in the drawing of the cube folding lines have been drawn on one side, while the
numbers were drawn from the opposite side. This produces the bas-reliefs on both sides of the piece.

Registration Marks:
The crop marks are places graphic references on 3 extreme corners of the design and are used to
properly align the top sheet with the lower one. Are 3 to eliminate alignment errors and their shape is
not critcal.

Crocini.png
Marks
Crocini.png (6.62 KiB) Viewed 419 times


[continue]
Ed
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Re: Photogravure

Postby Edgardo_Rosatti » Sun May 29, 2016 7:04 pm

The cusps:
We have said that the corrosion process is carried out for 50% of one side of the plate (upper) and for the remaining 50% on the lower side.
The corrosion process produces a side effect that you should take this into account.
The acid corrosion, by its nature removes the metal material in a gradual way and this makes the profile of the cut will never net.

Cuspidi.png
Cusps
Cuspidi.png (6.83 KiB) Viewed 417 times


For plates of 0.2 mm thickness it is difficult to see with the naked eye such defect, but I can guarantee that it exists.
Ergo: the higher the thickness of a slab and more obvious it will be the cusps.

The solution to compensate for this defect is to slightly increase the size of a particular as a hole.
For example, if we have to drill a hole of 2 mm in diameter as we draw of 2.05 mm. That is, we increase the diameter of the hole 5 hundredths of a millimeter and in this way the cusp effect is compensated allowing us to have a hole with an effective diameter of 2 mm.

The witnesses:
The witnesses are necessary to obtain an anchorage of the pieces with the sheet and to avoid that it breaks during the etching process of the sheet.

Testimoni.png
Witnesses
Testimoni.png (5.49 KiB) Viewed 417 times


For now I stop, because I realize that the post is very long.
Other interesting aspects of this technique should be conducted better.
If the topic is of interest to you are available for clarification and for me it will be a pleasure to speak of modeling.

PS: more pdf for example :wave:
Plafoniera.pdf
Street lamps, bus stops, barriers and accessories
(332.63 KiB) Downloaded 199 times


IMG_20160305_213429.jpg
Badoni prototype


IMG_20160305_213450.jpg
Badoni prototype (bottom)
Ed
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Re: Photogravure

Postby krokodil » Mon May 30, 2016 12:59 am

Hi Edgardo

Very nice description.
Last edited by krokodil on Mon May 30, 2016 1:09 am, edited 1 time in total.
Greetings from Austria
E.T.
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Re: Photogravure

Postby krokodil » Mon May 30, 2016 1:01 am

Hi Edgardo

Very nice description. Because I use similar technology since 1990 just few comments:
1.
The conversion into PDF is correct but before one does it, please set the PDF conversion to the highest resolution. Not all PDF converters offer such feature. (They simple convert the picture into PDF with 300dpi resolution - what is not always enough). For such tiny parts the required resolution is over 1000 dpi. (Check in the properties of the PDF printer.)
2.
The underetching, or cusps, depends on the used technology and chemicals. In my group we built a professional etching machine with 12 motor driven spray heads for A3 size experienced differences by using different etching chemicals. The ferrochlorid does exactly the same effect what you described, other chemicals works differently - I have no idea why., but they do rather underetching, ie. the surface is exact, deeper in the material the hole will be slightly bigger ( negative cusps?).
3.
What is important in overall drawing that the etched (lost) areas should be kept as small as possible. In Corel we have an option to increase the size of contur lines (outside/inside from the exact size). For eg you need a square of 10x10 mm add to the contur outside 2 mm, you will get a square of 14x14 mm with 2 mm wide border lines. If you assemble such parts together (practically you are etching around the parts with same wide border lines - I call it demarcation technology) the etching time will be shorter and most of the removed material requires the same etching time, ie the underetching will be almost the same overall on the plate. ( if you do it at home you can save lot of money on etching chemicals).
4.
If you have big holes in the drawing (inside of the windows, fans, rosettes :-) ) fill those holes with paint (no etching) and use just a narrow uncovered border for separation. Bind those parts with small witnesses to avoid them from falling out in etching machine. Everything what falls out from the etched plate eats the chemicals and rises the cost - plus one has to clean the etching machine.
Greetings from Austria
E.T.
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Re: Photogravure

Postby Edgardo_Rosatti » Wed Jun 01, 2016 5:52 pm

Thank you for your intervention Eugen.
The use of this technology only for about 3 years.
I try to answer your questions :-)

1: When I do a drawing with Corel Draw the export to pdf. Format that should ensure a good image quality. However, the press do not do it myself, but I can do from the service. it is unthinkable to get a good print quality with common commercial printers for these jobs. I tried even with more and laser prints on acetate via special service, but the quality is never comparable to the press (photolithography) of the service that makes the photo-etched.
I only spent money unnecessarily, but at least I gained experience.

2: Usually I make a initial project (prototype) which allows me to see the problem of the cusps and the size of the pieces. Then check the plate with a first test assembly.
Then I make a design change to eliminate defects and refer to service the modified project. That of PL FS I sent underwent two revisions ..

3 & 4: True, in Corel's this option, but when you make a service work the price does not change. If the blanks as windows or slots are realtive great, the service tolerates them and the work will still be good.

I apologize but some steps have not understood them well.

My advice to all who want to try this technique is to start with a simple project, and to experiment as much as possible.

Ed
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