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THE ORIGIN OF PAD
PRINTING
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 What is pad
printing? Pad printing is often compared to screen printing, hot
stamping, ink jet marking and ink stamping. While the final result may appear
similar, pad printing is fundamentally different from any of these processes.
Through a clever use of solvent evaporation, pad printing transfers an image
out of an etched plate, onto a silicone pad and then finally onto the part.
This makes pad printing quite a bit more complex that most other processes. Why
go through all the trouble, rather than using a more direct process? There are
a number of reasons, but in the first instance Swiss watch makers were looking
for a way to consistently print the faces of their product. The process
couldn't be done with a printing press, which was designed to handle paper.
Hand painting was a slow and tedious process, even for the Swiss. Traditional
hand stamping cannot reproduce fine lines. Thus confronted with a printing
quandary, some intrepid soul had an inspiration. Some gelatin, a few copper
plates and one would guess a few steins later viola, pad printing. Pad printing
was mainly limited to Europe until the early 1970's when a few manufacturers
introduced their products to the rest of the world. For the rest of that decade and
half of the next pad printing competed directly with the more traditional hot
stamping and screen printing for industrial decorating business. As people
began to see the benefits of pad printing versus the other two processes a nice
market developed.
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FIELDS OF
APPLICATION
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Pad printing is known to be the system to print on irregular surface, no plane,
concave or convex, of different shapes, but it prints well as on plane
surfaces. It is used where the multicolour printing in a unique cycle is
requested, where a fast drying is necessary (as on automatic line) and where
the highest printing quality is inquired, especially on thin line. With this
introduction the list of potential
applications for pad printing is seemingly endless. The average person probably
comes into contact with dozens of pad printed products on a daily basis without
knowing it. Here are just a few things that are commonly decorated with pad
printing: alarm clocks, shower heads, electric razors, toothbrushes, coffee
makers, coffee cups, toasters, oven knobs, radios, television housings, remote
controls, telephones, watch dials, computer housing, monitors, keyboards,
diskettes, pens, eyeglass frame, sky boots, washer panel, refrigerator drawer,
cosmetics bottle, baby's bottle....... the list goes on.
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PRINCIPLES
OF OPERATION
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printing process actually borrows from the screen, rubber stamp, but combines
them in a way that is entirely unique. In the beginning copper plates were
etched with recessed images similar to the photogravure process. Since then the
materials and methods have changed but the end result is still a plate etched
with an image. The surface of the plate is then flooded with ink and wiped
clean similar to the way a screen is flooded and squeegeed in the screen
printing process, but in pad printing this does not result in the immediate
transfer of an image. A pad is compressed first onto the plate to pick up the
image, then onto the substrate to transfer the image. This is similar to rubber
stamp printing, but notably different in the fact that the pad is a means of
transfer rather than the source of the image. All pad printing machines are
built around one of two basic principles of operation. Those two principles are
commonly referred to as the "open principle" and the "closed principle". The
following diagrams illustrate how the pad printing theory is applied in the
case of the open principle and how it works.
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OPEN PRINCIPLE
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| 1: FLOODING |
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| 2: DOCTORING
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| 3: IMAGE PICK UP |
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| 4: PRINT STROKE |
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| 5: INK DEPOSIT |
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| 6: PAD RELEASE |
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The image to be trasferred is etched into a printing plate, commonly referred
to as a cliché. Once mounted in the machine the cliché is flooded
with ink. The surface of clichés is then doctored clean, leaving ink
only in the image area, the ink's ability to adhere to the silicone transfer
pad increases. The pad is positioned directly over the cliché, pressed
onto the surface to pick up the ink and then lifted away. The physical changes
that take place in the ink during flooding (and doctoring) account for its
ability to leave the recessed image area in favor of the pad. After the pad is
lifed away from the cliché to its complete vertical height there is a
delay before the ink is deposited onto the substrate. At this stage the ink on
the pad surface undergoes physical changes as solvent evaporates from the
outside of the ink layer, making it tackier. The physical changes that take
place within the ink layer during the print stroke result in the ink developing
more of an affinity for adhereing to the substrate than to the pad. The pad is
then compressed onto the substrate transferring the image. Even though the pad
compresses considerably during this step, the contour of the pad is designed to
roll away from the substrates's surface rather than press flatly against it. A
properly designed pad, in fact, will never form a zero degree angle of contact
with the substrate. Such a situation would trap air between the pad and the
substrate, resulting in a incomplete transfer of the image. The pad lifts away
from the substrate and assumes its original shape. When the variables involved
with all five steps are properly controlled, the pad should lift away clean and
ready for the next print cycle.
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CLOSED PRINCIPLE
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| 1: FLOODING |
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| 2: DOCTORING
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| 3: IMAGE PICK UP |
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| 4: PRINT STROKE |
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| 5: INK DEPOSIT |
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| 6: PAD RELEASE |
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The closed principle differs from the open principle in that the ink is not
directly exposed to the air. In closed systems the ink is inside of a "sealed"
container, usually referred to as an "ink cup". Ink flooding occurs while the
ink cup is positioned over the etched image area. The sharp edge of the ink
cup, commonly referred to as the "doctoring edge" or " doctoring ring" acts
like the doctor blade in an open system. Doctoring occurs when the ink cup
slides across the top surface of the cliché. In either case the end
result is that the top surface of the cliché is doctored clean and the
recessed image area is left flooded with ink. From that point the theory is
applied the same as in open system.
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