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About Pad Printing
Pad printing is a kind of compressive rubber stamp
transfer printing process. It has been the major printing
method for the surface of 3-dimensional objects.
Methodology
The process is simple. The designed image is etched
into a printing plate called cliche. It is then flooded
with ink, through the silicon rubber pad, most of
the ink is then transferred to the subject.
Inking
the cliche
There are various method of inking the cliche. First,
flood the etched plate (cliche)with ink, the excessive
ink will be doctored clean by the doctor blade. The
solvents in the ink evaporates, ink dries and a plastic
film is form, that is easily adhered to the rubber
pad.
Ink pickup and printing subject
The pad is lift after picking up most of the ink,
at this time, part of the ink will be evaporated,
the remaining ink is then just enough to strike tightly
onto the printing subject from the rubber pad. The
design of the pad is able to roll along the surface
of the subject, to eject the excessive air between
the pad and the subject.
The working of ink and rubber pad
The ideal printing result is a complete transfer of
ink from the etched plate (cliche)to the subject,
however during the printing process (nearly ink of
10 micrometer or 0.01mm thick will be transferred
to the subject), pad printing is easily affected by
the air temperature and static electricity. The critical
point is able to keep the balance of solvent evaporation
speed and the dissolving speed. If evaporates too
fast, the ink film is dried before the transfer, if
too slow, the ink is unable to become a film for transferring.
Cliche
Major cliches :
- Thick steel plate
- Thin steel plate
- Plastic polymer plate ( sensitized )
The positive film image is exposed to a plate with
sensitized coating under the UV light, the imaged
area is hardened and washed away, usually with water
and alchol. The cost for this high quality etched
plate (cliche)making equipment is reasonable, approximate
around $3,000 according to customer's need.
If for metal plate, once the sensitized materials
is washed away, it's then soak into the erosive liquid
e.g. ferric chloride or nitric for etching depends
on the printing films. Thick steel plate is suitable
for production process but the printing result is
rather bad; thin plate is usually pre-sensitized with
coating, more practical, but the best printing result
should be the polymer one although the production
process is the worst one. Etching depth and quality
is very important, it affect the image, if defective
plate is used, it will be manifested on the printing
materials.
Silicon printing pad
It is made of compressible silicon, it can be in any
shape. It's functions to pick up the image film from
the cliche, and transfer it to the subject. The applicability
of the pad is based on the following effects :
- Shape
- Hardness
- Surface texture
- Material quality
Basically the pad is in the form of conical, rectangular
or square. These shapes enable the best contact between
pad surface and cliche, to roll and press out the
image. The size, shape and the stroke of the image
determines which type of pad is suitable. In principle,
the harder the pad, the better the printing effect.
The hardness depends on the amount of silicon oil
add into the rubber.For solvent based ink, likktle
silicon oil is required. Ideal pad should be smooth
but not glossy, for best printing result, wipe the
pad with a yarn soaked with solvent. Pad should be
replaced after 50,000 prints.
Ink
Pad printing requires ink of high content of pigments,
fast drying resins and solvents. As the ink consumption
in pad printing is minimized, the price is high, while
for screen printing, the ink is slow drying, the pigments
content only accounts for 5%, which is not suitable
for pad printing.
Closed Ink Cup Printing System
Relative to the open trough type, another choice is
the closed ink cup type. The ink cup is fixed upturned
and inking through pressurized stroke on the cliche,
the doctor blade then clean the excessive ink on the
cliche, as the ink cup is fine concealed, less solvent
evaporates, ink stability is maintained, which allows
longer working cycle. But the critical point falls
on the accurate blending of ink, as many factors will
also affect the evaporation of solvent. The operation
of the machine should be with care, otherwise, the
etched plate (cliche)will be easily erosed, and the
ink will be overflow the image. Price for a new ink
trough is high. The system becomes more popular on
automatic production line.
Key
Points for Pad Printing
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From
film to etched plate (cliche):
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A positive film image is exposed
to a polymer plate coated with photoresist, then
chemically etched. |
| From pad
to the cliche: |
The pad strokes onto the cliche, the
ink film of the image adhered onto the pad, and
lift up. |
| From pad
to subject : |
Pad strokes onto the fixed subjects,
release the ink. |
| Pad lift
away from subject : |
lifting of the pad, doctor blade
doctor clean the ink on cliche, pad can pick up
the ink of required part only. |
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