The process of Pad printing is the most
versatile of all printing processes with
its unique ability to print on three-dimensional
objects and compound angles. The theory
behind the pad printing process was derived
from the screen, rubber stamp and photogravure
This is how the process works:
Step 1: Flooding
The image to be transferred is etched into a printing plate commonly
referred to as a cliche'. Once
mounted in the machine, the cliche'
is flooded with ink. The surface
of the cliche' is then doctored
clean, leaving ink only in the
image area. As solvents evaporate
from the image area the ink's
ability to adhere to the silicone
transfer pad increases.
Step 2: Pick Up
The pad is positioned
directly over the cliche', pressed
onto it to pick up the ink, and
then lifted away. The physical
changes that take place in the
ink during flooding (and wiping)
account for its ability to leave
the recessed engraving in favor
of the pad.
Step 3: Print Stroke
After the pad has lifted away from the cliche' to its complete vertical
height, there is a delay before
the ink is deposited on the substrate.
During this stage, the ink has
just enough adhesion to stick
to the pad (it can easily be wiped
off, yet it does not drip). The
ink on the pad surface once again
undergoes physical changes: solvents
evaporate from the outer ink layer
that is exposed to the atmosphere,
making it tackier and more viscous.
Step 4: Ink Deposit
The pad is pressed down onto the substrate, conforming to its shape
and depositing the ink in the
desired location. Even though
it compresses considerably during
this step, the contoured pad is
designed to roll away from the
substrate surface rather than
press against it flatly. A properly
designed pad, in fact, will never
form a 0-degree contact angle
with the substrate; such a situation
would trap air between the pad
and the part, resulting in an
Step 5: Pad Release
The pad lifts away from the substrate and assumes its original shape
again, leaving all of the ink
on the substrate. The ink undergoes
physical changes during the head
stroke and loses its affinity
for the pad. When the pad is pressed
onto the substrate, the adhesion
between the ink and substrate
is greater than the adhesion between
the ink and pad, resulting in
a virtually complete deposit of
the ink. This leaves the pad clean
and ready for the next print cycle.
of Pad Printing
Versatile as it is,
the process does have a few limitations.
1. Image transfer is much more
efficient when solvent-based inks are
used. The use of water-based or UV curable
inks are not recommended.
2. The object to be printed needs
to have a higher surface energy than
the pad. This rules out printing on
silicone and other non-stick materials,
as well as wet, greasy parts. Additionally,
some types of plastic require pre-treatment.
3. Pad printing is limited to
relatively small /images/ compared to
screen-printing. Pad printable /images/
are usually less than 100 square inches.
Large opened areas (>4.0 sq. in)
can be difficult to cover with special,
4. Pad printing produces a finished
ink film thickness of approximately
.00025" to .0003" with a single pass.
By screen printing standards, this is
Open and Closed
There are two basic categories or
types of systems under which all pads
printing machines fall: open and closed.Open
systems feature an ink trough or basin
or some type which is at least partially
exposed to the air. Closed systems have
"ink cups" that keep the ink from coming
into contact with the air.
Basically open systems allow you to print
larger /images/, since more of the cliché’s
surface area can be used for the image.
Closed systems allow better process control,
especially for longer runs, since the
ink isn't exposed to the air and doesn't
require the frequent addition of thinner.
Closed systems typically use less ink,
allow for faster cliche' and color changes,
and are easier to keep clean.
are several basic types of pad printing
machines. The majority of machines in
use around the industry are referred to
as vertical machines. These machines can
be open or closed. In closed systems doctoring
is achieved by either sliding the ink
cup back and forth over the cliche' surface,
or by sliding the cliche' beneath a stationary
ink cup. The illustration below shows
a typical vertical machine with an ink
Some vertical machines can print on any angle using a special
pressurized ink cup. Instead of using
gravity to flood the cliche', pressurized
ink cups use an inflated diaphragm to
ensure the image is properly flooded.
Pressurized ink cups can be difficult
to install and maintain. The illustration
to the right shows how a pressurized ink
Closed vertical machines have
also been modified so that their ink cups
slide along the X-axis, allowing the printing
of wider /images/. This type of machine
is commonly referred to as a "sliding
ink cup" machine. The illustration below
shows this variation.
delaying the pad at the end of the print
stroke some vertical machines can print
360 degrees around a cylindrical object.
The image is transferred as a gear driven
nest rolls the part along the length of
the pad. The illustration to the right
shows how this works.
Without any special pad delays or gear
driven nesting fixtures vertical machines
can print a maximum of 120 degrees on
a cylindrical object. To accomplish this,
the pad's set-down point must be top,
dead center. The pad is then compressed
60 degrees in each direction from the
Horizontal pad printers are really
just another variation of vertical machines.
Horizontal machines flood and doctor like
vertical machines, then actuate the pad
to print using a horizontal stroke, rather
than a vertical one. The illustration
on the following page shows a typical
horizontal pad printer.
Horizontal machines are typically used
only when the part is too large to fit
into a vertical machine, or when the image
is too large to fit into a pressurized
Rotary pad printing machines are
very different from their vertical and
horizontal counterparts. Rotary machines
have cylindrical cliché’s, usually referred
to as "drums". The cliché’s are flooded
and doctored as they rotate. The pad also
rotates to pick up and transfer the image.
Rotary pad printers can be configured
to print vertically or horizontally. Vertical
machines can be self-standing for one-up
printing, or mounted on automations for
tandem or multiple color printing. The
illustration below shows a basic vertical
Horizontal rotary pad printers
are used to print on the vertical sides
of parts that are moving along a conveyor.
The following illustration is an example
of how cassette tapes would be printed
on two sides at once using a horizontal
rotary pad printer.
Carousel machines allow a part
to be printed with multiple colors without
ever being moved. Ink cups and cliché’s
are mounted on a rotary indexing table
which turns to locate each individual
cliche' under rotating pads for image
pickup. The pads then rotate around on
a separate carousel, locating over and
compressing onto the part to be printed.
These machines are typically slower than
their multiple color, standard vertical
counterparts, and are used only for printing
parts too large to fit a standard machine.
Pad printing machines are driven by a
number of different means. Most machines
are driven by electronically controlled
pneumatic systems and are thus referred
to as being electro-pneumatic.
Some pneumatic machines are controlled
by air-logic instead of electronics. Less
expensive than electro-pneumatic machines,
these can be more difficult to control,
especially if your incoming air pressure
isn't well regulated.
Electro-mechanical machines operate under
electrical power only. These machines
are usually more expensive than electronically
controlled pneumatic machines. They can
hold tight tolerances, even at high cycle
Electro-hydraulic machines are the most
expensive type. These machines are rare,
and are typically used only for large
format applications where additional compression
Manually operated machines are useful
for small format and short run applications.
Images are usually limited to less than
16 square inches, and multiple color printing
capability is limited. Many manufacturers
of manual machines offer trade in allowances
when upgrading to semiautomatic equipment.