Pads are a formulation of a silicone base material, silicone oil, and a
catalyst. The amount of oil determines how hard
the finished pad will be. The more oil, the
softer the pad. Manufacturers can have anywhere
from a few dozen to a couple of hundred shapes
and sizes of pads in varying hardnesses.
Size, shape and hardness are the three most
important considerations in choosing a pad for
a given application. Size is determined by the
size of the image to be printed, and by the
size of the machine. Measuring the image diagonally,
the pad should be 20% larger to prevent image
distortion. The machine needs to be able to
compress the pad far enough to pick up and transfer
the entire image in an even motion. In addition,
the pad must not interfere with the ink cup
or other parts, and it must not roll off the
edges of the cliche' during image pick up.
The shape of the part determines the shape of the pad. There are three
basic shapes from which all standard pads are
derived (with the exception of rotary pads.)
The three basic shapes of conical, rectangular,
and roof-shaped are illustrated to the right.
There are many variations of each of these three
basic shapes. To determining which pad works
best for a specific application have your supplier
test print your parts for you.
Once you have the correct pad you need to concern
yourself with setting the proper compression
and correctly locating the pad.
Compression should always be set for the minimum
amount necessary to pick up and transfer the
image. Over compression causes excessive pad
wear as well as poor transfer efficiency.
In set-up, the pad should be located so that
its point is not in the image area when the
image is picked up. The point is the first place
a zero degree angle will be created during compression.
Having the point in the image can result in
a void in the printed image in that location.
It is helpful to keep a record of set-down point
and machine compression settings if you're going
to run a job frequently, so as to expedite machine
Hardness (also referred to as shore or durometer)
is the last of the three main considerations
in choosing a pad. Hardness comes into play
for three main reasons. First, the same pad
of two different hardnesses requires different
amounts of energy to compress the same distance.
A given machine may not be able to compress
the harder of the two.
Second, a hard pad may damage the part you wish
to print, thus limiting you to pads of a lower
Third, textured surfaces are more successfully
printed using harder pads. (Some textures are
very difficult or impossible to print regardless
of what pad you use.)
How long a pad lasts depends upon its initial
quality, hardness, the aggressiveness of the
inks and solvents used, the amount of compression
it receives, the texture of the surface you
are printing, and what type of etch the cliche'
has. There is no magic number. I've seen pads
last anywhere from 20 to 250,000 impressions.
Basically it depends on the application.
More Information on
contracting a Pad Printer