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In
order to choose the right pad it is necessary to have a deep
knowledge of the printing process. The choice has to be made
taking the following factors into consideration:
-
the
dimensions and the type of graphics to be printed;
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the
shape of the surface of the object to be printed;
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the
type of ink to be used;
-
the
type of engraving in the plate;
-
the
pad printing machine available.
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The
pad has a typical pointed shape on a round base. The pointed shape is necessary
in order to correctly take up the ink from the engraving in the plate and to
avoid air bubbles which would compromise print quality (see illustration 1).
Pads
with sharper edges are generally used for flat surfaces, while
pads with rounded profiles are used for convex surfaces (see
illustration 2-3)
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The
ideal contact angle between the object surface and the pad
surface lies between 20 and 50 (see illustration 2). A pad
with a suitable height compresses itself easier than a lower
one limiting deformations in a better way. It is therefore
recommended that the pad dimensions are 30% larger than the
graphics to be printed.
In
general terms the following rules are valid:
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best print quality can be obtained with hard pads. Compared with a soft pad,
with the same engraving depth, a hard pad can take up a thicker layer of ink
and guarantee better coverage. Softer pads are suitable for large printing
surfaces, rounded surfaces and fragile objects.
It is important to consider that the pad can be
compressed only to a certain point. If compression is excessive, the pad tends
to "skid" on the piece and produces blurs on the print. |
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Further
factors to be taken into consideration:
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bar
pads are used for thin and long prints;
-
revolving
pads with square or rectangular base are generally used in order
to reduce the area of the plate or to reduce the use of space when
many shapes are used at the same time.
The
point of the pad is always a critical area, especially where engravings are not
screened.
In
general the point must always find itself outside the engraving. If ,
for special reasons, it is necessary to make the point fall onto
the engraving; this latter must at least be screened and the
pad point rounded.
A pad with an excessively pointed shape
(high contact angle >50) on a non screened engraving would create high pressure and uneven taking up of the ink
layer. On the contrary, a pad with a very flat shape (low contact angle
<20) would take up the ink layer with air gaps.
Screening
the engraving and choosing a pad with
an average contact angle and a sharp point could help to solve
these problems.
It is therefore important to have a wide range of
pads with different forms, rubbers and hardnesses such as the range offered in
the Elport-Import catalogue. This makes the pad printer's life
easier when choosing the most appropriate pad.
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