Impressions - February 2014 - 67

screen printing press, platens absorb heat
as the production run progresses. In time,
the platens absorb enough heat that it
will contribute to the gelling process of
the ink being ﬂashed. At this point, you
can decrease either the ﬂash unit's temperature settings or the dwell time. This
scenario becomes more critical on automatic presses where you are printing at
higher production speeds and may have
more than one ﬂash unit on the press. The
easiest way to determine the platen temperature as the job progresses is with a
non-contact pyrometer.

EFFECTS ON INK FILM
Quartz ﬂash units use a series of
high-wattage output quartz bulbs that can
reach 1,400˚F in a matter of seconds and can
cool down just as quickly. Quartz ﬂash units
typically have higher wattage outputs than
ceramic panel ﬂash units and offer greater
versatility from the standpoint of temperature control. This is important when dealing
with specialty fabrics where the heat exposure must be closely monitored.

reach 1,400˚F in a matter of seconds and
can cool just as quickly. Quartz ﬂash units
typically have higher wattage outputs than
ceramic panel ﬂash units and offer greater
versatility from the standpoint of temperature control. This is important when
dealing with specialty fabrics where the
heat exposure must be closely monitored.
Flash unit parameters include time, distance and platen temperature. Flash unit
parameters/settings to be determined are:
* Wattage output of the ﬂash unit
* Distance of the ﬂash unit from the
ink/substrate surface
* Dwell time required of the ink ﬁlm
to achieve a proper "gel"
* Type of ﬂash unit being used; IR
panel or quartz
The variables that must be considered
in regard to these settings are:
* Ink type being ﬂashed
* Ink-ﬁlm thickness being ﬂashed
* Ink color
impressionsmag.com

* Fabric content
* Fabric color
* Platen temperature
As there are a number of new ink types
being introduced to the market - including high-solids-content water-based, PVCfree plastisol and other new water-based
inks - the ﬂashing characteristics of each
must be determined at the facilities employing them to determine the optimum
ﬂashing characteristics for each.
The ink-ﬁlm thickness will have a direct effect on the ﬂashing parameters from
the standpoint of time and temperature
setting. Ink color will affect dwell time, as
the lighter the color, the slower the ﬂash.
Darker colors absorb heat more readily
and, thus, ﬂash faster.
Fabric content will determine the actual temperature setting and ﬂash dwell
time. The most versatile fabrics are 100%
cotton. Caution must be taken when
printing/ﬂashing specialty blended fabrics
that may contain Lycra spandex or acrylic.
Such fabrics are inherently heat sensitive.
Thus, take caution when they are exposed
to a heat source like a ﬂash unit.
The last of the aforementioned variables is platen temperature. Whether
printing manually or on an automatic

Most screen printers will begin with
a starting point of time, distance and heat
settings to determine the amount of time
required to properly ﬂash the ink ﬁlm. The
desired effect is for the ink ﬁlm to emerge
from the ﬂash unit dried (not cured) on
the surface and readily capable to receive
the next color to be printed onto it.
When printing plastisol ink ﬁlms, there
typically is a degree of after-ﬂash tack.
This is an inherent characteristic of plastisols. Consult your ink manufacturer to
determine if it offers a product that has
the best ﬂashing characteristics for your
applications.
A common mistake made in the ﬂashing process is using excessive heat. This
often occurs in the name of faster production. When ﬂashing a plastisol ink ﬁlm, it
is important not to heat the underbase
ink-ﬁlm surface to the cure point of 320˚F.
Should this occur, the color being printed
onto the underbase will not bond or fuse
with it.

This image represents the degree
to which an ink will absorb heat based on that
ink's color. Black will be the most absorbent
color when ﬂashing and silver the least absorbent due to its reﬂective nature.

February 2014 | Impressions

http://www.impressionsmag.com

Impressions - February 2014

Table of Contents for the Digital Edition of Impressions - February 2014