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What are
the advantages of a pure photopolymer emulsion?
Photopolymer emulsions have three main advantages
1.
The require no mixing of added sensitizer. They are used
“straight from the can”.
2. They have a shelf life measure in years
3. They expose in a fraction of the time required by either
dual-cure or diazo emulsions
For
large volume shops, the fast exposure times (typically less
than one quarter_ mean that hundreds of screens can easily
be produced every day. This attribute also keeps exposure
times reasonable when exposing large format screens with a
single lamp, or when only low power exposure lamps are available.
For the small shop, the long shelf life means that the emulsion
should be good to the last drop. Other notable benefits include
reduced sensitivity to environmental moisture, since, unlike
emulsions that contain diazo and must be bone dry before exposure,
photopolymer emulsions are unaffected by humidity.
What
is a “dual-cure”?
Stencil advancements created dual-cure emulsions that already
contain a light-sensitive photopolymer system (added during
manufacture), in addition to the normal diazo sensitizer added
by the user.
Having
two curing systems results in more effective stencil hardening.
This leads to superior mesh bridging and resolution, and permits
higher solids contents for reduced shrinkage and improved
print definition. With dual-cure technology, it is even possible
to create an emulsion that’s both solvent and water
resistant, yet easy to reclaim.
Dual-cure
polymer photoemulsions are ideally suited to the demands of
a screen-printing industry that has become more quality conscious,
sophisticated and competitive.
There
are many types of stencils that I can buy. What are the different
types?
| Commercial
available photostencils fall into four main categories.
The first is known as Indirect Film, where the stencil
imaging and developing process is carried out independently
of the screen mesh. The finished stencil is applied to
the mesh with gentle pressure, blotted with newsprint,
and dried prior to removal of the backing film. Although
capable of the highest quality reproductions, the thin
edge of the finished stencil is very fragile and easily
damaged, and therefore unsuitable for long print runs
or for printing on difficult substrates. Indirect film
is only suitable for use on finer mesh counts that are
capable of supporting the fragile stencil. |
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The
second type of stencil is known as Direct Film or Capillary
Film. In this case, a much thicker layer of pre-coated
photographic emulsion, which has been manufactured to
a precise thickness, is adhered to a wet screen mesh through
capillary action. After drying and removal of the backing
film, exposure and development produces a much stronger
and more firmly adhered stencil than in the previous case,
but still with the image quality associated with a film-based
product. |
| With
the third type of stencil, known as Direct/Indirect, the
film is laminated to the mesh with a layer of photographic
emulsion instead of water. Once this sandwich has dried,
processing is carried out the same as for capillary film,
but with the advantage that an even more firmly adhered
and durable stencil is produced. The downside is that
the stencil making process is more complicated and messy,
particularly in larger formats, and is also more costly
sing both film and emulsion are required. |
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That
brings us to the last, and most commonly used, type of
stencil, which is known as Direct Emulsion. In this case
the mesh is coated with a light-sensitive emulsion, which
when dry is imaged and then developed in the same fashion
as capillary film. This is by far the least expensive
method, in terms of material cost, and results in the
most durable stencils. However, it is also capable of
producing poorer print quality than any of the film-based
systems, unless the correct choices are made in terms
of emulsion type and methods of processing and bringing
several variables under control. |
What
are stencils made of? What is the chemistry of a stencil?
With the exception of indirect stencil films, which generally
are thin coatings of gelatin containing an iron salt sensitizer,
the other types of photostencil system, mainly direct emulsion
and capillary film, which is real pre-coated emulsion, are
based upon a resin known as polyvinylalcohol. Polyvinylalcohol
possesses an unusual combination of three properties that
make it uniquely suited to be used as the basis of most stencil
materials. First, it is a water-soluble polymer, which means
that stencil processing and developing can be carried out
with water, rather than organic solvents. Second, it is highly
solvent resistant, unlike most other water-soluble polymers
that tend to dissolve even more readily in solvents, and therefore
stencils are able to stand up to a wide variety of different
ink types. Third, polyvinylalcohol contains a link in its
polymer chain that is easily broken by the application of
dilute aqueous solutions of sodium metaperiodate (AKA emulsion
remover). This means that after printing, the mesh can be
recovered and reused by stripping the stencil without harsh
chemicals.
In
order to make capillary films and direct emulsion light sensitive,
there is a choice of three basic types of technology, Diazo,
Dual-Cure, or Photopolymer. In addition, other ingredients
such as fillers or bulking agents are added to increase the
solid content and improve wet strength of the stencil during
processing. The choice of sensitizers, and the type or combination
of fillers used will determine the properties of the end product.
Ancillary ingredients include pigments, surfactants that improve
coating quality, and defoamers to kill bubbles during processing. |
| Photopolymer
stencil products do not contain diazo, since they are manufactured
with a light-sensitive polymer. Emulsions are supplied presensitized
and ready-to-use with no mixing required, and both photopolymer
emulsion and film have a shelf life that is measured in years,
and not weeks or months. (Diazo is affected not only by light,
but also by heat and humidity.) The other distinguishing feature
of photopolymer is that exposure times are a fraction of what
would be used for either diazo or dual-cure products. This
is due to the very high sensitivity of the polymer that is
sued. The resistance properties of photopolymer fall into
the same categories as those for diazo sensitized material,
either solvent or water resistant. Having said that however,
the water resistance of commercially available photopolymer
emulsions does not yet rival that of diazo. Products designed
for garment printing are really more suited for use only with
plastisol inks, unless a hardener is used to reinforce the
screen. The very fast exposure times achievable with photopolymer
has also enabled the development of products that are suitable
for use with extremely weak light sources, such as projection
exposure.
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