Thermal transfer ribbons are constructed using a high strength carrier material onto which several "coatings", including the colorant ink, are applied. Typically from two to four separate material coatings on both sides of the carrier material are performed. The diagram below depicts the construction of a typical thermal transfer ribbon.
Back Coating
Every good quality thermal transfer ribbon has a material layer coated on the backside of the ribbon, the side that directly contacts the thermal print head. This back coating layer serves several functions:
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It provides a smooth, non-abrasive surface to the
thermal print head, minimizing wear and tear on the
heating elements. |
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It prevents the polyester base film carrier from sticking
to the thermal print head and tearing. |
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It helps to lower friction that in turn reduces the
buildup of static electricity. Excessive buildup of
static electricity can cause poor print quality and
in certain situations may even damage the thermal print
head. |
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It aids in minimizing ribbon wrinkle and ribbon slippage. |
Although seemingly innocuous, the back coating is a very important component of any good quality thermal transfer ribbon. Inferior back coatings can:
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Cause dirt and debris to build up on the thermal print head, increasing the cleaning frequency and increasing the potential for premature thermal print head failure due to abrasive damage. |
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Increase the thermal heat energy requirements for good quality printing by acting as an insulator. |
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Increase the potential of experiencing ribbon wrinkle. |
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Generate static electricity that attracts dirt and foreign particles as well as potentially damaging the thermal print head. |
Polyester Base Film Carrier
The carrier is an important component of any thermal transfer ribbon and must exhibit several characteristics. The carrier must be thin in order to conduct heat fairly well! The thermal heat energy must travel through the carrier (as well as through any intervening layers) in order to reach the ink layer. If the carrier is not thin enough and if it does not conduct heat well enough,
Sufficient heat energy will not reach the ink layer and will cause poor printing. The carrier must also be strong and resist tearing and must withstand high temperatures without distorting.
Thermal transfer ribbon carriers are almost exclusively
constructed from polyester films that are only from 2.5
to 8 microns (millionth of an inch!) thick with the standard
thickness being 4.6 microns. All polyester base film carriers
are not the same!
Thicker polyester films tend to be stronger and better resist wrinkling (ribbon wrinkling causes poor printing of bar codes) but also tend to generate lower quality images and support slower printing speeds. Very thin polyester films can support high print speeds but tend to wrinkle and tear easily. The major material costs of most thermal transfer ribbons reside in the cost of its polyester base film. Since good quality polyester film carrier is essential to good quality printing, always be wary of very inexpensive ribbons since they may utilize inferior grades of polyester base films.
Primer Layer
Some thermal transfer ribbons employ a coating of material directly onto the inked side of the polyester base film that serves a couple of purposes. First, the primer helps to "bind" the ink layer to the base film, ensuring even ink coatings. The primer coating also serves as a logical "separation point" for the ink layer. When the primer coating gets hot, it typically exhibits a very "weak bond" to the ink layer, thus allowing the ink layer to easily transfer itself to the label surface. Again, not all-thermal transfer ribbons utilize or require a "primer layer", but its use depends upon the inking material formulation and the coating methods used in the construction of the ink layer.
Ink Layer
This is where the rubber meets the road, so to speak. The ink layer contains the actual material that is transferred to the label to form the image. A wide variety of thermal transfer ribbons are available and it is very important to match your ribbon selection to your application. There are three basic formulations of ink used in thermal transfer ribbons which are:
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Wax-based inks are
the lowest in cost and are suitable for most applications.
Label images may be scratched in use or smear if the
temperature is too high. |
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Resin-based inks
produce label images that are much more resistant to
wear and extreme conditions. Some resin inks used on
certain face stocks can withstand temperatures over
1000 degrees. However, resin-based inks tend to be rather
expensive. Wax-resin inks" produce label images with
higher durability than wax-based inks but are lower
in cost than pure resin-based inks. The melting points
of each type of ink varies and determines the amount
of heat energy required to transfer the ink to the label. |
Wax inks tend to require the least amount of heat while the pure resin inks generally require the most. The thickness of the ink layer also affects the amount of heat energy required. The ink formulation and how it is actually coated onto the carrier determines, to a very large part, how the ink separates from the ribbon carrier and how it transfers and adheres to the surface of the label. These characteristics in turn determine the edge definition and density (darkness) of the printed image. High resolution printing (250 DPI and higher) typically requires a more sophisticated ink formulation and coating method than do moderate to low print resolution applications (203 DPI and below). There are many ink formulations and coating methods employed today to support thermal transfer printing on an enormous array of different label materials and for label applications that subject their printed images to high temperatures.
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Wax |
Wax/Resin |
Resin |
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Heat Energy |
Low |
Medium |
High |
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Print Speed |
2 - 12 IPS |
2 - 8 IPS |
1 - 6 IPS |
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Durability |
Low |
Medium |
High |
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Static Elect |
Low |
Low / Medium |
Medium / High |
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Face sheets |
Coated Paper
Uncoated Paper
Some Synthetics
Tag Stock
Some Films |
Coated Paper
Glossy Paper
Some Synthetics
Tag Stock
Films |
Synthetics
Films |
Typical Ribbon Type Characteristics
Components of labels & their types?
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Top Coatings
"Top coatings" are materials applied to the top of the
face sheet (the side of the face sheet that will be
printed upon) mainly to either allow printing to occur
or to increase the print performance. "Direct Thermal'
label stock is fabricated by applying a specific type
of "leuco dye" and "acidic developer" mixture onto the
surface of raw face sheet material. The chemical formulation
of the "leuco dye" and "acidic developer" mixture, as
well as its thickness (also known as "coating weight")
determines the labels printing characteristics, print
performance and environmental performance. For labels
destined for use with thermal transfer ribbons, top
coatings are applied to enhance the face sheets print
performance. Many face sheets by themselves exhibit
moderate to poor print performance using thermal transfer
ribbons. Thermal transfer printing requires a rather
smooth surface and a surface that bonds well to the
ink formulation that is to be used. "Clay" is a common
top coating applied to face sheets that significantly
improves the face sheets print performance. Resin accepting
top coatings are often applied to face sheets when printing
with resin ink formulations is intended. These resin
top coatings improve the bonding between the label and
the resin ink and provide an indelible image. |
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Face sheets
"Face
sheets"
are the actual base material that makes up the label
itself. All the other components are "laminated" or
are coatings that are applied to the "face sheet". There
are literally hundreds upon hundreds of different combinations
available. Face sheets are chosen based upon the properties
they exhibit and the needs of the particular label application.
Some face sheets are "naturals" for thermal transfer
printing while others are not. Face sheets can be manufactured
from either paper or synthetic raw materials. |
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Paper
Paper is the most commonly used face sheet and is
usually the lowest in cost. It is available in many
types, thickness, colors and sizes. However, paper
can be damaged by light, water, dirt and chemicals
and may be torn or scraped. Paper labels perform best
in controlled environments and in applications such
as product labeling, pricing and shipping. |
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Synthetics
Synthetic face sheets include, but are not limited to, polyester, polypropylene, polyolefin, vinyl and mylar. These stocks can provide very high print quality and are more likely to perform better than paper for labels exposed to harsh environments or subject to hard use. Polypropylene facestocks are available in many forms for a wide array of applications. Polyester is used in applications subjecting labels to very hard use and to extreme environmental conditions. Vinyl
facestocks are also very durable, especially on curved or irregular surfaces. |
Primer Coatings
"Primer Coatings" are applied to the BACK SIDE of the face sheet and serve three primary functions:
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Increase Label Opacity
"Opacity" measures the amount of light a material blocks
out (or absorbs) and can be thought of as the opposite
of "transparency". A face sheet with a low opacity allows
whatever is located behind the label to "show though".
To lessen or eliminate this tendency, primer coatings
with high opacities can be used. |
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Provide Anchorage for Adhesive
Coating
The primer coating also provides a firm anchorage for
the subsequent coating of adhesive material, ensuring
a secure and reliable bond between the face sheet and
the adhesive. |
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Prevent Adhesive Migration
Adhesives tend to have a nasty habit of "migrating".
With labels this is exhibited by the leaking of adhesive
from the sides of the label and can be a very troublesome
problem if not dealt with. Properly formulated and applied
primer coatings drastically reduce or eliminate adhesive
migration problems. |
Types of Adhesive Coating
What good is a label if it doesn't stick? The adhesive coatings function is to provide the right type and amount of "stick" for the labels. Many combinations of adhesive materials are available. Some applications require permanent labels that resist exposure to temperature extremes, high humidity, chemicals or outdoor use and be tamper-resistant or tamper-evident. These applications require a "permanent adhesive". Other labels must be easily removed without tearing, damaging the item or leaving a residue. These labels may also need to be removed and perhaps even re-applied. These label applications require a "removable adhesive"
or a "repositionable adhesive". Many surfaces are difficult to label (like porous surfaces) and may require specific adhesive formulations and/or custom thickness, or coat weights. There are two common types of adhesive materials used today which are:
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Rubber based Adhesives |
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Acrylic based Adhesives |
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