Embossing Basics
Definition of embossing:
In the broadest sense, it means to change a surface from flat to shaped,
so that some areas are raised relative to other areas.
On this website, I use the word "embossing"
only as it applies to thin and malleable materials.
In most cases, each raised area on
one surface of the thin material is matched by a recessed area on
the opposite side, and vice versa.
One exception to this is sometimes called "pattern pressing".
This occurs when areas of one surface are recessed while the
corresponding areas of opposite side remain flat or are also recessed,
causing the material to become thinner in those areas.
This is typical when embossing leather
or when using heated embossing on a nonwoven to create thermal bonding.
High pressure pinpoint pressing with very narrow wheels against a hard smooth cylinder
is used to create ply bonding (for facial tissue and bath tissue),
and this is sometimes called “edge embossing”.
Materials that are embossed:
Just about anything that is thin, flat, and malleable can be embossed.
This includes paper, plastic film, metal foil, nonwovens, textile fabric, leather,
and even glass. These materials may be provided in continuous form
(like paper unwinding from a roll), or in discrete form (cut into individual
sheets before embossing).
Purpose of embossing:
Sometimes embossing is done for purely decorative reasons. However,
in most cases, the purpose of embossing is to change the physical
characteristics of the material. Embossing a metal foil with a fine
texture pattern makes it much easier to handle the foil within the
machine that wraps it around a piece of chewing gum. Embossing a
plastic film changes its elastic properties dramatically.
Embossing tissue paper improves absorbency and flexibility, but almost
always at the expense of strength.
Embossing increases the overall thickness of the material.
In some cases, embossing is used to bond two or more layers of material.
Methods of embossing
are often determined by the properties of the material, and how it is provided.
The material may be malleable or fluid, or somewhere between.
It may be provided in continuous form (without breaks), or in discrete lengths or pieces.
- For malleable materials,
a permanent shape change is imposed simply by the
application of force.
This usually has a very significant effect
upon the mechanical properties of the material.
Most tissue paper is
embossed this way, while the paper
is completely dry.
- For fluid materials, the embossing
step starts out more like casting onto
a mold while the material is still
fluid, and then the material is changed
from fluid to solid.
This reduces the effect of the embossing upon the
strength and elasticity of the material.
In the case of tissue
paper, the fluid state is the suspension of
paper fibers in water, the mold is the
forming wire, and the material becomes more
solid as the water is removed.
- Some embossed materials are somewhere between
malleable and fluid. For instance, tissue paper
can be shaped after it is formed, but still very
wet. The results are much different from
traditional "dry embossing".
- When the material to be embossed has been cut into
discrete lengths, it is usually necessary to
employ an intermittent method like stamp
embossing, where the sheet is pressed between
two plates.
- When the material to be embossed is provided in
continuous form, without breaks, then the preferred
method is rotary embossing, where the
material is passed between embossing rollers.
Rotary embossing is much, much faster than any of
the intermittent embossing methods.
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Embossing, in Greater Detail
The remainder of this discussion will cover only continuous rotary embossing, and will
focus upon how this is applied to absorbent tissue paper in the dry state.
Much of this is also relevant to embossing other materials.
The fundamental mechanism in rotary embossing is the embossing nip, which is the area
where two embossing rollers come into contact. The simplest embossing applications use
only a single nip. Others may involve several
embossing nips, either in series or in parallel. Sometimes embossing is directly combined
with other finishing processes, such as printing or laminating (which involve other nips).
The types of embossing nips are named after the materials that have traditionally been used
for the surfaces of the embossing rollers. These materials are still the most common, but
newer materials are being developed.
- S/S (steel-to-steel): Both rollers are engraved with patterns
that are designed to engage each other in some way. The surfaces of these rollers
must be hard enough and durable enough so that the raised protuberances on each
is able to deform the paper. Traditionally, both surfaces have been steel, and
therefore this type of embossing nip is called a "Steel-to-Steel" or S/S
embossing nip.
- R/S (rubber-to-steel): Only one of the rollers is engraved,
while the other roller is covered with a elastic material like rubber. The
surface of the elastic material is smooth, except while it is being pressed
against the engraved roller in the embossing nip. Elastic recovery to its
original smooth shape is extremely rapid. The surface of the engraved roller
must be hard enough and durable enough to deform not only the paper that is
being embossed, but also must deform the elastic material of the opposing
roller (which requires much more force and energy than the paper does).
Traditionally, the engraved surface has been steel and the deformable surface
has been rubber. However, the engraved roller could have a laser engraved
surface made of very hard rubber, while the smooth roller could have a surface
made of an elastomeric plastic.
- P/S (paper-to-steel): There is another type of embossing nip
which is really a hybrid between the two described above.
It is mostly used only for paper napkins where the embossing must produce
bonding of multiple plies and/or high visual definition in the pattern.
In this case, the steel roller is engraved with the embossing pattern, while
the opposing roller is a paper-filled roll that is initially smooth. A
"run-in" period is required to transfer the pattern from the engraved steel
surface into the paper surface initially, and also to repair any damage that
may later occur to the paper surface.
Embossing nips may be combined in parallel or in series.
- Serial nips: This is sometimes used to superimpose one embossing
pattern over another, by passing the paper first through one embossing nip, and
then through another. It works best when the first pattern is a very fine-scale
pattern that has complete coverage over the paper (like a micro embossing
pattern), and the second pattern is composed of larger figures with large open
areas between them (like a spot embossing pattern). However, a very
similar effect can often be achieved in a single nip less expensively.
- Parallel nips: This is only used for products that have two or
more plies. In a two-ply product, one ply is passed through one nip while the
other ply is passed through the other nip, and then the two plies are brought
back together again, usually with some method of bonding the plies together.
This is most often employed in two-ply laminated towel products, which use
very carefully placed dots of glue to bond the plies together. The choice
of embossing patterns, how the pattern on each ply aligns with the pattern on
the other ply, and the placement of the glue are all critical elements in
the design of an embossed/laminated paper towel product.
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