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 involves 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 are
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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