Technology of Embossing - Embossing

Technology of Embossing

ABOUT THE TECHNOLOGY OF EMBOSSING
for Commercial Application
Embossing Technologies
The intent of this page is to educate the reader about the application of embossing in manufacturing, and also to convince the reader that Embossing Technologies is a valuable source for technical information in this field.
Embossing Basics
Definition of embossing: In the broadest sense, it means to change a surface from flat to shaped, so that there are regions that are raised up from a background. This meaning would seem to encompass areas that are normally not called embossing: carving figures into the face of a mountain (like Mt Rushmore), or engraving a steel block. On this website, I use the word "embossing" only as it applies to thin and malleable materials. Since the opposing surfaces of a thin material are very close to each other, each raised area on one surface is matched by a recessed area on the the opposite side, and vice versa. One exception to this is sometimes called "pattern pressing" or "blind embossing". This occurs when areas of one surface are recessed while the opposite side remains flat, causing the material to become thinner in those areas. This is typical when embossing leather, for example.
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.
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.

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.