Embossing of Tissue Products
Tissue is an extremely thin and therefore damageable paper grade with basis weights between about 12 and 25 g/m2 per ply. To fulfil the functions expected from finished products, these are produced as multiply products, i.e. mainly with 2, 3 or 4 plies. Along with a whole range of other converting techniques, tissue embossing is used for different reasons: achieving the required effects as regards ply bond and softness but also to enhance bulk and for decorative purposes.
Embossing fulfils an important function: thanks to both suitable embossing patterns and processes it contributes to the end consumer perceiving only the positive side of a multiply tissue product, just as wanted. As a result of insufficient ply bonding in toilet tissue rolls, consumers used to express the strange complaint the perforation was "shifted". In fact it was simply an isolated tissue ply not adhering to the others which happened to be turned over so that the perforation line of the single sheets was no longer exactly superposed! Today this problem has been solved with an embossing process suited to the function. Along with corresponding perforation techniques, special embossing patterns and processes nowadays ensure fully satisfying ply bonding for roll products such as toilet tissue and kitchen towel. Especially multiply kitchen towel is furthermore glued at the embossing points, which positively impacts the absorption volume and absorption speed of the product besides ply bond. Folded products such as handkerchiefs and napkins usually have edge embossing to ensure ply bond.
Another important purpose of embossing is to achieve softness in the finished product, i.e. especially the sensorily relevant "surface softness" along with the so-called "crumpling softness". But while enhancing softness, the embossing process is not to cause an excessive loss as regards tensile strength and puncture resistance of the product. This is a difficult task that can only be fulfilled through an optimal combination of embossing patterns and embossing processes. As far as multiply folded products with edge embossing such as handkerchiefs and napkins are concerned, embossing must neither harden nor too much elongate these edge areas. Single-ply napkins with a higher basis weight are embossed all-over to convey certain softness to these otherwise somewhat rigid papers.
Tissue embossing serves an extremely important purpose by creating bulk. Malicious gossips claim that tissue hygiene papers are nothing else than air packed in an optimized way with a minimum amount of pulp! A certain amount of bulk is however necessary for a whole range of product features. Not only are the different ways products are perceived in a quantitative way involved here, but also really measurable features such as absorption volume and absorbency. The latter criteria are of exceptional importance as far as the quality of kitchen towels is concerned. When it comes to taking the buying decision and finally using the product, a bulky product of course creates intended impressions supposed to induce the customer to buy again this particular product.
In the last years, embossing has played an ever growing part in distinguishing products through decoration. Producers of branded products very early started trying to clearly differentiate their products and make them recognisable with the help of special embossing patterns. Some of them, e.g. handkerchiefs, even used the brand name hand for that purpose. Quite a lot of embossing patterns are even protected by trademark rights and currently said to be defended with considerable commitment and vehemence. The violent debate that flared up on the kitchen towel market a few months ago also involves, among other things, the embossing patterns in use.
Embossing technique requirements
The high-performance machines used nowadays don't let embossing rolls much time to fulfil all the above described functions. The time spent by tissue material in the embossing nip is extremely short. Meanwhile modern toilet tissue production lines reach speeds up to nearly 1000 m/min. At the same time a rapid increase in working width can be observed. In 1997, the first rewinder worldwide with 5.4 m working width was successfully started up in Italy (cf. Papier+Kunststoff-Verarbeiter issue 6-97). Hanky and napkin lines too are getting quicker and quicker. The numerous tissue grades, differing in both product requirements and the origin of the material, make considerable demands on embossing roll surfaces too. No material is allowed to stick in relatively small embossing nips, which is achieved, among other things, with a special surface refinement.
A multitude of different embossing processes and patterns, some of them being protected, is now available to meet all the a.m. tissue embossing requirements. A distinction is to be made between simple processes, simultaneously embossing the layers of multiply tissue, and combined processes on the other hand, individually embossing the single plies. Combined processes are often associated to a directly following gluing unit to glue the 2 or 3 layers together.
Steel roll against paper roll
For this long known and relatively cheap embossing process, the positive engraving of the steel roll is impressed onto the opposite paper roll before starting production. To ensure that after each revolution, the steel roll hits again exactly the same spot of the paper roll, both rolls have to be linked together with toothed wheels, the so-called connecting gear. The tissue to be embossed abrades the run-in paper roll resulting both in a worn out paper roll after some time and in a decreased embossing quality. The embossing pattern must then be impressed again onto the paper roll during a production break. This operation is called "running in" and can be repeated several times. But once wear has reached a certain extent, the paper roll is too thin and must be replaced with a new one having the right diameter. This embossing technique is mainly used in the production of all-over embossed napkins.
Steel roll against resilient counter-roll
This embossing process is almost only used in the production of bottom-of-the-line toilet tissue grades and this only for rough patterns. The resilient counter-roll with rubber or polyurethane coating doesn't follow exactly the engraving shape while pressing on the positive steel engraved roll and springs back when pressure decreases. In the course of this the engraving is partly pressed out of the material. Ply bonding cannot be achieved with this process alone. But it is an unproblematic embossing process, although not claiming to achieve a good and sharp embossing pattern definition.
Positive steel roll against negative steel roll
This process in widespread used in the paper industry, also called union embossing, works with two steel rolls rotating against each other, the one with a positive engraving and the other a matched steel embossing roll with a negative engraving. This embossing process is basically similar to the one described above as steel-paper-embossing, with the difference that in the present case, the a.m. resilient counter-roll is replaced by a negative steel engraved counter-roll. This way an embossing quality is achieved that remains equally good much longer. Production stoppages due to the paper roll running in, its rewashing and regularly necessary replacing can be avoided with this process, provided that a higher initial investment is accepted.
Union embossing requires the operation of the rolls in the embossing unit of the production line to be absolutely free from backlash. The rolls must faultlessly rotate to each other. They are coupled with a special connecting gear excluding backlash and absorbing shocks.
Manufacturing such matched rolls is much more expensive than the steel/paper, steel/rubber or steel/polyurethane rolls described before. Today union rolls for napkins and handkerchiefs are most adequately produced in deep-hardened design. This way an absolutely faultless ply bond can be achieved between the tissue layers, often up to four of them. In the case of handkerchiefs and napkins with edge embossing, an otherwise unaesthetic elongation in the edge area, also impeding the packaging process, can be avoided to a large extent. As far as rolled hygiene products such as toilet tissue are concerned, the roll diameter can be influenced by the space between the rollers. The larger the space, the more inaccurate the definition, the narrower it is, the sharper the definition and the larger the possible winding diameter of the roll product.
Positive steel roll against plain steel roll
This is a relatively old embossing process, primarily used to achieve ply bonding. In the toilet tissue production, it is often combined with the above described embossing process "steel roll against resilient roll". This combination ensures the ply bond that is not to be achieved with the latter technique. On the finished product, it is recognizable from two embossing tracks. Unfortunately these are rarely placed symmetrically to the middle of the roll, which negatively impacts the product aesthetics.
If this embossing technique is used to emboss format engravings, the demands on the embossing rolls are of course much higher. It is essential for these embossing rolls to rotate to each other without any backlash at a defined embossing nip. The load alternation inevitably occurring between longitudinal and cross bar embossing must be compensated for. Furthermore the deflection due to the embossing pressure must be allowed for. This is the reason why UniMaTec developed and successfully launched a special nip adjustment. This method makes it possible to adjust and modify an exactly defined embossing nip under production pressure. This device offsets the bearing play in the load direction so that the load alternation has no effect on the material to be embossed. The possibility for adjustments is important when different material grades or numbers of layers are to be produced. The embossing rolls themselves are made of a special material with deep hardness penetration and considerable hardness. For this embossing process, tissue is impressed on one side, but not pushed through as opposed to the conventional process. Ply bond is produced within the tissue thickness towards one side. The advantage is the not palpable embossing however presenting visual two-sidedness.
Positive steel roll against positive steel roll
Two further embossing processes were developed in connection with the embossing of paper handkerchiefs, i.e. the "point to point" and "top to flat" versions. The 4-ply tissue web is led through two rolls with positive engraving to achieve good ply bonding. For the former version, "point to point", ply bond is achieved by pressing together the tissue layers right in the middle of the tissue. The advantage is that both tissue sides look identical. For the "top to flat" version, both embossing rolls are adjusted so that each embossing point hits the flat area between the embossing points of the counter-roll. Bonding points are no longer in the middle of the tissue but on each outer ply. From a visual point of view both sides of this tissue look yet identical too. Both versions of this embossing process require an exact adjustability of the embossing rolls in relation to each other, without backlash. Trademark rights were granted to the developers of both processes.
Combined embossing processes
More complex embossing processes, where tissue plies are embossed separately and glued afterwards, were developed specially for the kitchen roll production. After the remarkable success on the market of kitchen rolls produced this way, this process was adopted both for premium toilet tissue rolls and for some isolated folded handkerchiefs.
PCMC Green Bay, USA, was the first to launch a combined embossing process, which was called "nested embossing". For this process, two tissue webs are embossed separately in a double embossing unit, each of them by a positive steel engraved roll against a corresponding rubber roll and straight after glued together with the help of a further rubber roll ("marrying roll"). Both steel rolls being synchronized with a connecting gear, the two embossing patterns, which were produced separately, can be exactly matched. If the protuberances of a layer are located between two protuberances of the other layer (top to flat) in this case, we have to do with classic "nested embossing". But if the engraving tops of both webs are glued exactly on each other, this process is called "top to top" - or "punta-punta" embossing. If the gluing points are not exactly defined, but a casual result - both steel rolls being not matched -, this technique is called "Desl" embossing. The two main rewinder suppliers, PCMC Green Bay and Perini Lucca, Italy, have trademark rights for these combined embossing processes.
All these embossing processes impress tissue on both sides. Gluing points are either on one side or in the middle of the tissue. The advantage of all these embossing types is primarily to be found in the trouble-free embossing against a rubber roll. As far as both steel rolls are concerned, an accurate adjustability, free from backlash must be ensured. Gluing provides good ply bonding with air cushions between the layers, which enhance quality. Gluing being rather sensitive from a technological point of view, the achievable production speeds are clearly inferior to that possible with union matched rolls.
Conclusion and prospects
Innovative tissue converters have developed a whole range of other embossing processes in the last years, some of which have already been tested on the market or even launched. Let's just mention here "spot embossing" and the so-called "valley printing". These processes are mainly used to decorate tissue products. This product specification is obviously becoming more and more important, especially for branded suppliers. Tissue embossing certainly has still more facets than those described here. The near future will undoubtedly have a few more surprises in store for the industry and consumers.
Source of pictures and drawings:
UniMaTec Prägesysteme GmbH in D- 52320 Düren