The operation of a tissue production machine: Product features

We would like to continue our virtual voyage in the backstage of the converting process by analysing another production phase in paper machines.

In the previous issues we briefly described the start-up of a PM and the pulp preparation phase. In the current issue we are going to deal with one aspect connected to the operation of a PM: the product, its characteristics and how to modify them by setting parameters.

Perini Journal

Like the preliminary phases of the PM start-up, also running a PM working at full capacity is very complicated and difficult. The variety of different factors interacting requires highly skilled and experienced personnel attending the machine. The control and supervision of product pre-set parameters represent the main matter; maximum attention must be paid to prevent any parameter from undergoing a variation - which is not unusual - thus causing an alteration in the final product.

Once the machine has been started up and the product has been defined, the main task of technicians is that of running the machine keeping all parameters constant, aiming at the highest speed possible, always bearing scale economies in mind. The machine starts working very slowly and then speed is gradually increased up to the maximum potential efficiency, always making sure the parameters are kept constant.

THE MAIN FACTORS THAT DETERMINE AND MODIFY THE FINAL PRODUCT ARE: pulp composition, chemical additives, basis weight, thickness, transversal and longitudinal tensile strength, water content, softness, absorbency, creping and candling.

The product must conform to the so-called “Technical Chart”, which contains all the characteristics of the final product according to the customer’s specific needs. For toilet paper, for example, the following characteristics are defined: basis weight (usually 15.5 gr/square metre) and pulp composition, which can consist of 50% short fibres and 50% long fibres up to 75% short fibres and 25% long fibres; creping can vary in a range between 15% and 30%.

Specifications are useful in order to document and memorise products and to confer the finished product a constant quality (repetitiveness). In order to observe all such specifications, some special equipment and tools are used during the production process to modify and control the features of the product information.

LET US NOW ANALYSE THE MAIN CHARACTERISTICS IN MORE DETAIL: Pulp composition: It is a fundamental aspect of the product. There are kinds of pulp with elevated content in short fibres, usually destined for soft grades, and other types with high content in long fibres, destined for absorbent and strong grades, i.e. kitchen towels. Pulp containing a high percentage of recycled paper is used for low-quality grades.

Chemical additives: Usually glue, softeners, additives for strength enhancing, antifoaming and anti-resin additives to obtain softness, better wet and dry strength. Chemical additives can sometimes help to solve technical problems connected to machine operation.

Basis weight: It is the measurement unit of the product weight per square metre. It usually varies in a range between 14 and 25 gr/square metre. Such measurement is carried out through the Quality Control System1, an electronic scanner positioned between the yankee dryer and the pope reel.

Thickness: It is expressed in mm and is measured through mechanical or digital micrometers. Thickness is important to confer bulk to the finished product, thus favouring the converting phase.

Transversal and longitudinal tensile strength: It is measured by the use of dynamometers and it is paper resistance capacity to mechanical tension. This is a very important characteristic both during the converting phase and for the final use of the finished product.

Water content: It usually ranges from 2.5% to 7% and is obtained by calculating the amount of water left in the web. It is measured continuously by the use of QCS scanners placed on-line in the PM, between the yankee and the pope.

Softness: It is a highly oscillating feature. No tools currently exist to measure it according to ISO norms, so it is usually measured empirically, through the comparison of different samples. It is a very subjective and flexible parameter that mainly depends on the specific market and product.

Water absorbency: It is the capacity of paper to absorb a fluid, usually water, with respect to its weight. It is measured through equipment subject to ISO standard procedures.

Creping: It is the result of the speed difference between the yankee and the pope, and makes paper more versatile for its end use.

Candling: It is the uniformity of fibres distribution on the wire, and is empirically measured by holding the web up against the light.

All the above mentioned aspects can widely vary according to the product required. Many factors influence them and this makes the work of paper producers even harder.

LET US NOW HAVE A LOOK AT HOW THESE PARAMETERS CAN BE CONTROLLED AND MODIFIED DURING THE PRODUCTION PROCESS. Pulp composition can be modified by changing the percentage of short fibres, long fibres and recycled paper when the raw material is introduced into the pulper.

Also chemical additives are inserted in pulp directly in the pulper; a few are added during the passage of pulp from one stocking vat2 to another by the use of dispenser pumps and on-line dilution. During this passage, the pulp transits through some machines that modify and enhance fibre characteristics.

The percentage of additives is not pre-fixed but is defined during the process according to production needs.

Basis weight can be modified by increasing or decreasing the pulp flow before entering the fan pump, thanks to valves that measure it (QCS sensors).

Weight depends also on machine speed as, keeping speed constant and decreasing pulp concentration, a lower weight product will be obtained. So weight can be modified both through the weight valve and by changing speed, thus maximally exploiting the machine performances. Another possibility to change weight is by changing creping ratio between the pope and the yankee.

Thickness can be modified by changing the inclination of the creping blade with respect to the yankee dryer as well as by using bulky fibres or altering the paper’s water content. These are very important changes and a producer must know how to adjust and use all the mechanical options of the machine in order to obtain a high quality and profitable product.

Transversal and longitudinal tensile strength is modified by acting on the fibres recipe when cellulose is introduced in the pulper, as well as acting on the refiners and on the headbox jet when pulp is deposited on the wire. Refiners enhance paper tensile strength as they increase the bonds among fibres. Tensile strength can also be modified by directly acting on the headbox, exploiting the difference in speed between the wire and the headbox jet.

Water content can be changed by altering both temperature and air speed in the dry section hoods. It must be carefully controlled as it results in paper’s thickness and softness. By increasing the cycle rates of fans, exchange speed is increased too and consequently humidity is reduced. Hot air, rich in water for the evaporation from paper, is conveyed into heat exchangers that recover and recycle part of the heat; finally hot air is expelled in the atmosphere.

As previously mentioned, softness can be changed by acting on chemical additives, recipe and humidity. Chemical products are added at the beginning of the process, directly in the pulper, and then also distributed on the web before it reaches the yankee.

The recipe can be changed in the pulper, increasing the percentage of short fibres. Softness can be enhanced also by the inclination of the crepe blade, the stick-out3 and the coating4 applied to the yankee.

Changes on absorbency can be obtained by increasing the percentage of hydrophile fibres of pulp directly in the pulper; the same target can also be attained mechanically acting on deflakers and, only minimally, on refiners; finally, by using some chemical additives.

Creping is modified by the speed of the pope with respect to the speed of the yankee and by acting on the creping blade, on its thickness, on its inclination, and on the counterblade.

Finally, candling is modified by diluting pulp in the headbox, through changing the fan pump speed and opening and closing the headbox slice.

HERE TERMINATES OUR BRIEF VOYAGE through the various complex characteristics of the final product realised by the PMs. Ten parameters that interact and, thanks to a delicate balance, give shape to the raw material of converters - the paper sheet - and confer it the ideal consistency.•

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