"Behind the scenes" of the converting process: how a tissue paper machine works

In the field of paper, and specifically in the converting process, many times we forget the procedure that lies behind the production of the parent reel.

Perini Journal

Starting form this issue of the Perini Journal, we would like to discuss some of the "behind the scenes" processes that concern converting and that are more closely tied to those operators who work in the preparation phase of the parent reel, necessary first step in the converting process. It is a section that is reserved for "authorized personnel only" and is intended to serve as a support article, to satisfy the technical curiosities of those who work in this realm. And - given its simplicity of explanations - can also serve as an introductory article supplying basic information to all those neophytes among us. What is a paper machine?

In the global process of converting raw materials into finished product, paper machines represent the main instrument through which the parent reel is obtained. The manufacturing process of the parent reel can be simple from the descriptive point of view, but the complexity of the number of phases that comprise the process itself, the systems - specific for each phase - make its actual production a rather complex operation. Many problems can arise due to the numerous variables that come into play, such as the use of chemical products, the wear of the devices and equipment, percentage of humidity, high operation speeds, etc.

These potential problems substantially complicate the production phase. So even though we can say that the process of converting fibrous substances into finished product is rather simple to understand, it is, instead, complicated to carry out. The Big Bang of the paper making phase is given by the breaking up of cellulose - or recycled fibres in water. These are the indispensable raw materials for the start of the process and this phase is carried out in the pulper, a machine whose function it is to break down these fibres. The sheet of paper produced is then dried in the Yankee dryer. The fibrous mixture obtained from the crushing of cellulose through the addition of water - fundamental element of the process - is distributed onto a wire and is then transported to be dried. The water contained in the mixture, bonding element and means of transport of the cellulose pulp, will have to be subsequently removed. It is at this phase of the production process that the paper machine plays a determining role. The PM allows the continuous formation and detachment of the sheet of paper. One, continuous phase where the raw material is crushed, distributed uniformly on a wire and the sheet of paper is dried. For this reason, it contains many devices (headboxes, vacuum presses and Yankee dryer, etc.) that remove half of the water concentration contained in the pulp. The Yankee dryer, a large cylinder heated by vapour and aided in its activity by vacuum hoods, heats the sheet, thus terminating the drying phase through the total elimination of the residual water concentration. The sheet thus obtained is wound on a large cardboard core to become a parent reel. Here is where the work of the paper machines end and the converting machines come into play.

START-UP OF A TISSUE PM. As can well be imagined, the start-up of a tissue PM presents several small potential problems to be resolved and kept under control. Describing these is very complicated and complex, considering that we are talking about a dynamic field, in constant and fast evolution. For this reason, writing an exhaustive article is complicated and rather difficult.

In order to describe the complicated start-up phase of a tissue machine, we could use endless rivers of ink and we still would not have succeeded in being exhaustive. Therefore, we would like to limit ourselves to listing and describing some of the main factors that influence in particular the start-up procedure of the PM, especially the Crescent Former. This procedure can be thus summed-up as follows:



Measures to be take before the start-up

1. Adjustment and control of fibre distribution inside the headbox

2. Vacuum line control (vacuum boxes and press rolls, wire and felt rinses)

3. Yankee dryer pre-heating

4. Pre-heating of the hoods

5. Coating preparation on the Yankee surface

6. Humidity profile adjustment

ADJUSTMENT AND CONTROL OF PULP DISTRIBUTION INSIDE THE HEADBOX. This procedure consists in adjusting the aperture of the headbox, a mechanism that allows to uniformly distribute the fibre along the web width of the PM. The pulp, initially largely comprised of water and pulp (the pulp concentration varies between about 0.16 and 0.30%, and depends on the various types of paper and grammage to be produced), is sprayed on the wire from which it will subsequently migrate - through contact - and position itself on the felt. Controlling the aperture is necessary in order to align the jet speed of the pulp with the speed of the wire. By producing slight speed changes between jet and wire, we can act on the mechanical characteristics of the web of paper produced (longitudinal and/or transversal tensile strength) because the fibres comprising the pulp will be distributed more or less uniformly according to the speed difference between jet and wire.

VACUUM LINE CONTROL. The vacuum line is comprised of devices able to eliminate the greatest amount of water possible from the pulp originally deposited on the felt, until a pulp concentration - before the Yankee dryer - of about 36-42% dry matter is attained. These devices can be: pumps of various types that can generate a vacuum; vacuum boxes and presses that utilize the vacuum created by the pumps to eliminate the water in the pulp; vacuum boxes that, aided by rinses with clean water, maintain the felt in good working condition (permeability). Efficiency control of the vacuum line is necessary both for the correct functioning of the water extraction operations from the pulp as well as for felt cleaning. The latter must always be in optimal conditions of permeability both to absorb water and to allow a better deposit of the paper fibres on its fine surface.

YANKEE DRYER PRE-HEATING. Through evaporation, the Yankee dryer extracts the water (humidity) from the pulp. The web passes from the felt to the Yankee through contact. The internal heating of the machinery ensures an adequate temperature of its external surface. Entering into contact with the sheet, the machine further reduces - through vaporization - the water concentration of the pulp bringing it to about 3-7% (relative humidity of the sheet). The pre-heating phase of the Yankee, a large, thick metal cylinder, allows to obtain complete and uniform heat distribution, thus avoiding internal tensions of the materials comprising it.

PRE-HEATING OF THE HOODS. The hoods aid the activity of the Yankee as they contribute to the water vaporization within the sheet through air jets aimed directly on the sheet. The pre-heating phase is necessary for these machines because they are comprised of sheet metal tubing of various thickness and sizes.

COATING PREPARATION ON THE YANKEE DRYER SURFACE. The coating, an ensemble of chemical products uniformly distributed on the Yankee dryer surface, reduces the friction created between the Yankee and the creping blades used to scrape the paper from the cylinder. Some chemical products of the coating aid the creping blades in the proper release of the sheet from the Yankee (adhering and detaching products). The preparation in the proper percentages of such products and their correct distribution is very important for uniform release along the entire width of the sheet from the Yankee surface. The products are sprayed on the cylinder surface through fan nozzles located at a well-defined distance.

HUMIDITY PROFILE ADJUSTMENT. This operation is necessary in order to avoid the formation on the sheet width of areas having different humidity levels. This can be caused by an imperfect cleaning of the felt, malfunctioning of the headbox, vacuum press, etc. The sheet must have a uniform humidity content in order to avoid malformations of the sheet (bumps or depressions or wet ends) on the reel during final winding on the pope shaft. This phenomenon can create problems (wrinkles, breaks) during the converting process of the parent reel into finished product and must therefore be avoided. A solution is to act on the drying hoods located above the Yankee dryer, reducing the hot air flow and addressing it appropriately through a computer-controlled humidity profile.

Special attentions

1. The surface temperature of the Yankee must be kept under constant control according to the speed of the paper machine and the type of paper produced.

2. The time elapsed between the approach of the headbox and the vacuum press of the felt against the Yankee surface, and the opening of the pulp valves must be sufficiently reduced so as to avoid part of the coating that has formed on the Yankee migrating onto the surface of the felt; If the coating becomes detached from the Yankee, the paper sheet that is to adhere to it will not find a uniform anchoring, thus creating problems both for the creping blades as well as for reel formation.

3. The control of the chemical products used in the circuit (anti-foaming, cationizing, etc.) must be carried out with particular attention both in terms of operation and quantity in order to avoid mechanical and dimensional modifications in the paper characteristics. An excessive use of anti-foaming agents - aimed at avoiding the formation of air bubbles in the fan pump and in the headbox - may cause modification in the detachment of the sheet, thus altering its mechanical characteristics and its adherence to the Yankee.

4. For a hygienic use of the products obtained - toilet rolls, table napkins, handkerchiefs, baby diapers, etc. - products destined to come into direct contact with skin and with the human body - a constant control of the pH of the waters in the tissue machine up to the fan pump is necessary (min 6.8 - max 7.6, as close as possible to neutral). The chemical products used have been studied so as to maximize their efficiency in environments having such a pH. It is very difficult to be exhaustive when describing the "behind the scenes" of the converting process, but in these few lines we have tried to introduce in a simple way some of the many aspects that we will face in the next publications.

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