1. INTRODUCTION:
In printing and converting operations the cost of production and the quality demands on the finished products are increasing steadily. As a consequence the quality of printed paperboard must also improve. One important quality factor in converting is the out-of-plane deviation, or curl, of the paperboard. Curled paperboards are also generally difficult to handle and may reduce the production speed in converting operations.
It also affects the aesthetic look and flatness of the final package. Curl problems can cause obstruction or jamming at various stages in printing and converting equipment [1]. Due to the economic significance of the curl problem, understanding curl and the mechanisms behind it are increasingly important. In this paper the words paperboard, sheet and board are synonymous. Similarly the words warp and curl means the same.
2. OBJECTIVE OF THE PROJECT:
To eliminate the tendency of the uncoated paper board, to get curled.
3. GENESIS OF CURL:
Curl can be defined as either a curled condition of paper or a tendency of paper to adopt a non-flat shape when exposed to changes in humidity or temperature. It is mainly due to the release of stress that got built up into sheet during manufacture and subsequent processing. There are three basic types of curl,
3.1Mechanical curl:
Mechanical curl develops when one side of the paper is stretched beyond its elastic limits. An example of this is the curl in the sheet which forms near the centre of a roll [2].
3.2 Structural curl:
Structural curl is caused by two-sidedness in the sheet, which is a difference in the level of fines, fillers, fibre area density or fibre orientation through the sheet thickness [2].
3.3 Moisture curl:
Moisture curl can develop when the paper sheet is printed by offset lithographic process. One side of the sheet is subjected to pick up more moisture than the other. This side with higher moisture releases the built in drying strains and the paper will curl towards the drier [2].
The curling of paper and board may occur due to structural differences between the two sides of the sheet, detrimental factors in the making of the paper; or during the converting of the material[3][4][5][6].
4. PROBLEM DESCRIPTION:
Addressing curl problem requires that we look at our process and the very structure of paperboard to find out why curl occurs.
Curl during storage occur when layers within a sheet expand or contract unevenly accumulating stress within and resulting as uneven strain affecting the paper board dimensionally. Uneven contraction (or expansion) produces a bending moment because of uneven shear forces [7]. Paperboard can lose and subsequently absorb moisture depending on the environment that it is in. Fibers that make up paperboard expand and contract in diameter depending on moisture levels in the environment which on a macro scale gives dimensional changes in the closely laid fiber network.
Curl after printing can be expected if the moisture content of the paper board does not correspond to the equilibrium humidity condition that will be present when the paper is printed due to poor or non-existent air conditioning [1]. Higher moisture contents can result in curl away from the printed side.
Paper dampening curl develops in offset lithographic printing when the paper is subjected to excessive dampening, a situation where there is little ink coverage needing more than normal dampening. Initially, the side that is wetted expands excessively in the papers cross direction and the curl is towards the undampened side of the sheet with the axis of the curl parallel to the papers machine direction. When the sheet dries out the final curl is then towards the dampened side, the curl reversing because of shrinkage occurring after dried-in strains in the dampened side of the paper has been released [6]. With the press dampening curl, the axis of the curl is always parallel to the paper machine direction because fibers expand more in their width than they do in their length and fibers in machine made paper are mainly oriented with the latter dimension in the machine direction. Paper that is delivered dry is more prone to this type of curl.
MD-Curl - The axis of curl will be parallel to the machine direction of the paper.
CD-Curl - The axis of curl will be perpendicular to the machine direction of the paper.
SBS (Solid Bleach Sulfate) uncoated board of 170gsm was used in study of the curl.
5. FACTORS INFLUENCING BOARD CURL:
5.1 Moisture content of paper:
Moisture in paper, another source of trouble when it is above or below that of the pressroom, cannot be controlled unless pressroom and paper storage rooms have relative humidity and temperature control. Paper, by nature, is hygroscopic. This means that it will pick up moisture from its environment and it will release moisture into the environment to try to reach equilibrium with the moisture in the air surrounding it [5].A moisture curl can be controlled by establishing proper humidity conditions in the pressroom and warehouse and maintain the board to attain equilibrium. In practice, the press room maintains a temperature of about 27°C and relative humidity of about 55%. And it is necessary that the same is maintained in the warehouse. Moisture curl will disappear when the moisture content of paper reaches equilibrium with that of surrounding atmosphere.
The effect of relative humidity is directly related to temperature. Relative humidity is the amount of water in the air, in vapor form, relative to the amount of water that the air could hold at a given temperature [5]. The effect of water vapor in the air has a definite effect on paper and dimensional stability.
Main component of paper is cellulose. Due to its chemical composition and its capillary structure it can absorb and bind water molecules from atmosphere. At a given temperature the amount of bound water is dependent on the relative air humidity and changes with it. Paper contains maximum amount of moisture at 100% relative humidity. Cellulose contributes to the majority of moisture content in paper. In fact paper also contains other substance which cannot take up moisture [8].
The moisture content of a paper is in equilibrium with the relative humidity of the ambient air, which it establishes by the absorption or desorption of water vapor [6]. Any change of relative air humidity in the room or the temperature leads to a disturbance of the equilibrium, which resets very slowly in paper stacks. The connection between the moisture content of paper and air humidity at constant temperature (296K) can be shown in sorption isotherm graph (Fig. 2). The lower curve accounts for the behavior of paper at increasing relative humidity and the upper one for the decreasing relative humidity. The relations given in the curve are equilibrium values which are reached at a free hanging sheet after sufficient adaptation time. Moisture conditions of the paper remain constant as long as the climatic conditions do not change. Equilibrium relative humidity is the relative humidity of the atmosphere at a particular temperature at which paper will neither gain nor lose moisture. At equilibrium relative humidity, paper can be exposed to air without the environment effecting a change in its length and/or width.
Fig. 2 sorption isotherm of paper
Source: UNIDO - Influence of climatic condition on paper cited in “Material and Process of Graphic arts under special climatic condition”, Anonymous.
5.3 Drying condition:
When board is printed on an offset press, it passes through a dryer unit after printing. With higher temperature in the drying process the offset printing machine could run much faster, but at the same time the moisture content in the board drops quickly. This decreases the moisture rapidly producing curl towards the print side [6].
5.4 Sizing:
Sizing is used on cellulosic fibers during paperboard manufacture in order to curb their tendency to absorb liquids by capillary action. These additives are usually starch, gelatin amine-based. There are two methods of sizing: internal sizing sometime also called engine sizing and surface sizing (tub sizing). Internal sizing is applied to almost all papers and especially to all those that are machine made, while surface sizing is added for the highest grade bond, ledger, and writing papers. There are three categories of papers with respect to sizing: unsized (water-leaf), weak sized (slack sized), and strong sized (hard sized). Hard sized papers have the highest water resistance and show better dimensional stability [9].
6. COATED VERSUS UNCOATED BOARDS:
It is quite common that curl is predominant in uncoated stocks rather than coated ones. To find possible mechanisms controlling the curl response, coated and uncoated boards were compared by conducting moisture content and water absorption test as illustrated in experiment 1 and experiment 2.
6.1 Experiment 1: Moisture Release
An experiment conducted based on moisture content of the paperboards revealed the fact that moisture content of the uncoated sheets reduces by about 1% after printing unlike coated stocks where the moisture content is retained even after printing and drying.
Table 1 Moisture release determination
|
Board Type |
Uncoated Board(Moisture Content) |
Coated Board (Moisture Content) |
|
Unprinted |
6.64 % |
5.35 % |
|
Printed |
5.48 % |
5.22 % |
|
Moisture Release |
1.16 % |
0.13 % |
Fig. 3Digital moisture analyzer
Product : Paperboard
Mass of sample : 2g
Temperature : 103°c
Time : 3 – 5 mins
A digital moisture analyzer was used to measure the moisture content of the sample boards. At the initial stage of measurement, the device precisely determines the mass of object placed on its weighing pan. Following this, there is fast heating of the sample with halogen or IR lamps. This causes vaporization of water molecules from the tested sample. While sampling, the moisture analyzer is continuously checking the decline of mass till the complete vaporization of water molecules in the sample, and after calculation, it displays the moisture content of the sample on the display panel as percentage. Moisture content is calculated by comparing the initial weight of sample and weight after vaporization of water molecules. The entire process takes place in few minutes depending on the mass of the sample kept and its moisture content.
6.2Experiment 2: Water Absorbing Ability
In order to quantify the ability of the board to absorb water another experiment was conducted. A square board of dimension 10×10 cm was taken. The surface was subjected to a constant volume of water for a specific time at normal room condition of 30°C temperature and 70% relative humidity. The experiment was carried out on both coated and uncoated boards under similar conditions.
Fig. 4 Cob test - Experimental Setup
The readings are tabulated and a graph is plotted between weight of board after water absorption along Y-axis and time along X-axis (Fig. 5).
Table 2 Comparison of water absorption
(W1& W2>>Weight of Water Absorbed Board)
|
Time (min) |
Coated Board |
Uncoated Board |
||
|
W1 (mg) |
Moisture (%) |
W2 (mg) |
Moisture (%) |
|
|
0 |
842 |
- |
729 |
- |
|
15 |
1081 |
28.3 |
1103 |
51.3 |
|
30 |
1133 |
34.5 |
1254 |
72 |
|
45 |
1165 |
38.3 |
1490 |
104 |
Fig. 5 Water Absorbing Ability
Inference:
From the graph, it is very obvious that under similar conditions of temperature, pressure and relative humidity the absorbing nature of uncoated board is very much high unlike coated board. The sizing agents added to the coated stocks during the time of manufacture curb their tendency to absorb liquids by capillary action. For every fifteen minutes, after first fifteen minutes moisture content of uncoated board gets increased by about 21% whereas in coated board it was only about 5-6%. This explains the absorbing nature of uncoated board that leads to dimensional instability.
Experiment 3: Conditioning Board in Environmental Chamber
Exposing paperboard to changes in the ambient climate is used to predict how the paperboard behaves. An environmental chamber is an enclosure used to test the effects of specified environmental conditions on materials. An environmental test chamber artificially replicates the temperature and humid conditions to which machinery, materials, devices or components might be exposed. It is also used to accelerate the effects of exposure to the environment, sometimes at conditions not actually expected.
These conditions may include:
· Extreme temperatures
· Sudden and extreme temperature variations
· Moisture or relative humidity variations
An environmental simulation test chamber can be a small room used both to condition test specimens and to conduct the test. It typically consists of a sealed enclosure with the built-in or attached air conditioning system capable of both heating and cooling. The temperature in the chamber is usually controlled by an electronic controller that senses the temperature at some point within the enclosed space.
Fig 6 Environmental Test Chamber
Table 3 Paperboard behavioral in different conditions
|
Observations |
Environmental Condition |
Sample no |
Moisture content of board |
|
|
Temp |
RH |
|||
|
Test 1 |
25⁰C |
50% |
I |
5.89% |
|
II |
5.76% |
|||
|
III |
5.81% |
|||
|
Test 2 |
25⁰C |
60% |
I |
6.82% |
|
II |
6.77% |
|||
|
III |
6.86% |
|||
|
Test 3 |
25⁰C |
70% |
I |
7.55% |
|
II |
7.65% |
|||
|
III |
7.63% |
|||
Air within the enclosure is circulated by powerful fans that keep the temperature variations within the volume at a practicable minimum while also making it possible to change temperature rapidly.
The paperboard is subjected to various RH levels at constant temperature. The corresponding moisture content of paperboard is measured. The results of this experiment are tabulated in Table 3.
Inference:
From the above test result it is clear that for every 10% increase in relative humidity there is a 1% increase in the moisture content of board.
Experiment 4: Determination of Dwell time
The printed sheet is warmed by infrared radiation. It will raise the temperature of the prints effectively to 30 or 40 °C and ease the vapor to be sent out. And then warm air is applied on top. Warm air readily absorbs humidity, and so it carries the vapor away from the print, enabling more vapors to come out. The more hot air is applied to the sheet and the greater the speed at which it is delivered, the better the drying result. The air is heated and applied to the sheet via nozzles. The nozzles accelerate the speed of the air to 33 meters per second - fast enough to penetrate the ambient air drawn along by the sheet (laminar layer) and drive the water out of the paperboard. Hot air temperature should generally be between 70° - 100°C (158 F – 212 F).
The time the sheet is retained under the drying unit is called the dwell time (s). Dwell time of the sheet in the dryer of a sheet fed offset press depends mainly on two factors namely the dryer length and the machine speed. As production speeds increase, the dwell time becomes shorter. Varying the temperature in the dryer based on the dwell time is a key to efficient drying. These settings have to be adjusted accordingly to ink coverage, printing material and printing speed. The dwell time of the paperboard, while printing at the speed of 18,000 iph and 14,000 iph are calculated and tabulated as Table 4.
Table 4 Dwell time of board at different press speeds
|
Press speed (iph) |
Press speed (m/s) |
Length of drying unit (m) |
Dwell time(s) |
|
18000 |
4.5 |
2.2 |
0.4 |
|
14000 |
3.5 |
2.2 |
0.62 |
|
18000 |
4.5 |
4 |
0.8 |
|
14000 |
3.5 |
4 |
1.14 |
The length of the drying unit in use (Heidelberg XL 105) is 2.2 m. If the length of drying unit is increased to 4 m by using extended type dryer a considerable increase in dwell time is observed at similar speeds of 18,000 and 14,000 iph.
Quantifying curl:
The key to control of curl is good test methods that can provide meaningful measurements of curl. Warp measuring device is used to quantify the curl in paperboard. The basic measurement involved in warp measurement is distance.
Fig. 7 Warp Measurement Device
Warping device has two major units and each unit has its own components which are effectively programmed and assembled in such a way that the warping on the board can be measured.
1. Measuring unit
· Ultrasonic Sensor
· Stepper Motor
· Timer Belt
· Arms
· Sensor moving shaft
· Vertical and horizontal scales
2. Microprocessor controlling unit
· Electronic circuit board
· LED Display
· Operating switches
The device makes use of the phenomenon of reflection of the ultrasonic sound waves. Sound waves are defined as longitudinal pressure waves in the medium in which they are travelling. Subjects whose dimensions are larger than the wavelength of the impinging sound waves reflect them; the reflected waves are called the echo. If the speed of sound in the medium is known and the time taken for the sound waves to travel the distance from the source to the subject and back to the source is measured, the distance from the source to the subject can be computed accurately. This is the measuring principle of this device. Here the medium for the sound waves is air, and the sound waves used are ultrasonic, since it is inaudible to humans. The reflection medium is the board [10].
The sample is arranged so that its plane is horizontal. A reference point has to be selected to compare the differences between the normal and warped board. Once reference point has been set, sensor will start the measuring process. Each value of the height will remain displayed on the LED for one second to record, and the value will be stored automatically in to the memory [10].
Assuming that the speed of sound in air is 1100 x 304.8 mm/second at room temperature and that the measured time taken for the sound waves to travel the distance from the source to the subject and back to the source is ‘t’ seconds, the distance ‘d’ in ‘mm’ is computed by the formula,
Since the sound waves travel twice the distance between the source and the subject, the actual distance between the source and the subject will be d/2.
Warp of the board sample of size 297 x 210 mm is measured using the device and readings are tabulated in Table 5. Five sets of readings are taken across the board, starting from the top of the board to its bottom, at regular interval of 52.5 mm. Two sets of readings are used to represent the pattern of the curl as line graph which is shown in the Fig. 8. Similarly, other sets of values can also be represented in graph.
7. RESULTS AND DISCUSSIONS:
As mentioned earlier there are four major factors that directly or indirectly influence the board curl. Board Curl could be brought under control only by standardizing the factors that influence them. In an attempt to standardize them various experiments were conducted. The results of those experiments were tabulated above. Based on the results obtained following solutions are proposed,
7.1. Pre-Conditioning of Paperboard:
The paperboards tend to curl due to the temperature variations they are subjected to at the time of printing. In most of the cases there is large difference in temperature and relative humidity maintained in warehouse than that maintained in press room. This causes the paper to lose its moisture after printing which leads to dimensional instability of paper itself.
In order to avoid such problems paperboard has to be conditioned, well in advance to printing[8]. This could be done either in a separate room or press room itself. A single sheet of paperboard takes about five mins to attain ‘equilibrium relative humidity’ (ERH) at the given temperature. Equilibrium relative humidity is the condition at which the paperboard neither loses nor gains any more moisture content from the atmosphere. But paperboards are brought to press room as pile. Conditioning paperboards individually consumes more time. Hence it is better to make the entire pile to get acclimatized to the press room conditions at once.
With paper in a pile taking–up or giving-off water vapour can take place only at the edges of the sheets (from the front side of the pile), water molecules have to distribute from there over the entire paper volume by diffusion. This proceeds slowly, so it takes considerable time until the temperature in the entire pile has adapted to that of the room (Table 6) and the respective moisture value has been reached.