Moisture Control of Corrugated Boards
During production of corrugated boards, many have met the problems caused by inaccurate control of moisture, whether by higher (leading to collapse, flute-exposure) or lower moisture (leading to board breaking). Moisture control is a crucial issue in production and procession of corrugated boards. Too much water-content would cause boards to soften and submit to collapse and flute exposure after being produced. However, not enough water would make them to be crisp and easy to breakdown. Therefore, only by strict control of moisture, can we produce high-quality boards and boxes, but moisture control is a complex process affected by many elements.
In order to solve the problem, we shall：Control quality by checking moisture of in-factory base paper and demanding conforming base paper from suppliers. Store base paper in separate warehouses with temperatures within 15℃ and 20℃, and relative humidity within 30% and 40%。 Don’t expose them in the open air, otherwise the moisture of base paper would change, and moisture in the ends and middle part would vary from one another, which would make it harder for further procession. Temperature is a decisive element influencing the quality of corrugated boards. Moisture of base paper is affected by temperature which is also important for solidification of paste liquid. Therefore, a production line of corrugated board is usually equipped with many pre-heaters and drying plates which are used to adjust the water content of base paper and single-face board, solidify paste liquid, and to paste each layer of corrugated board. When machine speed is above 100m/min, the solidification temperature of paste liquid usually ranges from 160℃ to 180℃. That is to say, pressure of saturated steam of boilers shall reach 0.9MPa to 7.4MPa. A conforming corrugated board is processed through several equipments, each of which has different impact on the moisture content of the board during its forming process. See the following for detail. According to analysis of tensile force yielded while single-face corrugated board being shaped in single-face machine, we know that the water content entered into single-face base paper within 9% and 12% is best for board shaping. Therefore, we shall make full use of preheat roller, pre-heaters in the front and back of single-face machine and sprayer to adjust base paper moisture.
Solution: when preheat roller reaches required temperature and the moisture of base paper is too high, adjust the flexible paper-lead roller of preheat roller to increase wrap angle and area of preheat, and reduce moisture of base paper till reaching the standard. When preheat roller reaches its maximum preheat area, but moisture of base paper is still too high, you could lower down the machine speed.
If water content is too high and temperature of preheat roller can not meet the requirement, you shall increase the air pressure of boiler to obtain saturated steam with pressure within 0.9MPa and 1.4MPa.
If moisture is too low, you have to adjust the flexible paper-lead roller to cut preheat area. Meanwhile, use the sprayer of preheat roller and single-face machine to spay water onto base paper, so as to raise its moisture up to standard. You may also increase machine speed. Moisture and temperature of base paper could be measured by moisture meter and thermometer or manometer. Adjust them according to the measured result. Except meter measuring, we could also apply a conventional method. According to our experience, if single-face corrugated board on the overpass conveyor is in the shape of ∞, it means that the moisture of base paper and corrugated paper is appropriate. When there is no thermometer, you could spit onto the preheater. If saliva jumps high, the temperature has exceeded 150℃ and production could be started.
Temperature of the drier has big influence on moisture of corrugated board, for the drier’s heating plates are generally divided into three groups, including 4, 5 and 6 heating plates separately. Every group of heating plates is equipped with control valve which is used to adjust steam pressure to change temperature of heating plates and produce high-quality corrugated boards.
Solution: Single-face boards are kept on overpass for different duration, so their moisture would change before entering the drier. Moisture would increase especially after applying paste fluid in the single-face machine. Therefore we have to pre-heat single-face boards in the triplex preheat machine. During the preheating process, we shall also adjust the flexible paper-lead roller to change preheat area and control water content. After entering the drier, first ensure pasting of paste liquid, and then adjust reducing-valve to get proper temperature and water content. The length of cooling section of drier also has influence on board moisture. If the section is too short, water content can not be emitted in time, while blower may be used for help.
When temperature fails to be satisfactory, you could change machine speed. Lower speed when water content is high, and increase speed when water content is low.
Don’t stop the machine when drier is on operation. In case that the single-face machine ahead needs to change paper roller, it could run faster before the change. Accumulate more single-face board on the overpass. When the drier receives paper-changing command, it could reduce speed. If the drier stops, moisture of boards in it would all evaporate thus all boards are written off. Except for that whether saturated steam supplied by boiler reaches required pressure could affect the temperature of board production line, the drainage device of steam pipe system also accounts heavy, because after passing through all preheaters, base paper and single-face board have to loss a lot of heat. A proportion of saturated steam would change into condensed water which shall be drained in time, otherwise the preheat effect of preheaters would be affected.
Solution: when use preheat roller with rotary steam head, if temperature paper or temperature at two ends differs over 5℃, it indicates that the suction tube is damaged or is wrongly placed. Unpack it for check and repair. Outlet volume of the drainage valve is twice to thrice of condensed water. Change the drainage valve if drainage capacity is not enough. Check the discharge opening of drainage valve weekly and clean it in case of blockage. The paste fluid quality, water volume and consistency have great influence on moisture of boards.
If paste fluid contains large amounts of water, it would be sparse and permeate quickly, and moisture of board is high, in converse, water content is low.
Solution: strictly follow the water volume, mix ratio and mixing duration in paste making. Carefully control the viscosity of paste fluid. The mixing time for normal single-face machine is between 50 and 60 seconds, and for double-face pasting machine is about 100 seconds. What’s more, double-face pasting machine shall use fresh paste fluid made at the same day. Only good paste fluid can produce good board.
Pasting volume can affects water content of board too. When it reaches the maximum amount, moisture is high, in converse, moisture is low. Solution: for single-face machine, adjust the seam between gluing roller and doctor roller, and between gluing roller and main corrugated roller, being 0.2mm~0.25mm and 0.1mm~0.2mm respectively in most cases. The measuring method accords with that of feeler gauge. Two ends shall be kept in conformity.
In case that the seam between gluing roller and doctor roller of the double-face gluing machine is between 0.2mm and 0.25mm, the seam width between pressing roller and gluing roller shall be 0.05~0.1mm less than the height of corrugated board passed through single-face machine. Measurement shall be conducted by feeler gauge, with two end data in conformity. Width of glue line of corrugated board produced both by single-face machine and double-face gluing machine shall be between 1.5mm and 2mm.
If there is any damage or alien stuff on the gluing roller, gluing would fail to be even-distributed and thus the water content distribution would also be affected. Solution: change gluing roller if there is any scrape or damage to the roller. Stop the machine and clean the roller if alien stuff is found. Wash all gluing rollers before going off work and clean alien stuff by brush.
If the conveying belt of drier has weak ventilation capacity, or the belt is sprayed with water randomly, moisture of board would be high. Solution: analyze the causes for bad ventilation. Stop the machine for clean if it is stuck with waste paper. Change new belt if the old one has served for a long time. Don’t spray water randomly to the belt. If the conveying belt of each single-face machine on the overpass has accumulated too many single-face boards or for too much time, much of the water would evaporate and it’s easy to take too much moisture away from boards.
Solution: all operation stations on the whole line shall contact with one another from time to time. Operation speed shall be in conformity. The accumulated amount of single-face boards on the conveying belt is recommended to be as long as 2/3 or 3/4 of the belt.
Moisture is also influenced by climate and environment. In hot and rainy season, the relative humidity is high, while in winter, the fluidness of paste is poor and pasting volume is small. All these would influence the water content of boards.
Solution: in hot and rainy season, increase heating area, decrease spraying and water content of paste; raise temperature, fully use preheat rollers to adjust moisture. In conclusion, there are many elements that can affect the moisture of corrugated boards. In order to produce conforming corrugated boards, operators shall carefully observe and control every process of the production.
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