The company began to produce electrical insulation paperboard, is one of the earliest electrical insulation paperboard production enterprises in China, so far has 50 years of production history, the production of electrical insulation paperboard control and treatment of various paper defects accumulated rich experience.
Electrically insulated paper board was originally manufactured by cold pressing. Due to the limitations of production, it has the shortcomings of small specifications, large thickness tolerance and unequal warpage. With the development of electric power industry and the appearance of large capacity transformer, the electric insulated paper board produced by cold pressing method can not meet the needs of electric power industry, so that the imported paper board volume increases year by year. In 1979, the company introduced a large-scale hot-pressed paperboard production line with international advanced level from Germany, and began the production history of electric insulation paperboard by hot-pressing method.
In this production line, the large-scale hot press with length of 4m and width of 2m is the key equipment of production, and its control determines the quality of electrical insulation paperboard production. The hot press is controlled by hot-pressing curve in the whole production process, so it is very important to use and control the hot-pressing curve and its operation method reasonably in the whole production of electrical insulation paperboard.
Formulation of hot pressing curve
The so-called hot-pressing curve is a curve that reflects the pressure change with time at a certain temperature. The hot-pressing curve is the main factor in the process control of the insulation paperboard. The control of the hot-pressing curve requires not only the best function of the equipment, but also the quality of the pressed paperboard.
The hot-pressing process is completed by four stages: plywood boosting, holding pressure, pressure reducing and slow-pressing, pressure reducing and hoisting. These four stages are automatic. Pressure curve is to reflect the pressure in these four stages with the time changes in different stages of the situation. The key of its control lies in the determination of the length of each time period, the pressure and temperature in each time period. The necessity of its formulation is to reflect the best production condition to direct daily production.
Application of initial hot pressing curve
The initial hot pressing curve was formulated in this way, taking 2 mm electrical paper board as an example:
Plywood boost: 30s holding pressure: 43Min
Buck slow pressure: 5min step-down start board: 1min
Maximum gauge pressure: 16 MPa minimum gauge pressure: 13 MPa
After pressure: 8% full time: 49.5min
Analysis of hot pressing curve
1. Change of paperboard during hot pressing process
1> Changes in water content
The moisture content of the wet paperboard entering the hot press is generally between 75% and 8O%. In the process of pressing, the wet paperboard drains about 4O of moisture by mechanical pressure; in the other three processes, the remaining moisture is evaporated by the heat of the hot press. In order to facilitate dehydration, wet paperboard is lined with 40 meshes of copper mesh on both sides, so that the moisture in wet paperboard is squeezed out in liquid state at first, and soon into vaporization mixing state, and finally into full vaporization state. Therefore, in the control of the hot-pressing curve, water passes through three stages: liquid drainage drainage evaporation evaporation escape.
2> Heat consumption
The moisture content of wet cardboard undergoes three stages during hot pressing, and the heat consumption also undergoes three stages: the peak of heat consumption, the transient latency of unstable heat absorption and the stable heat conduction. Wet cardboard drains a lot of heat when it is pressurized, and the temperature of hot-pressed cardboard drops sharply, which consumes a lot of heat and forms a peak of heat consumption. The heat consumption tends to ease when the evaporation stage is changed, and the temperature of the hot-pressed board begins to rise, and the curve goes on to the later stage, that is, when the moisture content of the board is 6%-8%, the temperature of the board is close to the temperature of the hot-pressed board, the temperature difference is very small, and the heat consumption is in a stable stage.
3> The change of fiber
The most obvious phenomenon of wet cardboard during hot pressing is its shrinkage. Under the action of pressure and heating, when the moisture drops to about 67, the wet paper begins to shrink slowly. At this time, the wet cardboard is still in the step-up stage. At this time, the main removal of non-binding water between the fibers, and mainly in the form of liquid extrusion by mechanical force, with the gradual reduction of moisture in wet paper, the surface tension of water towing fiber, resulting in paper shrinkage, but the shrinkage rate is small. When the moisture content drops to 42%, the wet cardboard basically enters the stage of keeping pressure. At this time, the moisture begins to evaporate a lot. What evaporates is the binding water inside the cell wall. The fiber itself begins to shrink radially, which causes the shrinkage rate of the cardboard to rise sharply until the moisture content drops to about 15%, then the shrinkage slows down again. The most important meaning of shrinkage for paper sheet is that the bonding force between fibers is formed, that is, hydrogen bond is formed, so that the board has a variety of physical strength, at the same time, the paper sheet tightness and elongation are improved.
2. The influence factors of hot pressing and their coordination
Temperature, pressure and time are three indispensable factors in the control of hot-pressing curve, and they must be coordinated and adapted to each other in the control of the whole hot-pressing curve.
At the beginning of the curve, it is more important to control the pressure by the mechanical pressure during the dehydration period. The pressure is determined by the thickness, tightness and strength of the board. Proper pressure is conducive to the dehydration of the board, and it is also an important factor to form the best physical properties of the board.
Temperature of hot-pressed plate is an important condition that determines the hot-pressing effect and hot-pressing period of hot-pressed plate. Although there is no visual display on the graph of hot-pressing curve, its function and pressure are controlled in the pressure curve.