Medium Densiry Fiberboard hot pressing and post processing
MDF is hot pressed by dry process. The hot pressing process is a physical and chemical process that includes compacting the slab to increase the compactness of the fiber contact surface, raising the temperature to make the internal moisture evaporate, and solidifying the adhesive. The main factors that constitute the strength of MDF are the quality of fiber itself and the bonding of resin. In the process of hot pressing, the resin flows because of heating, dispersively permeates between fibers, and gradually polycondensates and solidifies with the increase of temperature and the extension of time, so as to obtain a stable glue between fibers moisture resistant mdf cut to size.
1.1 Relationship between factors and product quality of MDF Multi-layer hot pressing Medium density fiberboard temperature, time, pressure, slab moisture content and compression speed in hot pressing MEDIUM density fiberboard, each mdf thickness sizes.
Factors are closely related to product quality. In the formulation of hot pressing process must be based on the physical and chemical properties of different raw materials, appropriate selection of technological conditions.
1.1.1 Relationship between hot pressing process factors and product quality
(1), hot pressing temperature: the role of hot pressing temperature is to make the fiber plastic increase and soften after heating, easy to compress, slab water fractional evaporation; Accelerate the condensation and curing of adhesive. The selection of hot pressing temperature mainly depends on the performance of the board, the type of adhesive and the production efficiency.The hot pressing temperature is usually 160~180℃ for urea-formaldehyde resin adhesive and 185~195℃ for phenolic resin adhesive. The degree of temperature selection depends on other factors such as raw materials, tree species, slab moisture content, slab thickness, heating method, time and pressure. (2) hot pressing pressure: the role of pressure is to overcome the internal friction or rebound force of the slab, and compress the slab to the thickness specification to ensure the thickness and density of the product.The general hot pressing pressure in 2.5~ 3.5mpa, high strength products or in the case of hard wood fiber materials and other special circumstances, the pressure can reach 5.0~ 5.5 MPa.
(3) hot pressing time: when the slab is hot pressing, no matter the temperature and pressure, it takes a certain amount of time melamine sheets sizes bunnings.
Can ensure the heat conduction and pressure transfer, in order to achieve adhesive curing, the system of predetermined density and mechanical properties
Products that can be made. The determination of hot pressing time is related to adhesive properties, fiber quality, water content of slab, hot pressing temperature, pressure, heating method and slab thickness.
Heating time is generally expressed as the time required by 1mm finished plate thickness. Using hot pressing plate contact heating square, 1mm heating time is 18~22s.
(4) The moisture content of the slab: the moisture in the slab increases the plasticity and thermal conductivity of the fiber in the process of hot pressing, and promotes the resinization of lignin mdf board sizes in cm.
And reduce the melting point, so the appropriate slab moisture content, the quality of the board is guaranteed, the general slab moisture content is 8 ~ 12%.
(5) pressurizing speed: under the same hot pressing pressure, different pressurizing speed affects the performance and section density of the board the cloth. High compression speed, high static bending strength of the plate, reduced internal cohesion. The compression speed is slow, the static flexural strength decreases, the precuring layer thickens, and the internal bonding force increases supawood board sizes. But too slow will make the whole board loose less than quality standards.
1.1.2 Hot pressing Curve The change of hot pressing process parameters in the hot pressing process can be represented by the hot pressing curve. The hot pressing curve is the comprehensive implementation of various technological factors in the hot pressing process. Reasonable thermal pressure curve is to ensure production
The key to product quality. To develop a correct thermocompression curve, the following steps must be carried out, and the completion of the following steps constitutes a thermocompression cycle.
Start the press when the slab is loaded into the hot press.
(1) The hot pressing plate is rapidly promoted.
(2) When the surface of each slab is close to the upper platen, slowly close.
(3) The slab is accelerated after the pressing plate is closed.
(4) According to the thickness of the pressing plate, with the rise of pressure should be automatically controlled to adjust the pressure speed, in order to achieve the uniformity of the thickness and density of the plate.
(5) After the slab is compressed to the thickness limit, it is converted into the position control of linear position controller (such as LVDT, etc.) to maintain the heating time required by the process.
(6) After the time to hold, the pressure slowly depressurized.
(7) The hot pressing plate is opened slowly.
(8) The hot pressing plate drops rapidly.
(9) When the hot pressure plate falls close to the lowest position, it slowly falls to the bottom — a soft landing.
(10) End of hot pressing cycle, and prepare for the next cycle.
1.2 structure characteristics of xx fiberboard multi-layer hot press 6mm mdf cut to size.
1. The closing time of the pressing plate of the fast closing multi-layer hot press under load is generally about 10s.
But for medium density fiberboard production, because the hot pressing plate spacing is 1-3 times larger than the production of hard board, the cylinder stroke increases when the pressing plate is closed, and the amount of oil needed to supply in a short time increases.
2. variable speed pressure system after the press closed pressure stage, in order to meet the requirements of pressure speed change, hydraulic system equipment basically take two methods, one is the use of large flow variable pump through the servo mechanism, flexible stepless adjustment of the supply cylinder oil pressure and flow, to adapt to the parameters set by the process; Two is the use of multiple putong vane pump, through the valve group control, combined into several grades of pressure and flow, and adapt to adjust the requirements of the process parameters.
3. Thickness control It is difficult to control the thickness of MDF produced by multi-layer press. General press must be equipped with thickness gauge, which is the basic method to ensure the thickness tolerance of products. Thickness gauge width 40~50mm.
The thickness shall be added according to the finished product thickness 1.7~2.0mm, length according to the length of the platen section installation, material is low carbon steel or aluminum alloy strip,installed in both sides of the hot pressing plate under each layer.
Install the thickness gauge of the hot press, between the top and bottom two hot pressing plate and the upper roof, laminated mdf cut to size movable plate platform respectively, to pad a group of hard insulation strip, each (thickness × width) 20mm×60mm, mdf cut to size and delivered longitudinal arrangement, edge part of a hot pressing plate edge alignment, the rest evenly distributed, material heat resistance 250℃, pressure resistance 200MPa, Hard heat insulation between the soft insulation to fill. The hot press with thickness gauge, under any circumstances can not be no load or overpressure test, so as not to cause deformation of the pressure plate.
4. The hot pressing plate is made of the whole hot tied steel plate, the material is SM grade, and after careful ultrasonic testing. The thickness is about 100~127±1mm; The width is more than 300mm larger than the slab size. The diameter of the steam channel is φ 30~50mm, the best hole is longitudinal, reduce the loop elbow, to reduce the steam pressure loss.
Surface roughness, parallelism, flatness and temperature difference should be in line with the national standard of hot press GB6856-6857-86 class 0 standard. The processed hot pressing plate shall be subjected to a pressure test of 300MPa.
5. Simultaneous closure device At present, the simultaneous closure device of hot pressing plate of MDF multi-layer hot press is mostly applied to the lever type with spring compensation mdf cut to size and delivered.
1.3 characteristics of continuous flat pressing continuous flat pressing hot pressing production of XX fiberboard:
(1). High product quality. The surface of the plate is smooth and the texture is fine. The density gradient distribution of the plate can be adjusted reasonably according to the needs of the use.
(2). product specifications. After the slab is pressed into the plate by the continuous press, the length range of sawing is relatively large, and the width of the plate can be cut out of a variety of sizes under the condition of the maximum size of the press. Thickness can be in 2.5~35 mm.
(3). Low single consumption of wood. The precured layer on the surface of the sheet is extremely thin and has a small thickness tolerance, resulting in less sanding. The general sanding amount on both sides is 0.4 ~ 1.0 mm.
(4). Production continuity. Using continuous flat pressing hot press, really realize continuous production, effectively lift high stability of product quality and productivity.
(5). thickness tolerance is small. Because the continuous flat hot pressing machine not only in the longitudinal direction of each section of the pressure can be adjusted, and the transverse pressure along the hot pressing plate can also be precisely fine-tuned, to ensure that the vertical and horizontal thickness deviation of the plate is very small. And in the production process, the thickness of the plate can be adjusted quickly by adjusting the gap of the steel belt in the pressure holding section, which can be adjusted faster than the multi-layer press. Therefore, sheet thickness size is very accurate, general thickness tolerance is less than ± 0.15mm mdf board sizes in inches.
(6). Save energy. In the process of production, due to the continuous feeding steel strip slab, always contact press does not frequently open closed, to reduce heat loss, no pressure system with large fluctuation of peak pressure, make the system pressure is nearly constant, plus hot press different extents of pressure, temperature control individually, thus saving electricity province hot, installed capacity is small.
(7).Civil investment province. High plant with deep pit without multi-layer press can reduce the plant height by 35%.
(8). equipment a large investment. Because the continuous press design and manufacture difficult, material variety, high technical content, so the price is more expensive.
(9).Good comprehensive benefits. Although the continuous flat hot pressing machine is expensive, but because of the production of good product quality, high production efficiency, energy saving, material saving and a series of advantages, for manufacturers to create a good economic benefit. So continuous flat hot press has a good market, in recent years has been rapid development.
1.4 Medium Density fiberboard Continuous flat pressing Process In the whole hot pressing process, the slab is always under the action of a given temperature and pressure, and the pressing plate is continuous.
Heat transfer to the slab, water vaporization, exhaust and adhesive curing of a series of chemical and physical changes, so that the organic fiber together.
Hot pressing is one of the most important processes in MDF production. The first step is to choose the best pressure curve. It directly affects the density gradient distribution of hot-pressed section thickness control and physical and mechanical properties. The hot pressing curve mainly considers the plate type, raw material, fiber screen value, slab moisture content, sectional density distribution requirements and other factors.
Continuous flat pressing hot pressing process can be divided into preheating stage and main hot pressing stage.
1. Preheating and pressing stage:
a. Rapid pressing stage. After the slab enters the hot press with the steel belt, the gap between the steel belt becomes smaller to speed up the slab is compressed to a certain thickness and the pressure rises rapidly to the maximum pressure. At the same time, the surface temperature of the slab rises rapidly, and heat transfer to the slab, forming a large temperature gradient. Under the joint action of slab rebound, platen pressure and temperature gradient, the hard layer of plate surface is formed rapidly.
B. Decompression section. After the slab is compressed to a certain extent and the pressure rises to the maximum value, the pressure begins to decrease gradually in order to reduce the hard layer thickness of the plate surface and make the sectional density distribution more reasonable. At the same time, it is beneficial to the discharge of water inside the slab. The speed of decompression directly affects the thickness of the hard layer of the board, generally thick plate decompression speed is faster.
2. Main hot pressing stage:
The main hot pressing stage is about 75% of the total length of the compressor. Product thickness and glue curing is mainly finished in this stage Into.
A, low pressure section (also known as exhaust section). Maintain low pressure in the press for a certain length (a certain time) The force, the low pressure is generally 0.1 ~ 1.0mpa, in order to control the density distribution of the section more reasonable and conducive to the smooth discharge of water inside the slab. As the eduction of moisture and adhesive ceaseless solidify, overcame the rebound of slab, pressure rises gradually again 30mm mdf cut to size. To the state of the specified thickness of the wool plate.
B. Pressure retaining section. When the slab is compressed to a certain position, the press keeps the position for a long time, the gap between the steel belt is unchanged, and the pressure holding pressure is generally 0.2 ~ 1.0mpa, in order to control the thickness, after the basic curing of the internal adhesive of the slab, the wool board will produce dry shrinkage, and its pressure tends to zero. The wool plate moves forward with the steel belt, leaving the holding section, and the hot pressing ends.
1.5 Post-processing of MDF includes cooling, sawing, sanding and tempering
After cooling and hot pressing, the temperature of THE MDF is over 150℃. If the heat is directly piled up jumbo mdf sheet size. It is not easy to send out, the temperature of the initial board will increase, resulting in a darker color of the board. According to the experiment, hot plate stacking after 48 hours, its internal temperature drops to 67℃. This results in the thermal decomposition of urea-formaldehyde resin and the reduction of mechanical strength. Therefore, the hot pressed MEDIUM density fiberboard must be cooled first, and the temperature of the board can be reduced to below 60℃ before stacking or processing. High temperature board fiber softening, sawing, sanding easy to hair, saw edge is not smooth; Plate cooling is conducive to mechanical processing and improve accuracy, because of cooling to reduce the impact of thermal shrinkage deformation, processing specifications and size accuracy, deviation is reduced. In addition, some free formaldehyde still exists after hot pressing in the plate can be excluded.
Saw cutting for multi-layer hot-pressed products, the edge is sawed and the same head is processed into fixed size specification plate; For continuous hot pressing products after sawing, according to the standard size or order requirements sawing into different length (width) specifications.
Sanding removes soft pre-cured layers on the surface, improves the surface hardness of the board, and achieves precise thickness, good flatness and surface finish for direct application or reprocessing.
Quenching and tempering is mainly about the moisture content of the balance plate. After hot pressing, the water content of the plate is low, and the internal and external are different. After cooling, the water content of urea-formaldehyde resin glue products is 2-3% on the surface and 6-7% on the core. After stacking for a certain time, natural conditioning and tempering can achieve a balance moisture content of 5~8% adapted to the environment and climate.