First, the storage of raw materials

In order to ensure normal production, a certain amount of raw material reserves are needed in the factory. The storage of raw materials is determined by factors such as production scale, raw material supply and transportation conditions, and usually should be stored for at least one month. In order to establish reasonable process conditions, ensure the quality of the products and increase the productivity of the equipment, different raw materials must be stacked separately.

The material storage site should be dry, flat, and have good drainage conditions. In order to prevent fire and ensure good ventilation and drying of raw materials, it is also necessary to consider the convenience of loading and unloading work, and a certain interval must be left between the stacking of raw materials.

The size of the stockyard pile depends on the mode of handling. The piles that are manually handled are generally at a height of 2. 0 - 2. 5m. If the site is restricted, it can also be as high as 6-7m; by mechanical handling, it is allowed to reach more than 10m.

For the stacking of raw materials, it is necessary to keep the moisture content of the raw materials in the range of 10% to 15%. If the water content is too high, it will cause decay and heat, which will lead to spontaneous combustion of the raw materials.

Second, the preparation process

The preparation is the first section of MDF production. The specifications and quality of the raw materials after cutting directly affect the quality of the fibers and products.

Different raw materials should have different preparation processes and corresponding equipment. The preparation process depends on the raw materials produced, the selection of equipment and the requirements for the quality of the preparation. When using firewood and small-diameter wood as raw materials, the more complete preparation section includes the following processes:

Since most of the production of MDF is to use a variety of processing residues and unpeeled materials, it is necessary to eliminate debris. The raw materials are washed with water, and the metal and gravel are removed to extend the service life of the grinding discs and saw blades. Wood chip washing can effectively remove all kinds of sundries and improve the moisture content of the wood chips. However, in the process of washing, the wood chips must absorb water, which will increase the burden of fiber drying and lead to an increase in heat consumption. Therefore, wood chips are washed, and in dry production, there is a tendency to be replaced by dry purification. The principle of this type of equipment is the combination of oscillating screening and gravity separation (referred to as dry cleaning).

Large and medium-sized medium-density fiberboard production lines are generally a variety of raw materials. Therefore, multiple assembly lines are used for the preparation process. The process flow and process layout are shown in Figures 1--10 and Figure 1-11.

When using non-wood fiber as raw material, it is necessary to formulate specific majority according to the different characteristics of various raw materials, and must pay attention to strengthening screening. At the same time, it should also pay attention to installing dust removal facilities and reducing environmental noise to ensure civilized production.

Third, shaving

(1) Requirements for wood chips

The requirement for chip cutting is that the size of the chips is uniform, uniform, and the cuts are smooth and smooth. Wood chip specifications should be 16-30 mm long, 15-25 mm wide, and 3-5 mm thick. The composition of the selected wood chips is shown in Table 1-11.

Nbsp; (2) Factors affecting the quality of wood chips

1. Raw material quality raw materials are best in shape and shape, have a certain moisture content, and no decaying materials. The wood chips thus produced are uniform and uniform, have a high yield, and have less debris. The moisture content of the raw material is not less than 3500-40%, otherwise not only the scrap will increase, but also the power consumption will be high, and the tool life will be shortened.

2. Types of Chipper There are two types of chippers commonly used: disc chippers and drum chippers. Due to the different cutting forces, the types of raw materials suitable for processing are slightly different, and the cutting quality is different, which should be considered comprehensively.

3. The sharpness of the cutter The cutting insert has two kinds of single edge and double edge. Single-edged inserts are prone to chipping; double-edged inserts are suitable for cutting hard and frozen materials. Blades should be replaced and ground frequently to keep the edges sharp. Cutting with a blunt knife not only increases power consumption, but also makes the cutting chips uneven and the amount of debris increases. The grinding angle of the blade should be determined according to the quality of the steel. The hard material should be cut and the angle should be larger to prevent the tool from being damaged. The cutting soft material can be smaller. Under normal circumstances, the flying knife should be changed every 4-8 hours, and the bottom knife should be replaced every 10-20 days.

4. The length and thickness of the blade mounting wood are determined by the mounting position of the blade, so the blade installation requirements are high. Regardless of the disc or drum chipper, all blades should have the same amount of protrusion to maintain the distance between the flying knife and the bottom knife.

The slab of the slabs of the slabs of the slabs of the slabs of the slabs of the slabs of the slabs. The width is naturally broken and depends on the transverse shear strength of the material.

4. Screening and washing (or dry cleaning) of wood chips

(1) Screening

The chips in the wood chips after cutting should be screened out, and the large wood blocks must be separated and broken, otherwise the quality of fiber separation will be affected.

The wood chip screening machine has a flat sieve, a circular sieve and a shake sieve. The circular sieve is used less because of its low efficiency and large footprint. At present, small and medium-sized factories use flat screens. The efficiency of the shaker is high and the output is large, which is suitable for the factory with high output (Figure 1-12). The piece of wood falls onto the sieve cap and is evenly distributed through the guide plate.

It spreads to the edge and then falls onto the polygonal sieve plate. The sieve plate vibrates in a plane with low speed and large amplitude. The bottom of the sieve has three outlets, which are respectively finely divided, qualified wood chips and large pieces of wood chips. The sieve plate amplitude is 40 - 50mm. The area of ​​the sieve plate depends on the output. The machine is simple, sturdy, easy to operate, easy to install, and has good mechanical properties.

(2) Wood chips Washing (or dry cleaning) The proper moisture content of the raw materials allows the fibers to be smoothly fed when separated, and the moisture can absorb the frictional heat during grinding and reduce the softening temperature of the raw materials. The sediment in the raw material affects the quality of the product. The inclusion of stones and metal may damage the grinding disc. Therefore, before the wood chips enter the hot mill, a wood washing process should be added to replace the magnetic separation and reduce the screening burden.

Wood chip washing is generally carried out using a flushing water washing system. The wood chip washing machine is shown in Figure 1-13. After the wood chips enter the debris separator, they are immersed in water under the stirring of the drum mixing blades, and the water flowing from the lower water inlet is stirred to make the stones, metal fragments and wood chips in the wood chips. The sediment is separated and falls into the pipe at the bottom of the agitator. The lower part is periodically discharged with two alternately opened and closed valves. The washed wood chips are transported by a tilting screw conveyor, and the excess moisture is discharged by the inclined spiral perforated casing. The water circulation system consists of a water supply pipe, a pump and a slag remover, a float relay, and a control valve. The water flows into the water pump from the bottom of the inclined screw conveyor. The liquid level is controlled by the float relay. The water level is insufficient to open the water control valve. The water volume exceeds the small water valve on one hand and the overflow port is discharged. The water is pumped to the slag remover, and the slag remover continuously discharges the slag, and the clean water discharged from the slag is continuously recycled.

The wood sheet obtained by the wood chip washing machine has a water content of about 56%, and the flow rate of the back flushing water in the pipe under the drum stirrer does not exceed 0.8 m/s to ensure the separation effect of the wood chips and the sundries.

Dry cleaning is carried out using a wind washing machine (Fig. 1-14). The wood chip enters from the rear of the wind washing machine classification box. Since the classifying box and the oscillator are fixed together, the classifying box and the oscillator are oscillated up and down together, and the pipe from the outlet of the blower is divided into two air flows by the rotary valve device, alternating Blowing the bottom of the classification box, the classification box is used for up and down, front and rear vibration. The wood chips in the classification box are moved up and down and back and forth along with the classification box. The fine sand and small stones in the wood piece pass through the mesh in the classification box, from the front of the classification box. The face is discharged, dust and fine impurities are blown up, and the pipe is passed through the dust hood.

Figure 1-13 wood chip washing machine

1. Drum agitator 2. Drain valve 3. Float relay 4. Relief valve

5. Screw conveyor 6. Slag remover 7 slag valve 8. Pump

Entering the cyclone separator, dust and impurities are separated and discharged by the rotary discharger. The clean air enters the air inlet of the blower from the outlet of the cyclone separator and re-enters the next cycle. The front of the classification box is also equipped with a horizontal classification. A box for removing extra large pieces of wood, and a portion of the qualified chips are re-entered into the classifying box through the circulation device. A magnetic iron is placed on the head of the dust collecting hood to remove wires, caps, small iron pieces and the like from the wood chips. After the above steps, the wood chips basically achieve the purpose of cleaning.

5. Peeling and peeling

The bark content accounts for 600^20% of the trees, with an average of about 10%. The sheet metal material accounts for more than 10%, and the small diameter material and the branch are 1300^-28%, with an average of more than 20%. The fiber content in the bark is extremely low. Some of the fiber is crushed into pieces during the fiber separation process, and some are ground into powder, which affects the appearance of the product on the surface of the board and affects product strength and water resistance in the board.

When the bark content in the raw material exceeds 20%, the raw material peeling or the peeling treatment of the wood chips is generally required. It is more difficult to peel the small diameter wood and the branches and stems. Figure 1-15 shows

Foreign tool rotary peeling device for small-scale wood peeling. The peeling diameter of the cutter can be adjusted according to the log diameter level, so that it is suitable for peeling logs in a certain diameter range, and the effect is better.

Zhiya, small shrub strips and processing residues cannot mechanically reduce the bark content in the raw material. In addition to the exfoliated bark crumb particles separated by screening and washing, a large amount remains attached to the wood chips. Foreign countries have the peeling treatment process of steamed compressed wood chips as shown in Figure 1-16, which can improve the peeling effect of wood chips.

Figure 1-17 shows the structure of the wood chip compression and peeling device.

The skinned wood chips are input between the two pressure rollers by the holding device, and are compressed, so that the bark is detached from the wood chips to achieve the separation effect.

6. Transportation and storage of wood chips

The transport equipment for wood chips includes belt conveyors, scraper conveyors, bucket elevators and pneumatic conveyors. The first two are often used.

The angle of the belt conveyor is generally not more than 140-160. Otherwise, special belts are used to prevent the chips from sliding down. The inclination of the scraper conveyor should also be controlled below 450. When the inclination angle is 450^-900, a bucket elevator can be used.

The pneumatic conveying can be carried out in any direction, and the conveying density is 0. 35 -o. 5kg o The speed of the transporting wood chips is generally 15^-25m/s, and the air speed excluding the cyclone separator is 1 5-, -2m/s.

In order to ensure the continuous supply and preheating (or humidity control) of the chips, the chips should have a storage capacity of 1-2 days. The wood grain silos have metal structures, reinforced concrete structures and wooden structures.

Wood chip bins are divided into vertical and horizontal. Usually used in a vertical form, the upper part is charged and the lower part is discharged. The size of the discharge port shall not be less than 30ommX 300mm o When the moisture content of the wood chip is too large, freezing and poor screening, the discharge resistance will increase. In order to avoid the phenomenon of “bridge” in the silo, the inclination angle of the silo wall must be greater than the natural slope angle of the wood chip, which should be 550^-600. In addition, the outlet should be smooth to facilitate uniform feeding of the vibrator.

Seven, several problems in the treatment of non-wood materials

Non-wood materials are available in a wide variety of sources, resources and low prices. The use of non-wood raw materials for the production of medium density fiberboard, the process is simple, low energy consumption, good economic efficiency, the quality of the plate can meet the standard requirements. Several problems with the processing of non-wood raw materials are described below.

(1) In addition to the core material technology

The cotton stalk, the stalk, the bagasse, etc. in the non-wood raw materials have a soft pith structure, for example, the medulla content of the stalk of the cotton stalk is about 7.4%, the stalk is about 12%, and the bagasse About 10% of these compositions have strong absorbing adhesive properties, affecting interface cementation strength and reducing the physical and mechanical properties of the board. At the same time, the presence of the pith core also affects the water resistance of the board, resulting in a defect of high water absorption and thickness expansion. Therefore, the use of such raw materials cannot be considered without the removal of the pith. The removal of the pith core can be adjusted in the raw material preparation, and the wind speed and wind pressure can be adjusted and discarded in the cyclone separator. If the moisture content of the raw material is too low, the wood part may be broken and discarded too much. For the non-wood raw material containing the core, the raw material needs to be humidified, and the water content should be controlled at 250o~35%. If the water content is too large, the pulp core is easy to absorb moisture, which affects the effect of cyclone separation.

(2) In addition to the skin problem

The outer layer of cotton stalk and hemp stalk has a soft outer skin layer, which should be removed theoretically. If it is not removed, the following defects will be caused in the production: 1 The fiber bundle formed by the skin when the air is sent will wrap around the fan blade and affect the movement of the fan. Balance, causing fan noise and strong vibration. At the same time, the fiber mass blocks the air outlet of the fan and affects the air supply of the material. 2 The raw material without the skin is separated. After the fiber is separated, the retained leather fiber is long fiber or fiber bundle. During the sizing process, it is easy to curl and agglomerate, resulting in uneven sizing; 3 long (skin) fibers curled into a group, which are not easy to loose in the paving, resulting in uneven paving, and easy to form a plaque on the surface of the board. . Peeling process: cotton stalks are first cut into 4 - 8cm long sections, and the moisture content is controlled at 35 0o-50% during cutting. The cut cotton stalks are screened to remove some cotton stalks. About 25%. Then, the cotton stalk section is dried to a moisture content of 5% to 10%, and then, after being crushed by a crusher, a part of the ramie fiber is removed by air separation, and the remaining amount is left in the raw material, which does not produce a large production process. influences.

(3) Silica in raw materials

Some raw materials contain silica (SiO 2 ), such as the composition of bamboo. . 1% - 0. 2%, there is also a certain amount in the cane skin. The non-polar surface structure formed by silica affects the adsorption of the adhesive and the formation of hydrogen bonds, thereby affecting the bonding force between the fibers. Especially in the production of medium-density fiberboard, the tensile strength of the medium-density fiberboard is insufficient. When the product is subjected to secondary processing such as veneer decoration, it is easy to fall off. Therefore, when processing such raw materials, the surface area should be increased as much as possible to strengthen the fiber separation. To make it more conducive to the adsorption of adhesives.

(4) Dust removal problem

Due to the different harvesting and cutting methods, non-wood raw materials inevitably cause a large amount of dust and dust, which not only deteriorates the working environment of the preparation section, but also affects human health by dust and dust, and brings into the fiber to affect the effective use of the adhesive. There are many current methods for dust removal technology. Washing and strengthening screening have a good effect on removing dust and dust from raw materials.

In addition, all the raw materials can be used for pipeline transportation, and the dust collection measures in the sawing and paving sections can be strengthened, but this must meet the air exhaust volume required by the equipment, and ensure that the air inlet has sufficient wind speed in the cyclone separation system or sanding. The separation system should use an efficient dust collector.

(5) Storage of raw materials

Another commonality of non-wood raw materials is that the supply period is short and the seasonality is strong. It is only possible to maintain the annual production by storing raw materials. There are two problems: First, from the economic benefit, the storage of raw materials is large, which is not conducive to Capital turnover; Second, a large amount of storage of raw materials, anti-mildew, anti-corrosion and fire-proof work tasks. In terms of production process, the mechanical properties of the sheet are reduced with the storage time of the raw materials; the raw materials such as cotton stalk and hemp stalk are porous materials (porosity up to 80%), huge specific surface area, and are susceptible to natural storage during storage. Aging effects, such as photodegradation, reduce or weaken the activity of effective polar groups on the surface layer of the raw material, which is detrimental to the formation of hydrogen bonds and other binding forces and affects the bonding force.

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