Fiberboard production technology

Raw Material

The fundamental component of fiberboard is an individual wood fiber or a bundle of fibers. The raw materials for fiber production primarily come from forest residues like branches, tree tops, and small diameter logs, as well as waste from wood processing, including offcuts, shavings, and sawdust. Additionally, waste from the chemical processing of forest products, such as silicon and hydrolyzed residues, along with other plant stems, can be utilized to create fibers. Softwoods have a higher fiber content compared to hardwoods, with their fibers being 30 to 50% longer. If hardwood is used, it must undergo pretreatment, either by mixing it with softwood or chemically treating wood chips. Hot water and steam can also be applied. Prior to fiber separation, the raw material is cut into pieces measuring 20 to 30 mm in length, 3 to 5 mm in thickness, and 15 to 25 mm in width using a chipper. If the wood chips are too large, they can be difficult to soften evenly during preheating and refining, leading to lower fiber separation efficiency. Conversely, if the chips are too small, there will be a higher proportion of fragmented fibers, reducing the interlocking ability and ultimately affecting the strength of the fiberboard. The cut wood chips are sent to a silo after screening, recrushing, and washing, preparing them for fiber separation.

Production Process

There are three primary methods: wet, dry, and semi-dry. The wet production process uses water as the carrier for fiber transport. The mechanism involves creating a certain level of strength through the intertwining of fibers, the binding forces between fibers, and the adhesive forces generated between fibers to produce fiberboard. The dry production process uses air as the fiber transport carrier. Fibers are separated using a single separation method. Typically, the resin isn't finely ground, and adhesives are applied. The fibers are dried before the slab is formed, and are usually not heat-treated after hot pressing, similar to the wet method. The semi-dry process also uses airflow, maintaining high moisture levels in the fibers without drying them, and uses little to no adhesive. This method overcomes the main disadvantages of the dry and wet processes while retaining some of their advantages.

Basic Process Content

It involves fiber separation → slurry treatment → slab forming → hot pressing → post-treatment (see picture).

1 Fiber Separation. Also known as pulping, it is the process of breaking down raw materials into fibers. The fiber separation method can be divided into two major categories: mechanical and explosive. Within the mechanical category, there are thermal mechanical, chemical mechanical, and purely mechanical methods. The thermal mechanical method first treats raw materials with hot water or saturated steam to soften or partially dissolve the intercellular layers of the fibers, separating them into fibers through mechanical force under normal or high pressure, followed by fine grinding in a disc refiner (dry fiberboard pulping typically does not involve refinement). The fiber pulp produced by this method has complete fiber shapes, strong interlacing properties, good water drainage, and high yields. The yield of softwood pulp can reach 90-95%. The energy consumption is relatively low; however, after fine grinding, the fiber length decreases, the specific surface area increases, the outer layer and ends become deuterated, the water absorption properties improve, the softness and plasticity increase, and the interlacing properties are excellent. Thus, the thermomechanical method is the primary pulping method used in the domestic and international fiberboard industries. The chemical mechanical method uses a small amount of chemicals, such as caustic soda and sodium sulfite, to pretreat raw materials, damaging or dissolving lignin and hemicellulose to a certain extent, and then separates the fibers through mechanical force. The purely mechanical method directly crushes the fiber raw material into water after soaking it, dividing it into a log refining method and a wood chip refining method, but this method is rarely applied. The explosive method heats raw materials in a high-pressure vessel (4 MPa) for a short time (about 30 seconds) to soften lignin, partially hydrolyze carbohydrates, then raises the vapor pressure to 7-8 MPa, maintaining it for 4 to 5 seconds, and quickly opens the valve to blast the fiber raw material into fluffy fibers or fiber bundles.

2 Slurry Treatment. This refers to waterproofing, reinforcement, fireproofing, and防腐treatment according to the intended use of the product to enhance the performance of the finished product. Hard and semi-hard fiberboard pulps should be treated with paraffin emulsion to improve water resistance, while soft board pulp can use both rosin and paraffin-rosin emulsions. Waterproofing agents can be applied in the slurry tank or a continuous glue box. The reinforcing agent used for reinforcement treatment is water-soluble, can be adsorbed by fibers, and is suitable for the hot pressing or drying process of fiberboard, with hard fiberboard often made using phenolic resin glue. Refractory treatment commonly applies refractory agents such as FeNH4PO4 and MgNH4PO4. Adding pentachlorophenol or pentachlorophenol copper salt to the slurry serves as an antiseptic. The treated slurry is dried or dried for drying; or after adjustment to a concentration, it is directly introduced into a molding machine for wet forming to form a wet slab of a certain specification with preliminary compactness.

Dry production of fiberboard requires a fiber moisture content of 6 to 8% at hot pressing, and a slurry moisture content of 40 to 60% after sizing, so it is necessary to dry before molding. Fiber drying can be performed using two kinds of pipe airflow drying methods: the first stage has a temperature of 250-350°C and a duration of 5-7 seconds; the second stage first has a temperature of 160-180°C, reducing the moisture content to 20%, the second stage temperature is 140-150°C, reducing the moisture content to 6-8%, and the total time of the two stages is about 12 seconds. The drying equipment has three types: straight tube, pulse, and jacketed.

3 Slab Forming has two types: wet forming and dry forming. Soft boards and most hard boards are formed by wet molding; medium-density boards and some hard boards are dry formed.

The low-concentration slurry for wet forming is gradually dehydrated to form a slab. The basic methods include box-frame molding, long-net molding, and round mesh molding. Box-frame molding pumps the slurry with a concentration of about 1% from the slurry pump into a bottomless box frame placed on the pad net, performing vacuum dewatering at the bottom of the box and pressure dehydration at the top of the box frame. This method is mainly used for the production of soft fiberboard. The equipment used for long-net forming is similar to the long-net paper machine in the paper industry. The 1.2~2.0% concentration slurry is copied from the net front box to the long-net, forming the wet slab by self-weight dehydration, vacuum dehydration, and roller press dehydration, with a water content of 65-70%. Rotary net forming is also transplanted from the paper industry. In the production of fiberboard, vacuum single-web type is used. The slurry concentration is 0.75~1.5%. It is adsorbed on the circular net by vacuum working slurry and dehydrated by roller. And controls the thickness of the slab.

Most dry forming uses an airflow forming machine. The dried fibers to which paraffin wax and adhesive (phenolic resin) are applied are fed into the laying head by a gas stream, and the dry fibers are uniformly dropped on the mat to form a slab by the action of the fiber weight and the vacuum box under the mat. Semi-dry forming uses mechanical or airflow forming machines. By the action of mechanical force or airflow, the agglomerated fibers with high water content are dispersed and uniformly dropped to form a wet slab of a gradual structure or a mixed structure. However, it is difficult to completely disperse the wet fiber agglomeration phenomenon by mechanical force or airflow. In actual production, the slab density uniformity is poor, which easily affects product quality. In the early 1970s, successful research on dry fiber static orientation molding in the United States.

The soft fiberboard and the hard fiberboard using the wet forming dry hot pressing process (also known as wet drying method), the slabs are dried. Drying equipment is available in both batch and continuous types. Drying 1 kg of water consumes 1.6 to 1.8 kg of vapor. The final moisture content of the soft fiber slab after drying is from 1 to 3%. When the hard fiberboard is manufactured by the wet-drying method, the moisture content of the slab is not too high, otherwise bubbling is apt to occur during hot pressing.

4 Hot Pressing. The wet process for producing hardboard requires a pressure of 5 MPa and a dry method of 7 MPa. Above this pressure, the bending strength is reduced. The pressure required for the semi-dry method is between 6 MPa. The wet-formed slab is pressed into a hard fiberboard at a pressure of up to 10 MPa. The temperature used in the wet pressing method is 200 to 220 °C. There is no drying stage when dry pressing, the temperature is based on the rapid curing of the adhesive, generally 180 ~ 200 °C; when using hardwood as raw material, the hot pressing temperature can be appropriately increased, up to 260 °C. The semi-dry hot pressing temperature should not exceed 200 °C to prevent the melting of lignin and sugar pyrolysis coking in the slab, so that the strength of the product is significantly reduced. The temperature required to produce a hard board by wet drying is 230 to 250 °C.

During the hot pressing process, a temperature difference may occur between the surface layer and the core layer of the slab, and the temperature difference of the core layer of the medium-density slab having a large thickness may reach 40 to 60 °C, which affects the curing rate of the core resin. Conventional heating and high frequency heating can be used to eliminate temperature differences and shorten the hot pressing cycle.

5 Post Processing. The wet and semi-dry fiberboards are subjected to heat treatment and humidity conditioning after hot pressing, and the dry fiberboards are directly subjected to humidity conditioning without heat treatment. The surface of the medium-density fiberboard needs to be sanded, and the surface of the soft fiberboard sometimes needs to be slotted and punched. When the hard fiberboard is used as the interior wallboard, the surface can be opened with a "V" groove or a striped groove. The surface processing of fiberboard is usually done by two methods of coating and covering (see the surface decoration of wood-based panels). As for the embossing, indentation, and deep indentation of the surface of the rough sawing material, most of them are formed once during the hot pressing of the slab, which is not a reworking range.

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