Fiberboard production technology

Raw Material

The fundamental building block of fiberboard consists of individual wood fibers or bundles of fibers. The primary sources of these fibers include residues from forest harvesting like branches, tree tops, and smaller diameter materials, along with waste from wood processing such as sawdust, shavings, and trimmings. Additionally, byproducts from the chemical processing of forest products (like silicone and hydrolyzed residues) and other plant stems can also be utilized for fiber production. Softwood fibers tend to have a higher content compared to hardwood, with their lengths being approximately 30 to 50% longer than those of broadleaf species. If hardwood is used, it often requires preliminary treatment—either mixing it with softwood or subjecting it to chemical treatments such as hot water or steam. Wood chips, which are typically 20 to 30 mm in size, 3 to 5 mm thick, and 15 to 25 mm wide, are prepared by a chipper before fiber separation. These chips need to be cut into appropriate sizes to ensure they soften evenly during preheating and refining processes, thus achieving better fiber separation. After being screened, re-crushed, and washed, the chips are sent to storage silos in preparation for fiber separation.

Production Process

Fiberboard production can follow either a wet, dry, or semi-dry method. In the wet process, water serves as the carrier for transporting fibers, creating a certain level of strength through fiber entanglement, surface friction, and bonding forces. The dry process uses air as the fiber transport medium and involves preparing fibers via a single separation technique. Resins aren’t finely ground but are instead applied after the fibers are dried. Post-hot pressing, the board isn't usually subjected to further heat treatment, similar to the wet method. The semi-dry process forms fibers through airflow while maintaining high moisture levels without drying, either without or with minimal use of adhesives. This method combines the advantages of both dry and wet processes while overcoming their major drawbacks.

Basic Process Content

The process includes fiber separation → slurry treatment → slab forming → hot pressing → post-treatment (see image).

1 Fiber Separation. Known as pulping, this step separates raw materials into fibers. Methods include mechanical and explosive techniques. Mechanical methods further divide into thermal mechanical, chemical mechanical, and pure mechanical approaches. Thermal mechanical treatment first softens or partially dissolves intercellular layers using hot water or steam before mechanically separating fibers under normal or high pressure, followed by fine grinding through a disc refiner. Dry fiberboard pulping doesn’t involve refinement. Fibers produced by this method maintain complete shapes, offer strong interlacing, excellent drainage, and high yields, reaching up to 90-95% for softwood. Power consumption is low; however, fine grinding shortens fibers, increases specific surface areas, deuterates the outer layers and ends, enhances water-swelling properties, boosts softness and plasticity, and improves interlacing. Thus, the thermal mechanical method remains widely adopted globally. Chemical mechanical treatment employs small amounts of chemicals like caustic soda or sodium sulfite to damage or dissolve lignin and hemicellulose before mechanical separation. Pure mechanical methods directly pulverize soaked fiber materials into water, categorized by raw material shape into log refining and chip refining, though less commonly applied. Explosive methods involve heating raw materials in high-pressure vessels (4 MPa) for around 30 seconds to soften lignin and partially hydrolyze carbohydrates, then raising vapor pressure to 7-8 MPa for 4-5 seconds before rapidly opening valves to blast fibers into floc fibers or bundles.

2 Slurry Treatment. This involves enhancing product performance according to intended use by applying waterproofing, reinforcement, fire resistance, and防腐 treatments. Hard and semi-hard fiberboard pulps receive paraffin emulsion treatments for improved water resistance, whereas softboard pulps can use both rosin and paraffin-rosin emulsions. Waterproofing agents can be applied in slurry tanks or continuous glue boxes. Reinforcement agents, soluble in water and adsorbable by fibers, adapt to fiberboard hot pressing or drying processes; hard fiberboards often utilize phenolic resin glue. Fire retardant treatments frequently incorporate agents like FeNH4PO4 and MgNH4PO4. Adding pentachlorophenol or pentachlorophenol copper salts to the slurry serves as an antimicrobial agent. Treated slurries are dried or adjusted for concentration and directly introduced into a molding machine for wet forming to create slabs with specific dimensions and initial compactness.

Dry fiberboard production requires a 6 to 8% moisture content at hot pressing and a 40 to 60% moisture content after sizing, necessitating prior drying. Fiber drying occurs via two types of airflow methods: first-stage temperatures range from 250-350°C for 5-7 seconds, reducing moisture to 20%; second-stage temperatures reach 140-150°C, lowering moisture to 6-8%, taking about 12 seconds in total. Drying equipment comes in straight tube, pulse, and casing types.

3 Slab Forming. Wet and dry forming are two main types. Softboards and most hardboards employ wet forming; medium-density boards and some hardboards utilize dry forming.

Low-concentration slurries for wet forming dehydrate progressively to form slabs. Methods include box-frame, long-net, and rotary-net forming. Box-frame forming pumps a 1% concentration slurry into a bottomless box frame on a wire mesh, using vacuum dehydration at the bottom and pressure at the top. This primarily supports soft fiberboard production. Long-net forming machinery resembles paper industry long-net paper machines. A 1.2-2.0% concentration slurry is transferred to the long-net via a front box, forming a wet slab through self-weight, vacuum, and roller press dehydration, leaving a water content of 65-70%. Rotary-net forming similarly derives from paper technology, employing vacuum single-web type setups with a slurry concentration of 0.75-1.5%. The slurry adheres to the circular net by vacuum suction, dehydrates via rollers, and controls slab thickness.

Most dry forming uses airflow forming machines. Paraffin wax and adhesive-treated dried fibers are fed into the laying head via a gas stream, uniformly dropping dry fibers onto mats to form slabs through fiber weight and vacuum box action below. Semi-dry forming employs mechanical or airflow machines, dispersing high-moisture aggregated fibers by mechanical force or airflow to create wet slabs with gradual or mixed structures. However, fully dispersing wet fiber aggregations proves challenging, leading to poor slab density uniformity and affecting product quality. Static orientation molding of dry fibers achieved success in the U.S. in the early 1970s.

Soft fiberboards and hard fiberboards using the wet forming dry hot pressing process (also known as wet-drying method) dry slabs. Drying equipment is available in batch and continuous types. Drying 1 kg of water consumes 1.6 to 1.8 kg of steam. After drying, the moisture content of soft fiber slabs ranges from 1 to 3%. When manufacturing hard fiberboards by the wet-drying method, slab moisture content shouldn't be too high to avoid bubbling during hot pressing.

4 Hot Pressing. Producing hardboards via the wet process requires pressures up to 5 MPa, while the dry method demands 7 MPa. Pressures exceeding this reduce bending strength. Semi-dry method pressures fall between 6 MPa. Wet-formed slabs are pressed into hard fiberboards at pressures up to 10 MPa. Temperatures used in the wet pressing method are 200 to 220°C. Dry pressing lacks a drying stage, focusing on adhesive curing at temperatures typically 180 to 200°C; using hardwood as raw material allows for slightly higher temperatures, up to 260°C. Semi-dry hot pressing temperatures shouldn't exceed 200°C to prevent lignin melting and sugar pyrolysis coking, reducing product strength. Producing hardboards by the wet-drying method requires temperatures of 230 to 250°C.

During hot pressing, temperature differences may occur between the slab's surface and core layers, with large-thickness medium-density slabs experiencing core layer temperature differences of 40 to 60°C, impacting resin curing rates. Conventional and high-frequency heating can eliminate these differences and shorten hot pressing cycles.

5 Post-Treatment. Wet and semi-dry fiberboards undergo heat treatment and humidity conditioning after hot pressing, while dry fiberboards directly undergo humidity conditioning without heat treatment. Medium-density fiberboard surfaces require sanding, and soft fiberboard surfaces occasionally need slotting and punching. For hard fiberboards used as interior wallboards, a "V" groove or striped groove can be added to the surface. Surface processing of fiberboards typically involves coating and covering methods (see wood-based panel surface decoration). Embossing, indentation, and deep indentations of rough-sawn materials are usually formed during slab hot pressing, not as part of rework.

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