Particleboard is any board that is made from wood particles, as opposed to wood fibres or sheets of wood. The generic product spans many types of board, differing in particle size, orientation and position and includes particleboard, flakeboard, oriented strand boards (OSB) and waferboard. These products are mainly used for structural purposes (wall bracing and flooring). Some of the larger particleboards (eg OSB) are seen in appearance uses like wall lining and café tabletops.
These board products offer comparable structural properties to plywood and compete in the same markets.
Particleboard has traditionally been manufactured from coniferous wood, but non-wood fibre such a linen flax shives, bagasse from sugar cane, jute and bamboo can be used. About 5% of particleboards are made from non-wood material.
Manufacturing Process
Particleboard was developed to make use of wood processing waste. Planer shavings and sawmill slabwood, offcuts and sawdust are still used in some specific board types. Particleboards are generally made from low-density wood (350-500 kg m3). Higher density wood does not improve board strength it just adds to the weight.
Wood for chipping is first debarked, but because small amounts are allowed in the final board product, the debarking process does not have to remove all bark. Most pulp and wood panel mills use drum debarkers. These are large rotating steel pipes or drums that logs are fed into. The tumbling and rubbing of the logs against each other shears off the bark. The debarked logs fall out the other end and the bark falls through holes in the drum.
The type of board being manufactured dictates the ideal wood particle size. For structural boards such as OSB, particles should be as long as possible to maximise the amount of overlap between particles and, therefore, strength. Length and thickness are the most important dimensions to control. OSB typically uses particles 75 mm long, but can be up to 150 mm. In conventional particleboards, particles between 10 and 38 mm in length are used. Particle thickness of between 0.2 and 0.5 mm is desirable but the longer the particle, the greater the thickness can be without compromising board strength. Board thickness and ease of handling long particles are the only constraints.
Wood particles can be produced by a variety of methods. In the manufacture of conventional particleboards, where precise particle dimensions are not as important, hammer-milling and disc-chipping can be employed to produce the wood particles. Where precise particle size is important, disc-flaking and knife-ring flaking methods can be used. These are generally used to produce longer, thinner particles for structural boards. Disc chipping methods can also be used to produce particles of smaller dimension.
The resulting flakes (particleboard) or strands (OSB) are dried by blowing them through a drum in a stream of hot air. As they move through they are stirred with rods to accelerate drying.
Following drying, particles are sorted into sizes in two ways: through a series of screens; or by airflow separation. Airflow separation makes use of the weight-to-surface area ratios of the particles, in that smaller particles get blown further than heavy ones.
Resin mixing is important. The amount of resin in a product and how effectively it is mixed determines the strength of the board. Conventional particleboard contains about four times the resin of plywood, primarily because of the larger surface area, but also because of the larger voids in the board and the rougher surface of the particles. The process involves particles being stirred in a blender as liquid resin is sprayed on to them.
After resin mixing the panel mattress is formed. The glued particles are metered out of a feeding bin and fall on to a conveyor or tray. Depending on the particular process, either the forming station or the conveyor/tray move so that a long mattress is formed. Some processes form a three-layer mattress. A layer of fine surface particles is laid down first, then a layer of coarse core particles is formed on top, followed by a second fine surface layer. Other processes lay the mattress in a single action, but they do it in a way that separates the particles, putting the coarse core material in the middle and the smaller particles on the surfaces. The objectives of forming is to lay a mattress which is even in weight along its length and across its width.
Particleboard:
The formed mattress is conveyed into the press where heat and pressure are applied. The mattress is compressed, reducing it to the required pre-sanding thickness. The heat cures the resin, binding the particles together into a solid board. There are various types of particleboard press. Single opening flat presses have two flat plattens which press one discreet mattress between them. Multiple opening flat presses have several flat plattens positioned one above the other, and they press several discreet mattress between each pair of plattens. Continuous roll presses press an endless, continuously moving mattress between a large rotating drum and a tensioned steel belt.
Strand board and fibrefaced strand board:
These panels, which are the thickest of all panels made in New Zealand, are pressed in a single opening daylight press that injects steam into the matt to speed up the heating and curing of the adhesive. The matts are loaded between two stainless steel mesh screens which help spread the steam that is forced through thousands of small holes in the press platens. The steam softens the strands, and very quickly heats the matt to the temperature needed to cure the glue. A normal press would not be able to heat the thick matt for these products quickly enough to be cost-effective. Once the matt reaches the required temperature, the excess steam is sucked out of the press and the matt to ensure the moisture content of the board is correct. As with other boards, the panel is sanded, trimmed and packaged for shipment.