Fibreboard includes hardboards, insulation boards and medium density fibreboards (MDF). Each can be classified according to particle size, density and production method.
MDF is normally between 700 and 750 kg m3, but it has been produced for specialty uses down as low as 450 kg m3 – this is within the density range of solid wood which is 400 to 800 kg m3.
Hardboard is used for exterior cladding (outer walls of buildings), panelling and furniture. Insulation boards are used for cladding in buildings for thermal insulation. Some boards are produced with a coating of sealant to help protect from water.
MDF is primarily used in the joinery and furniture industry. It has several key advantages over other products:
• Surface Finish: smoothness allows patterns (e.g wood
grain) to be printed directly without the need for any
underlay.
• Fine Particles: form a smooth profiled edge when
machined or cut. The edges of other boards, like
particleboard, require filling or joining with products
to give a good finish.
• Uniform Density: delivers superior working qualities
such as screw withdrawal.
The MDF market has grown significantly worldwide in the last 10-20 years and New Zealand is one of the leading producers of radiata pine-based MDF.
In 2004, 77.3% of the fibreboard produced in New Zealand was exported with a total value of NZ$236 million. Hardboard and plywood have dominated the thinboard (<10mm) markets in Europe and Asia respectively, but that has been steadily eroded by MDF.
Manufacturing Processes
The two main production processes for fibreboard are called wet felting and dry felting. Wet felting is used for insulation board and hardboard. Dry felting is preferred for MDF and some hardboards.
Fibreboard can be produced from low-grade wood and residues. The wood is reduced to wood fibre from 'chips'. In more modern mills, this is achieved using disc refiners like those used in the thermo-mechanical pulp and fibreboard pulp processes. The chips are softened with heat before refining. The refiner is a pair of large discs rotating against each other at up to 1800 rpm. The chips are fed into the centre of the refiner where steam pressure and centrifugal force move it radially outward. Progressively finer knives break the chips into wood fibres which escape through a narrow gap in the refiner. The largest disc refiners are capable of producing 720 tonnes of dry fibre per day.
Board Formation
(Wet Felting & Forming)
This process involves washing the wood pulp to remove the soluble carbohydrate fraction. After washing, the pulp is suspended in water and laid on a moving wire mat through which the water drains. The remaining fibre 'mattress' is then dried in a hot press (or dried without pressing in the case of insulation board).
Plants producing 300 tonne-a-day of board use about 360 litres of water a second in the washing and mat-laying processes. Although this water is recycled, consumption is still significant. Water pollution can also be a problem with this process.
(Dry Felting & Pressing)
In this process, wood fibre is blown directly into the dryer where moisture is quickly evaporated. Because of the large surface-to-volume ratio and high temperature in the dryer, the smallest spark caused by a foreign object (e.g. a stone fragment) can result in spontaneous combustion. Large explosions have occurred in dryers and mills have been designed specifically to limit damage when this occurs. Resin is added to the fibre before it enters the dryer and air turbulence ensures that the resin is well mixed.
The fibre mattress is formed on a moving belt and is progressively pre-pressed to consolidate and remove any air pockets.
Hot Presses
(Multi-Opening Press)
Mats are stacked in this press which then closes. Metal blocks (or gauge bars) next to the mat control the thickness of the boards. Density is controlled by the amount of material in the mattress and the thickness of the boards. When the boards have been pressed long enough for resin curing, the press is opened and boards removed from the opposite side. Hot presses normally have 8 to 20 openings, and produce boards from 1.2 x 2.4 m to 2.5 x 10 m in dimension.
(Single Opening Press)
Have only one opening but produce huge boards (eg 2.15 x 52.5 m). This allows it to be cut into specific sizes and minimises waste. Because the outer edges of the boards don't cure well, they have to be trimmed.
(Continuous Press)
Utilises heavy heated rollers instead of presses. Pressing and curing are done between a large roller (3-4 m diameter) and smaller adjustable rollers. The smaller rollers are set to determine the board thickness, which ranges from 2.5-8 mm thick. Continuous presses eliminate the unproductive opening and closing part of the press cycle.
(Board Finishing)
As with other board manufacturing processes, the final stages involve sanding, trimming to size, grading and packaging.
High Performance Product
New Zealand is a key international manufacturer of high quality MDF - making around 850,000 m3 of the 12.3 million m3 total world production.
The light-coloured MDF made here is prized by joiners and furniture manufacturers because it is easier to apply synthetic overlays and veneers of optimum thickness. Kiwi MDF is also known for its smoothness and great machining properties.