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The Manufacturing Process Medium Density Fibreboard
(
Update:
25/06/2018
)
Introduction
MDF is a wood based composite. The primary constituant is a softwood that has been broken down into wood fibres; that is the very cells (tracheids, vessels, fibres and fibre-tracheids), which are far smaller entities than than those used in particleboard. In Australia the main species used in the production of MDF is plantation grown radiata pine, but a wide variety of softwood species will constitute a suitable base for MDF production, though if too many species are used too great a variation in the properties of the finished MDF will result.
Other materials successfully used have been waste paper, randomly collected waste wood and bamboo.
Mixing wood and other non-wood materials such as fibres of glass, steel, carbon and aramide have all resulted in successful MDF type products being produced.
The two major MDF manufacturers in Australia are Laminex , a subsidary of BTR Nylex, with MDF plants at Gympie, Queensland and Wagga Wagga, NSW and the CSR Company, with a plant at Oberon, NSW.
Debarking
Once the MDF plant has obtained suitable logs, the first process is debarking. The logs could be used with the bark, as could any fibrous material, but for optimisation of the final product the bark is removed to; decrease equipment damaging grit, allow faster drainage of water during mat formation, decrease organic waste load by 10-15 %, stabilise pH levels ( reduces corrosion of tools ) and increase surface finish. The most popular debarker used in MDF manufacture is a ring debarker (shown below), though rosser head and drum debarkers can be used.
In some manufacturing plants the debarking process is not important as the plant obtains chips rather than logs. The chips can come from the waste of another operation or from logs chipped in the forest.
A Cambio ring debarker can operate at feeds of up to one tonne per minute, the logs being typically 2 to 2.5 m in length. The tool heads, held in place pneumatically or by springs, rotate about the the logs and rip off nearly all the bark, and do little damage to the log. The waste bark can be sold, for landscaping or to power on-site furnaces (Environmental Considerations).
Chipping
Though some plants accept chips directly from other operations, chipping is typically done at the MDF plant. A disc chipper, containing anything from four to sixteen blades, is used. The blades are arranged radially on a plate and the spinning plate is faced perpendicularly to the log feed. The feed speed of the logs, the radial speed of the knife plate, the protusion distance of the knives and the angle of the knifes, control the chip size.
The chips are then screened and those that are oversized may be rechipped, and those that are undersized used as fuel. Stockpiles of several hundred tonnes of chips are maintained. There may be a blending of chips from different sources or timber species to enhance certain properties. For example the highly moisture resistant high density boards that many manufacturers make typically use a eucalypt content of 10%. The chips are washed, and a magnet or other scanner may be passed over to detect impurities.
Pulping
MDF takes much of its characteristics from the fact that it uses wood cells, (tracheids, vessels, fibres and fibre-tracheids), rather than particles. This can be done by a Masonite gun Process, Atmospheric or Pressurised Disk refiner. The Asplund defibrator pressurised disk refinement being that primarily used in MDF manufacture. The chips are compacted using a screwfeeder into small plugs which are heated for 30 to 120 seconds (this softens the wood), then fed into the defibrator. The defibrator consists of two counterwise rotating plates each with radial grooves that get smaller as they get closer to the circumference. The plug is fed into the centre and gets broken down as the centrifugal forces push it toward the outside of the plates where the groves are finer.The feeding devices at the entrance and exit to the defibrulator maintain suitably high pressure and temperature (about 150 C).
The high temperatures lower the energy required to defibrate wood as there is a softening of lignin that facilitates fibre seperation along the middle lamella. The steam is then seperated from the pulp, the total time in the defibrator is about one minute. They pulp may pass through a secondary refiner to ensure the fibres meet pre-determined levels of `freeness'.
The resulting pulp is light, fine, fluffy and light in colour. As the acompanying micrograph of an MDF sample shows the fibre walls are still intact.
The Blowline
After defibration fibres enter the blowline. The blowline is initially only 40mm in diameter with the fibres passing through at high velocity. Wax, used to improve the moisture resistance of the finished board, and resin are added in the blowline while the fibres are still wet, as dry fibres would form bundles, due to hydro bonding, and material consistency would be lost. The blowline now expands to 1500mm in diameter and fibres are dried by heating coils warming the blowline to about 550 F. The air-fibre ratio is about 500 cubic ft/lb with air speed of 500 ft/min though the air is still humid and the resin does not yet cure. The agitation of fibres in the blowline helps disperse resin consistently. Exit temperature is about 180 F. The fibres may be stored in bins for an unspecified length of time but the board making process is usually continous from here on. The Moisture Content of the fibres is 12%, and thus this is considered a dry process.
The blowline mixing process and the use of dry fibres are distinguishing characteristics of MDF.
Mat Formation
In order to form a continous and consistant mat the the following problems must be over come: the fact that considerable air velocities must be maintained to suspend fibres, fibre/air suspension does not flow laterally on a horizontal support and fibre form lumps. One way of overcoming this is a Pendistor.
Impulses of air act on the fibre as it falls down the shaft to a vacuum box at the start of the conveyor belt that carries the mat. The oscillatory action on the fibres spreads them uniformly into a mat and they begin their run on the conveyor belt at between 230-610 mm thick.
Pressing
The mat can either be laterally cut to size as it leaves the pendistor or it can be cut half way through its run by a synchronised flying cut off saw. The density profile of the the panel is critical to acheiving satisfactory strength properties. Concentrating mass, and hence load bearing abliliy, at the top and bottom of the board means that inertial properties are maximised and the greatest strength can be obtained for minimal weight. This is acheived by the press acting at impacted pressure initially and then slower pressure aplication. As an example for a 16mm board:
Press closed. 20 seconds to bring mat to 28 mm.
28 seconds at 26mm.
23 seconds at 25mm.
125 seconds at 18.3
Total time of 330 seconds to bring board to 16mm, then decompression time.
The pressure may reach 3500 MPa and be heated to over 200 C. Thicker boards may require up to 750 MPa and additional steam or radio frequency heating. The MDF plant at Wagga uses a hot press with seven daylight openings to accomadate reasonable production rates. Press sizes can be from 1.5-2 m wide by 5-20m long.
Finishing
After pressing boards are cooled in a star dryer, and final trimmed and sanded. They are given a few days storage to allow complete curing of resins. The boards are commonly given a coloured melamine laminate, though natural wood veneers and raw MDF are common.
