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MDF - Medium Density Fibreboard

Medium Density Fibreboard. It is man-made from wood fibres, which are compressed together with adhesive. It is great for making everything from shelves to furniture and can be used with most types of paint. You can buy it in different thicknesses and finishes - bendy MDF, which looks a bit like corrugated cardboard, is good for circular cabinets and tables. There have been some safety concerns about breathing in the dust particles created when cutting MDF, which may aggravate asthma and skin disorders. However, if you follow safety guidelines, such as cutting it outside in the open air or in a well-ventilated room, and always wear a mask and goggles, it should be safe.

An extremely versatile product that is suitable for many interior projects. It cannot be used for outside work exposed to wet weather conditions, but is an asset for shelving, cupboards and surfaces inside the home. It has a fine texture that is even throughout its length. This is due to the method used in manufacture. Bonding wooden fibres together under high pressure forms MDF.

Due to the fineness of the fibres, it is advisable to use a mask when sawing the boards to avoid inhaling the fine dust particles. The sheets are available in numerous thicknesses including 9mm, 12mm, 15mm, 18mm.

Medium Density Fibreboard (MDF) is a wood based composite material that draws on the usage of wood fibres, rather than particles or veneers to produce board or sheet products. It is typically made as a board type product, though it's use in mouldings and increasing use as a structural product will see beam type products proliferate. It is replacing the use of particleboard in uses such as furniture manufacture, cabinet making, joinery, craft work and flooring. Its advantages include high strengths (MOR of 80 MPa and MOE of 3000 MPa), ease of machining, good weathering properties, and the ability to be made from a wide variety of fibrous products (An Environmental plus).

Medium Density Fibreboard gets it's name from it's historical place in the production of fibreboards. Manufactured using a dry blowline process to a density of 750 kg/cum, this was originally heavier than existing low density boards such as caneite, and lighter than high density boards such as masonite. Nowadays the same MDF manufacturing proccess is used to produce light boards of 600 kg/cum, and tough high density moisture resistant boards (such as HD3) with densities of 1200 kg/cum.

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.

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.

MDF was originally developed exclusively for furniture. But it's weight strength and aesthetics have seen its proliferation to many uses. It is used extensively in kitchens and for mouldings, and in bathroom environments. It's use as an exterior cladding for housing has successfully been trialed, and structural applicationsare are increasing.

MDF has been able to take over from the traditional materials used (especially particleboard ) in the aforementioned applications because:

It's moderate overall density and suitable density profile give it excellent machining characteristics It has greater moisture tolerance than particleboard It has very strength, with a Modulus of Elasticity of 2500 MPa for standard boards to over 5000 MPa for higher density varieties of MDF. Modulus of Rupture is from 28 to 80 MPa depending on thickness and types. Aesthetically pleasing. There is a trend for furniture to become more ornate ( or a return to furniture being ornate) and the machinability, the laquering properties, strength and ability to maintain shape will be well suited to MDF. Door skins and architraves are easily moulded. MDF has better moisture sorption characteristics than timber The Fire resistance of MDF is also better than that of timber Although attractive enough in its raw form MDF is usually coated with a Melamine paper, which can provide a full palette of colours and artificial wood veneers. True wood veneers, other plastics, paints and aluminium will all adhere to MDF. The author was recently impressed by its use, in the raw form, as stands to set of wood carvings in a forest gallery.

The primary constituant of the MDF made in Australia in Radiata Pine (Pinus Radiata d.Don). Though up to 12% may be Eucalyptus (predominantly E.delegantensis) may be used, particularly in the higher density boards. Information on four MDF plants in Australia:

In 1993 the Laminex plant at Wagga Wagga took in chips from small scale mills, other laminaex operations, or compeditors such as Boral or CSR. This was due to the availability of chips and the then scarcity of roundwood, though the plant was capable of producing its own chips. They were also taking some Callitris chips but this is winding up The CSR plant at Oberon uses exclusively Radiata Pine, as it is widely available and first thinnings are often used, chipped on site The Gympie plant uses pinus Elliotti and Pinus Caribaea A new MDF is being opened up near Launceston, Tasmania. In NSW there is over 250,000 ha of Pine plantation, mostly in state forests. The largest areas are around Bathurst/Oberon, servicing the CSR mill at Oberon, and the Tumut/Batlow area servicing the Laminex operations.

The trend has been to use formaldehyde based resins for MDF, a practice that carried over from Particleboard technology. Though epoxy resins have superior properties their cost makes use of them unfeaseable. Originally Urea Formaldehyde was used but this is being replaced as concerns over its health risks and moisture sucseptability grow. The formaldehyde family is still primarily used. The Laminex MDF plant phenol formaldehyde is used

A feature of MDF that may well see its use become more popular is its "Environmental freindliness". Presently it is made primarily from plantation grown Radiata Pine, though it can be successfully made from almost any wood. Recycled paper, recycled scrap wood, bamboo and materials such as carbon fibres, polymers, steel and glass have all successfully been used in it's manufacture. MDF is able to utilise young forest thinnings and sawmill offcuts that would normally be of only limited value. The use of MDF over hardwood is being promoted due to a worldwide shortage of hardwood, and the areas where hardwood exists being touted as areas of conservation value.

One presently contentious issue is the use of formaldeyde resins in the manufacture of MDF, and the healt risks assocciated. This is being overcome by the use of lignin based adhesives. Presently standards require that the emission of formaldehyde, which can cause eye or throat irritations, from finished products that contain formaldehyde be less than one part per million. This standard is being adhered to by most major manufacturers prior to its legislation.