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Medium-density fibreboard (MDF) is an engineered wood product made by breaking down hardwood or softwood residuals into wood fibres, often in a defibrator, combining it with wax and a resin binder, and forming panels by applying high temperature and pressure. MDF is generally denser than plywood. It is made up of separated fibres, but can be used as a building material similar in application to plywood. It is stronger and much more dense than particle board.
Over time, the word "MDF" has become a generic name for any dry process fiber board. MDF density is typically between 500 kg/m3 (31 lbs/ft3) and 1000 kg/m3 (62 lbs/ft3). The range of density and classification as Light or Standard or High density board is a misnomer and confusing. Density of board when evaluated in relation to density of the fiber that goes into making of the panel is important. A thick MDF panel at a density of 700-720 kg/m3 may be considered as high density in the case of softwood fiber panels, whereas a panel of the same density made of hard wood fibers is not regarded as so. The evolution of the various types of MDF has been driven by differing need for specific applications.
There are different kinds of MDF, which are sometimes labeled by colour:
Although similar manufacturing processes are used in making all types of fiberboard, MDF has a typical density of 600-800 kg/m³ or .022-.029 lbs/in3, in contrast to particle board (160-450 kg/m³) and to high-density fiberboard (600-1450 kg/m³).
The trees are debarked after being cut. The bark can be sold for use in landscaping, or burned in on-site furnaces. The debarked logs are sent to the MDF plant where they go through the chipping process. A typical disk chipper contains 4-16 blades. Any resulting chips that are too large may be re-chipped; undersized chips may be used as fuel. The chips are then washed and checked for defects.
The chips are then compacted using a screw feeder, are heated for 30-120 seconds to soften the wood, and then fed into a defibrator. The defibrator maintains a high pressure and temperature while grinding the wood chips into a pulp.
From the defibrator the pulp enters a blowline where it is joined with wax and resin (often UF). The wax improves moisture resistance and the resin initially helps reduce clumping but ultimately is the primary binding agent. The material dries quickly when it enters an expansion chamber and expands into a fine, fluffy and lightweight fibre that is stored until needed at the forming line.
Dry fibre gets sucked into the top of a pendistor which evenly distributes fibre into a uniform mat below it, usually of 230-610 mm thickness. The mat is pre-compressed and either sent straight to a continuous hot press or cut into large sheets for a multi-opening hot press. The hot press activates the bonding resin and sets the strength and density profile.
After pressing, MDF is cooled in a star dryer, trimmed and sanded. In certain applications, boards are also laminated for extra strength.
The Environmental Impact of MDF has greatly improved over the years. Today many MDF boards are made from a variety of materials. These include other woods, scrap, recycled paper, bamboo, carbon fibers and polymers, forest thinnings and sawmill off-cuts.
As manufacturers are being pressured to come up with greener products, they have started testing and using non-toxic binders. New raw materials are being introduced. Straw and bamboo are becoming popular fibers because they are a fast-growing renewable resource.
MDF does not contain knots or rings, making it more uniform than natural woods during cutting and in service. However, MDF is not entirely isotropic, since the fibres are pressed tightly together through the sheet. Like natural wood, MDF may split when woodscrews are installed without pilot holes, and MDF may be glued, doweled or laminated, but smooth-shank nails do not hold well. Typical fasteners are T-nuts and pan-head machine screws. Fine-pitch screws do not hold well in MDF and screw retention in the edge is particularly poor. Special screws are available with a coarse thread pitch but sheet-metal screws also work well. Typical MDF has a hard, flat, smooth surface that makes it ideal for veneering, as there is no underlying grain to telegraph through the thin veneer as with plywood. A so-called "Premium" MDF is available that features more uniform density throughout the thickness of the panel.
MDF is often used in school projects because of its flexibility. It is also often used in loudspeaker enclosures, due to its increased weight and rigidity over normal plywood. Slatwall Panels made from MDF are used in the shop fitting industry.
When MDF is cut a large quantity of dust particles are released into the air. It is important that a respirator be worn and the material be cut in a controlled and ventilated environment. It is a good practice to seal the exposed edges to limit the emissions from the binders contained in this material.
Formaldehyde resins are commonly used to bind the fibers in MDF together, and testing has consistently revealed that MDF products emit free formaldehyde and other volatile organic compounds that pose health risks at concentrations considered unsafe, for at least several months after manufacture. Urea-formaldehyde is always being slowly released from the edges and surface of MDF. When painting, it is a good idea to coat all sides of the finished piece in order to seal in the free formaldehyde. Wax and oil finishes may be used as finishes but they are less effective at sealing in the free formaldehyde.
Whether these constant emissions of formaldehyde reach harmful levels in real-world environments is not yet fully determined. The primary concern is for the industries using formaldehyde. As far back as 1987 the U.S. EPA classified it as a "probable human carcinogen" and after more studies the WHO International Agency for Research on Cancer (IARC), in 1995, also classified it as a "probable human carcinogen". Further information and evaluation of all known data led the IARC to reclassify formaldehyde as a "known human carcinogen" associated with nasal sinus cancer and nasopharyngeal cancer, and possibly with leukaemia in June 2004.
Veneered MDF provides many of the advantages of MDF with a decorative wood veneer surface layer. In modern construction, spurred by the high costs of hardwoods, manufacturers have been adopting this approach to achieve a high quality finishing wrap covering over a standard MDF board. One common type of veneered MDF uses oak veneer. Making veneered MDF is a complex procedure which involves taking an extremely thin slice of hardwood (approx 1-2mm thick) and then through high pressure and stretching methods wrapping them around the profiled MDF boards. This is only possible with very simple profiles because otherwise when the thin wood layer has dried out, it will break at the point of bends and angles.
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