Medium-density fibreboard

Medium-density fiberboard output in 2005

Medium-density fiberboard (MDF) is an engineered wood product formed by breaking down softwood into wood fibers, often in a defibrator, combining it with wax and a resin binder, and forming panels by applying high temperature and pressure.

It is made up of separated fibers, (not wood veneers) but can be used as a building material similar in application to plywood. It is much more dense than normal particle board.

The name derives from the distinction in densities of fiberboard. Large-scale production of MDF began in the 1980s.

Types of MDF

There are different kinds of MDF, which are sometimes labeled by colour:

• Moisture resistant is typically green
• Fire retardant MDF is typically red or pink

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).

Another addition to the MDF range is a product named FX-Platform, produced by Norbord. It is a softwood plywood core, laminated on both sides with MDF, giving it working properties containing the advantages of both plywood and MDF. This product has met the acceptance criteria for compliance with the ANSI/HPVA HP-1-2004 Section 3.12 Formaldehyde Emission Requirements for industrial panels.

Lighter densities of fiberboard are commonly marketed as ultralight or LDF boards.

Manufacture

In Australia the main species of tree used for MDF is plantation-grown radiata pine but a variety of other products have also been used including other woods, waste paper and fibers.

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, and will be heated for 30-120 seconds to soften the wood; they are then fed into a defibrator which maintains high pressure and temperature. The pulp that exits from the defibrator is fine, fluffy, and light in weight and in colour.

From the defibrator the pulp enters a blow line where it is joined with wax (to improve moisture resistance) and resin (to stop the pulp from forming bundles). The material expands in size and is then heated by heating coils. When it comes out it may be stored in bins for an indefinite length of time.

After this drying period the board goes through a "Pendistor" process which creates 230-610 mm thick boards. Then it is cut and continues to the press. Here it is pressed for a few minutes, to make a stronger and denser board.

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, steel, glass, forest thinning 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.

Comparison to natural woods

Benefits of MDF:

• Is an excellent substrate for veneers.
• Is becoming an environmentally friendly product.
• Some varieties are less expensive than many natural woods
• Isotropic (no grain), so no tendency to split
• Consistent in strength and size
• Flexible. Can be used for curved walls or surfaces.
• Shapes well.

Drawbacks of MDF:

• Heavier than plywood or chipboard (the resins are heavy)
• Swells and breaks when waterlogged
• May warp or expand if not sealed
• Contains urea-formaldehyde which may cause eye and lung irritation when cutting and sanding
• Dulls blades more quickly than many woods
• Though it does not have a grain in the plane of the board, it does have one into the board. Screwing into the edge of a board will generally cause it to split in a fashion similar to delaminating.
• Subject to significant shrinkage in low humidity environments.

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Recent developments

A fairly recent development is flexible MDF sheets. These are sheets scored with multiple slots so that the material can easily be formed into curved shapes and then fixed. The manufacturer is Bendymdf.

Applications

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.

A sample of MDF

Slatwall Panels made from MDF are used in the shop fitting industry.

Safety aspects of MDF

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 wood 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 MDF together, and testing has consistently revealed that MDF products emit formaldehyde and other volatile organic compounds that pose health risks at sufficient concentrations, for at least several months after manufacture. Whether these chronic emissions reach harmful levels in real-world environments is not yet fully determined.

References

• Jesberger, Lee FX-Platform
• HCHO_Certificate Norboard
• Lignocellulosic Composites
• Spence, William P. (2005). The Home Carpenters & Woodworker's Repair Manual. New York: Sterling. ISBN 1402710550

External links

• Pro Woodworking Tips.com
• A video podcast from podcastschool.net
• The Production Process of Humewood MDF (Humewood is a product of Hume Fiberboards Sdn Bhd, a Malaysian fiberboard manufacturer)
• Wood Dust - dangers of exposure to wood dust, including MDF dust (from the Worker's Health Centre)
• Medium Density Fiberboard - from Investment New Zealand, a division of New Zealand's economic development agency.
• The Medium Density Fiberboard Home Page (although not updated or maintained, still a valuable reference)
• http://www.medite-europe.com