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What is MDF?

https://en.wikipedia.org/wiki/Medium-density_fibreboard

 

Medium-density fibreboard

From Wikipedia, the free encyclopedia
 
 
 
A sample of MDF

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.[1] 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 denser than particle board.[2]

The name derives from the distinction in densities of fibreboard. Large-scale production of MDF began in the 1980s, in both North America and Europe.[3]

 

 

Physical properties[edit]

Over time, the term MDF has become a generic name for any dry process fibre board. MDF is typically made up of 82% wood fibre, 9% urea-formaldehyde resin glue, 8% water and 1% paraffin wax.[4] and the density is typically between 500 kg/m3 (31 lb/ft3) and 1,000 kg/m3 (62 lb/ft3).[5] The range of density and classification as lightstandard, or high density board is a misnomer and confusing. The density of the board, when evaluated in relation to the density of the fibre that goes into making 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 fibre panels, whereas a panel of the same density made of hard wood fibres is not regarded as so. The evolution of the various types of MDF has been driven by differing need for specific applications.

Types[edit]

There are different kinds of MDF (sometimes labeled by colour):

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

Although similar manufacturing processes are used in making all types of fibreboard, MDF has a typical density of 600–800 kg/m³ or 0.022–0.029 lb/in3, in contrast to particle board (160–450 kg/m³) and to high-density fibreboard (600–1,450 kg/m³).

Manufacture[edit]

 
Medium-density fibreboard output in 2005

In Australia and New Zealand, 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 fibres.

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/steamed for 30–120 seconds to soften the wood, then joined with liquefied wax and 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 resin, often urea-formaldehyde. 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 or cooling carousel, trimmed and sanded. In certain applications, boards are also laminated for extra strength.

The Environmental Impact of MDF has greatly improved over the years.[citation needed] Today, many MDF boards are made from a variety of materials. These include other woods, scrap, recycled paper, bamboo, carbon fibres 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 fibres because they are a fast-growing renewable resource.

Comparison with natural woods[edit]

MDF does not contain knots or rings, making it more uniform than natural woods during cutting and in service.[6] However, MDF is not entirely isotropic, since the fibres are pressed tightly together through the sheet. 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 may be glued, doweled or laminated. Typical fasteners are T-nuts and pan-head machine screws.[7] Smooth-shank nails do not hold well, and neither do fine-pitch screws, especially in the edge. Special screws are available with a coarse thread pitch, but sheet-metal screws also work well. Like natural wood, MDF may split when woodscrews are installed without pilot holes.

Benefits[edit]

  • Is an excellent substrate for veneers.
  • Some varieties are less expensive than many natural woods
  • Isotropic (its properties are the same in all directions as a result of no grain), so no tendency to split
  • Consistent in strength and size
  • Shapes well.
  • Stable dimensions (won't expand or contract like wood)
  • Easy to finish (i.e., paint)

Drawbacks[edit]

  • Denser than plywood or chipboard (the resins are heavy)
  • Low grade MDF may swell and break when saturated with water.
  • May warp or expand if not sealed.
  • May release formaldehyde, which is a known human carcinogen[8] and may cause allergy, eye and lung irritation when cutting and sanding[9] associated with nasal sinus cancer and nasopharyngeal cancer, and possibly with leukaemia in June 2004.[10]
  • 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.
  • Trim (e.g. baseboards) comes pre-primed, but this is insufficient for fine finish painting. Painting with latex paints is difficult due to rapid water absorption. Most finishes appear uneven and nail holes tend to pucker.

Applications[edit]

 
Loudspeaker enclosure being constructed out of MDF

MDF is often used in school projects because of its flexibility. Slatwall Panels made from MDF are used in the shop fitting industry. MDF is primarily used for internal use applications due to its poor moisture resistance. it is available in raw form with fine sanded surface or with decorative overlay.

MDF is also usable for furniture such as cabinets, because of its strong surface.[11]