Medium density fiberboard (MDF) just might be the perfect material for building furniture, casegoods, fixtures, and interior millwork.
It is easily machinable with common woodworking tools, accepts many types of finishes and surface treatments, makes use of wood fiber that would otherwise be considered waste, and can be engineered to meet the demands of a wide range of applications and performance requirements.
Like raw wood, MDF can be carved and sculpted into almost anything a designer can imagine. Unlike raw wood, it is much more dimensionally stable and therefore less susceptible to changes in climate or environment.
MDF is the most refined member of the composite wood family, which includes particleboard, OSB, and hardboard. By “refined” we don’t mean the most sophisticated, although that case could be made based on the types of products it makes possible.
In this case, “refined” refers to the size of the basic elements within the finished panel. MDF is literally made from the most basic building block of wood, the lignocellulosic fibers that give structure to trees and all woody plants.
“Medium density fiberboard” is actually a generic term for a panel created by combining these fibers with a bonding resin system and subjecting the mix to heat and pressure, creating panels with a density between 31 and 50 pounds per cubic foot. Other additives may be added to the mix to give the finished panels specific properties.
MDF and other composite panels offer improved mechanical performance over natural wood in one very important way—dimensional stability. Wood fibers, when all aligned as they are in solid lumber, shrink and swell dramatically with changes in temperature and humidity. Composite panels reorient these fibers to offset those environmental responses.
Composite panels also drastically reduce consumption of forestlands by offering alternatives to solid lumber. Much of the wood we harvest for lumber is not be suitable for building. MDF and its complementary composite panels make use of the parts of trees that can’t be turned into furniture or pallets. These panels also give a second life to the pre- and post-consumer wood fiber wood from furniture, pallets, and other wood products.
Developed in the early 1960s in the U.S., MDF is actually based on a hardboard product first created accidentally by William H. Mason (of Masonite fame) in 1925. He was trying to turn wood chips discarded by lumber mills into an affordable insulation product. One evening he forgot to shut down his equipment, and instead of a lightweight sheet of insulation he created a thin, very durable composite wood panel.
The wood fiber, or “furnish,” for MDF comes from many sources. Most commonly it is pre-consumer wood waste that would otherwise be landfilled or incinerated—forest thinnings and wood residuals from lumber, plywood, and furniture plants. Additional sources include post-consumer items like wood pallets and retired wood furniture (after impurities are removed). It should be noted that different fiber sources may require different bonding systems.
A few vertically integrated forest products companies have streamlined their sourcing of furnish for MDF and other composite panels by optimizing transfer from their other lumber, engineered wood, and panel operations.
The remaining material is pre-steamed, where low-pressure steam is injected to heat and soften the lignin. After pre-steaming, fibers are fed into a plug screw feeder where they are compressed to remove the water from the steaming process. Fibers are then transferred into the pressurized vessel, or digester, and finally to a refiner where the material is separated into usable fibers by two grinding discs.
Resins and a wax emulsion are applied to the fiber at the inlet pipe to the drying tube. This is also the stage were additives to enhance flame retardancy, moisture resistance, or other properties are introduced. Ratios of resin, fiber, additives, and catalysts are carefully controlled by weighing each ingredient. Single- or multiple-stage tube dryers dry and blend the fibers.
To create a panel, the dried fiber is pushed through scalping rolls to produce a thick, fluffy mat of uniform thickness.
The mechanical stability of MDF is attributable to three primary variables: physical and mechanical properties of individual wood fibers, fiber-to-fiber stress transfer, and fiber orientation. These origins of fiber properties and stress transfer can be traced to the fiber generation method wherein fiber orientation is associated with mat formation.
A continuous or “feed-through” press equipped with a steel band running over large heated drums compresses the mat at a uniform rate, or a multi-opening vertical “daylight” press that creates several panels in a single pressing operation.
Modern MDF presses are equipped with electronic controls to prevent resin pre-curing, creating MDF with the desired density and uniform strength as efficiently as possible.
As the finished board emerges from the press it is cut to panel lengths using automated saws before the MDF cools.
After cooling, the panels are sanded on both sides by large belt sanding machines using either silicon carbide abrasives, or for finer surfaces, ceramic abrasives like zirconia alumina and aluminum oxide.
Most MDF plants use computerized process control to monitor each manufacturing step and to maintain product quality. Product consistency is maintained by a combination of continuous weight belts, basis weight gauges, density profile monitors, and thickness gauges. In addition, the American National Standards Institute has established product specifications for each application, as well as formaldehyde emission limits. As environmental regulations and market conditions continue to change, these standards are revised.
The standard for MDF, (ANSI A208.2-2016 Medium Density Fiberboard for Interior Applications), is the most recent version of this industry standard. This standard classifies MDF by density and use (interior or exterior) and identifies four interior product grades. Specifications identified include physical and mechanical properties, dimensional tolerances, and formaldehyde emission limits. Specifications are presented in both metric and inch-pound limits.
Physical and mechanical properties of the finished product that are measured include density and specific gravity, hardness, modulus of rupture, abrasion resistance, impact strength, modulus of elasticity, and tensile strength. In addition, water absorption, thickness swelling, and internal bond strength are also measured. The American Society for Testing of Materials has developed a standard (D-1037) for testing these properties.
MDF AND SUSTAINABILITY
Few materials on earth are as perfect for their purpose as wood. Trees grow essentially by building themselves, efficiently creating their own construction materials along the way. The lignocellulose fibers that form the essence of wood create a unique combination of strength, resilience, workability, and renewability that no other material can even come close to.
The inherent properties of wood are what make MDF and other composite wood panels an environmentally positive choice for furniture, fixtures, and interiors.
A brief overview:
- Wood is one of the planet’s most easily renewed resources.
- During the past 60 years, net growing-stock growth has consistently exceeded growing-stock removals in the United States.
- In terms of percent of standing volume, removals are at the lowest level in the past 60 years and growth has also slowed.
- The volume of annual net growth is currently two-times higher than the volume of annual removals.
- Source: “U.S. Forest Service Resource Facts and Historical Trends,” FS-1035, August 2014
- North American panel producers have proven themselves to be exceptional stewards of their resources.
- Composite wood panels make use of wood fiber left over from other manufacturing processes.
- This material would otherwise be destined for landfills and incinerators.
- These panels are more stable than solid wood, and may be engineered for specific applications and performance characteristics.
- Moisture resistance, fire resistance, strength, weight, machinability, etc.
- These properties ensure a longer useful life, requiring less frequent replacement.
- Composite wood panels have been shown to be “better than carbon neutral” in a recent lifecycle inventory analysis.
- The wood in composite panels acts as a carbon sink, sequestering more carbon than is expended in their production, transportation and installation.
- Source: “Cradle to Gate Life Cycle Assessment of U.S. Medium Density Fiberboard Production”; see sources section
- Rare and endangered wood species are spared by the use of decorative composite wood panels.
- High-definition printed and textured decorative surfaces offer the beauty of any wood, with better design consistency and durability.
- Carefully cut veneers maximize the decorative square footage of responsibly harvested trees.