Composite panel products, Part II: Medium density fiberboard

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Woodworking Network– MDF, as the name implies is made with fibers and small clumps of fibers. The raw material is logs (short, crooked are fine) of almost any species; cost and availability are key factors. Hence, the raw material cost for MDF is not high, compared to plywood veneers. The first steps in processing are cutting the wood into small pieces and then mechanically or chemically converting the wood into fibers or small clumps of fibers. If needed, the fibers are then dried.

Dry fibers are then coated with adhesive and sometimes wax is added to help with water repellency. The amount of adhesive is a key in determining the strength of the final product. The fibers are then spread on a mat (the quantity of fibers determines the board density) and then pressed to the desired thickness and density.

Overall, an MDF panel is very uniform in properties. This provides uniform matching and finishing.

MDF can be used alone or used as a core material for a wood veneer face and back. MDF can be embossed. MDF can be printed on. Edges can be machined with sharp edges. MDF ends can made with a variety of profiles including mortise and tenon-type joints.

MDF characteristics and processing hints

MDF has many advantages and a few disadvantages.

Advantages

–MDF is generally cheaper than plywood.

–The surface of MDF is very smooth, making finishing and even printing on it quite easy and it has an excellent appearance.

–MDF is very consistent throughout, so routed and sawn edges, including curved edges, can be very smooth with sharp corners and edges.

Disadvantages

–MDF is weaker than solid wood and plywood

–MDF is heavier than plywood.

–MDF dulls tools faster than plywood.

–MDF will swell in thickness when first exposed to liquid water or high humidity.

–Conventional fasteners in the face and edges are not strong.

A. Strength. For the same thickness, MDF tends to be not as strong and not as stiff as plywood or solid wood. However, as strength is controlled by thickness, panel density (which reflects the amount off wood per cubic inch, which is controlled by the amount of pressure used) and the amount of adhesive used, MDF panels have a range of possible strength and stiffness values. However, in general MDF panels are denser than plywood.

B. Gluing. MDF can be difficult to glue because of the added wax and the heat used in manufacturing. Also, the surfaces are usually quite smooth and appear burnished. Therefore, scuffing of the surfaces to be glued is suggested to achieve the strongest joint. In many cases, the glue joint strength, even with PVA adhesives, can exceed board strength. A hot-melt PUR is attractive. Other PUR-type adhesives may not work well as the product is often very dry; PUR adhesives need moisture to help fast curing. For strong 90 degree joints, consider wood blocking that is glued into the joint, using strong, short brad nails to hold the blocking in place until the adhesive cures.

C. Fastening with screws, nails and staples.  A variety of specialized mechanical joining systems have been developed to help provide fairly strong joints. These are often the best option for fastening. On the other hand, because the MDF panel is not exceptionally strong, it is fairly easy to make a glue joint that is stronger than the board itself.

Screws have less holding power in MDF than in solid wood. MDF screws use fewer threads per inch. Counter sink to allow head to be smooth and allow room for debris. Long skinny screws would be better than short stubby ones in most cases.

When the screw enters the dense MDF, it tears or pushes the fibers apart in order to make room for the screw. The screw, especially a larger diameter screw, is acting like a wedge so the board can actually be split apart around the screw. When this separation occurs, then the screw is held by these loose, torn fibers which do not have a lot of strength, as you might imagine. Even with predrilling, the strength of conventional fasteners is not high. I have seen a drop or two of epoxy put into the predrilled hole just before screwing. This seems to be worth trying.

The high density can make small brads and other nails difficult to use without bending or damaging the panel.

D. Machining. MDF’s uniformity provides precise machining without producing splinters or chipping. Compared to solid wood, MDF’s high density and the fairly hard adhesive used mean that machining is difficult and dulls conventional cutters. Carbide and diamond tools provide good, clean cuts.

Of special note is that abrasive debris (sand, for example) in the raw material also causes sanding and machining issues such as streaks due to nicks in knives and loss of sanding mineral. A quality MDF board will have very little extraneous debris. The amount of debris is measure with an “ash test.”

E. Warping. Moisture causes wood movement. However, in MDF, the added wax, the uniformity of properties and a vapor resistant finish all help to minimize moisture changes. So, in normal use conditions, warping is not an issue for MDF unless moisture changes are large and rapid.

Liquid water that gets into the panel through the finish or through other pathways such as a fastener hole creates serious problems with glue failures and unusually swelling.

F. Finishing. The smooth uniform surfaces with the absence of knots help with finishing. With any finish and a dense wood product, some experimentation and working closely with finish suppliers is essential to assure a well bonded, durable finish. With printing, the surface can be made to look like “real wood.”

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