While MDF is generally cheaper than plywood, it is not as hard and can sag under weight. MDF doesn't handle moisture very well so it is more suited for indoor use such as furniture. Moisture also affects the strength of plywood; so while exterior glued plywood can be used outdoors, it works best when moisture content remains low.
edit What is MDF?
Medium Density Fiberboard (MDF) is an engineered wood product formed from wood fibers obtained by breaking down hardwood and softwood using a defibrator. The wood fibers are then glued together using wax and resin adhesive and made into panels by applying high temperature and pressure.
edit What is Plywood?
Plywood is made from good log called peelers. Thin layers are peeled from the wood by rotating it along its horizontal axis. The sheets of veneer thus obtained are cut to the desired dimensions, dried, patched, glued together and then baked in a press at 140 °C (284 °F) and 1.9 MPa (280 psi) to form a plywood panel.
edit Strength and durability
MDF is not as hard as plywood and can be damaged easily when handled rough. It is not as stiff as plywood and can sag with weight when used as shelves without reinforcement.
The cross graining of plywood improves dimensional stability by reducing expansion and shrinkage equating. The strength of panels is consistent across both directions. Moreover, the odd number of sheets reduces warping. Another advantage of plywood is that extreme cold does not affect its dimensions or strength.
When driving a screw at the edge, the soft core of MDF splits easily. When a countersink drill bit is not used, the head of the screw might snap off before sinking in or it might push up chips. The smooth finish of MDF allows it to be painted and also makes it ideal for shaping or routing. Due to its non-directional grain structure, it is ideal for cutting, machining and drilling without splinters or chipping. The absence of knots makes it easier for finishing.
The cross graining of plywood reduces its tendency to split when nailed at the edges.
MDF is usually cheaper than plywood but a lot depends upon the grade of plywood. Higher grade plywood is more expensive. A 1/4 Bendy MDF Sheet (24 x 48) is about $20 on Amazon.com while a Grade BB plywood sheet (1/4 thick, 24 x 30) is $10 on Amazon.
edit Uses in construction
MDF is used for doors and internal paneling in construction. MDF was exclusively developed for furniture. MDF is mainly used for indoor applications due to its poor moisture resisting characteristics.
Plywood is used for doors, exterior stairs, external cladding, flooring, framing, interior rails and balustrades, interior stairs, internal paneling, shear walls, timber joinery products and timber portal frames in construction. Plywood bends easily with the grain so it is often used to create curved surfaces. For example, the top smooth surface over bent curves in skateboard ramps.
Sculptor Tom Sachs made this short film about his love for the versatile plywood.
edit Disadvantages and Risks
Plywood is porous and is susceptible to damage when exposed to water over time.
- MDF produces a large amount of dust when cut. It should be cut in a ventilated environment and a respirator is advisable when cutting MDF.
- The panels are heavy.
- It is not water resistant.
- MDF is not as stiff and bends or sags under weight.
- MDF products emit urea-formaldehyde and other volatile organic compounds that pose health risks at sufficient concentrations.