Woodsense: Manmade MaterialsComments (0)
This article is from Issue 10 of Woodcraft Magazine.
Composite wood products have many practical and cost-saving applications, particularly in cabinetry. We’ll help you decide which materials are suitable for your projects.
By Udo Schmidt
MAN-MADE WOODWORKING MATERIALS
Composite wood products, also called sheet stock, are wood materials adhesive-bonded together. The three main groups of these man-made materials are plywood, particleboard and fiber composites. Each of these products has its advantages and disadvantages and is used for different applications. Hardwood plywood is used for fine woodworking, while less-expensive particleboard is used for commercial casework. In general, plywood is stronger than particleboard and fiberboard, but not as stable. Like solid wood, plywood can warp with changing temperatures and relative humidity, though not as severely.
Most sheet stock is sold as a 4' x 8' panel, in ¼", ½" and ¾" thicknesses. Many retailers have a panel saw to cut the sheet to a more manageable size, but they do not necessarily sell partial sheets. Before you start any project with sheet stock, check the exact dimensions, especially thickness. Even though the material is sold as ¾" thickness, its actual thickness may only be 11/16" or less. This is because many composite wood products are imported and might have been manufactured under the metric system.
A number of adhesives are used in the manufacture of sheet stock: protein, phenol-formaldehyde and urea formaldehyde. Protein glue is extracted from plants and animals; typical ingredients include water, dried blood, soya flour, lime, sodium silicate, caustic soda and a formaldehyde donor for thickening.
Urea-formaldehyde glues are synthetic thermosetting glues; typical ingredients are water, defoamers, extenders (wheat flour) and urea formaldehyde resin. Phenol-formaldehyde resins are also synthetic thermosetting glues; typical ingredients include additives of caustic soda and soda ash. With their weaker resistance to water, both protein and urea-formaldehyde are interior glues, while phenol-formaldehyde is an exterior glue with good water resistance.
For decades, plywood has been the staple of cabinet shops. Plywood is a panel built up of sheets of veneer called plies, bonded together with grain direction perpendicular to the adjoining layers. The number of layers in a sheet is always uneven. They range from three layers in ¼" sheets to as many as 13 layers in ¾" Baltic birch plywood. Because of the crossbanding of the plies, each layer is mechanically restricted from swelling and shrinking, making plywood dimensionally stable and uniformly strong.
Plywood is also relatively lightweight because most manufacturers use lighter softwood for the core layers and hardwood for the face veneer.
Plywood comes in many grades and species for the face veneer, from low-cost poplar to expensive exotics. Whenever a large area of wood is called for, plywood is a preferred material. It is cheaper and easier to work than solid wood, more stable and more uniform in texture and color.
The face veneer of plywood can be put in two categories. Straight vertical slicing is the more expensive way of producing the face veneer. This type of veneer is most often bookmatched. If a less expensive rotary cut is taken, the veneer is peeled off the logs on a gigantic lathe.
A popular plywood often mentioned on TV woodworking shows is “apple plywood.” The core veneer is alder and the face veneer is hard maple. A ¾"-thick sheet is made up of 13 layers of ply, making it very stable and strong. It is probably the best plywood money can buy, but it is 50 to 80 percent more expensive than imports. Manufactured in the U.S., this product’s nickname came from the expression “as American as apple pie.”
The Invention of Plywood
Egyptian carpenters are credited for inventing plywood. A coffin dated 2750 B.C. from the Egyptian Saqqara Pyramid was made out of plywood. Its sides were constructed of six layers of 4 mm veneer, glued and doweled, with each layer in alternating grain direction like today’s plywood. Industrial production of veneer started about 1850, and the production of plywood panels in 1893.
Baltic Birch Plywood
Baltic birch plywood is a good choice for all applications where a composite material is called for. A high-density plywood because of its many layers it is used by the cabinet making industry primary for drawer sides. Compared to domestic birch plywood, baltic birch plywood is heavier, because the inner plies are birch too.
One of the drawbacks of this plywood is its standard size of 5' x 5'. It is difficult to handle and creates a problem if you need a cabinet size longer than 60".
Being imported also means it is manufactured using the metric system even though it is retailed in fractional measurements. That should not create any big problems unless you use dado and rabbet joints in your construction.
The least expensive sheet material for cabinet cases is particleboard. Particleboard is a recycled product from the waste of various wood manufacturing products. Made in three layers, the face layers are smaller wood particles than the core layer, for a smoother surface. With the low cost of these products come several drawbacks.
Particleboards are weak in strength across both directions compared to plywood. Screw and nail holding ability is very low and any exposure to moisture will swell the panel drastically. Particleboard is commercially used as a substitute for laminate, thin solid surface countertops or European-style cabinets.
The term melamine is commonly used for a sheet product coated with melamine-impregnated paper. The coating is applied to plywood or, more commonly, particleboard.
Melamine is used in most European-style cabinet cases and by most of the large cabinet manufacturers in various colors on traditional face frame cabinets.
This is a very economical product, about half the price of pre-finished plywood, with the same outcome: a pre-finished cabinet case. Most melamine products are manufactured in the U.S., which means that if you buy a ¾" sheet it is ¾" thick, not 18 mm.
Even though using melamine-coated particleboard will give you an inexpensive case that needs no finishing, it has the same drawbacks as plain particleboard: low strength and poor screw- and nail-holding capacity. The melamine coating is scratch resistant, but brittle at the edges. The industry uses special machines for a chip-free cut. A triple-chip laminate blade gives a satisfactory cut with standard table saws.
The paper-coated surface does not bond well with glue. Any glue joint needs to be rabbeted or dadoed.
Choosing a saw blade
Plywood and melamine boards are best cut with a triple-chip-ground saw blade, with 80 or more teeth, to prevent tearout. This blade should not be confused with an 80-tooth fine cut-off blade, which has an alternate top bevel grind.
MDF and HDF (medium- or high-density fiberboard) are made out of wood fibers, not particles like particleboard. This material is very stable and consistent. MDF is sold as paint-grade stock or veneered with popular veneers. Because of the dense core material, chipping is not a problem — the veneer has a solid contact with the substrate without voids. Screw- and nail- holding capacities are better than particleboard, but not as good as plywood. MDF gives the cleanest machined edges and surfaces due to the consistent density of the wood fibers in all directions. This makes it an excellent material for templates and jigs. MDF/HDF is also the preferred choice for painted cabinet parts such as doors or drawer sides. The major drawback of MDF is its sheer weight. A ¾", 4' x 8' sheet can weigh in excess of 90 lbs.
What's the Best?
If you ask five professional cabinetmakers, you probably will get five different answers.
As I pointed out, each sheet stock has its advantages and disadvantages. Another factor is the availability of the material. Most home improvement stores carry some sort of cabinet-grade plywood, so if you have one of these stores close by and the nearest wholesaler is 100 miles away, use common sense and get your supply around the corner. If you have access to all of the different composite materials, my recommendation is pre-finished birch plywood. If you want to save money, go with Baltic birch plywood. Cabinets built out of these materials are strong, lightweight and have a solid wood look.
Besides solid wood, wood veneer is the material most commonly used by woodworkers. Wood veneer has many advantages over solid wood. Per square feet of coverage, it is a lot cheaper, especially in rare wood species or grain patterns like birds-eye maple or burled walnut.
Wood veneer is very thin, glued to a paper backing. It is almost always used by gluing it to a substrate, which can be any of the materials mentioned above. Like composite material, veneer is sold as 4' x 8' sheets, but many retailers specializing for novices may sell smaller sheets. Unlike solid wood, a large coverage area of veneer can be mail-ordered without excessive shipping and handling charges.
Veneer is used to cover large surfaces where solid wood would be inappropriate because of its movement. Also because of its flexibility, veneer is used for irregular shapes or bent surfaces.
Even though there are special tools for cutting veneer, it is easily cut with a razor blade knife. For large, flat surfaces, contact cement is usually used to glue the veneer to the substrate. For irregular shapes, a vacuum press will do the best job with yellow wood glue and if allowed to cure for several hours.
— Udo Schmidt is a contributing editor to Woodcraft Magazine.
Hardwood Plywood Grades
Hardwood plywood grades are a voluntary standard of the industry. Manufacturers can choose whether to grade their products according to ANSI* (American National Standards Institute) hardwood plywood standards. Hardwood plywood is graded with a letter for the face of the panel and a number for the back of the panel. The best panel grade is A-1. Face grades range from A to E and back grades from 1 to 4. In ½" to ¾" thicknesses, a typical cabinet plywood grade is A-1 or B-2. In ¼" plywood a typical cabinet plywood grade is A-4.
• Slight mineral streaks and vine marks
• No sound or repaired knots
• Conspicuous burls maximum size: 3/8".
• Two 1/16" x 6" blended repaired tapering hairline splits
• May be bookmatched
• Slight mineral streaks
• Vine marks, small repaired knots, conspicuous burls
• Two 1/8" x 6" blended repaired tapering hairline splits
• Not bookmatched
• Mineral streaks and vine marks are acceptable
• No limit to pin knots and small burls
• Up to eight sound and repaired knots
• Up to four 3/16" x 8" blended repaired tapering hairline splits
• Up to 16 sound tight knots; no knotholes; sound tight burls; mineral streaks, and up to six repaired splits or joints
• Up to 16 sound tight knots; repaired knot holes; sound tight burls and mineral streaks; up to six repaired splits or joints
• Larger tight knots; larger repaired knots and larger repaired splits or joints
• Any species of hardwood; unlimited knot holes, tight knots and burls; can have open defects
*ANSI is a private, non-profit organization that administers and coordinates the U.S. voluntary standardization and conformity assessment system.
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