Plywood is a manufactured wood panel made from thin sheets of wood veneer. It is one of the most widely used wood products. It is flexible, inexpensive, workable, and re-usable, and usually can be manufactured locally. Plywood is used instead of plain wood because of plywood's resistance to cracking, shrinkage, splitting, and twisting/warping, and because of its generally high strength.
Plywood layers (called veneers) are glued together, with adjacent plies having their wood grain at right angles to each other, to form a composite material. This alternation of the grain is called cross-graining and has several important benefits: it reduces the tendency of wood to split when nailed at the edges; it reduces expansion and shrinkage, providing improved dimensional stability; and it makes the strength of the panel consistent across both directions. There is usually an odd number of plies, so that the sheet is balanced—this reduces warping. Because plywood is bonded with grains running against one another and with an odd number of composite parts, it is very hard to bend it perpendicular to the grain direction.
Plywood was invented about 3400 B.C. by the Ancient Mesopotamians, who attached several thinner layers of wood together to make one thick layer. They originally did this during a shortage of quality wood, gluing very thin layers of quality wood over lesser-quality wood.
Different varieties of plywood exist for different applications:
Softwood panel is usually made either of cedar, Douglas fir or spruce, pine, and fir (collectively known as spruce-pine-fir or SPF) or redwood and is typically used for construction and industrial purposes
Used for demanding end uses. Birch plywood is characterized by its excellent strength, stiffness and resistance to creep. It has a high planar shear strength and impact resistance, which make it especially suitable for heavy-duty floor and wall structures. Oriented plywood construction has a high wheel-carrying capacity. Birch plywood has excellent surface hardness, and damage- and wear-resistance
Tropical plywood is made of mixed species of tropical wood. Originally from the Asian region, it is now also manufactured in African and South American countries. Tropical plywood is superior to softwood plywood due to its density, strength, evenness of layers, and high quality.
Certain plywoods do not have alternating plies. These are designed for specific purposes
High-strength plywood also known as aircraft plywood, is made from mahogany and/or birch, and uses adhesives with increased resistance to heat and humidity. It was used for several World War II fighter aircraft. Although the British-built Mosquito bomber, nicknamed "The Wooden Wonder", was constructed of a plywood monocoque, this was formed in moulds from individual veneers of birch, balsa and birch rather than machined from pre-laminated plywood sheets.
Decorative plywood (overlaid plywood)
Usually faced with hardwood, including ash, oak, red oak, birch, maple, mahogany, Philippine mahogany (often called lauan, luan or meranti and having no relation to true mahogany), rose wood, teak and a large number of other hardwoods. However, Formica, metal and resin-impregnated paper or fabric bonded are also added on top of plywood at both side as a kind of ready for use in the decoration field. This plywood is a lot easier to dye and draw on than any other plywoods.
Flexible plywood is very flexible and is designed for making curved parts. In the UK this is sometimes known as "Hatters Ply" as it was used to make stovepipe hats in Victorian times .It is also often referred to as "Bendy Ply" due to its flexibility. However these may not be termed plywood in some countries because the basic description of plywood is layers of veneered wood laid on top of each other with the grain of each layer perpendicular to the grain of the next. In the U.S., the terms "Bender Board" and "Wiggle Board" are commonly used.
Marine plywood is manufactured from durable face and core veneers, with few defects so it performs longer in humid and wet conditions and resists delaminating and fungal attack. Its construction is such that it can be used in environments where it is exposed to moisture for long periods. More recently, tropical producers have become dominant in the marine plywood market. Okoumé from Gabon is now the accepted standard for marine plywood, even though the wood is not very resistant to rot and decay. Each wood veneer will be from tropical hardwoods, have negligible core gap, limiting the chance of trapping water in the plywood and hence providing a solid and stable glue bond. It uses an exterior Water and Boil Proof (WBP) glue similar to most exterior plywoods.
Other types of plywoods include fire-retardant, moisture-resistant, sign-grade and pressure-treated. However, the plywood may be treated with various chemicals to improve the plywood's fireproofing. Each of these products is designed to fill a need in industry.
Plywood is used in many applications that need high-quality, high-strength sheet material. Quality in this context means resistance to cracking, breaking, shrinkage, twisting and warping.
Exterior glued plywood is suitable for outdoor use, but because moisture affects the strength of wood, optimal performance is achieved in end uses where the wood's moisture content remains relatively low. On the other hand, subzero conditions don't affect plywood's dimensional or strength properties, which makes some special applications possible.
Typical end uses of spruce plywood are:
- Floors, walls and roofs in house constructions
- Wind bracing panels
- Vehicle internal body work
- Packages and boxes
There are many different types of wood-based man-made materials on the market. Each has its own purpose, drawbacks, and benefits. When most people hear the term plywood, they think of CDX (or whatever...) which is used to build houses. Yet in the wholesale wood products industry, plywood is a generic term for any sheet product regardless of construction (for example; MDF, VC, CDX.) The two terms are interchangeable, more-or-less.
MDF is made from fine wood dust mixed with a binder and heat-pressed into panels. The sheets can be sold as-is, or a veneer skin, like oak or maple, can be laid up on the sheet. (The veneered sheet is the most common form, but blank MDF sheets are available as Paint-Grade)This material is extremely stable to work with, and is typically very consistent from batch to batch. A 3/4" thick sheet purchased over a year ago is exactly the same thickness as a new sheet purchased today. The surface below the veneer is typically free of voids and blisters, resulting in a better veneer consistency and bond. With this better bonding of the wood veneer, there is less chipping during a crosscut operation. I have also observed that this material is easy to machine either by saw or router, and the cut edges are excellent for glue adhesion. (I have heard it mentioned that MDF is hard on cutters, but personally, I disagree with this statement. I feel that MDF is rather easy on the cutters.)The primary drawback to this product is weight. A 3/4" x 4' x 8' sheet can weigh as much as 70 to 90 pounds per sheet. The density of the core is expressed as the weight of a one cubic foot (1'x1'x1') block of the material. Therefore, an MDF sheet using a 48# (pound) core, will weigh 96 pounds. (48"x96"x3/4"= 2 cubic feet)Medium- and High-Density Overlay Plywood (MDO and HDO)
MDO and HDO consist of a core material, like laminated fir veneer, overlaid with a pressed fiber material. In short, this is a typical veneer core plywood (common plywood) with an MDF surface. This gives the best of both worlds; the weight is lower than a full MDF, but the surface is more stable than a veneer core plywood.
Veneer Core plywood is made from alternating layers of fir slices (common plywood) with a surface veneer of a finished woodgrain such as oak or maple. This construction gives VC plywood a distinct advantage over others in strength. This is a light weight material, and easy to handle.
- Voids in the core and face are common.
- VC is not always consistent in thickness from sheet to sheet, or within the same sheet.
- The pre-veneered surface is coarser, and does not accept veneer as well. This results in excessive chipping and tearout during machining.
- No matter how you cut it, you will be ripping some layers, and crosscutting others. This makes cutting this material with a fine laminate blade more difficult, with a greater tendency to burn.
- The saw-cut edges are not as clean and smooth as the other products, so this material does not take edge gluing as well.
- The inconsistency in the pre-veneered surface can result in thin spots in the veneer.
Lumber Core Plywood is manufactured from strips of solid lumber, typically basswood. The core is then surfaced and a veneer layer is applied. This is one of the most expensive plywood types to make, and is commonly used for applications where the edges cannot be concealed or need to be routed.As the popularity of this product diminishes, it is becoming more and more difficult to locate suppliers who are willing to carry high grade sheets. The quality of the core lumber is dropping in all but the best of grades. Most grades machine poorly. If the core is not glued up with consistent stock, voids can be present which will run the full length, or at least a portion of the full length, of the entire sheet.Because of this, care must be taken in selecting sheets if they are to be used for matched and sequenced door material, as a flaw in the core can wipe out an entire set of doors if they need to maintain grain matching from one to another.Particle Board Core Plywood (PBC)
PBC uses a coarser wood dust than MDF. Because of this, it has a slightly lower weight, but the edges and surfaces are not as smooth and consistent. Most melamine products use PBC as the substrate.
Melamine plywood is a thermally fused, resin saturated paper finish over a particle board core. It is highly stain and abrasion resistant. As a cabinet maker, I use a lot of this material. Even though glue manufactures claim to have developed an adhesive which bonds to the surface, I (personally) am not willing to take the chance; after all, this is a "paper" surface. (My personal recommendation to any one using this product is to dado the joints for better bonding.)Contrary to popular belief among many woodworkers, melamine is not the name of the paper finish; it's the name of the resin used to impregnate the paper liner (chemically C3H6N6). Even among manufacturers of this type of sheet product, however, it is still called melamine.This material comes in a variety of colors, is highly stain and mar resistant, and is commonly used in the cabinet industry for carcass construction.Depending on the grade of melamine, it can be brittle or soft, coarse or smooth. Typically, the higher grades of melamine are more brittle and will chip during machining but have a thicker surface and greater resistance to abrasion.I have found that the best blade for cutting melamine is a triple-chip laminate blade set with a blade height of about 1" above the top of the wood. With a higher blade height, there will be excessive chipping on the back of the sheet, and with a lower blade height, there will be some chipping on the top of the sheet. The reason for the top-side chipping with a low blade height is due to the teeth striking the surface veneer nearly perpendicular, and throwing chips forward.
High Density Maple/Birch (Baltic Birch or Appleply)
High density plywoods (HDP) typically come in either maple or birch specie. Unlike common plywood, HDP has many more plies, is generally void free, and uses a stronger species than fir. HDP is commonly used for drawer side material as it is strong, stable, and has a moderately attractive edgeBaltic Birch
Baltic birch is probably the most common type of HDP, and uses birch as the substrate. This will come in 5' x 5' sheets. For a 1/2" sheet, there are typically 7 to 9 plies. Being birch, the surface does not finish as nicely as the maple counterpart, and there is a tendency for splintering at the edge of a machined cut.Appleply
Appleply is a manufacturer's name for high density maple plywood. From a fabrication stand point, it is similar to Baltic Birch, in that it carries about the same number of plies, except Appleply comes in standard 4'x8' sheets. Because the surface is maple, there will be slightly more grain pattern on the surface, and the surface will sand much smoother. There is less splintering of the machined edges, and those splinters which do appear will be shorter and less inclined to align with the edge.