INTRODUCTION TO PLYWOOD

There are many different types of wood--based man-made materials on the market.  Each
has its own purppose, drawbacks and benefits.  Plywood is not just CDX which is used so much in home building.  In the wood products industry Plywood is the generic term for any sheet product regardless of construction.  MDF, VC, CDX are somewhat interchange-
able.

MDF (Medium Density Fiber Core Hardwood Plywood)
This product 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 a paint-grade.

This material is extremely stable to work with and 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.  This provides for better bonding of the wood veneer and less chipping during crosscut operation. 

The primary drawback to this product is weight.  A 3/4"x4'x8' sheet can weigh as much as 70 to 90 pounds.  The density of the core is expressed as the weight of a one cubic foot (1'x1'x1') block of material. 

MDO and HDO (Medium and High Density Overlay Plywood)
MDO and HDO consist of a core material, like laminated fir veneer, overlaid with a pressed fiber material.  This is a typicel veneer core plywood (common plywood) with an MDF surface.  The weight is lower than a full MDF, but the surface is more stable than a veneer core plywood.

VC (Veneer Core Hardwood 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, it is a light weight material and therefore, easy to handle.

The drawbacks of VC plywood are: voids in the core and face; the lack of consistency in thickness from sheet to sheet (and perhaps within the same sheet); pre-veneered surface is coarser and does not accept veneer as well which can result in more chipping and tearout during cutting; because of the differences in the direction of the layering of the product you can experience burning when using a fine laminate blade; the saw-cut edges are not as clean and smooth as with other products so it doesn't take edge gluing as well, and finally the inconsistency of the pre-veneered surface can result in thin spots in the veneer.

PBC (Particle Board Core Plywood)
PBC uses a coarser wood dust than MDF.  This results in a slightly lower weight but the edges and surfaces are not as smooth and consistent.  Most melamine products use PBC as the substrate.

MELAMINE
Melamine plywood is a thermally fused, resin saturated paper finish over a particle board core.  It is highly stain and abrasion resistant.  Contrary to popular belief, melamine is not the name of the paper finish; it's the name of the resin used to impregnate the paper liner,
however it is still called melamine.  It comes in a variety of colors, is highly stain and mar resistant, and is commonly used in the cabinet industry.  Depending on the grade of medalmine, 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.

HDP (High Density Maple/Birch - Baltic Birch or Appleply)
High density plywoods (HDP) typicaly 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 edge.

Baltic Birch is probably the most common type of HDP, and uses birch as the substrate.
This comes in 5'x5' 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 denisty maple plywood.  From a fabrication standpoint, 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.

OSD (Oriented Strand Board)
OSD plywood is produced by binding wood chips together with a mix of glue and resin.  The mixture is then cast and baked to produce a plywood-like sheet.  Because OSD does not use costly veneers it is as much as half the price of resular plywood.  It tends to have better waterproof characteristics, since the glue and resin make the wood chips impervious to moisture that can seep in between the plies of standard plywood.  This means that OSD is the perfect choice for applications such as exterior wall sheathing and roofing.

CDX
This plywood is produced by gluing together sheets of veneer, with each layer being glued the opposite grain to the one below it.  The result is a wooden sheet that is proportionally as strong as steel, without the weight that comes along with concrete or steel.  Because of this strength, CDX plywood is the most popular choice for interior flooring, CDX plywood will stand up to many years of use and abuse without warping or breaking.

DIFFERENCES BETWEEN THE TWO PLYWOD BOARD TYPES
CDX plywood is classified as an "exposure" type of sheating rather than an "exterior".  This means that CDX plywood should only be used inside or on surfaces that will not be exposed to water once the building process is complete.  Because of its chemical properties, however, OSD is perfect for exterior applications.  Another difference between the two plywood board types is appearance; if you are looking for a wood grain, choose CDX, since OSD plywood is generally flat on one side and rough on the other.  A good builder can save money by using both OSD and CDX plywood.