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.
Medium Density Fiber Core Hardwood Plywood (MDF)
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 Hardwood Plywood (VC)
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.
The drawbacks of VC plywood are:
- 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 edge
Baltic 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.