MDF FAQ - Table of Contents
Q: What does MDF stand for?
A: Medium Density Fiberboard.
Q: What is MDF?
A: MDF belongs to the hardboard family of products which are
made from wood fibers glued under heat and pressure. Medium Density Fiberboard
typically has densities between 33 and 50 pounds per cubic feet while High
Density Fiberboard (HDF) ranges between 50 and 80 pounds per cubic feet.
Hardboard was first produced in 1924 by W. H. Mason, founder of Masonite
Corp. The term Masonite is therefore often used to denote hardboard products,
especially HDF.
Physical and dimensional tolerances for MDF are specified in ANSI
A208.2-1986
Q: What properties does MDF exhibit?
A: MDF has many qualities that make it an ideal replacement for plywood
or particle board. It is dense, flat, stiff, has no knots and is easily
machined. Its fine particles provide dimensional stability without a predominant
"grain" (as is the case with lumber). Unlike most plywoods, MDF
contains no voids, and will deliver sharp edges with no tearout. MDF is
very well damped acoustically thus making it an ideal material for speaker
enclosures.
Below are some typical values for the modulus of elasticity (in million
pounds per square inch) and density (in pounds per cubic feet) for MDF,
Oak, Pine and Fir Plywood
- MDF - 0.53, 48
- Oak - 1.55, 38
- Pine - 1.3, 29
- Fir plywood - 1.2. 33
The modulus of elasticity (MOE), also called Young's modulus, is the
ratio of stress to strain, where stress is the force per unit area placed
on the item and strain is the deformation caused by the stress. The MOE
is therefore a measure of stiffness.
Q: What does MDF look like?
A: Here is an image of a birch veneered MDF board on top and for contrast
an image of veneered particle board below. Notice the much larger and obvious particles.
Q: Are there any drawbacks to using MDF?
A: While MDF has been in use for almost 30 years, it is only now becoming
available to the general public. Finding MDF may end up being the single
toughest part of using it. As its density implies, MDF is very heavy and
thus potentially difficult to handle. See the safety issues below.
Q: What are the safety issues to consider when working with MDF?
A: MDF is typically made with urea-formaldehyde resin totalling 9% by
weight. While most people will not be affected by this, people sensitive
to formaldehyde emissions should consider low formaldehyde or formaldehyde-free
MDF, or consider methods of controlling these emisions through proper finishing.
Finishes that work best at controlling formaldehyde emissions are solid
add-on surfaces such as high pressure laminates, vinyl covering, and finished
wood veneers. Less effective at controlling emissions are simple seal coats,
oil and latext paints, danish oil, and wax. Plum Creek makes low-formaldehyde
MDF, while Medite II and Medex from Medite Corp. are formaldehyde-free
MDF.
Dust is another MDF hazard. The large amount of dust released when working
MDF makes proper respiratory and eye protection mandatory. At a minimum
use a dust mask. A respirator is preferable. Shop dust collection (or even
a ShopVac) would greatly help the removal of dust from not only the air
but also the working surfaces, making them easier to see. Goggles should
always be worn while using tools.
Q: Is all MDF the same?
A: No. MDF from different sources will vary in texture, density, color, etc.
Q: How is MDF sold?
A: MDF is manufactured in sheets up to 8ft x 25ft. Typical consumer
level sheets are 4x8 or 5x8 and 1/2 inch, 3/4 inch and 1 inch in thickness.
Thicknesses can also be metric - an important consideration when considering
the use of English system tools (such as router bits). MDF is also available
with a variety of veneers and laminates pre-applied, which may affect its
actual thickness.
Q: What about MDO, particle board, hardboard, void-free plywood?
A: Medium Density Overlay and High Density Overlay
are plywood products with a resin impregnated paper coating. They are often
used for exterior painted surfaces. These are not fiber based products.
Likewise, particle board is not fiber based; it is a solid wood composite
product. Along with flakeboard and other engineered lumbers, composite
products are made from wood flakes, chips, splinters, etc., formed into
layers and held together by resin glues and heated under pressure. Being
layered and consisting of larger chunks, particle board does not have the
uniform texture of MDF.
While MDF is a hardboard, the term hardboard is often used to refer
to 1/8 or 1/4 inch thick HDF, usually containing a screen pattern on one
surface. As previously mentioned, this is commonly referred to as Masonite.
Plywood is made from an odd number of lumber plies, each layer having
a grain direction at right angles to the previous layer. This arrangement
provides a dimensionally stable product. Void-free plywood uses plies with
supposedly no holes, thus the completed plywood has in theory no voids.
Baltic birch plywood is often sold as void-free plywood though some users
have encountered small voids in these products. Be sure to ask specifically
for void-free plywood if this is what you are looking for.
Q: Can I build speakers with {MDF | particle board | plywood | solid wood}?
A: You can build speakers with whatever you like. However, MDF is often
the material of choice. Its stiffness and density yield good accoustical
damping properties. Particle board and plywood are cheaper and can still
be used if cost is an issue. Plywood (especially if void free) can be used
in the main baffle as a sandwich material to better hold fasteners. Plywood
is also a good material for making braces inside speaker boxes. Solid wood
(lumber) suffers from movement - the swelling and shrinking of wood due
to environmental changes such as humidity - and is therefore not a good
material for speaker enclosures. Lumber is also not as acoustically dead
as MDF.
Q: What should I use to cut and mill MDF?
A: MDF can be treated much like a fine grained hardwood. Its high glue
content means that steel cutting tools will dull VERY quickly; thus the
use of carbide tools is highly recommended. Always keep your tools sharp
for efficiency and safety.
The following recommendations are from the The National Particleboard
Association publication:
- For general shop or table saw use with decent cut and good blade life,
a 50 tooth, 10 inch combination blade may be used.
- For those demanding a better cut, consider a 60 tooth, 10 inch blade
with alternate top bevel (ATB) teeth at 15 degrees, 10 degree positive
hook, 5 degree side clearance, 10 degree outside diameter clearance, and
low approach angle (blade projecting no more than 0.5 inch through top
of material).
- For an even smoother cut, consider an 80 tooth, 10 inch blade with
15 degree ATB, 10 degree alternate face bevel, 15 degree positive hook,
and 7 degrees side clearance. This is costlier and may result in a shorter
blade life.
Q: Where can I find MDF?
A: Availability varies geographically so there is no simple answer to
this question. Hobbyists have found MDF from a wide variety of sources
including, but not limited to :
- large warehouse style supply dealers (Home Depot, Lowes, etc)
- small local lumber yards
- cabinet shops who buy in large quantities and are willing to part with some
- surplus building supply dealers
As MDF becomes more popular you will see it more and more in your
local hardwares. Sometimes, they will only have smaller 2x4 pieces or 1x4
pieces designed for use as shelving, so be sure to look around or ask.
Beware of clueless store clerks trying to pass plywood, particle board
or MDO as MDF ! Note that many lumber yards can special order MDF but may
not realize this, so it never hurts to ask. Ask them to check their price
book for availability.
Q: What kind of joints can I use?
A: Because MDF can be milled to just about any profile, there are many
possible joints. However, not all make sense in the context of speaker
building.
- butt - this simplest of joints may not be ideal for furniture but works
very well for building speakers, especially when combined with biscuits
(for alignment) and screws (for holding strength while the glue dries).
- miter - works well when using pre-finished MDF (veneered or laminated)
thus leaving no exposed unfinished surfaces.
- lock-miter, dovetail and other routered joints - works just like lumber.
These joints have limited use in most speaker enclosures.
- rabbets, dadoes, grooves and other saw cut joints - same as with hardwood.
Note that these can also be cut with a router. These joints may be useful,
depending on the design of the speaker.
- spline, biscuit, dowel - as with lumber, the glue joint is stronger
than the MDF. Dowel holes should be 0.002 to 0.003 inch larger than the
average dowel diameter, and 1/32 to 1/16 inch deeper than the actual depth
used. Plain or spiral grooved dowels are preferred over fluted or multigrooved
dowels. Biscuits are very handy for alignment of parts in addition to the
additional glueing surface provided.
MDF can also be edge glued to make larger surfaces, although this is
not likely to happen except with exceptionally large speakers. Panels can
be scarfed, doweled, tongue & grooved and finger jointed.
Q: How may various fasteners be used with MDF?
A: Some typical fasteners and their uses follow :
- staples - Do not staple within 3/4 inch of any corner. Coated staples
hold better than smooth staples. Use a finer wire staple if splitting is
a problem. Drive at right angle to the surface to avoid bending.
- nails - The same rules apply to nails as they apply to staples. Use
ring-shank nails to avoid fiber raising around the nail head; do not use
smooth nails.
- screws - Drill pilot holes between 85 % and 90 % of the root diameter
of the screw used and at least as deep as the screw. Untapered sheet metal
screws with constant size shank are good, as are some untapered wood screws.
Pilot hole sizes and minimum edge distances for common screw sizes are :
- #6 screw - 3/32 inch pilot - 1/2 inch edge distance
- #8 screw - 7/64 inch pilot - 5/8 inch edge distance
- #10 screw - 1/8 inch pilot - 1 inch edge distance
Do not rely solely on the above fasteners for building speaker enclosures.
This is especially true for butt joints. Combine glue with screws for a
simple and strong joint.
Q: What kinds of glues can I use with MDF?
A: Good glues to use are gap-filling glues such as polyvinyl acetate
(PVA) typically known as yellow glue, modified PVA glues like Titebond
II or white glues. Epoxy, urea and hot melt glues may also be used.
Q: How can I finish my MDF speaker?
A: For that finished look, there are many options requiring different
levels of woodworking skills.
- The box may be painted. Be sure to seal and prime the surface before
painting to ensure even absorption on all surfaces. A high gloss piano
finish can be made with combinations of spray enamel, spray lacquer or
other topcoats. A little experimentation at this juncture can be very rewarding.
- The box may be laminated. Options include melamine, Formica, or even
contact paper. Be sure the surface is clear of dust before applying any
laminate.
- The box may be veneered. Carried out properly, veneering can yield
a very professional looking speaker. Refer to the references below for
veneering info. A veneered surface can be finished with lacquer, varnish,
oil or wax depending on individual taste. Stains and dyes may be used to
modify the color as desired.
Note that raw MDF is very porous. Use a generous amount of glue to ensure
a proper bond.
Q: Are there any other tips for using MDF in speaker enclosures?
A: Some members of the DIY
Loudspeakers list have submitted the following tips for working with
MDF.
- Apply the finish to the enclosure after assembly. Then cut the speaker
holes. This gives the best appearance for the least work.
- Use shelf braces to stiffen the box and to further support edge joints.
Also use 3/4 x 3/4 inch lumber along the inside of edge joints for extra
strength and for stiffening.
- Use a table saw for dados and grooves when possible. This usually gives
better, straighter results.
- On large panels with no bracing or shelves attached, reinforce the
panel with one or more 3/4 x 3/4 inch ribs.
- If using particle board, fill exposed edges with spackle or wood filler.
Then sand all surfaces thoroughly to get a smooth surface. Prime before
painting.
- Two coats of yellow glue applied 10 minutes apart may be used to seal
MDF edges.
- Wood or autobody sealer may also be used to seal MDF edges.
- To avoid stripping threads when mounting drivers to MDF baffles use
threaded inserts such as T-nuts.
- Excess glue may be removed before it dries with a damp rag. For pre-veneered
MDF, care should be taken to avoid smearing glue into the wood grain. An
alternative method is to scrape off the glue in its semi-hard state.