Cutting MDF Compared to Plywood

Both MDF and Plywood can be very tough on the cutting tools.  This is why it is important to use saw blades that are specifically designed for cutting MDF and Plywood.  Below is a link to some tools specially designed for cutting MDF and Plywood along with some helpful tips on how to choose a saw blade for cutting MDF and Plywood.  The rest of this article explains why MDF and Plywood have such a high wear on cutting tools and the differences bewteen the two materials.

Saw Blades for cutting MDF and Plywood

MDF is much harder on tools than plywood is for a variety of reasons.  

1. The density – especially the varying density (see below)
2. The mechanism of separation.  Plywood shears while MDF ruptures. 
3. The amount and kinds of fiber
4. The amount of glue and additives used to stretch it.   

MDF is harder on cutting tools for several reasons.

1. There is just more material being cut.  Typically MDF is heavier than plywood because MDF uses wood fibers and then compresses them.  (See below) 
2. Plywood uses peeled wood while MDF can use a variety of fibers some much more abrasive.  
3. Plywood and MDF can both use the same woods.  However in plywood the cells are open or hollow.  In MDF the cells have been compressed flat.  Cutting hollow a straw is easier than cutting a solid piece of plastic because you can cut the straw one wall at a time. 
4. The binder is distributed differently in the two materials.  In plywood the binder is largely in the overlapping sheets.  MDF can be considered binder with particles in it. 

In analysis of worn tools we see several things. 

1. Just the amount of material being cut.   
2. MDF is more abrasive because it denser.  When cutting the fibers are severed but the cut material rubs on the side of the tooth causing wear similar to the effect of a stiff bush versus a soft brush. 
3. MDF seems to trap more heat in the cut and heat affects the saw tips.  Again this is a natural property of the density and the glue distribution.

 Suppositions

1. It is widely believed that MDF is “grabbier” on cutting tools and tests with ammeters have seemed to confirm this.   MDF draws more power than wood or plywood.      
2. It is widely believed that the glue in MDF is stretched with a clay like binder much as clay is used in paper.  This binder is much more abrasive than wood as clay is technically fine sand. 
3. MDF is made of wood fibers and any source can be used.  Although the fibers are broken down into cells they retain many of their base properties.  Bamboo cells have different properties than fir cells. 

The Materials 

Particleboard

This is the generic name for a panel manufactured from lignocellulosic or plant materials. Technically, this is can mean anything from straw or Bagasse (sugarcane stalks) to wood. This material is combined with a synthetic resin or other suitable binder, and then bonded together under heat and pressure. The basic raw materials for particleboard are plant residues or low-quality logs. Some recycled material, where it is economical to use such a substance, is now part of the raw material supply. 

MDF, or Medium Density Fiberboard

The same general procedure is employed to manufacture MDF, except that the panels are compressed to a density of 0.50 to 0.80 specific gravity in a hot press by a process in which the entire interfiber bond is created by the added [synthetic resin or other suitable] binder. A wide variety of raw material types can be handled in an MDF plant. These types range from pulp chips to planer shavings to plywood trim to sawdust. Other non-wood materials, such as bagasse, (sugarcane stalks) also make excellent MDF. 

Plywood

These panels are made by laying up layers, or plies, of wood so that the grain direction in each ply runs at right angles to the one next to it. Cross-grain construction is what gives plywood its strength and dimensional stability. Standard veneer-core plywood 3/4 in. thick consists of seven plies: two outer veneers, plus five hardwood or softwood plies between them.  The layer structure leads to more uniform properties than solid wood, since the effects of grain anisotropy are minimized. The properties of plywood vary with the quality of the constituent layers; typical values are listed below. 

Physical Properties 

MDF

Modulus of Elasticity of 2500 - 5000 MPa

Modulus of Rupture is from 28 to 80 MPa.

Density of 600-800 kg/m³

Particle board density 160-450 kg/m³

Hardboard (high-density fiberboard) 500-1,450 kg/m³

Color (visual) ……………… Beige

Density ………………………39 lbs./foot³

Internal Bond ……………….90 lbs/inch²

Moisture Content …………..5-8%

Hardness (Shore D)……….45-55

MDF weight 70 90- - 100 

Plywood

Tensile Strength, Ultimate 31 MPa 4500 psi  parallel to face;

Flexural Modulus 9.3 GPa 1350 ksi  8.2 - 10.3 GPa.

Modulus of rupture in bending is typically 0.06 GPa.

Compressive Yield Strength 31 - 41 MPa 4500 - 5950 psi  parallel to face

Shear Modulus 0.17 GPa 24.7 ksi  in plane (rolling shear)

Shear Modulus 0.7 GPa 102 ksi through thickness (edgewise shear)

Shear Strength 1.9 MPa 276 psi in plane (rolling shear).

Shear Strength 6.2 MPa 899 psi through thickness (edgewise shear) 

Thermal Properties

CTE, linear 20°C 6.1 µm/m-°C 3.39 µin/in-°F 

Medium Density Fiberboard - The Manufacturing Process 

Ø      Debarking

Ø      Chipping

Ø      Pulping

Ø      The Blowline

Ø      Mat Formation

Ø      Pressing

Ø      Finishing 

Introduction

MDF is a wood based composite. The primary constituent is a softwood that has been broken down into wood fibers; that is the cells (tracheids, vessels, fibers and fiber-tracheids), which are far smaller entities than those used in particleboard. A wide variety of softwood species will constitute a suitable base for MDF production, though if too many species are used too great a variation in the properties of the finished MDF will result. 

Other materials successfully used have been waste paper, randomly collected waste wood and bamboo. 

Mixing wood and other non-wood materials such as fibers of glass, steel, carbon and aramide have all resulted in successful MDF type products being produced. 

Debarking

Once the MDF plant has obtained suitable logs, the first process is debarking. The logs could be used with the bark, as could any fibrous material, but for optimization of the final product the bark is removed to; decrease equipment damaging grit, allow faster drainage of water during mat formation, decrease organic waste load by 10-15 %, stabilize pH levels ( reduces corrosion of tools ) and increase surface finish. 

In some manufacturing plants the debarking process is not important as the plant obtains chips rather than logs. The chips can come from the waste of another operation or from logs chipped in the forest. 

Chipping

Though some plants accept chips directly from other operations, chipping is typically done at the MDF plant. A disc chipper, containing anything from four to sixteen blades, is used. The blades are arranged radially on a plate and the spinning plate is faced perpendicularly to the log feed. The feed speed of the logs, the radial speed of the knife plate, the protrusion distance of the knives and the angle of the knives, control the chip size. 

The chips are then screened and those that are oversized may be rechipped, and those that are undersized used as fuel. There may be a blending of chips from different sources or timber species to enhance certain properties. The chips are washed, and a magnet or other scanner may be passed over to detect impurities. 

Pulping

MDF takes much of its characteristics from the fact that it uses wood cells, (tracheids, vessels, fibers and fiber-tracheids), rather than particles. This can be done by a Masonite gun Process, Atmospheric or Pressurized Disk refiner. The Asplund defibrator pressurized disk refinement being that primarily used in MDF manufacture. The chips are compacted using a screwfeeder into small plugs which are heated for 30 to 120 seconds (this softens the wood), then fed into the defibrator. The defibrator consists of two counter-rotating plates each with radial grooves that get smaller as they get closer to the circumference. The plug is fed into the centre and gets broken down as the centrifugal forces push it toward the outside of the plates where the groves are finer. The feeding devices at the entrance and exit to the defibrator maintain suitably high pressure and temperature (about 150 C). 

The high temperatures lower the energy required to deliberate wood as there is a softening of lignin that facilitates fiber separation along the middle lamella. The steam is then separated from the pulp; the total time in the defibrator is about one minute. They pulp may pass through a secondary refiner to ensure the fibers meet pre-determined levels of `freeness'.

The resulting pulp is light, fine, fluffy and light in color. As the accompanying micrograph of an MDF sample shows the fiber walls are still intact. 

The Blowline

After defibration fibers enter the blowline. The blowline is initially only 1.5 in. in diameter with the fibers passing through at high velocity.  Wax, used to improve the moisture resistance of the finished board, and resin are added in the blowline while the fibers are still wet, as dry fibers would form bundles, due to hydro bonding, and material consistency would be lost. The blowline now expands to 60 in. in diameter and fibers are dried by heating coils warming the blowline to about 550 F. The air-fiber ratio is about 500 cubic ft/lb with air speed of 500 ft/min though the air is still humid and the resin does not yet cure. The agitation of fibers in the blowline helps disperse resin consistently. Exit temperature is about 180 F. 

The fibers may be stored in bins for an unspecified length of time but the board making process is usually continuous from here on. The Moisture Content of the fibers is 12%, and thus this is considered a dry process. 

The blowline mixing process and the use of dry fibers are distinguishing characteristics of MDF. 

Mat Formation

In order to form a continuous and consistent mat the following problems must be over come: the fact that considerable air velocities must be maintained to suspend fibers, fiber/air suspension does not flow laterally on a horizontal support and fiber form lumps. 

One way of overcoming this is a Pendistor. Impulses of air act on the fiber as it falls down the shaft to a vacuum box at the start of the conveyor belt that carries the mat. The oscillatory action on the fibers spreads them uniformly into a mat and they begin their run on the conveyor belt at between 8 – 24 inches thick. 

Pressing

The mat can either be laterally cut to size as it leaves the pendistor or it can be cut half way through its run by a synchronized flying cut off saw. The density profile of the panel

is critical to achieving satisfactory strength properties. Concentrating mass and hence load bearing ability, at the top and bottom of the board means that inertial properties are maximized and the greatest strength can be obtained for minimal weight. 

This is achieved by the press acting at impacted pressure initially and then slower pressure application. As an example for a 16mm board:

• Press closed. 20 seconds to bring mat to 28 mm.

• 28 seconds at 26mm.

• 23 seconds at 25mm.

• 125 seconds at 18.3

• Total time of 330 seconds to bring board to 16mm, then decompression time. 

The pressure may reach 50,000 psi. and be heated to over 200 C. Thicker boards may require up to 110,000 psi. and additional steam or radio frequency heating.

Finishing

After pressing boards are cooled in a star dryer and final trimmed and sanded. They are given a few days storage to allow complete curing of resins. The boards are commonly given a colored melamine laminate, though natural wood veneers and raw MDF are common.