Forest Products Marketing Department

22 November, 1998

MDF is a wood based composite. The primary constituent is a softwood that has been broken down into fibres; that is the very cells (tracheids, vessels, fibres and fibre-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 and randomly collected waste wood. Mixing wood and other non-wood materials such as fibres of glass, steel, carbon and aramide have all resulted in successful MDF type products being produced.

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 optimisation 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%
• stabilise pH levels (reduces corrosion of tools)
• increase surface finish

The most popular debarker used in MDF production is a ring debarker, though rosser head and drum debarker can be used.

Fig.2: Cross-section of a five-knife ring debarker,

debarking a log

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

A cambio ring debarker can operate at feeds of up to one tonne per minute, the logs being typically 2 to 2.5 m in length. The tool heads held in place pneumatically or by springs, rotate about the logs and rip off nearly all the bark, and do little damage to the log. The waste bark can be sold or used for power on-site stations.

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. Following features govern the chip size

• the feed speed of the logs
• the radial speed of the knife plate
• the protrusion distance of the knives
• the angle of the knives

The chips are then screened and those that are oversized may be re-chipped, and those that are undersized used as fuel. Stockpiles of several hundred tonnes of chips are maintained. 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.

MDF takes much of its characteristics from the fact that it uses wood cells, rather than particles. This can be done by a Masonite Gun Process, atmospheric or pressured disk refiner. The defibrator pressurised disk refinement being that primarily used in MDF manufacture. The chips are compacted after using a screw feeder 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 counterwise 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 grooves are finer. The feeding devices at the entrance and exit to the defibrator maintain suitably high pressure and temperature (about 150 degree Celsius).

The high temperature lowers th energy required to defibrate wood as there is a softening of lignin that facilitates fibre separation along the middle lamella. The steam is then separated from the pulp, the total time in the defibrator is about one minute. The pulp may pass through a secondary refiner to ensure the fibres meet pre-determined levels of `freeness`.

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

Fig. 3: Micrograph of MDF fibres

After defibration fibres enter the blowline. The blowline is initially only 40mm in diameter with the fibres 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 fibres are still wet, as dry fibres would form bundles, due to hydro bonding, and material consistency would be lost. The blowline now expands to 1500 mm in diameter and fibres are dried by heating coils warming the blowline to about 290 ° C. The air-fibre ratio is about 32 kg/m³ with air speed of 150 m/min though the air is still humid and the resin does not yet cure. The agitation of fibres in the blowline helps disperse resin consistently. The exit temperature is about 85 ° C. The fibres may be stored in bins for an unspecified length of time but the board making process is usually continuos from here on. The moisture content of the fibres is 12%, and thus is considered a dry process.

The blowline mixing and the use of fibres are distinguishing characteristics of MDF.

In order to conform a continuos and constant mat the following problems must be overcome: the fact that considerable air velocities must be maintained to suspend fibres, fibre/air suspension does not flow laterally on a horizontal support and fibre form lumps. One way of overcoming this is a Pendistor.

Fig.4: The pendistor and mat formation of MDF

Impulses of air act on the fibre 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 fibres spreads them uniformly into a mat and they begin their run on the conveyor belt at between 230-610 mm thick.

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 synchronised 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 bottom and top of the board means that inertial properties are maximised and the greatest strength can be obtained for minimal weight.

Fig. 5: Compared density profile of 19mm MDF- and Particleboard (Spanplatte)

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

• Press closed. 20 seconds to bring mat to 28 mm
• 28 seconds at 26 mm
• 23 seconds at 25 mm
• 125 seconds at 18.3 mm
• Total time of 330 seconds to bring board to 16 mm, then decompression time

The pressure may reach 3,5 MPa and be heated to over 200 degree Celsius. Thicker boards may require up to 7,5 MPa and additional steam or radio frequency heating.

Presses can be divided into platen (of which there are single opening (daylight) and multi-opening (multi-daylight) types) and continuous types (e.g. Conti-Roll™ SIEMPELKAMP, Germany). Advantages and disadvantages may not be the same for each. Multi-opening presses are dominant in the industry at present. Continuous presses are being installed world-wide even for the process of thick panel manufacture.

Fig.6: Layout of MDF-Manufacturing-Process

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 coloured melamine laminate (e.g. High Pressure Laminates HPL), though natural wood veneers and raw MDF are common.

Nowadays MDF is mostly compared with particleboard (esp. particleboard with fine surface) or laminated boards. These are the main products which are used for joinery and cabinet manufacturing. In earlier times MDF was used as a substitute for massive wood, but today in America the share of furniture manufactured with MDF is higher than 45% (according to Deppe and Ernst, 1996)

Particleboard with fine surface is marked through a smooth surface which is achieved by using finer particles in the middle layers. Attempts of using finer surface layers did not reveal any positive results, since difficulties occurred when compressing the very fine particles homogeneously. As a result of still using small chips with cubic dimensions, the uneven shrinkage and swelling of the boards still remains a problem of keeping dimensions.

By manufacturing and using laminated boards it was attempted to decrease disadvantages of revising massive timber material like inhomogenous swelling and shrinkage and proneness to twist.

Special advantages of MDF in comparison to these products are good properties for moulding and sawing. MDF boards have smooth surfaces because of the fibre used, and they have what is known as a "tight edge". Shavings-type particleboard is somewhat porous in the core due to the chunky shape of the particles or shavings making up the core layer and because of the press cycle. As a result, direct moulding of edges and surfaces, lapsing of cover and edging strips, the possibility of lacquering or directly printing the surface are some examples for the superior properties of MDF .

Additionally, MDF has higher strength properties than Particleboard, whereas higher density means also higher weight. Especially for manufacturing of profiled fronts for furniture, especially kitchen- and wet-room-furniture MDF is often preferred to other materials due to the finer structure and the more homogeneous profile of density through the thickness of the board. The density of Particleboard various between 500 and 1200 kg/m³, whereas the density of MDF alternates in between 600 and 900 kg/m³. Whereas the surface of other materials must be painted with filling materials or must at least be sanded, the surface and profile of MDF does normally not require any further processing. If MDF is overlaid with thin films or light materials it must be considered that there is the danger that the board shines through. This can be avoided by using boards which are produced without any bark, so called light MDF.

It is possible to use additives to influence some properties of MDF. For example, Urea is used to neutralise or bound the share of formaldehyde, paraffin is used to increase the moisture resistance (by hydrophobing), Ammoniumphosphates are used as mean for fire protection and Kalihydrogenefluorids and Xyligels allow exterior use of the panels.

MDF-Boards can be sawn with all usual kinds of saws. Because of the high board density it is recommended to use tungsten carbide tipped sawing blades (compare Fig.7). A cutting speed of 40 to 80 m/s has revealed to be optimal for pre-cutting and final cuts.

When sawing overlaid MDF, the physical properties of the sawing equipment must be set according to the overlay.

Both feeding and cutting speed, rpm, number of teeth are linked together and the geometry of the blades must be considered for every new manufacturing process

Due to the splitting of timber into fibres and the high density of the boards the results of moulding can be compared to moulding massive timber.

When moulding and profiling MDF-strips and boards, tungsten tools should be used. Best results can be gained, when a high rotation speed of the spindle is combined with a speedy feeding speed. It is an advantage to use knives with small cutting angles. For profiling the faces normally routers and stencils or nowadays often CNC-machines are used.

Fig.8: Moulded MDF-doors for kitchen-cabinets, prepared for overlaying

For drilling, also for drilling rows of holes (e.g. for shelving), the same machinery and tools are used as for drilling plywood and particleboard.

In general the screw holding strength of MDF is at least two times higher than in normal particleboard, in some cases it is nearly the same as in some wood species. It is recommended to drill the holes before screwing to avoid cracking of the board. The best results are gained when the drill-diameter is as high as the core diameter of the screw. Of decisive influence for the screw holding strength is the distance to the edge of the board. This distance should be at least 20mm to the edge and the distance to the corner should be at least 70mm. The distance between screws should be more than 30mm.

Holes for dowels should be 0.1 to 0.2 mm bigger than the dowel itself. An uniform distribution of glue is guaranteed. This is supported by using moulded dowels.

MDF is very suitable for varnishing. But it is necessary to note a general order of steps:

1. Isolating

This is the key point of varnishing MDF. A badly carried out or even forgotten isolating leads to fissures of lacquer at the edges and/or the surfaces. This isolations are normally based on two components and are offered by all manufacturers of paints and lacquers.

2. Basic paint

After isolating the panel should be sanded once or twice with very fine sand paper with grids of 140 to 200 followed by painting it with a pigmented filler. If it is sold as high quality surface, a second basic paint should be applicated after sanding it again.

3. Putting on the lacquer

Many types of varnish are suited for painting MDF, every manufacturer of varnishes recommends his own products. A list of varnishes recommended can be ordered also by the manufacturers of the boards.

It is not a problem to overlay or veneer MDF, since it has an already sanded and plane surface. This is the reason, why the boards are specially suited for Overlaying and Veneering. In general it should be considered to use only formaldehyde-free materials as glues, overlays or lacquers. Further on the same rules are valid as for overlaying other materials like particleboard, blockboard or plywood.

Right after manufacturing boards are already sanded with grids of 120 to 140. In most cases it is possible to start with further processing (e.g. surface treatment) directly after delivery of the boards. For very high quality requirements the boards can be sanded with grids up to 400.

It is possible to get MDF already prepared for moulding. For this the panels are sliced on the backface so that bowing or cupping is possible without destroying the surface. They are offered in thickness of 8 or 10 mm. For higher thickness some panels should be glued together when moulding. Using this boards mouldings can be manufactured fast and cheap. When overlaying the moulded panels, it must be considered that the overlays also can follow the deformations of the MDF to avoid cracks or fissures in the overlay.

Fig.12: Prepared boards offered by some manufacturers for special mouldings

As a result it can be said, that the use of MDF-Boards must be discussed for every new field of application. Although there are all the above mentioned advantages like good strength properties, multiple possibilities for moulding and optimal core for overlaying, the disadvantage of MDF-boards is the high price. When these boards are used, possible alternatives must be considered (if high quality surface is required). It is possible and advisable to calculate also the decreasing manufacturing time when using MDF, since several steps of production are easier in comparison to particleboard usage. In future especially for high quality products and special design furniture MDF-boards

For these report we mainly used the following sources:

• different web-sites of the Internet
• Information from MDF-manufacturers:
• Hornitex Werke, Germany
• Fritz Egger Spanplattenindustrie GmbH & Co.KG, Germany
• Glunz AG, Hamm, Germany
• MDF, Medium Density Fibreboard, Deppe
• Plywood and Veneer-Based Products, Richard F. Baldwin, 1995
• Modern Particleboard & Dry-Process Fibreboard Manufacturing, T.M. Maloney, 1993