Medium-density fibreboard
The medium density fiberboard or medium density fiberboard , shortly MDF is a wood-based material in the field of fiber boards . The product was invented in the United States and became more widespread in Europe in the late 1980s. The area of use is similar to that of chipboard .
Finely shredded, mainly bark-free softwood is pressed into a wood-based material that is equally homogeneous in the longitudinal and transverse directions. The edges are smooth and firm and can be profiled without special edge banding. Thicknesses from 2 mm to 60 mm with densities from 600 kg / m³ to 1000 kg / m³ are commercially available. MDF boards have similar swelling and shrinking properties in all directions, but have a significantly lower strength than solid wood.
composition
Depending on the intended use, MDF panels are treated with additives. The following information is roughly valid for an MDF furniture board that has been manufactured with a urea-formaldehyde glue ( UF glue ) in accordance with emission class E1 : 80 to 83 percent wood, 9 to 10.5 percent glue, 0.5 up to 2.5 percent additives and 6 to 8 percent water.
Wood
Different types of wood are used, in addition to the classics pine, spruce and beech there are now also eucalyptus, birch, poplar, acacia and many more. The decisive factor is the price of the wood types rather than their suitability for optimal production.
Adhesives
In most cases, urea-formaldehyde resins ( urea resins , urea- formaldehyde UF) used for the gluing. These resins can be reinforced with melamine or phenol in order to improve the moisture resistance of the glue joint. Isocyanates (polymeric diphenylmethane diisocyanate - PMDI ) are only used as a glue system in rare cases . In newer UF glue systems, the molar proportion of formaldehyde compared to urea is continuously being reduced in order to meet the legal requirements regarding formaldehyde emissions from wood-based materials (Class E1, E2 in Europe, CARB in the USA or F **** ("F- 4-Star ") in Japan).
Additives
Additives are used for different reasons. Paraffin , which makes up the largest proportion of the additives, is used in the form of melted wax or paraffin emulsion to improve the swelling properties of the plate. Urea is sometimes used as a catcher in classic formaldehyde glues to reduce formaldehyde emissions from the board. Ammonium sulfate or ammonium nitrate are used as hardeners for faster setting of formaldehyde glues. Colors are often added to identify special properties such as swelling or fire protection, but can also be used for decorative reasons in the form of a complete coloration. If necessary, other chemicals are used which, due to the cost, are only used when necessary.
Fungicides or insecticides are sometimes used to protect the MDF boards against insect or fungal attack. Chemicals containing chromium or boron are mostly used for this. MDF treated with other additives meet building material class B1 according to DIN 4102 Part 4 in Germany and are considered flame-retardant. Numerous other additives can also be used.
water
Wood-based panels are hygroscopic and therefore swell when exposed to moisture.
Manufacturing
For the production of fiberboard, the wood is first shredded, glued and then pressed into panels.
processing
The raw materials used for wood fiber production are predominantly round wood (logs), wood chips , rind and, to a lesser extent, waste wood , residual rolls from peeling veneer production, veneer scraps and sawdust.
Wood chip production
In most cases, round wood is debarked and chopped into wood chips in disc chippers or, like the rinds or waste wood, in drum chippers. Before further processing, the wood chips are first sorted (sieved) and mostly cleaned of sand and stones either wet or dry. Bark waste and wood waste are mostly stored temporarily and later used to generate thermal energy in the plant.
Wood chip storage
A store for the wood chips produced is required to ensure a continuous supply of wood to the production plant. The size of the store depends on the expected duration of the interruption in the supply of wood chips. Interruptions can e.g. B. caused by knife changes on the chipper, bans on wood chip production due to noise emissions at night, on weekends and so on. The wood chips can be removed from the warehouse either automatically using screws or moving floor systems or by vehicles and fed to the downstream process.
Fiber manufacture
cleaning
In many plants, the wood chips are cleaned of foreign matter by being sent to a machine cleaning system. A distinction is made between “dry cleaning” and “wet cleaning”. In the dry cleaning process, the wood chips are freed of heavy bodies with the help of air. In wet cleaning, stones, sand and metals are separated from the wood chips in water.
Fiberization
The wood chips then go to a pre-steaming tank for hydrothermal pre-treatment, where they are “pre-steamed” without pressure and at a temperature of up to 100 ° C. This treatment softens the middle lamellae of the wood cells, which promotes the compressibility of the wood chips and also later the defibration. The partially plasticized wood chips get into the digester via the vibrating discharge floor and via the auger. At this point, the sawdust is usually added to the process first. The pitch of the screw spiral decreases continuously, which compresses the wood chips into a relatively pressure-tight plug. The so-called squeeze water is pressed out of the wood chips. With this plug the seal to the cooker takes place. Here the wood chips are “cooked” at a steam pressure (depending on the type of wood and the demands on the fibers) between 6 and 10 bar. The wood chips stay in the digester for around two to four minutes before they pass through a conveyor screw (discharge screw) and via the feed screw into the refiner (defiber).
In the refiner , two grooved grinding disks rotate relative to one another with a regulated distance of about 0.1 mm. A distinction is made between single disc refiners with only one rotating disc and double disc refiners in which both discs rotate. The softened wood chips get between these discs under the pressure of the screw conveyor and are ground up. The fiberized material is blown out of the refiner via an adjustable "blow valve" through the "blow line". The refiner also has 6 to 10 bar steam pressure. The steam forms the means of transport for the fibers on their way through the blow line into the dryer .
Drying
After defibering, the pulp is dried - in contrast to chips - with electric dryers (mostly heated directly with flue gases or burners). The pulp is added to the hot exhaust gas flow and dried in the drying channel during pneumatic conveyance. Other systems with indirect heating (air heating via steam or thermal oil heat exchangers) are also used less frequently. At the end of the process, the pulp with around 8 to 12 percent wood moisture (dry) is separated from the air flow in cyclones . With dry gluing (see below), the fibers are dried to approx. 2 percent (absolutely dry).
Gluing
There are three different methods of gluing:
- Mixer gluing
- oldest process, but leads to so-called "glue stains",
- Blow-line gluing
- currently the most common process, no glue stains, but increased glue requirement,
- Dry gluing
- State-of-the-art process, lower glue requirements, higher investments and higher operating costs.
Mixer gluing
After drying, the pulp is collected in a bunker and then glued in a mixer . The glue liquor must be mixed evenly under the fibers, for which circumferential waves with mixing arms, also called paddles, mix the material, while the glue is fed either directly through the outer wall into the trough or at the end of the paddle. Before mixing in, the glue is prepared by adding constant proportions of the various components to the glue liquor and mixing them. The glue system must not yet harden, but only network through activation under the heat in the press, which is why cooled mixers are also used. Depending on the system, the glued fiber is transported again in a heated air stream in order to dry off the additional moisture brought in with the glue. After gluing, the mixture reaches the forming station with a fiber moisture content of 9 to 11 percent .
Blow-line gluing
With blow-line gluing, the glue liquor is injected directly into the fiber stream in the blowline. The slight hardening of the glue on the fiber surface and on the surface of the glue droplet is accepted, since there is still enough liquid material available during pressing. This process leads to a very homogeneous glue distribution. However, the thermal stress on the glue in the dryer affects the “residual strength” of the glue, which ultimately leads to higher glue consumption.
Dry gluing
This represents the most modern process, but has so far only been implemented in a few systems. The glue is applied to the dried fibers by extremely fine spraying. Since the thermally intensive drying process essentially takes place before gluing, the glue consumption is significantly lower than with blow-line gluing with largely homogeneous glue distribution. Post-drying is also necessary with this system in order to reduce the moisture brought in with the glue.
Fiber sighting
In most of today's MDF plants, the fibers are cleaned in a fiber sifter after the drying process or - if available - only after re-gluing. The fibers are placed in a stream of air and largely freed of heavy parts (lumps of glue, rubber, metal, etc.) either through the formation of eddies, sharp deflections, impact sifting, ascending air sifting or a combination of several effects. The fibers are then separated from the air flow again via cyclone separators and fed to the forming station.
Forming and pressing
scattering
The spreading station consists of a dosing bunker, mat spreading and mat smoothing. The matting has changed significantly in the course of development. In the early days of MDF production, many spreading stations were still equipped with vacuum spreading. Later on, the mechanical spreading heads became more and more established due to their lower electrical power and more homogeneous spreading. The mechanical spreading machines were initially equipped with milling drums for smoothing the mats, which sucked off excess material after the spreading. In the modern spreading machines this suction can be dispensed with and the fiber mat is smoothed by mechanical equalization.
Molding line
After the scattering station, the fiber mat is weighed and the fiber moisture is measured. The molding then goes into the prepress . Here it is reduced in thickness during cold pre-compaction so that the hot presses can be loaded more efficiently and the risk of damage is reduced. In the case of in-line pre-compression, belt pre-presses are usually used according to the conveyor belt principle (less often with apron belt pre-presses, according to the armored chain principle, or with roller belt pre-presses, based on the principle of pyramid stone transport with round timber).
After pre-pressing, the mat is trimmed, where side strips are separated from the fiber mat and the desired panel width is produced. The side strips are fed back into the manufacturing process in front of the spreader. Measuring devices for density control or metal detection can follow, optionally also a mat spray to improve the surface quality or to accelerate the heating of the mat.
Hot press
This is followed by hot pressing , where work is done clocked or continuously.
- In the case of cyclic presses, a distinction must be made between single and multi-daylight presses. In order to achieve short pressing times, single-daylight presses are operated at a higher temperature, up to 230 ° C. The molding is covered at the top and bottom with a press plate and transported into the press, which then closes and ensures the transfer of heat to the mostly preheated press plate and finally to the material to be pressed. In almost all types of presses, the heat transfer into the press material is also improved by the steam chuff effect. Water from the moistened top layer evaporates due to the one-sided heating, diffuses to the center of the plate and condenses while giving off heat.
- Multi-stage cycle presses close simultaneously in order to achieve the same pressing times and thus consistent board quality. Only 150 ° C to 200 ° C are used here, as the pressing times can be comparatively longer with the same capacity compared to one-daylight systems.
- Continuously operating presses work with a press belt or with press plates through which the pressure and temperature are transferred. The belt is supported either by a roller carpet (Küsters), a stick carpet ( Siempelkamp , Dieffenbacher) or an oil cushion (Bison) opposite the heating plates, which are mostly heated with thermal oil (less often with steam). This press system now represents the state of the art and enables the production of panel thicknesses between 1.5 mm and 60 mm.
- Only thin chipboard or fiberboard can be produced on calender presses. The pressing takes place here with press rollers and an outer belt on a heated calender roller.
Finishing
After pressing, the panels are trimmed and trimmed in motion. A series of quality control measurements usually follows. These are usually a thickness measurement, a gap detection and a plate scale. A density profile measurement and a residual moisture measurement are less common.
One or more cooling stars are used to cool the panels down before they are stacked. After cooling, the panels are usually temporarily stored in a ripening store for around one to three days. Because of the hot surface of the press plates or bands, the plates have a pressed skin and insufficient thickness tolerances. Furthermore, the panel thickness changes due to the "ripening process". For this reason, the panels are sanded on sizing grinders after maturation. The division into finished dimensions usually takes place in the same processing line before the panels are packed and stored for transport. Grinding dust, trimming material and sawdust are stored temporarily and fed to the plant's thermal energy generation.
Energy system
The heat transfer media required for the production process are heated in the energy system, usually solid-fuel-heated steam boilers. Production waste is used as the primary energy source, which can cover 30 to 70 percent of the energy requirement, depending on the wood processing and panel type. Bought-in biomass (wood) or fossil fuels are mostly used to cover the additional energy requirements. The energy required for the production process is made available in the form of steam, hot thermal oil and, if necessary, boiler exhaust gases.
use
Raw panels are used in interior and roof construction. Coated fibreboards are used as carrier boards for laminate floors and in the furniture sector for furniture and kitchen fronts, in shop fitting and loudspeaker box construction. Entire furniture series are made from MDF panels. In addition to cabinets and cabinet systems, MDF boards have been used in powder-coated form as table and cover plates for several years. They are also well suited for creating profile decorations, for example on kitchen fronts. They are also popular in high-quality speaker box construction due to their high flexural and tensile strength. MDF boards can be treated in a variety of ways with paints and varnishes, resulting in a smooth, clean surface with profiled edges, millings and so on.
standardization
MDF boards were previously defined in the European Standard EN 316 (1999), Paragraph 3.2.3 as fibreboard manufactured using the dry process. In the following, a distinction is made between HDF (≥ 800 kg / m³), light MDF (≤ 650 kg / m³) and ultra-light MDF (≤ 550 kg / m³) in the mentioned version of the standard . However, this distinction has been removed in the current version from 2009. Board properties are specified in EN 622-1 (2003) (fibreboard: general requirements) and EN 622-5 (2010) (requirements for boards according to the dry process - MDF) for various purposes and board types (including for light MDF and ultra-light MDF). The currently valid standard for "light MDF" and "ultra-light MDF" defines properties, but the density range for these panels is no longer specified.
specialization
- MDF B1
- The flame-retardant MDF-B1 board is suitable for interior construction and the commercial sector where special requirements are placed on fire protection. The material can be profiled, milled, varnished, coated and veneered like conventional MDF.
- Depending on the panel manufacturer, the B1 panels can have a very reddish core, which is caused by deliberate color marking and which, depending on the processing (e.g. optical or acoustic slitting), can be disruptive.
- Top form
- Designation for a flexible MDF board (Glunz AG)
- conductive MDF
- The coating of MDF boards with powder coatings requires that the board itself has conductivity , as the coating process takes place via an electrostatic application. The results of coating standard grades fell far short of expectations. For this reason, various MDF manufacturers have carried out special pressings with the addition of special conductivity additives. The low market volume of conductive MDF boards has meant that only one supplier is involved in this area. The coaters are still working on solutions for a high-quality coating of standard qualities for MDF powder coating .
- MBPL
- Designation for a conductive MDF board (Egger company)
capacities
Industrial MDF production began in the 1960s and has since increased in line with a logistical growth curve because MDF first had to displace other materials.
The first production plants were designed for small capacities of 100 m³ or less per day. With the introduction of continuous presses, capacities increased to 2000 m³ per day.
In Germany, the MDF board has meanwhile achieved economic importance. In 2008 around 3.8 million m³ were produced. During this time, the previously leading producer USA was overtaken by China. Production capacity in Germany has been falling since 2008, and Germany fell from second place in 2008 to seventh place in 2011 in the production statistics, behind China, Turkey, Brazil, the USA, Poland and South Korea. The three largest production countries in 2016 were China (about 57.7 million m³), Turkey (about 5.1 million m³) and Brazil (about 4.2 million m³).
Individual evidence
- ↑ Guideline on the classification and monitoring of wood-based panels with regard to the release of formaldehyde. DIBt guideline 100, version June 1994, publisher: Deutsches Institut für Bautechnik - DIBt ( Memento of the original dated August 10, 2018 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 485 kB)
- ↑ CARB-Fact Sheet (English; PDF; 41 kB)
literature
- André Wagenführ, Frieder Scholz: Pocket book of wood technology. Specialized book publisher at Carl Hanser Verlag, Leipzig 2008, pp. 127–259. ISBN 978-3-446-22852-8 .
- M. Dunky, P. Niemz: Wood-based materials and glues . Springer, Heidelberg 2002, ISBN 3-540-42980-8 .
Web links
- Association of the German Wood Materials Industry V (German)
- European Panel Federation (English)
- MATERIAL ARCHIVE - Extensive material information and pictures