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Data Sheet 1
Sep 2001 |
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Scope
This data sheet describes the manufacture of resin bonded wood Particleboard
and Medium Density Fibreboard (MDF). The manufacturing process involves
producing wood particles, applying a binding resin, forming a mat and
then consolidating and curing the mat with applied heat and pressure.
The different product types are distinguished by the wood particle
shape (particles or flakes for Particleboard, fibres for MDF) and by
the binding resin. Different resin systems are used for Standard boards,
Moisture Resistant (MR) boards and for Flooring. These provide the
properties that are required to meet end-use applications and environments
in which the products are used.
Manufacturing processes are illustrated in Figure 1 for Particleboard
and figure 2 for MDF.
Materials
Wood
Wood fibres, particles or flakes comprise 80 to 90% of
the weight of MDF and Particleboard. Wood species are usually pines
such as Radiata Pine.
There are two main sources of wood raw material:
(a) Forest thinnings
(b) Sawmill residues such as slabwood, hacked or pulp chip,
dockings, planer shavings and sawdust.
Binders
The binder (resin or adhesive) and the amount used, play
a key role in the stability of the final board. Synthetic resins
are generally used which, because their formulation can be varied,
have the advantage of flexibility of curing time. In addition, they
are thermosetting and cure rapidly and irreversibly by the application
of heat.
The amount of resin used is usually in the range 4% to 12% of dry
wood. However this proportion may vary according to the type and size
of wood fibres or particles. For example, in three layer Particleboard,
the coarser core material may contain 4% to 10% resin, while the finer
surface layers may have 10% to 12% resin.
The three main types of resin in current use are:
- Urea-Formaldehyde (UF) Resin
Cheapest and easiest to use, and cures to a clear film. It is used
for Standard boards that are not exposed to moisture.
- Melamine-Formaldehyde (MF) Resin
These resins are moisture resistant and superior to UF but are much
more expensive. They are used to fortify UF resins to provide improved
moisture resistance. Moisture Resistant Particleboard and MDF uses
mixed MF/UF (or MUF) co-condensed resins.
- Phenol-Formaldehyde (PF) Resin
These resins are of the highest durability but are expensive
and dark in colour. PF resin and Tannin-Formaldehyde
(which is based on natural poly-phenotic materials) may
be used in the manufacture of Particleboard Flooring.
Additives
These can be added to the resins or sprayed on to the wood particles
to improve particular properties of the finished panels.
Paraffin wax is added in small quantities, either as an emulsion or
sprayed in the molten state, to provide water resistance and to control
swell caused by temporary wetting. It does not protect against high
humidity and continual dampness.
Hardeners and catalysts control rate of resin cure during pressing
and allow production of panels with the optimum properties.
Fire-retardant, insecticide and fungicide chemicals may also be added
in small quantities for specific products requiring protection.
Wood Furnish Preparation
Wood Particles
The properties and performance of Particleboard and MDF depends to
a great extent on the care taken in preparation of the wood particles
or fibres used in board manufacture.
Relatively thick wood flakes on or near the surface of Particleboard
may impair the dimensional stability and surface smoothness, resulting
in show-through on veneered or foil-surfaced boards and the possibility
of surface delamination. Decorative finishes on panels require a very
fine surface of finely ground particles, despite the fact that such
particles require an increased resin usage to coat the particles with
sufficient glue. Most Particleboard therefore is manufactured with
a three-layered structure – fine particles on both surfaces for
smoothness and coarser particles in the core for strength. These boards
are best suited to decorative finishes for furniture and cabinet manufacture.
Products that need better structural and bending strength such as
Particleboard Flooring are made from larger, more slender and thinner
flakes.
Medium Density Fibreboard is made from wood fibres carefully produced
to avoid oversize material (fibre bundles) and fines.
Oriented Strand Board (OSB) uses long strands with the long dimension
running in the same direction in each layer. As with plywood, each
layer runs at right angles to the adjacent layer providing strength
and dimensional stability. Waferboards use very large shavings
of wood, up to 50mm square, with each wafer overlapping the next in
the horizontal plane.
Production of Particles and Fibres
Particleboard raw material may be round wood, such as forest thinnings
or peeler cores, or sawmill residues ranging from slabs and offcuts
to planer shavings and sawdust. A wide range of chippers, flakers and
size reduction mills are used to convert the different raw materials
to the required particles. Screening is used to ensure tight control
of final particle size, with oversize being returned for further breakdown
while material that is too fine is usually used as an energy source
for drying.
Production of fibres for Medium Density Fibreboard requires chips,
so this product is more selective in its raw material. Thinnings and
slab wood are converted to chips which are often washed to remove dirt
and grit.
Fibre production involves preheating the chips to soften them and
then feeding them between two contra-rotating steel plates to tear
them apart into their component fibres. A further refining process
is used to ensure that fibre bundles are reduced and to eliminate material
that is too fine.
Heat Energy Plant
The Heat Energy Plant can provide hot gases, hot oil and steam for
various processes in the production of wood panels. Waste wood materials
are usually used for energy generation. Fuel sources can be bark, waste
chips and fibres, sander dust and saw trim as well as any reject boards.
These materials are sized and blended and stored ready for use in the
furnace. Hot furnace gases may be used directly for drying, can be
passed through a heat exchanger to produce hot oil for the hot press
or can produce steam for some pressing operations.
Drying
In Particleboard plants, particles are passed through driers that
reduce their moisture content to 3-5%. Most modern driers are direct-fired
in that the particles are dried by direct contact with hot gas from
the burners.
Particles are then passed through screens and wind-sifters to sort
the furnish into various size fractions, to remove heavy or thick particles
or excessive dust and to grade the particles into sizes if layered
boards are produced.
With MDF manufacture, glues or resins (see next section) are added
as the fibres are produced and so drying is carried out after this
blending process. Drying is similar to that described above. For both
products, control is essential to avoid over drying and charring of
the wood and to avoid pre-curing of the resin in MDF if fibres get
too hot.
Glue Addition
This operation is usually known as blending. Resin, in the form of
a liquid, is forced through nozzles and sprayed onto the particles,
in a separate Blender for Particleboard and after refining for MDF.
Prior to entering the nozzles, the other glue additives are correctly
proportioned and added to the glue flow and mixed thoroughly. Wood
furnish passes through weighing devices that automatically control
the resin flow rate. Continual checks on flow rates and particle moisture
contents ensure consistent blending with moisture contents increased
to 10-16% after blending.
Board Manufacture
Glued particles and fibres are formed into a mat, which is then subjected
to heat and pressure to cure the resin and produce a board of the required
thickness. Boards are sanded prior to sale or prior to prefinishing
with various surface and edge treatments. The rough panels of Particleboard
and MDF are trimmed after pressing and can be cut-to-size as required.
Mat Forming
Board quality depends on the quality of mat forming. For Particleboard,
glued particles pass from a silo into a weighing device that meters
the correct weight of flakes onto a moving belt. Various rotating rakes
distribute these evenly across the belt. This controlled wall of flakes
falls from the belt cascading onto the mat-forming device that is also
moving at controlled speed. The mat is built up to weight either by
several passes under these spreaders or by one pass under several spreaders
and is layered upon the mat transfer belt or caul which carries the
mats to the hot press.
There are various types of mat.
Single-layer: The mat is a consistent layer of the same particles
with fines and coarse evenly distributed throughout the thickness of
the mat.
Three-layer: The two outside
layers of particles are fine and contain more glue and
moisture than the
layer of coarser core particles. When pressed, the surfaces
of the boards are of higher density than the core. Three-layer
or sandwich construction of the mat is the most common
industry practice giving an opportunity to tailor the
board characteristics, e.g. fine surface for prefinishing,
flakes for strength.
Multi-layer: Similar to three-layer except for an increase
in the number of layers.
Graded-density: Special spreaders enable use of one furnish
source without pregrading the particles. Particles that
fall from the belt weigher cascade through wind-sifting
nozzles and screens which throw the fines further than
the coarse particles. The fines land on the advancing
mat former first and are covered by gradually increasing
sizes of particles. The other half of the spreader does
this in reverse with coarse arriving first to be covered
by chips that become finer. In this way, a graded mat
is formed with fine resinated particles on the surface
and grade evenly to the coarsest particles in the core.
Graded mats can also be formed using rollers of various
diameters with calibrated spaces between them. These
allow the fine material to fall between the first bank
of rollers and coarser particles carried forward to fall
between the wider gaps of later rollers. This forming
gives exact gradation of material through the mat from
finer to coarse then to finer again.
MDF mat forming does not have the grading options of Particleboard
and is aimed simply at producing as uniform a spread of fibres as possible.
Fibres are blown or raked onto a belt in multi-compartment spreaders
to assist with control. Fibres have a low bulk density and the very
thick MDF mats need to be prepressed to enable handling of the mats.
Pressing
The preformed mats of glued particles and fibres are transferred to
the hot press for pressing and curing. This operation is critical and
requires carefully controlled heat, pressure and timing. Prior to hot
pressing, the mats may be pre-pressed cold to reduce their thickness
and to make them easier to transport.
Pressing can be by batch ie mats formed then pressed using single
daylight presses or in groups in a multi daylight press. Single daylight
presses take large single boards at each pressing cycle, while the
multi daylight operation presses many boards at once.
The thick mats are compressed in the press with thickness controlled
either by thickness bars (stops) or other electronic thickness measuring
devices. As soon as heat is applied, the glue curing process begins
and full pressure is quickly applied to reach the desired thickness
before cure. Full pressure at stops is held for the prescribed time
then pressure is slowly reduced until the press is opened. Airing cycles
are important to allow the steam generated to escape thus preventing “blown
boards”. Typical pressures are 2-3 MPa, temperatures 140-2200
C and press-time 6-15 seconds per mm of board thickness plus the opening/closing
times of the press.
Recent technology is based on continuous presses with a continuous
process of mats being formed while boards are cured in the hot press.
Continuous presses are commonly used for MDF with fibre mats passing
through pressure frames where heat is applied to cure the resin. Continuous
presses range in length from 15m to 50m long with 2.5m being a common
width. These presses are capable of producing continuous panels with
a thickness tolerance of ±0.2mm.
Finishing
The hot hoards are removed from the press (or sawn across on continuous
presses) and further conditioned to equilibrate board moisture content
and to stabilise and fully cure the resin. This conditioning usually
follows cooling in star coolers for boards with urea formaldehyde resins.
Phenolic bonded Particleboard Flooring is usually hot stacked for
some days to ensure final cure of the resin.
Panels are then trimmed and sanded on both faces to tight thickness
tolerances. Sanded sheets are sawn to stock sizes or to suit special
orders.
Quality Control
All AWPA Members have extensive quality control and testing laboratories.
Quality control is an integral part of the production process. Regular
testing and statistical analysis ensures that all production meets
the requirements of AS/NZS 1859 or 1860. Additionally, each pack slip
is numbered giving an easy reference access to production and quality
control records.
Figure 1 Particleboard Manufacturing Process
Figure 2 MDF Manufacturing Process
Whilst the information contained herein is based on data which to
the best of our knowledge is reliable and accurate as of the date hereof,
no responsibility can be accepted by us for errors or omissions. Since
the information contained herein may be applied under conditions beyond
our control, no responsibility can be accepted by us for any loss or
damage caused by any person acting or refraining from acting as a result
of this information.
Published by the Australian Wood Panels Association Incorporated,
PO Box 158, Coolangatta Qld 4225, September 2001.
