Particle Board QC Inspection Checklist : Part 8 Manufacturing Process steps

Particle Board QC Inspection Checklist : Part 8 Manufacturing Process steps

1. Raw material preparation
2. Converted into particles
3. Particle drying
4. Particle classification
5. Blended with a resin and additives
6. Particle/resin/blend (“furnish”) is formed into a
7. Hot pressing to compact the particles together
and cure the resin
8. Cooled and finished

(Particles, adhesive & additives)

Raw material properties influence many board properties,
including board density, strength and stiffness etc.
• Can also influence the manufacturing process….
• Wood:
– Virgin wood, mainly softwood, but other species can be used
• Round wood
• Co-products (sawdust, slabs, etc.)
• Short rotation coppice (poplar)
– Recovered/recycled wood (“urban forest”)
• Pallet wood, packaging etc.
• Management can be problematic due to difficulties in identifying contaminants (polymeric – paints and varnishes; preservatives – CCA).
• Non-wood:
– Agricultural by-products
• Wheat straw, bagasse, hemp, rice straw
• Basically any fibrous lignocellulosic material!

Wood raw material
• Species:
– Density (affects the “compaction ratio” and the way the chip deforms
under compression during pressing), strength and stiffness
– Chemistry: pH, buffering capacity, affect resin cure
– Extractives: may “interfere” with the bonding process or lead to
emissions problems
• Recovered wood:
– Contaminants:
• Metallic (large! Easily dealt with, but in preservatives such as copper
chrome arsenate – CCA – more problematic)
• Inorganic (grit, sand, cement…)
• Organic (e.g. paints/varnishes – problems with pressing; creosote)
– Identification of contaminants (materials management)

Non-wood raw material
• Basically any lignocellulosic material!
• E.g. wheat straw:
– High inorganic content
– Waxes

Raw material for particleboa

• By-products
• Spruce – 80 %
• Birch – 15 %
• Pine – 5 %
• Saw dust
• Mini chips
• Cutter chips
• Chips/board
• 4,5 m3 / m3

• Primarily thermosetting resins
– Main type (90%) is Urea formaldehyde (UF)
• Low cost
• Rapid cure
• Adequate properties for many applications (brittle and susceptible to hydrolysis)
• “Low” formaldehyde emissions “E0”, E1 (E1 Classification <8m g HCHO/100g oven-dried
– “Fortified” UF (melamine urea formaldehyde)
– Phenol formaldehyde (PF)
– MDI (methylene diphenyl diisocyanate) and pMDI. Zero formaldehyde release, but
careful handling needed
• Other adhesives
– Inorganic (cement bonded)
– Thermoplastics
• Renewable resource-based adhesives
– Tannin-based adhesives
– Lignin-based adhesives
– Glues based on vegetable oils
– Soy flour-based adhesives
– Furan polymer-based adhesives
• Binder-less boards using “activated” chips/particles (enzymatic or chemical)

• Waxes (reduction in moisture uptake)
– Emulsion
– Molten wax
• Fungicides
• Flame retardants

1. Raw material preparation
• Debarking undertaken to remove
bark which can reduce the
properties of particleboard if
present in quantities more than a
few per cent
– Rotary debarker
– Drum debarker
• Sorting classification of waste wood
for further processing
• Removal of metallic and other
• Problems in identifying some
treatments that have previously
been applied to wood. In particular,
polymeric substances can cause
problems in processing

2. Conversion to particles (chipping)
• Aim to manufacture a
homogeneous product
• Need therefore to
produce chips of a
suitable size & shape to
form the furnish for the

Chipping Particles (chips)
• Depends on raw material. For example, if the raw
material is in log form or is already reduced in size,
e.g. shavings
• Reduction may therefore be a two stage process.
For example, primary reduction of debarked logs to
give large, coarse chips, followed by further milling
to produce more “engineered” chips
• Different equipment may be used to prepare chip of
the required type

Types of equipment
• Knife systems
– Use knives cut or slice chips form the raw material
– Disc/drum chippers
• Hammer mill systems
– Use hammers to grind, break, tear shred raw materials
into smaller particles
• Attrition systems
– Grind the wood raw material into fibre – similar to TMP
(thermomechanical pulp)

Particle engineering
• Each particle reduction (milling) process will produce
a range of particle sizes
• Different chip size/type used for surface and core
layers – fine chips or particles on the surface (good
finish), coarser chips in the core
• Important to produce chip surfaces with as little
damage as possible to improve particle-particle
– A mechanically weak surface layer will lead to possible
break-down of the glue-line, resulting in degraded board

3. Particle drying
• Why?
• High MC of chips from green wood too high (>100%) for
pressing – can lead to the development of high vapour
pressure in boards leading to delamination or “blows”
• Ensure that the “furnish” has a homogeneous MC
– Core and surface layers may have different MC
• Some moisture is necessary to assist in heat transfer during
the pressing process (see later lecture)
– Remember: most commonly used commercial resins (e.g. UF/MUF)
are water-based and this needs to be taken into account when
determining the final moisture content of the particles….. Typically for
UF around 65% solids content
– Also wax emulsions contain water!
• Typically final MC of chips in the region of 2.5% after drying,
but before resin blending
• Drying is a BIG consumer of energy!

Particle drying
• Optimisation of MC important: too much and this
will cause problems with board blows, too little will
lead to poor heat transfer in the press and
incomplete resin cure, resulting in poor mechanical
• Driers work on the principle of passing the particles
through a stream of hot air (180-200oC)

Dryer types
Various types of dryer e.g.
• Drum type (rotary/fixed)
• Belt dryers
• Paddle trough dryers
• Suspension-type
• “Flash” driers (high temperature)

4. Classification
• Why?
• Control particle size distribution – important so as to optimise
the position and size of the particles relative to their position
in the board – small particles on the surface, bigger particles
in the core
• Need to ensure that the board is “balanced”, i.e. the same
amount of surface particles on both faces of the board (c.f.
plywood). Otherwise may result in distortion of the board
• Note: very small particles (fines) will consume more resin
(high surface area to weight ratio)

Classification systems
• Sieving – pass particle through different sized sieves,
larger particles are retained, smaller particles pass
through to the next size down
• Air classification – suspended in an air stream; the
smaller, lighter, particles are carried away in the air
stream, leaving the larger heavier particles (lower
surface area to weight ratio)
– Can form part of the drying process – simultaneous drying
and classification

5. Gluing/blending

An adhesive or glue needs to be applied to the surface of the
particles to bind the particles together
• Different resins have different performance characteristics:
– UF; brittle and subject to hydrolysis
– MUF; better moisture resistance than standard UF
– PF; more resilient and less subject to hydrolysis
– MDI; highly reactive and bonds well
• The resins are generally applied as liquids in the form of an
atomised spray
– Note: can also be applied in solid (powder) form that forms a liquid on
heating, before reverting to an insoluble, infusible, solid form (e.g. PF)
• Additional mechanical blending can enhance the coverage of
the resin over the particle

Glue factors influencing board
• Amount of resin
– Generally, more resin, better properties (but more expensive!), therefore a
– Varies from resin type to resin type and is typically 8% (solids on oven dry
wood) for UF in PB (or MDF), but can be as low as 2% (for MDI in MDF)
• Resin properties
– Viscosity (depends on resin itself and “solids content”)
– Solids content (weight of solid resin to total weight of resin)
– pH and buffering capacity
• Distribution of resin
– Droplet size (spray type and nozzle design)
– Generally smaller droplets preferred, giving better resin distribution (see e.g.
Kelly, 1977)
• Cure characteristics
– Heat and/or catalyst needed to cure resin
– Gel time/cure time depends on several factors

More on glues & additives
• Resins frequently manufactured on site, particularly UF and
MUF. Storage conditions and duration important to prevent
degradation (short shelf-life)
• Other additives (e.g. waxes, fungicides, flame retardants) may
be blended in at the same time
• Typically, wax is added at around 0.5-1.0% (solid on OD wood)

• Laying up the “furnish” prior to pre-pressing and hot pressing
• Important process in the formation of the structure of the
board and therefore strongly affects the board properties:
– Improved bending properties
– Good surface finish
– Optimised density
• Boards may be “single layer”, “multi-layer” (3,5), or
• Must always be a “balanced” structure to prevent distortion
or poor properties (i.e. 2 or 4 layer no good)
• Surface layers of “fine” particles, “core” of coarser particles
• MC of surface layers 8-15%; core 4-8%
• Resin content of surface layers may also be higher to give
improved strength


Pre-pressing & hot pressing
• Pre-pressing of the furnish to
reduce the bulk and give some
mechanical strength to the
• Hot-pressing, to finally
consolidate the board and cure
the adhesive. Density profile
generated during hot-pressing
• Various types of press, the most
common being a continuous

• Part of the consolidation process
• Pre-pressing undertaken to:
– Reduce the thickness of the mattress (i.e. increase the bulk density of
the mattress
– Give the mattress some mechanical strength for handling
– Speed up the hot press process, reduce pre-cure
• Important that the mattress has some “tack” (slightly sticky),
provided by the glue
• Pre-pressing may be cold or hot, important not to pre-cure
the resin, but can help in getting heat energy into the

• Main mechanism for consolidation of the mattress,
the development of the board internal structure (in
combination with forming etc…) and curing the
adhesive binding the particles together
• Pressing may be batch-wise in a single or multiopening press, or in a continuous press (“contipress”)
• Press control vital in the development of board
properties and much research directed towards
understanding what is going on the in press and in
modelling this behaviour

Press types
• Opening presses:
– Single-
– Multi- daylight presses
• Continuous presses

What is going on in the pressing process?
• Heat transfer and flow of gasses
• Phase change of water
• Densification of material
• Adhesive cure
• Development of internal stresses

Basic steps
• Board (mattress) fed to the press
• “Daylight” closes
• Press closure to target (thickness, pressure) –
pressure increase
• Pressure/thickness maintained: resin cure
• Daylight opens: pressure reduced
– May be done in stages
• Boards removed

hot pressed
hot pressed

What happens to:
• Mat thickness
• Counter pressure
• Surface temperature
• Core temperature
• Core vapour pressure
During the pressing process?

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