Particle Board QC Inspection Checklist : Part 9 Factors that affect the pressing process and influence the final board properties

Particle Board QC Inspection Checklist : Part 9 Factors that affect the pressing process and influence the final board properties

Raw material
• Particle type and geometry – whether it is “granular”
(chips), flakes or strands
– Internal structure and ability of gasses to move around the
interior of the mat: easier if it is granular, less so if it is
composed of flakes
• Mat moisture content
– Overall
– Distribution
• Higher on surfaces
– Heat transfer, plasticisation of chips, but…
– …higher internal vapour pressure, possible delamination

Heat transfer mechanisms
• Conduction
• Convection
– Change of phase: liquid-vapour-liquid
• Radiation
– Negligible

Development of adhesive strength
• Gel time: liquid to solid….
• Time to reach full cure

Densification, internal stresses and
spring-back
• Wood undergoes both instantaneous and timedependent deformation when load is applied:
– Elastic
– Delayed elastic
– Viscous
– Plastic/microstructural failure

• “Trapped” elastic and delayed elastic stresses lead to
internal stresses

Development of density profile
• Higher face moisture content will favour relaxation
of the chips and so lead to higher face densities
• Faster press closure times will tend to result in
higher density surfaces
• Higher density surfaces:
– Increased bending properties and strength parallel to the
board surface ….
– … but reduced internal bond strength (transverse tensile
strength), inter-laminar shear strength and screw holding
characteristics

Edge effects
• Vapour/gasses in the mat during pressing will tend
to escape from the edges (not sealed)
• Leads to a lower gas pressure at the edges
– Difficulty of scaling: e.g. simulating processes on a
laboratory press

Limitations
• Thick panels
• Low density panels
• How to overcome these:
• External heating sources….
– Steam injection
– Radio Frequency (RF) and microwave heating

Cooling and finishing
• At exit from press, adhesive cured sufficiently to
retain board integrity
• Some further “curing” may take place (used to be
common to “hot stack” finished panels to cure the
resin further)
• Panels need to be cooled before finishing (e.g.
sanding) as if done too soon, board properties may
be affected and tools will become clogged
• On exit boards trimmed before cooling

Cooing & finishing
• Cooled in e.g. “star cooler”
• After cooling the boards are
sanded to the final thickness
and to give good surface finish,
especially if subsequent
processing takes place, for
example, lamination

Adding value
• Frequently the “raw boards” undergo further
processing to add value
• Example is lamination or overlaying a decorative
finish to the board
• Speciality boards (“moisture resistant”, “flame
proof”)

References and further reading
• Chapman, K. (2006). Zen and the Art of making MDF. In: proceedings of
the 10th European Panel Products Symposium, 11-13 October, Llandudno,
Wales, UK
• Halligan, A.F. (1970). A review of thickness swelling in particleboard.
Journal Wood Science and Technology, 4(4), 301-312
• Kavvouras, P.K. (1977). Fundamental Particleboard Process Variables, PhD
thesis, University of Wales, Bangor
• Maloney, T.M. (1993). Modern Particleboard & Dry-process Fiberboard
Manufacturing (available in library).
• Kelly, M. (1977). A critical review of relationships between processing
parameters and physical properties of particleboard (available at:
http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr10.pdf)
• Philip E. Humphrey (undated) Temperature and Reactant Injection Effects
on the Bonding Kinetics of Thermosetting adhesives

Further information
• Manufacturers:
– Egger (www.egger.com)
– Kronospan (http://www.kronospan.co.uk/)
– Norboard (http://www.norbord.com/)

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