Veneer quality control Part 3 : QUALITY ISSUES RELATED TO VENEER MOISTURE CONTENT

Veneer quality control Part 3 : QUALITY ISSUES RELATED TO VENEER MOISTURE CONTENT

Special thanks to the original source of  https://www.aciar.gov.au

QUALITY ISSUES RELATED TO VENEER MOISTURE CONTENT
Veneer moisture content has an important influence on product quality. The
veneer must be at the right moisture content for the following reasons:
◆ so that it can be glued successfully;
◆ to avoid biological degradation including fungal (stains and decay) and insect
damage;
◆ to ensure that the veneers are pre-shrunk prior to product manufacture;
◆ to ensure product dimensional stability and therefore prevent veneer and
panel distortion, e.g. waviness and warpage;
◆ to avoid splits and checking; and
◆ so that it is compatible with manufacturing processes such as the pressing
closed and open assembly times.
The target moisture content of the veneer is dependent on the adhesive and
manufacturing protocols, however a common target moisture content is 6% with
a range of 3–10% (e.g. for most phenol formaldehyde resins). Urea formaldehyde
resins are often more tolerant of higher moisture content and wider variation.
The two most common methods of measuring moisture in veneer are with a
resistance meter or a capacitance meter

Resistance meter (pin-type meter)
A resistance moisture meter uses two or more pins that penetrate the veneer at
a desired depth (Figure 7.10). Direct current travels between the pins through
the wood and the resistance is measured. Dry wood allows only little current
to pass whereas wood with higher moisture content permits more. The meter
reads how much resistance there is to the current and correlates the resistance to
wood moisture content. This type of meter becomes less accurate as the moisture
content increases. Pin-type meters are more accurate and effective in determining
the moisture gradient within wood (the difference between shell and core
moisture content) than other types of meter as the operator can control the pin
depth.
Capacitance meter (electromagnetic field meter)
A capacitance meter measures the moisture content of wood without
penetrating the wood (Figure 7.11). A sensor emits electrical waves that create
an electromagnetic field. This field behaves differently depending on how much
moisture is in the wood and the wood density. Capacitance meters measure the
capacity of the wood to store energy (capacitance), the amount of power the
wood absorbs from the field (power loss) or the wood’s resistance to the field
(impedance). This pinless meter type is less effective in measuring the moisture
gradient within wood compared to a resistance meter. Also, readings provided
by a capacitance meter are influenced more by surface moisture, so that readings
for material beneath the surface veneer are generally less accurate. They are also
affected by surface roughness. However, they are easy to use and do not damage
the veneer surface.
Table 7.2 compares the two types of moisture meters and the influence of
physical factors and various features.
Hand-held resistance moisture meters provide quick access to moisture
content information and are relatively easy to use in small-scale operations and
are especially useful to determine the moisture content of sheets outside the
production line. Commercial plants producing veneer on a large scale often
install ‘in-line moisture measurement systems’ as part of their production line
(Figure 7.12). These systems usually use the capacitance method. They are often
combined with automatic grading systems and form an essential part of a highly
automated process.

Figure 7.10.
A hand-held resistance moisture meter.

Figure 7.11.
A hand-held capacitance moisture meter.

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