Veneer quality control Part 1 QUALITY ISSUES RELATED TO PEELING
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Quality control is a system of process and product monitoring that aims to ensure procedures and protocols are being followed to produce a consistent product of the required quality. Such systems help to identify issues quickly and correct the manufacturing process if necessary. Optimised quality control systems also help to reduce manufacturing costs by reducing rejects and downgraded products, reducing repair and reprocessing costs, increasing recovery and reducing waste, and increasing productivity.
The objectives of quality veneer production are to:
◆ maximise veneer recovery;
◆ produce long and straight veneer ribbons;
◆ minimise buckling and waviness;
◆ minimise veneer breakage/splitting;
◆ ensure a smooth surface;
◆ produce uniform thickness.
While there are a number of steps involved in the production of veneer that affect veneer quality (e.g. pre-conditioning, peeling and drying), peeling has the most influence on the quality. For this reason many quality control checks are
necessary immediately after the lathe, so that any set-up issues are identified and rectified quickly (Figure 7.1)
QUALITY ISSUES RELATED TO PEELING
The ribbon should leave the lathe in a straight line onto the conveyer belt producing a consistent cylindrical peeler core (Figure 7.2). A straight-tracking ribbon will produce a peeler core with the same diameters at each end and at its
centre. Mis-tracking occurs if the ribbon tracks in an arc to either the left or the right side of the conveyer, depending on which end of the knife leads into the cut (Figure 7.3). The effect of mis-tracking becomes accentuated as the diameter of
the peeler core becomes smaller and might only be noticeable when peeling large diameter billets .
The conveyer belt is designed to take the ribbon away from the lathe in a straight line and mis-tracking can cause splits along the concave edge of the ribbon and waviness (see below), especially if the veneer ribbon is straightened. The ribbon
can break or split into unfavourably sized sections, thus reducing the overall veneer recovery. Mis-tracking veneer can also cause the clipper to jam. Mis-tracking indicates the need to realign the lathe settings (e.g. knife, nose bar,
rollers). Multiple adjustments and fine tuning might be necessary to achieve straight tracking.
A veneer ribbon is considered flat if the ribbon sits even and parallel with the conveyer belt and exhibits minimal gaps between the veneer and the belt’s surface. The presence of waviness in a freshly peeled veneer ribbon indicates
an issue with the lathe settings. Waviness within the ribbon may be linked to barrelling or cotton-reeling which is generally accompanied by thickness variation.
Barrelling and cotton-reeling
Barrelling occurs when the veneer ribbon exhibits buckles and wrinkles in the centre and its edges are tight and stretched. On a spindled lathe, barrelling is caused by insufficient back-roll pressure and/or when the horizontal opening
between the knife and pressure bar of a lathe is narrower at the centre of the log.
If all peeling parameters are optimised (e.g. billet pre-treatment and lathe settings) the green veneer thickness should have minimal variation. Uniformity in thickness (within and between veneer sheets) directly affects the manufacturing
process, and ultimately, the quality of the final product. In order to achieve efficiency in gluing and finishing, minimal variation in the panel thickness is required. Excessive thickness variation can cause the following problems:
◆ variation in the amount of adhesive spread on veneers resulting in poor bond quality of the finished product—for instance, thinner veneers will tend to have more glue spread on their surface than thicker veneers with the same
◆ variation in platen pressure in the press during product manufacture;
◆ undesired variation in the product thickness causing the product to be outside specifications;
◆ low pressure areas which cause poor localised bonding and can increase the blow rate.Variation in thickness for green veneer can be monitored by using a hand-held dial thickness gauge (Figure 7.7). To minimise measurement error, it is important for the operator to calibrate the gauge and use the same method for each assessment. Thickness measurements should be taken along both edges of the ribbon at opposite points. The system and frequency of measurement will vary depending on standards, production and product requirements; however, good practice would be to select a random sub-sample of veneer sheets and measure at 300 mm intervals on both sides along the veneer sheet edge (parallel to the grain)
with measurements taken on areas free of knots, pronounced grain deviations, decay and other major defects (Figure7.8).
Various standards provide thickness tolerances for peeled veneer for plywood production. For example, the American standard PS 1-95 (APA–The Engineered Wood Association 1996) states a thickness tolerance of 5% of the nominal dried
veneer thickness. Other standards, e.g. Vietnamese standard TCVN 10316:2014 (Standards Vietnam 2014) and Chinese standard LYT 1599:2011 (Standards China 2011), provide a list of thickness tolerances depending on the nominal
thickness of the veneer (Table 7.1)
A hand-held dial gauge used to measure veneer thickness.
Figure 7.8.Routine checking for veneer thickness variation