Veneer quality control Part 3 : Thickness variation

Veneer quality control Part 3 : Thickness variation

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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
machine set-up;
◆ 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 (Figure 7.8).
Thickness tolerances
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)

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