Veneer clipping and drying

Veneer clipping and drying

CLIPPING
Veneer clipping is usually done as part of the peeling operation. As the veneer ribbon leaves the lathe, it is transported along a conveyor to a clipper that clips (cuts) the veneer, parallel to the grain, into smaller, more manageable veneer
sheet widths. Alternatively, the veneer ribbon is coiled or reeled immediately after the lathe and is moved to a separate clipping station. Regardless of approach, any clipping strategy should aim to recover the highest amount of veneer of a
dimension and quality that aligns to the target end products (Figures 5.1–5.3).The simplest clipping systems are manual operations that utilise hand- or footoperated guillotines. These are relatively slow and rarely adopted given the availability of relatively cheap, more automated systems.The next level of technology utilises a mechanical clipper system, normally activated on a time interval. This means that the sheet width is determined by the speed that the veneer ribbon is travelling through the clipper and the time interval between the clipper knife being activated. Often with these operations, the clipper knife is fixed to a rotating shaft meaning that the knife blade clips the veneer at each revolution. This is a simple, low capital cost approach that is much more efficient than hand-operated guillotines; however, it provides little opportunity to maximise the recovery of specific grade qualities. This is because the clipping strategy does not consider veneer quality (e.g. defects such as knots, splits etc.) and focuses on veneer width only. This approach can result in a lowgrade recovery as unacceptable defects remain in the veneer sheets. While these defects can be removed during follow-up processes, this can lead to less than desirable and variable veneer sheets widths, and added costs.

Advanced systems incorporate veneer scanning technology that can be programmed to detect and measure defects within the veneer as it travels.

Figure 5.1.
Veneer sheets being clipped manually by guillotine in Vietnam.

Figure 5.2.
Veneer ribbon being coiled in preparation for a separate clipping operation.

Figure 5.3.
Veneer ribbon ready for clipping.

DRYING
The main reason for drying veneer is to remove excessive moisture so that the veneers are prepared for adhesive application and product manufacture. Drying soon after clipping is preferred, to prevent mould, veneer distortion
(e.g. buckling) and other degradation. Practical objectives are to dry at low cost, with short drying time, and to achieve appropriate quality.Veneer drying approaches have evolved over time. The simplest and lowest cost approach is to air-dry veneers (Figure 5.4), however this gives little control over drying time and quality. There are various types of mechanical drying systems, with the most common in larger commercial operations being the jet-box conveyer-type dryer (Figure 5.5). With this system, veneers are fed along a conveyor system into the dryer and passed through a series of chambers
where hot air is blasted across the veneers. Temperature and conveyor speeds are adjusted to ensure the veneer exits the dryer with the appropriate moisture content and suitable quality. A number of moisture monitoring systems are commercially available that identify under-dried (wet) veneers at the dryer exit, allowing these veneers to be separated and re-dried.The most important and common form of avoidable degradation during veneer drying is out-of-range and uneven moisture contents. The target moisture content and acceptable range both between veneers and within a veneer depend mainly on the adhesive to be used to manufacture the final product, but the species and
the manufacturing process also have an influence. A common target moisture
content is 6% with a range of 3–10%.Drying-induced defects include buckling, splitting and surface modification.
All these defects affect the veneer recovery, and the aesthetics and mechanical qualities of the final product. These defects are generally more common if excessively high temperatures are used or if the veneer has been over- or underdried.

The variables that effect drying time are veneer thickness, wood species, the mix of heartwood and sapwood, initial moisture content, drying temperature, air velocity and relative humidity. Air-drying may take several days or weeks whereas a jet-box dryer can dry veneer within minutes. Better utilisation of the dryer is achieved by batching veneer to take account of large variations in initial moisture content. Moist sapwood, drier heartwood and veneer already partially dried require progressively milder drying schedules.

Figure 5.4.
Air-drying veneer sheets in Vietnam.

Figure 5.5.
Jet-box conveyortype veneer dryer

 

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