Three is plenty of interest in the trade at the moment in ensuring plywood has suitable quality and meets the performance criteria of the standards – regardless of whether it is going into furniture, vehicle fit outs or used in shuttering applications. We have looked before at the adhesion and the way bond strength is measured in shear. One aspect of plywood quality that lies in the hands of the machine operator is veneer tightness. While some factors (knottiness, grain deviations – coming from the log itself) are outside his control, the veneer product, and the quality of the loose face can still be controlled through skill, and judgement in the machine settings.
Firstly, what do I mean by ‘loose face’? Well, a veneer from rotary peeling will have two different surface textures. This is a result of the way the wood splits off the log just ahead of the blade. The upper side of the veneer, or outer side of the log, will tend to be much smoother, as it doesn’t undergo as much stress during the cutting process. The inner side is where small cracks form, due to stress release. This gives it the term ‘loose face’, while the upper side is ‘tight’. A veneer will often curve more readily with the loose face outermost, and less with the tight face outwards.
When doing decorative work, like marquetry, it may be second nature to recognise these tiny differences between the loose face and tight face. Then the best one can be presented to the eye in the decorative surface. The small checks in looser side will also allow it to take adhesive better and ‘key’ slightly better giving a strong bond. A win-win situation. But can a loose face be too loose?
In plywood manufacture, especially with greater mechanisation, the operator may be further removed from observing these differences. In fact, there is a lot to gain from thinking about, and controlling, crack development on the loose face. Proper soaking or steaming of the log before peeling is one way to reduce some of the stress that results in cracking, making the veneer less brittle. Good choice of cutting angle can also affect the stresses ahead of the blade tip. Choice of this angle can reduce or increase size and frequency of these checks. The use of a nose bar, with pressure on the log just ahead of the blade meaning the wood is able to flex with the cutting action, reducing checking. And of course, certain species are favoured as they are known to peel well, with lower risk of checking…
Veneer thickness is also important – governing the risk of check formation. The stresses are less intense when peeling a thinner veneer. Greater control is needed for thicker veneers – where checks can either be numerous but small or sometimes become very deep. In the lab here we assess these effects using microscopy. Dyes can be used to highlight the fine checks in the face, to ensure they are accurately measured when looking at a cross section of the veneer thickness. For example, the number of checks, their lengths and angles could be relevant. Some very interesting research has come out of Finland in the past developing techniques to understand and optimise peeling.
The greatest level of care probably goes into thin veneers for up market decorative applications. The thick veneers for the core of plywood, or these days, even thicker veneers for LVL and related engineered wood products might be the ones where greatest care is needed. The integrity of the veneer ensures structural performance. However, a small degree of checking can be desirable to promote good adhesion – the mechanical keying effect mentioned earlier. The adhesive can infiltrate the crack during spreading, leading to better adhesion between plies in the panel. However excess loss of adhesive into deep checks in very ‘loose’ surfaces can be a problem for product manufacturing costs. So, there are many reasons to understand this issue!
