Analysis of small-log processing to achieve structural veneer from juvenile hardwood plantations
AuthorMcGavin, Robert Lee
AffiliationForest and Ecosystem Science
Document TypePhD thesis
Access StatusOpen Access
© 2016 Dr. Robert Lee McGavin
The majority of the hardwood plantation estate in Australia has been established and managed for pulpwood markets; however interest exists from both forest and wood processing industries as to whether the resource could be processed into higher value products. This situation is consistent with many other countries where hardwood plantations have been established. Previous wood qualities studies have confirmed that relatively young, fast-grown hardwood trees contain wood properties that are different from wood recovered from the same species which was sourced from mature, native forest trees. Despite this difference, the properties are generally in the range of desirable properties for many processing options and high-value end-products. Indeed, the reduction of some properties (e.g. density) may prove advantageous for some processes and end-products. Previous studies have also confirmed a large variation of properties within plantation trees as a result of the high proportion of wood which is formed in the juvenile phase of tree growth. Plantation trees are also known to yield relatively small diameter logs that contain a range of defects that affect the efficiency of processing methods and suitability for end-products. Sawmilling and veneer processing are attractive processing options for production of value-added products. Many processing studies have been previously completed to convert plantation hardwood logs into a traditional suite of sawn products, mainly using conventional production systems. The results of these studies have consistently shown that complications are encountered with persistent problems arising in recovery, drying, stability, durability and appearance qualities. Rotary veneer processing has the advantages of yielding significantly higher recoveries when compared with sawn timber processing and the relatively small piece size (mainly in thickness) can reduce some of the effects of properties gradients and growth stresses; and reduce lengthy and problematic drying. Difficulties exist however in utilising traditional veneer processing methods for processing plantation hardwoods, especially where log diameters are small and end-splitting is common. A relatively newly developed veneer processing method can remove the difficulties experienced when traditional methods are used. The objective of this PhD study was to analyse small-log processing through the application of lean spindleless rotary veneer processing methods to achieve rotary veneer for structural purposes from Australian-grown, juvenile hardwood plantations. Simple spindleless lathe technology was used to process 918 billets from six commercially important Australian hardwood species (Corymbia citriodora subsp. variegata, Eucalyptus cloeziana, E. dunnii, E. pellita, E. nitens and E. globulus). The study demonstrated that processing representative stands of the current Australian hardwood plantation estate using spindleless veneer lathe technology can overcome many of the problems present when using traditional solid wood processing techniques. Recoveries achieved during the study were in the order of two to six times what is usually achieved from processing similar resources using traditional solid wood processing systems. The graded veneer recovery was dominated by D-grade veneer, the lowest visual grade quality for structural veneer meaning the veneer is suitable for face veneers on non-appearance structural panels as well as the core veneers for the vast majority of appearance and non-appearance structural panels. The veneers contained a range of defects that impacted the final assigned grade. The presence of gum pockets, bark pockets and decay (mainly surrounding knots), encased knots, splits and surface roughness were identified as the main defects limiting veneer grades to D-grade, with other defects contributing to a lesser degree. Several scenarios were simulated including a relatively easy change to the grading standard rules for gum defects, and various outcomes resulting from the modelling of effective pruning. All scenarios demonstrated a positive outcome with veneer values increasing by up to nearly 23%. Variation in veneer and grade recoveries was found within species growing across different conditions and with different silvicultural treatments (thinning and pruning). The results however were not consistent, especially when resulting veneer value was assessed. While thinning and pruning is a common practise to target increased production of higher-value clear wood, the study demonstrated that clear wood is not always achieved which can result in a reduced veneer value. Veneer density, dynamic modulus of elasticity (MoE) and specific MoE evaluations revealed a wide variation of properties existed between species, within species and within a billet. Veneer density was found to not always be a good predictor of MoE, especially in commercial size samples which contain natural defects. Simple mathematical modelling, using sigmoidal curves, was demonstrated to be an effective method to model the evolution of key wood properties across the billet radius and along the resulting veneer ribbon with benefits for tree breeders and processors. Rotary veneer processing using spindleless lathe methods was shown to be able to efficiently process young, fast grown hardwood plantation trees with resulting veneers containing visual grade qualities and mechanical properties suitable for the manufacture of structural veneer-based products. Further research is necessary to determine appropriate end-products and optimised product manufacturing protocols. An economic evaluation is necessary to determine the potential profitability.
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