In this study, we conducted mill tests to: 1) measure the distributions of veneer clipping width and moisture content (MC) for different green veneer sorts; and 2) evaluate the tangential veneer shrinkage at different MC levels for common veneer species. We also performed lab tests to evaluate the effect of veneer species, log location, veneer thickness, sapwood and heartwood, drying conditions (temperature and humidity) and final veneer moisture (MC) on veneer width shrinkage.
Based on the mill measurements, although a positive trend exists between veneer clipping width and green veneer MC, the correlation is generally weak for common softwood species such as SPF and Douglas-fir veneer. This result contradicts the prevailing concept of veneer MC clipping. The primary reason could be attributed to the inaccurate measurement of green veneer MC with the current industrial MC sensors.
Based on both mill and lab testing results, a good correlation exists between veneer width shrinkage and final veneer MC. However, the correlation between veneer width shrinkage and green veneer MC is weak or at most fair for common softwood species. Also within each veneer sort, this correlation is very poor. These results bring up an issue as to what strategy to use to perform green veneer clipping.
Based on both mill and lab measurements, a shrinkage of 1.5 ~ 3% in veneer width occurs when final average veneer MC is still above fiber saturation point (FSP). This contradicts the well-established theory that shrinkage of wood only starts to occur when its MC drops below the FSP. The main reason is probably due to the MC gradient through veneer thickness, and MC variation across veneer width. Once the MC of the wood cells of the veneer surface drops down to FSP, it starts to shrink. However, due to the constraint caused by the wood cells of the inner part of veneer with MC above the FSP, the internal stresses develop and shrinkage decreases.
Veneer width shrinkage varies among species and logs, and also within the log, namely, from sapwood to heartwood. It is further affected by drying conditions, veneer thickness and final veneer MC. At fast or conventional drying conditions such as higher temperature and faster airflow, veneer drying rate increases, the veneer MC gradient through the thickness becomes significant. In this manner, constraints, resulting from uneven shrinkage and internal stress, occur between the outer and inner part of the veneer. As a result, veneer width shrinkage is reduced compared to slow or air drying conditions. For common softwood species such as SPF and Douglas-fir, the effect of drying temperature on tangential veneer shrinkage is more pronounced with heartwood veneer than sapwood veneer. In other words, at a higher drying temperature, shrinkage of heartwood veneer is reduced more than that of sapwood veneer. Meanwhile, the effect of humidity on veneer width shrinkage is negligible.
At the same conventional drying condition, for Douglas-fir sapwood veneer, thicker veneer (1/8”) shrinks more than thinner veneer (1/10”); In contrast, for Douglas-fir heartwood veneer, thicker veneer (1/8”) shrinks less than thinner veneer (1/10”). The shrinkage of thicker veneer (1/8”) is more considerably affected by the drying conditions compared to that of thinner veneer (1/10”). The difference in shrinkage between sapwood and heartwood veneer is smaller for thinner veneer (1/10”) than for thicker veneer (1/8”).
It is important to improve drying productivity since this process is the bottleneck in plywood production. To address this issue, we evaluated the veneer moisture content (MC) distribution and the accuracy of radio frequency (RF) sensors both in the laboratory and in mill trials in this study.
We analyzed the distribution of green veneer moisture content (MC) and density from sapwood to heartwood for lodgepole pine logs via veneer peeling in the Forintek’s composites pilot plant. The results show that the MC distribution appears to be a dual-peak pattern both for heartwood and for sapwood. The position of the first peak is more consistent whereas the second peak varies more among logs. This information is useful for determining the proper number of green veneer sorts and the optimum cut-off MC level for each sort.
Readings of the radio frequency (RF) sensor are affected by veneer species, veneer density, veneer thickness, temperature, green veneer MC, grain angle, and distance between the sensor source and veneer surface. Of these variables, veneer density, veneer species and the distance are found to be the three main factors affecting the readings of the sensor. Based on the measurement results from different species, we conclude that the RF sensor is only suitable for measuring green veneer MC below 80%.
The correlation between readings of the sensor and green veneer MC varies from species to species. In general, this correlation could be improved using an exponential or a power equation. The readings of mill sensor are more inconsistent than those of the lab sensor for the heart sort veneer. The lab sensor underestimates MC for the heart sort veneer and overestimates MC for the sap sort veneer. However, the mill sensor overestimates in both positive and negative ways for each sort. To improve the accuracy of the readings, the sensor needs to be calibrated based on the species and veneer thickness, and the distance between the sensor source and veneer surface has to be kept as small as possible. However, this is not practical for the spruce, pine and subalpine fir (SPF) group since these species are not separated on a commercial basis.
We conducted three mill visits to: 1) measure the distribution of green veneer MC for different veneer sorts, and 2) assess the accuracy of current green veneer sorting. The results demonstrate that the accuracy of moisture sorting differs among mills and species, and the species mix like SPF generates larger MC variation within each sort. In general, the heart veneer sort is well done, but there is a significant overlapping between light-sap and sap veneer sorts. This indicates that the current industrial RF sensors cannot ideally sort higher MC veneer. The results also show that there is potential for more accurate green veneer sorting which would result in a 5% increase in drying productivity. This improvement would generate more than $1 million in annual savings per mill.
In this study, we conducted systematic experiments on air permeability of aspen veneer and glueline in terms of panel compression ratio (or applied platen pressure), degree of glue cure (or pressing time), veneer type (sapwood or heartwood veneer) and glue spread level. We also compared the air permeability data of aspen veneer and veneer-ply (2-ply veneer panel) to aspen solid wood and aspen oriented strandboard (OSB). Based on this study, the following conclusions were drawn:
For laminated veneer lumber (LVL) and plywood panels, the compression ratio is the most important factor affecting the panel permeability, followed by veneer type (sapwood or heartwood veneer), glue spread and degree of glue cure (or pressing time). The air permeability of the glueline decreases in the course of glue curing; however, its order of magnitude remains the same as that of uncured glue. The reduction in panel permeability mainly results from small densification of each veneer ply instead of the sealing effect of the glueline. Therefore, during LVL/plywood hot-pressing, the glueline does not serve as a main barrier to the gas and moisture movement as commonly speculated. However, due to the substantial change in the magnitude of panel permeability merely within a 5% compression ratio, the convection effect on heat and mass transfer is considered to be very limited.
The air permeability of sapwood veneer is about twice that of heartwood veneer without compression. However, with compression, the air permeability of heartwood veneer drops much faster than that of sapwood veneer. The permeability of a sapwood veneer panel is 5.5 ~ 7.0 times higher than that of a heartwood veneer panel merely with a compression ratio in the range of 2.5% ~ 5%. In practice, it implies that 1) panels made from sapwood veneer are more treatable with preservatives; and 2) by controlling panel permeability through veneer incising, proper panel lay-up and densification, mills could reduce blows/blisters during hot-pressing.
The air permeability of aspen wood or veneer is not affected by wood density. The air permeability of aspen LVL/plywood panels is 1.5~ 2 times larger than that of aspen solid wood due to the existence of lathe checks, but is significantly lower than that of aspen OSB at the same density level of the panel. On average, commercial LVL/plywood panels have almost the same magnitude of air permeability as commercial OSB. However, due to the absence of voids and small horizontal density variation, LVL/plywood panels will be less permeable than OSB.