In this work, the properties of aspen veneer from two mills (A and B) were compared. The comparisons between the incised veneer and non-incised veneer for mill A were made in terms of veneer thickness, ultrasonic propagation time (UPT), density and MOE. The aspen veneer was further characterized for LVL/plywood products by tailoring veneer grades to the requirements of final veneer products. In addition, MOE-based veneer stress grading and UPT-based veneer stress grading were compared for the aspen veneer. The advantages of MOE-based veneer stress grading over UPT-based veneer stress grading were identified in terms of veneer grade MOE and volume breakdown. The main results are summarized as follows:
1) Aspen veneer properties change from mill to mill. The differences in aspen veneer density and MOE between mill A and B are significant with mill A producing denser and stronger aspen veneer.
2) For aspen veneer in the mill A, the distribution shapes of veneer thickness, UPT, density and MOE between the non-incised and incised veneer are quite similar. Although the differences in veneer thickness, UPT and density between the non-incised veneer and incised veneer are identified as significant, the difference in veneer MOE is not significant due to the effect of both veneer UPT and density. The incised veneer has a slightly higher variation in thickness and is also slightly thicker compared to the non-incised veneer. This could due to the change of lathe settings or the property variation of aspen species as indicated with the veneer density variation.
3) Of the aspen veneer from mill A, using the optimum UPT thresholds, about 27.5 ~ 30.9% can be extracted through veneer stress grading to make 2.0 million psi LVL; about 43.4 ~ 59.9% can be sorted out for 1.8 million psi LVL; and the remaining 12.6 ~ 25.7% can be used for 1.5 million psi LVL or for plywood. It was also found that the incised aspen veneer generates 3.4% less of top stress grade G1 but 16.5% more of stress grade G2 compared to the non-incised aspen veneer if performing the optimum UPT-based stress grading.
4) The MOE-based veneer stress grading not only results in a smaller variation in MOE of each grade, but also higher volume percentages of stress grades G1 and G2 compared to the UPT-based veneer stress grading. This smaller variation in MOE of each stress grade will be very beneficial to the industry and structural applications since higher design stress can be assigned for the wood structural components. Also the higher percentages of stress grades G1 and G2 with the MOE-based veneer stress grading has significant economical implications and should be recognized by the industry.
5) To maximize mill profits, veneer sheets need to be periodically sampled and analyzed using the VGrader software. The optimum grading thresholds for the specific veneer can be established for on-line veneer stress grading based on the current market and requirements of final veneer products, providing a real solution to characterize and make best use of the specific veneer for LVL/plywood products.
Characterizing aspen veneer for LVL/plywood products. Part 2. LVL pressing strategies and strength properties|Manufacturing characteristics and strength properties of aspen LVL using stress graded veneer
In this study, aspen veneer sheets were sampled from a Forintek member mill. Their attributes and properties were measured. Using the optimum stress grading strategy, aspen veneer was segregated into 3 distinct stiffness groups (stress grades G1, G2 and G3) and conditioned to 3 different moisture levels. An experimental design for 3-level four factors comprising veneer moisture content, veneer stress grade, mat pressure and glue spread (or resin level) was adopted. Based on the experimental design, LVL panels with different combinations of four factors were pressed until the target core temperature reached 1050C to achieve full cure followed by a stepwise decompression cycle. The LVL panel final thickness, density, compression ratio and relevant strength properties were measured. After that the effect of aspen veneer moisture, stress grade, mat pressure and glue spread and their relative importance on LVL compression behavior, hot-pressing and strength properties were evaluated using a statistical analysis program. The relationship between LVL panel properties and veneer properties was examined. Finally a method to enhance LVL modulus of elasticity (MOE) to make high stiffness LVL was discussed. From this study, the following results were found:
Aspen veneer is capable of making LVL products meeting 1.8 and 2.0 million psi MOE requirements. Optimum veneer stress grading and proper pressing schedule are two important keys to the manufacture of high-stiffness aspen LVL products. Further, a possibility to make high-grade aspen LVL meeting 2.2 million psi MOE exists by proper veneer densification and optimum veneer stress grading.
The roles of four factors affecting LVL pressing behavior and strength properties are quite different. Glue spread and mat pressure, rather than stress grade and veneer moisture content, are two main factors affecting hot-pressing time taken for the core to reach 1050C. With incised veneer, the moisture from the glue in the glueline affects the rise of core temperature more pronouncedly than the moisture in the veneer, and is more critical to the cure of the glue. High glue spread (44 lbs/1000ft2) not only significantly increases the hot pressing time taken for the core to rise to 1050C, but overall also decreases most LVL strength properties with the pressing schedule used. High mat pressure does not necessarily result in high LVL panel compression due to the high gas pressure that occurs in the core.
Veneer stress grade and veneer moisture are the two predominant factors that mostly affect LVL strength properties. LVL panels assembled with high stress grade result in increases in both flatwise and edgewise MOE and MOR properties rather than shear strength either longitudinal or through-the-thickness. Further, using high stress grade veneer can help make more efficient structural systems in terms of both stiffness-to-weight and bending strength-to-weight ratios compared to using low stress grade veneer. High veneer moisture at 6% impairs all LVL strength properties except edgewise bending MOE.
LVL compression ratio can help link veneer MOE with LVL panel edgewise bending MOE. Overall, every increase of 1% in LVL compression ratio would result in 1% increase in LVL and veneer MOE ratio. With regard to aspen LVL MOE enhancement, using high veneer stress grade gains slightly less than using low veneer stress grade. On average, every increase of 1% in aspen LVL compression ratio results in 0.82%, 1.05% and 1.20% increase in aspen LVL and veneer MOE ratio assembled with stress grades G1, G2 and G3, respectively. In practice, those conversion factors for any specific veneer can be derived based on the correlation between veneer MOE and MOE of target LVL/plywood products made with proper pressing schedules, and be further used to derive requested veneer MOE for each stress grade to perform the optimum veneer stress grading.
Pressing schedules show significant effect on aspen LVL compression behavior and strength properties. Using a pressing schedule with step-wise decompression cycles following the core temperature to rise to 1050C, an excessive compression of LVL in the range of 13.5% to 27.6% is generated which results in high-stiffness LVL with an average MOE of approximate 2.0 million psi for all experiments. Although this pressing schedule has slightly longer pressing time and off-target LVL thickness than current commercial LVL pressing schedules, it helps enhance the strength properties of LVL.
It is recommended that further work should include the effect of different decompression cycles and mat pressure on LVL panel compression ratio and strength properties.
This summary report presents the main findings of a study of systems for the production of aspen veneer logs in northern Ontario. The case studies, which took place in 2001 and 2002, covered four types of operations: in-woods chipping, satellite yards, roundwood merchandizing at roadside, and roundwood production at the stump. The advantages and disadvantages of each system for the production of veneer logs are given, along with production cost estimates for each system. A report presenting further productivity and quality data, and a detailed cost analysis (in an Excel spreadsheet) are available to FERIC members and partners on request.
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.
As increasing volumes of short-rotation hybrid poplar reach maturity, various sectors of the composites wood products industry have shown an interest in their potential as substitutes for aspen or other low-density species. Veneer manufacturers were particularly curious of the suitability of these hybrids for the manufacture of laminated veneer lumber (LVL) and plywood. This study was designed to provide some guidance to Forintek members.
As a result of its fast growth and abundant availability, aspen has become an increasingly important commercial wood species in the production of oriented strand board (OSB) and veneer-based composites such as laminated veneer lumber (LVL). The purpose of the study described in this report was to determine the effect of conditioning temperature on veneer quality using a 5/8" roller bar, and to determine the optimum bar gaps based on results from previous Forintek studies on aspen veneer peeling.