Laminated Veneer Lumber (LVL) and plywood are the two major veneer-based wood composite products. During LVL/plywood manufacturing, the hot pressing process is crucial not only to the quality and productivity, but also to the performance of panel products. Up to now, the numerical simulation of the hot-pressing process of LVL/plywood products is not available.
To help understand the hot-pressing process of veneer-based wood composites, the main objective of this study was to develop a computer simulation model to predict heat and mass transfer and panel densification of veneer-based composites during hot-pressing. On the basis of defining wood-glue mix layers through the panel thickness, a prototype finite-element based LVL/plywood hot-pressing model, VPress®, was developed to simulate, for the first time, the changes of temperature, moisture and vertical density profile (VDP) of each veneer ply and glueline throughout the pressing cycle. This model is capable of showing several important characteristics of the hot-pressing process of veneer-based composites such as effect of glue spread level, veneer moisture, density, platen pressure and temperature as well as pressing cycles on heat and mass transfer and panel compression. Experiments were conducted using several different variables to validate the model. The predicted temperature profiles of the veneer plies and gluelines (especially at the innermost glueline) by the model agree well with the experimental measurements. Hence, the model can be used to evaluate the sensitivity of the main variables that affect hot-pressing time (productivity), panel compression (material recovery) and vertical density profile (panel stiffness). Once customized in industry, the new model will allow operators to optimize the production balance between productivity, panel densification and panel quality or stiffness. This hot-pressing model is the first step in facilitating the optimization of the pressing process and enhanced product quality.
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, hot-pressing behavior of 5-ply Douglas-fir and spruce plywood was evaluated. Visual sorted Douglas-fir and spruce veneer (select) sheets were acquired from a Forintek member's mill. All veneer sheets were conditioned to a moisture level of 3%. Platen pressure and target core temperature were chosen as two control variables. Using a JMP statistical software program, their effect and sensitivity on plywood productivity, material recovery and panel bond quality were studied in terms of hot-pressing time, panel compression ratio and wood failure percentage. Second-order response surface models (RSM) were further established for these three criteria. The results showed that 1) the material variation within the same visual grade (select) for each species resulted in the variation of hot-pressing time, compression ratio and wood failure percentage. Therefore, the applied pressing schedules should be a bit conservative for thin-type 5-ply plywood products; 2) generally, the rate of temperature rise in 5-ply Douglas-fir plywood was faster than that in 5-ply spruce plywood. The difference in heat transfer speed for these two species was more pronounced when the target core temperature is higher than 1100C; 3) the hot-pressing time, compression ratio and wood failure percentage was not very sensitive to the platen pressure within the range of 155 psi to 190 psi for 5-ply spruce plywood; In contrast, in order to achieve the target wood failure percentage (80%) for 5-ply Douglas-fir plywood, the target core temperature had to be set at least 1100C along with a platen pressure higher than 200 psi; 4) the hot-pressing optimization was the balance of the hot-pressing time (productivity), panel compression ratio (material recovery) and wood failure percentage (panel quality). Overall, the optimum pressing conditions were veneer moisture dependent. At a 3% veneer moisture content level, the optimum platen pressure was 175 psi for 5-ply spruce plywood whereas the optimum platen pressure was 225 psi for 5-ply Douglas-fir plywood. For both species, the optimum target core temperature for the 5-ply panels was 1100C at a platen temperature of 1550C.
This report addresses issues about productivity, recovery and quality concerning veneer peeling in plywood mills. It was demonstrated that green veneer can be composed using a stitching technique. The maximum stitching speed was 50 ft/min which was slower than a current veneer composer. Stitched veneer did not have a significant effect on bending properties, but shear strength was slightly reduced which could be caused by the existence of stitching threads between the glueline.
The roller bar diameter size had a significant influence on veneer quality. In general, peeling veneer with a 1” diameter roller bar resulted in the smoothest veneer with the most uniform thickness. The veneer thickness and roughness between 1.0” and 2.56” diameter roller bars were significantly different, but the difference in veneer quality between 1.75” and 2.56” diameter roller bars was not significant. Further, the difference in veneer quality between 1.0” and 1.75” diameter roller bars was not significant except for veneer roughness.
Knife height also had a significant effect on veneer quality. Setting the knife at the spindle center proved to be the best. Veneer thickness at this setting was consistently closest to the target, and had the smoothest surfaces and smallest lathe checks. Average veneer thickness was lowest as well. While higher or lower settings created rougher veneer, higher settings were more forgiving than lower ones. For best results, the peeling knife should therefore be set at 0.0” to 0.015” above the spindle center.
Incisor teeth pattern affected veneer quality. Narrower teeth and a wider gap resulted in better veneer quality in terms of veneer curl-up (flatness) and green and dry veneer thickness variations. However, the effect of incisor teeth patterns on veneer roughness and lathe checks seemed to be negligible.
The validation tests revealed that an optimum lathe setting for the smooth roller bar was the following: pitch angle (PA) =89.50, vertical gap (VG)=0.425” and horizontal gap (HG) = 0.1”, and the optimum lathe setting for the incisor bar was the following: PA=90.50, VG=0.388” and HG=0.1” to 0.11” when peeling 1/8-inch veneer.
The peeling computer program VPeel® was successfully upgraded to allow users to define profiles of pitch angle and horizontal gap. This feature will help the veneer product industry to define optimum lathe settings.
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.
This project was placed top priority because veneer drying is the bottleneck and the biggest item for energy consumption in plywood mills. During the past three years, the Forintek veneer drying manual was first upgraded. Our research capabilities in drying were significantly improved by the establishment of a mini-dryer and the installation of pilot scale veneer dryer. While the former allows for in-situ monitoring of moisture content change in veneer during drying, the latter can be used to simulate industrial dryers such as jet dryers or longitudinal dryers. Both are capable of testing the effects of such parameters as drying temperature, humidity and air flow. Based on the improved understanding from the experimental tests and theoretical analyses, initial computer models (VDRY-L and VDRY-J) have been developed to simulate the drying processes for longitudinal dryers and jet dryers, respectively.
The combination of laboratory studies and computer simulation led to effective mill studies to evaluate the existing drying technologies, to optimize the existing dryers without capital cost and develop future opportunities.
The key findings from these studies were:
Lab tests showed that temperature and airflow rates are dominant factors affecting drying rate for the whole drying process. While veneer drying increases as drying temperature and airflow rates are increased, higher temperatures and airflow rates both have a greater influence on drying rate at the early drying stages compared to the final stages.
Lab tests also showed that veneer can be dried at high rates under high humidity and temperature conditions. A combination of high temperature and humidity is a good drying mode to save energy and increase output.
Mill case studies showed that the sorting of green veneer prior to drying was poorly done. Lab tests showed that a probable reason for this was due to the inaccuracy of RF sensors used for measuring moisture content of green veneers. The RF accuracy significantly dropped when veneer moisture content exceeded 30%.
Mill case studies showed that a sensitivity analysis of drying parameters is a very useful method for determining effective measures for optimizing dryer performance.
Mill case studies showed that by using higher temperatures and humidity levels in the early drying stages and lower temperatures in the final stages, veneer production can be increased and drying energy can be reduced.
An impact analysis showed that mills can potentially save up to 10% in energy costs and increase production by 10% by optimizing dryer performance.
It is recommended that mills use the results and methodologies from this project as guidelines for optimizing dryer performance. Further research should be undertaken to improve green veneer moisture sorting. The current computer simulation models of drying should be calibrated and used as a quality control tool for determining optimum dryer parameter settings.
On March 31, 2003, Forintek Canada Corp completed the project “Optimization of Veneer Drying Processes”. Forest Industry Investment and Forintek members funded this one-year project. The purpose was to develop practical methodology to transfer new Forintek knowledge about veneer drying and quantify the benefits of optimizing existing dryers. Currently in BC there are 15 softwood plywood mills producing approximately 1.7 billion square feet of plywood (3/8” basis) per year and employ over 2800 people.
In plywood production, veneer drying is one of the key manufacturing processes. Currently, dryers create a bottleneck in the mill, restricting plywood production. Dryers are expensive to purchase and operate. The results show that by optimizing existing dryer operations the potential for increased production and efficiency savings is significant. This provides BC manufacturers with the opportunity to reduce costs without investing capital for costly new equipment or modifications.
At the beginning of the project, five BC mills were visited to identify and quantify key variables affecting dryer operation. Discussions with mill personnel were important for determining specific limitations that could be made to the drying process for optimization and to understand the main problems and potential for improvement from a mill perspective.
At Forintek’s laboratory in Vancouver, small-scale drying tests were conducted to further develop the fundamental knowledge of commercial drying. For this purpose, a laboratory dryer was specially designed to simulate industrial drying processes. Testing focused on determining the drying rate relationships involving drying air temperature, airflow speed and humidity. Results showed consistently that the best set-up for maximizing drying rate was by operating the dryer at high temperature with high humidity and airflow speed. The lab study also showed that these conditions are most importantly applied at the early stage of drying. Near the end of the drying process, the effects of high temperature were diminished, suggesting that temperature can be reduced at this stage to economize on energy consumption without significantly affecting drying rate. As well, the lower drying temperature near the end of the drying process reduces the risk of surface inactivation (loss of bonding sites) in the veneer. Additional testing at Forintek was conducted to determine the accuracy of radio frequency moisture sensors currently used for sorting green veneer prior to drying. Results showed that moisture content measurement error increased significantly with moisture content above 30%.
Case studies were conducted at two BC mills to quantify the relationships of key drying parameters and to test methods for optimization. In both mills, prior to drying, measurements of green veneer showed that sorting was very inefficient with a large proportion of veneers incorrectly directed into the heart, light-sap and sap bins. Dryer control modifications demonstrated that dryer temperatures could be increased by the same order of magnitude as in the laboratory tests by restricting damper openings to raise humidity. This not only increased veneer feed speed, but also reduced steam consumption used to heat the dryer. At one mill, changes to temperature and humidity conditions resulted in an increase of the veneer feed speed from 9.3 to 10.8 ft/min. for light sap veneer, representing an increase of 16% in productivity through the dryer. At the same time, it was estimated that 10% of the annual energy use for the dryer could be saved based on reduced steam consumption.
Based on the case study results, it was conservatively estimated that the 15 plywood mills currently operating in BC could potentially increase plywood production by 5% or 85 million square feet per year. At today’s prices, this translates into approximately $31 million per year by implementing, at no cost, the methodology presented here to improve dryer performance. In addition, results also showed that dryer energy consumption could be potentially reduced by 10% A practical 5% reduction would amount to a savings of $3.0 million for the BC plywood industry. No additional implementation cost was required to achieve this energy savings. In addition, Forintek estimated that better green veneer sorting by mills could significantly reduce the moisture content variability of dry veneer, further improving dryer productivity. However, this would require more accurate moisture sensing technology than is now commercially implemented. It is recommended that new technology be developed.