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 report investigated the effect of veneer incising (incisor teeth patterns) on conventional hot-pressing time and compression behavior of spruce LVL/plywood. The 4 factors taken into account were panel type (LVL or plywood), number of panel layers, veneer moisture content and veneer type. Three veneer types were considered: non-incised veneer, lightly-incised (lathe-incised) veneer and heavily-incised veneer (similar to veneer incising in front of dryer). The three veneer moisture levels considered were 0%, 3% and 6%. The number of panel layers used was 5, 9 and 13. A mixed-level experimental design was employed. Using a statistical software program, JMP, the main factors affecting the LVL/plywood hot-pressing time were identified.
Veneer incising at the peeling lathe, a new technology developed at Forintek, has been increasingly applied in Canadian softwood plywood mills. Significant benefits include reducing veneer curl-up and spin-outs and increasing veneer recovery. However, a comprehensive study of the effect of veneer incising on the conventional hot-pressing process has not been undertaken so far. As part of the work on optimization of LVL/plywood hot pressing process, this report investigated the effect of veneer incising on the strength properties of Douglas-fir LVL/Plywood products. Lightly-incised Douglas-fir veneer peeled with a Forintek mini-lathe was used in comparison to non-incised Douglas-fir veneer. The three veneer moisture levels considered were 0%, 3% and 6%. The number of plies for plywood and LVL panels were 5 and 13, respectively. A mixed-level experimental design was employed. Using a statistical software program, JMP, the importance of factors affecting Douglas-fir LVL/plywood hot-pressing and strength properties were identified. Also the t-test was used to test the significance of the difference in panel mean strength. The results showed that:
1. For Douglas-fir species, the lightly-incised veneer (similar to that now used at mills) does not significantly affect the conventional hot pressing times for 1) 5-ply plywood panels if the target core temperature is 115 0C and 2) 13-ply LVL if the target core temperature is 105 0C, compared to non-incised veneer. The hot-pressing time increases with increase in veneer moisture content ranging from 0 to 6% for 5-ply plywood.
2. For Douglas-fir veneer, no occurrences of blows were observed after unloading the press even with 5-ply plywood panels at 6% veneer moisture content. Under the same conditions, the compression ratios of 5-ply plywood and 13-ply LVL using the lightly-incised veneer are slightly larger compared to non-incised veneer. Also higher moisture veneer results in higher panel compression ratio.
3. For 5-ply Douglas-fir plywood, there are no significant differences in plywood bending MOE and shear strength (lap-shear) between non-incised and lightly-incised veneer at either 0% or 3% veneer moisture content. However, at 6% veneer moisture content, the differences in mean plywood bending MOE and shear strength between non-incised veneer and lightly-incised veneer were identified significant; the mean bending MOE of 5-ply plywood using the lightly-incised veneer is about 10% higher compared to the non-incised veneer; on the contrary, the shear strength (lap-shear) of 5-ply plywood using the lightly-incised veneer is about 20% lower compared to the non-incised veneer. On a statistical basis, there is no significant difference in plywood mean bending MOR between non-incised veneer and lightly-incised veneer at each of the three moisture content levels.
4. For 13-ply Douglas-fir LVL, there is no statistical difference in edgewise bending MOE and MOR between the incised veneer and non-incised veneer at 3% veneer moisture content. Based on the limited number of replicates, the block shear strength through-the-thickness using the lightly-incised veneer is found to be approximately 10% higher compared to the non-incised veneer and this difference is found to be significant on a statistical basis. But the difference in block shear strengths parallel to grain between the incised veneer and non-incised veneer is not significantly different.
Veneer incising at the lathe, a new technology developed at Forintek, has been increasingly applied in the Canadian softwood plywood industry. The benefits include reducing veneer curl-up and spin-outs and increasing veneer recovery. However, a comprehensive study of the effect of veneer incising on veneer stress grading and LVL strength properties has not been thoroughly undertaken. In response to requests from our Forintek member mills, this report investigated the effect of veneer incising on the veneer stress grading and strength properties of spruce LVL products. Both incised veneer and non-incised veneer were peeled with blocks from same log using a Forintek mini-lathe equipped either with incisor bar or smooth roller bar. Then veneer sheets were randomly and proportionally sampled from the peeled veneer ribbon. These veneer sheets were stress wave tested and used to make LVL panels. The t-test was used to examine the significance of the differences in veneer stress wave time (equivalent to UPT) and LVL panel mean strength properties. The results showed that:
Veneer incising did not significantly affect veneer stress grading (identified by the measurement of veneer stress wave time or UPT), veneer density and veneer MOE.
Veneer incising also had no significant effect on the spruce LVL conventional hot pressing times for the core temperature to rise to 1050C and LVL compression ratio.
Further, there were no significant differences in LVL edgewise bending MOE, MOR and block shear strength parallel to grain between the non-incised and incised veneer. However, the difference in mean LVL block shear strength through-the-thickness between the non-incised veneer and incised veneer was significant. The block shear strength through-the-thickness using the incised veneer was slightly lower compared to that using the non-incised veneer using a glue spread level of 32 lbs/1000 ft2 per single glueline. A previous study showed that at higher glue spread levels normally used for LVL, 40 lbs/1000 ft2 per single glueline, the block shear strength through-the-thickness was slightly higher for the incised veneer compared to that using the non-incised veneer.
VGrader, Veneer Grading Optimizer, was developed at Forintek to assist mills to optimize on-line veneer stress grading operations. So far, more than 10 copies of VGrader 1.0 software have been delivered to Forintek member mills. The software can recommend the optimum grading thresholds through analyzing the properties of veneer to help mills deal with “what-if” scenarios when veneer species, log source and diameter as well as final veneer products change. By tailoring veneer grades to the market requirements of LVL/plywood products, the software serves as a useful tool to characterize specific veneer for end use and help optimize veneer on-line stress grading and products lay-up options.
During the past year, the VGrader software has been upgraded to deal with either UPT-based (ultrasonic signal propagation time) veneer stress grading or E-based (modulus of elasticity) veneer stress grading or veneer visual grading. The software has also been upgraded to accommodate UPT data either from mills or laboratory testing of veneer samples. A direct linkage between laboratorial measurement span and desired wheel-span of the on-line grading system was also setup. The current version of the software is VGrader 3.0. To help mills optimize current on-line stress grading operations, the proper procedures to find the optimum UPT thresholds were established.
The proper procedures are as follows:
1) Sample veneer sheets representative of veneer population in the mill and perform stress wave testing for sampled sheets using a portable stress wave timer. Alternatively, full-size veneer sheets can be sampled right after the on-line grading system with UPT data being recorded for each veneer sheet;
2) Measure other relevant veneer properties such as thickness, density, moisture and knots;
3) Calibrate the stress wave time (or UPT) to find its zero offset value;
4) Store all measurement data into a VGrader compatible database;
5) Use the upgraded VGrader software to examine the distribution of veneer attributes/properties such as thickness, UPT, density and MOE;
6) Derive required veneer MOE based on the performance requirements of target veneer products;
7) Establish stress grading constraints and using VGrader 3.0 to perform computerized veneer stress grading through adjusting the UPT or E thresholds and examining the change of statistical veneer MOE, densities and volume breakdown per grade until all the grading constraints are satisfied;
8) Convert the optimum set of UPT or E thresholds from the VGrader software into those used for on-line veneer grading system to perform stress grading;
9) Make veneer products and test them to validate the grading results.
An example of establishing the above procedures was also demonstrated.
Veneer incising at the peeling lathe, a new technology developed at Forintek, has been increasingly used in Canadian/US softwood plywood/LVL mills. Significant benefits include reduced veneer curl-up and spin-outs and increased veneer recovery. However, veneer incising and peeling is a very complicated process with a number of interactive variables: 1) lathe setting variables including vertical gap, horizontal gap and pitch angle; 2) veneer incising variables including overdrive percentage of the incisor bar and 3) lathe peeling speed. Therefore, optimization of veneer incising and peeling appears critical to mills to achieve good veneer quality and high veneer recovery.
In order to better simulate the industrial peeling lathe, Forintek’s laboratory mini-lathe was significantly upgraded. A statistical method (Response Surface Method) was used to investigate the effects of 5 main process variables: vertical gap, horizontal gap, pitch angle, peeling speed and bar overdrive percentage on the veneer thickness variation and curl-ups. More than 100 spruce and Douglas-fir blocks were peeled using either incisor bar or smooth roller bar. The effects of bar type on veneer curl-up and thickness variation were compared. A series of regression analysis models were generated for veneer thickness variations and veneer curl-up. The important variables affecting veneer quality were identified. The main results were:
1) Significant interactions existed between the main peeling variables. Veneer thickness variation and curl-up were lathe setting dependent. Pitch angle and horizontal gap were identified as the two most critical variables affecting veneer quality in terms of both thickness variation and veneer curl-up, followed by vertical gap, bar overdrive (incisor bar) and peeling speed.
2) There was a trade-off between veneer curl-up and veneer thickness variations. Based on a joint optimization method, the optimum lathe settings for either incisor bar or smooth roller bar veneer peeling have been established and validated for spruce, which can help industry achieve better veneer quality and higher veneer recovery as well.
3) The optimum lathe settings for smooth roller bar peeling and incisor veneer peeling for spruce were quite different, which demonstrated that lathe settings need to be adjusted after changing bar from one to another. The optimum lathe settings for 2.50” smooth roller bar peeling were pitch angle 89.50, vertical gap 0.425” and horizontal gap 0.1”. In contrast, the optimum lathe settings for incisor bar peeling were pitch angle 90.50, vertical gap 0.388”, horizontal gap 0.1” and overdrive percentage level 100.5%. Compared to smooth roller bar peeling, incisor bar peeling allowed a slightly tighter gap (or higher compression) due to applied bar overdrive. By comparing the respective optimum lathe settings, it was concluded that veneer incising at the lathe did not contribute to more thickness variation in veneer.
4) Compared to Douglas-fir, spruce tended to generate more serious curl-up veneer. For spruce veneer, higher moisture in the core contributed to a reduction in veneer curl-up. Using the optimum settings with the incisor bar for spruce, it was found that these settings also produced high quality veneer when peeling Douglas-fir.
5) While the above findings based on the laboratory lathe are useful for understanding the lathe operation mechanism, optimum settings are specific for industrial lathes. It is therefore recommended that studies using similar statistical approaches be conducted in mill trials.
This manual is an updated version of the 1982 veneer drying manual and presents new veneer drying technologies for use by dryer operators, foremen and managers. This manual is prepared for the plywood and LVL mills of both eastern (primarily hardwood veneer) and western (primarily softwood veneer) Canada.