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.
In this study, extensive veneer compression tests were conducted to examine the transverse compression behaviour of veneer at both ambient and controlled temperature and moisture content (MC) environments. Based on the results, a novel method was developed to characterize overall surface quality of veneer and other wood materials in terms of their bondability and compression behaviour.
The method would have significant implication in both theory and practice. In theory, the general wood compression theory would need to be modified. The revised wood compression theory would include four stages instead of commonly defined three. The first stage, which has long and so far been overlooked but is critically important, could be named as “non-linear conformation”. During this stage, the contact area increases nonlinearly with the load applied. It is this stage that directly reveals the interfacial bonding behaviour of wood materials such as veneer-to-veneer and strand-to-strand and their minimum compression required for achieving adequate contact (bonding). In practice, the method provides a fast and objective way of evaluating surface roughness/quality of veneer and other wood materials. The new method also establishes the maximum compression allowable for achieving the best panel performance in terms of bonding strength, stiffness and dimensional stability. Based on the concept of this method, it was further found that both minimum compression required and maximum compression allowable are independent of temperature and MC, which provides a direct benchmark to the material recovery during panel hot-pressing.
In a case study with Trembling aspen veneer, the variation of veneer surface roughness/quality and its effect on resulting material recovery were first revealed. Then, the optimum panel densification was identified for performance plywood and LVL products based on the frequency distribution of the minimum compression required and the maximum compression allowable. Finally, an overall veneer quality index was established to compare veneer overall quality for different species/thickness. The method shows good potential in practical applications for increased material recovery, reduced glue consumption and improved panel performance.
In this project, 5 species of veneer from 4 mills comprising aspen, hemlock, incised Douglas-fir and spruce/lodgepole pine veneer were sampled and evaluated. Also, non-incised Douglas-fir veneer was assessed. A portable Metriguard laboratory unit was employed to measure the stress wave time for each piece of veneer sheet. Other relevant veneer characteristics such as density, moisture content and knot area were also measured. All veneer samples were visually graded according to CSA Standard O151-M1978. A computer database was developed to record all measured data.
A practical user-friendly computer software package VGrader 1.0 was developed to assess veneer sorting strategies. This software provides users with panel lay-up options in connection with veneer grading results. Users can assemble their desired veneer products using either visual grades or stress grades by mixing species, grades and thickness. Further built into this software is an end product strength prediction model which was calibrated with experimental results obtained throughout this research. An electronic user help manual is built into the software, which guides users through the operation of this software. The intent of the software is to provide users with a tool to assist users understand the relationship between veneer visual grades, stress grades and performance of their final veneer products. The tool can assist those seeking to develop new veneer based composites with predictable strength properties for engineered applications. The software can give quick answers to questions such as what percentage of specific veneer can be used for making a target product, and what the optimum stress-grading thresholds are. It can be used to adjust and calibrate mill stress grading operations to meet the market requirements of final products. It can also serve as a management tool for mill managers to optimize products mix and keep track of mill production. Further, it can recommend appropriate adjustments of on-line production when veneer species, log source, log diameter and final veneer products change.
The key results from this research are as follows:
Veneer properties vary from species to species, stand to stand, and from mill to mill. They further vary with block positions and from sap to heart to core. According to this study, there exist two groups among the veneer species studied. One group is Douglas-fir, aspen and hemlock, which are suitable for making LVL and high strength plywood; the other is mixed spruce/lodgepole pine, which is suitable for making plywood or using it as inner layers for LVL manufacture.
There is little or no correlation between veneer visual grades and stress grades. Hence, it is not accurate to visually sort veneer on a strength basis. The stress grading operation is threshold-dependent, which differs from visual grading in both strength properties and percentages of grade volume. Compared to visual grading, stress grading can sort veneer into distinct strength groups with much smaller variation for quality assurance, and can extract more high-grade veneer for high value LVL manufacture. To maximize the value of veneer products, the best strategy is to extract the strongest veneer via stress grading to make market-demanding LVL and use the rest to make either low-grade LVL or plywood. It is also strongly recommended that veneer/plywood operations first perform stress grading to sort veneer, followed by veneer visual grading. By combining stress grading with visual grading, high-grade or high-value plywood can be produced, and veneer panels requiring high visual grade face veneer combined with strength can be manufactured.
A significant correlation exists between veneer MOE and LVL edgewise MOE and MOR for all the species tested. However, the correlation between LVL flatwise MOE and MOR, shear strength and veneer MOE is less or much less significant and differs from species to species, and from mill to mill. A calibration with experimental data is needed when trying to predict panel MOR and shear strength with veneer MOE. Good correlations between plywood MOE and MOR and average MOE of veneer layers parallel to the testing span were identified for all the species tested, which can set up a benchmark for predicting the strength properties of structural plywood panels for engineered applications using stress graded veneer.
Using VGrader 1.0 software, an optimum set of veneer stress grading thresholds can be established, which makes it possible for adjustment and calibration of mill on-line stress grading systems based on requirements of market-oriented veneer products. By periodically sampling veneer, mill operations can be diagnosed and optimized, and mill profits can be maximized.