A series of plywood and laminated veneer lumber (LVL) panels were prepared using veneers with higher than normal moisture contents in face and back layers. The purpose of the work was to evaluate the effects of self-generated steam on the pressing times and panel warpage. Panels made with 6% and 10% m.c. faces and backs were compared with control panels made with all dry veneer. Thirteen- ply 40 mm (1 5/8 inch) thick panels were evaluated for press times and thin 9.5 mm (3/8 inch) panels were evaluated for cupping and bowing. Normal plywood press temperatures and adhesives were used. All panels were made with incised 3.2 mm (1/8 inch) SPF veneers. The project demonstrated that substantially shorter press times and more dimensionally stable panels can potentially be made using higher moisture content outside veneers.
A series of plywood and laminated veneer lumber (LVL) panels were prepared using incised veneers in the second phase of this two year project. The primary purpose of the work was to evaluate the effects of steam injection on the pressing times. A secondary objective was to expand the study of warpage in three-ply and four-ply plywood which was begun in phase one. Thirteen-ply 40 mm (1 5/8 inch) thick panels were evaluated for press times and thin 9.5 mm (3/8 inch) and 12.5 mm (1/2 inch) panels were evaluated for cupping and bowing. Press temperatures of 150 degrees C, 175 degrees C and 204 degrees C were used with a commercial adhesive mix for the LVL study while normal plywood pressing conditions were used for the plywood. For the plywood warpage study, the effect of lathe check orientation and species mix were evaluated. The lathe check orientation had little effect while the surface veneer species had a pronounced effect on the warpage in the plywood. Steam used for injection was heated to 260 degrees C at 450 KPa (65 psi) with a super-heater. All panels were made with incised 3.2 mm (1/8 inch) SPF veneers. The project demonstrated that steam injection can shorten press times by fifty percent if incised veneers are used.
The project objective is to provide key data on the laminating properties of Canadian wood species to assist the secondary manufacturing industry to meet domestic and international customer expectations. This is a progress report to March 31, 1999.
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.