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Veneer grading strategy for plywood / LVL products

https://library.fpinnovations.ca/en/permalink/fpipub37539
Author
Dai, Chunping
Wang, Brad J.
Date
March 2000
Material Type
Research report
Field
Wood Manufacturing & Digitalization
Author
Dai, Chunping
Wang, Brad J.
Contributor
Canada. Canadian Forest Service.
Date
March 2000
Material Type
Research report
Physical Description
41 p.
Sector
Wood Products
Field
Wood Manufacturing & Digitalization
Research Area
Advanced Wood Manufacturing
Subject
Veneer
Plywood
Laminate product
Grading
Automatic control
Series Number
Canadian Forest Service No. 30;1967
W-1665
Location
Vancouver, British Columbia
Language
English
Abstract
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.
Veneers
Grading - Automation
Plywood - Grading
Lumber, Laminated veneer
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