Wood-General

Wood-General - Wood Forest Resources of the United States...

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Unformatted text preview: Wood Forest Resources of the United States 33% of the US lands are classified as forest lands. http://nationalatlas.gov/articles/biology/a_forest.html Global Forest Resources Global Industrial Roundwood Production, 2001 Percentage of Forest Area Globally by Region, 2003 Forest Resources in the U.S. All species Hardwood Softwood What a waste! .... ? North American Hard Wood Lumber Production and Estimated Uses (Billion Board Feet) 1998 N. America Prod. Utilization by Sector Furniture Cabinets Dim./Millwork Flooring Pallets/Crating Exports Misc. 1999 2000 2001 2002 2003 2004 (est.) 14.00 14.25 14.00 11.50 11.20 10.32 11.35 13.54 13.43 14.32 11.73 11.22 10.74 10.99 3.40 0.56 0.60 0.58 4.40 0.95 0.75 3.40 0.56 0.60 0.62 4.20 1.40 1.10 0.75 3.45 0.55 0.86 0.49 4.93 1.43 1.20 0.72 1.80 0.58 0.85 0.83 3.73 1.40 1.10 0.65 1.70 0.61 0.70 0.84 3.30 1.40 1.17 0.65 1.56 0.64 0.67 0.85 2.99 1.40 1.16 0.60 1.40 0.74 0.70 0.89 3.08 1.50 1.20 0.60 Lumber/Dist. Yards 1.40 Softwood and Hardwood Terms refer to the water-conducting cells in a living tree, NOT the hardness of the wood !ymnosperms (cone producing plants with seeds borne on open cones) - plants with needle like leaves called conifers, are softwoods Angiosperms (seeds produced within a closed pod) flowering or broadleaf plants, are hardwoods Common Hardwoods and softwoods Softwood Pine, Douglas Fir ,hemlock, cedar*, redwood*, spruce ,cypress* Hardwood Red oak, white oak*, birch, poplar, ash, cherry*, maple, walnut*, mahogany, elm Pine wood *Naturally resistant to decay Cedar - fence Redwood - greenhouse White oak - wine barrel Most Common Hardwoods Poplar Cherry White ash Red Oak White Oak Teak Most Common Hardwoods Birch Maple Walnut, American Mahogany Most Common Softwoods Redwood White Pine White pine Yellow pine Cedar Douglas fir Wood Structure Wood Structure Xylem tissue Quarter sawn Flat sawn Radial Tangential Longitudinal Phloem tissue Close, Non-porous vs. Open, Porous Grains Softwood also called close grain or non-porous grain Hardwood also called open grain or porous grain 1esin canal Cambium & Phloem Tissues Cambium is the only place where new growth takes place; produces phloem and xylem Phloem is a food conducting tissue which transport the sugars and other materials made in the leaves to all other living cells; i.e., unlike vessels and tracheids, phloem cells move food both upward and downward Xylem tissue Phloem tissue Wood Cells in Xylem Tissues Xylem is the vascular tissue thru which most of the water and minerals of the tress are conducted Softwood cells Tracheids (longitudinal), rays (radial) Tracheids play dual roles: transport sap upwards and also provide structure strength Rays are storage sites for carbohydrates Vessels (long), fibers (long), and rays (radial) Only vessels conduct sap, in upward direction Fibers provide the strength for the xylem tissues Hardwood cells Tracheids (softwood fiber) Radial Section Pits Tangential Section Softwood fiber (tracheid) Vessels (hardwood pores) Hardwood fiber Hardwood Birch Ash Red Oak Softwood Red pine Southern yellow pine Sugar pine Rays Longitudinal Section Radial Section (Quarter sawn) Rays (White pine) Tangential Section (Flat sawn) Typical Cell Wall Structure of a Fiber or Tracheid P-primary wall S1, S2, S3-layers of the secondary wall ML-middle lamella, the amorphous, high-lignin-content material that binds cells together Structural Components of Cell Wall The primary cell wall of green plants is made of cellulose; the secondary wall contains cellulose with variable amounts of lignin. Cellulose is the major constituent of paper and textiles made of cotton, linen and other plant fibers. Rayon is a very important fiber made out of cellulose and has been used for textiles since the beginning of the 20th century. During growth, cellulose molecules are arranged into ordered strands called fibrils. Fibrils organized into the larger structural elements that make up the cell wall of wood fibers. Cellulose ! 50% (dry weight), crystalline Lignin 23-33% in softwood, 16-25% in hardwood; Polymeric cementing agent which binds cellulose together Hemicellulose 23-32%, amorphous Present in cell walls along with cellulose and helps binds the cellulose fibers Has much smaller molecular size compare to cellulose =y elements: Carbon 49%, Oxygen 44%, Hydrogen 6%, others 1% Structure of Cellulose Fiber Cellulose Fibers Macrofibril Microfibril Chain of cellulose molecules Structure of Cellulose Fiber Some Interesting Facts Lignin has been used for making vanillin, artificial vanilla flavoring Amber fossil tree resin Tree resin A cake of amber violin bow rosin Mechanical Properties of Wood Wood is an orthotropic (or anisotropic) material Unique and independent mechanical properties in three mutually perpendicular directions Fiber or longitudinal direction has the strongest properties Modulus of rupture (stress at fracture) obtained from the bending test is an accepted criterion of strength How Lumber Is Sawn? Plain (Flat) Sawn Technique (Top Half) Quarter sawing produces both Rift and Quartered (Bottom Half) The arrangement of the annual growth rings will determine what face of a board will look like. Flat (Plain) Sawn Quarter Sawn Rift Sawn Plan or Flat Sawn Less waste but less stable Easier to kiln dry More shrinkage in width Less expensive Wider widths Quartered or Rift Sawn Most waste and therefore most expensive Most stable but narrow widths Shrinks more in thickness than width More difficult to kiln dry Reduces twisting, warping and cupping Logs Sawmill Operations Bucking Debarking Primary Breakdown Headrig Gang Sawing Trimming Grade Sorting Drying Surfacing Grading Final Sorting Edging Historic image of Logging, Lower Columbia, Oregon, 1905 Delivery @. =uckingC Logs entering the mill are bucked (cut) into sawlog length logs. F. Primary =reakdown HeadrigC Prepared logs are fed into the headrig. This is a computerized saw which makes a number of measurements using laser scanners to determine the logs size and shape. Then the computer decides the best location to place the saw to maximize the yield. The log is moved into the proper position, and the log is cut. Placing the headrig cut in the right place is very important: a sawline misplaced by only one-tenth of an inch can result in a loss of 25% or more of a log's potential yield. This kind of inefficiency in today's high-priced log market would be disastrous for any mill. D. EebarkingC The main reason for removing bark is to produce bark-free slabs that can be chipped for use in making pulp. Removing bark also reduces saw blade dulling. The bark itself can be used for fuel, or landscaping. (3 cont'd) The primary breakdown machine uses either two saw blades (twin bandmills) or four-saw (quad bandmills) depending on the size of the log. The most common pattern for cutting is illustrated here. H. !ang SawingC The center cant is transported out of the headrig, rotated flat and sawn on a rotary gang edger. This saw makes a series of cuts and can create several boards in one pass. Because many small logs have sweep (lengthwise bend) in them, modern mill technology uses curved sawing, in which the cant is cut into curved sawn boards. This increases yield, and the curves actually flatten out during the drying process! J. EdgingC Meanwhile the side boards (called flitches) on either side of the center cant are sawn in a board edger. These computerized edgers scan the flitches to determine how to cut for the best yield. L. TrimmingC Trimming is one of the final steps in creating lumber. The computerized trimming process removes defects from boards and cuts them into salable lengths. It uses multiple saw blades which can move in and out of position. First, boards are scanned for shape, and sometimes grade, using laser scanners. The computer program that controls the trimmer uses this information to optimize the value of each board. M. !rade SortingC Grade Sorting used to be done on the 'green chain', where people manually pulled lumber into sorted piles. But most lumber manufacturers now use an automated sorter which scans the lumber, and diverts different grades of lumber into different bins. N. EryingC Graded lumber is then dried in a steam-heated kiln. Lumber is stacked in layers separated by spacer sticks (stickers). Computers are used to monitor wood moisture and air temperature and control the drying process. Different species of lumber require different drying schedules. Highgrade Western hemlock may require over a week to dry, while Southern pine can dry in about 20 hours. O. SurfacingC After drying (or after sorting if its Douglas-fir), the lumber is sent through the planing mill, where it is surfaced on one to four sides, graded, trimmed and sorted. @R. !radingC Here, graders mark each board with a grade mark. @@. Pinal SortingC A grade-mark reader then checks if any additional trimming will upgrade the board. The board is stamped with a grade mark, and then sent on to the automated sorter be sorted by length. @D. EeliveryC Finally, the finished lumber is banded, and prepared for delivery. Moisture Content (MC%) in Wood Moisture content affects the structural properties of wood including weight, dimension, strength, and others. In the lab, a specimen of wood is weighed then placed in an oven set at @RR"C for HN hours (as per ASTM Standard D2395-93). The moisture content is calculated by the following equation: Moisture Content (MC%) Weight of moist wood - Weight of oven dry wood # x100% Weight of oven dry wood Moisture content may range from 0% (oven dry wood) to greater than 200% (a living tree). Wood Seasoning 1emoving moisture from freshly cut wood is referred to as seasoning air drying, kiln drying It is the most important steps in converting raw wood into finished products Imparts dimensional stability Increases most strength properties Increases fastener holding power and thereby joint strength Increases electrical resistance Improves paintability and glueability Improves the thermal properties Biological degradation (fungous attack) is prevented if dried below fiber saturation point Fiber Saturation Point (FSP) Moisture in wood exists in two forms: Free water, liquid filling the wood cell cavities Bound water, liquid or vapor chemically bound by hydrogen bonding to the cellulose of the wood cell walls FSP: The moisture content at which all of the free water is removed - the cell cavities are empty - but the cell walls are still completely saturated. MC% above FSP: physical and mechanical properties of wood remain constant as MC% changes MC% below FSP: physical and mechanical properties of wood change as MC% changes Species Ash, white Birch, yellow Douglas fir Hemlock, western Pine, loblolly Pine, longleaf Pine, red Spruce, red FSP(%) 24.0 27.0 26.0 28.0 21.0 25.5 24.0 27.0 Shrinkage of Wood Wood changes dimensionally as it gains and/or loses moisture at levels below the FSP =elow the PSP, all moisture is bonded to the cell walls, which act like sponges, swelling when water is added and shrinking as they dry. The anisotropic nature of wood causes it to shrink at different rates along each of its three principal axes. Tangential: greatest amount of shrinkage occurs in the direction of the annual growth rings Radial: about half of the tangential Longitudinal: very slight. Engineered wood panels Particle board Plywood OSB before pressing (Oriented Strand Board) OSB (Oriented Strand Board) Strand Cutting Engineered Wood I-Joist APA Specifications APA: Engineered Wood association (old name: American Plywood Association) Plywood Manufacturing Process (Example 1) Step 1 : Log Processing The log emerges from the barker after having been stripped of its bark. Step 2 : Log Conditioning The logs are conditioned using steam or hot water to improve peel quality. Step 3 : Lathing At the lathe, a sharp blade peels the log, now called a block, into a continuous sheet of veneer. Step 6 : Curing The veneer sandwiches are subjected to heat and pressure in the hot press until the glue is cured. Step 4 : Veneer Application Green veneer is dried in steam or gas heated ovens. Step 5 : Veneer Coating Veneers are coated with waterproof glue and laid up into sandwiches. Step 7 : Grade Selection After pressing, the plywood panels are trimmed, squared and selected for grade. Step 8 : Inspection Finished plywood panels are carefully inspected and graded. Step 9 : Quality Control Quality Control inspectors check finished plywood panels in inventory. Step 10 : Sample Testing Plywood samples, selected at random, are tested in laboratory. Step 11 : Boiling Tests Other samples are subjected to the boiling test specified by the CSA. Step 12 : Pressure Tests Some samples are subjected to vacuumpressure tests. Step 13 : Shear Tests Samples are shear tested to determine the strength of the glue bond. Step 14 : Test Results are Compiled and Reports are Circulated Lay-up of Plywood (cross-lamination) Superior dimensional stability Exceptional two-way strength Highly resistant to damage from impact Plywood Manufacturing Process (Example 2) 1. Debarked maple logs ready for lathe. 4. Remainder of log after turning is complete. 2. Log mounted on lathe and being turned. 3. Veneer being peeled from log and coming off the lathe. 6. Applying glue and stacking veneer for pressing. 5. Dried and cut to size veneer ready for further processing. 7. Finished product to be used as die boards for cutting paper. ...
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This note was uploaded on 04/06/2008 for the course ENTC 206 taught by Professor Fang during the Spring '08 term at Texas A&M.

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Wood-General - Wood Forest Resources of the United States...

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