Characteristics of Wood Materials

1. Characteristics of Wood Materials Chapter 11

2. Polymer production sounds like something that comes from large chemical plants • May polymers used in manufacturing come from farms, forests and oceans • For thousands of years the most important natural polymer has been wood

3. Unique Characteristics of Wood • Used throughout history without much modification • With careful harvesting and reforestation programs wood has become a bountiful resource Clear cutting – felling of all trees in a chosen area at once • Still practiced, but not environmentally friendly • Too much impact on ecosystem Selection management • Individual trees are marked and cut in rows • Small clearings allow growth of new trees • Natural reseeding • Cover remains for wildlife

5. Structure of Woods Industry • 2 million employees • 7 major industries: • Forestry • Lumbering • Millwork • Furniture making • Construction • Wood processing • Distribution

6. Wood as a Manufacturing Material • Can be easily formed, shaped and smoothed • Used to make: • Furniture • Cabinets • Construction materials • Prototypes • Molds • Jigs • Fixtures

7. New practice, impregnating wood with synthetic polymers (Engineered woods) • Increase stiffness • Water repellency • Strength and stability • More durable than hardwoods • More attractive than plastics • Hundreds of species of wood used commercially • Varieties of: • Grain patterns • Tones • Surface textures • Strength

8. Classifications of Wood • Classified according to cellular structure • Hardwoods and softwoods • Classification doesn’t dictate hardness of wood • Balsa considered a hardwood • Based on cellular structure • Wood fibers are hollow (polymers are solid)

9. Deciduous trees • Broad-leafed hardwood tree species that typically shed their leaves each fall • Most hardwoods harvested east of the Mississippi Conifers • Cone bearing softwood tree species that have needles and remain green all year round • Most softwoods are harvested in the southern and western regions of the U.S.

10. Composition of Wood • Two major ingredients: • Cellulose (approx. 70%) • Lignin • Hardwoods have vessels that carry sap vertically through the tree • When cut, vessels appear as holes, called pores • Open-grained woods – hardwoods with large pores • Ash • Oak • Closed-grained woods – hardwoods with small pores • Birch • Cherry • Maple

11. Softwoods have long tubular cells called tracheids to transfer sap • Thicker walls than cells in hardwoods • Cells arranged in more orderly rows than in hardwoods • Softwoods are easily dissolved using certain chemicals • Process for paper making

13. Effects of moisture • Related to cellular structure • More opened the grain is, the more moisture absorbed • Woods can absorb water during changes in temperature and humidity • Expands when moisture is absorbed • Shrinks when moisture is lost • Can affect construction • Build something while wood is wet, as it dries it will shrink and have gaps

14. Checking Moisture Resistance type moisture meter • Pin type probes are pushed into the wood • 1%-2% accuracy • Many variables can influence results

15. Oven dried method • Commercially used • Accurately measures the entire range of moisture content • Resistance meters only measure 7%-30% • Wood samples are weighed, then dried at 215°F until dry • Sample is weighed again • Difference in weight = how much moisture was present

16. Drying Methods • Green wood – lumber that has just been cut • Retains as much as 20% of it’s moisture • If used at this state it will undergo drastic changes as it dries • Seasoning will reduce moisture as much as 20% • Will not stop farther drying and warping • Warpage is bending and twisting in all directions

17. 4 types of warp • Bow warp – occurs with the length of the board • Crook warp – board twists in an arc or partial circle • Twist warp – twisting of the board from one end to the other • Cup warp – across the width of the board

18. Air Drying • Final moisture content around 15% • Normally done to wood that will be used outdoors Kiln Drying • Final moisture content around 7% • Relieves more stress in the wood • Wood that is intended for indoor use Radio Frequency (RF) Dielectric Drying • Place wood between plates (electrodes) • Uses an alternating arc field • Causes wood molecules to move, friction causes heat

19. Wood Grades Softwoods • Select lumber – graded from A to D • A is best quality and surface appearance • Common lumber • Graded by numbers 1 to 4 • 1 is best grade, no knots or knotholes • Each lower grade has an increase in defects • Defects include knots, stains, splits, holes and warping

20. Hardwoods • FAS –firsts and seconds • Highest quality called firsts • Next highest called seconds • At least 8’ long and 6” wide • 83.33% clear of defects • Select hardwood boards • At least 6’ long and 4” wide • One side must be FAS quality • Number 1 common • 66.66% clear • Number 2 common • 50% clear • Number 3 common • 33.33% clear

21. Industrial Stock Dimensional Lumber • Standard length boards up to 1” think and 12” wide • Ready to use without any preparation Factory and Shop Lumber • Remanufacturing purposes • Mills that make fabricated doors, cabinets, moldings and trim • Number 4 softwood (poorest grade) Structural Lumber • Includes light framing material (2x4) and wider boards • Construction industry

22. Wood stock made from wood chips • Particle board • Waferboard • OSB (oriented strand board)