Characteristics of Wood MaterialsChapter 11

• 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

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

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

Wood as a Manufacturing Material

• Can be easily formed, shaped and smoothed• Used to make:• Furniture• Cabinets• Construction materials• Prototypes• Molds• Jigs• Fixtures

• 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

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)

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.

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

• 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

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

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

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

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

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

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

Wood GradesSoftwoods• 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

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

Industrial StockDimensional 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

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