WOOD 474 Structural Panels. Plywood

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Sheena Farmer
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1 WOOD 474 Structural Panels Plywood 1

2 Oriented Strand Board Structural panels Plywood Produced from veneers glued together at right angles to maximise stability and strength. Oriented Strand Board (OSB) Made by gluing strands of wood together, to form a dense mat of fiber. 2

3 NA Panel Consumption Forest Products Society Forest Prod. J. 62(2): Trends Plywood production has trended down since the introduction of OSB. From 1985 to 2000 OSB grew 300%. In 2000 about 11 million m 3 produced in the USA and 8 million m 3 in Canada (OSB). In 2006 OSB was over 65% of structural board production. 3

4 What caused this growth? Plywood is produced from high quality veneer more expensive. Plywood is peeled from logs, which must be cut to specified length, shape and quality, and the efficiency is typically about 50-60%. What caused this growth? OSB can use lower value/quality fiber so it is cheaper to produce. OSB logs are cheaper (pulpwood variety) and recovery is about 85 to 90%. 4

5 Composites stages in production Wood element production Wood element drying Addition of adhesive Formation Hot-pressing Finishing Plywood production - overview (Walker) 5

6 Plywood constitution Crossband Grain direction Core Face Plywood constitution Face Cross-ply Core Cross-ply Back 6

7 Plywood production Logs are cut to length (rough 2.64 m (104 in)) allowing for final trimming. The bolts are normally heated (50 C) to soften the fiber. Heating produces higher yield, lower variation and higher grade. Peeling veneer Almost all veneer in structural products is peeled on a lathe Rotary lathes produce a continuous ribbon of veneer The pressure bar and knife assembly move together to peel the veneer 7

8 Plywood production Peeling veneer Lathe checks form during peeling. These lathe checks appear on the inside surface of veneer peeled from the bolt (the loose side) not on the surface from the outside of the bolt (the tight side). Lathe check Nose bar Knife 8

9 Veneer clipping and drying Veneer is clipped to width. Veneer is then dried. Most often drying is by using forced hot air (205C) for 8 to 10 min. Final MC is 6 to 8% for best gluing. Lay-up Labour intensive (but can be semiautomated) Application of adhesive spray or curtain coating Spray - low pressure nozzles apply adhesive Curtain coaters - box with a slit releases a curtain of adhesive onto veneer 9

10 Veneer curtain coater trough filter resin film veneer pump Roll Coating 10

11 Lay-up Pressing Cold pre-press prior to hot press Designed to tack veneer together Easier subsequent loading of hot press Use hydraulic heated multi-opening presses Produces 20 to 40 panels each cycle of 2 to 7 min at pressures of 750 to 1500 kpa 11

12 Pressing Plywood recovery factor 12

13 OSB production - overview OSB constitution Flakes in faces oriented roughly parallel to long axis of panel Flakes in core oriented either randomly or roughly perpendicular to long axis of panel 13

14 Waferboard vs. OSB 27 OSB production Long or short logs can be used Can use mix of species Strands are produced by a strander - disk, drum, ring 14

Strand production strands dimensions (approx):

75 mm length: 90 150mm width: 25 mm Drying of the

15 Strand production strands dimensions (approx): thickness: mm length: mm width: 25 mm Drying of the strands For good adhesion, MC must be 2 to 6% Rotary driers are most common. Hot air enters the dryer (600 C +) Evaporating moisture cools the air Exiting temperatures are lower (200 C) 15

often done separately Forming Process of depositing strands onto surface to form a mat is called forming To align the strands parallel to

16 Blending Addition of wax and resin adhesive, is called blending Wax 0.25 to 2% to add water repellency Resin 2 to 5 % by weight Resin applied by spray like spot welding Blending of surface and core furnish often done separately Forming Process of depositing strands onto surface to form a mat is called forming To align the strands parallel to the belt they drop through series of spinning discs onto belt below To align strands perpendicular to the belt they drop onto a series of rotating vanes (like paddle wheel) where they self align and land on the belt 16

17 Forming Common to use four forming heads First aligns strands 25% parallel Next two deposit the flakes 50% perpendicular Fourth deposits strands 25% parallel Final board thickness, dimension and density determined during forming To produce a 12.5 mm thick panel, loose mat is 100 mm thick Pressing In batch processed OSB loose strand mat is transferred to press charger Multi-opening presses used 3.66 x 12.2 m platen size and can produce sheets 2.74 x 7.62 m Platens conduct heat to mat (175 to 205 C) Press time for 3 to 6 minutes Use stored heat to ensure complete cure 17

18 OSB press Multi-daylight press Plywood or OSB? Both rated as structural panels in model building codes Same exposure durability rating Same span ratings Nail and staple performance the same 18

edges of OSB expand OSB does not work well in exposed uses OSB is denser (panels are

19 Plywood or OSB? Because of construction (cross banded veneers) plywood more stable Under moist conditions, edges of OSB expand OSB does not work well in exposed uses OSB is denser (panels are heavier) Plywood or OSB? OSB is stronger than plywood in shear, hence its use as webs in I-beams. 13 mm OSB costs less than comparable plywood, $700 on 2,500 ft 2 house 19

Structural Panels. Forestry 240 Wood Science. Plywood. Plywood Manufacturing. Rotary Lathe. Chargers. Chapter 14 - Structural Panels

Structural Panels. Forestry 240 Wood Science. Plywood. Plywood Manufacturing. Rotary Lathe. Chargers. Chapter 14 - Structural Panels Forestry 240 Wood Science Chapter 14 - Structural Panels Structural Panels Structural Panels have revolutionized the nature of light woodframe construction. Larger panel area (32 ft 2 ) facilitates faster