Engineered Wood Products: Plywood, LVL, PSL, LSL, OSL

Plywood

• A panel product constructed primarily of veneer sheets called plies.
• Consists of an odd number of layers with the grain direction of adjacent layers oriented perpendicular to one another.
• A layer can consist of a single ply or multiple plies laminated with parallel grain direction.

Plywood Components

• Face Veneer: The outer layer of the plywood.
• Crossbands: Layers with grain direction perpendicular to the face veneer.
• Veneer Core: The central layer made of veneer.
• Lumber Core: The central layer made of lumber.
• Back Veneer: The back layer of the plywood.

Plywood Manufacturing Process

• Forest: Logs are sourced from the forest.
• Log Yard: Logs are stored and sorted.
• Log De-barker: Bark is removed from the logs.
• Logs cut into peeler blocks: Logs are cut to proper sizes.
• Veneer Peeling: Thin sheets of veneer are peeled from the rotating log.
• Veneer Clipping: Veneer sheets are cut to the required size and defects are removed
• Veneer Drying: Veneer is dried to reduce moisture content.
• Glue Spreading: Adhesive is applied to the veneer sheets.
• Cold Pressing: Veneer layers are pre-compressed.
• Hot Pressing/Curing: Veneer layers are bonded under heat and pressure.
• Trimming: Plywood panels are trimmed to final dimensions.
• Sanding: The surface is sanded to achieve a smooth finish.
• Grading and Packing: Plywood is graded based on quality and packed for shipping.

Log Sorting

• Logs are sorted by dimension and species in the log yard.
• Heavy lift trucks, derricks, or cranes handle logs.

Log Conditioning

• Most timbers require conditioning before peeling.
• Conditioning softens the wood to facilitate peeling and improve veneer quality.
• Peeler blocks are exposed to heat and moisture through soaking in hot water vats, live steam, or hot water sprays.

Peeling, Reeling, and Clipping

• Plywood veneer is typically rotary cut.
• The peeler block rotates around its axis in a lathe, and a knife cuts a continuous veneer sheet.
• The veneer sheet is wound on spools or led to a multi-tray system for storage and surge capacity.
• Storage systems are synchronized with the lathe speed.
• Green veneer is clipped to size, graded, and stored in piles.
• Defects like knots and splits are removed.

Veneer Drying

• Veneer is dried to a moisture content of 2-10% to aid gluing.
• Methods include air drying or kiln-drying.
• Kiln-drying involves batch drying of stacked veneer or continuous drying on belts or rollers.
• Veneer drying consumes approximately 70% of the thermal energy in plywood production and about 60% of the mill's total energy.
• Dryer heating can be indirect (steam or thermic oil) or direct (fresh-air regulation).
• Normal drying temperatures range from $90-160^\circ C$, with some species dried at $175^\circ C$ to reduce drying time.

Veneer Coating (Glue Application)

• Plywood assembly involves jointing narrow veneer strips and edge-gluing them to form the required sheet size.
• Glue is applied to the inner plies or core, which is then placed between the outer veneers for bonding.

Curing/Hot Press

• Veneer sandwiches are subjected to heat and pressure in a hot press until the glue is cured.

Structural Composite Lumber (SCL)

• Family of engineered wood products made by layering dried and graded wood veneers, strands, or flakes with moisture-resistant adhesive into billets.
• Billets are then resawn into specified sizes.
• In SCL billets, the grain of each veneer or flake layer runs primarily in the same direction.
• SCL products outperform conventional lumber in face or edge loading.
• SCL is solid, predictable, uniform, and virtually free from warping and splitting.

Types of SCL

1. Laminated Veneer Lumber (LVL)
2. Parallel Strand Lumber (PSL)
3. Laminated Strand Lumber (LSL)
4. Oriented Strand Lumber (OSL)

1. Laminated Veneer Lumber (LVL)

• Widely used structural composite lumber product.
• Produced by bonding thin wood veneers together in a large billet, with all veneer grains parallel to the long direction.
• The LVL billet is sawn to desired dimensions depending on the end-use application.
• LVL is available in lengths far beyond conventional lumber lengths due to scarfed or lapped jointed veneers.
• Common applications include headers and beams, hip and valley rafters, scaffold planking, and flange material for prefabricated wood I-joists.

Laminated Veneer Lumber vs Plywood

Laminated Veneer Lumber (Historical Context)

• 1940s: LVL targeted the production of high-strength parts for aircraft structures using Sitka spruce veneer.
• 1970s: Research on LVL used veneer up to $12.7$ mm ($1/2$ in.) thick.
• Since the 1990s: Production of LVL uses veneers $3.2$ to $2.5$ mm ($1/8$ to $1/10$ in.) thick, hot-pressed with phenol formaldehyde adhesive.
• Lengths range from $2.4$ to $18.3$ m ($8$ to $60$ ft) or more.
• Today: Commonly used as flanges in composite I-joists.

Laminated Veneer Lumber - End Joints

• End joints may be butt joints, or veneer ends may overlap for load transfer.
• Some producers provide structural end joints using scarf or finger joints.

Laminated Veneer Lumber - Dimensions

• Sheets are commonly produced in $0.6$ to $1.2$ m ($2$ to $4$ ft) widths and a thickness of $38$ mm ($1.5$ in.).
• Continuous presses can be used to form potentially endless sheets, which are cut to the desired length.

2. Parallel Strand Lumber (PSL)

• Manufactured from veneers clipped into long strands laid in parallel formation.
• Strands are bonded together with adhesive to form the finished structural section.
• The length-to-thickness ratio of strands is around $300$.
• Used for beam and header applications requiring high bending strength.
• Frequently used as load-bearing columns.

Parallel Strand Lumber - Definition

• Defined as a composite of wood strand elements with wood fibers oriented primarily along the length of the member.
• The least dimension of the strands must not exceed $6.4$ mm ($0.25$ in.).
• The average length of the strands must be a minimum of $150$ times the least dimension.

Parallel Strand Lumber - Proprietary Product

• PSL is a proprietary product, sold as Parallam®.
• Often used for large beams and columns, typically as a replacement of solid-sawn lumber or glulam.

Parallel Strand Lumber - Manufacturing

• Manufactured using veneer about $3$ mm ($1/8$ in.) thick, clipped into strands about $19$ mm ($3/4$ in.) wide.
• Strands are commonly at least $0.6$ m ($24$ in.) long.
• The process utilizes material from roundup of the log in the veneer cutting operation and other less-than-full-width veneer.
• Thus, the process can utilize waste material from a plywood or LVL operation.

Parallel Strand Lumber - Species

• Species commonly used include Douglas-fir, southern pines, western hemlock, and yellow-poplar.
• There are no restrictions on using other species.

3. Laminated Strand Lumber (LSL)

• Similar to PSL, LSL is made from flaked wood strands with a length-to-thickness ratio of approximately $150$.
• Strands are combined with an adhesive, oriented, formed into a large mat or billet, and pressed.
• LSL is used in various applications from studs to millwork components.

4. Oriented Strand Lumber (OSL)

• Similar to LSL, OSL is made from flaked wood strands with a length-to-thickness ratio of approximately $75$.
• The wood strands used in OSL are shorter than those in LSL.
• Combined with an adhesive, the strands are oriented and formed into a large mat or billet and pressed.
• OSL resembles oriented strand board (OSB) in appearance.
• Unlike OSB, the strands in OSL are arranged parallel to the longitudinal axis of the member.
• OSL offers predictable strength and stiffness properties and dimensional stability that minimize twist and shrinkage.
• Natural defects such as knots, slope of grain, and splits have been dispersed throughout the material or have been removed altogether.