Knowledge of Plywood.
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Plywood:
Plywood is a sheet material manufactured from thin layers or "plies" of wood veneer that are glued together with adjacent layers having their wood grain rotated up to 90 degrees to one another. It is an engineered wood from the family of manufactured boards which includes medium-density fibreboard (MDF) and particle board (chipboard).
All plywood bind resin and wood fibre sheets (cellulose cells are long, strong and thin) to form a composite material. This alternation of the grain is called cross-graining and has several important benefits: it reduces the tendency of wood to split when nailed at the edges; it reduces expansion and shrinkage, providing improved dimensional stability; and it makes the strength of the panel consistent across all directions. There is usually an odd number of plies, so that the sheet is balanced—this reduces warping. Because plywood is bonded with grains running against one another and with an odd number of composite parts, it is very hard to bend it perpendicular to the grain direction of the surface ply.
Smaller thinner plywood and lower quality plywood may only have their plies (layers) arranged at right angles to each other, though some better quality plywood products will by design have five plies in steps of 45 degrees (0, 45, 90, 135, and 180 degrees), giving strength in multiple axes.

Production:
Plywood production requires a good log, called a peeler, which is generally straighter and larger in diameter than one required for processing into dimensioned lumber by a sawmill. The log is laid horizontally and rotated about its long axis while a long blade is pressed into it, causing a thin layer of wood to peel off (much as a continuous sheet of paper from a roll). An adjustable nosebar, which may be solid or a roller, is pressed against the log during rotation, to create a "gap" for veneer to pass through between the knife and the nosebar. The nosebar partly compresses the wood as it is peeled; it controls vibration of the peeling knife; and assists in keeping the veneer being peeled to an accurate thickness. In this way the log is peeled into sheets of veneer, which are then cut to the desired oversize dimensions, to allow it to shrink (depending on wood species) when dried. The sheets are then patched, graded, glued together and then baked in a press at a temperature of at least 140 °C (284 °F), and at a pressure of up to 1.9 MPa (280 psi) (but more commonly 200 psi) to form the plywood panel. The panel can then be patched, have minor surface defects such as splits or small knot holes filled, re-sized, sanded or otherwise refinished, depending on the market for which it is intended.
Plywood for indoor use generally uses the less expensive urea-formaldehyde glue, which has limited water resistance, while outdoor and marine-grade plywood are designed to withstand moisture, and use a water resistant phenol-formaldehyde glue to prevent delamination and to retain strength in high humidity.
Anti-fugal additives such as Xyligen (Furmecyclox)may sometimes be added to the glueline to provide added resistance to fungal attack.
The adhesives used in plywood have become a point of concern. Both urea formaldehyde and phenol formaldehyde are carcinogenic in very high concentrations. As a result, many manufacturers are turning to low formaldehyde-emitting glue systems, denoted by an "E" rating ("E0" possessing the lowest formaldehyde emissions). Plywood produced to "E0" has effectively zero formaldehyde emissions.

Sizes:
The most commonly used size for a sheet of plywood is 4x8ft(also know as 1,220×2,440 mm),which was first used by the Portland Manufacturing Company, who developed what we know of as modern veneer core plywood for the 1905 Portland World Fair. And the most common thickness range is from 1/8” to 1” (commonly as 1/8”,1/4”, 3/8”, 1/2 ”, 5/8”, 3/4”, 7/8”, 1”). Of course, the exact size and thickness can be available from the suppliers according to the buyers’ requirements.

Grades:
Grading rules differ according to the country of origin. Most popular standard is the British Standard (BS) and American Standard (ASTM). Joyce (1970), however, list some general indication of grading rules:

Grade Description
A Face and back veneers practically free from all defects.
A/B Face veneers practically free from all defects. Reverse veneers with only a few small knots or discolorations.
A/BB Face as A but reverse side permitting jointed veneers, large knots, plugs, etc.
B Both side veneers with only a few small knots or discolorations.
B/BB Face veneers with only a few small knots or discolorations. Reverse side permitting jointed veneers, large knots, plugs, etc.
BB Both sides permitting jointed veneers, large knots, plugs, etc.
C/D For structural plywood, this grade means that the face has knots and defects filled in and the reverse may have some that are not filled. Neither face is an appearance grade, nor are they sanded smooth. This grade is often used for sheathing the surfaces of a building prior to being covered with another product like flooring, siding, concrete, or roofing materials.
WG Guaranteed well glued only. All broken knots plugged.
X Knots, knotholes, cracks, and all other defects permitted.
WBP Weather and Boil Proof used in Marine Ply. Designation replaced by EN 314-3.
JPIC Standards
Grade Description
BB/CC Face as BB, back as CC. BB as very little knots of less than 1/4inches, slight discoloration, no decay, split and wormholes mended skillfully, matched colors, no blister, no wrinkle. Most popular choice for most applications.

Applications:
Plywood is used in many applications that need high-quality, high-strength sheet material. Quality in this context means resistance to cracking, breaking, shrinkage, twisting and warping.
Exterior glued plywood is suitable for outdoor use, but because moisture affects the strength of wood, optimal performance is achieved in end uses where the wood's moisture content remains relatively low. On the other hand, subzero conditions don't affect plywood's dimensional or strength properties, which makes some special applications possible.
Plywood is also used as an engineering material for stressed-skin applications. It has been used for marine and aviation applications since WWII.
Plywood is often used to create curved surfaces because it can easily bend with the grain. Skateboard ramps often utilize plywood as the top smooth surface over bent curves to create transition that can simulate the shapes of ocean waves.
Softwood plywood and Hardwood plywood is different from density and weight, and has their special applications-

Softwood plywood applications:

Typical end uses of spruce plywood are:
· Floors, walls and roofs in home constructions
· Wind bracing panels
· Vehicle internal body work
· Packages and boxes
· Fencing
There are coating solutions available that mask the prominent grain structure of spruce plywood. For these coated plywoods there are some end uses where reasonable strength is needed but the lightness of spruce is a benefit e.g.:
· Concrete shuttering panels
· Ready-to-paint surfaces for constructions

Hardwood plywood applications:

· Panels in concrete form work systems
· Floors, walls and roofs in transport vehicles
· Container floors
· Floors subjected to heavy wear in various buildings and factories
· Scaffolding materials

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