Plywood is among the strongest of all building materials. The strength is a result of individual grain layers placed perpendicular to each other bonded with glue under high pressure in the manufacturing process. This makes plywood virtually split-proof. Other properties that can add strength to plywood are type of glue, manufacturing process ,core veneer quality levels ,veneer core structure or core construction ,species and thickness.
The strongest of all plywood is used for concrete forms, stair risers and industrial flooring,Marine Container Flooring PLYWOOD . Mostly widely used plywood types include 3/4 inch (18mm +/-0.5mm ) , which is one of the most widely used of all plywood thicknesses to the world markets . A piece of 4’x8’x18mm Plywood can support up to 26kgs before bending ,can suport up to 145kgs (about two adult weight )without breaking problems. Plywood strength drops when thickness diminishes. For example; a piece of 5.2mm plywood will only support about 2.268kg. before bending.
Simple testing ,During the whole piece of plywood 1220x2440mmx18mm Inspection ,Let the board vacated, tilt angle of about 45 degrees, standing on the plywood board, gently jump and fibrillation, check when started to bend ,when started to break ….Nail in the edge (also the face/back surface) of the plywood,check if any delaminated/split .
Wood species ,Two types of plywood that are commonly used in the building industry are manufactured using two distinct species of wood. The conifers, or softwood plywoods, generally lack the strength of hardwood species. But even though hardwood plywood is much stronger than softwood plywood, manufacturers use exterior glue on some varieties of softwood plywood. This makes it much stronger over time than hardwood plywood since it will not absorb moisture that causes plywood layers to separate and weaken. But when testing equal sizes of hardwood against softwood that have both been manufactured using interior glue, hardwood plywood is significantly stronger.
In China,most of are local chinese poplar wood species and eucalyptus ,Eucalyptus is much stronger than poplar plywood ,if use the same types of glue . Different glue require different moisture content and different manufacturing process ,so the final strength can be different .
Flexibility is an overlooked asset of plywood. For exterior uses such as sheathing typically placed over two-by-four studs on the exterior of a home, flexibility is a definite advantage. Exterior grade 3/4- or even 5/8-inch fir plywood has enough flexibility to resist cracking when applied to uneven frame structures when they are being built, or during settling of the structure. This flexibility — also known as shear strength — adds incredible strength to frame structures. Before a frame structure has plywood nailed to it, it is very fragile and can be knocked down easily by a strong wind. But when fir plywood is added as sheathing, the shear strength of the structure improves dramatically, making the structure strong. The shear strength of fir plywood is unparalleled by any other building material.
Plywood is layered. The number of layers in plywood relates directly to strength. Individual layers can be counted by looking at the edge of any piece of plywood. If there are fewer than four layers in typical 3/4-inch-thick plywood, the plywood has the minimum amount of strength. This is often referred to as shop-grade plywood, the most affordable plywood. It will bend or break easier than almost any other type of plywood. If the plywood has between four and seven layers, the plywood has a moderate amount of strength and can be used for cabinets or almost any other project. If the plywood has more than seven layers, it is the strongest and can be used for special projects such as guitar bodies or router patterns.
Why Is Plywood So Strong?. The most commonly used material in woodworking and construction is plywood. The strength and durability of plywood stems from certain species of trees such as birch, pine or redwood oak. A finished plywood sheet typically is 4 by 8 feet. The numbers of layers determine the finish and thickness of plywood, ranging from 1/8 inch to 1 1/8 inches.
Plywood is strong because of the raw materials used and the machinery involved to create plywood panels. Strengthening plywood starts with the manufacturing procedure of slicing wood from a flitch or log to create thin rectangular sheets or layers called veneers. A lathe or slicer that cuts veneer is similar to a vegetable slicer that cuts thin slices of potatoes. The lathe preform a continual motion, cutting one sheet in one piece, which gives each sheet a different grain structure. This method contributes to the strength and reliability of plywood.
The layers and balance of the cross-laminate means plywood has a high-impact resistance against splitting, crumbling, chipping or cracking. The plies are cross-laminated at 90-degree angles, are parallel to each other, and lie against the grain. This system gives an even strength across the width and the grain of the plywood panel. Front and back veneers are added to the cross-laminate sheets,which then are glued and pressed together to form one panel of plywood. The minimum amount of plies is three and is used only for thin sheets. Generally, the more plies, the stronger and more stable the sheet is, according to Pro Woodworking Tips. For example, odd numbers of five or seven plies are used to make even stronger sheets of plywood.
Softwood plywood is made with waterproof glue called phenol-formaldehyde for exterior sheets that are installed for exposed areas or application, such as house framing. Phenol-formaldehyde resin is an adhesive used to bring strength and moisture resistance to finished plywood panels.
Hardwood plywood is used for interior installation. The layers are glued together with a strong adhesive called urea-formaldehyde resin. This resin withstands an enormous amount of moisture; this gives the plywood a sturdy balance. Hardwood plywood is used in construction of furniture, flooring and cabinets. Interior plywood made with IMG (intermediate glue) is resistant to bacteria, mold and moisture, but no interior plywood is made for outdoor use.
By Abhijit Phadke
Plywood is a strong material, and to understand why, we need to take a look at the way it is manufactured.
It is an engineered wood product, made by sticking together several thin layers of wood over each other. These individual layers of wood (plies) are called veneers, and they are produced from timber logs by using ‘rotary cut’ or ‘slice cut’ methods. The rotary cut method which is more common involves rotating the wood log on its longitudinal axis using machines, and peeling layers from the log wood using cutting tools.
Every sheet of plywood has at least a minimum of 3 such wood veneers glued together.
What makes plywood so strong is the way in which each of these layers are arranged over one another. Each individual layer (veneer) has a grain pattern, such that it is easy to tear the veneer along the grain but not across it. This peculiarity is used to maximum advantage while making a plywood sheet. The layers are arranged over one another in such a way that the grain patterns alternate and are always perpendicular to each other.
The image shown below should help in making this adequately clear.
A piece of plywood that was damaged during the Mumbai floods in the year 2005. Notice the individual layers of ply that are seen coming apart. Also notice the tear lines seen on the individual sheets, and how these tear lines (grain patterns) alternate and are perpendicular to each other in each successive layer of the plywood.
Quality of wood used
The next most crucial aspect that affects plywood strength is the type of wood used in its making. Hardwood (for example teak wood or gurjan wood) is better in quality and much stronger than softwood (for example pine wood or mango wood).
So plywood sheets that are made using good quality hardwood are stronger than those made using softwood.
Thickness of the sheet
Plywood is manufactured in a wide range of sizes and thickness. The general rule of thumb is that the thicker the sheet is, the stronger it will be. Needless to say the thicker sheets also cost more.
Quality checks for strengths
The Indian standards for plywood (IS:303 for general use plywood) lists specifications of several types of tests that should be carried out on ready plywood sheets to check their toughness, and durability.
Some of the quality tests carried out are mentioned below.
|Moisture content (percentage)||The moisture content of plywood can affect its strength. If the content is too low, the plywood sheet will be too dry and hence will be weak. On the other hand if the moisture content is very high the board will be susceptible to rotting and decay. Should be between 5% to 15%|
|Glue shear strength||Measures the tenacity with which the glue or bonding material holds the individual layers/plies together.|
|Mycological test||To check the resistance against micro-organisms such as fungi.|
|Water resistance test||Checks resistance to water. For BWR grades, samples are submerged in boiling water for upto 72 hours.|
|Modulus of elasticity||Is measured across the face grain as well as along the face grain|
|Modulus of rupture||Is measured along and across the grain|
|Screw and Nail Holding strength||Ability of the plywood to hold on to nails and screws|
Several other tests on the plywood are carried out, such as measuring specific gravity, tensile and comprehensive strength, water absorption etc.
Comparison with other wood products
Compared to Particle Board, LDF and MDF
Plywood is much stronger than other engineered woods such as Particle Board (made from wood chips/flakes) or LDF and MDF (Low and Medium Density Fiberboard, which is made from wood fibers).
If you have ever closely observed Particle board or MDF furniture, you must have noticed that instead of nails, the fittings are all done using screws, and this is because the nail holding strength of such woods is very low. Plywood is a tougher material and plywood furniture is likely to last many years longer than Particle board or MDF furniture.
Compared to Block boards
Good quality hardwood plywood is also stronger when compared to block boards or flush doors. (Block boards have a core made from blocks of softwood and plies are glued on either side. Solid core flush doors are also very similar).
However in certain cases such as in the case of long bookshelves or any application where pieces longer than 6 or 7 feet have to be used, block boards are preferred over plywood. This is because plywood has a tendency to bend or sag in the middle, when long pieces of it are used.
Compared to solid wood
The general consensus amongst carpenters is that solid wood is better because it is a homogeneous material and hence unlike plywood there is no question of layers coming apart. So solid wood is generally considered tougher than plywood, however it largely depends on the quality of the wood and the intended use.
Another advantage of solid natural woods over plywood is that its more natural and contains no chemical resins (adhesives). Solid wood (obtained from good quality hardwood such as Indian teak wood – called Saagwan) is usually preferred over plywood although in its pure form it is much costlier.
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