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Why Does Plywood Have Odd Number Layers?

Buying plywood is akin to going into a candy store. Hardwood, softwood, birch, oak and pine are just a few types. The core of plywood can be veneer, particleboard or medium-density fiberboard. We can easily find out why different woods are used. It's a little harder to figure out why they always use odd-numbered layers. In order to understand the odd number of layers we need to understand its history.

  1. Veneer

    • Veneer is a thin layer of decorative wood glued onto the suface.

      Bonding two pieces of wood together was first accomplished around 1500 B.C. Egyptians applied a thin layer of dark ebony wood over a casket made of cedar. In the 1600s, furniture was decorated with thin pieces of wood in a process called veneering.

      In 1890 doors were made by applying a decorative veneer to both sides of doors. It was not long before the idea of cheaper wood covered with decorative wood took off and plywood became standard. “(See References 1, Background)”

    Manufacturing Process

    • Harvested trees are put on a large lathe once the bark is removed and the trunk is heated to make the wood pliable. A large blade is placed against the length of the trunk as it is turned. This cuts a continual sheet of wood, which is later cut down to size. This allows more of the tree's wood to be used for large panels of wood. Gluing layers together makes the sheet thicker.

    Cost

    • Cost effective use of lumber for plywood uses less of the forest.

      Cutting 4-by-8 sheets of oak wood 1/2-inch thick is not possible from a tree trunk. Making plywood using cheaper types of wood and applying a veneer of oak on the front and back of the plywood gives the appearance of a single wide sheet of oak. Fewer trees are needed for the job and larger sizes can be made with a veneer of any type of wood required. This process greatly reduces the cost of fine wood while maintaining the appearance.

    Strength

    • Cutting a piece of wood along the grain of the wood is easier than cutting against the grain. Large thin sheets can snap along the grain, hence the need for several layers. Gluing a layer on top of another layer with the grains perpendicular to each other adds strength to the sheet of wood. With each new layer you create a stronger board.

    Dynamics

    • Each sheet of plywood starts with a center sheet. Another piece is glued onto that board with the grains crossing for strength. Two boards glued together like that will twist unless you put a second piece on the other side of the center piece. This method applies the same force to both sides of the center piece keeping it straight. With each new layer you must have equal force pulling on the center piece or it will twist and warp.

    Mathematics

    • Making stronger, cheaper wood while maintaining a good look is not as easy as it seems. Equal force to the front and back of the sheets must be maintained. The center layer is one. With each new cross grain layer you need two pieces, one on each side of the center piece. A good looking sheet is applied to the front and back for looks with the grain running the length of the sheet.

      Odd numbered layers keeps the center board straight. An even number of layers will twist the sheet, making it useless.

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