لخّصلي

Plywood

Title

together. The layers, called plies, are arrranged so that the direction of the wood grain, or Plywood is a building material usually made of an odd number of thin layers of wood dglued pattern in the wood, in one layer is at right angles to that of the layer next to it. The outside plies are called "faces" and "backs"/and the center ply or plies are called the "core." The simplest plywood is three-ply. However, five, seven, nine, or more plies may

Nam

be used.

Using glue to fix together an odd number of plies produces a very useful building materiar with several advantages. It is lightweight and workable, but it is also strong and rigid. Plywood shrinks and swells less than ordinary wood. Plywood can be produced in large panels and cut to exact sizes. This makes it strong, easy to work with, and appropriate for smooth surfaces. For these reasons, plywood is one of the main materials used in building houses. Flat rectangular lengths of plywood are used for laying floors, lining roofs, and panelling walls. Plywood is even used in the construction of ships and aircraft.

Plywood is also used to make inexpensive, yet attractive, furniture. Veneers of expensive woods, such as walnut or cherry, can be used for the faces because only thin sheets are needed. Alternatively, plastic or metal faces can be used to provide surfaces that resist scratching.

Plywood is not something new. We know it existed in ancient Egypt around 3,500 years ago when wooden articles were made from veneers glued together crosswise. This was originally done because of the shortage of fine wood. Thin sheets of high-quality wood were glued over layers of low-quality wood to make the product look better. The added strength was an additional benefit.

The process of making plywood involves three main steps: the log, the veneer, and the lay-up. Logs used for plywood are selected for straightness, roundness, and lack of knots and decay. After the bark, the outer covering of wood, is removed and the logs are cut to the right lengths, they are often steam-heated. This softens their surfaces. The logs are then placed in a rotary lathe, As the lathe rotates, a blade cuts the logs into long. continuous plies.

After the plies are dried, trimmed, and matched, a thick layer of glue is applied to each ply. Workers then lay up or place the plies with the grain in each ply opposite to that in the adjacent ply, Large hydraulic presses squeeze the plies together with heat and pressure. Then the finished plywood is dried again, trimmed, smoothed, and finished into sheets.