Felling the trees
1 Selected trees in an area are marked as being ready to be cut down, or felled. The felling may be done with gasoline-powered chain saws or with large hydraulic shears mounted on the front of wheeled vehicles called fellers. The limbs are removed from the fallen trees with chain saws.
2 The trimmed tree trunks, or logs, are dragged to a loading area by wheeled vehicles called skidders. The logs are cut to length and are loaded on trucks for the trip to the plywood mill, where they are stacked in long piles known as log decks.
Preparing the logs
3 As logs are needed, they are picked up from the log decks by rubber-tired loaders and placed on a chain conveyor that brings them to the debarking machine. This machine removes the bark, either with sharp-toothed grinding wheels or with jets of high-pressure water, while the log is slowly rotated about its long axis.
4 The debarked logs are carried into the mill on a chain conveyor where a huge circular saw cuts them into sections about 8 ft-4 in (2.5 m) to 8 ft-6 in (2.6 m) long, suitable for making standard 8 ft (2.4 m) long sheets. These log sections are known as peeler blocks.
Making the veneer
5 Before the veneer can be cut, the peeler blocks must be heated and soaked to soften the wood. The blocks may be steamed or immersed in hot water. This process takes 12-40 hours depending on the type of wood, the diameter of the block, and other factors.
6 The heated peeler blocks are then transported to the peeler lathe, where they are automatically aligned and fed into the lathe one at a time. As the lathe rotates the block rapidly about its long axis, a full-length knife blade peels a continuous sheet of veneer from the surface of the spinning block at a rate of 300-800 ft/min (90-240 m/min). When the diameter of the block is reduced to about 3-4 in (230-305 mm), the remaining piece of wood, known as the peeler core, is ejected from the lathe and a new peeler block is fed into place.
7 The long sheet of veneer emerging from / the peeler lathe may be processed immediately, or it may be stored in long, multiple-level trays or wound onto rolls. In any case, the next process involves cutting the veneer into usable widths, usually about 4 ft-6 in (1.4 m), for making standard 4 ft (1.2 m) wide plywood sheets. At the same time, optical scanners look for sections with unacceptable defects, and these are clipped out, leaving less than standard width pieces of veneer. The wet strips of veneer are wound into a roll, while an optical scanner detects any unacceptable defects in the wood. Once dried the veneer is graded and stacked. Selected sections of veneer are glued together. A hot press is used to seal the veneer into one solid piece of plywood, which will be trimmed and sanded before being stamped with its appropriate grade.
The wet strips of veneer are wound into a roll, while an optical scanner detects any unacceptable defects in the wood. Once dried the veneer is graded and stacked. Selected sections of veneer are glued together. A hot press is used to seal the veneer into one solid piece of plywood, which will be trimmed and sanded before being stamped with its appropriate grade.
8 The sections of the veneer are then sorted and stacked according to grade. This may be done manually, or it may be done automatically using optical scanners.
9 The sorted sections are fed into a dryer to reduce their moisture content and allow them to shrink before they are glued together. Most plywood mills use a mechanical dryer in which the pieces move continuously through a heated chamber. In some dryers, jets of high-velocity, heated air are blown across the surface of the pieces to speed the drying process.
10 As the sections of the veneer emerge from the dryer, they are stacked according to grade. Underwidth sections have additional veneer spliced on with tape or glue to make pieces suitable for use in the interior layers where appearance and strength are less important.
11 Those sections of veneer that will be installed crossways—the core in three-ply sheets, or the crossbands in five-ply sheets—are cut into lengths of about 4 ft-3 in (1.3 m).
Forming the plywood sheets
12 When the appropriate sections of veneer are assembled for a particular run of plywood, the process of laying up and gluing the pieces together begins. This may be done manually or semi-automatically with machines. In the simplest case of three-ply sheets, the back veneer is laid flat and is run through a glue spreader, which applies a layer of glue to the upper surface. The short sections of core veneer are then laid crossways on top of the glued back, and the whole sheet is run through the glue spreader a second time. Finally, the face veneer is laid on top of the glued core, and the sheet is stacked with other sheets waiting to go into the press.
13 The glued sheets are loaded into a multiple-opening hot press. presses can handle 20-40 sheets at a time, with each sheet loaded in a separate slot. When all the sheets are loaded, the press squeezes them together under a pressure of about 110-200 psi (7.6-13.8 bar), while at the same time heating them to a temperature of about 230-315° F (109.9-157.2° C). The pressure assures good contact between the layers of the veneer, and the heat causes the glue to cure properly for maximum strength. After a period of 2-7 minutes, the press is opened and the sheets are unloaded.
14 The rough sheets then pass through a set of saws, which trim them to their final width and length. Higher grade sheets pass through a set of 4 ft (1.2 m) wide belt sanders, which sand both the face and back. Intermediate grade sheets are manually spot sanded to clean up rough areas. Some sheets are run through a set of circular saw blades, which cut shallow grooves in the face to give the plywood a textured appearance. After a final inspection, any remaining defects are repaired.
15 The finished sheets are stamped with a grade-trademark that gives the buyer information about the exposure rating, grade, mill number, and other factors. Sheets of the same grade-trademark are strapped together in stacks and moved to the warehouse to await shipment.
Just as with lumber, there is no such thing as a perfect piece of plywood. All pieces of plywood have a certain amount of defects. The number and location of these defects determine the plywood grade. Standards for construction and industrial plywoods are defined by Product Standard PS1 prepared by the National Bureau of Standards and the American Plywood Association. Standards for hardwood and decorative plywoods are defined by ANSIIHPMA HP prepared by the American National Standards Institute and the Hardwood Plywood Manufacturers' Association. These standards not only establish the grading systems for plywood, but also specify construction, performance, and application criteria.