Marine plywood is an expensive, water-resistant grade that is more tightly constructed and glued than ordinary plywood. Its manufacturing process is best understood in comparison to that of lesser grades. Plywood, a lower-cost alternative to hardwood board, is made from at least three thin wood layers or "plies" that are held together with glue. The manufacturing process begins with the cutting of a variety of woods, including Douglas fir, pine, redwood, cedar and spruce for soft woods, along with teak, oak, mahogany and ash for the harder layers. At the plywood mill, cut logs are brought by chain conveyor to a debarking machine that uses grinding wheels and water jets to smooth the surface. Another conveyor transports the logs to a circular saw that shaves them into chunks, called "peeler blocks," that are just over 8 feet long. The peeler blocks next undergo a softening process to make them easier to slice into the thin veneers that make up the plies. The softening process, which can last from half a day to two full days, is done with either the application of steam heat or total immersion in water. Once heated, the peeler blocks are sent through a precision lathe with a cutting blade, which shaves a succession of veneers from the block until very little is left. Defective pieces are discarded at this point. The veneers, still a bit over 8 feet long, are next cut into widths of just over 4 feet so they can later be trimmed to standard 4x8-foot sheets. The veneer pieces are put through a heated dryer, after which they are stacked for later use.
The Gluing Process
The thin veneer layers are glued together with various types of adhesives intended for specific uses. Soybean or blood protein adhesives are typically used for interior plywood, while plywood intended for furniture building and other curved applications is often held together with urea-formaldehyde resin. Exterior plywood is glued with moisture-resistant phenol-formaldehyde resin. The direction of the grain is varied between adjacent layers, which may also alternate between hard and soft woods, to improve the strength of the finished board. The gluing machines are run with varying degrees of automation. Layers are added and glued together one at a time in these machines according to a complicated formula that differentiates between a wide variety of grades, types, finishes and uses. Once gluing is finished, the sheets are piled 20 to 40 high in hot-press machines that squeeze them together at 110-200 PSI while heat is applied in the range of 110-157 degrees C for several minutes. The sheets are next sawn to exact 4x8s. The manufacturing process ends with automated and manual sanding, removal of defects, and quality inspection. The plywood sheets are marked or stamped to indicate their differing grades and quality, and prepared for shipping.
Differences in the Marine Plywood Manufacturing Process
Waterproof wood is necessary for marine use since standard plywood grades rot after exposure to moisture. The lumber used in marine plywood differs somewhat from regular plywood. In addition to Douglas fir, spruce and cedar, marine plywood suitable for boat and dock construction is made from longleaf yellow pine, cypress, western larch, and eucalyptus. The main difference in construction is that the surfaces of the plies are better smoothed so as to reduce the "voids," the micro-spaces in between layers, to tightly adhering joints that do not let destructive moisture in. The adhesive is once again the water-resistant phenol-formaldehyde resin used in exterior-grade plywood. All parameters of finishing, grinding and cutting are done with closer tolerances than standard plywood, accounting for the very high cost of marine plywood.