Veneer, veneers

Four other groups of synthetic adhesives find uses in secondary processing, ie, overlaying, assembly gluing, etc, and in furniture and cabinet manufacture. Poly(vinyl acetate) (PVA) adhesives are widely used in appHcation of veneers and other overlays to panel substrates and in some unit-assembly operations. PVA adhesives are an emulsion of polyvinyl acetate in water and cure by loss of water. The PVA adhesives are somewhat... 

Another widely used overlay adhesive is the contact type. These specialized adhesives, in the same group as mbber cement, may be of the solvent-base or water-base types. They are often used to bond overlays such as wood veneer, vinyl (poly(vinyl chloride)) films, or high pressure laminates such as countertop overlays. 

Plywood is a panel made from wood veneers (thin shces or sheets) bonded to one another. Generahy each ply is oriented at right angles to the adjacent ply, and the two face pHes should have the grain direction parahel to each other. Thus most plywood wih have an uneven number of pHes, such as 3, 5, 7, or more. An exception to this is a four-ply constmction in which the two core pHes are oriented parahel to one another and perpendicular to the two face phes. 

Fig. 1. The hardwood plywood process using sHced decorative veneers.

Other veneers for backs and cores, or in the case of the tropical luauns, are almost always peeled on a rotary lathe. In this process, the log is rotated against along, stationary knife and the log is essentially "unroUed" in along, thin sheet. These sheets are called rotary-cut veneers. 

The stacks of veneer are moved to the panel lay-up area, where adhesive is appHed and veneers assembled into lay-ups ready to be pressed. Because core cross-veneers are only half-panel width, these stacks are cut into halves. 

Lay-up proceeds by laying down the veneer which is to be the back surface of the panel. Then a sufficient number of pieces of core veneer are passed through the glue spreader to form the next layer of cross-oriented veneer. The glue spreader commonly used in hardwood plywood manufacture is a roU coater in which a pair of opposing mbber roUs are coated with a thin layer of adhesive. As the veneer is passed between the roUs, the adhesive is transferred to the surfaces of the veneer. Adhesive is appHed only to the cross-pfles and in sufficient quantity to provide a continuous layer on both opposing faces of veneer. Thus, in the case of a three-ply panel, only the core layer is spread with adhesive and in that of a five-ply panel, the second and fourth layers both of which are cross-pfles, are spread with adhesive. Then the top surface veneer, which is normally the decorative surface, is placed on the assembly. 

After the allotted pressing/heating time, the pressure is released and the press moves to the open position. By this time, the loader is again ready and as each veneer assembly is moved into its respective opening, the pressed panels are pushed out into the unloader on the opposite side of the press. As the press begins the next cycle, the unloader moves to deposit the pressload of panels into a stack. 

The stacks are moved and again separated into individual panels where they pass a grading station. Those panels requiring touch-up or repair move to the repair stations. Panels then move to the trim saws where edges are trimmed to the final desired size, normally 1220 x 2440 mm (4x8 ft.). Panels are then touch-sanded to final thickness, and pre-finished as desired. Those panels with high quaUty decorative veneer faces are usually filled, stained to the desired tone, and finished with a clear finish. 

Uses and Treatments of Hardwood Plywood. Most early appHcations of hardwood plywood were those where the hardwood plywood was better adapted to the use than soHd wood. One of the most important early uses was in curved or formed parts, an appHcation particularly suited to the use of veneers which could be molded into intricate shapes during the pressing and bonding process. Then, as furniture manufacturers realized the inherently superior stabiHty of plywood compared to soHd wood, lumber-core or plywood panels began to be used for most flat-panel constmctions in furniture. 

Lumber core is a five-ply panel, usually about 19 mm (3/4 in.) thick, in which the bulk of the thickness, about 16 mm (5/8 in.) is edge-glued lumber. Yellow poplar and red gum are desired species for lumber core. Cross-pHes of lower value wood veneers are laid at right angles to the core grain direction, followed by two thin surface pHes of the decorative face veneer in a parallel direction to the core. This assembly is pressed and bonded to form a panel of exceptional quaHty, provided all steps are accompHshed in a desirable manner. 

Plywood furniture core panels, also about 19 mm (3/4 in.) thick, were normally made of a number of layers of relatively thick, 1.5—3.0 mm (1 /16—1 /8 in.) lower value wood veneers combined with thin surface pHes of the decorative veneer. These assembhes were laid-up from glued veneers and then pressed while the bonding occurred. Both lumber core and plywood core have been almost totally displaced in recent years by particleboard or medium-density fiberboard, both discussed herein. This change resulted from the increasing availabiHty and improved finishing characteristics of composites and from decreasing suppHes of core lumber or veneer of suitable quaHty. 

One type of thick hardwood plywood stiU available is imported from the northern Scandinavian countries and is generally known as Finnish birch. Characteristically, these plywoods are manufactured using multiple layers of veneer of the same thickness, about 1.5 mm (1 /16 in.), and bonded with a urea—formaldehyde or melamine—urea—formaldehyde adhesive. 

Thin hardwood plywood in the range of 4.5—6.0 mm (3/16—1 /4 in.) was normally a three-ply constmction with a thin, medium-quaHty back ply, a thicker lower value core, and another thin, high quaHty decorative face veneer. These panels were used as wall paneling, door facings, or for fumiture/cabinet appHcations requiring thin panels. Currently, only relatively small quantities of these types of panels are produced in the United States. 

The majority of thin paneling used today is imported from the Far East and is made from various tropical species of the luaun group, sometimes known as Philippine mahogany. These panels are normally finished using one of the processes intended to create the appearance and grain pattern of a decorative veneer or other patterns. 

Softwood is generally considered to be that coming from a coniferous tree, ie, an evergreen tree having needle-like or scale-like leaves. There are exceptions to the evergreen rule, however. In addition, many hardwoods also may now be used in softwood plywood as core veneers. 

After the block is chucked in the lathe, the lathe turns the block against the knife and peels the veneer in a continuous sheet as the knife moves toward the center of the block. When the knife cannot advance further without moving into the metal chucks, the lathe is stopped, the core of the block is dropped, the lathe is recharged, and the cycle repeated. 

In the automated lay-up system, each layer of veneer (with exception of the top surface veneer) passes under an automatic adhesive appHcation system. This may be a spray appHcation, a curtain coater, or an extmder, each of which is designed to apply a uniform adhesive spread on the upper face of each veneer. After all except the top veneer have been spread with adhesive and laid together, the top veneer is added. The me of the veneer—adhesive assembly at this point should be about 8%. 

There is a hybrid product available which has a veneer back, a layer of PF-coated wood particles, core veneer cross-ply, another layer of wood particles, and a top veneer. This assembly is pressed into a panel, trimmed to size, and sold into the stmctural-use panel market where it competes with plywood and oriented strand board. 

Economic Aspects. The stmctural plywood industry now has (ca 1997) about 105 operating mills, representing a significant decrease over the past several years. Production in 1994 was about 17.4 x 10 m (2), also representing a marked decrease over previous years. This decrease is a result of several factors, two of the most important being a decrease in availabiHty of suitable veneer logs, especially in the western states, and competition from the newer oriented strand board stmctural panel industries. 

Plywood requirements—includes wood species used, synthetic repair requirements, veneer grades, veneer layers and thicknesses, panel grades with respect to end-use, adhesive bond requirements, panel constmetion and workmanship, scarf and finger-jointed panels, dimensional tolerances, moisture content, and packaging and loading... 

A small amount of particleboard is made with a fire-retardant treatment for use in locations where codes require this material, as in some offices and elevators. Particleboards receive overlay and finishing treatments with ease. Wood veneers, melamine overlays, printed paper overlays, vinyl overlays, foils, and direct grain printing can all be done quite simply. A small amount of particleboard is also made in the form of shaped, molded articles such as furniture parts, paper roU plugs, bmsh bases, and even toilet seats. There is another small increment of particleboard made by the extmsion process. These products are made in small captive operations owned by furniture manufacturers which consume all of this production in their furniture. The extmsion process differs from conventional flat-pressed particleboard in that the wood furnish is forced between two stationary heated surfaces. The mats are formed from one edge and this edge is alternately formed and pushed between the heated platens, which are maintained at a distance equal to the thickness of board produced. This is an old, slow, small-scale process, but is stiU in use in at least one location. 

Laminated Strand Products. The most recent developments in the family of wood-based composites are a group of laminated strand products, made with strands oriented in the long direction of the product and marketed as stmctural composite lumber. One product is made with long, narrow strips of softwood veneer. The strips or strands are about 2.5 x 13 x 600 mm (0.1 x 0.5 x 24 in.), coated with a PRE adhesive, and pressed under heat and pressure into large blocks. After the resin is cured the blocks are resawn and planed into lumber dimension stock. 

The Hardwood Plywood and Veneer Association (HPVA), 1825 Michael Faraday Drive, P.O. Box 2789, Reston, VA 22090, (703) 435—2900. 

Laminates aie materials made up of plies or laminae stacked up like a deck of cards and bonded together. Plywood is a common example of a laminate. It is made up of thin pHes of wood veneer bonded together with various glues. Laminates ate a form of composite material, ie, they ate constmcted from a continuous matrix and a reinforcing material (1) (see also Reinforced plastics). 

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