Some Information On Veneer Manufacturing Process
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A veneer is a covering made of very thin wood placed over a layer of coarser wood or any other material. The lower layer is usually one of lesser decorative or aesthetic value. In the context of plywood manufacture (from hardwood trees that have broad leaves), veneer refers to the thin sheets that are sliced off from a hardwood log by using different processes. This article focuses on the decorative veneer surfaces of core panels rather than the less attractive material that are used for the inner plies. The decorative face and back veneers are generally made of hardwood such as maple, oak, cherry, and birch. Sometimes, they are also made from cone-bearing wood (softwood) such as pine or cypress or cedar because they lend an attractive appearance.
The machines that cut out veneers with differing grain patterns are basically of two types: rotary mills and slicing mills. Whereas rotary mills complete the entire veneering process at one location, the components produced by a slicing mill have to be further processed at another location. However, the log preparation procedure for producing the veneer faces is similar, though the type of cut is different.
During the preparation phase in the storage yard, the log is maintained in a moist condition using a water sprinkler or any other similar arrangement. This prevents drying and splitting of the log. Before the veneering process starts, the logs are placed in vats and cooked in steam to soften the fibers. This helps to ease the slicing and peeling of the log. After this, they are cut to a length of 4 to 12 feet, and the barks are stripped off. Any remaining bark debris is removed using the hands.
A majority of veneer that is produced today, hardwood, decorative veneers, face and back veneers, inner plies for veneer cores, pine and fir veneers are all rotary cut. When using this method, the entire log is held in place with the help of roller clamps or chucks at the ends in a spindle-less lathe. The modern lathes use a computerized method to determine the true center of the log in order to optimize the yield. An “XY” charging system is used to rotate the log as a set of laser beams are cast along its length. The data gathered is processed by a computer, which further repositions the log to create a maximum diameter cylinder bole. A block charger is used to determine the geometric center of the block.
After repositioning the log, it is rotated against a carriage-mounted knife on one side and a nose and pressure bar on the opposite side to cut sheets of consistent thickness. The first few cuts that are obtained when the log is rotated may produce sheets of varying lengths. This is called “round-up”. They can be used for different applications or may even be discarded. Soon enough the rotating log starts producing perfect cylindrical sheets of wood akin to paper sheets from a roll.
The veneer ribbon is then fed into a clipping line to obtain predetermined widths (say 50” for panel face veneer) and also to remove defects that include rotten areas, large knots, other damages, etc. Thereafter, the sheet is fed into a dryer to reduce the moisture content to a level between 6 and 12 percent. The dried sheets are conveyed to a grading section where the full width veneer sheets are separated and duly graded. The partial width sheets are moved away for splicing at a later point of time.
The 50” sheets (also called whole pieceface or 1-pieceface) of 100% heart oak or white maple are preferred for stock panel manufacture. However, the veneer sheets obtained that are free of defects and meet top-grade quality (AA or A) are limited. Though the sheets after the first few rotations are free of burls and knots, in cases of wood such as maple or birch, these sheets are largely of sapwood. In case of cherry or oak, it is a mixture of light and dark wood that are still not considered top grade. A large amount of wood must be spliced to obtain the full 50” width defect free sheets. After grading, the whole piece veneer is packaged which can then be shipped to a panel manufacturer.
The rotary cut veneer has a wild and variegated grain because the veneer is cut at a broad angle by the knife parallel to the growth ring (which is shaped like a tall slender cone). Veneer surfaces that have been rotary cut are best suited for components that make cabinets and smaller pieces of furniture. Rotary cut veneer is in demand in some markets because it is cheaper.
When a log is sliced, it displays a more pronounced grain pattern and for this reason sliced veneer is preferred to be used in the manufacture of wall panels, cabinets, furniture and cabinets of higher quality.
Prior to slicing, the bark of the log is removed, sawed in half longitudinally (roughly) and these sawed up halves or quarters (called flitch) are placed in vats of hot water or steam. The log is sliced by clamping the halves into slicing machines with the back or the flattest side against the slicer plate. The slicer alternately raises and lowers the sawn half at an angle between a fixed knife and a pressure bar. Whereas in some machines the slicing happens during the “down” stroke, in others it happens in the “up” stroke.
Once a veneer is cut, the log half or quarter is moved closer to the knife automatically by the machine. As the cut veneer falls off, it is stacked in pallets in the same order as it was cut. This order is maintained so that all the faces that are manufactured from a given sawn log of wood are alike. The cut veneers, nevertheless, display gradual changes in appearance and character.
The veneers are usually cut in a manner that best uses the width of the log, and the result is bundles of veneers of varying widths. At this point, veneer samples are sent to prospective buyers for evaluation. Typically, the samples from three locations within the flitch stack are sent to the buyers: first one at a one-third distance from the top of the stack, second one at half the distance from the top, and the third one from two-thirds into the stack. From a small stack, usually only two veneers are sampled. This helps the buyer to understand the character of the veneer from the face to the back of the log.
The outer parts of the veneers are then clipped to remove the sapwood, juvenile wood, knots, and discoloration. The clipped veneer lines are not parallel and as a result they are not ready to be spliced at this point in time. The clipped veneers can be packed neatly in a bundle and these bundles are shipped to a veneer splicer who processes the dried raw veneers into faces and backs.
As in the case of rotary cut veneers, sliced veneers are also passed through a dryer to reduce moisture content to about 6 to 12 percent before they are subjected to any further processing. Some slicing machines have additional capabilities (like splicing) to convert the veneers into panel faces.
Veneer Manufacturing Process
A plain slice is a cut that is tangential to the center of the log. The growth rings are conically shaped and, therefore, the cut at the bottom is parallel to the growth ring whereas the cut at the top is nearly at right angle to the growth ring. In a plain slice, the pattern of the grain assumes the shape of an inverted “V” (sometimes called a cathedral on some veneers). Many a time, the cathedrals are of the same height on most of the veneers and they are centered. A panel that is made up of such veneers has the design of a king”s crown on them. For the same reason, these veneers are also called “crown cut”. Clipping before the veneers are spliced can sometimes result in a partial cathedral component on the sheets. If these are matched, the resultant appearance is a split-heart cathedral. Plain sliced veneers may also alternately have a rift cut, quartered or rotary cut appearance. This is dependent on the individual log and the angle at which it is cut when sliced.
Veneers Quarter Slicing
The quarters from a log are usually cut with the help of a plain slicing machine. The log is cut into four longitudinal pieces instead of the usual two. The quarter log is placed on the slicing machine such that the blade, at every stroke, slices the flitch in a direction perpendicular to that of the growth rings and in a radial direction to the log”s center. The appearance of the grain is very straight in this case. Wood rays, found in all species, but more commonly in wood such as red and white oak, are exposed by quarter slicing and in these cases are referred to as fleck or quarter flake.
In cases where only the straight gain appearance is required minus the flake, the log is rift cut to eliminate the flake. For this, the flitch is placed on a machine called the stay log or half round. This machine functions like a rotary slicer but is different in that the log passes under the knife only for a minor part of the revolution. The log is cut perpendicular to the rays but not the growth rings. Rift cut machines make the appearance of flakes less prominent on veneers than quarter-sliced ones.
Plain sliced logs sometimes take on the appearance of quartered veneer or rift-cut veneer because, at the end of the slicing exercise, the machine cuts the log at right angles to the growth rings and thereby exposes the juvenile wood of the tree. This portion is softer, weak, has knots and is discolored. These are clipped off and the remaining straight grain from the sides of the log is visible. Such veneers are also called “bastard quarters”. They are few in numbers and are narrow.
The bastard quarters of red and white oak that contain flakes are sold as quarter sliced and those without flakes are sold as rift cut veneers. However, they should meet basic standards for the specified grade. Panel face components made from such veneers are also likely to be narrow.
Using a stay log machine is yet another method of cutting veneers from logs. This is commonly used on the wood of maple (it can be used on any wood species) as it produces lighter wood (more sapwood) than does a conventional log slicer. Keeping away from the dark heartwood of maple is a desirable characteristic. It works much like a rotary log slicing machine. A flitch is placed with an offset on the lathe in a manner that the knife comes in contact with the flitch for only half a rotation. The resultant cut resembles a plain slice with a broader grain appearance. Such veneers are sold as half round or plain sliced.
Whatever be the method of cutting the veneer, preparation of the log is a very important factor. The log should be adequately cooked. Under-cooking does not loosen the fibers and it results in the veneers displaying a ruptured appearance. Over-cooking, on the other hand, causes excessive breakdown of the fibers and the veneers tend to have a fuzzy appearance. This may also lead to red-colored discolorations in parts of the log.
Another important factor is the condition of the knife on the lathe. If the knife has a nick, it leads to formation of gouges on the veneer surface all across the grain. These marks are more obvious in wood that is hard and those that have a high mineral or ash content like hickory. They can, however, occur in any species. Lighter knife marks are removed by sanding the panel surface. Those that cannot be removed serve to reduce the grade and value of the veneer sheet.
In this context, it is important to note that any method of cutting the veneers produces different patterns of grain on the veneer surfaces. Whereas rotary cut wood sometimes appears like plain sliced, half round cut produces both the patterns. Quarter sliced and rift cut veneers have straight grain regions on the sides. The Hardwood Plywood and Veneer Association and Architectural Woodwork Institute standards therefore have a note under the veneer heading tables which clarify that the headings refer only to the patterns on the veneer face and not the method by which it is cut. The method of cutting is the prerogative of the mill unless otherwise specified by the buyer. An example would be “plain sliced to be cut on a either a vertical slicer lathe or a half-round rotary machine”.
Thickness of the Veneer
Different mills produce veneers of varying thicknesses. Whereas domestic mills (rotary) have veneer that are about 0.029” (1/34”) thick, rotary cut back and face veneers have thicknesses that vary between a range of 0.125” (1/8”) and 0.026” (1/38”). This is also dependent on the specifications of the customer, the grade of the wood, application and species of the wood. The veneer thickness of those cut by domestic plain slicing lathes are generally 0.028” (1/36”) or 0.024” (1/42”). However, rift cut or quarter-sliced veneers thicknesses range between 0.063” (1/16”) to 0.020” (1/50”). However, it depends on customer specifications and the application for which the veneers are cut. Prior to 1980, the standards were different and rotary cut veneers were generally between 0.042” (1/24”) and 0.033” (1/30”). Plain sliced veneers had thicknesses between 0.036” and 0.031”. Veneers with these thicknesses are rarely available today.
Tree Sources for Veneer Cuts
The trees from which veneers are cut are usually of the tall variety having straight trunks. They are trees that produce more than one log at a time. The origin of the log that is cut from the tree determines the method in which it will be processed. A tree may produce many logs of high quality and they can be cut using any of the known methods.
The first 8 to 20 feet are cut from the tree stump. This wood does not have many of the natural undesirable characteristics and is more suited to any of the cutting methods (plain slicing, quarter slicing, rotary cutting, etc.). Slicing this type of log results in a higher percentage of whole piece faces of the veneer.
The next cut is usually the next 8 to 15 feet of the trunk. This portion is likely to have more defects such as bark pockets and dead limbs. Burls from adventitious buds may also be present. This wood can be plain sliced or rotary cut. A third cut can be sourced from the remainder of the log. This portion can contain larger number of burrs and larger burls. This wood is typically cut for lesser grade rotary veneers. Sometimes, areas adjacent to the forks between major stems and limbs give rise to a feathered appearance termed as “crotch figure”. This is highly valued in veneer faces.
The tree stump is not usually processed but veneers from this portion of the wood are small sized and may contain figures that can be matched to many patterns. There is a lot of waste wood generated when veneers are cut from the stump. Small veneer faces may be jointed together on the sides to make full size panel face. These veneers are also called sketch faces.