Green Moisture Content Veneer is often cut from logs soon after the trees are felled. Such bolts or flitches have essentially the moisture content found in the living tree. This moisture content in the wood has a distinct effect on cutting. In general, wood with a moisture content above fiber saturation but not excessively high is best suited for cutting into veneer; this makes the wood more pliable than drier wood. In a number of studies we found that species with a natural uniform moisture content of about 50 to 60 percent cut well.
Some of the free water is forced out during cutting. This water apparently acts as a lubricant between the wood and the knife and pressure bar and aids the cutting process. The driest wood that we have cut successfully into veneer at the Forest Products Laboratory was a flitch of teak with a moisture content of 25 percent. Like all teak, this flitch had a waxy extractive that probably aided the cutting. We tried cutting even drier wood, but were not successful. This came about because a manufacturer wanted to slice air-dried planks of ponderosa pine into veneer VLG inch (1.50 mm) thick. The wood, which was at about 15 percent moisture content, was heated to about 200° F in water. Continuous sheets of veneer were produced from the flitches but the veneer had pronounced checks on the side that was next to the knife during cutting. After cutting, the veneer sheets immediately curled into tight rolls like window shades, so they were unsatisfactory. Because slicing of the wood at 15 percent moisture content was unsuccessful, we took sapwood air-dried planks from the same shipment, and pressure-treated them with water to a moisture content of over 100 percent. Veneer Vie inch (1.59 mm) thick was then successfully sliced from these planks. In other words, when water is put back into relatively dry wood, the wood can be cut into veneer. Some species have a higher moisture content in one part of the tree than another. For example, the sapwood of Douglas-fir has approximately three times as much water as the heartwood. Butt logs of redwood often have much higher moisture content than upper logs. In addition to requiring long drying times, wood having a very high moisture content is more difficult to cut into veneer than wood of the same species but with a lower moisture content. Examples are some western hemlock (as high as 215 pet), redwood (as high as 245 pet), and Douglas-fir (as high as 130 pet). In normal veneer cutting, the wood is compressed just ahead of the knife. Wood with a very high moisture content can not compress until some water is forced out. As water is relatively noncompressible, it is forced from the wood structure so fast that it ruptures the wood (fig. 1). Commercial experience indicates M 88966 Figure 1.—”Shelling” or shattering of redwood veneer that was rotary-cut from a “sinker” log. The wood shattered because water was forced out of the wood too fast during cutting. that high moisture content in “sinker” logs of species like redwood makes them undesirable for veneer because of cutting and drying problems. Likewise, for a long time sapwood veneer of Douglas-fir was not considered A-grade; part of the difficulty was in cutting it into smooth veneer as easily as the heartwood, which has a lower moisture content. Wood may be damaged by freezing if it is stored in a cold climate. For instance, southern pine sapwood was damaged when logs were stored outdoors during the winter in Madison, Wis. Even worse damage was observed in a sweetgum log stored through a winter at Madison when the temperature went from above freezing to as low as -20° F. The end of a bolt cut from this log is shown in figure 2. Ice was found in many of the cracks seen on this end section. Industry reports that walnut logs grown in California and shipped by rail to the East froze when crossing the Rocky Mountains. Veneer cut from those logs was nearly useless due to splits caused by freezing. Moisture content in the tree, then, is generally not a decisive factor in determining whether wood is suitable for use as veneer. Wood with a very high moisture content is usually more difficult to process than wood having a moderate moisture content such as 50 to 60 percent. On the other hand, it is very difficult or impossible to cut good veneer from wood below the fiber saturation point, approximately 30 percent for all species.如无特殊说明，文章均为本站原创，转载请注明出处。If there are no special instructions, the articles are original, if you want to use or reproduce, please indicate the original source www.plywoodinspection.com,If you find that our articles infringe on your copyrights and interests, please email us at firstname.lastname@example.org in time and we will delete it at the first time.