Did you know that Oriented Strand Boards (OSB) is replacing plywood as the material of choice for sheathing and flooring in most new homes built in North America? OSB is a material with good mechanical properties and low cost. It is also a more renewable product than plywood because it can be made from small, fast growing trees.
Control of moisture is critical in the manufacturing of consistent, high-quality OSB. The moisture content and its distribution influence the bonding of the wood strands. If moisture levels are too high, the resin will not bind sufficiently to the wood and delamination will occur. Over drying of the wood strands requires using more resin and wastes fuel, increasing costs, as well as adding the risk of flash fires.
Most OSB manufacturers use logs averaging 8 to 12” in diameter at breast height (DBH). Incoming logs are slashed, soaked and debarked, prior to being cut into strands ranging from 3.5 to 6” in length and 1” width. The green strands are then stored in wet bins waiting to be dried.
Since the moisture content of the wet strands ranges from 40 to 75%, they are dried to 5 to 10% moisture in a rotating single-pass or triple-pass dryer with an inlet temperature as high as 1250 degrees F. The dried strands are then screened and blended with a binder and wax, before entering the former where the strands are laid out as a continuous mat.
Dried strands are classified into two sizes using various sizes of screen. Large strands are used for the bottom and top face layers of the OSB panel while the smaller strands are used for the core layer. Face and core layers are oriented opposite to each other and sequentially laid onto a conveyor belt as a mat. The overall mat thickness depends on the desired thickness and density of the finished panel. The mats are converted to panels under high pressure and temperatures within a press.
Pressed panels are removed from the press line and conditioned on a rotating cooler prior to cutting them to final dimensions.
Moisture, resin and wax measurements can be done simultaneously, at multiple locations within the OSB manufacturing process, using Process Sensors Corporation (PSC) sensors.
Moisture of the green wood chips can be measured to calculate dryer load and to control the feedrate of the dryer. Without feedrate control the dryer can be easily overloaded.
After the dryer, moisture measurement can be done on the weigh belt before the blender. The measurements at this location are often used as an indicator of moisture level rather than an absolute moisture measurement, the aim being to narrow the final strand moisture content through tighter dryer control.
At the exit of the blender, moisture, wax and resin measurements can be measured simultaneously. Wax and resin measurements are often used as trends at this location.
On the forming line, measurements are normally done on the bottom, core and top layers.
A PSC sensor is typically mounted within 6 to 16” of the product and is either looking directly at the strands or through a glass window. The sensor’s analog outputs are normally connected to a process control system allowing the end user to monitor the process and log historical data.
If the plant layout makes it impractical to mount an online transmitter, a PSC QuikCheck benchtop analyzer is used in the laboratory for quick, accurate and reliable samples testing.
Incoming strands vary significantly in moisture content and composition. Measuring moisture at multiple locations with sensors from Process Sensors Corporation allows temperature and process speed to be adjusted automatically, ensuring that high quality, consistent panels are produced as efficiently as possible.
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Reliable moisture measurement for improved process control and consistent product quality in wood products manufacturing