Have you ever looked closely under your furniture or at an unfinished cabinet? If so, you may notice that the wood is uniform and without grain. That’s because it’s not natural wood but instead Medium Density Fiberboard (MDF). MDF is an engineered wood product that is easy to finish, paint, or apply a wood veneer to mimic natural wood. It is much less expensive than natural woods and is primarily for indoor use due to its poor moisture resistance.
Moisture is a very important parameter in the manufacture of MDF, as moisture content determines the overall strength and durability of the board. The surface appearance/quality of the board is governed by moisture distribution.
MDF Manufacturing Process
Debarking & Digester
Wood chips are prepared starting with logs that are debarked, chipped and screened. The chips are washed then softened in a steam-pressurized digester. The softened chips are transported to a pressurized refiner chamber where single or double revolving disks are used to mechanically pulp the softened chips into fibers suitable for making the board.
Drying & Blending
From the refiners, the fibers move to the drying and blending area. Single-stage or multiple-stage drying systems are used to reduce the moisture content of the fibers to the desired level. Resin, wax, and in some cases urea water are added to the softened fiber which is flash-dried while being conveyed to a bin.
Forming Line & Pressing
The dried fibers pass through a grader and are laid out in a mat which is consolidated at the forming pre-press. Next, the hot press converts the mat into a panel through the application of high pressure and temperature.
Finishing Line & Packaging
After pressing, the boards are cooled, sanded, trimmed, and sawed to final dimensions. The boards may also be painted or laminated. Finally, the finished product is packaged for shipment.
Quality Parameters and Measuring Points
Moisture, resin and wax measurements can be made simultaneously using an on-line Near Infrared (NIR) sensor at multiple locations within the MDF manufacturing process.
Moisture of green wood chips (pre-digester) can be measured to calculate dryer load and to control the feed rate of the dryer. Typical moisture range is between 45 and 150% dry weight, so without feed rate control the dryer can be overloaded.
Post-dryer moisture measurement is taken on the refined fiber at the exit of the refiner, and the moisture range is typically 30 to 55% dry weight.
On the preformed mat, the moisture range is 0 to 5%.
An MCT460 General Purpose On-Line NIR Sensor is typically mounted within 6 to 16” of the product and is either looking directly at wood chips and fiber or through a glass window. The moisture sensor can also be fitted with a snorkel sampler to measure moisture of fiber free-falling in a chute. The sensor’s analog outputs are typically connected to the process controls system allowing the end-user to monitor their process and log historical data.
If the plant layout makes it impractical to mount an online sensor, a benchtop NIR analyzer, such as the QuikCheck can be used in the laboratory for quick, accurate and reliable sample testing.
Value and Quality
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.