Particleboard, also known as chipboard, is used by millions of people every day. Particleboard is a composite material that is less expensive and more uniform than wood alone. Even though it is denser than wood, it is light and weak in comparison to other fiber boards such as MDF (medium-density fiberboard) and OSB (oriented strand board). Particleboard is typically used for indoor applications such as furniture and cabinets due to its expansion and discoloration when exposed to high humidity. It is also often sealed, painted or covered with attractive veneers.
Particleboard moisture levels are important in all phases of the manufacturing process. Moisture content determines the overall strength and durability of the board; and surface appearance and quality of the board are governed by moisture distribution.
Summary of the Manufacturing Process
Debarking & Milling
The furnish for particleboard normally consists of wood chips. Wood chips are delivered from offsite locations. If wood chips are prepared onsite, logs are debarked, chipped and screened prior to washing. Clean wood chips are then reduced in size in a one- or two-stage process, followed by further milling to produce more “engineered” chips. Each particle reduction (milling) process will produce a range of particle sizes. Fine particles are used for the surface layers and the coarser chips or particles str used in the core layer.
Drying & Blending
From the chippers, the wood particles move to the drying and blending area. Single-stage or multiple-stage tube drying systems are commonly used to reduce the moisture content of the particles to the desired levels. The wood particles are then mixed with an adhesive to bind them together. Melamine formaldehyde, urea or phenolic resins are blended in small rapid blenders or large slow vats. Additional additives such as fire retardants may also be added in this phase of processing. These adhesives and additives are typically 4–10 percent by weight of the finished board.
Forming Line & Pressing
This particleboard mix is then fed into a former with coarse particles in the core and finer particles on the surface to form an evenly distributed mat with the targeted weight. The resulting mat is sent to the pre-press and hot press and then to board finishing to create the end product particleboard.
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 done simultaneously using an NIR transmitter at multiple locations within the particleboard manufacturing process.
On the green wood chips (before being chipped at the hammer mill stage), moisture measurement can be done to provide information on the dryer load and to enable feed forward control of the dryer. The moisture range is typically between 45 and 150% dry weight.
Post-dryer, moisture measurement can be done on the milled wood particles at the exit of the hammer mill. The moisture range is typically 30 to 55% dry weight.
Then, on the pre-formed mat, the moisture range is 0 to 5%.
An MCT460 On-Line NIR Smart Sensor (NIR) transmitter is typically mounted within 6 to 16 inches of the product and is either looking directly at wood chips and particles or is viewable through a glass window. Sometimes, a snorkel sampler is used along with the MCT-460 transmitter to make the reading in a chute where the fiber is free falling. The MCT-460’s analog outputs are typically connected to the controls in the control room allowing the end user to monitor their process and log historical data.
The QuikCheck bench NIR analyzer is used in the laboratory for quick, accurate and reliable sample testing.
Value and Quality
Incoming green wood chips vary significantly in moisture content and composition. The different measurements at the different locations within the process are very important; it allows temperature and/or process speed to be adjusted automatically in order to remove the proper amount of moisture, as to produce high quality, consistent boards.