Wood Production: Manufacturing & Kiln Drying
Lumber mills turn trees into manufactured wood products. Throughout the process, the moisture content (MC) of the wood is an important factor for producer and end user alike.
The lumber manufacturing process generally follows these steps:
- Head Rig: The primary saw cuts the tree into sawn pieces or boards.
- Edging: Removes irregular edges and defects from sawn pieces or boards.
- Trimming: The trimmer squares off the ends of lumber into uniform pieces.
- Rough Lumber Sorting: Pieces are separated based on dimension and final product production, whether the finished piece will be unseasoned (known as “green”) or dry.
- Stickering: Lumber destined for kiln drying production is stacked with spacers (known as stickers) that allow air to circulate within the stack (green product skips this stage and the next).
- Drying: Kiln drying wood speeds up the natural evaporation of the wood’s MC in a controlled environment.
- Planing: Smoothes the wood’s surfaces and ensures that each piece has a uniform width and thickness.
- Grading: Assigns a “grade” to each piece of lumber that indicates its quality level, based on a variety of characteristics, including its MC.
Kiln Drying Wood for Maximum Value and Usability
In order to maximize wood’s value and strength, mills invest both time and money in the kiln drying processes to remove excess moisture from the lumber stack. In fact, kiln drying on some hardwood species can take up to (and beyond) a month, depending on the initial MC of the wood.
Properly dried wood has many advantages over green wood for both producers and consumers alike. It reduces waste in manufacturing and extends the service life and usefulness of wood products, giving the consumer a stable product that will last for years.
The kiln drying process can vary considerably, depending on the species and initial MC of the wood. In general, however, these are the steps in the process:
- Lumber producers carefully stack “green” wood, using spacers or “stickers” to create gaps for air to freely circulate throughout the stack.
- Once the wood is placed in the kiln, depending upon wood species, the kiln is heated to temperatures between 110 to 180 degrees (Fahrenheit) for conventional-temperature kilns and 230 to 280 degrees (Fahrenheit) for high-temperature kilns.
- Operators constantly monitor kiln temperatures and relative humidity (RH), as well as the lumber’s MC. The goal is dry the lumber to the correct MC for how it will be used.
The profitability of lumber manufacturing depends on the mill’s ability to maximize the wood’s quality throughout the entire lumber manufacturing process. If the wood in a mill’s production line is too wet or too dry, the finished product may receive a lower grade and will have a lower dollar value than a piece that has been properly kiln dried.
For the post-drying lumber processing, in-line moisture content measurement systems can easily identify and mark pieces that are either too wet or too dry. These pieces can then be pulled from the production line before further processing and possibly be re-dried or re-milled as necessary to create the highest grade lumber or wood component possible for that piece, thus increasing the mill’s profits and giving consumers access to better quality materials.
Many consumers and builders believe that concerns about the lumber MC end when the wood is shipped out of the mill. In fact, wood products should be constantly measured and monitored to assure that the MC levels are at optimum EMCs. Wood will naturally continue to gain or lose moisture until it reaches this EMC balance with the surrounding environment.
Wagner Meters has been manufacturing moisture measurement products and services for the wood product industry since 1965 and provides a full range of solutions for lumber producers and secondary wood products manufacturers as well as installers of wood products.
Ron is an application specialist for Wagner Meters, and has over 30 years of experience in instrumentation and measurement systems in different industries. In previous positions, he has served as a regional sales manager, product and projects manager, and sales manager with manufacturers involved in measurement instrumentation.