Commercial mills are usually specialized; they each create only a few specific types of lumber. This is because the tooling and processes are completely different for different types of wood and their respective products. Commercial mills are usually specialized; they each create only a few specific types of lumber. This is because the tooling and processes are completely different for different types of wood and their respective products.
One mill might be set up to run both plywood and structural lumber (ie 2x4s), since structural plywood and structural lumber are made from the same trees. In Washington, those trees are typically Douglas fir, spruce, hemlock and pine. The mill will specialize in sourcing and handling these trees, and not concern itself with things like molding or doors --- leaving that up to another mill. Logs come to the mill by the truckload. Usually, they come from the commercial logging operations up in the foothill/mountains. On occasion, a developer or homeowner will sell some trees (you can read this post to learn about selling your trees for money) to a commercial mill, but in general, the mill needs to procure logs by the truckload for it to be profitable. For less than that, its hard to justify the expense of transport. Logs are unloaded from the truck with the use of a claw on a crane. They are unloaded onto the conveyor system, and proceed down the line to be skinned (de-barked) and sorted. Then they’re ready to be milled. Plywood Plywood is made of multiple layers (plys) glued and compressed together. The layers run across the grain from the adjacent layers, which gives the sheet extra strength. Each log is turned into one long sheet, or “ply”. The log is rotated in place while a machine peels the outside layer off into one continuous ply. This leaves behind a small core at the end, when the workpiece gets so small that any layers peeled off would not be able to sit flat. The long sheet is then cut into smaller sections, which can be glued, stacked, compressed, and transported off to the lumber yard or hardware store. OSB Oriented Strand Board is a similar sheet material --- sold in the same dimensions but made via a different process. The wood is shredded into flat flakes, which are glued and compressed together. Though each flake is thin and offers very little strength on its own, they are strong together because they overlap irregularly. One advantage to OSB is that essentially no waste is produced. Dimensional lumber Dimensional lumber is about as straightforward as it gets. The logs are skinned, and then cut down to size with the use of a large bandsaw. The logs are transported to different stages of the process by conveyor. Some mills have automated much of the process, while others have heavy duty machinery that still requires human operators. Kiln drying The lumber needs to dried before it can be sold. This is a low tech operation; the kiln is just a chamber that functions as a low temperature oven. Heat evaporates the wood, which is then sucked up by a dehumidifier and expelled. Pressure treatment At this point, wood meant to be sold untreated is ready for distribution. Treated boards are placed in a vacuum sealed chamber, which is then drained of all air. This draws air out of the wood, which will allow the treatment to permeate all the way through the boards. To help with this, the chamber is pressurized, forcing the liquid treatment into the cells of the wood --- hence the name, pressure treated. Trim/molding and flooring Although molding and hardwood flooring might be processed in different mills, the process is similar. The boards come as “blanks”, which are produced by mills that run dimensional lumber. In essence, they are dimensional lumber --- long boards with straight flat edges. At the next stage, they’re run lengthwise through molding machines. This will cut the profile into trim boards, and the tongue and groove into flooring. |
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