OSB (Oriented Strand Board) is an engineered wood-based product, like particleboard, primarily used in the construction industry, and because of its inherent load-bearing properties, it is commonly used as sheathing for walls, floors, and decking for roofs.
According to the APA-The Engineered Wood Association OSB boards are graded by their mechanical and moisture resistance properties.
The four types of board classification used in this standard are:
- OSB/1 — Boards for interior fixtures (including furniture) and General-purpose boards for use in dry conditions
- OSB/2 — Dry conditions, Load-bearing boards
- OSB/3 — Humid conditions, Load-bearing boards
- OSB/4 — Humid conditions, Heavy-duty load-bearing boards
So, depending on the intended use for the OSB boards, the manufacturing process is altered to suit.
What is OSB Sheathing
Sheathing is the supporting structure that acts as a cover for the surfaces of a building. The main function of sheathing is to provide a surface where other materials can be applied to, either on either floors, roofs, or walls of structures. It also provides additional structural integrity to buildings.
Oriented strand boards are increasingly being used by builders for sheathing and have become more popular in the building and construction field. Sheathing is done to strengthen a structure or provide resistance to harsh weather conditions.
OSB sheathing is used in a broad range of applications. You can read more about the Uses of OSB Sheathing Here.
How is Oriented Strand Board made?
OSB is made from, sustainable, fast-growing trees such as poplar, aspen, yellow pine, or black poplar, which have an average trunk diameter of 10 to 12 inches, and from eight and a half feet to twenty-four feet long. When the trees have been felled any branches are removed, and they are taken to the mill and stacked ready to be processed.
Processing steps in the manufacture of OSB.
The first step is to remove the bark since the bark has no structural properties that can be incorporated into OSB. Short sections of the logs are washed and then put in a debarking machine, a high-powered industrial piece of equipment that strips all the bark from the log. High-speed, rotating blades or knives are used while the log is pushed through the machine, removing all the bark from the log.
The logs are then fed into a machine called “a strander”, which has anywhere from 30 to 50 knives, either in a ring or a fan configuration, where the logs are “sliced and diced” into strands. These strands measure approximately three to six inches in length, half to one inch in width, and 0.02 to 0.03 inches thick. The sizes of the strands can be adjusted depending on the specifications of the boards being produced.
Next, the strands have to be dried since the moisture content can range from 40 to 75 percent. Large rotating single-pass or multiple pass dryers are used to dry them down to three to five percent moisture content. The temperature of the dryer’s entrance may be as high as 1,500 degrees Fahrenheit and a mere 200 degrees Fahrenheit at the exit point.
The next process involves separating the dried strands into two sections by using different screen sizes. The bigger strands are destined for top or face layers of OSB boards and the smaller strands are used for the core of the boards.
The face layer strands are mixed separately from the core layer strands in a rotating drum blender with a mixture of wax and synthetic adhesive resins such as diphenylmethane diisocyanate, isocyanate-based glue, or phenol-formaldehyde resin to bind the strands together, ready to be sent to the orienters.
The mixture of wax and resins can be varied to suit the finished product’s application. If the OSB boards are to be used as roof sheathing, they will have a more water-resistant mixture of resins and wax than for a flooring board.
The orienters, which are spinning disks, align the two separate sized strands in opposite directions. The larger or face strands are aligned to be parallel with the finished board’s strength axis, and the smaller or core strands perpendicular to the axis.
7. Mat formation
In what is known as a “forming line”, the strands are dropped in sequence onto a conveyor belt, to form a layered, three to eight-inch mat. The forming line is generally six to eight feet wide but can be as wide as twelve feet depending on the OSB mill.
The finished product determines the number of core and face layers needed. For example, to obtain a half-inch thick OSB board, the layered mat would be approximately six inches thick. Also, the ratio of core to face strands will be changed depending on the type of board required.
8. Mat trimming
The mat is then transported to be pressed, but it must be trimmed to fit the mill’s presses. A cross-cut saw removes the excess mat before it enters the press. Once cut to length, individual mats are loaded into a thermal pressing machine, which can have from 10 to 20 layers.
After all the mats are in the press, it’s time to turn up the temperature. A combination of heat and pressure is applied to the mats for roughly four to six minutes. Approximately 600 to 800PSI of pressure causes the strands to come in close contact with each other, and the 400 to 425 °F of heat cures the resin that was applied to the strands.
The closed-in press machine openings each have heated rollers that compress the mats to a thickness of between 3/8-inch to just over an inch thick and when they exit the pressing machine, the temperature of the boards can be over 300 °F.
10. Cooling and cutting
From the press, the boards are put through a rotating cooling process before moving on to the cutting line where multiple saw blades cut each board to the required dimensions. The majority of OSB boards are cut to four feet by eight feet.
11. Further steps
After cutting, the boards may require additional treatment, such as if the boards are for flooring they get tongue and grooved. For roof applications, the boards can have a raised pattern to prevent slipping, or for exterior wall applications, a radiant barrier can be laminated to one side to increase energy efficiency.
12. Stack, seal and ship
Single boards are stacked into units and head to the paint booth to have sealant applied to the edges and the ends, then each of the units is strapped ready to be shipped.
From the time the logs are brought into the mill for debarking, until finished OSB boards are stacked, ready to ship, the whole process takes roughly about an hour to complete.
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