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Contents:
The timber resource
The first step in the process of production is the obtaining of a wood resource. One of the advantages of OSB is that it allows for the utilisation of small and relatively low grade timber. Small log and short rotation plantation forestry are likely to be popular trends in the future, making wood composite products of this strength and quality competitive players in the timber industry. One tree species commonly used to produce OSB is Aspen (Populus tremuloides) This was until recently treated as a weed (Beck et al. 1988). Now it is an important resource producing a valuable product.
The trees are felled, their branches are removed and they are cut to lengths suitable for transportation. They are then transported to the OSB mill (generally by truck) and stacked until they are needed. Finally they are loaded onto the log deck and the production process begins.
The Plantation Resource (Radiata Pine)
The logs length are loaded onto the production line (HFP video)
From logs to strands
The logs are debarked. Debarking takes place because bark is not wanted in the final product. It reduces the strength of the board and interferes with the setting of the resin. Strength of the board is reduced because the bark is weak and has fibres that are not the right length. There are a number of different debarking machines available for this purpose. Perhaps the most efficient one is the Cambio debarker such as that shown below. This machine efficiently removes bark without removing significant amounts of the usable wood fibre.
Logs being debarked by a Cambio Debarker (HFP video)
In the forest the logs were sawn into lengths that were practical for transportation. These lengths are too long for the machine that produces the wood strands so the logs are sawn into 'billets'. A circular saw is used to reduce the size of the logs to about 1.5m in length.
Logs being sawn into billets (HFP video)
The now short lengths of wood are able to pass lengthwise into a 'waferising' machine. These generally contain hundreds of serrated knives that rotate at several hundred rpm (Evans 1996). The waferiser reduces the logs into long, thin wafers (also called flakes or strands).
Logs being fed into a waferising machine (HFP video)
The strands produced from this machine (an example is a CAE disk cutter) are 0.5 - 0.7 mm thick and 19 - 38 mm wide and 76mm long. These dimensions allow the strands to be aligned further down the line. Wood fines less than 9.5 mm long are screened and 10% is added back to the mat furnish to create a smoother finish.
From here flakes go into a drum drier. The drum rotates and is usually heated by steam. In some processes the waferising machine produces long strips instead of flakes. These split during the drying process to the correct dimensions. The strands are dried down to a moisture content of 4.5- 6% as moisture affects resin uptake and bonding strength. Norbord Ltd (1991) OSB is conditioned to a minimum moisture content of 6% to minimise shrinking or swelling during use. They do not recommend that their be used in applications where it would be subjected to constant high humidity such as such as bathrooms and indoor swimming pools. (Picture source unknown)
Drum drier (HFP video)
Resin application
Phenol-formaldehyde is the resin generally used OSB manufacture is usually applied in powdered form. The resin is usually mixed the wood strands in a rotating drum. If it is a liquid resin it may be applied using high pressure sprays. The resin is applied at about 2-6% of the quantity of wood. Where the end product is to be used in an environment where there is a high relative humidity and low load, more resin should be used in order to reduce creep (Hse 1994) .The Phenol-formaldehyde resin most commonly used in is usually applied as a powder, while all other resins are applied in liquid form(Maloney 1977). The modulus of elasticity (MOE), Modulus of rupture (MOR), internal bonding (IB) and tensile strength (TS) of OSB improve as resin content increases from 4 to 6% (Avramidis 1989). However, quantities more generally used are around 2-3% (Maloney 1977). The resin does not actually coat the particles entirely, but are instead bonded together by small droplets that cover only small areas of the particles surface. A greater resin content would result in a stronger, more water resistant product. However, the resin is heavier and, more importantly, far more expensive than the wood component, so it is desirable to minimise the quantity of resin used. Waxes (at around 1% of binder) are often added to the resin to minimise the effect of moisture on the boards. Unfortunately these may interfere with the binder and reduce the strength of wet panels (Kubler 1980).Phenol-formaldehyde resin is applied in lower quantities than urea-formaldehyde (Maloney 1977).
Mat formation
The resin spotted wafers then have to be oriented either by an electrical field or, more commonly, are mechanically aligned by vibrating the particles through fins (Kubler 1980). Electrical alignment entails dropping the flakes through an electrical field formed between two oppositely charged plates. The electrical field then aligns them. The device that preforms this task is quite simple and is now being used in commercial applications. It also has the advantage over mechanical aligners as it has the ability to simultaneously align particles of many different types and sizes (Maloney 1977). The strands are aligned with the top and bottom layers running the length of the panel and the core strands running across the panel.
Strand alignment (HFP video)
A mat of strands is built up, with each layer being laid down separately along the conveyor belt. The final three ply mat (prior to pressing) is continuous and is between 100 and 200mm high. It is only loosely held together and has no strength. The resin will not begin to cure until it is heated.
Mat formation(HFP video)
Pressing and storing
The mat is then sawn into lengths and run through a prepress. This removes some of the air trapped in the mat. Both the prepress and the press vary their speed of compression to allow for air and water vapour to escape. Pressing forces the resin to spread over, and into, the wood fibres. It also makes the mat more stable as it enters the main press.
Prepressing the mat (HFP video)
The main press both heats the mats to around 205 degrees Celsius (Haupe 1994) and compresses them to a specified thickness. The heat and pressure produced by the press causes the resin coating the wood fibres to cure. The pressure also causes tangling of the fibres, increasing the boards strength. The panels are transformed from a thick, low density mats with almost no strength, to a high density, high strength boards within a matter of around 10 minutes (Avramidis et al. 1989). The panels are allowed to cool until the curing process is complete and an equilibrium moisture content is reached.
The final press
The boards are then stacked and packaged ready for sale. Additional processing of the panels depends on the customer the manufacturer sells to. If they sell to wholesalers, the product is not likely to be processed further than being cut to standard lengths. However, if they sell to more selective customers such as hardware stores, the panels may be made into more specialised products. Examples of specialised products may include painted panels, laminated panels and panels cut to non-standard sizes. Finally, the product is bought and used by the consumer. Click here for examples of end uses. (HFP video)
Stacking for storage (HFP video)
There are various hazards associated with producing Oriented Strand Board. Respiratory problems that may arise form inhaling the resin or wood fibres are covered by the site indicated. Procedures such as sanding or moulding of OSB are likely to present hazards of this kind. In these situations it is recommended that a face mask be worn.
References
Avramidis, S. and Smith, L.(1989). The effect of resin content and face-to-core ratio on some properties of oriented strand board. Holzforschung, 43(2), 131-133.
Beck, J., Constantino, L., Corns, I., Doucet, R., Phillips, W., Messmer, M. and Smith, S. (1988). Managing for aspen- a joint responsibility. Forestry Chronicle. 65(1), 16-35.
Evans, P. (1996). Wood Conservation and utilisation. Unpublished.
Haupe, R., Sellers, T., Kim, M., Miller, G. and Strickland, R. (1994). Comparisons of strandboard made with phenol-formaldehyde resin and resins modified with TVA acid- hydrolysis lignin. Forest Products Journal, 44(4), 63-68.
Kubler, H.(1980). Wood as a building and hobby material. Wiley and sons, Inc., Canada.
Maloney, T.(1977) Modern particleboard and dry-process fibreboard manufacturing. Miller Freeman Publications, Inc., California, USA.
Norbord Ltd (1991). Norbord Sterling- paints and finishes. Norbord Ltd. Norbord Ltd. Bristol.
HFP video. A promotional video for Highland Forest Products, Produced by Creative Television Co., UK.
Hse,C., Pu, J. and Tang, R. (1994). Creep behaviour of sweetgum OSB: effect of load level and relative humidity. Forest Products Journal, 44(11), 45-50.
Return to OSB Home Page
Created- 20/10/1996
Last Modified- 20/10/1996
URL: http://online.anu.edu.au/Forestry/wood/osb/Process.html