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Cross laminated timber element (Illustration)

Production process

Cross laminated timber

Cross laminated timber (CLT), BSP, also KLH in German, is a solid wood construction material produced out of at least three crosswise glued layers. The layers each consist of one-ply slat boards. According to the same principle as veneer plywood, the crosswise construction enables a high dimensional stability: Each wooden layer prevents dimensional stability changes to the individual boards from humidity deviations in the wood in the neighbouring layer that is at a right angle to the others.

CLT is mainly used as a ceiling and wall element in timber construction. It is also used in temporary path construction, such as when harvesting oil and gas away from paved ways (rig mats). In residential construction, as well as in building and industrial construction, it  is used as statically load-bearing and non load-bearing elements. CLT can be used not only for outer and interior wall construction, as well as roof and ceiling elements, but also for staircases and balcony slabs. It is frequently used for extensions on top of existing buildings because of its low weight. Insulation, facing shells, and facade elements can easily be attached to CLT.

CLT can also easily be combined with other materials, for example concrete. This makes it an ideal material for multi-storey wooden construction. In Vancouver, Canada, you can currently find the highest house made of CLT (March 2018). It is 18 storeys and 53 m high. Panels with three to seven layers and a total thickness of up to around 50 cm are customary. It is possible to produce panels up to 18 m in length and 3 m in width. Other common names on the market are KLH, thick wood or X-Lam.

If it is planned, built, and protected against weathering and humidity in the correct way, a CLT building can last for centuries. The CLT market is growing incredibly dynamically all over the globe. In 2016, production capacity was around 800,000 m³. The aim is to increase this by around 2 million m³ by 2020. In 2017 alone,  at least 30 new production sites were built. 

Source product

CLT is made of board slats, just like plywood boards, that have been dried to a certain wood humidity. Typical construction wood is used, e.g. conifers, such as spruce, pine, Scots pine, larch or Douglas fir tree. In rare cases, deciduous wood is used. 

Production Process

  • slat production

    Finger-jointed slat (Illustration)

    Pre-planing

    The board slats are pre-planed on four sides to ensure an even cross cut.

    Machines:
    Moulder

    Tools:
    Planer head

    Sorting timber

    The pre-planed and technically dried boards are sorted according to stability visually or by machine. The covering slats are maybe additionally chosen according to optical criteria.

    Machines:
    Sorting line

    Optimising

    When sorting the slats, flawed areas with stability decreasing or ugly deviations (e.g. large branches, pitch pockets, bark inclusions) are cut out, depending on stability and surface class.

    Machines:
    Optimising cross-cutting saws

    Tools:
    Circular saw blade

    Finger-jointing the slats

    The individual board slats are joined together longitudinally using a finger-shaped, frictional glued connection, a finger-joint. This makes an endless board out of individual short components.

    Machines:
    Finger-jointing line

    Tools:
    Finger-jointing router

    Planing finger-jointed slats

    The endless slats are planed to a uniform thickness to ensure a surface that can be glued.

    Machines:
    Moulder

    Tools:
    Planer head

    Slitting to panel dimension

    The endless slats are cut to length.

    Machines:
    Cross-cutting line

    Tools:
    Circular saw blade

  • Layer creation

    Layers of a cross laminated timber panel (Illustration)

    Laying board layers

    The board layers are laid manually or automatically.

    Machines:
    Laying station

    Surface gluing

    The board layers are glued. Common gluing systems are PUR (Polyurethane), MUF (Melamine-urea-formaldehyde-resin) and EPI (Emulsion-polymer-isocyanate).

    Machines:
    Coating line

  • Pressing

    Cross laminated timber element after pressing (Illustration)

    Press

    The complete cross laminated timber element is pressed hydraulically. The press predefines the element's maximum dimension. There are narrower presses in which standard measures can be mass-produced, but there are also very large presses, such as for example for complete wall elements that are floor high.

    Machines:
    Press (Vacuum press, HF press, hydraulic, pneumatic press)

  • Final processing

    CLT element (illustration)

    Calibrating (sanding, planing)

    The panel is sanded or planed to thickness. Remains from the pressing process, such as prints and surplus glue, are removed to create a clean surface. 

    Machines:
    Planer, Wide belt sander, Parquet sanding machine

    Framing

    The panel is optimised on all edges. Furthermore, other framing work, such as implementation of steel parts or cut-outs for windows/doors, are often already performed by the CLT producer.

    Machines:
    Framing line, Hand circular saw

Production process:Cross laminated timber

Tools for production process Cross laminated timber1

Besides machinery, also additional devices and gadgets are used in woodworking. Machine tools for Cross laminated timber production are listed here.

  • Cutters with bore, HW- / special steel-tipped

    E.g. used in:Wooden stairs, Cross laminated timber

Necessary peripheral devices for Cross laminated timber3

  • Chips and dust extraction systems (including filters)

    E.g. used in:Windows, Wooden stairs, Cross laminated timber

  • Conveyors for sawn timber (roller conveyors / tracks, cross-transfer units, etc.)

    E.g. used in:Glued laminated timber, Cross laminated timber

  • Infeed units

    E.g. used in:Carcass furniture, Furniture front, Flooring, Glued laminated timber, Cross laminated timber

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