Crosslam timber / CLT: Manufacturing process

1 Species selection  

The main species currently used for CLT is spruce. Though Scots pine, larch and Douglas fir are also available. Swiss stone pine (Pinus sylvestris) can be used in in an outer layer to provide a high quality finish to exposed panels.

2 Drying

Planed boards of between 12 and 45 mm thickness kiln-dried and conditioned down to a moisture content (MC) of 12% +/- 2%

3 Strength grading

Boards are graded in the range C16 to C24. C24 use is still more common, though C16 timber is more readily available in the UK – suggesting that CLT manufacturing might become more feasible in the future.

4 Visual grading

Surface quality is defined by BS-EN 13017-1 Solid wood panels. Classification by surface appearance.  

There are three different grades:

1. Residential Visible – planed and sanded
2. Industrial Visible  - planed and lightly sanded
3. Non-Visible – planed

5 Removing defects

According to the strength and final visual quality, defects such as large knots and resin or bark pockets are cut out.

6 ‘Endless’ lamellae

Individual boards can be joined to make, in principle, an endless lamella (layer) using finger joints to marry the boards. The lamellas are then planed to a uniform thickness.

(Layer assembly: Some manufactures assemble individual layers or plates at this point – ahead of forming the panel.  Layers are assembled by bonding along the edges of each lamella up to the desired dimensions. Panels are then built-up from these individual layers)

 

7 Panel assembly

Panel sizes vary by manufacturer and application, CLT panels can be manufactured in 3, 5, 7 or more board layers with typical widths of 0.5m, 1.2m, and 3m and lengths of up to 18m long. Transport by lorry is the ultimate limiting factor and in the UK restricts practical lengths to 13.5 m. Generally panels are manufactured up to 300mm in thickness, but larger dimensions are not unknown.

The outer layers of the panels are usually orientated to run parallel to the span direction. That is, for walls that are normally oriented, the outer layers of the CLT panels have the grain direction parallel to vertical loads to maximise resistance. Likewise, for floor and roof CLT panels the exterior layers run parallel with span direction.

The lamella strips are spread with adhesive and then adhered perpendicularly to the lamellas of the adjacent layer. Effective adherence is ensured through using either vacuum or hydraulic press techniques.

The completed CLT panel is trimmed along the edges.

8 Completion

The completed assembly is then planed and/or sanded before transfer to a machining station where a multi-axis machine cuts out openings for windows and doors in walls and staircase openings in floors.

 

Adhesives

Surface-bonding adhesive is usually applied mechanically and without contact either on (i) single lamellas in a continuous through-feed device or (ii) on complete CLT layers already positioned in a positioning or press bed. 

A range of glues have been used historically to bond engineered timber. The biggest initial development was that of waterproof phenol-resorcinol-formaldehyde (PRF ) resins in the 1940s. This allowed engineered timber to be used in external environments without threat of degradation.

In the 1970s melamine-urea-formaldehyde (MUF) was introduced which improved on the PRF resins but became known to form rather brittle bondlines.

1994 saw the introduction by Purbond AG of the first commercially successful PUR  (one-component polyurethane) adhesives. Since then other manufacturers have produced other PUR types enough to dominate the adhesive market.

The development of PUR was important because of a number of significant advantages over previous adhesives:

• One  component – no mixing and easy handling

• No off-gassing of volatile organic compounds (VOCs) including Formaldehyde

• Solid content without solvents

• Reduced processing time resulting from fast bonding and reduced press time

If timber processing technology is your thing, this is the video for you!