Technology Roadmap: Lumber and Value-Added Wood Products
7.3.2 Finger-jointed Lumber

Finger-jointing is a wood processing technique that consists of cutting finger profiles into the ends of wood pieces, and joining them into longer pieces with an adhesive. The finger-jointing process began in the United States in the 1920s, and was mainly used in the automobile industry. It was later adopted by the glulam industry to replace the more wasteful scarf joint. Finger-jointing has now become a way to convert short wood lengths and low-grade lumber into high-performance and highvalue products.

Finger-jointed material for both structural and non-structural uses is now commonly produced by the lumber industry and well accepted by the building industry and by trade organisations associated with these industries. This section will focus on structural finger-joints, while appearance joints will be covered in Section 8. In some instances, finger-jointed lumber is actually preferred to solid, unjointed lumber because of its superior stability.

The potential strength or performance of a finger-joint is an interdependent function of the slope, the pitch, the finger length and the tip thickness. Ideal joints of maximum strength are difficult to produce economically in wood but as the tip approaches zero, and the slope and length are such as to provide an adequate effective glue joint area, joint strength will approach the tensile strength of solid wood (around 90 percent).

There are three main types of structural finger-joints: vertical, horizontal and inclined at 45 degrees. Vertical finger-jointing (fingers cut parallel to the narrow face) is mainly used in Europe and permits operating the finger-jointing process at up to 60 blocks per minute, while horizontal jointing (fingers cut parallel to the wide face) is used in North America and permits 60 to 140 blocks per minute. Under normal production conditions, the vertical profile is generally stronger than the horizontal profile and it is used for glued laminated timbers; while the horizontal profile tends to be used for fingerjoined lumber.

While finger-jointing is most commonly performed on dry material, there are economic advantages to jointing blocks in the green state: increased value for trims blocks; reduced chip production; higher product value for low-grade material; and more efficient use of kilns (medium temperature). With modern adhesives technology, it is possible to finger-joint pieces of lumber with moisture contents up to 180 percent. It is even possible to finger-joint dry and wet pieces, whether or not they are frozen.

Raw Material

Existing Technology

Raw materials for finger-jointing are generally dry trim ends from the planer mill. Given current fibre supply constraints and waste disposal issues, green trim ends also represent a valuable resource base if they can be put to a better use than hog fuel or chips. Finger-jointing is typically performed on softwood lumber since hardwood lumber is hardly used in North America for structural building components other than composites. Hardwood could, however, be used in I-joists since it could provide superior tensile performance in the lower flange.

Incremental Technological Innovation

• Develop cost-effective techniques to manufacture vertical joints in green and dry lumber
• Adapt structural finger-jointing techniques to hardwood lumber, and have them accepted in the relevant standards.
• Develop and implement non-destructive evaluation techniques to grade short lumber pieces so they could be sorted out and assembled with other pieces of the same strength, thereby yielding a higher proportion of high-strength lumber. Other sorting criteria could include dimensional stability (for example, compression or juvenile wood, excessive slope of grain), moisture content, etc.

Material Handling

Existing Technology

Feeding blocks to the machine currently requires one or two operators, and stacking the boards at the outfeed requires one operator. Automatic feeders are used to place the blocks on the lugs to increase productivity. The trend in a number of mills has been to joint ever shorter blocks as lumber value increases and chip value decreases; this will further complicate the task of automating handling operations, but makes it even more necessary if productivity is to be maintained.

Incremental Technological Innovation

• Develop and implement semi-automatic or automatic infeed and outfeed systems capable of handling short blocks fast enough to make full use of jointer production capacity.
• Develop new methods to facilitate or automate machine set-up time. This will become even more critical when the infeed process is automated.

Block Preparation and Finger-joint Shaping

Existing Technology

Prior to shaping, blocks to be jointed need to be sorted and defects cut out to specific quality criteria, as end sections must be perfectly square and free of defects.

Finger-joint profiles are manufactured with cutting tools (a revolving head made up of a series of stacked knives, shaped saw blades or a head holding replaceable bits). Two major causes of poorly fitting finger-joints are improper finger length and improper cutter bevel. Critical cutting parameters are the cutting speed, the feed rate and the depth of cut.

Incremental Technological Innovation

• Develop and implement a two-stage finger-jointing technique, whereby lumber pieces are assembled in a preliminary jointing operation that develops enough strength for handling and reprocessing.
• Develop and implement a computer-assisted vision system to detect and reject pieces with defective fingers, and to check adhesive application and spread, in order to eliminate weak joints and facilitate maintenance and set-up.

Breakthrough Technological Innovation

• Develop and implement jointing technique combining finger-joints and another feature (for example, dowels, synthetic fibres, new adhesives) to reduce finger length in structural joints.

Adhesive Application and Pressing

Existing Technology

With soft roller applicators, the adhesive is continuously applied to grooved rollers. The adhesiveimpregnated rollers transfer the adhesive to the material to be coated. The quantity of adhesive is regulated by adjusting the distance and the pressure applied by the roller to the wood.

With spray adhesive applicators, the adhesive is atomised and transferred to the wood. Since the system is not sensitive to the position of the surfaces, it can reach the hidden horizontal planes of irregular pieces. The quantity is regulated by the system pressure and the characteristics of the adhesive.

With comb applicators, a profiled comb extrudes the adhesive to the wood. The spread is regulated by a computer which controls the starting and stopping times for adhesive application to each block.

In the most popular pressing method, the finger-jointed blocks are assembled in a continuous manner then cut to a fixed length, and pressed in the three plane directions into pieces of engineered material.

In large continuous operations, the preferred pressing method relies on the speed differential between two sets of rollers: the front rollers run more slowly than the back rollers, which has the effect of exerting a continuous pressure on the assembled engineered piece of wood.

Until recently, adhesives were water soluble and had to be applied to dry wood fibre. If water were present in the wood fibre cells, it would dilute the adhesive, causing it to wick beyond the joint and weaken the bond. Considerable research effort has gone into developing adhesives capable of gluing wet wood.

Incremental Technological Innovation

• Develop block positioning and assembly methods sufficiently accurate to eliminate the need for planing after the jointing operation (with attendant dimension problems).
• Develop and implement a method to determine and control actual pressure exerted on the joints.
• Explore the potential of surface activation as a means of improving adhesive performance (wetting of opposite surface).

Adhesive Curing

Existing Technology

The method commonly used in industry relies on radio-frequency (RF) heating. Its main advantage is that it tends to concentrate energy where there is conductive material, so that a water-containing adhesive constitutes an easy path for RF energy. The heat build-up within the glueline is highly dependent on moisture content and wood density, which means that wood moisture content needs to be sufficiently uniform to avoid power dispersion.

Some western Canadian operations rely on a curing technique developed by the Western Forest Products Laboratory (now Forintek Canada Corp.), which involves drying the fingers to an acceptable moisture content before the resin is applied and cured. Then the two pieces are assembled, pressed, and they achieve 90 percent adhesive cure within three minutes, which is enough to allow stacking. This method has been used in industry for 20 years, and it is currently being applied to the fingerjointing of green lumber shorts in two British Columbia operations. The resulting joints meet all the requirements of NLGA SPS 1 and SPS 3 standards.

Incremental Technological Innovation

• Develop new adhesive systems for dry lumber that combine ease of application and reduced energy usage, and meet the requirements of structural standards.
• Optimise adhesive systems under development for green lumber, and ensure adequate recognition in structural standards.
• Develop and implement closed-loop adhesive curing systems capable of adjusting energy as a function of wood moisture content and other critical factors.
• Develop on-line finger-joint evaluation systems to test every joint and eliminate the need for offline proof-testing.