By Kent Pitcher
End joint gluing of lumber is an important method for producing long length stock for fabrication into structural products such as trusses, rafters, beams, etc.
More recently, the technique has provided the means by which long pieces of material can be made from shorts and/or reclaimed off-grade stock for use in millwork products, furniture, and flooring.
Although there are a number of end joint configurations, the three common types are butt, scarf, and finger joints.
Butt joints are formed merely by butting the squared ends of material to be joined. A well prepared glued butt joint will exhibit at the most 25 percent of the tensile strength of clear lumber.
Consequently, butt joints are seldom glued and when used are located in multi-ply laminates in areas of little or no stress.
Scarf joints are made by chamfering, notching, or cutting away the ends of stock to correspond (or fit) to each other and subsequently securing them together by overlapping and gluing.
A well designed scarf joint, depending on the slope of the joint, will exhibit tensile strengths of 65 to 90 percent that of clear lumber.
Finger joints are defined as a series of fingers machined on the ends of two pieces to be joined which then are meshed together and held in place by an adhesive.
Generally speaking, the method of preparation for finger and scarf joints is accomplished by first squaring of the end with a cut-off saw and then cutting the joint with a high speed cutterhead. The joint must be machined to a close tolerance to insure even pressure during the gluing operation.
Voluntary Product Standard PS 56-73, "Structural Glued Laminated Timbers", is a commonly referenced standard for structural finger jointed stock. Under this standard, adhesives are classified according to the intended exposure conditions.
Wet use adhesives describes products expected to be exposed to moisture content conditions of 16 percent or greater.
For interior applications where moisture content exposures will be less than 16 percent, the term Dry Use is used to describe acceptable adhesives.
Wet Use adhesives must exhibit exterior durability as per ASTM Test Method D-2559, "Adhesives for Structural Laminated Wood products for Use Under Exterior (Wet Use) Exposure Conditions".
Dry Use adhesives, on the other hand, must meet the less stringent interior requirements of ASTM Method D-3024. Adhesives classified as Wet Use generally are those used in structural finger joint gluing.
Adhesives meeting the Wet Use requirements of PS 56-73 also will be suitable for finger joint gluing as per other specifications such as those of the Southern Pine Inspection Bureau or Western Wood Products Association for finger joint gluing of studs and joists.
For nonstructural finger or scarf joint gluing of products such as moldings, door stiles and railings, or window casements, adhesives can be classified as either Wet Use or exterior or Dry Use for interior applications.
Specifications by the Western Wood Molding and Millwork Producers (WWMM) and National Woodwork Manufacturers Association (IS 1-76 for Wood Flush Doors and IS 2-76 for Wood Windows) reference ASTM Method D-3110, "Adhesives Used in Nonstructural Glued Lumber Products", for adhesive performance requirements.
Wet Use under the context of ASTM D-3110 covers adhesives intended for use under exposure conditions of 16 percent equilibrium moisture content or greater, and temperatures as high as 165F (74C).
Dry Use covers adhesives where continuous service conditions are expected to be equilibrium moisture contacts of less than 16 percent and temperatures of no more than 140F.
The application of the adhesive to the joint is usually done automatically by a set of metal fingers or a metal fingered roll coater. In cases where this equipment if not available, the adhesive may be applied by a stiff bristled brush. Spreading usually is single, although in isolated instances double spreading may be required.
A light spread of thick glue will generally give a better bond than a heavy spread of thin glue. A light squeeze out is evidence of sufficient spread.
Joints should be assembled and pressure applied as soon as possible after spreading to reduce excessive end grain penetration.
Methods of Curing
The glue line of the end joints may be cured at room temperature, by hot platens, such as a Panel Mint, or by radio frequency heating. Scarf joint glue lines are cured by use of a heated "hot shoe" or more frequently by radio frequency heating.
Cold Set (Room Temperature Curing)
Joints are usually assembled mechanically by some type of crowder unit. The assembled joints are held under pressure for only a short period of time. The jointed members are then stacked until curing is complete.
At least 24 hours should elapse before machining. The adhesive used in the production of cold set finger joints should have the following characteristics:
1. room temperature curing
2. good initial tack
3. fast setting
Since pressure is not maintained throughout the entire curing period, cold set finger joints will seldom meet AITC requirements for bending strength and wood failure.
Polyvinyl resins have proved most successful in the bonding of cold set finger joints for nonstructural applications (molding, door jambs, etc.).
Radio Frequency Heating
The two types of radio frequency units that are presently in use are the tunnel or oven type and the double platen press. Finger jointed stock often is cured using an R.F. tunnel. In this unit, jointed members are pushed together by a crowder roll unit and passed through the R.F. tunnel. An R.F. field is continuously generated in the tunnel. The curing cycle is regulated by the speed with which the joint is passed through the tunnel, i.e. the length of time the joint actually stays in the R.F. field.
An R.F. press may be used for curing the glue line of either finger or scarf jointed stock. In units of this type, the joint to be glued is held together between two R.F. electrodes. Horizontal thrust and top pressure are maintained in the joint by a series of clamps and air cylinders throughout the entire curing cycle. The curing operation of this unit is similar to that of perpendicular heating R.F. machines used in the manufacture of curved plywood. Unfortunately, it is almost impossible to calculate curing cycles required because of the small amount of wood and adhesive involved in the actual heating operation.
The best indications of a completely cured joint are a dry joint on the outer fingers of the assembly and a brittle squeeze out of glue.
Should arcing occur, the following should be checked:
1. See that the platens are clean (often particles of squeezed out glue will build up on the platen causing a short).
2. Has the lumber been treated with a salt preservative? Salt treated lumber sometimes is extremely difficult to bond in an R.F. unit.
3. Is the moisture content too high?
4. If the arc is through the glue line - reduce the R.F. power and lengthen time cycle accordingly or use a less selective glue mix.
Stream or electrically heated platen presses such as a Panel Mint, although not commonly used, may be employed to cure the glue lines of finger or scarf joints.
Liquid Life of Adhesive
Keeping the mixed glue at a low temperature, 40-50F, will increase liquid life considerably. The resin itself can be kept in cold storage compartments. Cold water, 40-50F, can be used for mixing. The completed mixtures can then be kept in cold cabinets and water-cooled reservoirs.
Although very cold glue mixes require extra time or increased R.F. power for curing, this increase will be negligible. Pumping the adhesive through an applicator continuously may shorten the pot life somewhat.
This article was provided by Kent Pitcher of Custom-Pak Adhesives, Inc.