Edge and face gluing solid lumber puts even the strongest adhesive to the test – and makes it all the more imperative for furniture manufacturers and cabinetmakers to properly prepare the joints to be bonded. Poor preparation can cause weak, ill-fitting joints that compromise product quality.
Here are some simple steps to help ensure strong and durable edge-and-face joints for well-built furniture and cabinetry.
Preparing the Lumber
Good joint preparation begins with good stock. Store lumber at 30 to 40 percent relative humidity to ensure a moisture content of 6 to 8 percent. Higher humidity can lengthen the clamp time and cause the panels to shrink as they dry, possibly causing cracks or end-joint delamination.
Further, the joints should be straight and square, free of saw marks from ripsaws. Likewise, moulded or jointed stock should be free of knife marks. Ideally, woodworkers should prepare and glue the joints the same day to reduce the risk of moisture affecting the cut wood and prevent oil from surfacing in freshly cut oily wood, such as teak, before gluing.
Applying the Adhesive
The amount of adhesive spread matters. Too little can result in weak joints, while too much glue wastes money, requires additional clean-up and can cause ill-fitting joints. As a rule of thumb, 35-50 pounds of adhesive per 1,000 square feet – or 170-250 grams per square meter – of glue line gets the job done.
Most plants will use a conveyorized spread to ensure proper adhesive application. A small bead of adhesive squeeze-out around the perimeter of the bottom panel in the stack typically is a good indication of proper application.
Selecting the Clamping Pressure
The type of wood and level of joint preparation dictates the amount of pressure to be applied to the glued stock. A compressometer accurately measures pressure to the gluing area, and the clamping pressure chart (left) provides general guidelines for clamping pressure with woods of low, medium and high wood densities.
The ideal clamp time is based on a number of factors, such as the type of adhesive, kind of wood, moisture content, environmental conditions and glue-line thickness. Clamp times can vary from 45-90 minutes and are best determined under plant conditions that replicate actual circumstances.
Troubleshooting Joint Problems
The causes can vary greatly. A weak joint, exhibiting little or no wood failure, might result from poor machining or under-cured or frosted joints, whereas too abrasive a grit or dull saw blades are typical culprits behind weak joints that fail along the glue lines. On the other hand, imbalanced pressure or uneven adhesive application are probable causes for partial failure in some joints. Likewise, possible causes of under-cured glue lines range from too short of a cure cycle, too low of a plate current setting, too high of a moisture content or insufficient pressure.
If, after following the above guidelines there continues to be bonding problems, consult the adhesive supplier for technical support.
Author: Jaye Schroeder is vice president, Wood Adhesives Market Development North America, at Franklin Adhesives & Polymers. For more information visit FranklinAdhesivesAndPolymers.com
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