Joint preparation

Successful edge and face gluing

By Jaye Schroeder

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

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A small bead of squeeze-out around the perimeter of the bottom panel in the stack is always ideal.

Good joint preparation begins with good stock. Store lumber at 30 to 40 percent relative humidity to ensure moisture content of six to eight percent. Higher humidity can lengthen 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, molded or jointed stock should be free of knife marks. Ideally, woodworkers prepare and glue joints the same day. It’s best to prepare and glue joints the same day to reduce the risk of moisture affecting the cut wood and prevent oil to surface in freshly cut oily wood, such as teak, before gluing.

The amount of adhesive spread on the wood pieces matters. Too little of it can result in weak joints, and too much wastes glue, and money, requires additional clean-up and can cause ill-fitting joints. As a rule of thumb, 35 to 50 pounds of adhesive per 1,000 square feet, or 170 to 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.

Franklin Feature-1Selecting the right clamping pressure

Type of wood or other material and level of joint preparation dictates the amount of pressure to be applied to glued stock. Beyond that, though, most experts recommend direct contact with the gluing surfaces during clamping to ensure the strongest bond. A compressometer accurately measures pressure to the gluing area, and the clamping pressure chart (see below) provides general guidelines for suggested clamping pressure with woods of low, medium and high wood densities.

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 to 90 minutes and are best determined under plant conditions that replicate actual circumstances.

Troubleshooting joint problems

The causes behind specific bonding problems vary greatly. A weak joint, exhibiting little or no wood failure, may be the result of poor machining or under-cured or frosted joints. Too abrasive of a grit or dull saw blades are typical culprits behind weak joints that suffer glue failure along the glue lines.

On the other hand, imbalanced pressure or uneven adhesive application are probable causes for partial wood 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 the woodworker has followed the above guidelines for joint preparation and gluing and continues to experience bonding problems, a good next step is to consult the adhesive distributor or manufacturer for expert technical support.

Jaye Schroeder is vice president of wood adhesives market development in North America for Franklin Adhesives & Polymers.

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