Impression Finger Jointing of Lumber

Impression finger joints are formed by pressing small, precisely dimensioned fingers of a heated die into end grain of wood under high end-wide pressure. Tips of the die fingers cleave wood along the grain and thereby develop parallel-grain gluing surfaces. Wood between the die fingers is compressed laterally and densified by wedging action. Heat from the die serves to plasticize the wood, facilitating compression and stabilizing the densified wood. Thus, wood fingers with thin tips are molded to the same precise geometry as the die, and highly efficient gluing is possible. Two distinct types of impression joints were developed. Type 1 joints are formed by simple die impression in square-cut end grain of wood, while Type 2 impression is preceded by a joint preshaping operation. With both types, the ends of the fingers may be either perpendicular or parallel to the wide face of the board. As a result of promising preliminary research, a pilot program was sponsored at Washington State University. A pilot machine was designed and built for end-jointing 2 by 4’s of low-density species, such as redwood and western redcedar, with Type 1 joints and structural species, such as Douglas-Fir and western larch, with Type 2 joints. Strength values for Type 1 joints in redwood and cedar averaged about 5,000 to 6,000 psi. Thus, the Type 1 joints in these species would serve for many uses in which maximum strength is not a prime criterion. Type 2 finger joints met the most stringent requirements for high strength in laminating member. The average tensile test values for Type 2 joints in Douglas-fir and western larch ranged from 9500 to 9700 psi. These values exceed the average strength required by the A.I.T.C. for qualification of end joints in select structural grade lumber. Moreover, test values in each of the experiments with. Douglas-fir and western larch exceeded all the A.I.TC. requirements for structural end joints.

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