eee THE WESTERN CANADIAN LUMBER WORKER IN PLYWOOD INDUSTRY ULTRASONIC BLISTER DETECTION (Editor’s note: The Lumber Worker wishes to credit the magazine Forest Industries [July 1968 issue] for the text and pictures on the subject of Ultrasonic Blister Detec- tion in plywood manufactur- ing.) Glueline blisters are one of the softwood plywood indus- try’s hardest-to-spot manufac- turing defects, but not at Anacortes Veneer, Inc. For more than six months, this worker - owned plywood plant at Anacortes, Wash., has been using a new elec- tronic device to detect blis- ters and, according to gen- eral manager John H. Mar- tinson, virtually eliminate them as a factor in customer callbacks. The machine, manufactured by Automation Industries, Inc., of El Segundo, Calif., is known simply as the ‘“Ply- wood Blister Detector.” It is believed to be the first pro- duction line, nondestructive testing unit for that purpose. It is now available commer- cially. Blisters are air pockets be- tween the cross - laminated veneers in plywood. They can be caused by a poor applica- tion of glue, but more often they are the result of mois- ture in the wood. If not short- stopped before leaving the plant, a blistered panel is sus- ceptible to delamination on the job. “By the time the cus- tomer’s complaint gets back to the plant,” said Martinson, “you're helpless to pin down the exact cause.” “You can only wonder how many other defective panels might have been shipped that day,” he added. The blister detector re- places visual inspection as the most effective way to find blistered panels before they leave the plant. The old way, Says Martinson, was good for finding blisters along the edges, but it often failed to locate the ones concealed else- where in the panel. This is what the Plywood Blister De- tector, installed between the sander and the sorter-grader, does with far greater relia- bility and at a speed manual labor could never match. How does it work? The key is ultrasonics, says Automation Industries. The heart of the detector is two banks of rotary trans- ducers. The plywood panel travels flat through the ma- chine. Rolling over the upper surface of the panel is a row of 12 transducers which act as ultrasonic transmitters. The signal passes through the panel and is received by an- other row of transducers roll- ing along the bottom of the panel. A change in the den- sity of the panel, i-e., a blow, a knothole or a void, results in a reduction of the signal strength to the receiving transducer. Changes in signal strength are registered in the electro- nic unit in the control con- sole. This unit can be set to ignore voids under a certain size. A setting of four inches, for instance, will- mean that a three and one-half inch knothole will be ignored by the machine. But when a void larger: than the setting is registered, relays are triggered and the defect is automatically mark- ed by air and dye spray valves according to its size and shape. Seeing a marked panel approaching on the con- veyor, the sorter- grader shunts it into the reject pile. The machine, according to Automation Industries, can cope with plys varying from three: to seven layers. It will run at speeds ranging from 100 to 500 feet per minute. Top speed through the detec- tor is roughly one 4 x 8-ft. panel a second. It will detect defects ranging in size from 2-12 inches in length and the same in width. Martinson said installation was a one-day job. On an idle Sunday, when there would be CHANGES IN SIGNAL STRENGTH from the transmitting to the receiving transducers are monitored in ic unit of the above pictured control console, can be sent to ignore voids under a certain size. of four inches, for instance, will mean that a one-half inch knothole will not be dye-marked. \ no disruption of production, a two-foot length of the con- veyor was removed to make room for the machine. A con- trol console was installed at the sorting-grading station to permit that man to ‘turn the detector off and on without leaving his post. In other industries, ultra- sonic inspection has been widely-used for quality con- trol, particularly in the metals industry for examining brazed joints, and for inspecting welds, plate and billet stack. TWO BANKS of twelve transducers each Detector. One bank rolls over the surface of the machine transmitting ultrasonic signals to of the panel. A change in the density of the a blow results in a reduction of the signal st! But its application to plywood is new and required exten- sive development in Automa- tion Industries’ research lab- oratory at Boulder, Colo.. Some way had to be found to transmit ultrasonic energy through the wood without first immersing it in a liquid, as is the case with materials ultrasonically tested by the metals industry. The solution was the small, rubber-tired transducer, which eliminates the need for liquids, yet has the speed, abrasion-resistance and sensi- tivity characteristics required for plywood. | Prime mover behind the project was the Plywood Re- search Foundation, a subsid- iary of the American Plywood Association, Tacoma, Wash. PRF, under executive direc- tor Harold Evans, spelled out the basic need, searched out a developer and when the machine was ready, made ar- rangements for testing at Anacortes Veneer. at 8S: KEY COMPONENT: in ultrasonic blister detection in plywood panels is the small, rubber-tired “transducer” pictured above which transmits ultrasonic energy through- out the wood with the speed, abrasion-resistance and sensitivity characteristics required for plywood and at the same time eliminates the need for liquids as in the case with materials ultrasonically tested in the metals industry. form the heart of the Plywood Blister a plywood panel travelling “flat” through the opposite bank rolling along the bottom panel caused by a knothole, a void and/or rength from the transmitting transducer to the receiving transducer. Thus relays are triggered and the defect is automatically marked by air and dye spray valves. : : Bs DYE SPLOTCH wood industry’s (shown by arrow) j naar nnlity ) indicates control device. glueline blister detected by y ¥