While out West Bob and the crew visited the Simpson Overlaid Plywood mill in Shelton, WA—about 50 miles across Puget Sound from Seattle.
65% of the wood used at this plant is Douglas Fir, the rest is Hemlock—both because these woods are readily available and also because of their strength characteristics and the grade requirements of some of the high end products manufactured by Simpson.
95% of the logs used by Simpson are second growth harvested from company owned land. Over fifty years ago Simpson began an active tree planting program working with nature to grow, harvest, replant, and grow trees again.
The logs which arrive by truck, railroad, or water are carried by a Wagner (or large log loader) to the infeed deck of the debarker. Although the Nicholson Ring Debarker can handle logs up to 60" in diameter, today the average log is approximately 10". Once the logs are stripped of their bark, they are cut into 103" long peel blocks using a 60" diameter radial arm cut-off saw.
Next the debarked logs are carried by log loader to the steam vaults where they are conditioned and softened. 185 degree water is sprayed on the logs for anywhere from 10 to 48 hours depending on their diameter until their core temperature is 120 degrees and they are ready to be delivered inside the plant to the lathe infeed for the peeling process.
The veneer is peeled from the logs by a lathe as the log spins at up to 1500 revolutions per minute. Between 400 and 500 logs are peeled in an average eight hour shift. From the lathe the veneer travels down multi-level trays at 275 feet per minute for cooling on its way to grading stations. There electronic scanners detect defects which determine how the veneer will be cut and sorted.
Once sorted and stacked the veneer is hand-fed into the dryer where it is dried at up to 375 degrees F down to the required moisture content. For softwood this takes about 4 minutes. Once dried, the cream of the crop goes to the pluggers where all knots are cut out and replaced with football shaped veneer patches of equal thickness. This process ensures a smooth, solid surface for the face sheets that go directly under the paper overlay.
At the next station, the veneer is passed through the spreader where phenolic glue, made out of phenolic resins and additives, is applied to both sides at the same time. (The phenolic resin adhesive is what makes the MDO plywood waterproof and classified as exterior grade.) The weight of the glue is measured for accuracy by a digital scale to ensure consistency.
In a seemingly choreographed display of teamwork and timing the layup crew stacks the veneer alternating the grain direction: backsheet, crossbands, centersheet, crossbands, and so on until the necessary layers are assembled. The final layers are a face sheet and overlay paper topped with a tin separating sheet and then the process begins all over again.
Once stacked the panels are transported to the hydraulic press where they are loaded into press openings and compressed at 200 psi for a predetermined time at 285 degrees. After a cooling cycle it's off to the Saw Line where they are trimmed for size. After trimming they go to the grading station where they are inspected on both sides to ensure quality. Now ready for packaging, they are transported to the warehouse or on to a truck or into a boxcar for shipment anywhere in the world.
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