Hot-pressing of oriented strandboard by step-closure

The vertical density profile (VDP) is an important panel property that desc ribes the change in density through the panel thickness. The panel industry has effectively used press closure rate to manipulate the VDP to alter pro duct performance. Using a slow press closure rate to manipulate the density profile has some obvious limitations related to resin precure, density pro file symmetry, and total press cycle time. Traditional press schedules cons ist of a single closing step until the press reaches final board thickness or position. In this research, OSB mats were pressed using both two- and th ree-step closing schedules. Mats were pressed to 110, 105, or 95 percent of target panel thickness and held at that position for times ranging from 20 to 120 seconds before closing to final thickness. For the three-step sched ules, the mat was compressed to an additional intermediate position before final closure. The step closure schedules significantly changed the traditi onal shape of the vertical density profile of the laboratory-made OSB panel s. The step schedules resulted in multiple surface densification peaks of t he VDP rather than the traditional two density peaks of the VDP associated with conventional pressing. Step pressing schedules are one processing meth od that can be used to alter the densification process in the panel and sub sequently influence panel physical properties. Results are unique in that b oth in-situ density profiles and density profiles measured after pressing a re shown for different step-closure conditions and a comparison is made to traditional closing schedules. The physical properties for panels produced from each step-closure schedule are also shown. The step-closure procedure is one method that can be used to balance the structure of the panel densit y within the mat and in the resulting panel.