Effect of pre-press pressure on the dry shear strength and wood failure of two different type plywoods.

The qualities of plywood especially on its mechanical properties are usually varied by the species of veneer wood used, their respective characteristics and the ways of their compositions; the type of glue applied and its spreading; the set conditions for the pre-press and hot press processes etc.. When the glue, its spreading, and the later set conditions for the hot pressing are kept unchange, the set pre-press (or cold press) condition should be a key factor influencing the properties of plywood produced. Generally, there are a few considerations on the proper setting of pre-press condition for the most plywood plants. For example, there have been many different settings on the pre-pressing at this plant. The applied pressure and the time for pre-pressing the assembled packages are usually varied between the shifts or plants. The time for prepressing however, often set, coincide with the required time for package assembling. The pre-press pressure however, is required to maintain the most appropriate level, in order to avoid glue spillage or improper anchoring. This experiment was purposed to investigate the effect of pre-press pressure on the properties of plywood produced, based on its dry shear strength and wood failure, and also it was intended to introduce the most optimum pressure for two line pre-presses in this plant. The experimental packages for 1st line were all 7-plys aspen poplar species, industrial grade, 17 mm plywood purposed with the pre-press pressures of 3000, 2500, 2000, 1500 and 1000 p.p.a. (or 190, 159, 127,96 and 63 p.s.i.); the packages for 2nd line were all 5-plys aspen poplar core and cross bands with Summomo (South America species) for two faces, industrial grade, II mm plywood proposed with the pre-press pressures of 2750, 2250, 1750, 1250 and 750 p.p.a. (or 175, 143, 111,79 and 48 p.s.i.). The time for pre-pressing the respective package for two lines was set at 10 minutes, the glue spreading was 80 gr./Sq.ft (two sides), hot press temperature was 275°F with 2800 p.p.a. (138.2 p.s.i.) pressure for 8 minutes identically. The first (bottom) and the eighth panels in the hot press for each condition for two lines, were selected for dry shear and bonding tests. The results found that the most optimum pre-press pressure was around 1500 p.p.a. or 95 p.s.i. for either the I st line or the 2nd line as Figure 2 showed. The specimens after the application of shear stress failed mostly at the weakest portion, regardless of its layer located in the core, cross bands or on the faces as in the case of 5-plys Summomo face plywood, diagrammed in Figure 3. The pre-press pressure consequently affects the dry shear stress positively by means of increasing glue anchoring inside the wood tissues of the veneer layers withe the raising of pre-press pressure.