A series of plywood and laminated veneer lumber (LVL) panels were prepared using veneers with higher than normal moisture contents in face and back layers. The purpose of the work was to evaluate the effects of self-generated steam on the pressing times and panel warpage. Panels made with 6% and 10% m.c. faces and backs were compared with control panels made with all dry veneer. Thirteen- ply 40 mm (1 5/8 inch) thick panels were evaluated for press times and thin 9.5 mm (3/8 inch) panels were evaluated for cupping and bowing. Normal plywood press temperatures and adhesives were used. All panels were made with incised 3.2 mm (1/8 inch) SPF veneers. The project demonstrated that substantially shorter press times and more dimensionally stable panels can potentially be made using higher moisture content outside veneers.
A series of plywood and laminated veneer lumber (LVL) panels were prepared using incised veneers in the second phase of this two year project. The primary purpose of the work was to evaluate the effects of steam injection on the pressing times. A secondary objective was to expand the study of warpage in three-ply and four-ply plywood which was begun in phase one. Thirteen-ply 40 mm (1 5/8 inch) thick panels were evaluated for press times and thin 9.5 mm (3/8 inch) and 12.5 mm (1/2 inch) panels were evaluated for cupping and bowing. Press temperatures of 150 degrees C, 175 degrees C and 204 degrees C were used with a commercial adhesive mix for the LVL study while normal plywood pressing conditions were used for the plywood. For the plywood warpage study, the effect of lathe check orientation and species mix were evaluated. The lathe check orientation had little effect while the surface veneer species had a pronounced effect on the warpage in the plywood. Steam used for injection was heated to 260 degrees C at 450 KPa (65 psi) with a super-heater. All panels were made with incised 3.2 mm (1/8 inch) SPF veneers. The project demonstrated that steam injection can shorten press times by fifty percent if incised veneers are used.
Development of two-stage thermo-reforming technology for the manufacturing of cup-shape fibreboard. Part I. Investigation of effects of different resin systems and secondary hot pressing on panel properties
In this study, hot-pressing behavior of 5-ply Douglas-fir and spruce plywood was evaluated. Visual sorted Douglas-fir and spruce veneer (select) sheets were acquired from a Forintek member's mill. All veneer sheets were conditioned to a moisture level of 3%. Platen pressure and target core temperature were chosen as two control variables. Using a JMP statistical software program, their effect and sensitivity on plywood productivity, material recovery and panel bond quality were studied in terms of hot-pressing time, panel compression ratio and wood failure percentage. Second-order response surface models (RSM) were further established for these three criteria. The results showed that 1) the material variation within the same visual grade (select) for each species resulted in the variation of hot-pressing time, compression ratio and wood failure percentage. Therefore, the applied pressing schedules should be a bit conservative for thin-type 5-ply plywood products; 2) generally, the rate of temperature rise in 5-ply Douglas-fir plywood was faster than that in 5-ply spruce plywood. The difference in heat transfer speed for these two species was more pronounced when the target core temperature is higher than 1100C; 3) the hot-pressing time, compression ratio and wood failure percentage was not very sensitive to the platen pressure within the range of 155 psi to 190 psi for 5-ply spruce plywood; In contrast, in order to achieve the target wood failure percentage (80%) for 5-ply Douglas-fir plywood, the target core temperature had to be set at least 1100C along with a platen pressure higher than 200 psi; 4) the hot-pressing optimization was the balance of the hot-pressing time (productivity), panel compression ratio (material recovery) and wood failure percentage (panel quality). Overall, the optimum pressing conditions were veneer moisture dependent. At a 3% veneer moisture content level, the optimum platen pressure was 175 psi for 5-ply spruce plywood whereas the optimum platen pressure was 225 psi for 5-ply Douglas-fir plywood. For both species, the optimum target core temperature for the 5-ply panels was 1100C at a platen temperature of 1550C.
This report summarises a research project carried out at Forintek Eastern Laboratory on Investigation of MDF Press Strategies to Reduce Press Time. The scope of the research includes theoretical analysis of MDF hot pressing process, development of a computer simulation model for MDF continuous hot press, parametric study of hot pressing process using the simulation model, and experimental works on microwave pre-heating of the fibre mat. As results of the research project, a comprehensive computer simulation model was developed. The following conclusions can be made based on the theoretical and experimental works of this project:
The computer simulation model is capable of analysing the hot pressing process and predicting the evolution of important pressing parameters including temperature responses, gas pressure, gas density, gas flow velocity, moisture content, load and stress, and density profile development in different location of the fibre mat. The predicted data agree well in trend with the observations from the industrial MDF continuous presses.
The parameters significantly affecting the hot pressing time include panel density, initial fibre moisture content, initial fibre mat temperature, pressing temperature and width of the fibre mat.
Other hot pressing parameters, such as the initial closing speed and the temperature difference among different pressing zones, have no significant effects on the hot pressing efficiency.
Microwave pre-heating of the MDF fibre mat, not only increase the initial mat temperature, but also re-distribute the moisture towards the mat surface.
Microwave pre-heating induces an earlier increase of the core temperature in the MDF fibre mat and a greater rate of temperature increase.
As a result of microwave pre-heating, the pressing time can be substantially reduced, leading to an increase in production efficiency.