Laminated Veneer Lumber (LVL) and plywood are the two major veneer-based wood composite products. During LVL/plywood manufacturing, the hot pressing process is crucial not only to the quality and productivity, but also to the performance of panel products. Up to now, the numerical simulation of the hot-pressing process of LVL/plywood products is not available.
To help understand the hot-pressing process of veneer-based wood composites, the main objective of this study was to develop a computer simulation model to predict heat and mass transfer and panel densification of veneer-based composites during hot-pressing. On the basis of defining wood-glue mix layers through the panel thickness, a prototype finite-element based LVL/plywood hot-pressing model, VPress®, was developed to simulate, for the first time, the changes of temperature, moisture and vertical density profile (VDP) of each veneer ply and glueline throughout the pressing cycle. This model is capable of showing several important characteristics of the hot-pressing process of veneer-based composites such as effect of glue spread level, veneer moisture, density, platen pressure and temperature as well as pressing cycles on heat and mass transfer and panel compression. Experiments were conducted using several different variables to validate the model. The predicted temperature profiles of the veneer plies and gluelines (especially at the innermost glueline) by the model agree well with the experimental measurements. Hence, the model can be used to evaluate the sensitivity of the main variables that affect hot-pressing time (productivity), panel compression (material recovery) and vertical density profile (panel stiffness). Once customized in industry, the new model will allow operators to optimize the production balance between productivity, panel densification and panel quality or stiffness. This hot-pressing model is the first step in facilitating the optimization of the pressing process and enhanced product quality.
Based on elasticity plate theory and earlier models of mat formation (MatForm), pressing (MatPress), and OSB properties (OSB-Pro), a comprehensive computer model (OSB-CSL) was developed to simulate the spatial distribution of deflections and stesses as well as ultimate load (UL) of OSB panels under CSL loading. Laboratory tests were conducted to validate the model. This model simulates the stochastic variation of ULs due to heterogeneous mat formation and localized strength properties. The following conclusions can be drawn:
1) CSL loading in OSB typically goes through four stages: linear elastic deformation, yielding due to initiation of localized debonding, ductile deformation due to multiple debonding and crack propagation, and catastrophic failure in the form of punch shear failure.
2) CSL performance is highly dependent upon the uniformity of mat formation and the consistency of board properties, particularly bending MORs. Due to the inherent forming variations, a certain percentage of samples will be deemed to fail, which often makes it difficult to meet the current performance standard.
3) Smaller loading heads result in lower loading capacity due to stress concentration.
4) Localized MOR has a direct impact on UL. Variability in MOR from quality control tests can be used as a predictor for CSL performance.
5) Strand orientation has a strong impact on bending and CSL properties. Improving orientation in both face and core layers can lead to increases in ULs.
6) CSL properties can be improved by: using longer and thinner strands, increasing resin content (mainly for increasing UL), reducing fines content and increasing panel density.
This report reviews firstly some of the recent work on modeling mat consolidation. For the first time, the concept of elasto-plastic behaviour of composite mats under pressing is introduced. The implications of the mat elasto-plasticity on formation of vertical density profile is analyzed. Finally, theoretical models, which capture such mat deformation behaviour and mat heat and moisture transfer are developed. Both two-dimensional (2-D) and three-dimensional (3-D) models are capable of accurately predicting variations in mat temperature, gas pressure, moisture content and vertical density profile during hot pressing. Typical predictive results from the 2-D and 3-D models are presented and compared. Based on these comparisons, because of the complexity of the 3-D model, it is recommended that the 2-D model is sufficiently accurate enough to be used for simulating commercial OSB pressing.
A new green veneer moisture measurement method was developed based on the principle of light transmission. Compared to current radio frequency (RF) moisture measurement, the new method shows improved accuracy in green veneer MC detection for regular softwood veneer. As well, an off-line portable veneer testing system was developed based on the light transmission, which was successfully used in the mill trials to evaluate the accuracy of current green veneer moisture sorting. Meanwhile, a laboratory dry veneer moisture measuring system was successfully modified to measure the variation of dry veneer MC. Further a pilot-scale veneer dryer was upgraded to simulate the mill drying conditions. These developments ensure us to promptly transfer the technologies to the industry.
The effect of drying temperature, air flow velocity and air humidity on dry veneer final MC was investigated. It was found that drying at high temperature was faster than at the low temperature. The difference in final MC between low temperature drying and high temperature drying was greater when the initial MC was higher. However, the variation of final MC was smaller with low temperature drying compared to high temperature drying. The effect of air velocity was more apparent when the initial veneer MC was higher. When the initial veneer MC was below 70-80%, the effect of air velocity on veneer drying was very small, which may indicate that for heart veneer drying, the velocity was not a dominant factor whereas for sap and light-sap veneer drying, the air velocity played a significant role. In general, the effect of air humidity on veneer drying was drying temperature dependent. At low drying temperature, low humidity helped veneer drying. In contrast, at a drying temperature higher than 150 °C, the effect of air humidity on veneer drying rate was not significant. However, to improve the veneer drying quality and material recovery, high humidity could be used to reduce veneer brittleness for easy handling.
During stacking period, the moisture spread from the wet area to the dry area. The variation of MC between and within veneer decreased with the stacking time before reaching an equilibrium state. The equilibrium MC depended on the initial MC of veneer and stacking conditions. Compared to lower temperature stacking, the higher temperature stacking accelerated the moisture spread. Therefore, the allowable maximum MC of wet spot on veneer before stacking could be determined based on target dry veneer MC, ambient temperature (season) and stacking time.
A hot stacking model was developed to simulate the change of veneer MC during hot stacking. The prediction results agreed well with the experimental results. This model will be incorporated into the existing VDry models to simulate the effect of post-drying on final veneer MC.
It is recommended that the light green veneer moisture scanner should be further developed for industrial applications. The VDry model is a useful tool which should be applied for mill customization and optimization.
One of Forintek’s most valued services to the oriented strandboard (OSB) members is our use of computer modeling tools to assist members optimize their manufacturing processes and quantify the potential benefits of new technology developments This project substantially upgraded our hot pressing model and added two new models to Forintek’s suite of modeling tools.
Forintek’s 2-D hot pressing model describes the relationship between hot pressing schedules and panel attributes. In this project the 2-D hot pressing model was upgraded to a 3-D model. The 3-D model improves the prediction accuracy of heat and moisture transfer and vertical density profile during the hot pressing process. The model can be used to quantify the costs of current hot pressing processes and predict the potential cost implications of changes to schedules and equipment. It can also be used to assess the need for, and impacts, of developing new pressing technology.
Two new models were developed: one for continuous pressing and one that predicts the mechanical properties of panels from specific manufacturing options. Continuous pressing is an emerging technology which is increasingly used by the composite board industry. This project marks the completion of our first model for continuous pressing. The model simulates the distributions of temperature, moisture content, gas pressure and density profile along the press during pressing based on input data that describe panel structure, layer structure, flake geometry and pressing schedule. While the modelling tool provides many insights into the continuous pressing operation, some basic laboratory tests and mill trials are still needed to optimize individual members’ pressing operations.
The first model to predict the mechanical properties of OSB end products from manufacturing process attributes has been completed. The model predicts end product panel modulus of elasticity (MOE) from density profile, flake geometry, strand orientation, fines content and layer structure.
The models completed in this project will be used as a guide for research and development of new products and process optimization for existing products. Use of these models results in significant savings in R&D costs and higher efficiency in problem solving for members.
A study was carried out based on a previously developed continuous MDF hot press model (Deng, 2004) to further refine and advance the model and to validate the model with actual MDF mill data. A significant amount of work was also conducted on the software development for easy application.
The scope of the work in this study included:
MDF continuous hot press model modification
MDF continuous hot press model validation
MDF continuous model software design and development (MDFContiPress)
PB continuous model development
Software operating menu and documentation
Based on the results of this study, the following conclusions are made:
The model for MDFContiPress can be used to predict distribution and variation of most physical variables in the mat during hot pressing.
Coupling effect between mechanical and thermal procedures which exists in the hot pressing of the mat is considered in the model.
Validation of the model by experimental results shows its accuracy and flexible modeling ability.
Graphic user interface (GUI) is adopted to make the MDFContiPress software more user-friendly.
Appropriate design for data exchange and database interface makes simulation and visualization more efficient in a multi-platform software environment.
Parametric study and optimization become much easier to conduct through a series of simulations for different press configurations, processing parameters and environmental conditions by MDFContiPress.
Major characteristics which differentiate the PB model from the MDF model are panel structure and particle geometry, which affect material behaviour during hot pressing.
Batch hot pressing is more suitable for further validation of the hot pressing models for both MDF and PB.