Modeling the Transient Effects during the Hot-Pressing of Wood-Based Composites

Type of Document Dissertation
Author Zombori, Balazs Gergely
Author's Email Address bzombori@vt.edu
URN etd-04262001-172039
Title Modeling the Transient Effects during the Hot-Pressing of Wood-Based Composites
Degree PhD
Department Wood Science and Forest Products
Advisory Committee
Advisor Name Title
Dr. Frederick A. Kamke Committee Chair
Dr. Charles E. Frazier Committee Member
Dr. Elemer M. Lang Committee Member
Dr. J. Daniel Dolan Committee Member
Dr. Joseph R. Loferski Committee Member
Dr. Layne T. Watson Committee Member
Keywords

* heat and mass transfer of porous solids
* wood-based composites
* hot-compression
* process modeling
* Monte Carlo simulation

Date of Defense 2001-04-20
Availability unrestricted
Abstract

Modeling the Transient Effects during the

Hot-Compression of Wood-Based Composites

by

Balazs G. Zombori

Frederick A. Kamke, Chairman

(Abstract)

A numerical model based on fundamental engineering principles was developed and

validated to establish a relationship between process parameters and the final properties of wood-based

composite boards. The model simulates the mat formation, then compresses the

reconstituted mat to its final thickness in a virtual press. The number of interacting variables

during the hot-compression process is prohibitively large to assess a wide variety of data by

experimental means. Therefore, the main advantage of the model based approach that the effect

of the hot-compression parameters on the final properties of wood-based composite boards can

be monitored without extensive experimentation.

The mat formation part of the model is based on the Monte Carlo simulation technique

to reproduce the spatial structure of the mat. The dimensions and the density of each flake are

considered as random variables in the model, which follow certain probability density

distributions. The parameters of these distributions are derived from data collected on industrial

flakes by using an image analysis technique. The model can simulate the structure of a three-layer

oriented strandboard (OSB) mat as well as the structure of random fiber networks. A grid

is superimposed on the simulated mat and the number of flakes, the thickness, and the density of

the mat at each grid point are computed. Additionally, the model predicts the change in several

void volume fractions within the mat and the contact area between the flakes during

consolidation. The void volume fractions are directly related to the physical properties of the

mat, such as thermal conductivity, diffusivity, and permeability, and the contact area is an

indicator of the effectively bonded area within the mat.

The heat and mass transfer part of the model predicts the change of air content, moisture

content, and temperature at designated mesh points in the cross section of the mat during the hot-compression.

The water content is subdivided into vapor and bound water components. The free

water component is not considered in the model due to the low (typically 6-7 %) initial moisture

content of the flakes. The gas phase (air and vapor) moves by bulk flow and diffusion, while the

free literature