Technology for Wood and Natural Fiber-Based Materials

Research Project

Simulation-aided development of medium-density fiberboard for lightweight construction

The competition for wood as a raw material and the thereby resultant increasingly-limited availability of wood, together with the accompanying high cost pressure, are forcing companies from the wood-fiber material industry to seek new methods for product development and process optimization. The specific development of materials and the optimization of their characteristics by means of computer-aided simulation is becoming increasingly important. A simulation-aided development of fiber materials with improved mechanical properties at lower material densities makes a significant contribution towards sustainable management and improved resource efficiency.

The properties of medium-density fiberboards (MDF) depend on many different factors which can influenced during the manufacturing process. Microstructure properties (such as, for example, fiber volume proportion, fiber orientation, fiber length, rigidity of individual fibers) significantly determine the macroscopic mechanical properties of MDF. Until now, the fiber orientation during the manufacture of MDF has not been specifically configured. The objectives of a joint project between the Fraunhofer Institute for Wood Research (Fraunhofer WKI) and the Fraunhofer Institute for Industrial Mathematics (Fraunhofer ITWM) are:

  • Simulation-aided development of resource-conserving and cost-efficient lightweight MDF with high rigidity for application in furniture manufacture, building, transport and exhibition construction as well as, if necessary, for direct painting and printing,
  • the development of a complete micro-macro simulation model for MDF,
  • the development and testing of methods for the targeted fiber orientation in lightweight MDF for the stiffening of the pore spaces.

With new imaging techniques (micro-CT) in combination with modern modeling approaches (multi-scale methods), the relationship between microstructure and macroscopic properties should be determined. For this, multi-scale simulations are necessary in order to make favorable selections from the multitude of possible MDF variants with oriented fiber layers, which can then be subsequently produced and characterized.

© Fraunhofer WKI
Abb.1: Beispiele für Lösungsansätze zum Ausrichten von MDF-Fasern; links: V-Nuten aus Edelstahl; rechts: Steghölzer.
© Fraunhofer WKI
Abb.2: Faservlies, links ohne gezielte Vorzugsrichtung, rechts mit gezielter Vorzugsrichtung (vertikale Richtung).
© Fraunhofer WKI
Abb.3: Querzugfestigkeiten für unterschiedliche Streuvarianten: orientiert gestreute MDF (O-MDF), vereinzelt und wirr gestreute MDF (V-MDF) und konventionell hergestellte MDF (S-MDF) bei den Zielrohdichten 620 kg/m³ und 800 kg/m³

 

As part of a diploma thesis1, various approaches were taken at the Fraunhofer WKI to investigate the alignment of conventional MDF fiber materials (Fig. 1).

For this, mechanically-operating testing setups (pincushion, slotted plate, V-groove,wooden dividers/sticks) were designed and subsequently implemented as a testing setup on laboratory scale. The orientation results were both qualitatively (primarily) and quantitatively assessed by means of digitalized images of fleece surfaces (Fig. 2).   

Using oriented fibers and polymeric diisocyanate (PMDI), MDF with two gross density variants was produced. As a reference, MDF with no orientation of the fibers was used. In the case of MDF with fiber orientation, a flexural strength increase of around 25% in the orientation direction was determined. The increase in the flexural moduli of elasticity was around 32% (Fig. 3). According to the results of the studies conducted, the fiber orientation has no influence on the transverse tensile strength, or the swelling and water absorption of the MDF after 24 hours of immersion in water. In the longitudinal fiber direction, the relative change in length values after storage in 20/30 climate (20°C and 30% relative humidity) and 20/85 climate (20°C and 85% relative humidity) were lower than those transverse to the fiber orientation.

 [1] Lippe, D. 2013: Verfahrensentwicklung zum experimentellen Nachweis von Eigenschaftsänderungen mitteldichter Faserplatten mit ausgerichteten Fasern. (Process development for the experimental demonstration of property changes in medium-density fiberboard with aligned fibers.) Eberswalde University for Sustainable Development (FH) in collaboration with the Fraunhofer WKI.

 

Abb. 4: µCT-Aufnahme der MDF-Mikrostruktur (4 µm Auflösung, phoenix|x-ray, GE Sensing & Inspection Technologies GmbH)
Abb. 5: Faserrichtungsanalyse, rot: Faserbündel mit parallelen Fasern
Abb. 6: Volumenelement mit Modell der MDF-Mikrostruktur, Strukturgenerierung mit Hilfe der Software GeoDict (Math2Market) und der Software FeelMath (ITWM)

Results of the two-scale simulation

For the project, 2 scales for modeling and simulation are observed in order to analyze the influence of micro-structural parameters (such as, for example, fiber length distribution and fiber orientation) on the properties of MDF or building components. For the characterization of the microstructure, a fiber network in a volume element of a few cubic millimeters is used. Three-dimensional μCT images (Fig. 4) with a resolution of 4 µm are suitable for capturing a representative section (at least half the plate thickness) and for a simultaneous sufficiently-good resolution of the individual fibers.

For the modeling of the geometric microstructure, the length and thickness distribution of the fibers is determined from the applied fiber material, and the fiber orientation is determined from the μCT images. To determine the orientation, automatic mathematical algorithms for fiber-direction analysis are applied (Fig. 5). This method also enables the determination of the position of fiber bundles (red region). The result of this process step is a so-called stochastic geometry model.

For the stochastic geometry model, realizations (Fig. 6) in the form of microstructures in volume elements can be generated. In the generated volume element, for each point belonging to a fiber the local orientation this fiber has in this point is clearly defined. In contrast, in a binarized image, only the information as to whether a point belongs to the solid material (cellulose) or to the pore space would be captured.

In the next step, the anisotropic elastic properties and the anisotropic strength properties for the fibers are selected. Following this, 6 non-linear elasticity problems (tension in 3 directions, thrust in 3 directions) are solved using the FeelMath software developed at the ITWM, in order to determine the effective mechanical properties.

 

These effective material parameters represent the final result of the microstructure analysis and are applied in the second scale (macro-scale or component scale) as a material parameter. The simulation on the macro-scale is executed using a standard method (FEM). In the project, the results of the macro-simulation, e.g. a simulation of the flex test, correlated very well with corresponding measurement results (Fig. 7).

The advantage of the two-scale simulation lies in the fact that in the stochastic geometry model, individual parameters can be systematically selectively varied in order to study their sensitivity to the macroscopic stiffness and strength, without having to produce the relevant material variants. Whilst the computer simulation is elaborate, it takes considerably less time than measurements; a very large number of MDF variants can therefore be simulated. An example of this can be seen in Fig. 8. For the same fiber length distribution and the same fiber volume content, only the orientation has been changed. The result is an improvement in the rigidity in the direction in which the fibers are aligned. The extent of the quantitative improvement is demonstrated in the diagram in Fig. 8.

 

 

Abb. 7: Biegetest: Vergleich von Simulation und Messung für eine MDF mit isotroper Faserorientierung, Spannungs-Verzerrungskurve für die Außenschicht
Abb. 8: Simulation eines Biegetests: Bei orientierten Fasern steigt die Steifigkeit in Orientierungsrichtung um ca. 25% im Vergleich zu Platten mit isotroper Faserverteilung, Spannungs-Verzerrungs-Kurve für die Außenschicht.

Funding

Federal Ministry for Economic Affairs and Energy (BMWi) via the Federation of Industrial Research Associations "Otto von Guericke" of the International Association for Technical Issues Related to Wood (iVTH).

IGF Project No.:     17644 N

Duration:    1.2.2013 – 31.12.2015