Fibre Orientation
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Fibre orientation; Glass fibre orientation
Orientation of reinforcement fibres
Fibre orientation refers to the alignment of reinforcing fibres in the matrix. Fibres are used in many engineering plastics to increase the strength and stiffness (see also: tensile test compliance) of the material. The injection moulding process orients these fibres in relation to the flow direction. This means that the material properties in the component are no longer isotropic but anisotropic (see: anisotropy). This means that the material has different properties in the fibre direction and across it.
It is therefore necessary to systematically investigate the relationship between fibre orientation and material properties as a function of material composition.
The 5-layer model of fibre orientation
In components manufactured using the injection moulding process, the fibres align themselves along the direction of flow of the matrix during mould filling, particularly in areas with high shear gradients and due to the low flow resistance of the fibres. For this reason, the fibres are oriented parallel to the weld lines and do not contribute to increasing the strength of the matrix material in the weld line (Fig. 1).
| Fig. 1: | Light microscope image of a cross-section, component made of PA66-CF in the area of the weld line [1] |
The flow processes during injection moulding cause a layer structure of fibre orientation due to the parabolic velocity profile. At the edge, the melt solidifies very quickly and the fibre orientation is random. In many thermoplastics, an almost fibre-free edge zone with thicknesses of up to 300 μm is also observed. In the centre of the cavity, i.e. in the laminar flow region, the fibres are aligned perpendicular to the flow direction. Between the edge layers and the core layer, the melt is sheared, and the fibres orient themselves in the flow direction. A 5-layer model of fibre orientation was developed by MENGES and GAISBÜSCH (Fig. 2) [2].
| Fig. 2: | Velocity profile and fibre orientation during the injection moulding process according to Menges [2] |
Mechanical properties and toughness characterisation
In the 5-layer model, the main orientation of the glass fibres is parallel to the flow direction, which is also reflected in the mechanical properties (Figs. 3 and 4).
| Fig. 3: | Tensile tests – Stress–strain diagrams of polypropylene depending on the injection direction (longitudinal and transverse) and the filler content (20, 30, 40 m.-% glass fibres and 10 m.-% glass fibres + 30 m.-% mineral) [3] |
| Fig. 4: | Toughness characterisation of polypropylene depending on the injection direction (longitudinal and transverse) and the filler content (20, 30, 40 m.-% glass fibres and 10 m.-% glass fibres + 30 m.-% mineral) in the instrumented Charpy impact test, a) maximum force, b) deflection at maximum force, c) J-value, d) crack opening displacement [3] |
See also
- Fibre agglomeration
- ICIT with AE
- Fibre-reinforced plastics fracture model
- Fibre-reinforced plastics fracture model
- Interlaminar shear strength
- Fibre-reinforced plastics
- Fibre–Matrix adhesion
References
| [1] | VDI 3822, Blatt 2.1.2 (2024-06): Failure Analysis – Defects of Thermoplastic Products Made of Plastics Caused by Faulty Processing |
| [2] | Menges, G., Geisbüsch, P.: Die Glasfaserorientierung und ihr Einfluss auf die mechanischen Eigenschaften thermoplastischer Spritzgussteile – Eine Abschätzmethode. Colloid & Polymer Science, 260 (1982) 1, pp. 73–81; https://doi.org/10.1007/BF01447678 |
| [3] | Jäger, S.: Einfluss der Faserorientierung auf das mechanische Kennwertniveau medial und thermisch beanspruchter Polypropylen-Glasfaser-Verbunde. Martin-Luther-Universität Halle-Wittenberg, Master Thesis (2010) (see AMK-Library under B 3-168) |