Glass Fibre Orientation

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Glass fibre orientation; see also: Fibre orientation

General

The orientation of short glass fibres is of essential importance for the design and dimensioning of plastic components, as, alongside fibre geometry – specifically the aspect ratio (l/d) – and the fibre volume fraction, it has a significant influence on the stiffness, strength and deformation behaviour, as well as on the processing shrinkage and warping of the components [1].

Layer structure in injection moulding of fibre-reinforced plastics

In the injection moulding process, the filling process within the tool results in a characteristic cross-sectional layer structure, which can vary depending on the distance from the gate and the injection conditions [2, 3]. In the simplest case, the layer structure can be described by a 3-layer model (see also: short-fibre reinforced composites) with a core and two edge regions, which can be explained by the flow behaviour within the cavity. In the core region, a virtually constant laminar flow velocity dominates, resulting in a transverse to random orientation of the fibres. Near the edge of the tool, the contact between the melt and the already solidified edge zone results in high shear rates, which, due to the resulting shear forces, create fibre orientations in the direction of injection (Fig. 1).

Fig. 1:

Fluid flow conditions within the cavity during injection moulding [4]

If one defines a transition zone between transversely and longitudinally oriented layers and takes into account an outer layer that is low in fibres and disoriented, the 3-layer model gives rise to a 7-layer computational model, which can be extended to 9 to 11 layers by including a transition layer from the edge to the highly oriented zone (Fig. 2).

Fig. 2:

9-zone layer model of fibre orientation in short-fibre reinforced thermoplastics

Dimensioning of plastic components

Provided that the properties of these layers, in particular the fibre orientation and fibre content (see: ashing method), are known, this model can be used for the design and dimensioning of plastic components using finite element analysis (FEM) software. A fundamental theoretical determination of fibre orientation is provided by the use of mould flow simulations, e.g. M.Tec Moldflow, Mold Flow Plastic Inside [5] or Autodesk® MoldFlow®, can be calculated in advance and represented as tensors in a property matrix. This calculated data can then be transferred directly to the FEM programs and, together with the local stiffness matrix, is used to determine the dimensions and simulate the behaviour of the component under complex loading conditions.

Verification of orientation using direct and indirect testing methods

Regardless of the method used to predict fibre orientation, however, this must be verified experimentally, particularly where dynamic weld lines are present.

Various indirect and direct testing methods exist for determining fibre orientation, but these are generally relatively time-consuming and costly.

Indirect testing methods include the determination of the modulus of elasticity in bending or tensile tests, or the measurement of the shrinkage force or shrinkage (see also: shrinkage test) using thermal stress analysis (TSA) or thermal strain analysis [3, 6]. By determining the stiffness or shrinkage of test specimens with defined preferred orientations, the fibre orientation can then be determined recursively.

The oldest direct method involves preparing micrographs of the cross-sectional surface of test specimens or using ashed and detached fibres, followed by microscopic analysis using light or scanning electron microscopy. The fibres are visible on the cross-section as circles or ellipses and can be analysed using image analysis methods [13] with regard to their position (angle of inclination Θ relative to the normal) and orientation angle Φ (Fig. 3), whereby plastic-specific contrast agents must be used to ensure optimal visibility of the phase boundaries between the matrix and the fibre.

Fig. 3:

Diagram illustrating how to determine fibre orientation from polished images

Equations (1) to (3) can then be used to calculate the components of the fibre orientation:

$a_{x}=sin^{2}\theta \cdot cos^{2}\theta$

(1)

$a_{y}=sin^{2}\theta \cdot sin^{2}\theta$

(2)

$a_{z}=cos^{2}\theta \!$

(3)

with a_x + a_y + a_z = 1.

More modern testing methods are based on the use of non-destructive testing techniques such as laser diffractometry [7], X-ray refractometry [8, 9], micro-radiography [10], microwave testing [11] or various ultrasonic testing methods [12]. One testing technique that is being used with increasing frequency is X-ray-based µ-computed tomography (CT) [14], which directly provides the orientation tensor that can be used in FEM calculations. The CT represents the dependence of the absorption coefficient µ on the path of the radiation x through the material as a greyscale image. As the absorption coefficient µ is highly dependent on the density of the heterogeneous material, information regarding the position and the respective density can be determined, thereby enabling the reconstruction of the volume and the determination of the fibre orientation (Fig. 4).

Fig. 4:

Micro-CT analysis of a section of test specimens, showing the corresponding Cartesian coordinate system and the orientation of glass fibres in greyscale and false-colour images, following micro-CT scanning and analysis using VG STUDIO MAX V2.2 software [4]

See also

• Fibre orientation
• Fibre agglomeration
• Anisotropy
• Multiple fracture UD tapes
• Short-fibre reinforced plastics
• Fibre-reinforced plastics fracture model
• Fibre–Matrix adhesion

References

[1]	Menges, G., Geisbüsch, P.: Die Glasfaserorientierung und ihr Einfluß auf die mechanischen Eigenschaften – Eine Abschätzmethode. Colliod Polym. Sci. 260 (1982) 73–81
[2]	Hegler, R. P., Altstädt, V., Ehrenstein, G. W., Mennig, G., Scharschmidt, J., Weber, G.: Einfluss stofflicher Parameter auf die Faserorientierung beim Verarbeiten kurzfaserverstärkter Thermoplaste. Kunststoffe 76 (1986) 766–771
[3]	Pflamm-Jonas, T.: Auslegung und Dimensionierung von kurzfaserverstärkten Spritzgussbauteilen. Dissertation, Technische Universität Darmstadt, 2001 (Dissertation) (last access on March 19, 2026)
[4]	Illing, T.: Bewertung von mechanischen und thermischen Eigenschaften glasfaserverstärkter Polyamid-Werkstoffe unter besonderer Berücksichtigung des Alterungsverhaltens von Bauteilen in der Automobilindustrie. Dissertation, Martin-Luther-Universität Halle-Wittenberg (2015), Shaker Publishing GmbH, Aachen (2016), (ISBN 978-3-8440-4212-2; see AMK-Library under B 1-27)
[5]	Lutz, W., Lasko, G., Schmauder, S., Predak, S., Bullinger, O., Gerhard, H., Busse, G.: Spritzgieß-Simulation eines glasfaserverstärkten Formteils mit Bindenaht zur Berechnung von Faserorientierung und resultierenden mechanischen Eigenschaften. 19th Stuttgarter Kunststoff-Kolloquium 2005, Proceedings 4V5
[6]	Bierögel, C.: Prüfkörperherstellung. In: Grellmann, W., Seidler, S. (Eds.): Kunststoffprüfung. Carl Hanser, Munich (2025) 4th Edition, pp. 15–37 (ISBN 978-3-446-44718-9; E-Book: ISBN 978-3-446-48105-3; see AMK-Library under A 23)
[7]	Maurer, C.: Versagensmechanismen von PVD-Beschichtungen auf CFK unter Erosionsverschleiß. Dissertation, RWTH Aachen, 2014 (Dissertation) (last access on March 19, 2026)
[8]	Bullinger, O., Busse, G.: Röntgen-Refraktions-Topographie − Ein zerstörungsfreies Werkzeug zur Charakterisierung von Schädigungen in Kunststoffen. DGZfP DACH-Jahrestagung, Salzburg, Proceedings 2004
[9]	Günzel, S.: Analyse der Schädigungsprozesse in einem kurzglasfaserverstärkten Polyamid unter mechanischer Belastung mittels Röntgenrefraktometrie, Bruchmechanik und Fraktografie. Dissertation, Universität Berlin, 2013
[10]	Bürger, C., Ehrenstein, G. W.: Darstellung verstärkter Kunststoffe mittels RE-Detektor (REM) und Mikroradiografie. Practical Metallography: 43, 4 (2006) 161−183
[11]	Busse, G.: Zerstörungsfreie Kunststoffprüfung. In: Grellmann, W., Seidler, S. (Hrsg.): Kunststoffprüfung. Carl Hanser, Munich (2025) 4th Edition, pp. 445–509 (ISBN 978-3-446-44718-9; E-Book: ISBN 978-3-446-48105-3; see AMK-Library under A 23)
[12]	Reinfurth, M.: Geführte Ultraschallwellen zur Bewertung der Schadensakkumulation in Faser-Kunststoff-Verbunden. Dissertation, Universität Stuttgart, 2013 (Dissertation) (Last access on March 19, 2026)
[13]	Beller, T.: Erkennung von Faserorientierungen in Kunststoffen mittels Bildanalyse. DGZfP DACH-Jahrestagung, Bremen, Proceedings 2011
[14]	Kastner, J., Schlotthauer, E., Angermaier, D., Zitzenbacher, G.: Quantitative Messung von Faserlängen und -verteilung in faserverstärkten Kunststoffteilen mittels μ-Röntgen-Computertomographie. DGZfP DACH-Jahrestagung, Konstanz, Proceedings 2007