Weld Line
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Weld line
Generally differentiation
A basic distinction is made between static and dynamic weld lines. Static weld lines are created, for example, during the welding process when joining thermoplastic moulded parts. A dynamic weld line is created in a plastic component during the injection moulding process by the confluence of at least two mass flows, e.g. behind cavities, through differences in wall thickness or through several gates of the tool.
Weld line and flow front
A weld line is always a potential weak point in the component. The flow fronts meet perpendicularly due to volume expansion and weld together. The lower the pressure and temperature, the lower the strength of the weld line. Reinforcing fibres orient themselves parallel to the weld line due to the shear acting during the injection moulding process and the flow conditions (Fig. 1 and Fig. 3).
Notch effect of the weld line in components
If the melt has already cooled down so much that the melt fronts that meet can no longer be completely welded together, an inhomogeneous structure appears in the weld line area (Fig. 2a) and the weld line can be recognised on the surface as a V-shaped notch (Fig. 2b) [1].
If tensile stresses occur in this area, the notch effect leads to an increase in stress at the weld line, which then acts as a predetermined breaking point. The design of plastic components should be such that weld lines in the main stress area are avoided.
| Fig. 1: | Light microscope images of micro-sections in the area of the weld line [2], a) component made of PA66-CF, b) component made of PA66-GF 6-CF, b) Part of PA66-GF material |
| Fig. 2: | a) Polarisation microscope image of a thin section, component made of PA6 in the area of the weld line [2], b) reflected light microscope image of a component made of polyoxymethylene (abbreviation: POM) with circular weld lines [3] |
| Fig. 3: | Weld line morphology of long-fibre-reinforced polypropylene (steel fibres), a) light microscope image of a thin section, b) steel fibre network in the weld line area after ashing [4] |
See also
- Fibre orientation
- Glass fibre orientation
- Polarisation optical examination
- C-shaped test specimen
- Processing shrinkage
- Shrink voids
- Sink mark
References
| [1] | Myer, E.: Plastic Failure Guide – Cause and Prevention. Carl Hanser, Munich Vienna (1996); (ISBN 978-3-446-15715-6; see AMK-Library under D 1) |
| [2] | VDI 3822 Part 2.1.2 (2024-06): Failure Analysis – Defects of Thermoplastic Products Made of Plastics Caused by Faulty Processing |
| [3] | Kurr, F.: Praxishandbuch der Qualitäts- und Schadensanalyse für Kunststoffe. Carl Hanser Munich (2014) (ISBN 978-3-446-43775-3; see AMK-Library under D 6-2) |
| [4] | Roth, S.: Spritzgegossene Abschirmgehäuse aus stahlfasergefüllten Thermoplasten – Materialeigenschaften, Verarbeitung, Gestaltung. Technische Universität Chemnitz, (Dissertation) 2007 |