Sink Mark

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Sink mark

In the injection moulding process of plastic components, a volume contraction ( shrinkage) occurs during cooling. In areas of mass accumulation or large wall thicknesses, this volume contraction of the melt is increased, whereby the reducing volume cannot be replaced by new melt as the injection channel has already solidified. Tensile residual stresses are therefore built up inside. If there is a sufficiently thick surface layer, central cavities can form as a result of the internal tensile stresses. However, if the solidified surface layer is not yet stable enough to counteract the internal tendency to contract, the surface is deformed inwards and sink marks or depressions form on the surface of the moulded part.

These sink marks occur in particular on larger mass accumulations or with greater wall thicknesses [3].

Sink marks can be avoided by designing the construction to be plastic-compatible, i.e. by making the wall thickness as thin as possible, providing equal wall thicknesses and avoiding mass accumulations.

Fig. 1:

Sink marks at injection moulded components in the range of material accumulations, a) Example of [1], b) Example of [2] PA11-Clip

Fig. 2:

Sink marks at injection moulded components of PC/ABS in the range of material accumulations (rips)

See also

• Shrink voids
• Tensile mechanical stresses orientations
• Weld line

References

[1]	VDI 3822, Part 2.1.2 (2012-01): Failure Analysis – Defects of Thermoplastic Products made of Plastics caused by Faulty Processing and Corrigendum Concerning Guideline Part 2.1.2 (2012-04)
[2]	Kurr, F.: Praxishandbuch der Qualitäts- und Schadensanalyse für Kunststoffe. Carl Hanser Verlag, München (2014) (ISBN 978-3-446-43775-3; see AMK-Library under D 6-2)
[3]	Ezrin, M.: Plastics Failure Guide – Cause and Prevention. Carl Hanser Munich, 2nd Edition (2013) (ISBN 978-1-56990-449-7; see AMK-Library under D 7)

Additional literature references on plastic diagnostics/damage analysis

• Ehrenstein, G. W.: Kunststoff-Schadensanalyse – Methoden und Verfahren. Carl Hanser Munich, Vienna (1992) (ISBN 978-3-446-17329-3; see AMK-Library under D 2)
• Ehrenstein, G. W.: SEM of Plastics Failure – REM von Kunststoffschäden. Carl Hanser Munich (2010)(ISBN 978-3-446-42242-1; see AMK-Library under D 5)
• Engel, D., Klingele, H., Ehrenstein, G. W., Schaper, H.: Rasterelektronenmikroskopische Untersuchungen von Kunststoffschäden. Carl Hanser Munich, Vienna (1978) 1. Edition (ISBN 978-3-446-12560-5, see AMK-Library under D 8)
• Brostow, W., Corneliussen, R. D.: Failure of Plastics. Carl Hanser Munich, Vienna (1986) (ISBN 978-3-446-14199-5; see AMK-Library under D 10)