Shrink Voids
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Schrink Voids
General information
The terms “vacuoles”, “shrink holes” and “vacuences” are also used synonymously in Anglo-Saxon literature.
During the injection moulding process, a volume contraction (shrinkage) occurs when the moulded part (see: moulding compounds) cools in the mould, whereby a residual compressive stress is built up in the edge areas and a residual tensile stress in the interior. This volume contraction of the melt is increased in areas of mass accumulation, as the reducing volume cannot be replaced by new melt.
Process of shrink voids (vacuoles) formation
Shrink voids (vacuoles), also known as blowholes (cavities) in non-destructive testing (NDT), occur when the solidified surface layer has sufficient stability to counteract the internal tendency to contract. Due to the residual tensile stresses acting internally, the moulding compound tears open in the centre of material accumulations and holes are formed (Fig. 1). As a result of the negative pressure, the holes have a jagged surface and are irregularly shaped. They differ very clearly from the gas bubbles (Fig. 2).
Shrink voids are formed in particular when the processing conditions or the mould and/or component design are not optimal. The main causes for the formation of shrink voids are too low a mould temperature, too low an effective holding pressure, a holding pressure time that is too short, bottlenecks in the flow path, an incorrect gate position (in the thin-walled area) or a gate that is too small [1, 2].
| Fig. 1: | Shrink voids at injection moulded components in the range of material accumulations, a) Example of [2] PBT-GF, b) Example of [3] PA/PTFE-piston ring |
| Fig. 2: | Shrink voids on the fracture surface of a clip of polyoxymethylene (abbreviation: POM), a) light microscopical picture, b) REM picture |
Very small shrink voids (vacuoles) are called micropores.
See also
- Sink mark
- Micropores
- Hole formation films
- Hole formation plastics
- Failure analysis plastic products, VDI Guideline 3822
- Weld line
References
| [1] | VDI 3822, Blatt 2.1.1 (2024-06): Failure Analysis – Defects of Thermoplastic Products Made of Plastics Caused by Faulty Design |
| [2] | VDI 3822, Blatt 2.1.2 (2024-06): Failure Analysis – Defects of Thermoplastic Products; Made of Plastics caused by Faulty Processing and Corrigendum Concerning Guideline Part 2.1.2 (2012-04) |
| [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) |
Additional literature references on plastics diagnostics/damage analysis
- Ehrenstein, G. W.: Präparation. Unverstärkte, hochgefüllte und verstärkte Kunststoffe – Ätzen für Strukturunterbrechungen. Erlanger Kunststoff-Schadenanalyse. Carl Hanser Munich (2019) (ISDN 978-3-446-40382-6; e-Book ISBN 978-3-446-46054-6; see AMK-Library under F 27)
- 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, L., Klingele, H., Ehrenstein, G. W., Schaper, H.: Rasterelektronenmikroskopische Untersuchungen von Kunststoffschäden. Carl Hanser Munich, Vienna (1978) 1st 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)
- Ezrin, Myer: Plastics Failure Guide – Cause and Prevention. Carl Hanser Munich, 2nd Edition (2013) (ISBN 978-1-56990-449-7; see AMK-Library under D 7)