Processing Shrinkage

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Processing shrinkage

Definition

The term "shrinkage" or "dwindling" generally refers to an irreversible physical process caused by the cooling of an injection-moulded plastic part or the curing of a thermoset plastic. In this process, the volume and dimensions of the moulded part decrease in comparison to the original mould or moulding nest, which is a specific parameter S for the plastics used.

In principle, shrinkage depends primarily on the chemical composition of the plastic (amorphous or semi-crystalline) and the resulting morphology, but is also influenced by fillers and reinforcing materials as well as the orientation ratios.

In the case of semi-crystalline plastics, shrinkage depends to a large extent on the degree of crystallinity and the temperature gradient or profile in the mould. With homogeneous and isotropic plastics, shrinkage is identical in all spatial directions. By adding fillers, nucleating agents, colour pigments or reinforcing fibres, the absolute amount of shrinkage can be significantly reduced, but then an anisotropy of the shrinkage dimension is often observed, which can lead to warpage of the plastic moulded part due to inhomogeneous temperature distribution and residual stress distribution. The distortion manifests itself in component warpage and angular changes.

Shrinkage or processing shrinkage S is defined as follows according to Eq. (1).

$S=\left({\frac {\left(D-d\right)}{D}}\right)\cdot 100\,\%$

(1)

with:	D	–	dimensions of the mould cavity
	d	–	dimensions of the moulded part

The total shrinkage is made up of the processing shrinkage and the post-shrinkage, which is caused, for example, by post-crystallisation processes. Shrinkage reduces existing residual stresses in the moulded part.

Definition of terms

In contrast to shrinkage, the process shrinkage is associated with a decrease in volume. The mass, on the other hand, remains constant during this process.

Explanation of terms

The term shrinkage thus generally describes the loss of volume of a material due to drying processes or due to the manufacturing process, such as the casting of metals or the injection moulding of plastics. In the case of plastics, shrinkage is understood to be the geometric change of a moulded part during cooling from the molten to the solid state, whereby a volume contraction occurs. In contrast, the shrinkage process maintains the volume. The shrinkage process is only defined below the glass transition temperature:

$T<T_{G}\!$

(2)

Shrinkage in plastics can occur both in the manufacturing process and in operational use. Fixed measures of shrinkage, as with metals and metal alloys, cannot be given because the plastic properties are sensitively dependent on the manufacturing parameters and various additives specific to the application. However, shrinkage also depends on time, external conditions (the process parameters such as pressure and temperature) and also on the internal structure of the plastic, which includes residual stresses, orientations and crystallinity as well as reinforcements (fibres and fillers). Partially crystalline and reinforced plastics show a higher tendency to shrinkage than amorphous, injection-moulded plastics. If the plastic is homogeneous and isotropic, the shrinkage percentages are the same in all directions (Fig. 1).

Fig. 1:

Dimensions of the (a) moulding nest and (b) the cooled injection moulded part

However, the shrinkage that occurs is not only influenced by the plastic, but also by the wall thickness of the moulded part, the processing temperature, the temperature of the mould and the cooling speed as well as the injection and post-holding pressure and their duration of action. The more complex the component is with regard to the sprue design, geometry, wall thickness and undercuts, the more complicated the shrinkage behaviour of the plastic moulding. As a rule, shrinkage then occurs directionally (longitudinally and transversely to the flow direction) and can cause distortion of the moulded part (Fig. 2).

Fig. 2:

Warpage of the cooled injection moulded part due to orientations

After the injection moulding process and the ejection of the moulded part, the reduction of residual stresses due to post-shrinkage begins. This is significantly influenced by the following factors:

• uneven wall thicknesses and wall thickness jumps,
• varying orientation states,
• uneven tool temperatures,
• non-uniform post-holding pressure, post-holding pressure time and
• different flow path lengths.

By annealing at temperatures < T_G, the post-shrinkage and residual stresses that have arisen as a result of the forming process can be reduced more quickly. However, this only possible if there are no large wall thicknesses or wall thickness jumps, as then the residual stresses can no longer be relieved and stress cracks can develop as shown in Fig. 3.

Fig. 3:

Formation of stress cracks (blowholes) in the centre of the component

Types of shrinkage

There are three different types of shrinkage:

• demoulding shrinkage,
• processing shrinkage and
• post-shrinkage.

The demoulding shrinkage, as a result of volume contraction, is measured immediately after the moulded part has been ejected from the mould by means of a standardised plate. If the cooling process of the plastic takes place in a mould nest (tool), defined demoulding inclinations must be applied to the moulded part in order not to destroy the part during the demoulding process by the ejectors.

The processing shrinkage S_VS is determined from the difference between the tool dimensions and the moulded part after 16 hours of storage in a standard atmospheres (see DIN 16901 replaced by DIN ISO 20457). In order to ensure the required design dimensions, both shrinkage parts must be taken into account in the dimensioning of the component by means of a corresponding oversize.

The subsequent post-shrinkage S_NS in operational use is caused by the relaxation process, which is characterised by the partial dissolution of the residual stresses, in a post-crystallisation as well as in structural entropy-dominated realignments of the molecular chains as a result of reorientations.

The calculation for all types of shrinkage, including time-dependent shrinkage, is also carried out according to equation (3), whereby the time-dependent moulded part volume V_F (t) must then be used.

Determination of shrinkage

The shrinkage S_V is calculated using the following general equation as a dimensionless or percentage relative characteristic material value.

$S_{V}={\frac {V_{W}-V_{F}}{V_{W}}}$

(3)

Here V_W is the volume of the moulding tool at standard temperature (23 °C) and V_F is the volume of the moulded part. Typical percentage values of shrinkage are listed in the following Table 1 (according to KAISER):

Table 1: Generell shrinkage measures for plastics

Plastics	Processing shrinkage (%)	Past-shrinkage (%)
Thermoplastic
amorphous	0.2 – 0.8	≈ 0
semi-crystalline	0.2 – 3.0	0.2 – 2.0

See also

• Tensile test residual stresses orientations
• Relaxation plastics
• Sink mark
• Dynamic-mechanical analysis (DMA) – General principles

References

• Die Maßhaltigkeit von Kunststoffformteilen. HERA AG Kunststofftechnologie, Firmen-Schrift (2006)
• Zöllner, Olaf: Grundlagen zur Schwindung von thermoplastischen Kunststoffen. Anwendungstechnische Information ATI 1120. Bayer MaterialSience, Firmenschrift (2003)
• Kaiser, Wolfgang: Kunststoffchemie für Ingenieure. Carl Hanser Munich Vienna (2006) p. 80 ff. (ISBN 978-3-4462-2069-0; see AMK-Library under N 12)
• Rudolf, Nathalie: Druckverfestigung amorpher Thermoplaste. Dissertation, Universität Erlangen-Nürnberg (2009) ISBN: 978-3-931864-45-3 file:///C:/Users/Grellmann/Downloads/NatalieRudolphDissertation.pdf
• Baur, E., Brinkmann, S., Osswald, T. A., Schmachtenberg, E.: Saechtling Kunststofftaschenbuch. Carl Hanser Munich Vienna (2007) p. 733 ff. (ISBN 978-3-446-40352-9; see AMK-Library under G 4-2)
• Illig, A.: Thermoformen in der Praxis. Carl Hanser Munich Vienna (2008) p. 30 ff. (ISBN 978-3-446-40794-7)
• DIN 16901 (1982): Plastics Mouldings – Tolerances and Acceptance Conditions for Linear Dimensions (withdrawn 2009-10; replaced by DIN 16742 2013-10 (withdrawn)
• DIN 16742 (2013-10): Plastics Moulded Parts – Tolerances and Acceptance Conditions (withdrawn; replaced by DIN ISO 20457)
• DIN ISO 20457 (2021-06): Plastics Moulded Parts – Tolerances and Acceptance Conditions
• DIN EN 1842 (1997-11): Plastics – Thermoset Moulding Compounds (SMC - BMC) – Determination of Compression Moulding Shrinkage
• ISO 294-4 (2018-12): Plastics – Injection Moulding of Test Specimen of Thermoplastic Materials – Part 4: Determination of Moulding Shrinkage
• Dangel, R.: Spritzgießwerkzeuge für Einsteiger. Carl Hanser Munich Vienna (2017) (ISBN 978-3-446-45043-1)
• Maier, R.-D., Schiller, M.: Handbuch Kunststoff-Additive. Carl Hanser Munich Vienna (2016) (ISBN 978-3-446-22352-3; see AMK-Library under G 83)
• Ehrenstein, G. W.: Mit Kunststoffen konstruieren. Carl Hanser Munich Vienna (2007) (ISBN 978-3-446-41322-1; see AMK-Library under G 42)
• Bruder, U.: Kunststofftechnik leicht gemacht. Carl Hanser Verlag Munich Vienna (2016) (ISBN 978-3-446-44957-2)
• Hellerich, W., Harsch, G., Baur, E.: Werkstoff-Führer Kunststoffe – Eigenschaften, Prüfungen, Kennwerte. Carl Hanser Munich Vienna (2010) (ISBN 978-3-446-42436-4; see AMK-Library under G 37)
• Käufer, H.: Arbeiten mit Kunststoffen. Springer Verlag, Berlin (2013) (ISBN 978-3-642-81201-9)