Shrinkage in TPE  
Influencing factors, analysis and modern measurement methods 

In the injection molding of thermoplastic elastomers (TPE), shrinkage is a key aspect that has a direct impact on function, design and dimensional stability. Precise prediction and control of this effect are crucial - not only for the functional properties of the component, but also for reproducibility in series production. In our August newsletter, I shed light on the key influencing factors, the practical benefits of filling studies and the measurement of component shrinkage by optical testing.

Factors influencing shrinkage

Several factors have a direct or indirect effect on shrinkage. The following are particularly relevant: 

1. Demolding temperature: the higher the demolding temperature, the higher the residual energy in the molded part - and the greater the likelihood of subsequent dimensional changes. 

2. Wall thickness: Thick component walls store more heat and cool down more slowly. This leads to uneven shrinkage, especially in the core area. Different wall thicknesses also lead to different shrinkages and can be responsible for internal stresses in the component.

3. Holding pressure and holding pressure time: The holding pressure compensates for the shrinkage volume of the melt. Together with the amount and duration, it is the most influential parameter in the injection molding process. If the holding pressure is too low or the holding pressure phase is too short, cavities will form and shrinkage will increase uncontrollably. This can become apparent through sink marks or vacuoles. 

4. Proportion of fillers: Higher filler contents reduce shrinkage, as they stabilize the material volume or can even reverse the direction of shrinkage. At the same time, they influence the flowability, processing temperature and mechanical properties.

In addition to these main factors, parameters such as mold temperature, melt temperature, injection speed and cooling time also influence the shrinkage behavior - their weighting depends heavily on the selected TPE type and the component geometry.

Shrinkage measurement

Tactile methods are often used to measure shrinkage. Although this method is widely used, it has certain limitations with very soft TPEs. Even the smallest compressive forces during scanning can cause minimal deformation, which leads to measurement distortions and can ultimately affect the shrinkage values in the finished component.

To meet these challenges, KRAIBURG TPE has been using a non-contact optical measuring device since 2023. This method is based on precise distance measurement in accordance with DIN EN ISO 294-4, which is specifically designed to determine the processing shrinkage and post-shrinkage of thermoplastics. Standardized injection-molded parts measuring 60 x 60 x 2 mm are used as test specimens, which are measured both in the direction of flow and at right angles to it. Optical detection allows precise measurement - regardless of the Shore hardness of the material - and therefore provides a reliable basis for tool design and material selection.

Practical recommendations from everyday life

Finally, I would like to share a few practical rules from our experience with TPE projects:

• The demolding temperature has an often-underestimated influence - it is better to demold conservatively than too early.
• Carry out filling studies before systematically optimizing - everything else is a guessing game.
• Dimension the cold runner thick enough to allow the holding pressure to take effect.
• Compounds rich in fillers have a lower shrinkage - but may require adjustment during injection. 

We are happy to provide advice and assistance here and offer alternative compounds.

Conclusion

Shrinkage in TPE is a complex principle that can only be reliably controlled through the interaction of material understanding, reliable tool and process technology and suitable measuring instruments. Filling studies and targeted parameter variations in simulation and reality are the key to understanding the process - modern optical measuring systems open up new possibilities in terms of precision and efficiency.

How do you deal with shrinkage in your practice? I look forward to your insights and discussion in the comments.