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SIGMA-crystallization: Semicrystalline Thermoplastics - Understanding Post Shrinkage

Semicrystalline thermoplastics perform a phased transformation during solidification. The simply structured macromolecules will form crystalline structures adjacent to each other (spherulites and Shish-Kebab morphologies). This process is defined by the characteristic transition region of the classical pvT diagram and is characterized by a significant drop of the specific volume.

To what extent is the measured pvT isobar dependent on the chosen measuring conditions and what is the real pvT behavior of a polymer in the injection molding process? What is the impact of the highly dynamic cooling behavior of the polymer, especially the cooling rates?

Some laboratories now have the ability to measure pvT curves at increased cooling rates (Figure 2). It can be seen that with an increased cooling rate the rising of the isobars in the transition region flattens (tendency to amorphous behavior). The crystallization is impeded, the specific volume does not decrease as much, and the potential for post-crystallization develops.

SIGMA-crystallization displays this behavior. The crystallization simulation has three advantages:

  1. A consistently accurate calculation of the shrinkage & warpage for semicrystalline thermoplastics and the important shrinkage mechanism is understood and considered.
  2. The crystallization process is transparent and can be visualized. The crystallization rate of a part after ejection and complete cooling down is shown as well as those areas where high cooling rates impeded the complete crystallization. This is a clear indicator for post shrinkage.
  3. If there is a high probability for post shrinkage then the production cycle needs to be changed or a tempering process should be considered.

Pic. 1. Cooling-rates at the beginning of the packing-phase

Pic. 2. Cooling-rate depending pvT-data for a PA6 polymer