## 4 Conclusions  
In this study, calcium carbonate-polyamide 12 composites were produced via twin-screw compounding using a calcium carbonate blend of discrete particle sizes as functional filler material. The filler particle size distributions and filler ratio were improved to optimize the dual functions seen in the previous investigations. The melting, as well as crystallization behavior of polyamide 12, can be manipulated, dependent on the particle size distribution and filler surface area introduction.  
The effect of the mineral filler on the thermal properties can potentially be optimized with a combination of a precise amount of coarse and fine calcium carbonate filler.  
* The increase of the melt flow, due to the introduction of a coarse filler fraction, can be reduced with the introduction of a finer filler into the blend. This increase in melt viscosity generated by the coarse filler can be reduced up to 60% with the precise amount of fine filler in the blend. If the fine filler fraction exceeds the optimum range of 80-130 m\({}^{2}\) per 100 g polyamide 12, the improvement on the melt viscosity is reduced or even lost totally;
* The degradation temperature of the polymer compound can be increased by up to 6% with the implementation of at least 10 m\({}^{2}\) of the coarse and 80 m\({}^{2}\) of the fine filler fraction within 100 g of polyamide 12;
* With the combination of 10 m\({}^{2}\) of coarse and 130 m\({}^{2}\) fine filler in 100 g of polyamide 12, the melting transition can be sharpened up to 25% (reduced time required);
* Due to the higher single-particle-volume of the coarse fraction, the effect on the overall degradation and melting properties is reduced if the coarse filler fraction falls below the optimum threshold of 10 m\({}^{2}\) per 100 g polyamide 12. The same counts for when the finer filler fraction is increased significantly;
* The crystallization process at 170 \({}^{\circ}\)C can be accelerated approx. 60% with the introduction of enough additional nucleation points, independent of the ratio of coarse and fine filler material in the blend. The crystallization point is significantly increased by the higher particle number of the fine filler fraction, in comparison with the coarse filler introduction only.  
The thermal and flowability properties of a polyamide 12 matrix can potentially be optimized with a combination of a precise amount of broad size distribution coarse and narrow size distribution fine calcium carbonate filler. Out of the tested functional filler blends, \"Filler C\" and \"Filler D\" showed the most beneficial improvements in the overall thermal properties.  
These improvements found under twin-screw extrusion compounding of polyamide 12 demonstrate the possible usage of optimized calcium carbonate as a functional filler in additive manufacturing, which can potentially be transferred via a subsequent investigation into the selective laser sintering process.  
Conceptualization, F.I., G.H., T.C., P.G.; validation, G.H., T.C., P.G.; investigation, F.I.; resources, F.I.; writing--original draft preparation, F.I.; writing--review and editing, G.H., T.C., P.G.; visualization, F.I.; supervision, G.H., T.C., P.G.; project administration, F.I. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to company ownership.  
The authors gratefully acknowledge the support and Omya International AG for financial support as well as providing filler raw material and experimental laboratory facilities.  
The authors declare no conflict of interest.