## 3 Results and Discussion
### Thermal Properties
#### 3.2.2 Melt Properties  
As it was already shown in the previous investigation, the increase in the melt flow due to the introduction of a solid mineral filler shows a significant dependency on the filler particle size as well as the processing-induced filler surface in the compound matrix. Figure 6 shows how the melt flow increased more significantly if only 10 m\({}^{2}\) of coarse mineral filler is implemented into 100 g of polyamide 12 instead of finer mineral filler. Blending the same amount of coarse filler material with a finer fraction showed no significant increase in the  
Figure 4: SEM images of the filler distribution within the polyamide 12-CaCO\({}_{3}\) composites: A.5; B.5; C.10; D.10; E.10; F.10 (from left up to right down).  
melt flow. Therefore, the overall increase can be reduced by approx. 50%. Sample 'Filler F10' shows how the melt flow increases up to 20% if the fine mineral filler fraction exceeds the optimum range. This effect was shown by using finer filler particles with a smaller single-particle volume than that of coarser filler particles. This results in a better melt flow in comparison with the implementation of only coarser calcium carbonate particles. This results in a better melt flow during a selective laser sintering print, hence higher final part density.  
Figures 7 and 8 show how the melting point can be manipulated with the introduction of a precise amount of coarse mineral filler into the polymer matrix. The possible melt-point reduction of approx. 1 \({}^{\circ}\)C with the implementation of 10 m\({}^{2}\) of coarse mineral filler (Filler A.5), can also be achieved with Filler C.10, Filler D.10, as well as Filler F.10. All three filler combinations introduced approx. 10 m\({}^{2}\) of coarse filler material, where the finer filler fraction varied and did not show any significant effect on the melt point manipulation. Referring to the particle size distribution of the coarse fraction in Figure 1, the sensitivity of melting point to the use of a small amount of coarse filler A in the blend relates to particles significantly larger than the volume median, d\({}_{\rm v50}\), which control the thermal properties more strongly. Filler E.10 showed how the melting point is not affected if the coarser mineral filler fraction is reduced below the optimum range of 10 m\({}^{2}\) per 100 g polymer matrix. The same effect can be seen in Figure 7, which shows the melting transition width behavior, given by the peak width of the total time over which the melt  
Figure 5: Thermogravimetric analysis–degradation temperature comparison.  
Figure 6: Melt properties–melt viscosity comparison at 190 \({}^{\circ}\)C, 2.16 kg.  
transition occurs. The melt peak width can be reduced by up to 25% if the compound is filled with 10 m\({}^{2}\) of coarse filler material within 100 g of polyamide 12. Filler E.10 and F.10 showed how an extensive amount of the fine filler fraction masks the benefits of the coarser fractions. Considering the difference in the thermal properties between the functional filler and the polymer matrix (higher thermal conductivity and lower specific heat of calcium carbonate), the energy taken up by the filler material is taken up quickly and released more rapidly to the polyamide 12, surrounding the filler particle. More thermal energy can be transmitted due to the greater single-particle volume of the coarser calcium carbonate particles, in comparison with a multitude of single fine filler particles. This results in a greater influence on the melting behavior of the compound with the inclusion of a few coarse instead of many fine calcium carbonate filler particles [1]. The extensive number of fine filler particles take up all the energy and therefore hinder the more ready distribution of the stored energy by the coarser fraction. This results in the loss of the melt energy efficiency effect given by the coarse calcium carbonate filler particles.