First, the bulk density of the used substrate materials, a thermoplastic elastomer with a volumetric median particle size of 77 μm, was analyzed. As substrate material, virgin powder and powder that had passed through one processing cycle were chosen. The virgin powder shows an average bulk density of 0.39 g/cm3. With increasing numbers of processing cycles, the bulk density decreases to a value of 0.36 g/cm3. The packing density of the bulk material can be calculated by dividing the bulk density and the density of the solid material. Thus, the packing density of the used material is 41% and 37% for a solid material density of 0.95 g/cm3.
Besides the influence of the porosity of the substrate material, the influence of the used liquid resin and thus the surface tension and its viscosity on the infiltration behavior was determined. Therefore, the surface tension of the resins Araldite GY 764 and Araldite GY 793 was measured via the pendant drop method and calculated according the Laplace-Young equation. The resin with the trade name Araldite GY 764 shows a value of ± 37 Nm/m higher surface tension than the resin with the trade name Araldite GY 793 (± 22 Nm/m). Nevertheless, the surface tension cannot be linked directly to the infiltration speed because of the influence of the viscosity and contact angle on the infiltrated volume expressed in Eq. 3.
The viscosity and surface tension determine the absorption behavior of liquids in porous structures according to Eq. 2. For this reason, the temperature-dependent viscosity of the resins was analyzed. Figure 4 shows the complex viscosity dependent on temperature for a heating rate of 2 K/min. With increasing temperature, the viscosity shows a first decrease, which can be linked to the higher mobility of chain segments with rising temperature. For higher temperatures, the crosslinking or rather curing process takes place and the viscosity rises. The minimum viscosity is reached in a temperature interval between 100°C and 120°C for both resin and hardener mixtures. The curing reaction starts for a heating rate of 2 K/min at a temperature of 100°C. This value depends on time and temperature, and thus the heating rate plays a major role. Comparing the two resins, the curing behaves almost equally. The system with the trademark Araldite GY793 shows a slight delay in the viscosity increase.
The infiltration behavior of the used resins is shown in Fig. 5. Therefore, the droplet height is plotted against the interaction time. Medium values of at least ten droplets are represented. For a temperature of 20°C and a bulk density of 0.39 g/cm3, a total infiltration of the droplet into the substrate cannot be detected for either resin. The initial droplet height is higher for the resin with the trademark Araldite GY 764. This leads to the assumption that the volume of one drop of the resin GY 764 is greater than the volume of the GY793 droplet. The droplet height decreases more quickly during the first 5–10 s than during the remaining time. With increasing temperature, the infiltration time decreased dramatically for both systems. For a temperature of 60°C, the total infiltration takes only approximately 2 s. This acceleration of the infiltration process goes along with the reduced viscosity and thus the higher volume of liquid material, which can infiltrate the porous structure according to Eq. 2. For layer times of approximately 40 s in selective laser sintering, an infiltration time of only 2 s is favored because the infiltration itself must not enlarge the processing time.
The infiltration behavior of the resins in a substrate shows a higher porosity or rather lower bulk density of 0.36 g/cm3 (Fig. 5, right). The bulk density of the substrate changes from 0.39 g/cm3 to 0.36 g/cm3. This increase of porosity leads to an increase of infiltration speed. At room temperature, the resin with the Araldite GY764 trademark infiltrates the porous structure in < 60 s, whereas for a substrate bulk density of 0.39 g/cm3 after 80 s a droplet height of 0.5 mm is still measurable. For the substrate with the bulk density of 0.36 g/cm3, the deviation between the minimum and maximum values is larger than for the powder with a bulk density of 0.39 g/cm3. This may be a result of inhomogeneous packing and thus an irregular infiltration. Comparison of the two resins demonstrates a slightly faster infiltration for Araldite GY 793. As shown before, the infiltration speed increases with rising temperature because of the reduced viscosity of the resins. For temperatures > 60°C, the absorption of the liquid is even faster and the droplet height cannot be detected any more.
Besides the experimental evaluation of the infiltration or rather absorption behavior, the model-based description is of main interest. Therefore, the Washburn equation was used to calculate the decrease of droplet height depending on time. As a simplification, the decrease of droplet height was equalized with the infiltration length. This is incorrect because of the volume of the infiltrated particles itself. The form factor in the modified Washburn Eq. 3 is not determined but assumed to be between 0.2 and 0.4 according to the rough surface of the particles and resulting in uneven pore structure. Figure 7 shows the comparison of the measured and calculated droplet height dependent on the infiltration time. The pore diameter was calculated by using the following equation:17
$$ D_{\text{Pore}} = \frac{2}{3} \cdot \frac{\varepsilon }{1 - \varepsilon } \cdot d_{50,3} $$
(6)
with the porosity ε and the median particle diameter d50,3.
The viscosity value was taken from the measurements in Fig. 4, and the contact angle was idealized as 85°. To determine the droplet height dependent on time, the initial droplet heights of the infiltration measurements were used as default values. For 20°C, the calculated cure for a form factor of 0.4 fits well to the measured curve. With increasing form factor, the deviation from the measurement becomes bigger. For surrounding temperatures of 40°C and 60°C, the curves with the form factors 0.2 and 0.3 are much closer to the measurements. Therefore, a form factor of 0.3 was chosen for the further calculations (Fig. 6).
A comparison of the modeled and measured infiltrations curves for the different resins, temperatures and bulk densities is represented in Fig. 7. The calculated and measured values partially demonstrate a high level of consistency. Nevertheless, some deviations between the measured and calculated values are visible. This can on the one hand be traced back to the measurement uncertainty of the infiltration experiments. Especially for high temperatures, the recording rate of the used camera system is too low. On the other hand, according to the Washburn equation the pores in the bulk materials are assumed to be cylinders even if a form factor was implemented. However, it is a three-dimensional network with different types of pore geometries. Nevertheless, for the simplification and assumptions carried out, the Washburn equation can be used to predict the infiltration behavior of a specific combination of bulk material and liquids.
Besides the calculated and measured infiltration behaviors, the feasibility of the aimed for process must be analyzed. Therefore, Fig. 8 shows microscopy images of a cross section of a sample of PEBA with the GY 764 resin. The samples were prepared in a DTM Sinterstation 2000 selective laser sintering system. The building chamber temperature was 130°C (see DSC diagram Fig. 4), and the liquid droplet and the powder material were exposed to a CO2 laser. The images show that the infiltration depth of the liquid is higher than the laser material interaction zone, which is visible as white areas. The used resin is absorbed into a thickness of approximately 1000 μm. In regions far away from the surface, the PEBA is not molten, and the particles glue together because of the surrounding epoxy material. Nevertheless, these experiments show a principle feasibility of the represented hybrid process.