In a previous article, the process window in the Selective Laser Sintering (SLS) process with polyamide 12 (PA12) powder was determined with dynamic DSC measurements. It was shown that the onset of melting and crystallization are important parameters in the selection of suitable materials as well as in the determination of the process settings. Read the article here! Furthermore, the onset of crystallization is time-dependent and, therefore, isothermal DSC measurements can be used for more advanced studies of SLS materials.
During the SLS process, the molten portions of the part are kept in the molten state to reduce the effects of warpage. However, because the build takes several hours to complete, changes in temperature as well as the prolonged time can lead to crystallization. Read our introduction to the SLS process here!
How to set up the isothermal measurement
The isothermal crystallization behavior of a PA12 powder was studied using a NETZSCH DSC 214 Polyma.
Step 1: The sample was heated from room temperature to above melting at 200°C at 20 K/min. It was kept there for 1 minute to erase the sample history.
Step 2: It was then rapidly cooled to the isothermal temperature step (168, 167, 166, 165, 164, 163, 162°C in Figure 1) using a high cooling rate of 125 K/min to prevent reorganization processes that occur with PA12 at slow cooling rates. Both the ability to achieve a fast cooling rate with regular sample sizes and the ability to precisely control the temperature are features of the DSC 214 Polyma that are extremely valuable for this analysis.
Step 3: Next, the sample was kept at the isothermal temperature for 30 minutes to study the crystallization process.
Step 4: The sample can then be cooled down OR the sample can be heated back up to 200°C at 10 K/min (as was done here) to get the full picture and observe the melting behavior after the isothermal crystallization step. All other measurement conditions are summarized in the following table:
Table 1: Measurement conditions
|Pan||Concavus Al, unpierced|
|Sample weight||5 mg|
|Gas flow rate||250 ml/min|
|Temperature steps for |
isothermal measurement at 165°C
|25°C to 200°C (20 K/min), |
constant for 1 min,
200°C to 165°C (125 K/min),
constant for 30 min,
165°C to 200°C (10 K/min), cool down
Analyzing the crystallization peak temperature
Figure 1 shows the isothermal crystallization behavior at different temperatures from 165°C to 162°C right below the build envelope temperature. The crystallization peak temperature, tmax, is analyzed as the peak of the curve from the start of the measurement. Therefore, the values depicted here were normalized in the Proteus® software for the actual start of the isothermal step.
Figure 2 shows the corresponding normalized temperature profile. The isothermal temperatures were reached about 10 minutes after the measurement start. Even at these high cooling rates of 125 K/min, the temperature only overshoots by ± 0.1 K and hits the set temperature in less than 30 s.
What does that mean for my Selective Laser Sintering (SLS) process?
These results highlight that even at a build envelope temperature of 168°C, crystallization starts after about 10 minutes (figure 1) and reaches its peak after 23.7 minutes. While the top layers will be reheated closer to the melting temperature with each additional layer, it becomes obvious that lower layers will eventually stay at 168°C or could even cool further. Thus, given the long build durations of typically several hours, crystallization will occur and has to be taken into account.
To further understand the crystallization rate as a function of time and temperature as well as to model the process – for example, to determine warpage or residual stress build-up – the crystallization kinetics can be studied. How to set up and interpret these analyses will be shown in future articles.