Advanced Additive Manufacturing Material and Part Properties – Reduced Refresh Rates and Cooling Process Regarding Laser Sintering

With the use of PA 2221 material it is possible to use very low refresh rates of about 30% compared to 50% with the standard nylon 12 material PA 2200. Maintaining similar part quality characteristics, the powder consumption and waste are thus reduced significantly. As a result, the cost efficiency of the laser sintering process can be increased. The influence of the thermal loading on the powder age is analyzed with various methods, for example the melt volume rate and DSC analysis. Experiments are conducted along a test series with a rising number of build cycles using the refresh rate as well as the MVR value to adjust a defined used/virgin powder mixture ratio. Thereby, a representative ageing state for circulatory PA 2221 powder is achieved.

Since the ageing behavior of PA 2221 is known and characterized, experiments are performed to determine advanced, temperature-dependent material data, for example the mechanical, physical, thermal, electrical or impact part properties, which can be used for part design and FE analyses. These results are compared to the standard material PA 2200.

In previous investigations it has been shown that the position of a part within the part cake strongly influences its quality characteristics due to different temperature histories. Next to the part position, important job parameters like the part packing density or the build height influence the temperature history. Nevertheless, the temperature distribution of the inner part cake during the build process (warm-up, build and cooling down phase) and its influence on part and powder properties is less known yet.

In a first step, a temperature measurement system is developed and installed into an EOSINT P395 laser sintering system. Therefore, the build frame and the lift mechanism of the machine is modified. More than 50 thermocouples are attached to tubes and measure the temperature of the inner part cake during the whole build and cooling process. In a second step, the influence of important job parameters like the build height and the used layer thickness is analyzed for part-free build jobs. In addition, different temperature histories are correlated with powder and part properties, for example the powder age (determined by melt volume rate).

The results of the temperature measurements are used to develop a Finite Element (FE) model to simulate the cooling process. In this way, important thermal parameters of the bulk powder are analyzed. Different cooling down strategies can be tried out without the need of further experiments.

An optimization of the cooling process, which is vital for better and more constant part qualities, as well as a correlation between individual cooling rates and part crystallinity (and thereby shrinkage, warpage and mechanics) is in focus for future work.