Systematic review of the non-isothermal powder bed fusion process for polymers

Laser based powder bed fusion of polymers (PBF-LB/P) is considered one of the most relevant industrial process in the field of polymer additive manufacturing (PAM). In the current state of PBF-LB/P processing, the polymer powder bed surface is preheated using infrared (IR-)lamps close to its melting point, for semi-crystalline polymers, and the energy required to fully melt the polymer is delivered by a laser beam. As the temperatures of the powder in the build chamber only changes for a short time (1-2 s) during the energy input, this is referred to as a "quasi-isothermal process". This process can be controlled for several polymers to produce end-use parts on an industrial scale. However, this process route has several drawbacks, such as powder ageing, which leads to reduced recyclability. It is also subject to long preheating and cooling periods that lead to high production times and high energy consumption. Parts produced via this technique have rough surfaces, which hinder industrial applicability. It is also disadvantaged by time-consuming, depowdering, and post-processing. To address these issues, several studies have shown results with lowered powder bed temperatures, where a larger proportion of the energy required for melting the polymer is induced by the laser beam instead of IR-lamps. This type of process control is referred to as a "non-isothermal" or "low-temperature" process. This review summarizes the different process strategies, focusing on the increased energy input required and the materials investigated, and identifies the current challenges and limitations. The results are promising in terms of industrialization, although questions regarding the economic viability of the approach remain unanswered.