HIPS (High Impact Polystyrene): 

HIPS is an engineering thermoplastic with good dimensional stability, good impact resistance, excellent machinability, excellent aesthetic qualities, and it is easy to paint and glue. Within traditional manufacturing processes, HIPS applications include machined prototypes, low-strength structural components, housing and covers. 

Within the 3D printing community it retains all the traditional applications. It is also simmilar to ABS and some times used in ABS-HIPS blends. With the advent of dual extruder 3D printers, HIPS gains popularity as a dissolvable support material for PLA, ABS, PC, and PC-ABS parts, thus allowing for true free form fabrication of otherwise impossible to print 3D parts. 

HIPS 3D printing:

HIPS, like many engineering thermoplastics, will contract when cooled, therefore it will develop interlayer stress at the interface between a new hot layer 3D printed on top of a cold layer or bed surface. That can result in layer adhesion problems with the printing bed or in between 3D printed layers, and/or curling (upwards bending of the part corners and sides). This curling is more pronounced as the 3D printed object size is increased. This stress release behavior can be avoided by maintaining the entire 3D printed object at a temperature close to the glass transition temperature of the material and then cool it in its entirety after the 3D printing process is completed. Glass transition temperature is where the thermoplastic softens and becomes plastic (malleable, soft).

3D printing requirements:

HIPS 3D prints in similar conditions as ABS. A heated bed that can operate at at least 120 °C is required, along with an enclosure around the printed volume to prevent warping and a hot end that can sustain 250 °C continuously. 

HIPS is available in 1.75 mm and 3 mm.

Pyra 2 uses 3 mm filament, while the newer 3FXtrud 20 Uno, 3FXtrud 25 Uno, 3FXtrud 25 Duo, and 3FXtrud 30 Duo use 1.75 mm.