Polystyrene (PS) for Prototype Development on CNC Machines and 3D Printers:
Polystyrene is available in sheet stock, rod stock, and in various shapes. It is a great candidate for subtractive machining processes on a CNC machine. Colors are usually limited to clear, white, and black although colors can be added and it is an excellent candidate for external painting.
Polystyrene (PS) for Injection Molding:
General Purpose Polystyrene (GPPS) and High Impact Polystyrene (HIPS) are probably the most commonly used PS resins for injection molding. GPPS is clear, but brittle (think of a CD jewel case) while HIPS is opaque and much less brittle.