particles of a fine powder of a thermoplastic synthetic resin having a suitable broad range between its initial glass transition temperature and its final t g , are partially coated with an anti - caking coating powder by blending at an elevated temperature to form &# 34 ; mottled &# 34 ; particles . the elevated temperature is defined in relation to the initial and final t g s as : ° c . but below the final t g . mottled particles , without being encapsulated , have the unique ability to maintain their individual particulate characteristics at a temperature at least 10 ° c . above the initial t g of the uncoated host powder , and flow freely under applied pressure . such pressure is applied by a rolling drum which spreads the mottled particles onto a target bed where they are selectively sintered with a controlled high energy beam , usually a laser beam . sequentially sintering contiguous layers of the mottled particles results in a porous sintered body . such a body may be used to make a prototype of an article , which may be difficult or too expensive to machine , then to form a shell mold for use in investment casting . mottled particles , when stored above their initial t g , form a bed from 10 % to 65 % more dense than a bed of uncoated host particles , even when the coating powder coats less than 50 % of the area of the particles of host powder .