Milled fiber: Initially, an end use was sought for short lengths of carbon fiber, but has now developed into a defined end product. Fiber is sized, cut into a short staple form and milled to give a carbon fiber powder, which is sieved and has become important for incorporating in resin matrices, to confer EMI/RFI shielding, and in automobile air bags, to provide an electrical path for rapid ignition of the explosive charge.
Milled carbon fibers are typically 30-3000 um long, averaging about 300 um with a mean L/D ratio of 30.
Chopped carbon fiber: Chopped carbon fiber is generally cut 3 or 6mm long and sized with glycerol for incorporating in processes using a water carrier or a thermoplastic size such as polyurethane for integrating with thermoplastic matrices. The fiber can be chopped with a reciprocating knife, or a rotary blade. Cutting to length of 1mm can be achieved by chilling the fiber in liquid air prior to cutting.
Unidirectional fabrics: Definitions do vary, but a unidirectional fabric is normally taken as a fabric with at least 90% of the fiber weight in one direction, which can be in either the warp or the weft direction. The primary fiber can be held in position by bonding using a meltable thermoplastic, hot melt or thermosetting adhesive, weaving or stitching. Fibers are straight and uncrimped and mechanical properties are only bettered by prepreg, which has no secondary material other than the resin matrix system holding the tows in position.
Non-woven UD fabrics: A non-woven carbon fabric is made from fibers held in position by a binder, which can include melt bonding powders and meltable fibers, thermosetting resins and thermoplastic polymers.
Woven UD fabrics: Warp UD fabric: This fabric has the primary fiber in the 0 direction.
Weft UD fabric: This fabric has the primary fiber in the 90 direction.