Long Fiber Reinforced Thermoplastic Composites
A general purpose screw that utilizes a typical three-zone style is best for processing Long Fiber Compounds. The screw has three sections: feed, compression, and metering. Metering should be done with an L/D ratio of 16:1 to 22:1, with a low compression ratio of 2:1 to 2.5:1.
A 100% ‘Free Flow’ fluted screw tip valve assembly is required to allow smooth melt flow. The nozzle/sprue orifice should have generous dimensions (0.250in/5.56mm) and be without sharp edges or severe convolutions. Do not use internally tapered tips or tips without a constant diameter pathway.
Material construction for screws and barrels can be the same as for standard reinforced materials.
The Right Screw
General purpose metering screw — L/D ratio 16-22:1 — 40% feed, 40% transition, 20% metering. Mixing screws should not be used
Low compression ratio — 2:1 to 2.5:1
Deep flights — Metering zone 3.5 mm, feed zone 7.5 mm
Free flowing check-ring — three-piece ring valve
Nozzle configuration should be general purpose with a minimum orifice diameter of 7/32 in (5.6 mm). Full tapered nozzle designs (nylon tip) are not recommended for use with long fiber reinforced materials
Three Piece Screw Tip Ring Valve
Screw Speed and Back Pressure
Slow — 20-50 rpm
Adjust to the lower end of indicated speed range
Adjust to low to moderate back pressure — 25-50 psi (0.17-0.35 MPa)