The goal in processing long fiber thermoplastics is to keep the glass fibers as long as possible as this enhances mechanical properties. Amut and RCT have jointly developed a disc pump to preserve long fibers during processing. The new pumping device is expected to achieve this by wetting out continuous fibers with a pre melted polymer, cutting them to a specific length, and then feeding them back into the downstream section of a single or twin-screw extruder.
The rotary-channel pump can operate as a side-stream device, taking melt from the extruder upstream and returning wetted, chopped glass into the same extruder downstream. Alternatively, the pump could be situated between two separate extruders; one upstream for resin melting and the other downstream to disperse fibers before pelletizing or shape extrusion. The Nexxus Channel F (fiber) pump can process continuous fiber to any length, and can wet out glass fibers with even very-high-viscosity resins, down to fractional melt indexes of 0.2 MFI.
The Nexxus pump uses two or more narrowly spaced discs with a straight-sided groove or channel in the rim like discs in a pulley. Melt and continuous fiber enter this channel simultaneously, then travel around an arc as the disc turns inside a tight-fitting cylindrical housing. A thin film of melt coats the channel all the way around the disc. Fibers are pulled into the melt coating under tension from drag flow as the discs turn. Tension spreads the fiber bundle into a flat tape at the bottom of the channel, wetting fibers out rapidly and evenly. The square or C-shaped channel is enclosed in a heated casing, comparable to an extruder barrel. The difference is that the pump channels align melt flow at a 0� angle to the channel walls and casing, so melt infiltrates the fibers evenly. There is no forward motion along the axis of rotation, as there is in an extruder screw. The melt and fiber are scraped out of the disc channel of the pump with a wedge on the opposite side from where they enter. Melt and fiber are then pushed by the wedge into a discharge channel. The current prototype of the disc pump has a discharge channel that widens out so that the liquid slows down, causing the uncut fibers to bunch up like an accordion. This may result in longer cut fibers after the roving passes the internal cutter, which can cut at preset intervals. Besides glass, the cutter can handle nylon, PET, carbon, aramid, and natural fibers like jute and hemp. The long-fiber compound then flows back into the main single- or twin-screw extruder.
A �needling system� for gentle dispersion of fibers during the final extrusion step was also developed. This system uses inter-raking needle-shaped elements to separate the fibers inside the extruder. In a single-screw extruder, the splined needling element fits on the end of the screw inside a barrel extension with internal pins to counter-rake the needles on the screw. In a co-rotating twin-screw extruder, intermeshing �needling� elements in a mid-section of the screw counter-rake each other.
RCT�s research prototype pump has three disc channels, each capable of pumping about 44 lb/hr. For higher throughputs, Ponzielli plans to add more discs of the same diameter. For example, 24 discs could put out about 500 kg/hr. The experiments with long-glass extrusion on single-screws have been conducted since 1999. Continuous-fiber compounding trials with the Nexxus pump so far have focused on direct extrusion of glass-reinforced pipe, sheet, and long-fiber pellets using PP, HDPE, LDPE, and ABS with loadings of up to 60% glass.
The Nexxus pump setup in Amut�s lab has a single-screw extruder with 48 mm diam. and 35:1 L/D. The pump is installed at about 12 D, and fiber-filled melt re-enters the extruder 2D further down. Amut and RCT have extruded PP with 30-60% glass and found excellent mechanical properties