Polymeric Composites with Different Constituents - Introduction to Polymeric Composites with Rice Hulls - page 79

materials that are also cost-effective: polymeric LFRT are providing them with great
possibilities.
LFRT are a recent development in which PP or other thermoplastic polymers are
compounded directly with long glass fibres and the moulded. These new polymeric
composites are the 'buzz' of the automobile industries (particularly in the USA and
Europe) and considered to be one of the most important trends in the plastics industry.
Glass fibres of length ≈12≤50 mm give much higher stiffness, strength and toughness
than the 3-mm fibres used up to now.
LFRT have excellent mechanical properties and stiffness:weight ratios, which is of
great interest to the automobile industry. In-line compounding processes for long-
fibre materials offer users more flexibility because they can compound and process
such materials in accordance with their own formulations and also use ready-made
compounds. For this process, gravimetric feeders are used to feed the main polymer
as well as the additives. Key features of a feeder to achieve high accuracy over short
intervals are resolution of its weighing device and the response of its process controller.
Lengthy performance timescales permit a given blending accuracy to be attained with
a poor-performing weighing system. However, to achieve the same accuracy in a short
interval (a characteristic of continuous mixing operations) a much higher weighing
performance is required.
Three processing technologies are used for production: E-LFT, D-LFT and S-LFT.
E-LFT is direct in-line compounding and extrusion of LFRT as profiles or sheets.
D-LFT is direct in-line compounding and compression moulding of long-fibre
thermoplastics. S-LFT is direct in-line compounding and injection moulding of LFRT.
All three processes use gravimetric continuous feeders for accurate continuous feeding
into the compounding extruder. Long fibre-thermoplastic composite technology uses
special screw configurations for single and twin extruders.
The base polymer is, in general, fed with a single screw extruder or vibratory loss-
in-weight feeder. The integrated vacuum receiver is responsible for the steady refill
of the feeder. Additives such as colourants and stabilisers as powders or pellets are
added via smaller loss-in-weight feeders (single or twin screw, vibratory feeders or
bulk solid pump feeders). Edge trimmings or recycled materials can be shredded and
fed back into the process at a predetermined ratio using vibratory loss-in-weight
feeders. Glass-fibre filaments are pulled into the extruder from continuous glass
rovings on bobbins by rotation of the screw shaft. The throughput of the fibreglass
is volumetric and should be quite steady. Each fibre strand is monitored by a sensor.
The gravimetric throughput can also be checked by the bobbins mounted on scales
and by measuring weight loss over time.