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Long fiber reinforced thermoplastics are a recent development.
Newer technologies, wherein PP or thermoplastic material is compunded
with long fiber and moulded in one operation are quite exciting.
This new approach called Direct Compounded form with long fiber
is known as DLFT. The market of DLFT in 2001 was approximately around
20,000 tonnes. However it is expected to grow to almost 35,000 tonnes
by 2006. This exciting new technology has got several new developments
that will drive the growth of DLFT. Some of the new developments
are described here.
DLFT was so far predominently used for larger parts weighing at
least 2 kgs. Two new technologies have been developed.
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The first one developed by University of
Maryland along with New Castle Industries, uses counter
rotating twin screw extruder with injection moulding machine.
The twin screw compounding is built into the reciprocating
mechanism. The first 3/4th of the barrel is a counter
rotating non intermeshing twin screw extruder. The fibers
are added to melt through a downstream port in a special
mixing section that is quite gentle to fiber because of
the absence of kneading. One of the two screws is longer
than the other. The plasticated shot is accumulated in
front of the longer screw, which has a non-return valve.
After the plastification, the screw stops turning and
the longer screw rams the shot into the mould, while the
shorter screw is stationary. |
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The second technology specially developed
by Composite Products Inc. is based on single screw compounding
joning the injection moulding step. The new development
of "shooter" cylinder directly transfers the
molten shot to injection moulding. |
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Several new developments using twin screw
equipments have taken place to extrude sheets with long
fiber between 20-40%. Berstoff and Leistriz have carried
out these developmental work. |
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Long fiber is hard on screw and barrel.
Stellite coating is specially developed because it is
more easily repairable compared to nitrided screws. |
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Most of the present work is with PP. Polyolefin
grafted malice anhydride is used as coupling agent to
improve adhesion. Some new work at pilot plant levels
is being done with ABS, Polyamide 66 and PET. |
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