Important Stages of the Production Process - Introduction to Polymeric Composites with Rice Hulls - page 87

the ability of equipment used to the cost-effectiveness of the composite. For this
purpose, the process engineers and composite producers are aware of the latest mixing
technologies available.
7.4 Single-screw Compounders
Twin-screw compounding extruders are the preferred choices for large industrial
compounding processes. However, a 'quiet revolution' has been taking place over
recent years with sustained research and development, and many single-screw
compounding extruders are now coming on the market. For the first time in decades,
several genuinely efficient single-screw compounders are available, some of which are
the result of advances in flow simulation and computer-aided design, whereas some
are improvements in metal cutting. Some of these machines can be used in-line with
extrusion and injection moulding.
Until recently, screw designers have tried to improve single-screw mixing with
variations on decades-old designs such as fluted cylinders, densely flighted screws,
and the 'pin' or 'pineapple' mixer, among others. Compounders who want more
dispersive action could also add separate devices onto the end of an extruder such as
a cavity-transfer mixer, which has pockets in the rotor and barrel that push and pull
the melt for extensional mixing or a planetary gear mixer, in which a short screw is
ringed by six smaller 'orbiting' screws.
Among the newest single-screw mixers are devices with floating rings. These mixers
have a smooth cylinder that 'floats' around the screw and has rows of holes around
the circumference. The ring rotates more slowly than the screw. The rotor inside
the cylinder has hemispherical 'dimples' or depressions, corresponding to the holes
in the ring. The combination of holes and depressions combine to force the melt to
move forward and backward. More rotor-stator systems, plus barrier designs and
other types of mixing sections also number among recent developments. The new
mixing designs do not claim to match the efficiency of twin-screw mixing but they
do a better mixing job than previous single screws, thereby boosting the properties
of finished parts, allowing higher colour concentrations and also permitting quicker
colour changes. They are supposed to also improve the homogenisation of ingredients
and give better output by ≈25%.
7.5 Twin-screw Compounders
In addition to the already established suppliers of twin-screw extruders, the advent
of new companies makes choices for the buyers more complicated. For example,