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and other impurities. By measuring the transmitted
light through a polymer film, the CCD camera
processes the image and identifies discontinuities
in the film.
The camera scans an area of 400 mm2 at a rate
of 4 pictures per second, with a resolution to 60 μ
m. There are options for higher resolution on a
smaller area or scanning a larger area at a lower
resolution.
The results can be displayed and analysed on
different formats. This first display logs the counts
of different particle sizes over time.
This analysis gives the distribution of particle sizes
over the test.
Finally this analysis shows the distribution of
particles by size and frequency.
Other applications of the twin-screw use the
dedicated ancillary equipment that designed to
make up the EUROLAB System.
biennial 07-08
Pelletising
When samples are required in pellet form, then a
strand die and traditional cooling bath and
pelletiser are used. This system includes a variable
length pelletiser, where the pellet length can be
selected from 1mm to 3mm in 0.5mm steps. If
mini-pellets are needed for micro-moulding, them
a combination of high draw-down with high cutter
rotation will give 1mm X 1mm pellets.
When working with highly filled compounds, or
water sensitive products, an air quench, face cut
system is available. Here the product is cut on the
face of the die, propelled into an air stream and
conveyed to a cyclone for collection. This method
is also very useful for new water sensitive,
biodegradable polymers being studied today.
Data Logging
When many different materials need sampling, it is
very useful to be able to record operating
conditions so that reproducible samples can be
prepared. This can be even more reliable if the key
operating conditions can be recorded and stored
as a recipe for future use. The touch screen
interface and PLC control allow this to be done
simply, with added security of password
protection.
Scale-up
In any laboratory system, the operator is always
concerned with scale-up to larger production scale
equipment. Form our experience the most critical
element in this scale-up, is heat transfer capability.
Most twin-screw extruders are variable speed, and
so shear rates can be matched between laboratory
and production extruders. Residence time in the
extruder depends on a combination of product
feed rate and screw speeds. Because generally
output is determined by the motor power installed,
and that is usually scaled to the volume of the
extruder, which again gives a good scale up.
However, heat transfer in dependent on the
surface area to volume ratio. This means that the
ratio of surface area to volume is inversely
proportional to the barrel diameter. i.e. if you
double the diameter of the screws, then volume
(and hence output) will increase by 8 times while
surface area, and (hence heat transfer capability)
will increase only 4 times.
Surface Area, Free Volume
relationships
Conclusion
In a modern masterbatch plant, quality control and
product sampling are key factors in the efficient
operation. By using small-scale extruders, fitted
with suitable ancillaries, and with reliability in
scale-up, the operator can have confidence in
developing new materials and formulations. This
paper has shown availability of such equipment.
A copy of this article is available on request by
quoting LR-51
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