TPT July 2013

99 / 104

July 2013 97
Article
Molecor Tech
Madrid, Spain
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At this point there were still important issues to be accomplished
from a technical point of view. The pipe conditioning was one
of them. The pipe had to be heated up homogeneously with
extreme accuracy in the three cylinders coordinates: axial,
hoop and radial. Other kind of heating methods like radiation
or contact were discarded because they were not able to
guarantee these specifications.
A special oven was designed for that purpose, with high
internal air movement speed, allowing close to a hundred
renovations per minute. Isolation was very carefully designed
to prevent heat loss and noise. Up to this point the prospects
of output and homogeneity were fulfilled.
The speed of expansion is very important in orientation. This
was another important challenge to deal with. Whereas the
speed of expansion is quite easy to control using water with
air this aspect is more sophisticated.
The expansion is executed by communicating tanks of
compressed air at higher pressure than the one needed to
expand. In the first place, the pipe to expand is at atmospheric
pressure, and the tanks are at high pressure. Graph 1
represents the flow that is able to pass through a hole of
certain diameters depending on gradient of pressure between
two different pressures.
As shown in the experimental graph, in order to maintain
constant, for example, a certain flow of 3,000 litres per
minute, from a deposit of nine bars to the pipe at atmospheric
pressure, the passing orifice has to change from a certain
diameter to a diameter four times bigger.
Thanks to the electronic regulation and fast answer pressure
detector and servo valves, this required flow can be maintained
during the main part of expansion stage, and a constant
speed of expansion is guaranteed.
Air chilling was another drastic change in the PVC-O concept.
It consists of freezing the pipe from orientation temperature to
a temperature below Vicat without water.
Following this a research project just for freezing was started.
Aspects such as speed of flow, geometry of pipes or even
air humidity were considered and the results obtained, as
shown in Graph 2, were used to design the final equipment,
optimising energy consumption and output.
A real machine was built to produce pipes from 160mm to
400mm Ø and tested in regular production with an extrusion
line based on a Cincinnati Argos 93. The mixure used
was based on PVC resin from Solvin and Naftosafe from
Chemson. The following results were obtained:
These huge savings resulted from maintaining constant the
mould temperature and avoiding the use of water as fluid for
the process. The energy for conditioning is only applied to the
plastic to be processed. The equipment is so compact that
the length of the whole line for manufacturing up to 400mm Ø
can be of 45m and the width is less than 4m. The line is fully
automatic, so no extra labour is needed.
Other positive features were also implemented such as
swiftness in mould changing, an operation that now takes less
than one hour for two people. All the technical developments
were implemented in a recipe system; in this way the
manufacturer only has to choose the pipe he wants to produce
and all the parameters change automatically to the new
conditions.
The global situation with ever increasing prices of raw
materials and the standardisation activities in many countries
are making PVC-O one of the most attractive investments
in the plastic industry. The superb quality of the product, in
addition to the important raw material savings that can reach
50 per cent in many cases, and access to more profitable
markets for high pressure applications, forecasts a bright
future for this material.
Graph 1:
Flow passing
through
some holes
of different
diameters,
and real curve
obtained after
regulation with
a consignee
flow of 3,000
litres/min
Graph 2:
Results from
a simulation
process of
freezing a
pipe internally
with air.
Map of
temperatures