Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS AND PLANT FOR DEHUMIDIFICATION, HEATING AND INJECTION MOULDING OF
GRANULATED PLATSTICS MATERIALS
Description
Technical field
The present invention relates to a process of dehumidification and
of injection and moulding for granulated plastics materials according to
the characteristics set forth in the preamble to independent Claim 1.
The invention is also directed towards a plant arranged to operate in
to accordance with the process, according to the preamble to independent
Claim 10.
Technological backgiround
In the field of methods for the processing of granulated plastics
materials by injection and moulding, it is known that the granules have
is to reach the injection and moulding press with a moisture level which is
as low as possible (or in any case is controlled in dependence on the
type of final product to be produced). For this purpose, the granules are
normally subjected to a dehumidification step immediately upstream of
the injection and moulding step.
2o This requirement is particularly necessary for plastics materials
which have a high hygroscopic capacity such as, for example,
polyethylene terephthalate (PET), to the processing of which the present
invention is preferably but not exclusively applied.
In known processes, the step of dehumidification of the granules
2s is carried out in a hopper by stripping of the moisture contained in the
CONFIRMATION COPY
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granules with air which is caused to flow as a counter-current. To
promote the dehumidification process, the air admitted to the hopper
(process air) has an extremely low moisture content and a relatively
high temperature.
s With regard to this latter characteristic, the need is known to
maintain the temperature as high as possible, not only in view of
possible improvements in terms of the drying of the granules but also in
view of a reduction in the cycle times of the injection and moulding press
which is typically disposed immediately downstream of the
to dehumidification step.
In this apparatus, in fact, before the granules are injected into the
mould, they have to be brought to the melting point of the polymer in a
suitable heating chamber, requiring a certain amount of time which
forms part of the overall cycle time of the press.
is The increase in temperature within the dehumidification step is
limited, however, by the fact that high temperature promotes undesired
oxidative degradation reactions of the polymer, compromising the final
quality of the product.
It is therefore necessary to carry out the dehumidification process
2o at a temperature adapted to the conflicting requirements for quick and
effective drying of the granules and for maintenance of the qualitative
integrity of the polymer.
A range of temperature values which is normally used as an
optimal compromise between the requirements set out above is between
2s 180~C and 190~C. At these temperatures, the PET granules are
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supplied to the injection and moulding apparatus which brings them to a
temperature of about 280~C - 290~C, ready to be injected into the
mould.
Description of the invention
s The problem underlying the present invention is that of providing
a process of dehumidification and of injection and moulding for
granulated plastics materials, as well as a plant arranged to operate in
accordance with the process, which are designed structurally and
functionally to respond to the requirements set out above, overcoming
to the limitations of the prior art mentioned.
This problem is solved by the present invention by means of a
process of dehumidification and of injection and moulding, as well as a
plant operating in accordance with the process, according to the
appended claims.
is Brief description of the drawings
The characteristics and the advantages of the invention wilt
become clearer from the detailed description of a preferred embodiment
thereof which is described by way of non-limiting example with reference
to the appended drawing in which the sole Figure 1 is a schematic view
20 of a plant for the dehumidification and the injection and moulding of
granulated plastics materials, arranged to operate in accordance with
the process of the present invention.
In the appended drawing, a plant for the dehumidification of
granulated plastics materials and for the injection and moulding thereof,
2s formed in accordance with the present invention, is generally indicated
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1.
In the preferred embodiment described herein, the plant 1 is
intended for the dehumidification of polyethylene terephthalate (PET)
granules and comprises, as a whole, a dehumidification unit is for
s dehumidifying the granules, and an injection and moulding unit ib
disposed downstream of the dehumidification unit la.
The dehumidification unit 1a in turn comprises a hopper 2, a
granule-loading system 3 for loading the granules into the hopper 2, as
well as a circuit 4 for the admission and treatment of the process air for
to removing moisture from the granules inside the hopper 2.
The hopper 2 is loaded with granules from above by means of the
loading system 3 which comprises a suction line 5 for the pneumatic
transportation of the granules. The suction line 5 opens at a first end in
a granule storage .tank 6 and is connected to a cyclone 7, which is
is disposed at the top of the hopper 2 and in which the granules are
separated from the transportation air and are allowed to fall into the
hopper, and then to a vacuum pump 8.
Inside the hopper 2, the granules come into contact with process
air which is admitted from the bottom 2a of the hopper 2 as a counter-
2o current relative to the granules, at a dehumidification temperature.
The process-air admission and treatment circuit 4 comprises a
dehumidifier 10, for example, of the type with molecular sieves, which is
connected downstream to a heater 11 and then to a line 12 for
admitting the air to the hopper 2. The circuit 4 is completed by a line 13
2s for recovering the process air from the hopper; the line 13 connects the
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top of the hopper 2 to a pair of filters 14 and then to a heat recovery
unit 15 and finally to the dehumidifier 10.
The operation of the dehumidifier 10 is controlled by a control unit
16.
s The above-mentioned components are dimensioned and
controlled in accordance with procedures known in the technical field in
question and will not therefore be described in greater detail.
The bottom 2a of the hopper 2 is connected downstream to the
injection and moulding unit lb which comprises, for example, an
injection and moulding press arranged for the manufacture of PET-based
articles or semi-finished products.
The injection and moulding press in turn comprises a heating
chamber for bringing the granules to a moulding temperature which, in
the case of PET, is between 280~C and 290~C, and a mould into which
is the molten plastics material is admitted. These presses are also known
in the technical field in question and a detailed description thereof will
therefore be omitted.
According to a principal characteristic of the invention, granule-
heating means, generally indicated 20, are interposed between the
2o dehumidification unit 1a and the injection and moulding unit 1b.
The granule-heating means 20 comprise a hopper 21 and a circuit
22 for the heating and admission of a hot gas into the hopper 21 in
order to heat the granules contained therein to a supply temperature for
supply to the injection and moulding unit.
2s The hopper 21 is connected at its top to the bottom of the hopper
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2 by means of a duct 23 and at its bottom to the injection and moulding
unit ib by means of a duct 24 and is preferably provided with stirring
means 21b for keeping the granules inside it in motion.
The circuit 22 comprises gas-movement means, for example, a
blower 25 which is connected by means of a line 26 to a heater 27,
preferably of an electric type, and to the bottom 21a of the hopper 21,
where the gas is put in contact with the granules, flowing as a counter-
current relative thereto, towards the top of the hopper 21. From there,
the gas is led back to the blower 25 by means of the return line 28, after
passing through a filter 29.
The gas used in the circuit 22 is a substantially oxygen-free gas,
that term being intended to indicate generally a presence of less than
about 5% of oxygen. The gas is preferably technical nitrogen and can be
introduced into the circuit 22, for example for operations for the
~s replenishment thereof, by means of a coupling 30 provided in the line
26.
The granule-heating means 20 further comprise a control unit 35
which is arranged for the control and overall management of that step of
the process, as described in greater detail in the following paragraphs.
2o The process according to the present invention comprises a first
step for the dehumidification of the granules with process air. The
granules are loaded into the hopper 2 at the top thereof by means of the
loading system 3 and the process air is admitted from the bottom 2a as
a counter-current, at a temperature such that the granules inside the
25 hopper are at a dehumidification temperature of between 180°C and
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190~C. The process air output from the hopper 2 preheats the process
air output from the dehumidifier 10 in the heat recovery unit 15 and is
sent to the dehumidifier 10 in order to be treated in the molecular sieves
thereof. At the output from the dehumidifier 10, the process air has a
very low moisture level with a dew point typically of between -50~C and
-60~C.
Before the process air is admitted to the hopper 2, it is then
preheated in the heat recovery unit 15 and is brought to the final
temperature by the heater 11.
to The granules are then supplied, by falling through the duct 23,
into the hopper 21 where they are heated by contact with the gas
admitted through the circuit 22 to a supply temperature between the
dehumidification temperature and the moulding temperature.
The granules are preferably brought to a supply temperature of
1s between 200~C and 250~C, even more preferably between 220~C and
230~C.
It should be noted that the granules are heated to a supply
temperature lower than the softening point of the granules which, for
PET, is of the order of 270~C, so as not to create problems when they
2o pass from the hopper 21 to the injection and moulding press due to
sticking of the granules. In any case, the mixing to which the granules
are subjected by virtue of the action of the stirring means 21b is such as
to prevent the establishment of local hot spots which could lead to the
above-mentioned sticking problems.
2s The granule-heating means 20 are of dimensions such that the
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granules remain in the hopper 21 for sufficient time to reach the supply
temperature indicated above.
Granules are thus supplied to the injection and moulding unit at a
higher temperature and the unit thus requires less tii-ne to bring them to
s the moulding temperature.
The heating step thus promotes the process of the plasticization
of the granules within the injection and moulding press, also leading to
less thermal degradation of the PET and a lower energy consumption for
the operation of the press.
to As stated above, the gas is preferably technical nitrogen. It is
found that undesired degradation reactions of the PET are thus not
triggered, in spite of the high temperature reached.
Although the use of technical nitrogen remains the greatly
preferred option, the gas may be constituted by air with a low oxygen
is content. The gas may even result from the use, in closed circuit for a
suitable period of time, of process air drawn from the circuit 4 (variant
shown in Figure 1 by means of the discontinuous connecting line 31). If
the process air is recirculated continuously in the hopper 2l, the fraction
of oxygen therein will in fact reduce over time because of oxidative
2o reactions with the plastics material present therein until it reaches the
percentages indicated above which enable further possible degradation
reactions to be prevented. Naturally, during this recirculation stage in
which the oxygen fraction is still high, a lower temperature of the gas
will have to be maintained unless a generally lower quality of the
2s granules output from the hopper 21 is acceptable.
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The present invention thus solves the problem discussed above
with reference to the prior art mentioned, at the same time offering
many further advantages, amongst which is an increase in the
productivity of the dehumidification and injection and moulding plants
s without compromising the quality of the final product.
Another advantage is that operation in an environment that is
substantially free of oxygen also minimizes degradative reactions which
might lead to the undesired formation of aldehydes, particularly
formaldehyde and acetaldehyde which, with their strong and pungent
to odours, can adversely affect the flavour of any beverage contained in
bottles produced from the granules of plastics material.
