The present invention relates to a method and an apparatus
for the continuous production of expanded plastic material for forming panels and
It is known that expanded polyurethane foams are typically
obtained by pouring a so-called reactive, liquid mixture, constituted by the reagents,
i.e., isocyanate and polyol, water, catalysts and so forth, and by any blowing agents
such as hydrocarbons, hydrochlorofluorocarbons, hydrofluorocarbons, and the like.
In a few seconds, the reactive mixture starts to react,
generating heat and carbon dioxide produced by the water-isocyanate reaction.
The carbon dioxide, together with the gases generated by
the evaporation of any blowing agents, due to the heat generated by the chemical
reactions, causes the expansion of the reactive mixture so as to generate the foam.
On the basis of this type of method, plants have been available
for many years which allow the continuous production of low-density rigid foam panels
for thermal insulation having, in cross-section, a rectangular shape or otherwise
variously shaped; these panels can be produced in different lengths.
The foam of the lower and upper surfaces of the panels
is coupled to substrates of various kinds, such as for example paper, bituminized
paper, wood, metal plate and the like, which form outer claddings of said panel.
These panels are widely used, in particular, as insulators
in the field of industrial building or in the field of industry or refrigeration.
Plants for manufacturing the panels generally have a series
of dosage lines which dose out the components in the intended ratios to a fixed
or movable mixing head which feeds the lower substrate of the cladding with the
mixture produced by said lines, optionally by means of appropriately provided distribution
In another application, the reactive mixture can be applied
by deposit on the upper substrate.
The reactive mixture enters a step of growth and polymerization
and is conveyed, together with the upper and lower claddings, by appropriately provided
movable conveyance and containment systems such as, for example, double conveyor
belts, to a cutter for cutting the panels to the intended length.
A fundamental aspect in obtaining good-quality panels is
the manner in which the reactive mixture is distributed by the mixing head onto
the substrate of the cladding.
Accordingly, several systems have been proposed according
to the type of panel to be produced and to the production rates; these systems use,
for example, multiple fixed heads or a head which performs a reciprocating motion
on a guide which is perpendicular to the panel production axis.
In the latter case, it is particularly important that the
head be lightweight and compact, so as to allow easy placement of the injection
head between the lower and upper substrates of the panel before entering the double
In any event, however, containment of the weight and dimensions
of the heads is a desideratum to achieve plant optimization.
In most cases, the production of thermal insulation panels
requires the use of the cited blowing agents, and therefore such blowing agents
are preferably added continuously at the time of production, with the advantage
of being able to control their dosage according to the physical characteristics
of the panel to be produced.
The addition can be performed directly in the mixing head
or by premixing, in the exact amount required, by means of static or dynamic mixers,
the blowing agent with one of the reactive components, for example the polyol, before
said component, drawn from the storage tank, reaches the head in which final mixing
occurs in order to prepare the reactive mixture to be deposited by pouring onto
the substrate of the panel.
What has been described above for the blowing agent can
in any case be extended to other components, catalysts and the like that are involved
in the formation of the reactive mixture.
The concept already well known is that it is not convenient
to perform the addition directly in the injection head, since an immediate consequence
would be an increase in dimensions and weight, in addition to an increase in constructive
If high-pressure injection heads are used, problems are
encountered in relation to the premixing of the various components, which is performed
in the low-pressure part of the circuit, i.e., upstream of the high-pressure delivery
This aspect causes considerable problems, since an inevitable
seepage of the polyol high-pressure pump occurs which, exactly in the case of low-pressure
premixing, can cause continuous changes in the ratio among the various components.
It is also known, in polyurethane foaming technology, that
precise dosage and optimum thermostatic control of the components fed to the mixing
head throughout the process are extremely important for obtaining products having
optimum and constant characteristics.
For this purpose, for example, it is useful to provide,
before the step of pouring the reactive mixture begins, a recirculation of the components
which allows to obtain the required temperature, flow-rate and pressure conditions,
thus avoiding transients, in the initial periods of the pouring process, in which
these parameters are not ideal, consequently producing panels which do not have
the intended quality characteristics.
In the current state of the art, therefore, the possibility
to provide effective recirculation becomes an important factor in ensuring that
the physical conditions of the components of the reactive mixture remain constant
both during the pouring steps and during the recirculation steps.
DD 133 642 A discloses a device (4) for mixing gas in a
polyol. A polyol feed circuit (1-3,6-7) feeds and recirculates the polyol to a mixing
device (5) at which a reciprocating cone (8) is arranged and about which there are
disposed mixing grooves (10-12). A gas is also fed to one of the mixing grooves
(10) through a gas line (16-19).
The aim of the invention is to eliminate the above noted
drawbacks, by providing a method for the continuous production of expanded plastic
material for forming panels and the like which allows to perform a step of recirculation
in the head at high pressure which allows to avoid altering the ratios among the
various components, maintaining at all times the optimum ratios and preventing the
occurrence of incorrect mixing.
Within this aim, a particular object of the invention is
to provide a method in which it is possible to perform high pressure recirculation
despite having a mixing head or pouring assembly in which only two injection assemblies
are available, thus reducing to a minimum the dimensions and volume of the mixing
Another object of the present invention is to provide an
apparatus for the continuous production of expanded plastic material which thanks
to its particular constructive characteristics is capable of giving the greatest
assurances of reliability and safety in use.
Another object of the present invention is to provide an
apparatus which can be easily obtained starting from commonly commercially available
elements and materials and is furthermore competitive from a merely economical point
In accordance with the invention, there is provided a method
for the continuous production of expanded plastic material for forming panels and
the like, as defined in the appended claims.
Further characteristics and advantages will become better
apparent from the description of a method for the continuous production of expanded
plastic material for forming panels and the like and of the apparatus for carrying
out the method, illustrated with the aid of the accompanying drawings, wherein:
- Figure 1 is a schematic view of the apparatus during the recirculation step;
- Figure 2 is a schematic view of the apparatus during the pouring step.
With reference to the figures, the apparatus for the continuous
production of expanded plastic material for forming panels and the like comprises
a mixing head, generally designated by the reference numeral 1, which in a per se
known manner defines internally a pouring channel 2 in which a self-cleaning piston
3 can move hermetically, with recirculation slots 4 which are arranged at a first
injection assembly 5 and at a second injection assembly 6.
The piston 3 can perform a translation movement by means
of a piston 7 in order to position the slots 4 in the recirculation position, as
shown in Figure 1, or in the pouring position, in which the piston 3 releases the
injection assemblies in order to perform mixing and pouring of the foam.
The first injection assembly 5 is connected to a duct 10
for introducing a mixture of components which can include, for example, polyol mixed
with blowing reagents, catalysts and the like.
For this purpose, there is a polyol tank 20, downstream
of which there is a high-pressure pump 21 which introduces the polyol in a first
delivery branch 22 and a second delivery branch 23, respectively controlled by a
first two-way valve 24 and by a second two-way valve 25.
The first branch 22 is connected, with the interposition
of a first calibrated one-way valve 26, to the delivery duct 10 in close proximity
to the mixing head, while on the second delivery branch 23 there is a mixer 30 for
introducing the remaining components of the mixture.
The second branch 23, downstream of the mixer 30, has a
second calibrated one-way valve 31 connected to the delivery duct 10, which is also
in close proximity to the mixing head 1.
At least one duct for introducing the remaining components,
such as blowing agents, catalysts and so forth, is connected to the mixer 30.
The specific example illustrates two ducts 40 on which
a respective three-way valve 41 is arranged which connects a product delivery duct
42 and a recirculation duct 43.
The second injection assembly 6 is connected to a second
delivery duct 50 for introducing, for example, the isocyanate or in any case another
component of the mixture.
A first recirculation duct 60, located at the first injection
assembly 5, and a second recirculation duct 61, located at the second injection
assembly 6, are further provided on the mixing head.
In practical operation, when the apparatus is in the initial
recirculation conditions, the polyol is drawn from the storage tank 20 and is introduced,
by means of the high-pressure pump 21, toward the delivery branches 22 and 23.
In recirculation conditions, the valve 25 is closed and
the valve 24 is open, and therefore the polyol flows through the first branch 22
and, by means of the one-way or check valve 26, it is introduced in the mixing head,
where by flowing through the correspondingly arranged slot 4 it is introduced in
the first recirculation duct 60, which returns the polyol into the tank 20
The one-way or check valve 31 prevents from flowing the
polyol toward the mixer 30, thus isolating it and the corresponding delivery branch
from the mixing head, which is of the self-cleaning type.
Clearly, instead of the one-way valves 26 and 31, manually-
or automatically-operated isolator valves can be used.
In this manner, a complete recirculation is obtained which
occurs through the head for mixing the polyol alone without blowing agents, catalysts
or other components of the reactive mixture, which are recirculated by means of
the three-way valve or equivalent valves, which provide to divert the flow that
arrives from the delivery duct 42 toward the corresponding recirculation duct 43
before introduction in the mixer.
It should also be added that the apparatus can return the
polyol directly at the intake of the high-pressure pump 21 instead of directly into
the storage tank 20, thus excluding from the recirculation the storage tank and
any possibility of introducing blowing agent, catalysts and/or other components
in the reactive mixture.
Clearly, the other component of the reaction, i.e. the
isocyanate, is also in the recirculation step and is recycled in a conventional
manner by means of the second delivery duct and the second recirculation duct 61
with the passage through the corresponding slot 4.
In order to pass from the recirculation step to the pouring
step, shown in Figure 2, first the mixing head is opened by the action of the self-cleaning
piston 3, which is translated by the piston 7, thus opening the valve 25 and then
closing the valve 24, so that the polyol is introduced in the second branch, which
sends it toward the mixer 30.
The three-way valves 41 also switches, so that the other
components of the mixture, such as blowing agents, catalysts and the like, are mixed
The beginning of this sequence, with the opening of the
mixing head, interrupts the step of recirculation of the polyol before the blowing
agent, the catalysts or the components of the reactive mixture to be premixed with
the polyol are introduced in the mixer, thus making it impossible for said components
to reach the storage tank 20.
Opening the valve 25 before closing the valve 24 provides
protection from the danger of overpressures due to the simultaneous closure of both
At the end of this sequence of operations, the polyol is
taken from the storage tank 20 and is sent, by means of the high-pressure pump 21
through the valve 25 to the mixer 30, which also receives the blowing agent, the
catalysts and any other components of the reactive mixture through the respective
three-way valves 41.
The polyol, after being premixed with the blowing agent,
the catalysts or any other components of the reactive mixture by the mixer 30, through
the second one-way valve 31, which is also arranged very close to the mixing head,
is introduced in the first injection assembly 5, which injects the mixture into
the pouring channel simultaneously with the isocyanate introduced by the other injection
assembly, thus providing the reactive mixture, which exits from the pouring channel
The one-way valve 26 in practice isolates the first branch
22 from the flow of premixed polyol.
The above disclosed sequence of operations must be performed
as quickly as possible, optionally even with a partial overlap of the execution
times of some consecutive operations, insofar as this is allowed by the components
used, following in any case the described sequence, in order to minimize the amount
of reactive mixture that is poured without having optimum quality.
At the end of the pouring time, in order to return the
plant to the initial recirculation conditions, first the three-way valves 41 that
introduce the blowing agents, the catalysts or the other components to be added
are switched and the valve 24 is simultaneously opened; then the valve 25 is closed.
The mixing head is then closed by moving the self-cleaning
piston 3, which places its slots 4 at the injection assemblies.
This transition to the recirculation step can optionally
be performed with a short delay in order to flush with pure polyol the delivery
duct 10, which is common for the polyol and the remaining components.
The switching, as a first operation, of the three-way valves
41, and the closure, with optional delay, of the mixing head, as final operation
of the sequence, together with the action of the second one-way valve 31, which
in practice isolates the second branch 23 from the mixing head, prevents the recirculation
to the storage tank 20 of amounts of blowing agents, catalysts and other components
of the reactive mixture.
The above disclosed operations must of course be performed
as quickly as possible, optionally also with partial overlaps of the times of some
consecutive operations, so as to minimize the amount of reactive mixture that is
poured, at the end of production, without the necessary requirements that ensure
optimum quality of the produced panel.
Another important aspect is that the one-way valves 26
and 31 must be fitted as close as possible to the mixing head or directly on said
mixing head, thus minimizing the path shared by polyol and other components of the
mixture, so as to minimize said flushing time and the amount of reactive mixture
that is poured before the mixing head closes.
It should also be added that it is possible to provide
other structural solutions for the apparatus, such as for example the replacement
of the first and second isolator or cut off valves 24 and 25 with a single three-way
valve, without altering the types of the operating steps being performed.
From the above description it is evident that the invention
achieves the intended aim and objects, and in particular the fact is stressed that
the described solution allows to have a high-pressure mixing head with recirculation
in the head which allows to always have optimum component dosage despite having
a mixing head with only two injection assemblies, and allows to perform mixing of
the polyol with the other components of the mixture downstream of the high-pressure
pump, so as to always maintain precise values of the percentages of the various
The system has also the advantage that during production,
if the percentage of the components of the reactive mixture varies, the effect is
immediately evident on the panel being produced; whereas with the conventional system
the transient would be longer and therefore more finished product would be rejected.