Note: Descriptions are shown in the official language in which they were submitted.
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PRESS SYSTEM
The field of the invention
The present invention relates to a press system, more in detail the invention
relates to a press
system for production of partially expanded polymer bodies.
Background of the Invention
Today, PVC based rigid foam polymer materials are being widely used, mainly as
core
material in sandwich structures in the naval or aeronautic sector, or as
thermal/acoustic
insulators in the building sector. In a sandwich structure the core separates
two structurally
more rigid materials, such as fibre reinforced plastics (FRP), metal or the
like. Such sandwich
structures have many advantages compared to more traditional single layer
structures, such as
lower weight, insulation properties etc. Whilst other rigid foam polymer
materials, such as
foamed polyurethane etc. can be produced using streamlined continuous
extrusion methods,
the production of PVC based rigid foam polymer materials involves moulding of
discrete
partially expanded bodies (hereafter referred to as embryo bodies) under high
pressure in a
press. The embryo bodies are subsequently subjected to a chemical-physical
treatment to
obtain the rigid foam polymer material.
More in detail, the production process of a PVC based rigid foam polymer
material initially
involves formation of a plastisol paste consisting of a mixture of powders
(PVC and other
compounds) and liquid substances (in particular isocyanates). The paste filled
in a closed
mould cavity and is subjected to a heating and subsequent cooling process
under high
pressure resulting in a partially expanded embryo body. The embryo body is
then further
expanded through an additional heat treatment in water and/or a steam oven.
The formation of
the final rigid foamed material is a result of a hydrolysis reaction of the
isocyanate groups
present in the material, with subsequent build up of a polymer which crosslink
the chemical
structure.
At present, the methods for the production of embryo bodies involves filling
the each mould
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with an excess amount of paste with respect to the polymer content in the
finished product.
The excess amount is then allowed to leak out from the mould during the
moulding process.
The moulding process comprises heating the plastisol in a closed mould,
whereby a high
pressure is created by the thermal expansion of the plastisol and the
activation of the blowing
agent dissolved therein. During this expansion step, the excess amount is
allowed to leak out.
The plastisol is kept at elevated temperature a predetermined time to allow
the plastisol to
gelatinize, where after the mould cavity is cooled to a temperature that is
low enough to
remove the embryo body from the mould. The excess amount is approximately
equal to about
8%, in terms of weight of the product leaving the mould.
The excess paste emerges from the top edge of the mould. Consequently there is
a non-
recoverable wastage of material, since the PVC gelatinizes and some of the
blowing agent
substances deteriorate at high temperature.
US 6352421, Olivier Giacoma, filed 2000-02-15, solves the problem of escape of
paste from
the mould during the heating step by providing a secondary mould compartment
into which
the excess of paste is fed during the heating step, and from which a small
part of the paste is
allowed to escape into a perimetral waste-collecting groove. According to the
disclosed
method, paste is top-filled in the primary mould compartment, during heating
the paste
expands about 8% and excessive paste is fed to the secondary compartment by
connecting
grooves. The secondary compartment has a volume that is slightly less than 8%
of the primary
compartment. Hence, the amount of waste paste is reduced to about 8% of the
volume of the
secondary compartment, which is approximately equal to 0.64% of the volume of
the primary
compartment.
US 2768407, Lindemann, Filed December 5, 1950, relates to production of closed
cell
cellular bodies from thermo-plastic masses. It is stated that a problem in the
prior art is that it
is in practice impossible to keep a mould filled with a mass containing an
expansion agent
completely sealed during the heating stage. A solution to this problem is
proposed: by, after
the gases have been dissolved under pressure in the mass and the mass has
gelatinised
completely, expanding the volume of the mould by 1/5 to 2/5 of the original
volume. Use of a
mould with a moveable die is proposed, and it is stated that it is necessary
to apply a high
pressure e.g. 150-300 atm (bar) to slow down the decomposition of the
expansion agent and
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to cause the gas to dissolve. It is also stated that suitable thermo-plastics
include polyvinyl-
chloride.
Summary of the Invention
The object of the invention is to provide a new press system for production of
rigid expanded
polymer embryo bodies which overcomes the drawbacks of the prior art. This is
achieved by
the press system and the method as defined in the independent claims.
Like in all materials production processes, important parameters for moulding
embryo bodies
when producing rigid expanded polymer materials are e.g. materials
consumption, energy
consumption, work flow and throughput time. The proposed press system is
superior to the
prior art with respect to at least one of these parameters.
Embodiments of the invention are defined in the dependent claims.
Brief Description of the Drawings
The invention will be described in detail below with reference to the
drawings, in which:
Figs. 1 a to 1 e schematically show a cross sectional view of a press system
for production of
partially expanded polymer bodies, in different stages of moulding an embryo
body.
Fig. 2 is a schematic diagram of selected process parameters of a mould
process in a press
system according to the embodiment of figs. l a to 1 d.
Figs. 3a to 3d schematically show another embodiment of the press system.
Fig. 4 is a schematic diagram of selected process parameters of a mould
process in a press
system according to the embodiment of figs. 3a to 3d.
Figs. 5a to 5c schematically show another embodiment of the press system.
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Fig. 6 is a schematic diagram of selected process parameters of a mould
process in a press
system according to the embodiment of figs. 5a to 5c.
Figs. 7a and 7b schematically show another embodiment of the press system.
Fig. 8 schematically show another embodiment of the press system.
Fig. 9 is a flow chart over a method according to the present invention.
Detailed Description of Preferred Embodiments
Figs. 1 a to 1 d schematically show a cross sectional view of a press system
10 according to the
present invention in different stages of moulding a partially expanded polymer
body.
According to one embodiment, the press system 10 comprises a mould cavity 20
of variable
volume, temperature control means 30, counter pressure means 40 arranged to
counteract
expansion of the mould cavity 20 during moulding, wherein the pressure applied
by the
counter pressure means 40 is arranged to increase in response to expansion of
the mould
cavity 20. In the embodiment disclosed in figs. l a to 1 d, the press system
10 further
comprises a press arrangement comprising a press base 120 and a press top 50
with a
moveable press member 130 urged in the downwards direction by the counter
pressure means
40, and a replaceable mould too160 arranged between the press base 120 and the
moveable
press member 130. The press base 120 and the press top 50 are firmly
interconnected by
clamps 140 to avoid relative movement.
In the embodiment disclosed in figs. l a to 1 d, the mould cavity 20 is
provided as a
replaceable mould too160 comprising a first mould member 70 and a second mould
member
80. A recess in the first mould member 70 defines a portion of the mould
cavity volume 20
and sealing means 90 is fitted in between the mating side flanges 100 and 110
respectively.
As can be seen in the figs. la and lb, the first mould member 70 and the
second mould
member 80 are moveable with respect to each other during moulding, and the
sealing means
90 is arranged to provide an essentially hermetic seal between the first 70
and second 80
mould members during at least a portion of such a volume expansion of the
mould cavity 20.
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In other embodiments, as will be shown below, the mould cavity 20 of variable
volume may
be formed as an integrated part of the press system 10, or in other suitable
ways.
The temperature control means 30 are provided to effectively control the
temperature of the
5 plastisol in the mould cavity 20 during the moulding process. Initially, the
formation of the
partially expanded polymer body requires heat to activate the blowing agent
and to initiate the
gelatinization of the plastisol, thereafter when the gelatinization has
reached a certain point
the heat created by the process exceeds the amount consumed and the plastisol
has to be
cooled in order to avoid overheating. This will be discussed more in detail
below. According
to one embodiment, the temperature control means 30 comprises conduits for a
heating/cooling media, such as water or the like. Alternatively, the
temperature control means
may be provided as separate heating and cooling means, e.g. electrical heating
means and
cooling conduits for cooling media.
In the embodiment disclosed in figs. l a to 1 d, the mould cavity 20 is formed
to produce
embryo bodies of flat rectangular panel shape that in later stages of the
process are further
expanded and cured to form panels of rigid polymer foam material with
excellent mechanical
properties. Depending on the application of the finished rigid foam material,
the mould cavity
may be of different shapes, such as spherical, tubular, cylindrical etc.
According to the disclosed embodiment, each one of the first and second mould
members 70
and 80 of the replaceable mould too160, comprises a major wall 140 and 150
respectively,
parallel with and arranged to be adjacent the respective first and second
press members 120
and 130 respectively. The first mould member 70 comprises an essentially
perpendicular side
flange 100 that circumscribes the major wall 140, and the second mould member
80
comprises a corresponding side flange 110 that mates with the side flange 100,
defining a
narrow gap there between, wherein the sealing means 90 is fitted. As mentioned
above, the
replaceable mould too160 may be integrated with the press 10, by forming the
first mould
member 70 and the press base 120 as a press mould base and by forming the
second mould
member 80 and the moveable press member 130 as a moveable press mould member.
In figs, 1 a to 1 d the counter pressure means 40 are formed by an arrangement
of spiral springs
45 that urge the moveable press member 130 in the direction opposite the
expansion direction
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of the mould cavity 20. In the disclosed embodiment, the expansion direction
is upwards but
the press system may also be designed so that the expansion direction is
downwards sideways
or anything there between. A counter pressure means 40 in the form of a spring
arrangement,
will apply a counter pressure that increases as the expansion commences, but
which cannot be
actively controlled during a step of moulding. The counter pressure means 40
may be
arranged so that the pressure applied increases essentially linearly or
exponentially in
response to expansion of the mould. The counter pressure means 40 may be
arranged so that
the pressure applied increases stepwise at one or more points of expansion.
Further, the
counter pressure means 40 may be arranged to provide any suitable combination
of linear,
exponential or stepwise increase of the applied pressure. Hence, the expansion
of and the
pressure in the mould cavity can be controlled accordingly.
Other types of passive counter pressure means 40 include any types of
arrangements that
applies an increasing non controllable force on the moveable member 130.
According to
another embodiment, the counter pressure means 40 may be comprised of an
arrangement that
allow the force applied on the moveable press member 130 to be controlled
according to a
predetermined scheme during the expansion process. One example of such a
counter pressure
means 40 of controllable type is a hydraulic press system, wherein the applied
pressure can be
controlled by rising or lowering the hydraulic pressure in the system. Such a
hydraulic press
system may be controlled by a pressure controlled relief valve according to
the description
below.
As mentioned above, the mould cavity 20 is filled with plastisol in fig. la.
In order to avoid
surface defects on the embryo body, it is of great importance that essentially
all air is
evacuated from the mould cavity 20 before the moulding cycle is initiated. In
order to achieve
evacuation of air trapped between the mould members 100 and 110 when the mould
cavity 20
is filled with plastisol, the second mould member 110 (being the top one) in
the disclosed
embodiment is provided with a small evacuation opening 160. The evacuation
opening 160 is
formed to allow air to pass, but to prevent plastisol to escape from the mould
cavity 20.
According to one embodiment, the evacuation opening 160 is so small that the
plastisol itself
closes the opening due to the high viscosity, whereby only a small amount of
plastisol is
allowed to leak out of the mould cavity 20. However other types of self
closing evacuation
openings may be used, such as valve type openings, wherein the plastisol act
on a valve body
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to close the opening. In order to facilitate removal of the embryo body from
the mould
members 100 and 110, the evacuation opening 160 is formed to avoid that the
gelatinized
embryo body get stuck therein. One way to avoid this is to make the evacuation
opening 160
of conical shape with the broad end open to the mould cavity 20 and a small
top opening open
to the outside of the mould too160. As is shown herein, the mould too160 is
adapted to be
used in a press system with parallel press planes, whereby the small top
opening is covered by
a press plane 130 and the open area is further reduced.
Fig. 2 is a schematic diagram of some process parameters of a mould process in
a press
system 10 according to the embodiment of figs. la to ld. The mould process of
fig. 2
comprises heating the plastisol, T control, in a closed mould cavity 20,
whereby a high
pressure P is created by the thermal expansion of the plastisol and the
activation of the
blowing agent dissolved therein. By selecting suitable characteristics for the
counter pressure
means 40, the pressure P in the mould cavity 20 will, as a result of the
heating T control,
exceed the pressure applied by the counter pressure means 40, whereby the
volume V of the
mould cavity 20 will increase. The plastisol is kept at elevated temperature a
predetermined
time to allow the plastisol to gelatinize, where after the mould cavity 20 is
cooled to a
temperature that is low enough to allow removal of the embryo body 170 from
the mould
cavity 20. As is disclosed above, the gelatinization process produces heat,
and must be cooled
in order to avoid over heating. In fig. 2 it is indicated that the pressure P
continues to rise
during a short period after the T control has been switched to cooling, which
is a result of the
heat produced by the gelatinization process. The point of maximum pressure is
indicated by
the dotted line in fig. 2. For the same reason the mould cavity 20 volumes
will continue to
increase until the pressure has reached its maximum. Once the cooling process
has reached
the point of maximum pressure, the process of cooling leads to a reduction of
the volume of
the mould cavity 20, mainly corresponding to the resulting negative thermal
expansion of the
embryo body 170. Fig. lb shows the press system 10 when the volume of the
mould cavity 20
has reached its maximum volume defined by the characteristics of the counter
pressure means
40 the type of plastisol mix and the process parameters that are used.
Typically, the volume
expansion corresponds to 5 to 20 percent or more compared to the filling
volume.
Fig. 1 c shows a step of unlocking the press system according to this
embodiment, wherein the
elastic properties of the partially expanded polymer body 170 is utilized to
release the
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interconnection clamp members 140. A compression force exceeding the final
pressure in the
mould cavity 20 is applied on the press top 50, whereby the counter pressure
means 40 and
the embryo body 170 are compressed so that the clamps 140 can be withdrawn to
unlock the
press system 10.
Fig. ld shows the press system 10 when the press top 50 and the second mould
member 110
are lifted off the first mould member 100 and the press base 120, whereby the
compressed
embryo body 170 is starting to pop out from the mould 60 by the internal
expansion forces,
and fig. l e shows the relaxed embryo body after it has popped out from the
first mould
member 100. In the Figs. lb to ld, the resulting relative movement of the
first and second
mould members, 100 and 110 respectively, is exaggerated for illustrative
purposes, whereby
excess material 180 formed between the upper surface of the side wall 80 and
the major wall
70 of the second mould member represent an significant volume of waste
material that has to
be removed. However, in production scale mould tools 60, the excess material
180 will be
less than the previously accepted leakage volume of 8 %. The evacuation
opening 160
produces a nipple 190 on the gelatinized embryo body, which is removed
together with the
excess material 180.
The mould members 70, 80, the press base 120 and the moveable press member 130
may be
comprised of any suitable rigid material with reasonable thermal conductivity.
They may e.g
be comprised of a metal such as aluminium, stainless steel or the like.
Alternatively, or in
combination they may be comprised of a composite material, such as fiber
reinforced plastics.
Due to the high pressure in the mould during the mould process; up to and
exceeding 200 atm,
all parts of the press system must be designed accordingly.
As disclosed above, the expansion of the plastisol during the moulding process
is between 5
and 20% and during this process it is important that the high pressure is
preserved in the
mould cavity 20. However under certain circumstances, the pressure build up in
the mould
cavity 20 can reach extreme levels due to improper plastisol mix or
overfilling. According to
one embodiment, the mould is designed so that the moveable mould member 110
provides an
essentially hermetic sealing effect at an increase of mould cavity 20 volume
of a
predetermined value between 6 and 20% with respect to a filling volume, where
after the
sealing effect is arranged to be reduced to avoid overpressure in the mould
cavity. According
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to an alternative embodiment, the sealing effect is gradually reduced.
Moreover, one of the
mould members may be designed to provide an adjustable expansion limit.
Figs. 3a to 3d show an embodiment of a press system, wherein, the press base
121 and the
press top 51 are attached by a hinge arrangement 200 along one side and by
interlocking
means 210 on the opposite side. In the disclosed embodiment, the interlocking
means 210 is
shown as a rotary lock mechanism, but it may be any suitable interlock
mechanism. In the
embodiment of figs. 3a to 3d, the counter pressure means 40 is comprised of
compression
springs 220 and secondary compression members 230. The counter pressure means
40
according to this embodiment is arranged to allow an initial expansion of the
mould cavity 20
under a counter pressure from the compression springs 220, followed by a
secondary
expansion under an elevated counter pressure in form of the combined force
from the
compression springs 220 and the compression members 230. In figs. 3a to 3d the
counter
pressure means 40 are shown as a combination of compression springs 220 and
compression
members 230 in the form of solid members of a flexible material, e.g. rubber
or the like, as an
illustration that the counter pressure means 40 may formed by any combination
of means
capable of applying a counter pressure that increases in response to expansion
of the mould
cavity. Fig. 4 is a schematic diagram of some process parameters of a mould
process in a
press system 10 according to the embodiment of figs. 3a to 3d. In fig. 4 the
point when the
expansion of the mould cavity 20 has reached the secondary expansion is
indicated by the left
dashed line. As is indicated by fig. 4, the increase in pressure P during the
secondary
expansion results in reduced expansion rate of the volume V.
Fig. 3c illustrates the step of unlocking the press system 11, wherein a force
is applied on the
left hand side of the press top 51 to compress the embryo body 170 to allow
unlocking of the
rotary locking mechanism 210 where after the press system can be flipped open
as is shown in
fig. 3d.
Figs. 5a to 5c show an embodiment of a press system 12, wherein the mould
cavity 20 is
integrated with the press base 122 and the moveable press member 132. By
integrating the
mould cavity 20 in the press system 12, the handling of the separate mould
tools 60 is
omitted. In the disclosed embodiment, the press base 122 and the moveable
press member 132
are illustrated as a plunger type arrangement wherein the mould cavity 20 is
formed by a
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recess in the press base 122 and the press member 132 is formed as a mating
plunger defining
the upper wall of the mould cavity 20. As in the above embodiments, there is
provided a
sealing member between the press base 122 and the press member 132 to achieve
an
essentially hermetic seal there between.
5
In the disclosed embodiment, the press member 132 is arranged to be moveable
between a
lower position set by a lower shoulder 260 and an upper position set by
expansion termination
means 250 hindering further movement of the press member 132 in the expansion
direction.
Thus, in the disclosed embodiment, the final predetermined mould cavity volume
is set by the
10 position of the expansion termination means 250, and the resulting peak
pressure depends on
the volume of plastisol filled into the mould. Thus, the volume of the mould
cavity 20 is
defined by the expansion termination means 250, which corresponds to a volume
expansion
of 5 to 20 percent or more compared to the filling volume depending on the
type of plastisol
mix and the process parameters that are used.
Fig. 6 is a schematic diagram of some process parameters of a mould process in
a press
system 12 according to the embodiment of figs. 5a to 5c. Like in the
embodiment of figs. 3a
to 3d, the counter pressure means 40 of the press system 12 are arranged to
allow an initial
expansion of the mould cavity 20 under a counter pressure from the compression
springs 240,
followed by a secondary expansion under an elevated counter pressure caused by
the
expansion termination means 250. In this illustration, the compression springs
240 are
arranged to provide an essentially linear increasing counter pressure,
followed by a counter
pressure peak, caused by the terminated volume expansion.
Therefore, in order to be able to produce partially expanded polymer bodies
with the same
characteristics, it is of great importance that the same volume ofplastisol is
filled into the
mould for all such bodies. Partially expanded polymer bodies with different
characteristics
can thus be achieved by altering the volume of plastisol filled into the form
and/or by altering
the final mould cavity volume by changing the position of the expansion
termination means
250. According to one embodiment, the expansion termination means 250, and
thus also the
final mould cavity volume, are adjustable.
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Figs 7a and 7b show still another embodiment of a press system 13, wherein the
mould cavity
20 is at least partly defined by a flexible wall member 300 separating the
plastisol in the
mould cavity 20 from the counter pressure means 40 in the form of a hydraulic
pressure fluid
310 applying the desired counter pressure over the membrane surface. Like in
the
embodiment of figs. 5a to 5d, this embodiment comprises expansion termination
means 320
comprised of a rigid termination member or a rigid wall on the pressure fluid
side of the
flexible wall. In the disclosed embodiment, the press system 13 is comprised
of a press top 53
and a press base 123 that are firmly interconnected by clamps 140. The press
top 53
comprises, temperature control means 30, the flexible wall member 300 and
hydraulic
conduits 350 connecting the hydraulic fluid side of the flexible wall member
300 with a
source of hydraulic pressure 330 illustrated by an arrow. The pressure applied
by the
hydraulic pressure fluid 310 is supplied and controlled by the source of
hydraulic pressure
330. The press base comprises a lower section of the mould cavity 20 and
control means 30.
As is shown in fig. 7b, high pressure in the plastisol forces the pressure
fluid from the fluid
side of the flexible wall (via conduits to a reservoir or the like) until the
flexible wall abuts the
expansion termination means 320 defining the final mould cavity volume. This,
embodiment
makes it possible to produce partially expanded polymer bodies of complex
shapes in net
shape, that in later stages are further expanded and cured to rigid foamed
polymer objects.
According to one embodiment, disclosed in fig. 8, the counter pressure applied
by the
hydraulic pressure fluid 310 is controlled by a pressure controlled relief
valve 340. The
pressure controlled relief 340 valve is arranged to open when a predefined or
controllable
pressure threshold is reached, and the hydraulic fluid from the valve is fed
to a reservoir 350.
The pressure threshold may be a static pressure, or it may be controllable in
order for the
pressure in the mould cavity to be controlled according to a predetermined
scheme.
There is also provided a method of moulding a partially expanded polymer body
in
accordance with the above embodiments, comprising the steps:
preparing a plastisol comprising a blowing agent,
filling the plastisol into a mould cavity of variable volume
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heating the plastisol to activate the blowing agent
allowing the mould cavity to expand under a counter pressure arranged to
increase in response to expansion of the mould,
preserving the plastisol at elevated temperature a predetermined time to allow
the plastisol to gelatinize and transform into a partially expanded polymer
body,
cooling the partially expanded polymer body,
opening the mould cavity, and
removing the partially expanded polymer body.
The above method may be performed with the press system according to the
present
invention, but it may also be performed in large facility press system, using
mould tools of
variable volume. The counter pressure may either be actively controlled by
e.g. a hydraulic
pressure system, or passively controlled by selection of counter pressure
members with
predetermined characteristics.
As mentioned above, the counter pressure may increase linearly or
exponentially during the
expansion, or it may be varied according to a predetermined scheme during the
expansion.
According to one embodiment, the counter pressure increases stepwise.
According to one embodiment, the method comprises the step of:
terminating the expansion of the mould cavity after a predetermined volume
expansion.
By terminating the expansion at the appropriate volume pressure in the mould
cavity is
allowed build up, and a desired high pressure is achieved.
