Note: Descriptions are shown in the official language in which they were submitted.
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DEVICE FOR HEATING A PRESS TOOL, PRESS HAVING SUCH
DEVICE, AND METHOD OF MANUFACTURE
The present invention relates to the heating of
a press tool mounted in a press. More specifically, the
invention concerns a device which is adapted to magne-
tically heat a press tool directly in a press, with the
possibility of a subsequent rapid cooling of the tool
in the press. The invention also concerns a press and
a method for manufacturing products wholly or partially
of plastic or composite.
When manufacturing products wholly or partially of
plastic or composite, use is made of a press, the press
tool of which must be heated. This normally takes place
by removing the press tool from the press, placing it in
a preheating device, such as a furnace, and after heat-
ing, again mounting it in the press. A preheating device
is disclosed in e.g. US-A-5,023,41~. The handling of the
press tool is very time-consuming, which means that the
number of finished products per hour will be small.
With a view to reducing the time consumed, it is
known to provide the press with hot plates, by means of
20 which the press tool is heated in situ in the press. It
takes about 10 min to heat a tool of l kg to a tempera-
ture of 200~C by using heat conducted from the hot plates
of the press. This is too long for economy in large-scale
manufacture of components which are wholly or partially
25 formed of composite.
After heating, the tool must be cooled. Such cool-
ing is normally carried out with water or, when tool
inserts of sintered material are involved, with carbon
dio~ide which is gasified such that a temperature of
P 30 about -60~C is achieved, or with liquid nitrogen. If the
cooling operation is slow or difficult to control, and
the chemical process in the composite therefore cannot
be accurately controlled, it is impossible to achieve an
acceptable quality of the finished products.
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In the manufacture of products wholly or partially
of composite, economy has up till now been possible only
in the manufacture of expensive components, since the
technigue of today permits but a few components being
manufactured per hour.
One object of the invention is to overcome the
above-described drawbacks of prior art, i.e. to pro-
vide a device which gives rapid and uniform heating of
a press tool mounted in a press, without necessitating
the removal of the press tool from the press. The heating
device should be of simple design, have high manufactur-
ing capacity and yield final products of high and uniform
quality.
The inventive device should also allow rapid, repro-
ducible and economic cooling of the press tool mounted in
the press after the above-mentioned heating operation.
One more object of the invention is to provide an
energy-saving heating device which can be operated by
normal circuit voltage.
It is also desirable to provide a press intended
for the manufacture of products wholly or partially of
plastic or composite, and a manufacturing method, which
achieve the above objects.
According to the invention, these and other objects,
which will appear from the following specification, have
now been wholly or partially achieved by a device, a
press and a method according to the accompanying claims
1, 10, 11 and 12. Preferred embodiments of the invention
are defined in the subclaims.
The invention will now be described for the purpose
of e~emplification and with reference to the accompany-
ing drawings, which schematically illustrate currently
preferred embodiments. In the drawings, like parts in
the various embodiments have been provided with the same
reference numerals.
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Fig. 1 is part-sectional view of a first embodi-
ment of the invention, in which the poles of the core
are recessed in the tool.
Fig. 2a is a cross-sectional view of a second embo-
diment, in which the tool comprises plates with toolinserts. Fig. 2b is a cross-section taken along line B~B
in Fig. 2a.
Fig. 3 illustrates a third embodiment, in which the
tool has tool inserts of a gas-permeable material.
Fig. 4 illustrates a fourth embodiment, in which the
magnetic core consists of two movable, horizontal plates
which are in contact with two fixed, vertical plates.
Fig. 5 illustrates a fifth embodiment of the inven-
tion with two pole coils each encompassing a pole arrang-
ed against the tool.
Fig. 1 shows a heating device according to the in-
vention, comprising a core 1 of some suitable magnetic
material, such as a package of laminae of such electric
sheet steel as used in transformer cores. The core 1 is
mounted between and abuts against two mutuall~ movable
jaws 2a, 2b of a press 3. A press tool 4, which consists
o~ two halves, is arranged within the circumference of
the core 1. Each half comprises a shell tool part 5a,
which at least partly consists of metal, and an outer
~5 layer 5b of electrically nonconductive ceramic material.
The tool 4 is adapted to receive a workpiece 6, which
consists of a material arranged in the tool and/or
injected under pressure.
The core 1 is in the shape of a box with open ends
and consists of two parts 7a, 7b which are joined toge-
ther and movable in relation to one another. The core 1
has on the inside two opposite projections 8a, 8b, which
project into the tool halves and form magnetic poles
when the heating device is activated. A first coil 9 is
arranged around one narrow side of the core 1, and a
second coil 10 is arranged around the other, opposite
narrow side of the core 1. A third coil 11 is arranged
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inside the core 1 around the tool 4. The coils 9-11 are
connected in series and to a voltage source 12, which
operates with one phase and the zero of the electric
mains and gives e.g. 200 V at 50 Hz. Alternativelyr two
or three phases can be used, as will be described in more
detail below.
The press tool 4 has in the outer layers 5b cooling
ducts 13, which are designed for the passing of a liquid
or gaseous working medium having a lower temperature than
the tool 4. By thermal conduction, heat is conducted from
the tool 4, which thus is cooled. Preferably, use is made
of a heat exchanger 14, which on its primary side is
passed by a cooling agent, for instance gaseous carbon
dioxide with a temperature of -65~C, and which on its
secondary side is passed by a working medium, for
instance oil.
The inventive press 3 is operated in the following
fashion. First a material, such as a glass fibre fabric
preimpregnated with polymer material, preferably thermo-
plastic material, a so-called prepreg, is supplied to the
press tool 4, and then the jaws 2a, 2b are pressed toge-
ther. As a result, also the parts 7a, 7b of the core 1,
the poles 8a, 8b and the tool halves are pressed toge-
ther. The heating device is then activated by applying a
voltage to the coils 9-11, which generate magnetic fields
which are conducted into the core 1 and in the circuit
formed by the tool 4 and which also appear to some extent
outside the core 1. The magnetic fields generated by the
coils 9-11 and conducted through the core 1 will be con-
ducted in the tool 4 and there cooperate for the heatingthereof. The magnetic field outside the core 1 will heat,
by induction, the more peripheral metal portions of the
tool 4. By using a relatively low frequency of the
applied voltage, preferably in the range 16-500 Hz, since
higher frequencies tend to result in great losses in the
core, a satisfactory penetration depth into the tool and,
thus, a uniform heating thereof can be obtained. The fre-
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~uency should be adapted to the thickness of the tool. I-f
a penetration depth of about 2 cm is desired, a fre~uency
of about 25 Hz is used. If the tool, as will be described
below with re~erence to Figs. 2a-b, has inserts of sin-
tered material for the purpose of letting through gaseswhich are formed during the treatment of the workpiece or
which are used for the cooling thereof, the frequency is
suitably ad~usted such that also the inserts are heated
to a uni~orm temperature.
The inventive device can be used for heat and pres-
sure treatment of a large number of combinations of mate-
rials. In addition to the above-mentioned prepreg, the
workpiece 4 may consist of, for instance, a combination
of fibre cloth and foils of polymer materials or of a
compound of ground fibres, thermoplastic material and a
filler. Alternativel~, the workpiece may comprise a pre-
fabricated fibre body, which is placed in the tool,
whereupon a liquid plastic material, for instance a ther-
moset plastic such as polyurethane or epoxy, i5 injected
into the tool. In this case, the workpiece is normally
not cooled ;mm~i ately after the treatment in the press
but is postcured in a separate furnace.
It has been found, for instance, that the embodi-
ment illustrated in Fig. 1 can heat a 50 kg press tool
to 200~C in about 5 min, which should be compared with
a corresponding heating time of about 60 min by using
conventional technique, i.e. heating via hot plates
mounted in the press. By means of the inventive device,
the tool can then be cooled to 40~C in about 2 min. With
a process time of about 2 min per workpiece, about 8
workpieces can thus be manufactured per hour by using the
invention.
Correspondingly, experiments have been made with a
press tool of about 1 kg comprising shell tool parts with
a wall thickness of about 2 cm. The poles of the core
were recessed in the electrically nonconductive ceramic
outer layers into contact with the shell tool parts. The
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outer layers comprised cooling coils. A thin polypropy-
lene-impregnated glass fibre fabric was placed between
the shell tool halves, whereupon the tool was closed.
By using an inventive device, the tool could be heated
directly in the press to about 200~C in about 5 s, such
that the plastic material melted and formed a surface
layer against the tool which had a surface coating of a
suitable release agent. After about 20 s at process tem-
perature, the tool was cooled within 10 s to about 40~C
via the cooling coils by using carbon dioxide. Subse-
guently, the fin; .~h~rl product was removed and new mate-
rial was supplied to the tool. Thus, up to 120 workpieces
could be treated per hour. With conventional technique,
only about 6 workpieces could be treated per hour in the
same tool.
According to a second embodiment as illustrated in
Fig. 2a, the tool 4 comprises two plates 15a, 15b which
are coupled to the nonconductive ceramic outer layers 5b.
The poles 8a, 8b of the core 1 extend through the outer
layers 5b and are in contact with the plates 15a, 15b.
The magnetic field which is conducted through the core 1
will also be conducted through the plates 15a, 15b. Since
each plate 15a, 15b forms a secondary winding of one turn
each, also the plates 15a, 15b are heated by the magnetic
field. The outer magnetic field from the coil 11 heats
the peripheral parts of the tool 4 and the plates 15a,
15b by induction.
Each plate 15a, 15b is provided with a tool insert
16a, 16b of a material which lets through gases from the
workpiece and gases that are used for cooling of the tool
inserts 16a, 16b and the workpiece 6. The plates 15a, 15b
can be provided with cooling ducts (not shown) for rapid
and reproducible cooling of the plates 15a, 15b and,
thus, the workpiece 6.
If the outer layers 5b are made of a conductive
material, it may be desirable that only the plates 15a,
15b and not the outer layers are heated when the coils
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9-11 are activated. This can be achieved by each outer
layer 5b being designed so as not to form a secondary
winding, as shown in Fig. 2b.
Fig. 3 shows a third embodiment comprising gas-per-
meable tool inserts 16a, 16b which are mounted in a re-
cessed mAnne~ in the tool halves. The poles 8a, 8b of
the core 1 extend through the tool halves into contact
with the tool inserts 16a, 16b. When activating the coils
9-11, the tool inserts 16a, 16b are rapidly and uni~ormly
heated to process temperature.
Fig. 4 shows a fourth embodiment, where the core 1
consists of four plates 17a, 17b, 17c, 17d, of which two
plates 17a, 17b are movable and arranged to abut against
the jaws 2a, 2b of the press 3 and two plates 17c, 17d
are fixed and arranged perpendicular to the movable
plates 17a, 17b. Thus, the tool halves can be pressed
together by means of the press 3 during working of the
workpiece 6, while the magnetic fields are conducted in
the core 1, independently of the thickness of the work-
piece 6.
In the above embodiments, the poles 8a, 8b pro;ectinto the tool 4, which in most cases is preferred since
the height of the heating device thus is m~ n; m; sed.
Fig. 5 shows a fifth embodiment, in which the poles 8a,
8b are arranged against the outside of the tool 4. This
arrangement is usable, for instance, when the tool 4 is
relatively thick and comprises shell tool parts 5a with-
out any ceramic outer layer.
The coil arrangement in Fig. 5 further comprises two
pole coils lla, llb which are each arranged about a pole
8a, 8b. This coil arrangement renders it possible to
mount the pole coils lla, llb close to the tool 4 for
optimum heating thereof. The extent of the coils lla,
llb should be adapted to the extent of the tool 4. It
has been found that the coils lla, llb should be arrang-
ed close to the press tool 4, preferably at a distance
of some centimetres or less. The losses will be reduced
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drastically if the coils lla, llb are arranged in contact
with the press tool 4.
It should be emphasised that the invention generally
has the additional advantage that it also allows rational
manufacture of structures of plastic or composite which
comprise metallic inserts, such as reinforcing steel
tubes. The metallic inserts will be heated by the applied
magnetic field. This can hardly be achieved with conven-
tional technique.
A further advantage of the invention resides in the
fact that the magnetic fields generate vibrations in the
press tool, which yield an increased heat transfer and
reduces the occurrence of air bubbles in the workpiece.
In all the embodiments described above, the core 1
encompasses the press tool 4. This is advantageous since
the encompassing core 4 forms a cage which catches a
great part of the generated magnetic fields, which are
concentrated ad;acent the press tool 4 by means of the
poles and heat the press tool. It is certainly conceiv-
able to design the core 1 so as to ~n~ompass but a por-
tion of the press tool 4. In that case, however, the
power consumption will be higher and the losses will be
greater.
The press tool 4 is thus heated by magnetic fields
being concentrated therein. Moreover, the pole coil 11
gives an inductive heating of the peripheral parts of the
press tool 4. Eddy currents around the contact surface
between the poles 8a, 8b and the tool 4 also result in a
certain heating of the tool.
In the embodiments shown, the projections or poles
8a, 8b are in direct contact with the shell parts 5a,
the plates 15a, 15b or the tool inserts 16a, 16b. This
can result in mutual short-circuiting of the steel
sheet laminae of the core, which may cause losses and
high power consumption. It is therefore preferred that
the tool 4 comprises a thin insulating material which is
arranged between the poles 8a, 8b and the outside of the
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shell parts 5a, the plates 15a, 15b or the tool inserts -
16a, 16b.
It will be appreciated that the coil arrangement
9-11 can be varied within the scope of the invention.
For example, the core coils 9, 10 can be arranged in some
other fashion around the core 1. A larger number of core
coils can also be used. Further a larger number of pole
coils lla, llb can be used, for example one around each
pole 8a, 8b of the core 1. It is also conceivable to use
only pole coils or alternatively only core coils.
The relative displacement of the poles 8a, 8b can be
achieved in some other m~nn~r . For instance the projec-
tions forming the poles can be movable in relation to the
rest of the core 1 and be in contact with the jaws 2a, 2b
of the press 3. The press force exerted by the jaws 2a,
2b is thus transferred directly via the poles 8a, 8b to
the press tool 4.
According to a preferred embodiment (not shown), the
press has three heating devices which are connected to a
common voltage source and use one phase each thereof. It
is also conceivable to arrange in one and the same press
an optional number of individually controllable heating
devices each heating a part of one or more press tools
which extend through the heating device.
It is also possible to provide the core in a heat-
ing devlce with several projections or poles arranged
in pairs and to arrange around each pair of poles one or
more pole coils. ~or instance, the core may have three
pairs of poles with associated pole coils each using one
phase of a common voltage source.
