Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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INJECTION COMPRESSION MOULDING
Field of the invention
The present invention relates to injection-compression
moulding and is concerned in particular with an improvement
of the mould described in PCT Publication W002/058909.
Background of the invention
In the injection-compression moulding technique to
which the present invention relates, an accurately measured
quantity a plastics material is injected. into a.-mould cavity
before it has been fully closed. As the parts of the mould
are brought together, the injected plastics material is
compressed and made to fill the cavity by the force applied
to close the mould rather than by the pressure applied to
inject the plastics material into the mould. As a
consequence, it is possible to achieve much higher length to
thickness ratios than achievable by conventional injection
moulding, even when using lower cost plastics materials
having high viscosity. This enables the technique to be used
in the manufacture of such items as cups and margarine tubs
which have hitherto needed to be manufactured by other
methods, such as by vacuum or pressure forming of a heat
softened sheet material. A further advantage is the greatly
reduced cooling times due to lower processing temperatures
and improved packing allowing for faster heat loss through
the cavity (conventionally 70o of the heat loss is expected
3o through the core due to shrinkage away from the cavity
wall).
W002/058909, which is believed to represent the closest
prior art to the present invention, describes a mould for
mounting between the platens of an injection moulding
machine for injection compression moulding of a thin walled
article. The mould comprises a female mould half mounted on
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the stationary platen of a moulding machine and a pressure
plate which is mounted on the moving platen and carries a
core for closing the mould (there is no reason why these two
parts cannot be interchanged if desired). The core passes
through, and is sealed relative to, a cylindrical bore in a
rim closure ring, arranged between the female mould half and
the pressure plate. In use, as the pressure plate is
advanced towards the stationary mould half, the rim closure
ring is used to seal the mould cavity before the core
1o reaches its end position. Thus, when the plastics material
is injected into the mould cavity, it is fully sealed even
though the core has yet to be fully advanced into the mould
to reduce its volume to its smallest-size.
l5 It has been found in practice, however, that it is
difficult to form a rim closure ring with a cylindrical bore
that effectively seals around the core and yet allows the
core to pass freely through it. The clearance required to
permit reliable and free movement of the core relative to
2o the rim closure ring does not permit creation of a perfect
seal and results in an unacceptable witness line around the
rim of the moulded article.
Summary of the invention
With a view to mitigating the foregoing disadvantage,
the present invention provides a mould for injection
compression moulding of an article, comprising a female
mould part and a core movable relative to one another
between an open and a closed position, and a sealing ring
surrounding the core for effecting a seal between the female
mould part and the core, the core the female mould part and
the sealing ring together defining a closed mould cavity
when the core is in the closed position, characterised in
that the sealing ring has a tapering surface that seals
against a tapering surface on the core only after the core
has reached the closed position, there being a venting
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clearance between the tapering surfaces of the sealing ring
and the core as the core approaches the closed position.
The material that is to be moulded is conventionally a
molten thermoplastic material. It should however be clear to
the person skilled in the art that the invention will also
find application in moulding molten metals, resins and
thermosetting materials. Indeed the material can be any
material that is initially sufficiently fluid to be capable
to of being injected and that will subsequently harden, be it
by cooling, heating or chemical curing.
In the present invention , the purpose of the tapering
surface of the sealing ring that seals against the core is
to maintain a gap between the core and the sealing ring
until the mould is fully closed allowing free movement of
the sealing ring relative to the core. This is to be
contrasted with tapering surfaces provided on sealing rings
for alignment purpose, as exemplified by US 6,500,376, where
the surfaces meet and leave no gap between them before the
mould cavity is fully closed.
The term "sealing ring" is not, of course, to be
construed in the present context to be restricted to a
circular ring, as its outline will in each case be dictated
by the outline of the article to be moulded.
Preferably, the angle of taper measured relative to the
direction of movement of the core is small, typically less
3o than 5°, so that only a small gap is present between the
core and the sealing ring during the last few millimetres of
movement of the core. The width of the small gap that
remains as the core approaches the closed position will not
allow the injected material to penetrate into it but allows
air to escape from the mould cavity.
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Because air can escape from the cavity at any time
before it is fully closed, it is possible to dispense
entirely with the venting gaps that are normally required
when injection or injection-compression moulding an article.
The ability to dispense with venting gaps has many important
advantages. First, because gas has to escape through a vent
that is too small to allow the injected material to flow
through it, it is heated to a high temperature with the
result that the venting gaps require extensive maintenance
1o and can reach a sufficiently high temperature to scorch the
plastics material. Second, the back pressure resulting from
pumping air through the venting gaps reduces the speed of
movement of the injected material and the filling speed of
the mould.
The sealing ring is preferably mounted on the core by a
connection that allows it a limited degree of movement
relative to the core and the sealing ring is urged by at
least one of a spring and gas pressure in a direction to
increase the size of the gap between tapering surfaces.
Brief description of the drawings
The invention will now be described further, by way of
example, with reference to the accompanying drawings, in
which .
Figure 1 is a perspective view of a female mould part
looking into the mould cavity,
Figure 2 is a perspective view of a core part to fit
3o into the female mould part of Figure 1,
Figure 3 is a perspective view of the two mould parts
of Figures 1 and ~ in the closed position of the mould,
Figure 4 is a section through the core part of
Figure 2,
Figure 5 shows part of the section of Figure 4 drawn
to an enlarged scale, and
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Figure 6 is a section through the mould when in its
closed position.
Detailed description of the preferred embodiment
The figures show a single core/cavity set for an
injection-compression mould for making an article in the
form of a drinking cup having a generally flat base, a
frustro-conical side wall and a lip in the form of an
1o inverted "U" surrounding the mouth of the cup. It will be
appreciated that the core cavity/set may be one of many in a
multi-cavity mould and that the different sets can be
- - arranged side by.side and/or stacked back to back. The
ensuing description will, however, be refer for simplicity
to a single cavity mould.
The mould comprises a female mould part 10 and a core
part 12 which fit into one another in the manner shown in
Figure 3 to leave between them a mould cavity having the
desired shape of the drinking cup to be moulded.
In conventional injection moulding, articles are
moulded by first fully closing the mould. Next a plastics
material is injected into the mould cavity to fill it
completely. After the plastics material has been allowed to
cool sufficiently, the mould is opened, the moulded article
is ejected and the cycle is repeated. The above technique
however places a limitation on the length to thickness ratio
of the moulded article. The minimum wall thickness that can
3o be achieved varies with the viscosity of the plastics
material and even to produce an article having greater wall
thickness than is necessary for the structural integrity of
the moulded article requires the use of more expensive low
viscosity plastics materials.
By contrast, in injection-compression moulding, at the
end of the injection of the plastics material the core is
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not in the fully closed position of the mould cavity. As the
core is advanced towards the closed position, the injected
plastics material is forced to fill the mould cavity. The
plastics material is made to flow by the compression of the
mould cavity rather than by the injection pressure and this
has many advantages that are documented in the prior art.
Injection-compression moulding does however present
certain problems that are not encountered in conventional
l0 injection moulding. The first of the problems is concerned
with the alignment of the mould parts. Conventionally,
conical mating surfaces are provided on the different parts
of the mould which centre them relative to one another when
the mould is fully closed. However, before the mould is
fully closed, the mould parts may not be fully aligned, that
is to say they not be concentric or they may not be co-
axial. The guiding that is achieved by the tie bars, or
other guiding systems used by machine manufacturers of the
injection moulding machine, may not guarantee alignment to
the required accuracy, especially when it is noted that the
main purpose of using injection-compression moulding is to
achieve very large flow length to thickness ratios in
articles such as cups, margarine tubs or dustbins.
To overcome these problems, in the illustrated
embodiment of the invention three flat guide fingers 14 are
firmly secured to the core part 12 to surround the central
mould core 16 in accurately predetermined positions. Each of
the guide fingers 14 has two parallel sided locating
sections. One pair of locating sections 14a is provided near
the base of each guide finger 14 and the other pair 14b is
provided at its free end. The width of the locating sections
l4a exceeds the width of the locating sections 14b. The
portion of each guide finger in between the locating
sections 14a and 14b is shown as tapering gradually, but it
may have any shape provided that its width never exceeds the
width of the locating sections 14a. Mating parallel locating
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sections 18a and 18b are provided as part of U-shaped
recesses defined by inserts 18 similarly secures to the
female mould part 10.
In use, the mould parts that is not connected to the
injection system, usually the core part 12, is mounted in
such a manner as to allow it a small degree of lost motion
relative to its machine platen. Strong spring pressure is
used to retain the mould part in position but, if sufficient
force is applied to it, the mould part will moue laterally.
The first time the mould is fully closed, the fingers
and inserts may not mate perfectly-with one another and this
will apply a force to the core part to push it into
alignment with the female mould part. When the mould is then
closed fully, the parts of the mould are brought into
perfect alignment with one another in the conventional
manner. During subsequent operating cycles, the locating
sections 14a and 14b will interact with the surfaces 18a and
18b before the mould is fully closed and will effect in two
different and axially separated planes any minor relative
displacement of the mould parts that is necessary to assure
correct alignment of the mould parts both in terms of
concentricity and parallel alignment before the mould is
fully closed.
Even though the guide fingers 14 and the inserts 18
ensure the concentricity of the mould parts at both ends of
the mould cavity, they do so without the use of locating
sections having an axial length matching that of the mould
cavity. Instead, no force is applied to align the mould
parts until the mould is nearly fully closed. This is
important as it avoids excessive wear to the locating
sections.
One could consider using conical rods in place of the
flat fingers 14 but the latter are preferable because each
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finger is only called upon to effect a correction in one
plane thereby simplifying the positioning of the locating
sections on the mould parts.
In a multiple cavity mould, it is preferred to provide
guide fingers around each individual core/cavity set to
allow for possible movement of the sets relative to one
another. On smaller products, it may be possible to provide
guide fingers around two or four cavities if they are
tightly grouped together.
Though the illustrated embodiment of the invention uses
- three guide fingers l4 to align each mould, it is possible -
to use more, four being preferred.
A further problem that has to be overcome in injection-
compression moulding is that of containing the plastics
material within the cavity as its volume is being reduced.
In the prior art, this has been achieved by using a rim
2o closure ring to close off the cavity in the female mould
part and by the core passing through a cylindrical hole in
the rim closure ring. This is not a satisfactory solution
because it is difficult to form a rim closure ring with a
cylindrical bore that effectively seals around the core and
yet allows the core to pass freely through it. The clearance
required to permit reliable and free movement of the core
relative to the rim closure ring does not permit creation of
a perfect seal and results in an unacceptable witness line
around the rim of the moulded article.
In the preferred embodiment of the invention (see
Figures 4 to 6) a sealing ring 20 surrounds the core 16. The
sealing ring is held captive on the core part 12 and is
capable of only a small degree of movement relative to the
core part in the direction of the axis of movement of the
core part 12. Strong springs (or gas pressure) capable of
withstanding the pressure within the mould act to hold the
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sealing ring 20 against the top surface of the female mould
part 10 so that no plastics material can escape laterally
from the top of the mould cavity.
The sealing surface between the sealing ring 20 and the
core 16 is not cylindrical, as in the prior art, but is
formed of two contiguous tapering sections 22 and 24 of
which the section 24 has a very small angle of taper, less
than 5° and preferably of the order of 1°, and the section
to 22 has a larger angle of taper. In both sections, sealing
contact between the sealing ring 20 and the core 16 does not
occur before the mould cavity is fully closed.
In a typical operating cycle, the core 16 is first
fully advanced into the cavity in the female mould part 10
to exclude most of the air from the cavity. Next, an
accurately measured quantity of plastics material is
injected into the mould cavity to form a biscuit at the base
of the mould cavity. During this time, the core recoils
slightly from the female mould part either by the action of
the injection pressure or by movement of the core cavity.
The sealing ring may or may not come into contact with
the front of the female mould part before the recoiling
movement commences depending on the maximum stroke of the
ring and the thickness of the biscuit that is injected into
the mould. If the sealing ring does contact the front of the
female mould part, then depending on the stroke of the ring
relative to the core and the amplitude of the recoiling
movement, it may remain in sealing contact with the female
mould part during the whole or only the initial part of the
recoiling movement.
During the recoiling movement, the core 16 is
maintained in alignment with the female mould part by the
action of the guide finger 14. The axial movement of the
core 16 relative to the sealing ring 20 opens a gap between
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the two because of the tapering sealing surfaces 22 and 24.
However, because of the steepness of the angle of taper of
the section 24, the width of the gap that is created is only
wide enough to act as a vent to allow gas to escape from the
cavity.
When the core 16 is next advanced into the cavity of
the female mould part, the biscuit of plastics material is
compressed and is forced to flow up the side walls of the
1o cavity towards the rim of the container. During this time,
gas is expelled from the mould cavity first through the gap
between the sealing ring and the cavity then through the gap
between the sealing ring 20-and the core 16. As the core
reaches its end position, the gap between it and the sealing
ring 20 is closed fully so as to prevent any egress of the
plastics material from the mould.
Because of the accurate axial alignment which is
achieved by using the tapered fingers 14 and the U-shaped
sections 1~, the plastics material flows at an even rate
around the entire periphery of the cavity and reaches the
end of the cavity at substantially the same time. This
reduces the distance that the sealing ring 20 needs to move
relative to the core 16.
It will noted that the sealing ring not only closes the
mould cavity efficiently to avoid any flashing but it does
so without rubbing against the core. Furthermore, the
sealing ring provides a vent which decreases in cross
sectional area as the core reaches the closed position.
Thus, at the commencement of the compression stroke when air
needs to be expelled from the cavity, the air can pass
freely first between the sealing ring 20 and the cavity 10
then between sealing ring 20 and the core 16. This avoids
high temperatures being reached in the vent and reduces air
damping of the movement of the core. By the time the gap is
finally sealed off, all the air will have been evacuated
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from the mould cavity and the seal will prevent any flashing
of the plastics material.
The female mould part and the core have been
illustrated in greater detail than is necessary for an
understanding of the invention but the parts that have not
been described in detail are generally conventional and
their function will be understood by the person skilled in
the art without the need to detailed explanation. In
l0 particular parts have been illustrated which are associated
with such functions as the injection of the plastics
material into the cavity, the cooling of the moulding, the
ejection of the mould article from the mould and the-
mounting of the parts in the mould tool that is mounted to
s5 the platens of an injection moulding machine.
It will also be appreciated that the mould as described
needs to be mounted in a moulding machine that moves the
core and the female mould part relative to one another at
2o the appropriate rate while applying appropriate pressures.
It has been found that the pressure/distance profile of a
toggle operated moulding machine is ideally suited to the
injection compression moulding process but other machines
can be programmed to achieve a similar pressure/distance
25 profile. When a machine is not capable of changing smoothly
from a low pressure large displacement mode to a high
pressure small displacement mode, a further possibility
would be to include a module between the mould and the
machine platens that is capable of delivering the desired
30 distance/pressure profile.
