PROCEDE DE MOULAGE PAR INJECTION

INJECTION MOULDING PROCESS

Abrégé français

L'invention concerne un procédé de moulage par injection, selon lequel une matière plastique sous pression est injectée à partir d'une préchambre obturable (1), après ouverture de l'obturateur (2), dans une cavité de moule (3) qui est ensuite remplie sous pression. Le volume de la préchambre (1) et la pression régnant à l'intérieur de cette dernière, lors de l'ouverture de l'obturateur (2), présentent des valeurs en présence desquelles au moins la moitié de la pression atteinte dans la cavité (3) lors de la mise en oeuvre du procédé est produite lorsque le volume de la préchambre (1) est maintenu à un niveau constant pendant l'injection.

Abrégé anglais

The invention relates to an injection-moulding method, in which pressurised
synthetic material is injected from a sealable antechamber (1), after the
removal of a seal (2), into a moulding cavity (3), filling the latter under
pressure. The volume of the antechamber (1) and the pressure which prevails
therein when the seal (2) is removed, have values which result in the
formation of at least half the pressure achieved during the moulding process
in the moulding cavity (3), if the antechamber (1) volume is maintained at a
constant level throughout the injection-moulding process.

Revendications

5
CLAIMS
1. An injection moulding process in which plastic material under pressure
is injected from an antechamber (1) which can be shut off, into a mould cavity
(3)
after opening of a shut-off means (2), and fills the mould cavity under
pressure,
characterised in that the volume of the antechamber (1) and the pressure
prevailing
therein, at the opening of the shut-off means (2), are of values, at the
existence of
which at least half of the pressure achieved in the mould cavity (3) in the
process
occurs even if the volume of the antechamber (1) is kept constant during the
injection operation.
2. A process according to claim 1 characterised in that the pressure in
the antechamber (1) at the opening of the shut-off means (2) is over 1000 bars
and
preferably over 1500 bars.
3. A process according to claim 1 or claim 2 characterised in that the
volume of the antechamber (1) at the opening of the shut-off means (2) is at
least
twice as great as the volume which is downstream of the shut-off means (2) and
which includes the mould cavity (3).
4. A process according to one of claims 1 to 3 characterised in that the
volume of the antechamber (1) is kept constant during the injection operation
so that
the total pressure in the mould cavity (3) is produced by expansion of the
plastic
material which initially fills only the antechamber (1).

Description

CA 02381175 2002-02-05
1
Injection moulding process
The invention relates to an injection moulding process in which plastic
material under pressure is injected from an antechamber which can be shut off,
into
a mould cavity after opening of a shut-off means, and fills the mould cavity
under
pressure.
The operation of injecting the plastic material is usually effected by means
of
the screw which serves to plasticise the plastic material and which acts as a
piston in
the injection operation. There have however also been many proposals for
passing
the plasticised plastic material into a separate antechamber from which it is
expelled
by a piston which is independent of the plasticising screw.
Particularly in the production of small and thin-walled components, it is
essential for the injection operation to be terminated rapidly as otherwise
partial
hardening of the plastic material already occurs in the cooled mould, during
the
injection procedure. In order to achieve high injection speeds, hitherto the
advance
speed of the screw or a separate injection piston which is possibly provided
has
been increased more and more. The invention is based on the thought that
further
development along those lines is not meaningful as, at high screw advance
speeds,
the molten material in the antechamber is primarily compacted, whereas that
achieves little in terms of filling the mould cavity.
Therefore the object of the invention is to rapidly fill the mould cavity,
particularly when injection-moulding thin and small mouldings, in which
respect the
speed of the injection piston is to play no part or only a subordinate part.
In accordance with the invention that is achieved in that the volume of the
antechamber and the pressure prevailing therein, at the opening of the shut-
off
means, are of values, at the existence of which at least half of the pressure
achieved
in the mould cavity in the process occurs even if the volume of the
antechamber is
kept constant during the injection operation.

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Insofar as hitherto a pressure which is comparable to the pressure in the
mould
interior has akeady been built up upstream of the shut-off means, prior to
opening of
the shut-off means, the only result of that was that the mould was initially
partially
filled by expansion of the plastic material in the antechamber until after
some delay
the action of the screw advance movement came into effect. In contrast, the
invention
is based on the notion that the entire mould cavity is filled merely by
expansion of the
supply of plastic material which has accumulated in the antechamber and which
is
under pressure. If in that respect a movement of the screw or other injection
piston
takes place intentionally or unintentionally, that only results in a
modification of the
adiabatic expansion process which in itself governs the filling of the mould.
In
practical terms that means that the pressure in the antechamber, which
conventionally
does not exceed 800 bars, is typically increased to over 1500 bars when
carrying the
invention into effect, and in particular that the volume of the antechamber is
not
reduced to a very substantial extent, as is usual, in the injection operation,
but is
entirely or predominantly maintained.
Details of the invention are described hereinafter with reference to the
drawing
showing a view in diagrammatic cross-section of an apparatus which is
conventional
in itself, for carrying the process into effect.
The process according to the invention can be carried into effect on any
conventional injection moulding apparatus insofar as the feed flow to the
mould
cavity is controllable by a shut-off means. Therefore only the parts of an
injection
moulding apparatus which are essential to the process of the invention are
described
and illustrated here.
In the illustrated apparatus thermoplastic material is plasticised in the
cylinder
8 by a screw 5 and passes into the antechamber 1. The antechamber extends
through
the bores 13 almost to the mould 7. Its front opening 14 is closable towards
the mould
7 by means of a shut-off means 2, whereas a back-flow of the plastic material
out of
the antechamber 1 is prevented by a back-flow closure means 6 at the tip of
the screw
5.

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An essential functional part of the shut-off means 2 is in per se known manner
a closure needle 9 which, under the pressure of the plastic material in the
antechamber
1, has a tendency to move towards the right in the drawing. The needle 9 is
acted
upon in the closure direction by a lever 11 which is pivotable about the pin
10 and
which is subjected to the action of a controllable hydraulic unit 12.
It is essential for the invention that a substantially higher pressure is
built up in
the plastic material disposed in front of the shut-off means 2, than was
hitherto usual,
and that filling of the mould cavity 3 and the gate region 4 in front thereof,
is effected
by expansion of the plastic material in the antechamber 1. That does not
exclude a
movement of the screw 5 for increasing or reducing the pressure in the mould
cavity 3
also being effected, to influence the pressure pattern in the mould cavity 3,
after
opening of the shut-off means 2. Opening of the shut-off means 2 can also be
effected
in a controlled manner in order to modify the pressure pattern in the mould
cavity 3
which is determined primarily by the adiabatic relief of pressure of the
plastic material
in the antechamber 1.
In order to be able to suitably determine pressure and volume in the
antechamber 1 for a given mould cavity 3, desirably firstly a volume which
substantially exceeds the volume of the mould cavity 3 is selected for the
antechamber
1. Then the desired pressure in the mould cavity 3 is selected. The pressure
in the
antechamber 1 which leads to that result can be ascertained in a simple series
of tests.
With knowledge of the state equation of the plastic material used, it is also
readily
possible to calculate that pressure. This will be subsequently demonstrated by
means
of an embodiment.

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Example:
Material used: Polystyrene 143 E
Density at ambient temperature: 1.047 g/cm3
Volume of the antechamber 1 in front of the shut-off means 2: 45.6 cm3
Volume behind the shut-off means 2:1.37 cm3, of which 1 cm3 actual mould
cavity 3
The plastic material is under a pressure of 2000 bars, and its temperature is
around 30° above the desired operating temperature of 220°C.
The shut-off means 2 is now opened, whereby the plastic material expands to
the entire available volume, that is to say into the region 4 of the gate and
into the
mould cavity 3. Due to that adiabatic expansion, cooling by 30°C takes
place and
there is a pressure drop to the desired final range of 500 bars. That pressure
is
generally easily sufficient for production of the desired product, but it can
be
increased by displacement of the screw 5, subsequently to opening of the shut-
off
means 2, or reduced (by displacement towards the left in the drawing).

Dessin représentatif