Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Hot runner nozzle having a ceramic shut-off needle
The present invention relates to a hot runner nozzle having a shut-off needle
for
injecting thermoplastic moulding compositions into moulds, the shut-off needle
being wholly or partly of ceramic.
Hot runner nozzles are used in the prior art for manufacturing mouldings of
thermoplastic moulding compositions by means of injection moulding. Hot runner
nozzles serve to inject the melt of the thermoplastic moulding composition
into the
cavity of the mould without the melt solidifying. In order to keep the
moulding
composition above the melting point, the nozzle can be heated by means of
electric
heating elements.
Hot runner nozzles can be constructed with or without a shut-off needle.
Nozzles
having a shut-off needle serve to close the mould cavity at the end of the
injection
procedure. When the mould is opened, no more melt from the thermoplastic
material
can then emerge from the hot runner nozzle. Shut-off needles also make it
possible
to manufacture mouldings which have a smooth surface at the injection point. A
smooth surface at the injection point can represent a substantial quality
feature in the
corresponding mouldings.
Known hot runner nozzles having shut-off needles, such as those described in
DE 42
758 and EP 0 765 728, substantially comprise a nozzle housing surrounded by
25 electric heating elements and enclosing a melt runner, a nozzle orifice
which can be
shut off by a shut-off needle, and a needle drive unit. Known hot runner
nozzles use
hydraulic or pneumatic cylinders in order to be able to move the needle either
directly or by way of a lever and open or close the shut-off point. When the
cavity is
filled, the hot moulding composition flows along the opened needle in the hot
runner
30 nozzle. This warms the needle. After the needle has been closed, it forms a
hot spot
which favours adhesion of the moulding composition. By way of the surface
through
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which the shut-off needle bears against the cooled mould wall, the needle tip
is then
cooled again. In order to achieve a high cooling capacity, a material of high
thermal
conductivity is conventionally used to manufacture the needles.
The needle of lmown hot runner nozzles is made for example of hardened steel.
Known hot runner nozzles having steel shut-off needles made have a number of
disadvantages. Shut-off needles made of steel are subject to wear if the
thermoplastic
moulding compositions contain solid fillers, e.g. abrasive powders or mineral
fibres.
Using steel shut-off needles with needle gates of large cross-section may
result in
adhesion of the moulding composition. When the mould is opened and the
moulding
removed, this adhesion of the moulding composition then results in instances
of
unevenness on the moulding in the region of the injection point, which reduces
the
quality.
Normally, problems with adhesion to the shut-off needle are solved by
extending the
cooling period in the process cycle. A longer cooling time also extends the
overall
process cycle time and in the end results in increased manufacturing costs for
the
mouldings. It is also possible to reduce the cross-section of the needle gate
to
overcome this problem. However, small cross-sections are disadvantageous for
the
injection procedure and hence limit the size of the moulding which can be
manufactured without defects. Problems with adhesion can also be solved by
reducing the temperature of the temperature-controlled mould in order to cool
the
needle better. However, reducing the mould temperature is only possible to a
limited
extent, since many moulding compositions only give high-quality mouldings
within
a very narrow temperature range.
Ultimately, the surface through which the needle bears against the cooled
mould
wall can be made larger to avoid adhesion. This salution has the aim of
improving
the cooling of the needle tip.
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The object of the invention is to construct a hot runner nozzle having a shut-
off
needle such that high-quality mouldings can be manufactured in the mould while
minimising wear of the needle and adhesion of the moulding composition and
avoiding the other disadvantages of known arrangements.
This object is achieved in accordance with the invention in that a ceramic
shut-off
needle is inserted into the hot runner nozzle. Surprisingly, only ceramic
needles of
particular materials of low thermal conductivity give the desired result. This
result is
all the more surprising since previous experiences with needles made of steel
had the
aim of improving the cooling of the shut-off needle and the use of materials
of high
thermal conductivity. A shut-off needle of low thermal conductivity should
rather, in
accordance with the current view, make cooling of the needle worse and thus
also
make the quality of mouldings worse at the injection point.
1 S The invention relates to a hot runner nozzle having a shut-off needle, for
the purpose
of manufacturing mouldings from thermoplastic moulding compositions, having a
heatable nozzle housing which encloses a melt runner and can be shut off by a
shut
off needle by means of a movable needle drive unit, characterised in that the
shut-off
needle is of a ceramic material having a thermal conductivity of less than 7
W/m.K
at 100°C.
In a further preferred embodiment of the present invention, only the tip of
the shut-
off needle is of ceramic, with the needle shaft itself being of metal, in
particular of
steel or hardened steel. This embodiment has the advantage that the coupling,
between the needle and the hydraulic cylinder or the drive lever, and the
needle
guide can be manufactured of steel. This reduces the risk of breaking the
needle
when it is installed or during injection moulding.
All the current methods for manufacturing metal/ceramic bonds can be used to
attach the ceramic tip to the needle made of steel. This bond should, however,
be as
resistant to pressure as possible. Preferably, the ceramic tip is manufactured
by
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shrinking the metal needle shaft onto a ceramic pin at the opposite end of the
ceramic shut-off needle.
The shut-off needle or ceramic needle tip is preferably of densely sintered
and, to as
large as possible an extent, non-porous ceramic material having a thermal
conductivity of at most 7 W/m.K at 100°C, preferably at most 3 W/m.K at
100°C.
Examples of suitable ceramic material are zirconium oxide, porcelain,
forsterite and
steatite.
Shut-off needles or needle tips of sintered zirconium oxide or partially
stabilised
zirconium oxide are particularly preferred. Sintered zirconium oxide has a
thermal
conductivity in the region of 2 to 2.5 W/m.K. Sintered partially stabilised
zirconium
oxide, such as ZrOz partially stabilised by MgO, Ca0 or Y203, is particularly
preferred as the ceramic material because of its high flexural strength.
The hot runner nozzle according to the invention can be used in a variety of
ways in
the injection mould, in particular in injection moulding machines for
injection
moulding thermoplastic polymers and other thermoplastic moulding compositions.
In particular, where hot runner nozzles having shut-off needles of large cross-
section
are required, the use of the hot runner nozzle according to the invention
contributes
to a reduction in the cycle time and hence better economy in the injection
moulding
process.
A particular application of the hot runner nozzle according to the invention
is in the
injection moulding of thermoplastic moulding compositions containing high
quantities of ceramic powders. The injection moulding of these so-called
ceramic
moulding compositions, which contain in particular from 50 to 70 per cent by
volume of ceramic powders, is generally known. 'Che mouldings are subsequently
liberated from the organic constituents by the action of heat and then
sintered at a
temperature of >800°C to give dense ceramic bodies. These ceramic
moulding
compositions are very abrasive, wear conventional metal shut-off needles
rapidly
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and have a particular tendency to temperature-dependent adhesion. The hot
runner
nozzles according to the invention having shut-off needles made of ceramic
make it
possible to manufacture mouldings of high quality from ceramic moulding
compositions too, with a minimum of wear and adhesion to the shut-off needle.
The following can be mentioned as advantages of the hot runner nozzles
according to the invention having ceramic shut-off needles. Among other
things, the needle cross-section can be made bigger and hence the pressure
during injection moulding can be reduced without problems with adhesion
occurring. The injection moulding cycle time is also shorter than can be
achieved with a steel shut-off needle. The ceramic needle according to the
invention is more wear-resistant than the standard steel needle.
The invention will be described in more detail with reference to the following
examples, without restricting the details of the invention.
In the drawings:
Fig. 1 shows a longitudinal section through the hot runner nozzle according to
the
line A-B in Fig. 2,
Fig. 2 shows a longitudinal section through the hot runner nozzle according to
the
line C-D in Fig. 1,
Fig. 3a shows a shut-off needle with a ceramic tip,
Fig. 3b shows a shut-off needle according to Fig. 3a with the ceramic tip
separate
from it, and
Fig. 4 shows an injection mould with the hot runner nozzle according to the
invention.
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Examples
Example 1
An injection mould for manufacturing a moulding weighing approximately 120 g
was constructed, as shown in Fig. 4, with a hot runner nozzle. The hot runner
nozzle
1 was provided with a ceramic shut-off needle 6 (see Figs. 1 and 4). The hot
runner
nozzle 1 guides the injection moulding composition along the runner 4, through
the
electrically heated steel block 3 and 3a centrally to the base of the moulding
11.
During injection moulding, the needle 2 or 6 is opened and closed by the
laterally
disposed cylinder 5 using compressed air by way of the bars 9. A separately
cooled
ante-chamber 12 is disposed around the hot runner nozzle. The diameter of the
shut-
off needle tip 13 was 6 mm.
The shut-off needle 6 was constructed, as shown in Figs. 3a and 3b, from a
steel
needle 8 having a tip 7 of ceramic. The ceramic tip 7 comprised sintered
zirconium
oxide, partially stabilised with MgO, having a thermal conductivity of 2.5
W/m.K at
100°C, a density of 5.9 g/cm3 and a flexural strength of 500 N/mm2. The
ceramic tip
7 was attached, as shown in Figs. 3a and 3b, to the needle shaft 8 of hardened
steel
by shrinking the tube 13 of the needle shaft 8 onto the pin 14 of the ceramic
tip 7.
Injection moulding tests were carried out on an Arburg Allrounder 370 C
injection
moulding machine with a locking force of 100 tonnes and a 35-mm injection
unit. A
thermoplastic moulding composition comprising 84% by weight of ceramic powder
and 16% of a thermoplastic polymer composition having a melting point of
94°C
was used. The injection moulding composition had a viscosity of 620 Pa.s at a
temperature of 130°C.
The following operating parameters were used to injection mould the moulding:
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Composition temperature, extruder 160°C
hot runner 160°C
Mould temperature, wall 62°C
ante-chamber 50°C
Injection time 0.7 s
Holding pressure time 4 s
Holding pressure 250 bar
Remaining cooling time 15 s
Total cycle time 31 s
No adhesion to the shut-off needle made of zirconium oxide was observed. The
mouldings obtained were completely free of defects.
Comparison example 2
Comparison tests using the same hot runner nozzle 1 but a shut-off needle 2
manufactured from hardened steel, and with otherwise the same injection
moulding
conditions as those described in Example 1 above, resulted in pronounced
adhesion
of the moulding composition to the shut-off needle 2. This adhesion has the
effect
that when the moulding 11 is removed from the mould it has an irregularly
shaped
depression. This depression cannot easily be dealt with by secondary
finishing.
Lowering the mould temperature of the ante-chamber to 35°C or
lowering the
temperature of the moulding composition in the hot runner nozzle to
140°C could
not completely prevent the adhesion of the moulding composition to the shut-
off
needle. Even extending the remaining cooling time by 5 to 10 seconds did not
give
defect-free mouldings.
