Note: Descriptions are shown in the official language in which they were submitted.
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Description
Injection-moulding machine with torpedo plunger
plastification
The invention relates to a plastification unit of an
injection-moulding machine for melting and conveying
the plastic material, comprising a cylinder and a
torpedo plunger with a nonreturn valve arranged on its
front end, wherein the conveying of the plastic is
ensured by two opposing filling plungers, which operate
laterally to the left and right at the rear end of the
cylinder alongside the torpedo plunger, with a divided
feed shaft, and which are each driven by a working
cylinder, wherein a slide plate respectively closes
part of the feed shaft over the filling plunger moving
in the direction of the nozzle with a stuffing effect
and a filling channel valve opens the associated
filling channel, wherein the filling channels each
continue in the form of a longitudinal groove in the
shaft of the torpedo plunger and open out into
downstream shearing and mixing parts of the torpedo
plunger, wherein the flow cross sections of the
shearing and mixing parts narrow in the direction of
the nozzle, and wherein the torpedo plunger is
connected to a central injection cylinder, or a number
of decentral injection cylinders, for the axial drive
and generation of back pressure and injection pressure.
Injection-moulding machines of the type in question are
known from the prior art and operate in such a way that
a conveying screw connected to a rotary drive
transports the plastic material in the screw cylinder
from the feed opening into the accumulating space in
front of the nonreturn valve and is thereby forced back
in the screw cylinder against a back pressure and
subsequently pushed again in the direction of the
nozzle by one or more injection cylinders. As a
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result, the melted plastic is pressed through a nozzle
into the cavity of a mould. The energy for melting the
plastic material is made up of the thermal energy of
the set of heater bands on the screw cylinder and the
frictional heat of the conveying and packing process.
The rotation of the conveying screw produces metallic
friction between the screw cylinder and the parts of
the nonreturn valve, which inevitably causes wear, and
consequently limits the service life of this system.
The metallic abrasion caused by the wear has adverse
effects on the visual quality of the plastic part. The
conveying capacity of a screw is strongly dependent not
only on the rotational speed but also on the conditions
in terms of friction and fit between the screw cylinder
and the screw. These conditions deteriorate as a
result of wear and reduce the conveying capacity.
Furthermore, an ultra-high pressure plastification unit
is described in the literature (Plastverarbeiter 23,
1972 issue 10, pages 671-687), in which a plunger in a
cylinder forces the plastic from the feed opening in
the direction of the nozzle with extremely high
pressure through a narrow nozzle. The energy for
melting is intended to be obtained not only from the
heat of the set of heater bands but also from the
resistance to the flow through the nozzle. In the case
of a variant of this form of construction, the plunger
is operated with a pulsating pressure, which is
intended to bring about additional heating of the
plastic material.
This arrangement requires for the cylinder and plunger
material strengths that are difficult to realize. The
portion of the plastic that is pressed into the gap
between the cylinder and the plunger is lost from the
process as leakage. Exact metering of the amount of
plastic for the injection operation is not possible
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with the feed opening being exposed by the return of the
plunger.
An object of the invention is therefore to develop a
plastification unit further in such a way as to avoid wear
such as in the case of a rotating screw and accomplish a
continuous process comparable to that of a screw and achieve
conveying and homogenizing of the plastic material that can
be controlled better, without increased leakage.
The present invention provides a concentric torpedo plunger,
comprising a shaft and a combination of shearing and mixing
parts and a nonreturn valve secured in front of the latter,
is arranged in a cylinder, and two opposing filling plungers
in the end of the cylinder, on both sides alongside the
torpedo plunger, bring about the conveying of the plastic by
the plastic being pressed through a respective associated
filling channel, a closable filling channel valve and a
filling grove in the shaft of the torpedo plunger to beyond
the gap between the cylinder bore and the shearing parts and
through the mixing parts into the accumulating space, and
thereby homogenized and melted.
More specifically, the present invention provides a
plastification unit for an injection unit of an injection-
moulding machine, comprising a heated cylinder, secured to
the front end of which is a cylinder head with a nozzle,
mounted in a housing, which is connected to an injection
cylinder, and a nonreturn valve, which prevents the plastic
from flowing back during injection, wherein that an axially
displaceable torpedo plunger is arranged in the central bore
of the cylinder and said plunger, comprising a shaft and a
combination of shearing parts and mixing parts,
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is surrounded by the flowing plastic, which is thereby
melted and homogenized, wherein two opposing filling
plungers are arranged in bores laterally alongside the
central bore and bring about the conveying of the plastic
through the filling channels, through the filling channel
valves and through the filling groove in the shaft of the
torpedo plunger.
Dispensing with a rotary drive achieves the effect that the
injection cylinder is of a simpler construction and the
overall injection unit is made shorter, because it does away
not only with the length required for installing the rotary
drive but also with all the components for the transmission
of a torque.
The end of the cylinder has two feed openings, which open
out into the working spaces of the filling plungers. If the
rear filling plunger starts the stuffing operation, a slide
plate closes the feed
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opening over this filling plunger and at the same time
exposes the front feed opening. This rules out the
possibility of the plastic granules being forced back
into the material hopper, but instead includes them
fully in the filling operation. In addition, the rear
filling channel valve opens and the front filling
channel valve is closed. This prevents the plastic
from being pumped from the rear filling channel into
the front filling channel.
The length of the filling groove in the shaft
corresponds to the maximum working stroke of the
torpedo plunger and the distance from the point where
the filling channels open out into the inner bore to
the rear end of the cylinder likewise corresponds to
the maximum stroke length. This achieves a shorter
overall length of the cylinder in comparison with screw
plastification, since the feed openings are located in
the end of the cylinder.
Various views of an exemplary embodiment are shown in
the drawings, in which:
Figure 1 shows an injection unit with torpedo plunger
plastification, including the housing and the
injection cylinder, in horizontal section and
Figure 2 shows a plastification unit with a torpedo
plunger as a plan view, with the filling
plungers, and
Figure 3 shows a plastification unit with a torpedo
plunger in vertical section, with a material
hopper, and
Figure 4 shows a plastification unit, with a torpedo
plunger with toggle lever drives for the
filling plungers.
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The injection unit represented in Figure 1 shows a
cylinder 1 with a cylinder head 2 and a nozzle 3, in
which a torpedo plunger 4 that is connected to an
5 injection cylinder 5 is arranged. The cylinder 1 is
mounted in a housing 6, wherein the annular face of the
cylindrically thickened end of the cylinder la
introduces the injection force into the housing 6.
Arranged in this end of the cylinder la, on both sides
alongside the bore for the torpedo plunger, are two
bores of smaller diameter, in which the filling
plungers 7 are guided. These bores continue as filling
channels 8 into the filling channel valves 9. It is
essential here that the filling channel valves 9
alternately open the filling channel 8 of the filling
plunger 7 that is respectively driven by a working
cylinder 10 in the direction of the nozzle and release
the connection to the filling groove 11 in the shaft 4a
of the torpedo plunger 4 and close the opposite filling
channel 8. This is a prerequisite for the stuffing
filling plunger 7 not to force the plastic in the space
in front of the returning filling plunger 7 and allow a
back pressure to build up in the accumulating space 12
in front of the nonreturn valve 13. The lengths of the
filling grooves 11 correspond to the maximum stroke of
the torpedo plunger 4 and go over into an annular gap
between the central bore of the cylinder 1 and the
shaft 4a. In the direction of the nozzle, the shaft 4a
is adjoined by a combination of shearing parts 4b and
mixing parts 4c, with narrowing flow cross sections. A
nonreturn valve 13 is secured to the end face of the
torpedo plunger 4.
The plan view of an injection unit represented in
Figure 2 shows a cylinder 1 with two feed openings 14
in the end of the cylinder la over the bores of the
filling plungers 7. It is essential here to link the
timing of the activation of the filling plungers 7,
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filling channel valve 9 and slide plate 15 in such a
way that, when the movement of the rear filling plunger
7 in the direction of the nozzle begins, the rear feed
opening 14 is closed by the slide plate 15 and the rear
filling channel valve 9 is opened and the front filling
channel valve 9 is closed; the slide plate 15 has then
inevitably released the front feed opening 14 and the
front filling plunger 7 starts to move into the
starting position. This has the advantage that the
entire volume filled with plastic granules in front of
the filling plunger is included in the conveying
process and metering problems do not occur.
The guiding of the slide plate 15 between a cassette 16
and a cover 17 is represented in Figure 3. A material
hopper 18 is secured on the cover 17. The cassette 16
is preferably seated in a transverse groove of the end
of the cylinder la.
Figure 4 shows filling plungers 7 driven by toggle
levers 19. This has the advantage that the compressive
force increases the further a filling plunger 7 is
moved in the direction of the nozzle, which corresponds
to the required reaction forces.
List of reference numerals
1 cylinder
la end of the cylinder
2 cylinder head
4 torpedo plunger
4a shaft
4b shearing part
4c mixing part
5 injection cylinder
6 housing
7 filling plunger
8 filling channel
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9 filling channel valve
working cylinder
11 filling groove
12 accumulating space
5 13 nonreturn valve
14 feed opening
slide plate
16 cassette
17 cover
10 18 material hopper
19 toggle lever
