Note: Descriptions are shown in the official language in which they were submitted.
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PISTON RING ASSEMBLY FOR AN INJECTION UNIT
TECHNICAL FIELD
The present invention relates, generally, to injection units for molding
machines. More specifically,
the present invention relates to a support structures for a piston assembly.
BACKGROUND OF THE INVENTION
The injection molding process typically comprises preparing a polymeric (or
sometimes metal)
material in an injection unit of a molding system, injecting the now-melted
material under pressure
into a closed and clamped mold, solidifying the material in its molded shape,
opening the mold and
ejecting the part before beginning the next cycle. The molding material
typically is supplied to the
injection unit from a hopper in the form of pellets or powder. The injection
unit transforms the solid
material into a molten material (sometimes called a"melt"), typically using a
feed screw, which is
then injected into a hot runner or other molding system under pressure from
the feed screw or a
plunger unit. A shut off valve assembly is typically provided to stop and
start the flow of molten
material from the barrel to the molding system.
Some examples of known molding systems having such an injection unit are: (i)
the HyPETTM
Molding System, (ii) the QuadlocTM Molding System, (iii) the HylectricTM
Molding System, and (iv)
the HyMetTM Molding System, all manufactured by Husky Injection Molding
Systems Ltd.
SUMMARY OF THE INVENTION
According to a first aspect of the invention, the present invention provides a
piston assembly for
injection unit of a molding machine, comprising:
a piston housing, defining a piston chamber and a first aperture and a second
aperture on
opposing ends of the piston chamber, the piston housing being selectively
pressurizable;
an injection piston, slidably located at least partially within the piston
chamber and extending
through both the first aperture and the second aperture, the injection piston
being movable between
a retracted position and an extended position by selective pressurization of
the piston chamber; and
support structures for the injection piston located within the piston housing
proximate the first
aperture and the second aperture, the support structures operable to maintain
pressure in the piston
chamber.
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A piston assembly for injection unit of a molding machine is provided. The
piston assembly
includes a piston housing defining a piston chamber that has apertures at
opposing ends. An
injection piston is slidably located at least partially within the piston
chamber and extends through
both the apertures. The injection piston is movable between a retracted
position and an extended
position by selective pressurization of the piston chamber. Support structures
for the injection piston
are located within the piston housing proximate the first aperture and the
second aperture, the
support structures operable to maintain pressure in the piston chamber.
BRIEF DESCRIPTION OF THE DRAWINGS
Exemplary embodiments of the present invention will now be described with
reference to the
accompanying drawings, in which:
Fig. 1 is a sectional view of an injection unit in accordance with an aspect
of the present
invention;
Fig. 2 is a perspective view of the injection unit shown in Fig. 1, shown in a
pivoted position;
Fig. 3 is a cross sectional view of a barrel connector for the injection unit
shown in Fig. 1;
Fig. 4 is an exploded view of a screw connector for the injection unit shown
in Fig. 1;
Fig. 5 is a partial-cutaway view of an alternative piston assembly for the
injection unit shown
in Fig. 1;
Fig. 6 is a front cross sectional view of a drive assembly for the injection
unit shown in Fig. 1
taken along lines D-D;
Fig. 7 is a perspective view of a barrel collar for the injection unit shown
in Fig. 1;
Fig. 8 is a cross sectional view of a piston assembly for the injection unit
shown in Fig. 1; and
Fig. 9 is a cross sectional view of a drive assembly for the injection unit
shown in Fig. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
Referring now to Fig. 1, an injection unit for a molding machine in accordance
with a non-limiting
embodiment of the invention is shown generally at 20. Injection unit 20 is
mounted on a base
structure 22 and includes a barrel assembly 24, a nozzle assembly 26, a piston
assembly 27 and a
drive assembly 28.
Referring additionally to Fig. 2, base structure 22 typically sits on
vibration pads (not shown) on the
factory floor and typically houses the controls, electronic cabinets,
hydraulic tanks, and other
common equipment (none shown) as is known to those of skill in the art. A
swivel plate 30 is
pivotally mounted to a planar surface 32 on base structure 22 along the
longitudinal axis of base
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structure 22. When swivel plate 30 is pivoted, a large cross-section of the
swivel plate is still
supported by the planar surface 32, so that the load of injection unit 20 is
distributed across base
structure 22.
A rail 34 is fixedly mounted along the longitudinal axis of swivel plate 30.
As will be described in
greater detail below, the barrel assembly 24 is slidably mounted to the rail
34 via a pair of rail
carriages 60, and operable to translate between a fully-forward position,
i.e., towards a mold, and a
fully-back position, i.e., away from a mold (not shown). While a single, wide
rail 34 is preferred,
two narrower rails could also be used to mount injection unit 20.
An axle 36 defines a mounting interface for swivel plate 30. Swivel plate 30
pivots about the axle
36, which is located by the swivel plate at or near the centre of gravity for
injection unit 20.
Preferably, swivel plate 30 includes at least two mounting interfaces for axle
36 in order to locate
the injection unit at different positions relative to base structure 22.
During normal use, swivel plate 30 is immobilized and prevented from pivoting
by at least one
fastener 38, and preferably a plurality of fasteners 38 that extends through
aligned apertures in both
swivel plate 30 and base structure 22. Preferably, fasteners 38 are jack
screws, and can thus adjust
the vertical angle of injection unit 20 relative to the primary melt channel
in a runner system (not
shown). When injection unit 20 is down for servicing, the fasteners 38 are
removed, permitting
swivel plate 30 to pivot. Slots 39 are provided in the sides of swivel plate
30. Pins 41 extending
from base structure 22 are located within slots 39 and delimit the degree of
rotation permitted for
injection unit 20. Thus, the injection unit 20 can be pivoted away from the
mold (not shown),
permitting easier serving or replacement of components such as barrel assembly
24 or drive
assembly 28.
Referring back to Fig. 1, barrel assembly 24 includes a barrel 40 and a barrel
housing 42. Barrel
housing 42 can be integrally formed (such as through casting) or it can be
assembled from smaller
pieces for ease of manufacturing. A screw 46 is rotatably and slidably located
within a channel 50 in
barrel 40. Injection material, such as plastic pellets is stored in a hopper
54 that is mounted over
barrel housing 42. The injection material is fed to channel 50 through an
integrally-formed feed
throat 56, where it is plasticized by screw 46. Heater bands 48 are provided
along the exterior of
barrel 40, and help to melt the injection material. The plasticized material
is expelled through an
orifice 52 in nozzle assembly 26 into a mold (not shown). The flow of the
plasticized material can
be metered by a valve 58, which is moved between an open and closed position
by an actuator 66.
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The barrel housing 42 is slidably mounted to rail 34 via rail slides 60. The
barrel 40 extends through
a central aperture 62 defined by barrel housing 42 and extends into a chamber
64 in barrel housing
42. The sidewalls of central aperture 62 provide a wide stance for the barrel
mounting. In the
presently-illustrated embodiment, barrel housing 42 is cast as a single
component, but it is
contemplated that multi-component versions could also be manufactured for ease
of assembly. An
access window 67 (best seen in Figs 2 and 5) provides egress into chamber 64.
A control box 68
provides power to heater bands 48 on the barrel 40.
Referring additionally to Fig. 3, a locating groove 70 is defined on a portion
of the exterior surface
of barrel 40 within chamber 64, proximate an end 72 of the barrel (Fig. 1). A
barrel coupler 74,
comprising a pair of semi-annular half-couplers 81 (Fig. 7) that cooperate
together to retain barrel
40 within barrel housing 42 and prevent its withdrawal. The barrel coupler 74
is located around the
end 72 on barrel 40. Fasteners (not shown) are threaded through aligned
apertures 75 (best seen in
Fig. 5 and Fig. 7) defined in each the two half-couplers to clamp the two
parts together. Bolts 77 are
threaded through aligned apertures 79 (partially seen in Fig. 7) to mount the
barrel coupler 74 to
barrel housing 42 and prevent its rotation via that of the screw 46. On a
first end 76 of the barrel
coupler 74, an annular tab 78 depends radially inwards, extend into locating
groove 70 and is sized
so that it abuts against the sidewalls of the locating grove 70, and further
abuts against a sidewall 80
of chamber 64. A second end 82 of barrel coupler 74 extends to the end 72 of
barrel 40 so that the
two cooperatively define an end surface 84. Optionally, second end 82 and end
72 can be
substantially co-planar. As is described in greater detail below, end surface
84 defines a piston stop
for an injection piston 85 of piston assembly 27.
Referring now to Fig. 8, the piston assembly 27 is described in greater
detail. Piston assembly 27
includes a piston housing 44 and the injection piston 85. The piston housing
44 can be integrally
formed (typically through casting) or it can be assembled from smaller
components for ease of
manufacturing. Optionally, piston housing 44 can be integrally formed along
with barrel housing 42.
Piston housing 44 includes a piston chamber 86 that includes a pair of
apertures on opposing ends
of piston chamber 86. A first aperture, namely piston aperture 200 is coaxial
and in communication
with chamber 64 in barrel housing 42. A second aperture, namely shaft aperture
120, is coaxially
and in communication with in drive assembly 28. A portion of the injection
piston 85, namely
piston cylinder 90 is sized to be in slidable engagement within piston chamber
86 and chamber 64,
so that the injection piston 85 can move between a retracted position and an
extended position (each
described in greater detail below).
A first end 92 of piston cylinder 90 includes a screw connector 94 (described
in greater detail
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below) for coaxially mounting an end 98 of screw 46. When piston cylinder 90
is in the extended
position, first end 92 of the piston cylinder 90 abuts against end surface 84,
stopping the forward
motion of screw 46. A second end 104 of piston cylinder 90 includes a piston
shaft 112 that extends
coaxially out through a shaft aperture 120 in piston housing 44 into drive
assembly 28 (and is
described in greater detail below). Piston shaft 112 can be integrally formed
as part of piston
cylinder 90, or can be separately attached. When piston cylinder 90 is in the
retracted position, the
second end 104 abuts against an endwall 124 of piston chamber 86, stopping the
return motion of
screw 46.
Referring now to Figs. 3 and 4, screw connector 94 is shown in greater detail.
A recess 96 is
coaxially defmed on the first end 92 of the piston cylinder 90. Recess 96
includes a opening portion
100, a frusto-conical portion 102, and a base portion 106 sized to fit a
collet 108. A collet 108 for
retaining screw 46 is adapted to be inserted within recess 96, and includes a
flange portion 110 sized
to fit within opening portion 100 and a tapered portion 116 sized to fit
within frusto-conical portion
102 of recess 96. Collet 108 further includes a central bore 114 sized for
screw 46 to pass
therethrough. Apertures 118 are concentrically distributed around flange
portion 110 and align with
apertures 122 in collet 108.
To connect screw 46, collet 108 is placed within recess 96 so that tapered
portion 116 is located
within frusto-conic portion 102. A screw base 126 of screw 46 is inserted
through central bore 114
so that it bottoms out against the end of base portion 106. Fasteners 128 are
inserted through aligned
apertures 118 and 122 to secure collet 108 to piston cylinder 90. As the
fasteners 128 are tightened,
a wedging action between tapered portion 116 and frusto-conic portion 102
causes the tapered
portion 116 to securely grip the screw base 126 of screw 46. While the
presently-illustrated
embodiment shows only a single collet 108, it is contemplated that two or more
collets 108, sized to
fit within recess 96 could also be used. Alternatively, a portion of the
collet 108 could extend
outside of recess 96, provided that the wedging action occurred substantially
within recess 96.
The inventors have determined that the piston design in previous injection
units, such as in the one
described in the 2005/0048162 application, are prone to misalignment as they
can include long
unsupported portions when fully extended. In contrast, the present invention
provides fixed, non-
telescopic support structures for the piston 85 at each end of piston housing
44. Referring back to
Fig. 8, a first support structure, namely wear collar 130, is located in a
wear niche 132 at the edge of
piston chamber 86 adjacent to barrel support chamber 42. A second support
structure is defined by
sidewalls 135 of shaft aperture 120. Both the wear collar 130 and the sidewall
135 provide a tight-fit
engagement which maintains the piston 85 in coaxial alignment with screw 46
over the full range of
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travel without sagging. Additionally, both wear collar 130 and the sidewall
135 include a number of
sealing niches 134 for locating 0-rings in order to maintain fluid pressure
within piston chamber 86.
Additional piston seal niches 134 are defined within a piston guiding
structures 136 defined on the
exterior surface of by piston cylinder 90. The guiding structures 136 are
concentric portions of
piston cylinder 90 having a wider diameter than the rest of the cylinder.
Preferably, the gap between
guiding structures 136 and the sidewall of piston chamber 86 is slightly
greater than that between
either wear collar 130 or the sidewall 135 and the adjacent portion of the
piston 85 so as to facilitate
the insertion of piston 85 into and through piston housing 44 while the wear
collar 130 is removed
from the wear niche 132.
During assembly of injection 20 (or reassembly after servicing, such as
replacing the 0-rings), the
piston housing 44 is detached from barrel housing 42 to expose piston aperture
200. Wear collar
130 is also removed. The piston 85 is inserted into piston chamber 86, piston
shaft 112 first. As the
piston shaft 112 is threaded through shaft aperture 120, the guiding structure
136 helps maintain
alignment of the injection piston 85. Once the injection piston 85 is in
place, the wear collar 130 can
be concentrically mounted around the piston cylinder. 90 within wear niche
132. Remounting the
wear collar 130 will also help to correct the alignment of the injection
piston 85. The piston housing
44 can now be connected (or reconnected) to barrel housing 42.
Thus the present invention provides non-telescoping support for the piston
cylinder 90 over a major
portion of its length (while either extended or retracted), so that alignment
with screw 46 is readily
maintained both during operation and during regular maintenance.
Injection unit 20 can be configured to use a first piston, namely a dual
action injection piston 85A
(shown in Fig. 1), or a second piston, namely single action injection piston
85B (shown in Fig. 5).
As known to those of skill in the art, dual action injection pistons use
hydraulic pressure to both
extend and retract the piston. The piston cylinder subdivides the piston
chamber into two fluid-tight
portions, and forward or rearward motion is effected on the injection piston
by pressurizing one
portion or the other single action injection pistons do not subdivide the
piston chamber, and are
moved to the extended position via hydraulic pressure in the piston chamber,
but are returned to the
retracted position by depressurizing the chamber and the plasticizing of the
resin to push back the
screw. Generally speaking, dual action cylinders provide higher performance
(i.e., lower cycle
times), but an increased complexity and cost. Unless explicitly noted, the
following features are
common to both dual action injection pistons 85A and single action injection
pistons 85B.
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For a dual-action injection piston 85A, a first piston cylinder, namely piston
cylinder 90A (i.e.,
configured for dual action) has an outer diameter adapted for a fluid-tight
fit against a portion of
sidewall 138 in piston chamber 86, which divides piston chamber 86 into a
first portion 142A
(shown here having a narrower diameter) and second portion 142B (shown here
having a wider
diameter). Piston cylinder 90A is moved to the extended position by
hydraulically pressurizing
portion 142A of piston chamber 86 (i.e., the portion of piston chamber 86
between second end 104
of the piston cylinder and the endwall 124 of the piston chamber). The
hydraulic fluid acts upon
second end 104, urging piston cylinder 90A towards end surface 84. Piston
cylinder 90A is returned
to the retracted position by hydraulically pressurizing portion 142B of piston
chamber 86. The
hydraulic fluid acts upon a return surface 144, urging piston cylinder 90A
towards endwall 124. For
the purposes of clarity, the ports, valves and lines for the hydraulics system
have been omitted from
the illustration.
Referring now to Fig. 5, an injection piston 85B is shown in greater detail. A
second piston cylinder,
namely piston cylinder 90B (i.e., configured for single action) has an outer
diameter sized smaller
than sidewall 138, but rather, is sized for a fluid-tight fit against wear
collar 130. Thus, piston
chamber 86 is not subdivided into two portions. An annular cylinder insert 144
is located within
wider sidewall 140, reducing the volume of piston chamber 86. Piston cylinder
90B is moved to the
extended position by hydraulically pressurizing piston chamber 86 (which is
undivided). The
hydraulic fluid acts upon second end 104, urging piston cylinder 90B towards
end surface 84.
Injection piston 85B is returned to the retracted position by the plasticizing
of the resin against
screw 46 (Fig. 1), urging piston cylinder 90B towards endwall 124. Again, for
the purposes of
clarity, the ports, valves and lines for the hydraulics system have been
omitted from the illustration.
Thus, the present invention provides for a modular configuration in which
piston cylinders can be
exchanged while using the same piston housings.
Referring now to Fig. 9 and additionally to Fig. 6, drive assembly 28 will be
described in greater
detail. The inventors have determined some deficiencies in prior art drive
units, such as the drive
assembly described in the U.S. 2005/0048162 application. The aforementioned
drive assembly is
relatively expensive because it requires a unique hollow motor design that
incorporates the motor
into the injection unit itself. In contrast, in the drive assembly described
herein obviates some of
these deficiencies.
Drive assembly 28 includes an outboard-mounted, motor housing 146, a stator
148. a rotor 150 and
an end cap 152. Motor housing 146 is mounted to an end wall 154 of piston
housing 44 via
fasteners 156. A stator 148 is concentrically located within motor housing
146. A control box 162 is
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attached to an exterior surface of motor housing 146.
The hollow rotor 150 is coaxially mounted within a void defined by stator 148
and rotates freely
therein. Rotor 150 defines a axial void 166. A spline sleeve 164 is located
within the axial void 166,
and securely mounted at a first end to rotor 150 by fasteners 157, and is
supported (but not
mounted) at a second end to end cap 152. A concentric gap 153 is provided
between rotor 150 and
spline sleeve 164 between the first and second ends. Spline sleeve 164 defines
a plurality of
inward-facing splines 168. The axial void 166 is partially filled with
lubricating oil and is sealed by
seals 170. Alternatively, the plurality of inward-facing splines 168 could be
defined directly on an
inner surface of rotor 150.
As is described earlier, piston shaft 112 extends through the shaft aperture
120 in piston housing 44
into drive assembly 28. Bearings 172 are provided in piston housing 44
adjacent to shaft aperture
120 to facilitate the movement of piston shaft 112. An end portion 174 of
piston shaft 112 defines
an integral spline 176. End portion 174 can be an integral portion of piston
shaft 112, or
alternatively, could be an extension that is separately mounted to piston
shaft 112.
A spline insert 178 is located over end portion 174 by integral spline 176.
The spline insert 178
includes an inner-facing spline 180 which meshes with integral spline 176 on
the piston shaft 112,
and a plurality of outward-facing splines 182 which meshes with the plurality
of inward-facing
splines 168 on spline sleeve 164. The spline insert 178 is secured to the end
portion 174 by means
of a hex bolt and lock washer assembly 184. Apertures 179 are provided in a
spline insert 178
permit the lubricating oil to freely circulate within the axial void 166.
Through the intermeshed splines, the rotational movement of rotor 150 is
kinematically coupled to
screw 46 through piston cylinder 90. When injection unit 20 is in operation,
the rotor 150 rotates
spline sleeve 164 while molding material is being fed into the screw 46. The
rotational movement of
spline sleeve 164 causes spline insert 178 to rotate. Spline insert 178
transmits rotational motion to
the integral spline 176 on piston shaft 112, thereby causing screw 46 to
rotate.
Furthermore, as piston 85 translates between the retracted position and the
extended position (either
by hydraulic pressure or the flow injection material in screw 46), the spline
insert 178 slides within
spline sleeve 164. It will thus be appreciated that spline sleeve 164 should
be sized to be at least as
long as the distance of screw travel between the retracted and extended
positions. As such, the
present invention provides a compact and robust drive assembly 28 for
injection unit 20. The
combination of a short spline on the spline insert 178 and a long spline on
the spline sleeve 164
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provides a very compact drive assembly 28 that provides good support for all
moving parts so that
machine alignment is maintained.
Alternatively, the spline insert 178 could be made long and the spline sleeve
164 shortened. This
approach has the benefit of easier manufacture since it is much easier to
manufacture an element
with an external spline than it is to manufacture an element with an internal
spline. However, if the
spline insert 178 was made longer it would be necessary to increase the
distance between the end
cap 152 and the end wall 154 on piston housing 44 to accommodate the extra
length of the spline
insert 178. The increase in the distance would be directly related to the
amount of the increase in
the length of the spline insert 178.
The drive assembly 28 is readily configurable or replaceable. By removing
fasteners 156, the entire
drive assembly can be removed from injection unit 20 without disassembly of
the other components.
Servicing of drive assembly 28 and its components can easily be more easily
accomplished. In
addition, field upgrading of components can be readily done. For example, if
the entire drive
assembly is to be replaced, a more powerful (and typically larger) drive
assembly 28 could be
mounted to the back of piston housing 44. Alternatively, if a larger or
smaller piston is to be used,
the spline sleeve 164 and/or spline insert 178 could be exchanged, while still
using the rest of the
original drive assembly 28, such as the and use the original motor housing
146, stator 148 and rotor
150.
A position sensor 188 is mounted to end cap 152, and is operable to determine
the current position
of the piston between its fully retracted position and fully extended
position. Preferably, the position
sensor 188 uses a temposonic sensor, but other types of linear positioning
sensors could be used. As
can be seen in Fig. 9, the end cap 152 defines a concavity 192 within the
hollow of rotor 150. A
sensor housing 190 for position sensor 188 (containing the sensor head and
electronics) is bolted to
the exterior facing surface of concavity 192, and preferably, is fully
recessed within motor housing
146. Position sensor 188 further includes a sensor tube 194 which extends
through the centre of
axial void 166 into a bore 196 defined coaxially within the end portion 174 of
piston shaft 112. A
position magnet assembly 198 mounted to the distal end of end portion 174,
concentrically around
bore 196.As sensor tube 194 does not move, a greater or less portion of sensor
tube 194 will be
located within bore 196, depending on the current position of the piston shaft
112 (between its fully
retracted and fully extended position). The movement of position magnet
assembly 198 creates a
magnetic strain pulse which travels along sensor tube until it reaches the
sensor head within sensor
housing 190, thereby indicating the current position of screw 46.
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It will, of course, be understood that the above description has been given by
way of example only
and that modifications in detail may be made within the scope of the present
invention.
The description of the non-limiting embodiments provides non-limiting examples
of the present
invention; these non-limiting examples do not limit the scope of the claims of
the present invention.
The non-limiting embodiments described are within the scope of the claims of
the present invention.
The non-limiting embodiments described above may be: (i) adapted, modified
and/or enhanced, as
may be expected by persons skilled in the art, for specific conditions and/or
functions, without
departing from the scope of the claims herein, and/or (ii) further extended to
a variety of other
applications without departing from the scope of the claims herein. It is to
be understood that the
non-limiting embodiments illustrate the aspects of the present invention.
Reference herein to details
and description of the non-limiting embodiments is not intended to limit the
scope of the claims of
the present invention. Other non-limiting embodiments, which may not have been
described above,
may be within the scope of the appended claims. It is understood that: (i) the
scope of the present
invention is limited by the claims, (ii) the claims themselves recite those
features regarded as
essential to the present invention, and (iii) preferable embodiments of the
present invention are the
subject of dependent claims. Therefore, what is to be protected by way of
letters patent are limited
only by the scope of the following claims:
