Note: Descriptions are shown in the official language in which they were submitted.
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CANADA
TO ALL, WHOM IT MAY CONCERN:
BE IT KNOWN THAT I, JOHN BRTKA, of Toronto, Canada,
have invented: APPARATUS FOR REDUCING WEAR ON
INJECTION MOULDING GIBS
of which the following is a specification.
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FIELD OF INVENTION
The present invention relates generally to the art of injection moulding and
more
specifically to an apparatus and method for reducing wear in inj ection
moulding equipment,
especially gibs which support parts that reciprocate transverse to a mould
axis.
BACKGROUND OF THE INVENTION
Injection moulding machines are used for the manufacture of a variety of
plastic products.
A typical injection moulding machine comprises two portions or halves which
reciprocate along
a mould axis between a mould-closed position and a mould-open position. One of
the mould
halves may comprise cavities for defining the exterior of a hollow plastic
article while the other
half may comprise elongate cores mounted to a plate for defining the interior
of the article when
the mould halves are in the mould-closed position. In addition, one or more of
the mould halves
may be provided with additional members moving along an axis transverse to the
mould axis.
for instance, the core plate may carry mating re-entrant dies for defining
exterior portions of the
article. The dies slide transversely on the core plate in a direction
transverse to mould axis and
allow the plastic articles to be easily ejected from the machine.
One of the primary concerns in the art of injection moulding is the ability to
produce large
numbers of uniform articles satisfying strict tolerance specifications. For
this reason, the injection
moulding equipment must be precisely machined, and the moving parts thereof
must be precisely
aligned. However, the considerable frictional and impact forces encountered by
the machine
during operation will eventually cause various parts to wear out and misalign.
Consequently, the
machine may produce imperfect articles, for instance, articles exhibiting
visible seam lines.
The prior art has been primarily concerned with reducing the wear and tear
caused by the
heavy mould portion which typically travels along guide bars extending from
the fixed plate (or
body of the moulding machine) in a direction parallel to the mould axis. See
for example, U.S.
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Patent No. 3,782.,874 to Hehl, U.S. Patent No. 5,328,346 to Kodric, or U.S.
Patent No. 4,453,912
to Hehl.
However, the moulding equipment members which move along an axis transverse to
the
mould axis can also cause significant wear and tear on the guides or gibs
which support and
S guide these transversely moving parts. In particular, if the transversely
moving members are
mating re-entrant; feature dies for defining an exterior portion of the hollow
plastic article, such
as neck threads, the wear and tear on the gibs is likely to produce a
misalignment that will cause
seam lines in the resulting article. Depending on the weight of transversely
moving members,
tile guides or gibs, which are typically formed from a relatively soft
material such as bronze, have
a. relatively short operating life, e.g. three months. Re-machining or
replacing the gibs is a costly
operation, particularly in view of production interruptions.
An attempt to ease the problem of access to guide means which guide
transversely moving
members is disclosed in U. S. Patent No. 4,174,199 to Benninghaus. The
external guide means
in Benninghaus are composed of standard, inexpensively manufactured profiles
that can be taken
apart so that the individual surfaces thereof can be readily re-machined.
While providing for less
expensive guide means and easier access to remachinable surfaces, Benninghaus
does not address
the problem of extending the relatively short operating life of the guide
means and the costs
associated with frequent production interruptions to remachine the guide
means.
Therefore, there remains in the industry a need for solutions that would
extend the
operating life of injection moulding equipment such as gibs and reduce costly
interruptions to
production, allowing at the same time for a quick and inexpensive replacement
of parts when
such a replacement becomes necessary.
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SUMMARY OF THE INVENTION
According to one broad aspect of the invention, a moulding apparatus is
provided
comprising a pair of opposed mould members which reciprocate relative to each
other along a
mould axis. At least one moulding element is mounted to one of the
reciprocating mould
S members and is moveable relative thereto along an axis transverse to the
mould axis, and means
are provided far retaining the transversely moving mould element axially
relative to the
reciprocating mould member. A load bearing member is mounted to one of (a) the
reciprocating
mould member and (b) the transversely moving element, and is engageable with
the other thereof,
for carrying forces transverse to both the mould axis and to the transverse
axis between the
reciprocating mauld member and the transversely moving element.
In the preferred embodiment, the aforementioned load bearing member is a
replaceable
wear strip, and the other of the aforementioned reciprocating mould member and
transversely
moving element has at least one feature for riding against said load bearing
member. The feature
may comprise one or more rollers.
In the preferred embodiment, the retaining means includes at least one gib
which supports
the wight of the transversely moving member and the load bearing member is
located to
discourage wear to the gib. The invention thus significantly increases the
life of the gibs and
reduces the costs involved in replacing and/or remachining the gibs of prior
art injection
moulding machines. Moreover, in the preferred embodiment, the load bearing
member and the
feature which it engages can be quickly and expensively replaced without
extensive disassembly
of the moulding equipment.
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BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other aspects of the invention are explained in greater
detail below
having reference to the accompanying drawings, provided for the purposes of
description and not
limitation, wherein:
Fig. 1 is a perspective view of an injection moulding machine having two mould
members
which reciprocate along a mould axis;
Fig. 2 is a partially exploded, axonometric view of a portion of one of the
mould members,
including a stripper plate assembly, in accordance with a preferred embodiment
of the invention;
Fig. 3 is an exploded, axonometric view of the stripper plate assembly;
Fig. 4 is a perspective view of a portion of the stripper plate assembly in an
assembled
state incorporating neck ring members, the view being taken from the front
along an outline
labelled "A" in Fig. 2;
Fig. S is a detail, exploded view of a portion of the stripper plate assembly
in the vicinity
and direction of arrow "B" in Fig. 3, in accordance with the preferred
embodiment;
Fig. 6 is a detail, exploded view of a portion of the stripper plate assembly
in the vicinity
and direction of arrow "B" in Fig. 3 in accordance with an alternative
embodiment of the
invention; and
Fig. 7 is an illustration of a hollow plastic article.
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DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The invention relates to an apparatus for reducing wear on injection moulding
equipment.
Although the invention can be embodied in substantially all types of moulding
machines having
members which move transversely with respect to a mould axis, the following
detailed
description is provided with reference to an injection moulding machine 10
shown in Fig. 1 for
manufacturing plastic bottle preforms formed from polyethylene terephthalate
(PET). Fig. 7
shows an example of a preform, which comprises a body and a neck having a
thread. (The body
is typically blow moulded to full size in a separate operation to produce an
end-user container).
The injection moulding machine 10 comprises interlockable mould members 11A
and
11B which reciprocate along a mould axis 20 between a mould-closed position
and mould-open
position for each injection moulding cycle of the machine. Mould members 11A
and 11B are
mounted on guide bars 15.
Figs. 2-4 show a stripper plate assembly employed in the mould member 11A in
greater
detail. Referring to these figures, the stripper plate assembly comprises a
plurality of projecting
core members 14 affixed to a substantially vertically orientated core plate
12. The core members
14 resemble a series of proj ecting fingers which extend through holes 16 in a
stripper plate 18,
discussed in greater detail below. Each projecting core member 14 defines the
inner surface of
one preform, i.e. each core member 14 functions to mould the interior surface
of the hollow
plastic article shown in Fig. 7.
The stripper plate 18 is also substantially vertically orientated and mounted
on guide
means for movement relative to the core plate 12 along mould axis 20. The
guide means can
comprise any known apparatus, such as horizontal guide bars extending from the
core plate 12
or, if the machine 10 has a base or a main body onto which the core plate 12
is fixedly mounted,
guide bars extending from such a base or main body. The guide means can also
comprise roller
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bearings rolling on horizontal tracks mounted on a platform extending from the
main body. In
the preferred embodiment, the guide means comprise horizontal guide bars 15
(see Fig. 1 )
extending through cylindrical openings (not shown) in the stripper plate 18
which openings are
provided with bushings.
A plurality of gibs 24 are mounted to the stripper plate 18. The preferred
embodiment
comprises upper, middle and lower gibs, each identified by reference numeral
suffix U, M or L.
1=;ach pair of Bibs 24, i.e. gibs 24U/24M and gibs 24M/24L, support and guide
a plurality of
slides 22 comprising left and right slide members 22A and 22B which move
relative to each other
along an axis 26 transverse to the mould axis 20. There is a considerable
amount of friction
produced when slides 22 slide along gibs 24U, 24M and 24L. In order to reduce
the wear and
prolong the life of the slides 22, the gibs 24 are formed from a softer
material than the material
used to construct the more complex (and hence more expensive) slides 22. Such
a softer material
may, for example, comprise bronze, when the slides 22 are formed from steel.
The stripper plate
18 also comprises wear plates 30, also made from a softer material than the
stripper plate and
slides, in order to absorb the friction caused by movement of the slides 22
along the surface of
the stripper plate 18.
Each slide 22 also carries a plurality of mating re-entrant feature dies, such
as split neck
rings 28 (shown only in Fig.4) comprising left and right members 28A and 28B
respectively
mounted to left and right slide members 22A and 22B. In the mould-closed
position, i.e., when
slide members 22A and 22B abut one another, the neck rings 28 function to
mould the necks of
the preforms, including the threads thereof. The slides 22 also include wear
members 30 (Figs.
2 & 3) for absorbing friction caused by the neck rings 28.
Mould portion 11B (Fig. 1 ), is mounted on the same guide means as the core
and stripper
plates and comprises an arrangement of generally cylindrical cavities (not
shown) which matches
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the arrangement of the core members 14. The cavities are designed to receive
the proj ecting core
members 14 and function to mould the exterior surfaces of the preform bodies.
Cam members 32 are fixed to the core plate 12 and extend therefrom through
openings
34 in the stripper plate 18 and openings 36 defined between left and right
slide members 22A and
22B. Each cam member 32 includes a wedge-shaped head, and each opening 36 has
corresponding tapered sides {seen best in Fig. 3) disposed on left and right
slide members 22A
and 22B. The openings 36 function as cam followers as described below.
In operation, when mould portions 11A and 11B are in the mould-open position,
it is
necessary to open the slides 22 and the neck rings 28 in order to enable a
transfer mechanism
such as carrier plate 38 (Fig. 1 ) to withdraw recently moulded preforms from
mould portion 11A.
T'he slides 22, which are biased in a closed position by a biasing means (not
shown) such as
spring loaded, hydraulic or pneumatic mechanisms well known to those skilled
in this art are
forced open by a motive means (not shown) which urges stripper plate 18 to
move away from the
core plate 12 along mould axis 20. This causes a caroming action between the
wedge-shaped
heads of the cam members 32 and the corresponding tapered sides of slide
openings 36, to
thereby force the slides 22, and hence neck rings 28, open.
The injection moulding machine 10 generally operates in a vertical position
and in the
prior art the weight of the slides 22 and neck rings 28 rest on the centre and
lower gibs 24M and
24L. The wear surfaces 40 of these gibs have relatively small surface areas
and are subject to
extreme wear. This is particularly so of wear surface 41 of centre gib 24M,
seen best in the
exploded view of Fig. 3, which is very narrow due to the cam follower
structure 42 disposed
between gib 24M and stripper plate 18. When gibs 24M and/or 24L wear out, a
misalignment
problem arises between the neck rings 28 and the cavities of the mould member
11B on one side
of the mould as well as between the neck rings 28 and the core members 14 on
the other side of
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the mould. A misalignment of this type will cause visible seam lines to appear
on the preform.
In a typical mould, the gibs, particularly centre gib 24M, have to be replaced
or re-machined
every couple of months at considerable expense. The costs are further
exacerbated due to the fact
that the machine 10 cannot be used to produce any hollow plastic articles
during this time period.
The preferred embodiment of the invention considerably reduces the wear on and
prolongs
the life of the gibs 24 by providing inter-engageable features such as wear
strips 44 and slide
inserts 46. The wear strips 44 are replaceably secured (preferably bolted) to
the stripper plate 18,
while the slide inserts 46 are replaceably secured (also preferably bolted) to
slides 22. The wear
strips 44 and slide inserts 46 support the weight of the slides 22 and neck
rings 28, which can be
substantial. The maintenance requirements of the gibs are thereby
substantially reduced as
compared to the prior art moulding equipment where the load of the slides 22
and neck rings 28
rest exclusively on gibs 24M and 24L.
The invention may be embodied in the manufacture of new injection moulding
equipment
and in retrofitting existing injection moulding machines. In the preferred
embodiment, the wear
1 S plates 30 are segmented and the wear strips 44, which preferably have a
thickness equal to the
thickness of the wear plates 30, are mounted onto the stripper plate 18 in the
rebates between
such segments. The slide inserts 46 preferably resemble a flattened U-shaped
bracket as
illustrated and are mounted in channels 48, seen best in the detailed view of
Fig. 5, which are
milled into the rear faces of the slides 22. The portion of each slide insert
46 which extends
beyond the surface of the slide preferably has a thickness which almost equals
the thickness of
the wear plates 30 and wear strips 44.
In the preferred embodiment, the wear strips 44 and slide inserts 46 are
formed from
materials suitable for wear resistance and strength as known to those skilled
in this art. The wear
strips 44 are preferably made of a dissimilar material to that of the inserts
46 to prevent galling
and are preferably a few Rockwell points lower on the hardness scale. It is
preferred to make the
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wear strips 44 softer so that they will wear out before the inserts 46 since
the wear strips 44 are
easier to replace, and faster and less expensive to manufacture.
In an alternative embodiment, the slide inserts have a rectangular shape and a
thickness
similar to the wear strips 44. An example of such alternative slide inserts
are illustrated in
phantom in Fig. 3 and referenced by numeral 46'. In this embodiment, it is not
necessary to mill
channels 48 into the rear faces of the slides 22, since the alternative slide
inserts 46' can be
mounted directly to the rear face of slides 22. Accordingly, this embodiment
is particularly
suitable for retrofitting applications since no re-machining of the stripper
plate 18 or slides 22 is
necessary.
In either of these embodiments, the wear strips 44 and slide inserts 46 (or
46') can be
manufactured to be symmetrical with respect to a bisecting plane between the
upper and lower
surfaces thereof. In this manner, when the wear strips 44 and slide inserts 46
(or 46') wear out,
they can be rotated so that the upper face becomes the lower face and vice
versa. The members
44 and 46 may thus be discarded only after both faces have worn out.
In a second alternative embodiment, shown in the detail view of Fig. 6, a
series of rollers
50 are used instead of slide inserts 46. The rollers 50 are replaceably
mounted directly onto the
rear face of slide 22 and roll along corresponding wear strips 44. The rollers
include a thread
which makes their replacement very simple. The rollers are commercially
available from, for
example, Mcgill Manufacturing Company Incorporated, Valpaaraiso, Indiana, as a
pre-machined
part with a material hardness of about 60 Rockwell. The wear strips 44 in this
embodiment are
also preferably made from a dissimilar material to that of the rollers 50, and
preferably have a
material hardness of about 50 to 56 Rockwell.
It will be seen from the foregoing that the invention significantly increases
the life of the
gibs and reduces the costs involved in replacing and/or remachining the gibs
of prior art injection
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moulding machines. Moreover, either the wear strips 44, slide inserts 46 or
rollers 50 can be
quickly and expensively replaced without extensive disassembly of the moulding
equipment. In
addition, the wear strips 44 and slide inserts 46 can be quickly and
inexpensively manufactured
due to their simple profiles. Alternatively, replacement rollers can be
provisioned at a low cost.
The preferred embodiments of the invention have been described in detail for
the purposes
of description but not of limitation. A number of variants are possible. For
example, while the
wear strip is described as being substantially as thick as the wear plate, it
is possible to provide
a thicker wear strip which would be partially received in the recess provided
in the slide. In
addition, the stripper plate may be provided with a channel or recess in order
to accommodate
thicker slide inserts or rollers. Also, the number of slide inserts mounted on
each slide and the
corresponding number of wear strips mounted on the stripper plate is limited
only by design
considerations in the case of each specific injection moulding machine. Those
skilled in the art
will be aware of other possible modifications and embodiments to the preferred
embodiment
without departing from the spirit and scope of the invention.
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