Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DTSTRTBUTING MATERTAL IN BLOW MOLD HEAD
FIELD OF THE INVENTION
The invention relates to blow molding of thermoplastic
materials, and more particularly, to features of a blow mold head that encourage5 discharge of a uniform tubular thermoplastic material for subsequent expansion into a desired shape.
BACKGROUND OF THE INVENTION
Existing blow mold heads typically have a mandrel or multiple
mandrels that define one or more flow channels ~or producing a tube or
10 concentric tubes of thermoplastic material. A former shapes and discharges the
single tube or alternatively forms ehe multiple concent~ic tubes into a single tube
with a particular cross-section. Even distribution of molten therrnoplastic
material around each flow channel is important to ensuring product uniformity
and avoiding warpage and fracturing on product setting. To that end, each flow
15 channel will normally be associated with a supply chamber. Molten
thermoplastic material is injected under pressure axially into a particular region
of the chamber (in the general direction of the flow channel) and distributed
around the upstream end of the associated flow channel. Spiral grooves are
sometimes formed in the exterior of a mandrel to receive material from the
20 supply chamber and enhance distribution throughout the associated flow
channel. However, axial injection tends not to produce an optimal distribution
of material around the chamber and ultimately in the flow channel.
The present invention introduces inter ~_ methods and ~ -
apparatus for injecting thermoplastic materials transversely into a blow head and
25 dividing such flows so as to produce a more even distribution of material
throughout flow channels. It also addresses the problem of providing such
transverse injection for multiple concentric mandrels.
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SUMMARY OF TI~E INVENTTON
In one aspect, the invention pro~ides a blow head for use in
blow molding thermoplastic material. The blow head has an axis and a hollow
casing with a circurnferential sidewall surrounding the axis. A mandrel is
S mounted within the casing in general alignment with the axis. An exterior
surface portion of the mandrel cooperates with the casing sidewall to define a
Mow channel with upstream and downstream ends. A multiplicity of spiral
grooves are formed in the exterior surface portion. Each groove has a
downstream end terminating within the flow channel and an upstream end. A
10 circumferential recess is formed in the mandrel upstream of the flow channel
and of the spiral grooves. The recess cooperates with the casing sidewall to
de~me a chamber into which open the upstream ends of the flow channel and the
grooves. A material inlet extends through the casing sidewall and discharges a
flow of thermoplastic material into the chamber in a direction transverse to the15 axis. A flow divider attached to the mandrel and positioned within the recessconfronts the flow of thermoplastic material discharged from the inlet. The
flow divider is shaped to divide the flow of thermoplastic material into
substantially equal flows in circumferentially opposing directions around the ;~chamber. A former downstream of the flow channel forms thermoplastic
20 material discharged from the downstream end of the flow channel into a tube
with a preselected cross-section. This can be expanded according to
conventional blow molding techniques to produce a required product.
In another aspect, the invention provides a comparable blow
head, but comprising a plurality of hollow mandrels including at least an
25 innermost mandrel and an outermost mandrel. The mandrels are dimensioned
and shaped to seat within one another in a predetermined concentric
~; arrangement. Each mandrel has a circumferential sidewall surrounding the axis
of the blow head and an external surface portion defined by its circumferential
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sidewall. Each mandrel is associated with a flow channel external to the
mandrel. The exterior surface portion of the outermost mandrel cooperates with
the circumferential sidewall of the casing associated with the blow head to
define its associated external flow channel. The exterior surface portion of each
5 inner mandrel cooperates with the circumferential sidewall of an immediately
outer mandrel to define its associated external flow channel. The exterior
surface portion of each mandrel comprises a multiplicity of spiral grooves, eachgroove comprising an upstream end and comprising a downstream end
terminating within the external flow channel associated with the mandrel. Each
10 mandrel has a circumferential recess upstream of its associated external flowchamlel and of its spiral grooves. The recess associated with the outermost
mandrel cooperates with the sidewall of the casing to define a chamber, and the
recess of each inner mandrel cooperates with the sidewall of an immediately
outer mandrel to define a similar chamber. The ups~eam ends of the external
15 flow channel and spiral grooves associated with each mandrel opening into the chamber associated with the mandrel.
A plurality of material inlets are each associated with a different
mandrel and shaped to discharge a flow of thermoplastic material into the
chamber associated with the mandrel in a direction transverse to the axis. The
20 material inlet associated with the outermost mandrel includes a passage section
extending through the sidewall of the casing. The material inlet associated witheach inner mandrel comprises alignable passage sections formed in and
extending through the casing sidewall and each mandrel surrounding the -
mandrel. The passage sections of the material inlets are formed in
25 predetermined locations and orientations such that the passage sections alignsimultaneously to form the passagPs of the material inlets when the plurality ofmandrels are arranged in the predetermined concentric arrangement.
Accordingly, despite the presence of multiple concentric mandrels, each
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mandrel can receive thermoplastic material by injection transverse to the axis of
the blow head.
Each mandrel comprises a flow divider. The flow divider is
positioned within the recess of the mandrel to confront the flow of thermoplastic
material discharged from the material inlet associated with the mandrel. Each
flow divider is shaped to divide the incoming flow of thermoplastic material into
substantially equal flows in circurnferentially opposing d~rections around the
chamber associated with the mandrel. A former downstream of the flow
channels forms thermoplastic materials discharged from the downstream ends
of the flow channels into a single tube with a preselected cross-section.
Other aspects of the invention including methods and features
associated with a blow head will be apparent from a description below of a
preferred embodiment and will be more specifically defined in the appended
claims.
DESCRTPF~ON QEI~ DRA~NGS
The invention will be better understood with reference to
drawings in which:
fig. 1 is a cross-sectional view of a blow head in a vertical plane
containing the general axis of the blow head;
fig. 2 is an enlarged fragmented elevational view illustrating a
chamber and flow divider associated with an intermediate mandrel of the blow
head;
fig. 3 is a fragmented elevational view in partial cross-section
showing further detail of the intermediate mandrel, including the flow divider
and portions of the supply chamber associated with the outer mandrel.
DESCRIPI~Q~ OF PRE~ RRED EMBODIMNTS
Reference is made to fig. 1 which illustrates a blow head 10 for
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use in blow molding thennoplastic material. The blow head 10 has a central
axis 12 which is generally aligned with the direction of material flow. An arrowD indicates a downstream direction of material flow. An arrow U indicates an
upstream direction of material flow. This particular embodiment of a blow head
5 10 is intended to produce a three-layered tube 14 (layering not illustrated) that
might consist of inner and outer layers of virgin thermoplastic material and an
intermediate layer of recycled plastic. A typical application for the blow head 10
might be production of plastic bottles.
The general construction of the blow head 10 will be described
10 with reference to fig. 1. A hollow caslng 16 comprises an upstream end
structure 18. It also comprises a circumferential sidewall 20 surrounding the
axis 12 and defined by a downstream end plate 22 and a generally cylindrical
section 24. An assembly of hollow mandrels, including an innermost mandrel
26, an interrnediate mandrel 28, and an outermost mandrel 30, is located within
15 the casing sidewall 20 in general alignment with the axis 12. The mandrels 26,
28, 30 are formed with complementary interlocking internal shoulders (not
numbered) that permit the mandrels 26, 28, 30 to be bolted to the upstream end
structure 18 to form a unitary assembly in a predeterrnined concentric
relationship, all of which will be readily apparent from fig. 1. A former 32 is
20 oriented in general alignment with the axis 12 downstream of the mandrel
assembly. The former 32 shapes thermoplastic material discharged downstream
of the mandrels 26, 28, 30 into the tube 14 ultimately discharged from the blow
head 10. The former 32 comprises an outer former portion 34 which is bolted
to the downstream end plate 22 of the casing 16 over an aperture formed in the
25 end plate 22, and a hollow inner former portion 36 ~readed onto a hollow
support rod 38 extending through the interior of the innermost mandrel 26. The
rod 38 can be displaced axially by a hydraulic cylinder 40 to adjust the
cross-sectional thickness of the tube 14. A central passage 42 through the rod
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38 communicates with an air supply chamber 44 in the upstream end structure
18. This permits air under pressure to be applied to the interior of the tube 14(through the rod 38 and inner former portion 36) downstream of the former 32.
The air under pressure is used in a conventional manner to expand the tube 14
5 against a mold (not illustrated) to produce a product (not illustrated) of a
particular shape.
The casing 16 and mandrel assembly cooperate lo define three
flow channels. In general terms, each mandrel has a circumferential sidewall
surrounding the axis 12. An external surface portion of that sidewall cooperates10 with an outer structure to define a flow channel associated with the mandrel and
external to the mandrel itself.
More specifically, the exterior surface portion 46 of the
outermost mandrel 30 cooperates with the circumferential sidewall 20 of the
casing 16 to define an outermost flow channel 48 associated with the outermost
mandrel 30. The exterior surface portion 50 of the intermediate mandrel 28
cooperates with a circumferential sidewall 52 of the outermost mandrel 30 to
define an intermediate flow channel 54. The exterior surface portion 56 of the
innermost mandrel 26 cooperates with the circumferen~ial sidewall 58 of the
intermediate mandrel 28 to define an innermost flow channel 60. Each of the
20 flow channels will ultimately produce a substantially tubular flow of
thermoplastic material.
The exterior surface portion of each mandrel is formed with a
multiplicity of spiral grooves. The grooves associated with the intermediate
mandrel 28 are typical and are identified collectively by the reference numeral
25 62. These are best illustrated in the views of figs. 2 and 3. The various
grooves 62 are oriented in parallel relationship ar.d are substantially identical.
Each groove has an upstream end (such as the upstream end 66) that terminates
smoofriyattheexteriorrufaceporiin5Uoftheintermediatemandrel28
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within the intermediate flow channel 54. Each groove has an enlarged
downstream end (such as the downstream end 68) that is open-ended. The
depth of each groove decreases substantially continuously from its downstream
end to its upstream end. This arrangement forces thermoplastic material
supplied under pressure to the upstream ends of the grooves 62 to blend
smoothly into flows otherwise occurring in the intermediate flow channel 54.
Alternatively, the grooves 62 may have a uniform depth and may incline
progressively further away from the axis 12 from their downstream ends to
their upstream ends. The configuration of spiral grooves for such purposes is
known in the art and will not be described further.
Each of the mandrels 26, 28, 30 has a circumferential recess
upstream of the associated external flow channel and its spiral grooves. The
function and shape of the recess 70 of the intermediate mandrel 28 is typical. It -
is specifically identified only in the views of figs. 2 and 3. The recess 70
cooperates with the sidew~ll 52 of the outerrnost mandrel 30 to define a
chamber 72 for receiving thermoplastic material under pressure. That chamber
72 distributes the received materials to the intermediate flow channel 54 and toeach of the grooves 62 associated with the intermediate mandrel 28. More
specifically, the interrnediate mandrel 28 comprises first and second axially
opposing surface portions 74, 76 that bound its recess 70. The first surface
portion 74, which is discontinuous, has been indicated in edge view with a
dashed horizontal in fig. 3. It should be noted that the upstream end of the
intermediate flow channel 54 and the enlarged upstream ends of the grooves 62
of the intermediate mandrel 28 intersect that first surface portion 74 and
consequently open into the chamber 72 to receive material. Similar chambers
78, 80 are defimed by recesses associated with the other two mandrels 26, 30
and bear similar relationships to the external flow path and grooves associated
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with thcir respective mandrels. The recess associated with the innermost
mandrel 26 cooperates with the sidewall 58 of the intermediate mandrel 28 to
define the chamber 78 while the recess associated with the outermost mandrel
30 cooperates with the sidewall 20 of the casing 16 to de~me the chamber 80.
Three material inlets 82, 84, 86 are provided to receive
thermoplastic material under pressure. Each inlet discharges matexial into a
chamber associated with a different one of the mandrels 26, 28,30 in a
direction transverse to the blow head axis 12. The material inlet 84 associated
with the intermediate mandrel 28 includes a passage section 88 extending
through the sidewall 20 of the casing 16 and an aligned passage section 90
extending through the outexmost mandrel 30. The material inlet 86 associated
with the outermost mandrel 30 comprises a single passage section 92 extending
through the sidewall 20 of the casing 16. The material inlet 82 associated wi~h
the innennost mandrel 26 has a passage comprising three passage sections 94,
96,98 fo~ned respectively in the casing 16, the outermost mandrel 30, and the
interrnediate mandrel 28, that align to communicate with the chamber 78
associated with the innermost mandrel 26. The various passage sections are
radially oriented and circumferentially positioned in the mandrels 26,28,30
such that they align simultaneously to constitute the material inlets 82, 84,86
when the mandrel assembly is in the predetermined concentric arrangement
shown in fig. 1. The mandrels 26, 28,30 may simply be rotated relative to one
another, prior to bolting to the upstream end structure 18, to achieve the
particular concentric arran~ement necessary to align the various passage
sections. Accordingly, in this particular aspect of the invention, it is possible to
supply materials simultaneously by transverse injection into the chambers
associated with each of a multiplicity of concentric mandrels.
Each of the mandrels 26, 28,30 is associated with a flow
divider. The flow divider 100 associated with the intermediate mandrel 28 is
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typical and is most apparent in figs. 2 and 3. It is positioned within the recess
70 associated with the intelmediate mandrel 28 to confront the flow of
thermoplastic material discharged from the material inlet 84. It divides the flow
of the thermoplastic material into substantially equal flows in c~rcumferentially
S opposing directions (as indicated by arrows in fig. 3) around the chamber 72 of
the mandrel. The flow divider 100 is substantially heart-shaped with a crux
102 and a tip 104, the tip 104 being positioned axially upstream from the crux -
102. The particular shape has been determined empirically to optimize
distribution of materials to both the flow channel and the various grooves
10 associated with a mandrel. An alternative shape that also produces an effective
flow divider is semi-circular with the "flat end" positioned upstream, the
peripheral edges of such a divider, of course, being smoothed. Thermoplastic
flows are not particularly predictable and no comprehensive explanation can be
offered for the effect observed. However, any divider shaped to encourage
15 substantially equal and opposite flows about such a chamber will enhance
ultimate distribution of materials. Flow dividers 106, 108 associated with the
innermost and outermost mandrels are identified in fig. 1.
The shape of the chamber 72 associated with the intermediate
mandrel 28 should be noted. The first surface portion 74 bounding the
20 associated recess is substantially planar and perpendicular to the axis 12. The
second surface portion 76 is substantially planar, but inclined relative to the axis ,`
12 as apparent in fig. 2. This causes the chamber 72 to have a maximum axial
length proximate to the associated inlet 84 and a minimum axial length radially
opposite from the inlet 84. Tapering the axial dimensions of chamber 72 in
25 such a manner has been found particularly important to processing of nylon and
polyvinylchloride. These materials tend otherw;se to remain resident in a
portion of the chamber 72 and become subject to burning (the blow head being
heated in a conventional manner). The arrangement is believed to prevent
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pooling in portions of the chamber 72 radially opposite the material inlet 84 and
axially opposite the upstream ends of the grooves. With other matenals, such
tapering of the chamber is not necessary to eliminate burning, but very
significantly enhances distribution of the materials circumferentially about theflow channels, producing a more uni~orrn final product. The chambers 78, 80
associated with the other mandrels 26, 30 are similarly configured.
Annular notches may be formçd in the mandrels upstream of the
spiral grooves to provide a measure of pooling and smoothing of flows.
Notches 110, 112 have been provided in the intermediate and outermost
mandrels 28, 30, but have not been used in connection with the innermost
mandrel 26. Use of such notches is well known and will not be described
further.
How the blow head 10 is used to produce molded product will
be readily apparent to those skilled in the art. The blow head 10 will normally
b,e oriented with its axis 12 vertical and the forrner 32 facing downwardly. Thehydraulic cylinder 40 will be operated to adjust the position of the inner former
portion 36 and consequently the thickness of the tube 14 discharged from the
former 32. Material is delivered under pressure to the three inlets 82, 84, 86
and three tubular concentric streams are discharged downstream of the mandrels
26, 28, 30. The former 32 combines the flows to produce a single tube 14 with
a sandwich-type construction. As mentioned above, a mold will be located
about the tube 14, and air under pressure will be delivered to the interior of the
tube 14 through the central rod 38 and inner forrner portion 36 to expand the
tube 14 within the internal cavity of the mold. The finished product can then beremoved. Details o~ this process will be apparent to those skilled in the art
The general object of the invention in such blow molding
operations is to encourage even distribution of flows circumferentially in the
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~low channels and consequently to discharge a tube of vel~ uniform wall
thickness. The spiral grooves encourage even flows in a well-known manner.
The injection of raw material transverse to the axis 12 of the blow head 10,
rather than conYentional axial injection, and use of flow dividers immediately
5 confronting the injeceed flows, ensures more even delivery of materials to theupstream ends of both the spiral grooves and the flow channels. This itself
significantly enhances the uniforrnity of the tube 14 ultimately discharged fromthe former 32. The tapering of the axial dimensions of the chamber further
encourages even supply of material to the grooves and flow path. Use of flow
10 dividers in the preferred heart-shape with upstream location of their tips appears
to enhance the uniformity of the tube 14 further.
It will be appreciated that a particular embodiment oi the
invention has been described and that modifications may be made therein
without departing from the spirit of the invention or necessarily departing from the scope of the appended claims. ~ , i
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