Note: Descriptions are shown in the official language in which they were submitted.
~723~:
PQE~ 020 P2 -1-
TURBULENT FLOW GATE FOR THE
INJECTIOM MOLDING OF TUBULAR PARTS,
I~CLUDING PARISONS SUITABLE FOR F~LOW MOLDING
_
Back~round of the_Invention
It has long been recognized that obstructions
upstream of a gate, in the molding of tubular parts, such
as parisons, about a core or pin, can result in flow lines
in the part. In the case of a parison, the flow lines are
carried to the blown part. Such flow lines are caused by
the parting of the plastic material, such as a polymer, as
it flows around an obstruction, such as a core-support
pin, positioned upstream of the orifice or gate. At the
least~ flow lines result in poor appearance of the blown
piece. However, such lines can severely impair the
strength of a finished part. Strength impairment is par-
ticularly critical in the forming of parisons from thermo-
plastic polymers which have high melt viscosities and
which, when parted, do not readily flow or mix together,
or rejoin, in a laminar state. While the finished part
may present only a slight marring when physically
examined, with many such polymer materials it has been
found that the finished part is also severely weakened
along these lines and can lead to premature failure. Such
premature failure is particularly critical in blown auto
motive bellows and boots which are subject to mechanical
as well as environmental stresses.
The problem is particularly acute in the manufac-
ture of boots and bellows and the like from parisons which
are formed about cores, where the latter are supported by
conventional radially extendiny core support pins at the
inlet end of the mold. There accordingly exists a need
PQB 02~ P~ -2-
for apparatus and method for the remixing of such polymer
material on the downstream side of such core support pins
to eliminate such weld lines and lines of iMperfection or
weakness in the completed part.
Sum~y of the Invention
This invention relates to the molding o~ tubular
parts and more particularly to the molding of parisons ~or
use in the blow molding of finished parts, such as boots
and bellows for automo~ive applications and the like.
More particularly, the invention relates ~o the elimina-
tion of weld line problems caused by upstream obstructions
in the polymer passageway, such as core support pins.
It is believed that the sensitivity of certain
polymers to reform or reweld after having been separated
by flow around a core pin or the like is due at least in
parS to the chilling or surface cooling of the polymer,
such that it does not readily reweld downstream of the
pin. As an example, while the polymer may be injected at
450 F, the metal making up the mold and mold parts may be
substantially cooler, such as 340, thus providing a chil-
ling effect to the surface of the material where it has
been parted.
It has been found that the material may be
remixed to eliminate the weld line or line of disconti-
nuity by causing the material to undergo shear, such as by
causing the material to be folded back upon itself. The
added turbulence and resulting heat dissipates the rela-
tive chill at the contact line, and permits the remixing
of the polymer across the line boundaries, thereby reweld-
ing the material as a homogeneous massD
More particularly, the invention is adapted to
.. . . . .
,.~ . , ' ,
,
- : ' ,
' '
~23~
PQB 020 P2 -3-
the manufacture of parisons, to eliminate weld lines which
are caused by the interruption or splitting of the flow of
the plastic melt, between the injector and the gate open-
ing into the parison cavity, by reason of one or more
interposed core support pins which extend through the
polymer passageway leading to the gate, upstream of the
gate. A generally radiall~ formed remixing chamber com-
municates at its inner circumference with the gate and
with a generally radially outwardly directed flow passage
downstream of the core support pins. The plastic material
flows outwardly into the chamber into an expanding volu~e,
where the flow is reversed accompanied by radially inward
flow through into a contracting volume leading to the
gate. The resulting shear has a tendency to increase the
temperature of the polymer, and the resulting mixing by
reason of the flow of the plastic first radially outwardly
into the mixing chamber and then radially inwardly to the
gate, effectively remixes the material across the weld
lines so that the material entering the parison cavity
through the gate is circumferentially homogeneous.
It is accordingly an important object of this
invention to provide for the remixing of plastic material
leadiny to an extrusion gate, in the manufacture of a
parison.
Another object of the invention is the provision
of injection apparatus and method in which a generally
radially directed remixing chamber is positioned inwardly
of an annular gate to a parison cavity.
Another object of the invention is the provision
of molding apparatus in which weld seams otherwise caused
by the division of flow of a polymer material to a cavity
~27;~36~
PQB 020 P2 -4
are eliminated by remixing.
A still further object of the invention is the
remixing of plastic material which has been flow divided,
in a generally radially arranged cavity, for delivery of
such remixed material to a parison cavity through a gate
free of weld lines or lines or weaknesses.
These and other objects and advantages of the
invention will be apparent from the following description,
the accompanying drawings, and the appended claims.
Brief Description of the Drawings
Fig. 1 is a fragmentary sectional view through a
die head for the injection molding of parisons, incorpora-
ting a mixing chamber in accordance with this invention;
and
Fig. 2 is a transverse section looking generally
along the line 2--2 of Fig. 1.
Description of the Preferred Embodiment
Referring to Fig. 1, a center fed or mold die
head is illustrated generally at 10, as supporting a core
2~ pin 12, which core pin extends axially from the head 10
into the interior of a mold 15, for the extrusion or
injection forming of a parison 20. The die head 10 is
formed with a central melt passageway 22, which passageway
is divided by the rear conical portion 24 of the core pin
12 into a circumferentially annular diverging passageway
section 25 and to a cylindrical annular passageway section
26.
The core pin 12 itself is conventionally sup-
ported within the mold 15 on generally radially extending
core support pins 30. The core support pins 30, which maybe four in number, are shown in Fig. 2 as supporting the
.
. ~ :
PQB 020 P2 -5-
core pin generally centrally of the passageway 26 and
centrally of the mold cavity 3? defined between the body
of the core pin 12 and the mold 15. It will be understood
that the central or inlet passage 22 is fed by a polymer
resin or me.lt through a suitable adapter from a plastic
inject~ion machine, to form the parison 20. Center-fed
molds of the kind shown are in common use s:ince they have
the advantage of providing a rather uniform distribution
of the plastic melt around the core. However, the flow of
polymer material is caused to be divided as it flows
around the core support pins 30 and to the downstream side
of the pins as defined by the passageway section 26.
The invention provides apparatus which causes the
remixing of the polymer material downstream of the core
pins 30 to eliminate the weld line which would otherwise
be formed in the polymer material, as previously noted.
Por this purpose a generally radially-extending mixing
chamber 40 is formed in the head 10 between the passageway
26 and an axially tapered gate 42 leading in~o.the mold
15. The chamber 40 is connected to the passageway portion
36 through a radially out-turned section 45, which directs
the polymer material radially outwardly into the cavity
40. As shown in Fig. 1, the mixing chamber defined by the
cavity 40 i5 annular and is generally rectangular in cross
section, and communicates with the polymer at an inlet
region 47 at the outer end of the radial section 4S and
communicates with the gate 42 at an outlet portion 48
axially spaced from the inlet port or region 47.
It will be noted that the mixing chamber 40 has a
3~ volume which is at least twice the volume of the flow
channel defined by the passageway section 26 downstream of
'' ' . ' ' :.. ' ` :
~2~3~
P~B 020 P2 -6-
the impediments or pins 30. Further, it defines an ofset
which at least equals the radial dimensions of the channel
portions 25 and 26, and in effect provides a region of
expanding volume as the plastic material enters through
the entrance region 47 and flows radially outwardly to the
outer wall 50. The direction of movement o:E the material
is reversed within the chamber 4~ as the material proceeds
to the outlet region 48 and to the gate ~2 for in~ection
into the mold cavity 3Z to form the parison 20. The
~ expanding volume of the chamber 40 in the radially outward
direction, and the corresponding contracting volume occu-
pied by the polymer material as i~s direction of flow
reverses and moves radially inwardly effectively remixes
the ma~erial across the weld lines or lines of parting,
caused by the core pins 30. Further, the move~ent within
the chamber 40 induces frictional heat into the material
by reason of the shear on the material which overcomes the
resistance of the material to reweld and reform across the
shear lines otherwise caused by the chilling at the pins
30. When the material exits the chamber 40 through the
region 48 and gate ~2, it has been thoroughly remixed and
no weld lines are visible thereafter.
While the method herein described, and the form
of apparatus for carrying this method into effect, con-
stitute preferred embodiments of this invention, it is to
be understood that the invention is not limited to this
precise method and form of apparatus, and that changes may
be made in either without departing from the scope of the
invention, which is defined in the appended claims.
What is claimed is:
, .
.
: .
