Plasticating screw

Details Plasticating screw Plasticating screw

1998-09-17

2001-03-27

Soohoo, Tony G. (Department: 1723)

Agitating

Rubber or heavy plastic working

Stirrer is through-pass screw conveyor

C366S081000

Reexamination Certificate

active

06206559

ABSTRACT:

TECHNICAL FIELD
This invention relates to a plasticating screw suitable for use in an injection molding machine or extruder which is adapted to inject a resin material such as plastics into a mold or to extrude the same sub-sequent to plastication.
BACKGROUND ART
Injection molding machines and extruders, which perform injection molding or extrusion of a solid feed (resin material) subsequent to melting and plastication, are widely known for many years. Such injection molding machines and extruders are provided with screws for melting and kneading a resin material, and rotation of the screws promotes melting and mixing of the melted and plasticated resin material.
As an illustrative example of techniques relating to such screws, there is the technique disclosed in JP kokoku 43-24493. According to this technique, as is illustrated in
FIG. 7
, a mixing head
107
is arranged on a free end of a screw
101
, and a narrow clearance is formed between a cylinder
103
and the mixing head
107
.
Further, as is shown in
FIG. 8A
, plural grooves
107
a
,
107
b
are helically formed on an outer peripheral wall
107
d
of the mixing head
107
, and are alternately arranged so that they do not communicate with each other.
FIG. 9
is an A—A section of
FIG. 8A and
, as is depicted in
FIG. 9
, the grooves
107
a
and grooves
107
b
are alternately formed at equal intervals in an approximately central part of the mixing head
107
.
A description will now be made about these grooves
107
a
,
107
b
with reference to FIG.
8
B and FIG.
8
C. Incidentally, FIG.
8
B and
FIG. 8C
are A
1
—A
1
section and A
2
—A
2
section of
FIG. 8A
, respectively.
As is shown in
FIG. 8A
, an end of each groove
107
a
extends to a tapered portion
107
c
on a side of a forward end of the mixing head
107
(on a left side as viewed in the drawing), and as is illustrated in FIG.
8
A and
FIG. 8B
, its opposite end terminates in the outer peripheral wall
107
d
without reaching a tapered portion
107
e
on a side of a rearward end of the mixing head
107
(on a right side as viewed in the drawing).
In contrast with the grooves
107
a
, each groove
107
b
, as depicted in
FIGS. 8A and 8C
, terminates at an end thereof in the outer peripheral wall
107
d
without extending to the tapered portion
107
c
on the side of the forward end of the mixing head
107
. Further, as is shown in
FIG. 8A
, this groove is formed so that at an opposite end thereof, it extends to the tapered portion
107
e
on the side of the rearward end of the mixing head
107
.
A brief description will next be made about operation of the plasticating screw
101
having the mixing head
107
described above. When an unillustrated feed is supplied from a feed hopper
108
shown in
FIG. 7
, the cylinder
103
is heated to a predetermined temperature by a heater free of illustration. Rotation of the screw
101
by an unillustrated screw drive unit propels the resin feed from the feed hopper
108
toward a forward end of the cylinder
103
. By heat from the heater via the cylinder
103
and shear heating of the feed in the grooves of the screw
101
, the resin feed in the cylinder
103
undergoes gradual melting.
In some instances, kneading and dispersion of the resin feed would not be achieved fully if the screw
101
were solely relied upon. Owing to the provision of the above-mentioned mixing head
107
, sufficient melting and kneading can be performed. Described specifically, the feed which has entered the grooves
107
b
from the tapered portion
107
e
on the side of the rearward end of the mixing head
107
moves into the grooves
107
a
under rotation of the mixing head
107
, and is then propelled from the grooves
107
a
to the tapered portion
107
c
on the side of the forward end of the mixing head
107
. As a result of the movement of the feed from the grooves
107
b
into the grooves
107
a
as described above, the feed is subjected to further shear force and dividing action so that good kneading and dispersion can be achieved. By the way, this mixing head
107
is generally called the “Maddock type” and besides the above-mentioned one, a variety of Maddock mixing heads are known.
In the case of the Maddock mixing head
107
disclosed in JP kokoku 43-24493, etc., the clearance between the mixing head
107
and the cylinder
103
is very small. In the case of a resin feed insufficiently melted by the screw
101
or a resin, such as PVC (polyvinyl chloride), melted considerably at the mixing head
107
, the resin pressure may become off-balanced in the direction of a periphery of the mixing head
107
, so that a metal-to-metal contact may take place between the mixing head
107
and the cylinder
103
, resulting in so-called galling wear.
The mixing head
107
is arranged to improve the melting, kneading and dispersion of a resin feed. Melted conditions may however differ locally in the direction of the periphery of the mixing head
107
. Resin pressure which is acting in an angular direction on the mixing head
107
may be off-balanced accordingly.
This results in the occurrence of force so that the mixing head
107
is strongly urged in a radial direction. Therefore an inner wall of the cylinder and the mixing head
107
undergoes a metal-to-metal contact, leading to problems that galling wear may occur on the mixing head
107
and the resin may be deteriorated due to localized over-heating.
To cope with these problems, a technique is disclosed for the prevention of such galling wear, for example, in JP kokoku 6-84035.
According to this technique, a flight
202
of a screw
201
shown in
FIG. 10
is provided at a top portion thereof with a tapered land
204
′ as depicted in
FIG. 11
or with a stepped land
204
as illustrated in FIG.
12
. Owing to the provision of the land
204
or
204
′, lubricating pressure is forced to occur by a melted resin material between a cylinder
203
and the screw flight
202
, thereby avoiding galling wear between the screw flight
202
and the cylinder
203
. Incidentally,
FIG. 13
diagrammatically illustrates the state of occurrence of such lubricating pressure.
Further, JP kokoku 4-77611 also discloses a technique which makes it possible to avoid the above-mentioned galling wear. According to this technique, as is illustrated in
FIG. 14
, a flight is formed broader in a barrier start section or end section of a so-called barrier flight screw or in both of the sections, and this broadened top portion is provided with a tapered (sloped) or stepped recess
301
the radius of which changes from a small radius to a large radius in the direction of rotation of the screw, whereby lubricating pressure is produced by a melted resin to avoid galling wear.
However, if a top portion of a screw flight is provided with such a land only on a main part of a screw like the techniques disclosed in JP kokoku 6-84035 and JP kokoku 4-77611 described above, a mixing head rotates under poorly-balanced load with a small distance left, thereby involving a problem in that galling wear at the mixing head cannot be avoided completely.
The present invention has been completed in view of these problem, and has as an object thereof the provision of a plasticating screw which can avoid a metal-to-metal contact between an inner wall of a cylinder and a mixing head even when resin pressure becomes off-balanced at the mixing head.
DISCLOSURE OF THE INVENTION
To achieve the above object, the present invention provides a plasticating screw rotatable within a cylinder and having a mixing head with plural groove portions formed therein, characterized in that recesses are formed on an outer peripheral surface of the mixing head other than the groove portions so that the recesses are in communication with the groove portions located forward thereof as viewed in a direction of rotation.
Owing to the features as described above, rotation of the plasticating screw forces a feed, which has entered the groove portions, to move onto the outer peripheral surface of the mixing head via the recesses, thereby bringing about

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