Plastic and nonmetallic article shaping or treating: processes – Mechanical shaping or molding to form or reform shaped article – Shaping against forming surface
Reexamination Certificate
2000-10-23
2003-09-02
Heitbrink, Jill L. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Mechanical shaping or molding to form or reform shaped article
Shaping against forming surface
C264S328170
Reexamination Certificate
active
06613265
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a back-flow prevention apparatus and to a method of operating an injection apparatus.
2. Description of the Related Art
Conventionally, an injection molding machine has an injection unit. The injection unit has a heating cylinder in which a screw is disposed to be rotatable and to be movable in an advancement/retraction direction. Drive means rotates and advances or retreats the screw. In a metering step, the screw is rotated in one direction or in the forward direction, thereby melting resin supplied from a hopper and accumulating resin melt in a space located forward of the screw head. In an injection step, the screw is advanced so as to inject the resin melt from the space located forward of the screw head via an injection nozzle.
FIG. 1
shows a sectional view of a main portion of a conventional injection unit.
In
FIG. 1
, reference numeral
11
denotes a heating cylinder. The heating cylinder
11
has an injection nozzle
13
at its front end (left-hand end in FIG.
1
). In the heating cylinder
11
, a screw
12
is disposed to be rotatable and to be movable in an advancement/retraction direction (left/right direction in FIG.
1
). Unillustrated drive means rotates and advances or retreats the screw
12
. Notably, an injection cylinder, a motor, or a like device serves as the drive means.
The screw
12
extends rearward (to the right in
FIG. 1
) within the heating cylinder
11
. The screw
12
is connected at its rear end to the drive means and has a screw head
14
at its front end. A spiral flight
15
is formed on the surface of a metering portion
18
of the screw
12
to thereby form a groove
16
along the flight
15
.
An unillustrated hopper is disposed at a predetermined position located at a rear portion of the heating cylinder
11
. Resin pellets are charged into the hopper.
In the thus-configured injection unit, when the screw
12
is rotated in a forward direction through drive of the drive means in a metering step, resin pellets contained in the hopper drop into the heating cylinder
11
and are advanced (to the left in
FIG. 1
) through the groove
16
. At the same time, the screw
12
is retreated (is moved to the right in FIG.
1
).
An unillustrated heater is disposed around the outer circumference of the heating cylinder
11
. The heater heats the heating cylinder
11
so as to melt resin pellets contained in the groove
16
. Accordingly, when the screw
12
is retreated by a predetermined amount, a predetermined amount of resin melt for a single shot is accumulated in a space located forward of the screw head
14
.
Upon completion of the metering step, suck-back is performed; i.e., the screw
12
is slightly retreated without being rotated, to thereby prevent the resin from oozing from the front end of the injection nozzle
13
.
Next, in an injection step, the drive means is activated so as to advance the screw
12
. The resin accumulated in the space located forward of the screw head
14
is injected from the injection nozzle
13
and charged into the cavity of an unillustrated mold apparatus, thereby filling the cavity with the resin.
In order to prevent backflow of the resin accumulated in the space located forward of the screw head
14
in the injection step, a back-flow prevention apparatus is disposed.
Specifically, the screw head
14
has a conical head body portion
21
formed at its front section (at the left portion in
FIG. 1
) and a small-diameter portion
19
formed at its rear section (at the right portion in FIG.
1
). An annular back-flow prevention ring
20
is disposed around the circumference of the small-diameter portion
19
, thereby defining a resin passageway
24
between the small-diameter portion
19
and the back-flow prevention ring
20
. A seal ring
22
is disposed at the front end of the metering portion
13
such that the seal ring
22
can contact or separate from the rear end of the back-flow prevention ring
20
. Moreover, a cut
25
extending in the axial direction is formed at a plurality of locations on the circumference of the head body portion
21
.
Accordingly, when the screw
12
is advanced in the injection step, the resin accumulated in the space located forward of the screw head
14
is urged to move rearward. However, resin pressure causes the back-flow prevention ring
20
to move rearward with respect to the screw
12
. Thus, the rear end of the back-flow prevention ring
20
abuts the seal ring
22
, so that the communication between the spaces located on the front and rear sides of the seal ring
22
is broken to effect sealing. As a result, the resin accumulated in the space located forward of the screw head
14
is prevented from flowing rearward.
In contrast, when the screw
12
is rotated in the forward direction in the metering step, the screw
12
is retreated. However, due to pressure of the resin, the back-flow prevention ring
20
moves forward with respect to the screw
12
. Thus, the front end of the back-flow prevention ring
20
abuts the rear end of the head body portion
21
, so that the communication between the spaces located on the front and rear sides of the seal ring
22
is established. At this time, resin moves forward, while passing through the cuts
25
, so that resin flow is not hindered.
However, in the injection apparatus, the amount of resin accumulated in the space located forward of the screw head
14
changes during the suck-back operation, because resin at the metering portion
18
moves to the space located forward of the screw head
14
upon performance of the suck-back operation.
Further, since sealing is effected through an operation of advancing the screw
12
to thereby move the back-flow prevention ring
20
rearward, the timing of completion of the sealing operation varies depending on the state of kneading and dispersion of resin, resin viscosity, resin temperature, and the rate at which the screw speed is increased at the time of starting the injection step. Thus, the amount of resin that flows back varies.
Accordingly, even when the injection stroke of the screw
12
for each shot is accurately controlled, the amount of injected resin varies. Further, when the amount of resin that moves during the suck-back operation is excessively small or the timing of completion of the sealing operation is delayed excessively, molded products suffer short shot or like defects. When the amount of resin that moves during the suck-back operation is excessively large or the timing of completion of the sealing operation is advanced excessively, molded products suffer burrs or like defects.
In order to solve the above-described problem, there has been proposed a back-flow prevention apparatus which includes first and second rings disposed on the rear side of the screw head. When the screw is rotated forward, first and second resin passages formed in the first and second rings, respectively, communicate with each other, and when the screw is rotated in the opposite direction; i.e., rotated in reverse, the communication between the first and second resin passages is broken to thereby effect sealing.
In this case, effecting sealing before performance of the suck-back operation prevents resin from moving from the metering portion to the space located forward of the screw head. Therefore, the amount of resin accumulated in the space located forward of the screw head can be stabilized.
Further, since sealing can be effected without advancement of the screw, the timing of effecting the sealing can be stabilized.
However, the conventional back-flow prevention apparatus has a problem such that in some cases the communication between the first and second resin passages cannot be broken through reverse rotation of the screw, resulting in failure to effect sealing.
In such a case, resin accumulated in the space located forward of the screw head flows back during the injection step, with the result that a proper amount of resin necessary for providing a cushion effect cannot be secured. Accordingly, the amou
Fontaine Monica A
Heitbrink Jill L.
Squire Sanders & Dempsey L.L.P.
Sumitomo Heavy Industrie's, Ltd.
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