U.S. patent application number 09/988187 was filed with the patent office on 2003-05-22 for shutterless injection molding method and apparatus.
This patent application is currently assigned to Takata Corporation. Invention is credited to Kono, Kaname.
Application Number | 20030094257 09/988187 |
Document ID | / |
Family ID | 25533914 |
Filed Date | 2003-05-22 |
United States Patent
Application |
20030094257 |
Kind Code |
A1 |
Kono, Kaname |
May 22, 2003 |
Shutterless injection molding method and apparatus
Abstract
There is provided a method of injection molding a metal part and
an injection molding apparatus which reduces or eliminates drooling
of metal from a nozzle. The method includes the steps of (A)
separating an injection nozzle of an injection chamber from
contacting a mold surface, (B) retracting a plunger in the
injection chamber to create a suction in the injection chamber, (C)
closing an inlet to the injection chamber to seal the injection
chamber, (D) maintaining melted metal in the injection chamber with
a pressure difference and surface tension without substantial
drooling from the injection nozzle, (E) placing the injection
nozzle in contact with the mold surface and (F) advancing the
plunger in the injection chamber to inject the metal into the
mold.
Inventors: |
Kono, Kaname; (Tokyo,
JP) |
Correspondence
Address: |
FOLEY AND LARDNER
SUITE 500
3000 K STREET NW
WASHINGTON
DC
20007
US
|
Assignee: |
Takata Corporation
|
Family ID: |
25533914 |
Appl. No.: |
09/988187 |
Filed: |
November 19, 2001 |
Current U.S.
Class: |
164/113 ;
164/312 |
Current CPC
Class: |
B22D 17/30 20130101;
B22D 17/007 20130101; B22D 17/10 20130101 |
Class at
Publication: |
164/113 ;
164/312 |
International
Class: |
B22D 017/10 |
Claims
What is claimed is:
1. A method of injection molding a metal part comprising: (A)
separating an injection nozzle of an injection chamber from
contacting a mold surface; (B) retracting a plunger in the
injection chamber to create a suction in the injection chamber; (C)
closing an inlet to the injection chamber to seal the injection
chamber; (D) maintaining melted metal in the injection chamber with
a pressure difference and surface tension without substantial
drooling from the injection nozzle; (E) placing the injection
nozzle in contact with the mold surface; and (F) advancing the
plunger in the injection chamber to inject a metal into the
mold.
2. The method of claim 1, wherein the metal is maintained
substantially without drooling by a pressure difference between
outside atmosphere and the injection chamber and surface tension in
step (D) by a size of an opening of the injection nozzle being
sufficiently small to substantially prevent the metal from drooling
from the injection nozzle.
3. The method of claim 2, wherein the plunger retraction in step
(B) is started before or while the injection nozzle is separated
from the mold in step (A).
4. The method of claim 3, wherein the inlet to the injection
chamber is closed in step (C) after the step of retracting the
plunger in step (B).
5. The method of claim 4, wherein the metal in step (D) comprises a
liquid metal.
6. The method of claim 5, wherein a portion of the liquid metal in
a tip of the injection nozzle solidifies after injection; and the
solidified metal remelts when the nozzle is separated from the
mold.
7. The method of claim 6, wherein an injection pressure in the
injection chamber does not decrease during the step of advancing
the plunger in step (F).
8. The method of claim 7, wherein step (E) precedes step (F).
9. The method of claim 1, further comprising repeating steps (A)
through (F) a plurality of times.
10. The method of claim 2, wherein the size of the opening of the
injection nozzle is 15 mm or less.
11. The method of claim 1, further comprising: (G) providing liquid
metal into a temperature controlled barrel; and (H) providing the
liquid metal from the barrel into the injection chamber through the
inlet during step (B).
12. The method of claim 11, further comprising: (I) stirring the
liquid metal in the barrel by rotating a ram in the barrel; and (J)
advancing the ram in the barrel to close the inlet to the injection
chamber with a tip of the ram in step (C).
13. The method of claim 1, wherein the suction in step (B)
maintains the melted metal in the injection chamber without
substantial drooling from the injection nozzle.
14. The method of claim 13, wherein the suction in step (B) draws
in the melted metal from a temperature controlled barrel through
the inlet.
15. The method of claim 1, further comprising maintaining a
temperature of the injection nozzle of the injection chamber above
a liquidus temperature of the melted metal such that no plug forms
in the nozzle after step (A).
16. The method of claim 12, wherein the plunger in the injection
chamber retracts to create suction in step (B) before the ram
advances in the barrel in step (J).
17. The method of claim 16, wherein the barrel is located above the
injection chamber to allow gravity to assist passage of the melted
metal from the barrel into the injection chamber.
18. The method of claim 5, wherein the metal comprises a magnesium
alloy.
19. A metal or metal alloy article made by the method of claim
1.
20. The method of claim 10, wherein a diameter of the opening is 10
to 13 mm.
21. A method of injecting melted metal into a mold comprising:
introducing the melted metal into a barrel; allowing at least a
first portion of the melted metal to pass through said barrel into
an injection chamber; and injecting the melted metal from the
injection chamber into the mold, wherein during the step of
injecting, a pressure in the injection chamber does not
decrease.
22. The method of claim 21, further comprising advancing a ram in
the barrel to seal an outlet port between the barrel and the
injection chamber with a portion of the ram.
23. The method of claim 22, wherein the advanced ram prevents the
melted metal and gases from flowing between the barrel and the
injection chambers during the step of injecting.
24. The method of claim 21, further comprising maintaining a
temperature of an injection nozzle of the injection chamber above a
liquidus temperature of the melted metal.
25. The method of claim 21, wherein said allowing step comprises
creating a suction in the injection chamber to draw the portion of
the melted metal from the barrel into the injection chamber.
26. The method of claim 25, wherein a plunger in the injection
chamber retracts to create suction that draws the melted metal.
27. The method of claim 21, wherein the barrel is located above the
injection chamber to allow gravity to assist passage of the melted
metal from the barrel into the injection chamber.
28. The method of claim 21, wherein the melted metal is in a liquid
state.
29. The method of claim 28, wherein the metal comprises a magnesium
alloy.
30. A metal article made by the method of claim 21.
31. The method of claim 21, further comprising the steps of: (A)
separating an injection nozzle of the injection chamber from
contacting the mold surface; (B) retracting a plunger in the
injection chamber to create a suction in the injection chamber; (C)
closing an inlet to the injection chamber to seal the injection
chamber; (D) maintaining melted metal in the injection chamber with
a pressure difference and surface tension without substantial
drooling from the injection nozzle; (E) placing the injection
nozzle in contact with the mold surface; (F) advancing the plunger
in the injection chamber to inject the melted metal into the
mold.
32. The method of claim 31, wherein the metal is maintained
substantially without drooling by a pressure difference between
outside atmosphere and the injection chamber and surface tension in
step (D) by a size of an opening of the injection nozzle being
sufficiently small to substantially prevent the melted metal from
drooling from the injection nozzle.
33. The method of claim 32, wherein the plunger retraction in step
(B) is started before or while the injection nozzle is separated
from the mold in step (A).
34. The method of claim 33, wherein the inlet to the injection
chamber is closed in step (C) after the step of retracting the
plunger in step (B).
35. The method of claim 34, wherein the melted metal in step (D)
comprises a liquid metal.
36. The method of claim 35, wherein a portion of the liquid metal
in a tip of the injection nozzle solidifies after injection; and
the solidified metal remelts when the nozzle is separated from the
mold.
37. The method of claim 36, wherein an injection pressure in the
injection chamber does not decrease during the step of advancing
the plunger in step (F).
38. The method of claim 37, wherein step (E) precedes step (F).
39. The method of claim 31, further comprising repeating steps (A)
through (F) a plurality of times.
40. The method of claim 32, wherein the size of the opening of the
injection nozzle is 15 mm or less.
41. The method of claim 31, further comprising: (G) providing
liquid metal into a temperature controlled barrel; and (H)
providing the liquid metal from the barrel into the injection
chamber through the inlet during step (B).
42. The method of claim 41, further comprising: (I) stirring the
liquid metal in the barrel by rotating a ram in the barrel; and (J)
advancing the ram in the barrel to close the inlet to the injection
chamber with a tip of the ram in step (C).
43. The method of claim 31, wherein the suction in step (B)
maintains the melted metal in the injection chamber without
substantial drooling from the injection nozzle.
44. The method of claim 43, wherein the suction in step (B) draws
in the melted metal from a temperature controlled barrel through
the inlet.
45. An injection molding apparatus comprising: an injection
chamber; a plunger in the injection chamber; and an injection
nozzle in fluid communication with the injection chamber having an
opening sufficiently small to substantially prevent melted metal
from drooling from the injection nozzle by a pressure difference
between outside atmosphere and the injection chamber and surface
tension.
46. The apparatus of claim 45, wherein a diameter of the opening is
15 mm or less.
47. The apparatus of claim 45, further comprising: a temperature
controlled barrel; a ram in the barrel; and an inlet between the
barrel and the injection chamber.
48. The apparatus of claim 47, wherein the ram has a shape that is
capable of blocking the inlet port to prevent a flow of the melted
metal between the barrel and the injection chamber.
49. The apparatus of claim 48, wherein a tip of the ram is shaped
such that it seals the inlet port when the ram is in a fully
advanced state.
50. The apparatus of claim 49, wherein: the barrel is located above
the injection chamber; and the plunger retracts to create suction
that assists in drawing into the injection chamber at least a
portion of the melted metal from the barrel through the inlet
port.
51. The apparatus of claim 50, wherein the plunger is advanced at a
rate at which pressure in the injection chamber does not
decrease.
52. An injection molding apparatus, comprising: an injection
chamber containing an injection nozzle; a first means for
separating the injection nozzle from contacting a mold surface and
for placing the injection nozzle in contact with the mold surface;
a second means for injecting a melted metal from the injection
chamber into the mold and for creating a suction in the injection
chamber such that the melted metal is maintained in the injection
chamber without substantially drooling from the injection nozzle
between injection steps due to a pressure difference between an
outside atmosphere and the injection chamber; a third opening means
in the injection nozzle for maintaining the melted metal in the
injection chamber without substantially drooling from the injection
nozzle between injection steps due to surface tension; and a fourth
means for closing an inlet to the injection chamber to seal the
injection chamber.
53. The apparatus of claim 52, wherein the third means comprises an
opening in the injection nozzle that is sufficiently small to
substantially prevent the melted metal from drooling from the
injection nozzle due to surface tension.
54. The apparatus of claim 53, wherein the second means retracts at
a same time or before the injection nozzle is separated from the
mold.
55. The apparatus of claim 54, wherein the inlet to the injection
chamber is closed when the second means retracts.
56. The apparatus of claim 55, wherein the melted metal comprises a
liquid metal.
57. The apparatus of claim 56, wherein a portion of liquid metal in
a tip of the injection nozzle solidifies after injection; and the
solidified metal remelts when the nozzle is separated from the
mold.
58. The apparatus of claim 57, wherein an injection pressure in the
injection chamber does not decrease when the second means injects
the melted metal from the injection chamber into the mold.
59. The apparatus of claim 53, wherein the size of the opening of
the injection nozzle is 15 mm or less.
60. The apparatus of claim 52, further comprising: a fifth means
for providing liquid metal into a temperature controlled barrel;
and a sixth means for providing the liquid metal from the barrel
into the injection chamber.
61. The apparatus of claim 60, wherein the suction draws in the
melted metal from a temperature controlled barrel through the sixth
means.
Description
FIELD OF THE INVENTION
[0001] The invention generally relates to a method and apparatus
for manufacturing metallic parts, and more particularly to a method
and apparatus for manufacturing metallic parts by a process
involving injection of a melted metal into a mold.
BACKGROUND OF THE INVENTION
[0002] One conventional method used to produce molded metallic
parts from melted metal is by die casting. A typical die casting
machine and method is described in U.S. Pat. No. 5,983,976, hereby
incorporated by reference. Die casting methods inject liquid metal
into a mold.
[0003] Semi-solid methods for producing molded metallic parts
differ from the die casting methods by injection molding a metal in
its semi-solid state rather than in its liquid state. Semi-solid
methods are disclosed in U.S. Pat. Nos. 3,902,544 and 3,936,298,
both of which are incorporated by reference herein.
[0004] Both liquid die casting and semi-solid injection molding
methods require that the metal to be sufficiently fluid in order to
flow into the mold. Further, in conventional injection molding
machines (FIGS. 1a, 1b) the injection chamber is oriented
horizontally. The result of this combination of features is that
conventional injection molding machines tend to drool melted metal
out of the injection nozzle between injection steps.
[0005] In response to this problem, several techniques have been
developed to reduce drooling. In the prior art apparatus
illustrated in FIG. 1a, metal is prevented from drooling out of the
injection nozzle 90 between injection steps by forming a metal plug
91 in the exit opening 92 of the nozzle 90. However, the plug 91 is
undesirable because it is injected into the sprue cavity in the
mold, thus blocking the metal from flowing into the mold cavity.
This tends to adversely affect the filling of the mold cavity. For
example, the plug 91 or its fragments entangled in the sprue cavity
may hinder the flow of molten metal, and the molten metal is
disturbed when injected into the mold cavity.
[0006] To overcome the problems associated with the formation of a
plug 91, a shutter mechanism is developed. A typical shutter
mechanism 95 is illustrated in FIG. 1b. Metal is prevented from
drooling out of the injection nozzle between injection cycles by
closing the shutter 95 between the exit opening 92 of the nozzle 90
and the mold 94. Although the shutter 95 reduces metal from
drooling out of the exit opening 92, its use increases injection
cycle time. The use of the shutter results in a relatively long
injection cycle time. Further, unless the shutter 95 is exactly
flush with the tip of nozzle 90, some metal may drool out of
opening 92.
[0007] Therefore, an improved system for injection molding which
does not require plug formation is desirable. Preferably, the
system is also capable of operating without the shutter. In
addition it is preferable that the system is simple, fast and
reliable.
SUMMARY OF THE INVENTION
[0008] A preferred aspect of the present invention provides a
method of injection molding a metal part comprising: (A) separating
an injection nozzle of an injection chamber from contacting a mold
surface, (B) retracting a plunger in the injection chamber to
create a suction in the injection chamber, (C) closing an inlet to
the injection chamber to seal the injection chamber, (D)
maintaining melted metal in the injection chamber with a pressure
difference and surface tension without substantial drooling from
the injection nozzle, (E) placing the injection nozzle in contact
with the mold surface and (F) advancing the plunger in the
injection chamber to inject the metal into the mold.
[0009] Another preferred aspect of the present invention also
includes a method of injecting melted metal into a mold comprising:
introducing the melted metal into a barrel, allowing at least a
first portion of the melted metal to pass through said barrel into
an injection chamber and injecting the melted metal from the
injection chamber into the mold, wherein during the step of
injecting, a pressure in the injection chamber does not
decrease.
[0010] Another preferred aspect of the present invention also
includes an injection molding apparatus comprising an injection
chamber, a plunger in the injection chamber, an injection nozzle in
fluid communication with the injection chamber having an opening
sufficiently small to substantially prevent melted metal from
drooling from the injection nozzle by a pressure difference between
outside atmosphere and the injection chamber and surface
tension.
[0011] Another preferred aspect of the present invention includes
an injection molding apparatus, comprising an injection chamber
containing an injection nozzle, a first means for separating the
injection nozzle from contacting a mold surface and for placing the
injection nozzle in contact with the mold surface, a second means
for injecting a melted metal from the injection chamber into the
mold and for creating a suction in the injection chamber such that
the melted metal is maintained in the injection chamber without
substantially drooling from the injection nozzle between injection
steps due to a pressure difference between an outside atmosphere
and the injection chamber, a third opening means in the injection
nozzle for maintaining the melted metal in the injection chamber
without substantially drooling from the injection nozzle between
injection steps due to surface tension, and a fourth means for
closing an inlet to the injection chamber to seal the injection
chamber.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The foregoing and other features, aspects and advantages of
the present invention will become apparent from the following
description, appended claims and the exemplary embodiments shown in
the drawings, which are briefly described below. It should be noted
that unless otherwise specified like elements have the same
reference numbers.
[0013] FIG. 1a is a side view of a first prior art apparatus.
[0014] FIG. 1b is a side view of a second prior art apparatus.
[0015] FIG. 2 is a schematic illustration of a side view of an
injection molding system according to a preferred embodiment of the
invention.
[0016] FIG. 3 is a schematic illustration of a side view of an
injection nozzle according to a preferred embodiment of the
invention.
[0017] FIG. 4a is a plot of the pressure profile during the
injection step of an embodiment of the invention.
[0018] FIG. 4b is a plot of the pressure profile during the
injection step according to a prior art method.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0019] In the discussion of the preferred embodiments which
follows, a metal part is produced by injection molding a magnesium
(Mg) alloy in a liquid state. The invention is not limited to
processing of Mg and is equally applicable to other types of
materials, metals and metal alloys, in a liquid or semi-solid
state. A wide range of such metals and alloys are potentially
useful in this invention, including magnesium (Mg), Mg alloys,
aluminum (Al), Al alloys, zinc (Zn), Zn alloys, and the like.
[0020] The terms "melted metal" and "melted material" as used
herein encompasses metals, metal alloys and other materials in a
liquid or semi-solid state which can be processed in an injection
molding system. The term "without substantial drooling" means
completely without drooling or with only minimal drooling, such as
only with a few drops of melted metal (rather than a steady stream
of melted metal) drooling from the injection nozzle between
injection shots.
[0021] Specific temperatures and temperature ranges cited in the
following description of the preferred embodiments are applicable
to the preferred embodiment for processing a Mg alloy in a liquid
state, but could readily be modified in accordance with the
principles of the invention by those skilled in the art in order to
accommodate other metals and metal alloys in liquid or semi-solid
state. For example, some Zn alloys become liquid at temperatures
above 450.degree. C., and the temperatures in the injection molding
system can be adjusted for processing of Zn alloys.
[0022] The present inventor has determined that metal drooling out
of the tip of an injection nozzle can be reduced or eliminated
without using a plug or a shutter. More specifically, by creating
and maintaining suction in the injection chamber between injection
steps, forces are created which tend to hold the metal in the
injection chamber. Further, by providing a nozzle with an
appropriately sized opening (i.e., aperture or hole), the various
forces acting on the metal, i.e. suction, air pressure and surface
tension, may be balanced to reduce or prevent drooling. Thus,
drooling can be reduced or eliminated without resorting to use of
plugs or shutter mechanisms.
[0023] FIG. 2 is a schematic illustration of a side view of an
injection molding system 10 according to a preferred embodiment of
the invention. The system 10 includes an injection molding
apparatus 12 and a mold 13. The apparatus 12 is mounted on wheels
and or rails (not shown) such that it may be retracted from the
mold 13 after each injection step and advanced toward the mold 13
before each injection step by a motor or hydraulics (not shown). A
feeder 23 is provided with at least one heating element 25 disposed
around its outer periphery. The heating element 25 may be of any
conventional type. The heating element 25 operates to maintain the
feeder 23 at a temperature high enough to keep the metal alloy
supplied through the feeder 23 in a liquid state. For a Mg AZ91
alloy, this temperature would be about 610.degree. C. or greater.
Preferably, sufficient metal should be kept in the feeder 23 to
supply about 20 times the volume needed for one injection cycle (or
shot). This is because the amount of time required to melt the
metal necessary for one injection cycle is longer than the
injection cycle time, which depends on the volume of the mold
cavity, diameter of the injection chamber and actions of the
machine operator.
[0024] In a preferred aspect of the invention, the feeder 23
further contains an outlet screening element 24. For example, as
illustrated in FIG. 2, the screening element 24 may comprise at
least one non-horizontal wall 26, a top cover or portion 28 and an
outlet port 29. Preferably, the outlet port 29 is located in one of
the walls 26 instead of in the top 28 of the screening element 24.
The screening element 24 may contain one wall 26 if the element 24
has a cylindrical shape, or plural walls 26 if the element 24 has a
polygonal shape. Furthermore, the non-horizontal wall 26 is
preferably exactly vertical or substantially vertical (i.e.,
deviating by about 1-20 degrees from vertical). The screening
element 24 prevents solid metal pieces or ingots as well as other
residue present in the melted metal from clogging the outlet port
29 because the outlet port 29 is raised from the bottom of the
feeder 23. However, the screening element 24 may be omitted, if
desired.
[0025] The melted metal is subsequently supplied into a
temperature-controlled barrel 30 by way of gravity through a feeder
port 27 which may optionally be supplied with a valve serving as a
stopper (not shown). Preferably, no valve is present. A ram 32 is
arranged coaxially with the barrel 30 and extends along the center
axis of the barrel 30. The outer diameter of the ram 32 is smaller
than the inner diameter of the barrel 30 such that melted metal
flows in the space between the ram 32 and the barrel 30. The ram 32
is controlled by motor 33 for axial movement in both retracting and
advancing directions along the barrel 30 and for rotation around
its own axis if stirring of the melted metal is desired inside
barrel 30.
[0026] In the preferred embodiment of the invention, the ram 32
includes supporting ribs or fins 34. The fins 34 are preferably
attached to the ram 32 and can slide on the inner circumference of
the barrel 30, both coaxially with the length of the barrel and or
in a circular motion about the barrel axis. The movement produces a
rotation of the fins 34 around the inner circumference of the
barrel 30. Alternatively, the fins 34 may be attached to the inner
circumference of the barrel 30 in such a manner as to allow the
bare ram 32 to slide by. The fins 34 can be made of the same
material as the ram 32 or from a different material that can
withstand the required process temperatures. The fins prevent the
ram 32 from tilting and wobbling away from the barrel axis. They
also second enhance the uniform temperature distribution of the
melted metal.
[0027] The ram 32 as shown in FIG. 2 has a pointed tip, but any
shape may be used, including a blunt end or a rounded end.
Preferably, the tip of ram 32 has a shape capable of blocking inlet
port 37 to prevent the flow of melted metal between barrel 30 and
injection chamber 50, when the ram 32 is fully advanced inside
barrel 30. The injection chamber 50 contains a plunger or piston 45
and an injection nozzle 57. The plunger 45 is advanced in the
injection chamber 50 by a motor or hydraulics (not shown) to inject
the liquid or semi-solid metal from the injection chamber 50
through the nozzle 57 into a mold cavity 15 in mold 13. The plunger
45 contains a seal, such as O-ring(s) 41, to form an air tight seal
with the inner surface of the injection chamber 50. This allows the
plunger 45 to create a suction in the injection chamber 50 when the
plunger 45 retracts.
[0028] An injection molding method using system 10 will now be
described. After injection (i.e. after a shot), the nozzle 57 is
separated from the mold 13. Preferably, this is accomplished by
moving the injection molding apparatus 12 away from a stationary
mold 13 die. After or during the time the injection molding
apparatus 12 is retracted, the ram 32 is retracted in the barrel 30
(but may continue rotating if rotation is being used to stir the
melted metal inside barrel 30). The plunger 45, which is housed in
the injection chamber 50, begins retracting (moved to the right as
shown in FIG. 2) to expand the volume of the injection chamber 50
to a desired volume according to the dimensions of the desired
molded part before or while the apparatus 12 is retracted. The
plunger 45 retraction is stopped when the volume of the injection
chamber 50 becomes equal to the desired injection volume. The
plunger 45 may be retracted while that ram 32 is being retracted or
after ram 32 has been retracted to a desired position. While being
retracted, the plunger 45 acts like a pharmaceutical syringe that
draws in liquid from a container of liquid. Specifically, as the
plunger 45 retracts, it creates a suction to draw in melted metal
from the barrel 30 into the injection chamber 50 through port 37.
The suction prevents or reduces the drooling from nozzle 57.
[0029] After plunger 45 retraction is stopped, the ram 32 is
advanced downward. As a result, any metal collected in a lower
portion of barrel 30 is pushed into the injection chamber 50
through the inlet port 37. The ram 32 preferably advances through
barrel 30 until its end closes off inlet port 37. The ram 32
preferably remains in this position to keep inlet port 37 sealed
off until injection is complete and the next shot cycle is started.
The advanced ram 32 prevents metal and gases from flowing between
barrel 32 and chamber 50.
[0030] In the preferred embodiment of the invention, some suction
remains in the injection chamber 50 after inlet port 37 is sealed
by the ram 32. Thus, the whole back side of the injection chamber
50 is sealed off. The ram 32 seals off the inlet port 37, while the
seal 41 on plunger 45 seals off the back of the injection chamber
50. Thus, because the backside of chamber 50 is sealed off, a
pressure difference is created between the outside air pressure on
the front of the metal located in the nozzle 57 and the back of the
metal in the injection chamber 50. The pressure difference acts to
maintain the liquid or semi-solid metal in the nozzle 57. In
addition, the inventor has recognized that there is a capillary
force in the nozzle 57 acting on the metal due to the surface
tension. This also tends to maintain the liquid metal in the nozzle
57. Thus, the inventor has determined that by providing the
injection nozzle 57 with a sufficiently small exit opening or
aperture 58, the pressure difference between the outside air
pressure and the suction pressure combined with the surface tension
of the melted metal can reduce or prevent the metal drooling out of
the opening 58.
[0031] To inject the metal into the mold 13, the plunger 45 is
advanced in chamber 50, to force the metal in chamber 50 through
the nozzle 57 and the sprue cavity 14 into the mold cavity 15.
After a pre-set dwell time, the two mold 13 die are separated
(i.e., opened) and the molded metallic part is removed, so that a
new cycle can begin.
[0032] Initially, when the tip of the nozzle 57 separates from the
mold 13, parts of the solidified sprue may extend into the nozzle
57. However, because the tip of the nozzle 57 is heated above the
liquidus temperature of the metal, any solid metal in the nozzle 57
is quickly melted while the apparatus 12 is being retracted, such
that no plug forms. The solidified sprue, if present, may assist in
reducing or preventing drool between the injection step and plunger
retraction step. However, the solidified sprue is not the same as
the plug 91 in FIG. 1a, because any portion of the sprue is
remelted as the nozzle 57 is retracted from the mold. In contrast,
the plug 91 is maintained in the nozzle 57 during the entire period
between injection steps. The plug 91 is injected into the sprue
cavity rather than being remelted.
[0033] FIG. 3 illustrates a side view of an injection nozzle 57
according to a preferred embodiment of the invention. The injection
nozzle 57 is provided with an exit opening 58 of a predetermined
size selected based on the amount of suction that will be used for
the injection molding of a particular metal part. The amount
suction is generally determined by the size of the part, i.e. the
amount of metal required. By knowing the weight of the metal to be
injected, the surface tension of the metal, the ambient air
pressure, and the suction force, a desired exit opening 58 size can
be determined. Thus, the size of the opening 58 is sufficiently
small to substantially prevent melted metal from drooling from the
injection nozzle 57 by a pressure difference between outside
atmosphere and the injection chamber 50 and by surface tension. The
maximum opening size which reduces or prevents drooling is
preferably 15 mm or less, most preferably 10-13 mm. However, other
opening sizes may be used depending on the processing
conditions.
[0034] As shown in FIG. 2, heating elements 25 and 70a-70j are
provided along the lengths of the feeder 23, the barrel 30 and the
injection chamber 50. The temperature in the feeder differs
depending on the material present in the feeder. For the AZ91 Mg
alloy, heating elements 25 are preferably controlled so that the
temperature in the feeder 23 is about 640 to 670.degree. C. near
the upper surface of the melted Mg alloy and about 660 to
690.degree. C. near the lower region of feeder 23. Heating elements
referenced and prefixed by the numeral 70 are preferably resistance
heating elements.
[0035] In the barrel 30, the temperature is preferably maintained
at about 620 to 680.degree. C., preferably about 660 to 670.degree.
C. for the AZ91 Mg alloy. While only three heating elements 70a-70c
are illustrated adjacent to the barrel in FIG. 2, there may be four
or more heaters which heat the barrel 30.
[0036] In the injection chamber 50 and nozzle 57, the temperature
is preferably maintained at about 620 to 700+ C., preferably about
660 to 690.degree. C. for the AZ91 Mg alloy. While only three
heating elements 70h, 70i and 70j are illustrated in FIG. 2, there
may be more than three heating elements which heat the injection
chamber and nozzle. For example, there may be four heating elements
which heat the nozzle and two heating elements which heat the
portion of the injection chamber 50 in front of the seal 41.
Preferably, the temperature in the nozzle is 10-30 degrees higher
than in the injection chamber, and the nozzle tip is maintained at
the highest temperature.
[0037] The temperature near heating elements 70g and 70f behind the
seal 41 is preferably maintained at below 610.degree. C., such as
at about 600 to 570.degree. C. for the AZ91 Mg alloy. The lower
temperature behind the seal 41 helps prevent the metal from flowing
past the seal 41. It should be noted that the liquid metal is
prevented by the seal 41 from entering the portion of the injection
chamber 50 adjacent to heating elements 70f and 70g, even when the
plunger 45 is in a fully retracted position.
[0038] If desired, one or two additional heaters may be placed
adjacent to the port 37. The port 37 may be maintained at about 620
to 670.degree. C., preferably about 640 to 660.degree. C.
[0039] The temperatures described above are sufficiently high to
maintain the melted metal entirely in the liquid state from the
time it exits the feeder 23 into the barrel 30 to the time the
melted metal is injected into the mold 13 from the injection
chamber 50. The temperatures may be varied depending on the type of
metal part being molded.
[0040] Using the preceding temperatures at these locations permits
molding of the AZ91 Mg alloy in the liquid state. Molded metallic
parts having extremely smooth surfaces and minimal porosity can be
produced, which allows them to be painted directly without any
further machining. The molded metal parts also have extremely
accurate dimensions and consistency, and can be produced with
thicknesses of less than 1 mm when the part roughly has the
dimensions of a DIN size A4 sheet of paper (21.0 cm by 29.7 cm).
Preferably, the range of thickness of molded parts produced
according to the preferred embodiment of the invention is between
0.5 and 1 mm for parts that have roughly the dimensions of a DIN
size A4 sheet of paper. With known die casting and semi-solid
methods, thicknesses no less than about 1.3 mm can be obtained for
parts that have roughly the dimensions of a DIN size A4 sheet of
paper.
[0041] As discussed above, use of the system and method of the
preferred embodiment of the present invention, results in reduction
or elimination of drooling. Another benefit is illustrated in FIGS.
4a and 4b. FIG. 4b illustrates the pressure profile of an injection
step using prior art injection molding with plug 91 formation as
shown in FIG. 1a. Initially, there is a rapid increase in pressure
due to the plug 91 being located in the nozzle exit opening 92.
When the pressure builds up high enough to dislodge the plug, the
plug bursts out of the nozzle into the sprue cavity. Then, there is
a rapid decrease in pressure as metal shoots into the mold. This is
followed by a gradual increase in pressure until the mold cavity is
filled. Although this method works well for simple parts, the wide
variation in pressure as a function of time has an adverse effect
on the quality of more complicated parts, especially those with
thin sections.
[0042] FIG. 4a illustrates the pressure profile of an injection
step according to the above described preferred embodiment of the
invention. Initially, there is a rapid increase in pressure until
point 102, where the sprue cavity 14 is filled. Then the pressure
rises rapidly again until point 104, where the mold cavity 15
fills. After the mold cavity 15 fills, there is one final pressure
surge at point 106. The pressure then settles to a final value when
the advancement of plunger 45 is stopped. The displacement of the
plunger 45 is illustrated as the dashed line in FIG. 4a. Unlike the
prior art, the pressure in the present embodiment of the invention
does not decrease while the plunger is being advanced. The small
drop in pressure after the pressure surge 106 occurs as or after
the plunger 45 advancement has stopped (point 108).
[0043] The foregoing description of the invention has been
presented for purposes of illustration and description. It is not
intended to be exhaustive or to limit the invention to the precise
form disclosed, and modifications and variations are possible in
light of the above teachings or may be acquired from practice of
the invention. The drawings and description were chosen in order to
explain the principles of the invention and its practical
application. It is intended that the scope of the invention be
defined by the claims appended hereto, and their equivalents.
* * * * *