Method and apparatus for pore-free die casting

In a hot chamber method of pore-free die casting, a gas which reacts with the cast material flushes air from a passage in a nozzle, sprue bushing, and at least a portion of a gooseneck through which the material fills the die cavity, as well as from the die cavity. The reactive gas replaces the air in the passage so that air is not forced into the die cavity by the material when it fills the die cavity. Air in the die cavity would form pores in the cast material. Hot chamber apparatus for practicing the method has a bore communicating with the passage for flushing the passage with the gas. A valve closes the bore when the material fills the die cavity so that the material only enters the die cavity. A heating element heats the valve to prevent the material from solidifying about the valve and thereby interfering with its operation.

BACKGROUND OF THE INVENTION 
This invention relates to die casting and more particularly to an improved 
method and hot chamber apparatus for pressure die casting. 
Die casting is a well known way of shaping articles in which a liquid 
material, such as molten metal, is placed in a cavity which is formed in 
the shape of the desired article between separable die members. The liquid 
material fills the die cavity and solidifies therein in the shape of the 
desired article. The die members are then separated and the article 
removed from the die cavity. In pressure die casting, the liquid material 
is forced or injected into the die cavity under pressure. 
Although die casting is a relatively easy way of forming articles, 
particularly articles having complex exterior surfaces which otherwise 
would be difficult to form, pores often form throughout the casting to 
significantly weaken the cast article. Such weak, porous articles are not 
suitable for many applications and articles for these applications have 
had to be manufactured by other, more expensive techniques. 
Even though die casting and the problems presented by the porosity of cast 
articles have been long known, it was not until U.S. Pat. No. 3,382,910 
issued on May 14, 1968 in the names of Radtke and Eck that a novel way of 
avoiding pores in cast articles became known. This patent, which is 
assigned to the same assignee as this application, describes a method of 
pore-free die casting in which the die cavity is purged of air (or other 
non-reactive gas or vapor) with a gas which reacts with the material to be 
cast in the die cavity. Such gas is herein called a reactive gas. 
When the material is then cast, the reactive gas in the die cavity reacts 
with the material to form solid compounds therewith rather than the pores 
or bubbles which the non-reactive components of air in the die cavity 
would have formed in the cast article. For example, the patent describes 
flushing the die cavity with oxygen which reacts with a molten metal as it 
is cast in the die cavity to form small particles of oxides of the cast 
metal rather than pores or bubbles of trapped non-reactive gas. 
Even though the practice of the method described in U.S. Pat. No. 3,382,910 
produced articles which were significantly more pore-free than articles 
produced by conventional die casting, it has been found that some pores 
continue to be formed in articles cast according to the patented method in 
which only the die cavity is purged. 
One attempt to control the formation of pores produced in cast articles 
suggests that portions of the article which first solidified shrank from 
still liquid portions of the material to form pores in the article. It 
then proposes to control, but not eliminate, the formation of such pores 
by controlling the places at which the cast article first cools and 
solidifies in the die cavity so that the pores from shrinkage of the 
material are formed in a portion of the cast article which can tolerate 
weakness from the pores or which may be removed from the finished article. 
A proposal for further reducing the pores in an article cast in a die 
cavity which is filled with a reactive gas suggests placing a constricted 
gate at the place where the material enters the die cavity. The 
constricted gate produces a turbulence in the material injected into the 
die cavity to mix the injected material more completely with the reactive 
gas in the die cavity. Even if this proposal is more successful in 
reacting the gas with the material, it alone does nothing to eliminate 
non-reactive gas which may be injected into the die cavity with the 
material. Such non-reactive gas in the die cavity then forms pores in the 
cast article in the same way as if the die cavity had not been purged with 
the reactive gas before casting the material. 
Two types of die casting apparatus are known. One type, often and herein 
called cold chamber apparatus, has a chamber just large enough to fill the 
die cavity once. This chamber is generally open to receive the individual 
shots of material to be cast. The open chamber can be easily flushed with 
a reactive gas (along with the die cavity) so that non-reactive gas is not 
forced into the die cavity ahead of the material to form pores in the 
casting. 
The other type of die casting apparatus, often and herein called hot 
chamber apparatus, however, has a chamber or furnace holding a continuous 
supply of the material to be cast. This chamber is connected to the die 
cavity by an enclosed passage for filling the cavity for successive 
castings. Inasmuch as the passage is enclosed and blocked at one end by 
the supply of material to be cast, it cannot be readily flushed with 
reactive gas. 
Flushing the die cavity with a reactive gas merely traps non-reactive gas 
in the passage between the cavity at one end and the material at the 
other. Filling the die cavity then forces the non-reactive gas into the 
die cavity where it forms pores in the casting. 
Opening the passage so that it could be flushed like cold chamber apparatus 
cannot be done because the material would then escape through the opening 
as it fills the die cavity. In cold chamber apparatus, a plunger usually 
pushes the material away from the opening as it forces the material into 
the die. In hot chamber apparatus, however, a plunger which pushes the 
material along the portion of the passage in which non-reactive gas is 
trapped (a gooseneck, nozzle and sprue bushing, as later described) would 
not reach the passage to push the material away from an opening for 
flushing the passage. Indeed, the plunger would push the material toward 
and out of such an opening. For these and other reasons, the same 
techniques with which cold chamber apparatus is purged are not suitable 
for hot chamber die casting apparatus. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a method and apparatus for 
casting articles with greatly reduced porosity in a hot chamber die 
casting machine which is, more particularly, applicable to die casting 
zinc alloys. 
The invention will be described with reference to hot chamber pressure die 
casting apparatus which injects molten metal into a die cavity from a 
continuous supply through an enclosed passage. The enclosed passage 
comprises a sprue bushing in the die which communicates with one end of a 
hollow nozzle and a gooseneck which communicates with the other end of the 
nozzle. The gooseneck curves downwardly from the nozzle into a holding 
furnace full of molten metal and upwardly in the furnace to form a chamber 
for a plunger which injects the molten metal into the die cavity. A bore 
at the upper end of the chamber, but initially below the plunger, admits 
molten metal from the furnace into the chamber. Downward movement of the 
plunger then closes the bore from the chamber and injects the metal 
through the gooseneck, nozzle, and sprue bushing for filling the die 
cavity. 
In the method of the invention, pore-free die castings are formed when a 
reactive gas flushes air (or other non-reactive gas or vapor) from the 
sprue bushing, nozzle, and at least a portion of the gooseneck, as well as 
from the die cavity. The reactive gas replaces the air in the sprue 
bushing, nozzle, and portion of the gooseneck (and the die cavity) so that 
minimal air is forced from the sprue bushing, nozzle and gooseneck into 
the die cavity as the material is injected therethrough into the die 
cavity. Air or other non-reactive gas in the die cavity would form pores 
in the article cast from the material. 
It has been possible heretofore to purge the means which fill a die cavity 
only in cold chamber die casting apparatus. The chamber on this apparatus 
is open to receive individual shots of material to be cast and could thus 
also receive easily a purging reactive gas. In hot chamber apparatus, 
however, the passage in the gooseneck, nozzle and sprue bushing through 
which the die cavity is filled is enclosed. There has thus been no way of 
introducing a purging reactive gas to the passage. 
Hot chamber apparatus, however, is desirable for casting certain metals 
such as zinc. Individual shots of the metal have to be ladled into the 
chamber in cold chamber apparatus. In hot chamber apparatus, on the other 
hand, the holding furnace forms a continuous supply of metal for casting. 
Hot chamber apparatus thus saves the time and difficulty of ladling 
individual shots of metal. Thus, ever since the pore free method of die 
casting was introduced by the Radtke and Eck patent in 1968, there has 
been an unresolved need to combine the advantages of hot chamber apparatus 
with the advantages of the pore free die casting technique. Applicant has 
now filled this need. 
Hot chamber apparatus for practicing the method has a bore which 
communicates with the passage in the gooseneck through which, in 
cooperation with the nozzle and sprue bushing, the material is injected 
into the die cavity. A valve which seats in one end of the bore opens the 
bore while the reactive gas flushes air from the sprue bushing, nozzle, 
and portion of the passage in the gooseneck between the nozzle and the 
bore, as well as from the die cavity, and closes the bore while the 
material is being injected into the die cavity so that the material is 
injected only into the die cavity. 
In one mode of operating the apparatus according to the method, the passage 
receives the reactive gas from means communicating with the die cavity for 
flushing air from the die cavity, sprue bushing, nozzle, gooseneck through 
the bore, the valve then being open to exhaust the air through the bore. 
After purging the die cavity and passage, the valve closes and the 
material is injected into the die, the closed valve then preventing the 
material from escaping through the bore so that the material is injected 
only into the die cavity. 
In another mode of operating the apparatus, the reactive gas is introduced 
past the valve for purging the gooseneck, nozzle, sprue bushing, and die 
cavity, the air purged therefrom escaping through an outlet from the die 
cavity. After flushing the air from the gooseneck, nozzle, sprue bushing, 
and die cavity, the valve closes to prevent the material then injected 
into the die cavity from passing along the bore into the apparatus which 
supplied the reactive gas. 
Those in the art will readily appreciate from the description of the 
apparatus that the method can best be carried out when the bore 
communicates with the passage at a point where the portion of the passage 
between the nozzle and the bore is a substantial portion of the length of 
the passage between the nozzle and the material to be injected. When the 
apparatus is so designed, a substantial portion or all of the passage is 
purged by the reactive gas to minimize the amount of air which may be 
trapped in the passage between the bore and the material to be injected. 
The preferred form of the apparatus additionally comprises a heating 
element which heats the valve (or its cooperative valve seat at one end of 
the bore) for preventing material which may reach the valve through the 
bore during injection from solidifying between the valve and its seat. 
Such solidified material could prevent operation of the valve.

DESCRIPTION OF THE PREFERRED EMBODIMENT FOR THE PREFERRED PRACTICE OF THE 
METHOD 
The drawing schematically illustrates portions of known hot chamber 
apparatus for injecting molten metal 10 from a holding furnace 11 into a 
die cavity 12 through an enclosed passage 22. The die cavity is formed 
between two die members 14 one of which is pressed against the other, as 
by piston and cylinder 16, for substantially closing the die cavity. A 
sprue bushing 18 extends from the die cavity and communicates with one end 
of a hollow nozzle 20; the other end of the nozzle communicates with a 
gooseneck 24 to together form the passage. The gooseneck extends into the 
molten metal and curves upwardly at one end to form a chamber 26 in a 
portion of the passage 22 remote from the nozzle. A plunger 28 is 
initially positioned in the chamber 26 above a bore 30 which admits the 
molten metal 10 into the chamber. The plunger is connected to a piston and 
cylinder device 32 for moving the plunger downwardly into the chamber to 
first close the portion of the chamber 26 below the plunger from the bore 
30 and then to inject the molten metal through the passage into the die 
cavity. As earlier described, apparatus of this type for injecting molten 
metal into a die cavity is well-known. 
The preferred embodiment additionally comprises a bore 40 which extends 
through the gooseneck 24 from one end at the passage 22 to an opposite end 
which forms a seat 42 for a valve stem 44. Inasmuch as the preferred 
embodiment is intended for die casting molten metal, and particularly 
because the bore 40 and valve seat 42 are formed integrally with the 
gooseneck which is bathed in the molten metal, heat will be conducted 
through the gooseneck to the valve seat 42 and valve stem 44. A piston and 
cylinder device 46, which is connected to the valve stem 44 for raising 
and lowering the valve stem from the valve seat, is therefore thermally 
insulated from the gooseneck and valve stem to protect seals and other 
structure of the piston and cylinder device. 
A bracket 48 which supports the cylinder of piston and cylinder device 46 
on the gooseneck 24 has thermal insulation 50 at one or both ends of the 
bracket. A piston rod 52 of the piston and cylinder device is operatively 
connected to the valve stem 44 with additional thermal insulation 54. The 
piston and cylinder 46 is thus maintained at a lower temperature than the 
gooseneck 24 so as to avoid heat deterioration of the piston and cylinder 
device 46. 
The valve seat portion 42 of bore 40 forms a chamber which communicates 
with another bore 55. A bellows seal 56 extends between this chamber and 
the thermal insulation 54 on the valve stem to enclose the chamber (except 
for the bore 55). A heating element 58 surrounds the portion of the 
gooseneck in which the valve seat portion 42 is formed for maintaining the 
valve seat portion 42 at a selected temperature even higher than that 
produced by conduction of heat through the gooseneck from the molten 
metal. 
A passage 60 extends through at least one die member 14 for communicating 
with the die cavity 12. As will be quickly apparent from the following 
description of the operation of the preferred embodiment, there may be 
more than one passage 60 (only one shown) for assuring that each portion 
of the die cavity 12 is purged of non-reactive gas by the reactive gas. 
Description of Operation 
In one mode of operating the preferred embodiment, a reactive gas at a 
pressure above atmospheric pressure is supplied to the end of the bore 55 
remote from the chamber defined by the valve seat 42; for example, a tank 
(not shown) of compressed oxygen can be connected to the end of the bore 
55. The piston and cylinder device 46 raises the valve stem 44 from the 
valve seat 42 to admit the reactive gas to the bore 40. The pressure of 
the reactive gas forces the gas along the bore 40 into the passage 22. 
Some of the reactive gas may flush air from a portion of the passage 22 
which extends between the bore 40 and the molten metal 10, but most of the 
reactive gas flushes air from the portion of the passage 22 which extends 
between the bore 40 and the nozzle 20 because the molten metal blocks the 
other end of the passage. Flow of the reactive gas toward the nozzle is 
not blocked because the passage 60 opens the die cavity and thus the 
passage 22 to the atmosphere. The pressure of the reactive gas then 
carries the gas along the passage 22 to the die cavity 12 to flush air 
from the passage 22, as well as the die cavity, through the passage 60. 
After purging the passage and die cavity with the reactive gas, piston and 
cylinder 46 presses the valve stem 44 against the valve seat 42 to prevent 
further flow of the reactive gas. The piston and cylinder 32 then injects 
the molten metal into the die cavity 12 through the passage 22, the valve 
stem 44 on the valve seat 42 preventing the molten metal from escaping 
through the bore 40. 
In another mode of operating the preferred embodiment, the pressurized 
reactive gas, for example oxygen from a tank (not shown), is supplied to 
the end of the passage 60 remote from the die cavity 12 (an inlet valve 
(not shown) at passage 60 would be desirable to prevent flow of oxygen 
while the dies were open). Little or no reactive gas flows into the die 
cavity while the valve stem 44 is seated on the valve seat 42 because the 
valve stem blocks the bore 40 and the molten metal blocks the passage 22. 
Piston and cylinder 46 then raises the valve stem 44 from the valve seat 
42 to permit the reactive gas to enter the die cavity 12 and flow along 
the passage 22 to the bore 40 to flush the air which had been in the die 
cavity, sprue bushing, nozzle, and gooseneck out the bore 40 and through 
the passage 55 to the atmosphere. After flushing the air from the passage 
22, piston and cylinder 46 again seats the valve stem 44 on the valve seat 
portion 42 to prevent further flow of the reactive gas and piston and 
cylinder 32 operates the plunger to inject the molten metal into the die 
cavity, the pressure of the molten metal during its injection into the die 
cavity 12 being higher than that of the reactive gas. As before, the valve 
stem 44 on the valve seat 42 prevents the molten metal from escaping 
through the bore 40, as well as preventing further flow of the reactive 
gas. Relatively little molten metal, if any, will escape through the 
passage 60 because the molten metal quickly solidifies and plugs the 
relatively small passage 60 in the die. 
In both modes of operation, the sprue bushing 18, nozzle 20, and portion of 
the passage 22 between the nozzle and the bore 40, as well as the die 
cavity 12, are all purged of air and filled with the reactive gas before 
the molten metal is injected into the die cavity. When the molten metal is 
injected into the die cavity, it forces all this reactive gas (and such 
little air as may be trapped in the passage 22 between the molten metal 
and the bore 40) into the die cavity 12. There then being substantially 
only reactive gas in the die cavity, all the gas in the die cavity reacts 
with the metal to form a pore-free casting. 
In both modes of operation, the heating element 58 maintains the valve seat 
42 and the valve stem 44 at a temperature above the melting point of the 
metal so that any metal which may reach the valve stem during injection 
will not solidify between the valve stem and the valve seat to prevent 
operation of the valve when it is next desired to purge air from the 
passage 22. The heat from the heating element 58 is insulated from the 
piston and cylinder 46 by the thermal insulation 50, 54, as before 
described. 
Although the invention has been described with reference to one preferred 
embodiment for flushing air from a sprue bushing, nozzle, and gooseneck, 
as well as from the die cavity, it will be understood that other hot 
chamber apparatus may be adapted for practicing the invention.