Die cast machines

In a die cast machine of the type wherein the piston of an injection cylinder is advanced by pressure gas admitted into the rear chamber of the cylinder while discharging pressurized liquid in the fore chamber, and the piston is retracted by pressurized liquid admitted into the fore chamber while forcing the gas in the rear chamber back into a gas accumulator, a control valve device is provided in the path of the pressurized liquid discharged from the fore chamber, and the control valve device is controlled so as to vary the flow quantity of the discharged pressurized liquid in accordance with the position of the piston during its forward movement. There are also provided two gas accumulators which are connected to the rear chamber of the cylinder through a valve device which is controlled so as to adjust the rate of increase of the pressure applied to the molten metal at the final stage of the injection casting operation.

BACKGROUND OF THE INVENTION 
This invention relates to an improvement of a die cast machine, more 
particularly an improved fluid pressure operating circuit for the 
injection cylinder described in U.S. Pat. No. 3,891,126 dated June 24, 
1975. 
According to this patent, the energy of the compressed gas contained in an 
accumulator is used for moving the piston of the injection cylinder of a 
die cast machine at an extremely high speed without accompanying an 
objectionable water hammering phenomenon thereby decreasing the time 
required for raising the pressure of cast molten metal and eliminating the 
problems of dimensional inaccuracy and fins caused by the water hammering 
phenomenon. 
However, as a result of further investigation it was found that there 
remains problems to be solved as follows. 
More particularly, in most (more than 90%) of the metal moulds now being 
used commercially a high rate of rise in the pressure applied to the 
molten metal at the final stage of injection is desirable but several % of 
the metal moulds have gaps between the mating surfaces of the mould halves 
due to a wear, deformation and manufacturing error of the metal moulds. 
When such metal mould is used if the rate of rise in the pressure applied 
to the molten metal were too high fins or flashes would be formed. 
Accordingly, it is necessary to slightly slow down (of the order of 1/1000 
second) the rate of pressure rise at the sacrifice of the quality of the 
casting. 
It has been considered that it is desirable to inject the molten metal at a 
constant speed into the mould cavity by the injection plunger and the die 
cast machine has been designed to meet this requirement. Thus, in the 
first stage of the injection, for the purpose of preventing a wave from 
being formed on the surface of the molten metal poured into an injection 
sleeve and exhausting the air in the injection sleeve to the outside of 
the metal mould through an air vent, it is usual to move the injection 
plunger at a relatively slow constant speed or to gradually accelerate the 
plunger from a relatively low initial speed so that the plunger will 
attain a constant high speed when the molten metal reaches the gate of the 
metal mould. 
However, the temperature and fluidity of the molten metal decrease with 
time so that the resistance to the movement of the injection plunger 
increases with the injection stroke. Accordingly, the injection plunger of 
the prior art machine does not move at a constant speed during the 
injection stroke but decelerates as the stroke proceeds. With such 
injection speed characteristic, as the speed of the molten metal flowing 
through the gate of the metal mould during the later stage becomes smaller 
than that of the molten metal flowing during the early stage with the 
result that the molten metal injected into the mould cavity becomes 
discontinuous thus entraining air bubbles in the casting, dislocations 
(phenomenon wherein separated metal portions do not fuse again), and 
surface defects of the cast products. 
SUMMARY OF THE INVENTION 
Accordingly it is an object of this invention to provide an improved die 
cast machine capable of obviating the difficulties described above. 
Another object of this invention is to provide an improved die cast machine 
wherein the speed of the injection piston can be varied in accordance with 
a predetermined position thereof, or a predetermined time corresponding to 
said predetermined position during the forward movement of the piston 
thereby improving the quality of the casting. 
Still another object of this invention is to vary the pressure applied to 
the molten metal injected into a mould cavity at the end of the forward 
movement of the injection piston. 
According to this invention, these and other objects can be accomplished by 
providing a die cast machine including an injection cylinder and a piston 
contained therein for operating an injection plunger for injecting molten 
metal into a mould cavity, said piston dividing the interior of the 
cylinder into a fore chamber and a rear chamber, wherein pressurized gas 
from a gas accumulater is admitted into the rear chamber for advancing the 
piston and the injection plunger for injecting the molten metal into the 
mould cavity, and pressurized liquid is admitted into the fore chamber to 
retract the piston thereby forcing the gas in the rear chamber back into 
the gas accumulator, characterized in that a control valve means is 
connected in the discharge path of the pressurized liquid from the fore 
chamber, that the flow quantity of the pressurized liquid discharged form 
the fore chamber and flowing through the valve means is controlled in 
accordance with the position of the piston during the forward movement 
thereof thereby varying the speed of the piston, that valve means is 
connected between the rear chamber of the injection cylinder and a 
plurality of gas accumulators, and that the valve means is controlled so 
as to adjust the rate of increase of the pressure applied to the molten 
metal at the final stage of the injection casting.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A preferred embodiment of this invention shown in FIG. 1 comprises an 
injection cylinder 1 containing a piston 2 provided with a piston rod 3. 
The piston 2 divides the interior of the cylinder 1 into a fore chamber A 
and a rear chamber B. Pressurized liquid is supplied from source 4 to the 
fore chamber A via a check valve 6, conduits 9, 10 and 11 and a direction 
transfer valve 7 which is used to change the direction of flow of the 
pressurized liquid supplied by the source 4. The transfer valve 7 is 
actuated to move from position b to position c by an electrical signal 
from a limit switch or timer, or a mechanical signal from a cam (all not 
shown) which are produced when the piston rod 3 advances to a 
predetermined position. The fluid passage at position c is made larger 
than that at position b. In other words, the rate of flow is larger at 
position c than at position b. 
For the purpose of varying the rate of flow of the pressurized liquid 
supplied to the fore chamber A of cylinder 1 the lefthand section of 
transfer valve 7 is constructed as shown in FIG. 7. Thus, the transfer 
valve 7 comprises a casing 71, and a spool having three spaced lands 72, 
73 and 74. Port 75 is communicated with the fore chamber A of cylinder 1, 
port 76 with source 4 and port 77 with a reservoir 5 to be described 
later. The intermediate land 73 is provided with a reduced diameter 
portion 73a and a conical portion 73b. In the position shown, the chamber 
A is communicated with the reservoir through ports 75 and 77. When 
pressurized liquid is supplied to port 78 under the control of an 
electromagnetic valve, not shown, or a mechanical force is applied to the 
lefthand side of the land 72, the spool is moved toward right. Then, a 
small quantity of the pressurized liquid is supplied to chamber A from the 
source 4 via the reduced diameter portion 73a. As the spool is moved 
further, the flow rate of the pressurized liquid is increased gradually by 
the conical portion 73b. Finally, the land 73 is moved to a position 
intermediate of ports 75 and 77 thus fully communicating ports 75 and 76 
and interrupting the communication between ports 75 and 77. 
Accordingly, in the coarse of transferring from position b to position c, 
the flow rate is controlled in accordance with the stroke of the spool. If 
the speed of the spool is controlled, it is possible to control as desired 
the rate of increase in the flow quantity per unit time. Further a flow 
control valve 8 is provided for controlling the flow quantity of the 
pressurized liquid which flows from the valve 7 to a reservoir 5 when the 
transfer valve 7 is moved to position c so that the maximum speed of the 
liquid will not exceed a predetermined value during the forward movement 
of the injection piston. A low pressure gas accumulator 15 is connected to 
the rear chamber B through a check valve 18 and a conduit 23 whereas a 
high pressure gas accumulator 14 is connected to the low pressure gas 
accumulator 15 through a pressure reducing valve 16 and a conduit 24 for 
supplementing energy when piston 2 advances. The accumulators 14 and 15 
are connected to the rear chamber B via conduits 21 through 24. A check 
valve 17 is connected in conduit 25 for passing gas from chamber B to high 
pressure accumulator 14 while check valve 18 passes the gas from the low 
pressure accumulator 15 to chamber B. In series with conduits 21 and 22 
are provided a gas interception valve 19 operated by an electric coil (not 
shown) or fluid pressure or an external force to intercept the flow of gas 
to the chamber B from the high pressure accumulator 14 and a flow control 
valve 20. 
The operation of the die cast machine will now be described. At first the 
relationship between the pressures of the gasses in the high and low 
pressure gas accumulators 14 and 15 will be described. When the pressure 
receiving area A.sub.1 of piston 2 in the fore chamber A is made to be 
smaller than that B.sub.1 in the rear chamber B, and when the pressures of 
the liquid and gas are denoted by P.sub.L and P.sub.G respectively, to 
retract (to move toward right) the piston 2, it is necessary to establish 
a relation PG &lt; PL .times. (B.sub.1 A.sub.1). Further, the pressure 
reducing valve 16 is adjusted such that it sets the pressure of the gas 
from the low pressure accumulator 15 and acting in the rear chamber B to a 
predetermined value during the forward movement of piston 2. Further, the 
gas interception valve 19 is maintained in the closed state as shown in 
FIG. 1. 
After setting the pressures of the liquid and gas in this manner, the 
source 4 of the pressurized liquid is operated. Then the pressurized 
liquid is supplied to the chamber A via conduit 10, check valve 6, conduit 
11, transfer valve 7 at position a and conduit 9, so that the piston 
retracts until it is stopped at a limit position. During this retraction 
stroke the gas in chamber B is forced back into the high pressure 
accumulator 14 through check valve 17 and conduit 25 thereby storing 
energy. 
To inject molten metal into the mould cavity, the direction transfer valve 
7 is switched to position b where it restricts the flow quantity. Then, 
the liquid in the fore chamber A is discharged into reservoir 5 via 
conduit 9, transfer valve 7, conduit 12, flow control valve 8 and conduit 
5. On the other hand, the gas in the low pressure accumulator 15 flows 
into chamber B and the gas discharge from accumulator 15 is supplemented 
by the gas from the high pressure accumulator 14 through pressure reducing 
valve 16. Consequently the gas pressure in chamber B is set by valve 16 so 
that the piston 2 is advanced at a relatively low speed. The speed of 
advancement is controlled by the degree of opening of transfer valve 7 at 
position b and the flow control valve 8. 
When the piston rod 3 reaches a predetermined position the direction 
transfer valve 7 is switched from position b to position c permitting 
larger flow quantity so that the quantity of the liquid discharged from 
chamber A increases thus advancing the piston 2 at a higher speed to 
inject at a higher speed the molten metal in an injection sleeve 27 into 
the cavity of a metal mould, not shown, by an injection plunger 26 secured 
to the outer end of pistion rod 3. 
It is a feature of this invention that the flow quantity of the liquid 
discharged from chamber A is increased during an interval while the 
transfer valve 7 is switched from position b to position c, and that the 
flow quantity per unit time is also controlled. Accordingly, by switching 
the direction transfer valve 7 from position b to position c while the 
injection plunger 26 is injecting the molten metal into the cavity of the 
metal mould, that is while the piston 2 is moving at a high speed, it is 
possible to move the injection plunger at any desired acceleration rate. 
Accordingly, the discontinuity of the flow of the molten metal injected 
into the mould cavity through the gate can be prevented efficiently thus 
producing a high quality casting. The flow quantity of the liquid is 
controlled by control valve 8 so that the forward speed of piston 2 will 
not become excessive. 
In the pressurizing step in which the injection plunger 26 applies a 
pressure to the molten metal filled in the mould cavity, that is in the 
step in which the piston 2 transmits pressure to the injecton plunger 
after the high speed advancement of the piston 2 has been completed gas 
interception valve 19 is opened to supply high pressure gas into the rear 
chamber B from the high pressure accumulator 14 via conduits 12 and 22 and 
valves 19 and 20. 
This arrangement provides another feature. The variation in the gas 
pressure in chamber B and the variation in the speed of piston 2 with 
reference to an interval T in which the piston 2 starts its forward 
movement at a low speed and completes its high speed movement are shown by 
the graphs depicted in FIGS. 2 and 3 respectively in which the ordinate 
represents the gas pressure P in chamber B and the speed V of piston 2 
respectively, and the abscissa represents the interval or time T during 
which piston 2 and injection plunger 26 advance. A solid line Pa shown in 
FIG. 2 shows one example of the variation in the gas pressure in chamber B 
whereas solid line Va in FIG. 3 shows one example of the variation in the 
speed of the piston 2. 
As can be noted from FIG. 2 during the intervals t.sub.o - t.sub.1 and 
t.sub.1 - t.sub.2 in which the piston 2 is advancing at low and high 
speeds respectively the pressure in rear chamber B is substantially the 
same as that in the low pressure accumulator 15. However at point t.sub.2 
where the high speed forward movement of piston 12 completes, that is the 
injection of the molten metal into the mould cavity has been completed and 
the piston stops, the gas in the high pressure accumulator 14 is supplied 
into the chamber B through gas interception valve 19 and flow control 
valve 20 so that the pressure increases rapidly. By opening valve 19 
before or after point t.sub.2 it is possible to increase the pressure at a 
point before or after point t.sub.2 as shown by dotted lines in FIG. 2. 
Further, it is possible to adjust the time required for pressure rise by 
adjusting the degree of opening of the flow control valve 20. In this 
manner, as it is possible to increase the pressure in the rear chamber, at 
or before or after the point of completion of the high speed forward 
movement of the piston 20, no water hammering phenomenon occurs in the 
cylinder chamber B in any range of adjustment. 
To retract the piston 2, direction transfer valve 7 is returned to position 
a and the gas valve 19 is closed. Then the pressurized liquid is supplied 
to the fore chamber A from source 4 via conduits 10, 11 and 9, check valve 
6 and transfer valve 7 so as to retract piston 2. Then, the gas in chamber 
B is returned to the high pressure accumulator 14 through check valve 17 
and conduit 25 thus storing energy. As described above, the gas in the 
high pressure accumulator 14 is supplemented to the low pressure 
accumulator 15 each time the piston advances. 
FIG. 4 shows a modified embodiment of this invention in which elements 
corresponding to those shown in FIG. 1 are designated by the same 
reference numerals. In this modification, a cylinder 28 is connected to 
the fore end of cylinder 1 to form a chamber C. Passage 29 and a sealing 
packing 30 are provided for the rear end of the piston rod 3 and piston 2 
so that when the piston 2 approaches its forward limit the pressurized 
liquid supplied to chamber C via conduits 44 and 45 and check valves 6 and 
58 is supplied to the inside of sealing packing 30 via passages 29 to 
provide an efficient seal between the inner wall of cylinder 1 and the 
periphery of piston 2. For the purpose of preventing misoperation of the 
piston 2 there are provided a pilot check valve 31 and a safety check 
valve 32 which are connected to be opened by the pressurized liquid from 
source 4. Thus, when the supply of the pressurized liquid is stopped due, 
for example, to the interruption of electric supply, safety check valve 32 
is closed by pilot valve 31. A pilot check valve 33 is provided for 
advancing piston 2 at a high speed. When piston 2 advances to a 
predetermined position, an electric, hydraulic or mechanical signal is 
generated to operate a direction transfer valve 34 for discharging the 
pressurized liquid in the check valve 33, thus opening the same. 
Accordingly, the pressurized liquid in cylinder chamber A is discharged 
into reservoir 5 via conduits 47 and 51 and valves 32 and 33 thus 
permitting piston 2 to advance at a high speed. The check valve 33 is 
provided with a maximum flow quantity controlling handle 35 for adjusting 
the degree of opening of the valve 33 and hence the maximum speed of the 
piston. A flow control valve 36 is included between conduits 55 and 56 
from transfer valve 34 for controlling the flow quantity of the 
pressurized liquid passing through check valve 33 thus fastening or 
delaying the opening of the check valve 33. An accumulator 37 having a 
large capacity and an accumulator 38 having a small capacity are connected 
to cylinder chamber B through conduits 39, 40 and 41, and a check valve 18 
is connected between accumulators 37 and 38. A gas interception valve 42 
is connected in the conduit 40. Valve 42 is opened and closed by the 
pressure of the pressurized liquid in chamber A or the pressure of the gas 
in the small accumulator 38. When valve 42 is opened the gas in the large 
accumulator 37 is supplied to chamber B but when valve 42 is closed the 
gas in the small accumulator 38 is supplied to chamber B. Gas interception 
valve 42 is provided with a flow rate controlling handle 43. Thus, by 
controlling the speed of opening the gas valve 42 the rate of pressure 
rise in chamber B can be controlled. 
The modification shown in FIG. 4 operates as follows. Similar to FIG. 1, 
the relationship between the liquid pressure of source 4, and the gas 
pressures in the large and small gas accumulators 37 and 38 is established 
to satisfy a relation (A.sub. 1 /B.sub. 1).times. P.sub.L &gt;PG where 
A.sub.1, B.sub.1, P.sub.L and P.sub.G have the same meaning as above 
described. Further, the operating coil (not shown) of the direction 
transfer valve 34 is deenergized, check valve 33 and gas interception 
valve 42 are maintained in the closed position. 
Then the source 4 is started to supply the pressurized liquid to pilot 
check valve 31 via check valve 6, conduits 44 and 58, thus opening the 
pilot check valve 31. Consequently safety check valve 32 is also opened. 
Assume now that the direction transfer valve 7 is held in position a under 
these conditions. Then the pressurized liquid is supplied to chamber A 
through conduits 48 and 49, transfer valve 7, conduit 50, safety check 
valve 52 and conduit 47. On the other hand, since chamber B is 
communicated with small gas accumulator 38 via conduits 41 and 39, piston 
2 is retracted. Under these conditions the gas in chamber B is forced back 
into large and small accumulators 37 and 38 thus storing energy. 
When the direction transfer valve 7 is moved to position b, the liquid in 
chamber A is discharged into reservoir 5 via conduit 47, safety check 
valve 32, conduit 50, and through the controllable passage in transfer 
valve 7 at a relatively small flow rate. Accordingly, the piston 2 begins 
to advance at a low speed by the gas from the small gas accumulator 38. 
The speed of the piston can be controlled according to the degree of 
opening of the transfer valve 7. 
When the piston rod advances to a predetermined position, the direction 
transfer valve 34 is actuated to open pilot check valve 33. Then the 
liquid in chamber A is discharged into reservoir 5 via conduit 51, check 
valve 33 and conduit 57 so that piston 2 moves at a high speed. In this 
modification, the opening speed of check valve 33 is controlled by the 
degree of opening of the flow control valve 36 so that it is possible to 
gradually or rapidly change the speed of piston 2 from low speed to high 
speed and to vary the acceleration of the piston. The maximum speed of the 
piston under various conditions is determined by the maximum flow quantity 
controlling handle 35. 
Considering the liquid pressure in chamber A and the gas pressure in 
chamber B during the forward stroke of the piston, as the pressure 
receiving area A.sub.1 of the piston 2 is smaller than the pressure 
receiving area B.sub.1 the liquid pressure is higher than the gas 
pressure in reverse proportion to the ratio of areas A.sub.1 and B.sub.1. 
Accordingly the gas interception valve 42 is maintained in the closed 
condition by the liquid pressure applied thereto through conduits 47 and 
52. While the piston is moving forwardly as the gas in the small 
accumulator 38 is continuously supplied to chamber B, the gas in the 
accumulator 38 expands and decreases its pressure. 
When the injection plunger 26 completes filling of the molten metal in the 
sleeve 27 into the mould cavity, that is when the piston completes its 
high speed forward movement and stops, the pressure in chamber A instantly 
decreases to atmospheric pressure so that the pressure in conduit 52 
decreases also. Consequently, the gas interception valve 42 is opened by 
the gas pressure in the small accumulator 38 and acting through conduit 59 
whereby the gas in the large accumulator 37 flows into chamber B. 
In this embodiment, the time of pressure rise in chamber B can be adjusted 
to any desired value by varying the degree of opening of gas interception 
valve 42 by flow quantity control handle 43 thus varying the quantity of 
gas flowing into cylinder B. Thus, when the degree of opening of the gas 
interception valve 42 is reduced by the manipulation of the flow quantity 
control handle 43, the gas pressure in chamber B increases slowly, whereas 
when the degree of opening of the gas interception valve is increase, the 
gas pressure in chamber B increases rapidly. 
The variations in the gas pressure in chamber B and the speed of piston 2 
with reference to time are shown by the graphs shown in FIGS. 5 and 6 
where the ordinate represents the gas pressure P (FIG. 5) and piston speed 
V (FIG. 6) and the abscissa represents the time T. Solid line Pb in FIG. 5 
shows one example of the variation in the gas pressure in chamber B and 
solid line Vb in FIG. 6 shows one example of the speed variation of piston 
2. 
As can be noted from FIG. 5, inasmuch as the gas pressure in chamber B is 
governed by the expansion of the gas in the small accumulator 38, the 
pressure decreases with the advancement of piston 2 and rises rapidly when 
the high speed forward movement of the piston completes. The rate of 
pressure rise can be controlled variously by the manipulation of the 
control handle 43 as shown by dotted lines. As shown by solid curve Vb 
shown in FIG. 6, the forward high speed of piston 2 is caused to increase 
with time and accelerated near the end of the forward stroke by the 
manipulation of the maximum flow quantity control handle 35. This rate of 
speed increase can be varied variously as shown by dotted lines. In this 
manner it is possible to impart an ideal motion to the injection plunger 
26 for injecting the molten metal into the mould cavity and applying a 
suitable pressure to the injected molten metal. 
To retract the injection plunger 27 the direction transfer valves 7 and 34 
are switched to the position shown. Then the pilot valve 33 is closed by 
the pressurized liquid supplied thereto through conduit 53 and the 
pressurized liquid is supplied to chamber A from source 4 via check valve 
6, conduits 48 and 49, transfer valve 7, conduit 50, check valve 32 and 
conduit 47. As a result, piston 2 is moved to the right and the gas in 
chamber B is forced back into the large and small accumulators 37 and 38 
to store energy. 
In this embodiment, although the connection between the cylinder 1 and the 
gas accumulators is slightly different from that shown in FIG. 1 injection 
plunger 26 is operated in the same manner. 
Furthermore, instead of introducing gas in chamber B and liquid in chamber 
A, the same object can also be accomplished by introducing gas in chamber 
A, liquid in chamber B and moving cylinder 1 while maintaining piston 2 
stationary. 
As above described, according to this invention it is possible to fasten or 
delay the time of pressure rise in the mould cavity when the injection 
plunger completes its high speed forward movement and to vary the rate of 
pressure rise. It is also possible to cause the injection plunger to 
smoothly transit from a low speed to a high speed forward movement and to 
vary the rate of acceleration during the high speed movement. Consequently 
it is possible to improve the quality of the cast product. According to 
this invention materials that could not be die cast can be satisfactory 
cast.