Low pressure die casting system

Details Low pressure die casting system Low pressure die casting system

2000-09-12

2001-07-10

Lin, Kuang Y. (Department: 1722)

Metal founding

Control means responsive to or actuated by means sensing or...

Control of feed material enroute to shaping area

C164S312000

Reexamination Certificate

active

06257313

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is generally directed toward low-pressure die casting and, more particularly, toward a pressure control system and method for controlling the pressurization of low pressure die casting machines.
2. Description of Related Art
With reference to
FIG. 1
, a conventional die casting machine
10
is shown to include a crucible
12
, a movable upper die
14
, and a fixed lower die
16
. The upper die
14
moves from an upper position spaced from the lower die
16
to a lower position abutting the lower die
16
. Seals (not shown) fluidly seal the upper die
14
to the lower die
16
. Sand-based cores
20
are received between the upper and lower dies
14
,
16
, and are used as the mold for forming cast parts.
The crucible
12
, which receives and contains molten aluminum
22
, has a pressurization input
24
by means of which pressurized air is introduced into a chamber or space
26
in the crucible
12
relatively above the molten aluminum. One or more riser tubes
28
conduct molten aluminum
22
from the crucible
12
upwardly to the dies
14
,
16
. Pressure developed in the space
26
forces the molten aluminum upwardly through the riser tubes
28
and around the sand cores
20
. Typically, air pressure of between about 3-20 psi is used in low pressure die casting operations.
With reference to
FIG. 2
, a conventional control system for the die casting machine of
FIG. 1
is schematically illustrated. The conventional control system includes a master programmable logic controller (master PLC)
30
, a pressure controller
32
, and a series of remote sensing and input/output stations or units
34
.
Each of the remote units
34
provide input/output transfer and signals indicative of a sensed parameter, such as temperature and physical condition of various components (i.e., dies open/closed, valve open/closed etc.). The master programmable logic controller
30
(master PLC) holds the main program for die casting machine control. The pressure controller
32
receives, from the master PLC
30
, signals generated by the remote units corresponding to various sensed parameters to start pressurization.
With reference to
FIG. 3
, a typical mass production recipe is illustrated. The mass production recipe program
38
includes an initial pressure ramp-up period (A), a subsequent constant-pressure period (B), and a pressure exhaust or release period (C). During the initial pressure ramp-up period, pressure within the dies increases. During the constant-pressure period (B), pressure within the dies should remain constant or static. During the pressure release period, air pressure is released from the crucible. Thereafter, the dies are opened, the cast parts are removed, and the dies are prepared for a subsequent casting cycle.
With continued reference to
FIG. 3
, the pressure ramp-up period includes a first portion (A′) and a second portion (A″). During the first portion (A′), pressurized air is introduced into the space
26
in the crucible
12
above the molten aluminum
22
and begins to force the molten aluminum up the riser tubes
28
toward the dies
14
,
16
. During the second or subsequent portion (A″) of the ramp-up period (A), molten aluminum is forced out of the riser tubes
28
and between the dies. The second portion (A″) of the ramp-up period (A), which immediately precedes the constant pressure period (B), essentially ends when the dies
14
and
16
are full of molten aluminum.
During the constant-pressure period (B), the molten aluminum in the core
20
solidifies. Following the constant-pressure period (B), pressure is exhausted (C) from the crucible, the dies
14
,
16
are opened, the formed part and cores
20
are removed from the dies, and the dies are prepared for a subsequent molding operation. The pressure ramp-up period (A) is much shorter than the constant pressure period (B). Typically, the pressure ramp-up period (A) is between about 10-20 seconds in length whereas the constant pressure period (B) is between about 200-400 seconds in length, depending upon the part being cast.
The great disparity between the relative length of the pressure ramp-up and constant pressure periods (A, B) has resulted in the prior art system not being able to numerically display or track pressure during the pressure ramp-up period (A). Accordingly, as shown in
FIG. 3
, the user has no numeric display of the difference between actual and desired pressure
40
,
38
during the pressure ramp-up period (A). Rather, the system only shows the actual pressure
40
during the constant pressure period (B).
The aforementioned control system and method has generally worked satisfactorily in the past, but suffers from several disadvantages. Firstly, low-pressure die casting machines have numerous seals that have a tendency to leak over time. Unfortunately, the conventional system is ill equipped to compensate for such leakage. Accordingly, there tends to be wide variations in the actual pressure as compared to the desired or recipe pressure. Typically, a variation of ±8% between the actual pressure and desired recipe pressure occurs with the conventional system.
Also, in the conventional system, there is no means to monitor the system for gross pressure loss or lack of pressure at the beginning of the pressurization cycle (during the ramp-up period A), which would be indicative of potential catastrophic failure. As noted previously, the actual pressure
40
is not numerically displayed during the pressure ramp-up period, and no control action is taken if the actual pressure deviates significantly from the desired recipe pressure. Catastrophic failure could be the result of, for example, misalignment of the dies
14
,
16
, a missing sand core
20
, or failure of the seals between the dies. Therefore, in the conventional system it is possible for molten aluminum
22
to be introduced into the dies
14
,
16
and to leak from the dies and out of the casting machine
10
, possibly causing a fire or explosion.
Finally, in the conventional system, if there is a malfunction of the pressure controller, which is proprietary, the entire die casting machine is inoperable. Such a malfunction could simply be a loss of the display unit for the pressure control system. Therefore, it is necessary to retain in inventory replacement components that are specifically dedicated to the conventional pressure control system in order to avoid or minimize costly machine downtime.
Therefore, there exists a need in the art for a pressure control system that will more accurately control the actual pressure to track the desired pressure. There also exists a need in the art for a pressure control system that will anticipate and prevent catastrophic failure. Finally, there exists a need in the art for a low pressure die casting control system that uses standard, commercially available components.
SUMMARY OF THE INVENTION
The present invention is directed toward a control system for a low-pressure die casting machine wherein the pressure within the dies more accurately tracks the desired recipe pressure. The present invention is also directed toward a control system that tracks initial pressurization in the dies with real time data collection and stops the casting operation should the detected pressure be indicative of a catastrophic failure. The present invention is also directed toward a method for controlling the die casting system to minimize the occurrence of catastrophic failures. The present invention is further directed toward a control system that has generic, easily replaceable components and, thus, can be quickly repaired to reduce machine downtime should any component fail.


REFERENCES:
patent: 4741381 (1988-05-01), Nishida et al.

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