The Effect of Intensification on
Die Casting Quality
The purpose of intensification in cold
chamber die casting is to reduce "gas and shrinkage porosity" to
its minimum level. That is, intensification does not eliminate porosity, it
merely compresses it to an acceptable level.
Intensification is a controlled increase
of the metal pressure at the end of the die cast "shot" immediately
following impact or "cavity full". It is accomplished by increasing
the hydraulic pressure above the "nominal" pressure by one of the
following means: shifting to alternate relief valves, opening high pressure
accumulators, or operating "multipliers" also called cylinder intensifiers.
A less common approach is referred to as "PRE-FILL" with a variation
of that being the regenerative circuit with a regulated "back pressure".
First some definitions.
Cavity pressure: Cavity pressure is the
hydraulic pressure of the molten metal acting on the die cavities and components
such as slides and ejector pins. For example 6000 P.S.I..
Projected area: The net area of the die
cavities, overflows, chill blocks, gates, runners, biscuit, and heavy flash,
which has molten metal injected into it under pressure. For example 280 square
inches.
Tonnage: Normally the amount of pre-load
the toggle linkage mechanism applies to the die halves when the tie bars are
"stressed". This "tonnage" is the result of stretching
the tie bars. On an 800 ton machine this stretch is as much as .125 in. over
the length of a 7 in. diameter "4140" high tensile tool steel tie
bar!
Blowing the die: The die cavity is an
air tight enclosure with only vents to atmosphere to remove captive air. Flash
or die blow occurs when the tonnage of the machine is exceeded by applying
excessive cavity pressure to the "projected area". For example 280
sq. In. X 6000 P.S.I. = 1,680,000 P.S.I./2,000 LBS./TON = 840 TON. When
the tonnage is exceeded a "fuse" will "blow", relieving
the excess tonnage. In most cases this fuse is the tie bars stretching, but
in some cases is the platen, linkage, or the die holder block distorting.
Once the excess pressure is relieved the die faces try to reseat, however
most times this can not be done because flash is now tightly "coined"
onto the face of the die.
Delay time: Delay time is defined as the
amount of elapsed time from "impact" or the end of the cavity filling
cycle to the point where the "intensifier pressure" begins to rise.
This time is normally measured in milliseconds with an acceptable range being
15 to 200 milliseconds. This is best defined by the Process Engineer at the
time of sample. The procedure is to start out with the fastest rise time that
doesn't blow and is otherwise stable.
Rise time: Rise time
is defined as the time required for the intensifier to reach its maximum pressure.
Some times it is acceptable to consider intensification complete when it reaches
80 to 85% of maximum.
Intensifier pressure:
As stated the intensifier pressure is the maximum hydraulic pressure generated
by the circuit at the end of the shot in an appropriate period of time.
The effectiveness of the intensifier is
directly related to the total die, machine, and process design working in
harmony.
Following is a brief detail of each of
the most common types of intensifier circuits found in American die casting
plants.
1. The oldest type
of hydraulic intensifier is the "pump" intensifier.
This system is often referred to as a
"high pressure shot". This circuit sometimes utilizes a timer to
initiate a relief valve to "shift" the system into a higher than
normal pressure. A limit switch instead of a timer provides improved repeatability.
This circuit is subject to the limitations of the pump design working with
the fire retardant fluids required in die casting. Normally the maximum pressure
with this type of system is 2,000 P.S.I. Also the circuit is very slow, with
response normally in the 250 to 2500 millisecond range. Manufactures who used
this type of intensifier typically used larger shot cylinders to develop the
required force at the 2,000 P.S.I. maximum pressure. The larger cylinders
required for this circuit placed a large demand on the fast shot circuit and
in many cases also limited the fast shot velocity due to the limited availability
of large P.O. check and directional valves.
2. A more popular type of intensifier
is the "multiplier".
A hydraulic cylinder is coupled, some
times in "piggy back" style to the top of the shot cylinder. The
output from this cylinder comes from the rod or in some cases "piston
minus rod" area of the cylinder. An intensifier is rated as a ratio of
the piston (input area) to the rod (output), such as 4::1. For example a 4::1
which is common on "CASTMASTER"tm and "HPM"
tm diecast machines would develop 4,000 P.S.I. output from a 1,000 P.S.I.
input. The early multipliers drew their fluid supply from the shot cylinder
piston side during and at the end of the shot. The only controls were a flow
control valve which restricted the speed of the cylinder extending, therefore
the "rise time". Multipliers with only flow controls can be very
responsive, approaching the 20 to 100 millisecond range however they have
been shown to be unreliable, as a dragging tip can cause the multiplier to
extend prematurely, resulting in an un-intensified casting. More sophisticated
multipliers used "pressure reducing valves" to regulate the maximum
input pressure, therefore limiting the output pressure, however the pressure
reducing valve is extremely slow and creates a 150 to 250 millisecond delay
which is never stable. The addition of a solenoid operated directional control
valve operated by a limit switch will increase the repeatability of the process.
2a. An option to the above is a multiplier with
its own dedicated accumulator bottle, pressure control, and directional control.
These have several advantages including: Response in the 15 millisecond range,
adjustable to 500 milliseconds as desired. Also the output pressure is matched
to the capabilities of the particular job and machine combination.
Advantages to circuits with multipliers are that as a result of the
higher potential pressures the shot cylinders were sized with smaller diameter
bores by the manufacturers. This resulted in less gallon per minute demand
on the fast shot valving and in most cases higher fast shot capabilities than
systems using larger shot cylinders.
Disadvantages were that the higher pressures result in accelerated
wear in piston seals and rings. Thus requiring more frequent maintenance.
3. Another popular type of intensifier
is the pump charged accumulator or "bottle".
These are also responsive by nature, with
typical performance in the 15 to 500 millisecond range. As in the example
above pressures can be matched to the die by regulating the pump pressure.
They are subject to the pressure limitations of the pump manufacturer, which
may be as low as 2,000 P.S.I. A flow control valve must be utilized to control
the rise time, as it is possible to create a secondary "impact spike"
on the die causing excessive flash. The shot cylinder bore sizing is similar
to that used on pump intensifier circuits with similar limitations on fast
shot performance.
4. The most unusual
type of intensification circuit now being used is on "UBE"
tm machines with a "RUN AROUND" circuit.
The shot cylinder on these machines makes
the "FAST SHOT" approach in "REGENERATIVE" OR "RUN
AROUND", that is, both the piston and rod side have bottle pressure open
to them simultaneously. Because of this the metal pressure is limited by the
differential area of the piston to the rod side. At impact, a pilot operated
check valve, piloted by a sequence valve, shifts to allow pressure to "EXHAUST"
to the tank through a "BACK PRESSURE" relief valve. The "BACK
PRESSURE" relief valve is adjusted in order to achieve the desired cavity
or metal pressure. The lower the "BACK PRESSURE", the higher the
cavity pressure. At this time, the only other adjustment which can be made
is the "delay" time, at which the pressure decay will begin. "Rise
time" or in this case "pressure decay time" is not adjustable
in the sense of changing the shape of the curve.
5. The "PRE FILL" circuit is
popularly found on "LESTER" tm die cast machines and
was also used in very limited production by "PRINCE" tm
In this design the shot cylinder rod is
"hollow" and "telescopes" over a stationary tube,
which isolates it from the "main" piston. For fast shot the rod,
which has a much smaller area, and there fore requires a much smaller fluid
supply, is fed by the accumulator. During the fast shot, oil supply
from a large "PRE-FILL" valve mounted to the "head" of
the shot cylinder. The "PRE-FILL" valve has a small reservoir mounted
above it which "gravity" feeds, on demand, fluid to the main piston.
Due to the large diameter of the shot piston utilized there is a huge gallon
per minute demand on the system. The "PRE FILL" delivers that with
a minimum of "horse power" since only the rod area is receiving
"high pressure" fluid. At impact the "PRE FILL" is closed
and "high pressure" is allowed to "charge" the piston
area of the cylinder. This "high pressure" applied to the piston
generates the forces needed for intensification.
If you need assistance understanding or
troubleshooting your hydraulic or intensifier circuits contact us by one of
the following methods.
By:
Bob McClintic
Contact
Bob @: RMcClintic@DrDieCast.com
Other technical articles on die casting by Bob McClintic
Copyright Bob McClintic & Associates 1998 to 2024
Revised: June 28,2024