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.
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 diecasting 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
Copyright Bob McClintic & Associates 1998 to 2017
Revised: February 2017
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