Under influence of high hydraulic pressure with the die loaded on

machine (HPDC).

Components of a clamp mechanism

• Foundation and levelers • Tie bars • Tie bar adjusting nuts • Die height mechanism • Platens • Moving platen shoes or guides, also known

as traveling plate and rails. • Toggle linkage • Cross head • Closing cylinder • Hydraulic system

MAIN PARTS OF A HPDC M/C

Hydraulic system

A level machine begins with a solid floor/machine pad: – An un-level or weak floor that does not support the machine at all points will

create stress points in the machine frame and clamp system. • This will accelerate machine wear and can cause operating problems in

the die. • Frequent hydraulic leaks from cracked hydraulic lines are common.

• Leveling pads may compress over a period of time. – Recheck machine level annually to insure that the machine is not stressed

and remains within tolerance of 0.002 in/foot. (0.0508 mm/0.305 meters)1 • Tie bar has end play.

– Cover end: The tie bar should be secure and have no end play. If end play is allowed the tie bar nut will “coin” both the nut and the platen each cycle. If this is only occurring on one tie bar, eventually the reduction of pre-load on this tie bar will result in an uneven stress on all the tie bars. This can cause flashing dies, and failure of the other tie bars. As the tie bar nut becomes coined it will be difficult or impossible to remove or adjust. In addition, coining the hole through the platen can reduce the hole diameter making the tie bar difficult to remove. Damage to the platen is permanent and requires extensive welding and/or machining to repair.

• An “end bell” is used on some cover end tie bars. A pre-load is essential to prevent end play. Machining or welding may be required to eliminate end play and prevent future wear.

– Adjustable end: The tie bar adjusting nut covers should be secure and proper operating clearance maintained.

• Loose covers: – This is often the sign of a more serious problem. It is a common

practice to loosen the covers in order to free a locked up die height system. The cause of the bind is overlooked and the covers remain loose further accelerating the wear.

– The most frequent reason covers are allowed to operate loose is an out of square machine caused by worn linkage.

– The failure mode is similar to the cover end. If allowed to operate loose for extended periods, the gears, nuts and platen will become coined and require machining or replacement of the tie bar and nuts and welding and machining to repair the damaged platen.

 Die Height mechanism:

– See above  • Tie bar bushings:

– Tie bar bushings provide both lateral and vertical alignment. They are not, however intended to support the weight of either the platen or the ejector die. That is the function of platen shoes/guides.

– Regular lubrication is essential. – See platen shoe/guides below for additional information.

•  Moving platen shoes/guides: • The platen shoes and wear plates must support the platen and ejector half of

the die. Die carriers are recommended for larger dies 600 ton and above. This will reduce the wear on the tie bar bushings and guide pins in the die. – Adjust the shoes as required to maintain platen support. – Replace rail or wear strip if necessary. – Regular lubrication is essential.

• Platens: – Platens should provide a smooth parallel surface to

support the die. • Surfaces that are coined or not structurally sound can

allow the die to flex during the fast shot, impact and intensification.

– Short term solutions include welding and hand grinding the surface (if small enough area) to create a flat supporting area. For larger surfaces see below.

– Machine the platen to re-qualify the surface using portable milling equipment. (For more on portable machining see next months issue.)

– Longer term solutions include tear down, removal, welding and Blanchard grinding.

• Toggle linkage: – The toggle linkage uses mechanical advantage to develop the lock

up force by placing the linkage, platens and die in compression while elongating the tie bars.

• Worn pins and bushings result in excess clearance and uneven lock up. Wear accelerates as the clearance increases, further coining them with each cycle.

• Problems with uneven lock up/out of parallel: – Flash (wasted metal) – Inconsistent casting thickness – Slide blow (flash) and stuck slides. – Safety issues from flash – Die damage from above. – Damaged die guide or leader pins. – Frequent interruptions to clean up flash.

– In addition to placing a relatively equal force on the entire surface of the die, it also must maintain parallelism while opening the dies.

• Problems from out of parallel opening. – Broken or shearing link bolts on Harvill machines – Drags on castings – Broken or damaged core pins – Castings sticking to the cover half of the die. – Galled die guide or leader pins – Cracked castings from out of parallel ejection. Toggle systems live on lubrication. A constant supply of clean

lubricant is essential for reliable service. – Maintenance keys:

• Routine check and maintain the level in the automatic toggle lubrication system.

• Daily, weekly and monthly checks of the lubrication hoses, lines and metering blocks.

Cross head: – The cross head and bushings maintain the linkage

alignment. Depending on the manufacturer of your machine, the cross head bearing surfaces could vary greatly in design. Regardless of design, the cross head must maintain its center throughout the lock up in order to develop force on all four corners of the platens. • Four corner lock up designs such as B&T, Prince and

Quantum have a center bearing. • Other linkage systems use stationery guide bars

attached to the bumper or knock out plate on one end and the adjustable platen on the other. Examples include Birch, Cleveland, Ajay/Dejay, National/Avnet and Kux.

• Moving cross head guide bars are attached to the moving platen and move through bushings in the adjustable platens. Examples include Harvell, HPM and UBE.

– Worn cross head components also create excess side loads on closing cylinder rod bushings and seals. This results in frequent blown seals, lost fluid and more down time. •Other problems created by worn cross head

components include “over-locking”. In this situation, the main linkage is pushed past center and must tighten during the opening or “un-lock” portion of the cycle. In extreme cases, the cylinder may not have enough force to open the die and is stuck with a casting in the closed position.

– Cross head maintenance and lubrication requirements are similar to the toggle linkage.

 • Closing cylinder and hydraulic system:

– The closing cylinder and hydraulic system do not affect balance, but they do affect force and cycle time. Examples include: •Closing cylinder piston rings/seals must be

maintained to prevent blow-by. A defective piston seal can reduce the lock-up force by as much as 50%.

•An improperly adjusted hydraulic system can also reduce the locking force. An example would be a regenerative circuit that remains active also reduces the lock-up capability of the machine by 50%.

•Improperly adjusted pressure controls can drop off early resulting in reduced lock up force or excessive lock up time.

 In summary, the maintenance of the mechanical system of the machine is essential to producing quality casting consistently year after year. While it does require effort and investment, the cost is minimal in comparison to the cost of neglect.

• If you observe any of the following delays in your operation, chances are, it is due to a die that is not shutting off correctly.

• If the operator waits after the die is fully locked before pouring metal, the die is cooling and the cycle time is extended.

• If the operator delays hitting the shot button or the machine waits after the auto ladle pours, the metal is cooling in the cold chamber. This is a technique used by die cast operators around the world, to reduce flash.

• What is happening in each of the above events is that the operator is compensating for a die that is not sealing properly. The operator has learned that by cooling the metal, it will not flash as bad or maybe not at all. However, he is costing the operation money. You are throwing away BTU. The process is designed to operate at an optimum temperature. It is also designed to operate with a die that is sealed off except at the vents. When the shot is delayed, you are paying to heat the metal, and then he is cooling it below the proper operating temperature. While this may eliminate or reduce the amount of flash, this results in a casting which can have "cold" defects. The "cold" defects can result in scrapped castings, rework, customer complaints and lost business.

• The choice is to stop the process and repair the die and/or the machine to prevent the flash. Including one or more of the following:

• Squaring the machine or repairing the platen or linkage.

• "Spotting the die", Welding and resurfacing the die parting lines or faces, or slide shut off.

• Welding or resurfacing of the holder block if the vents are cut too deep or are damaged.

• Shimming the die inserts to create a shut off.

• The above have proven to reduce flash, improve part quality and increase productivity by reducing cycle time and scrap.

• If you need assistance reducing flash and improving profits, contact us by one of the following methods.

• "FLASH", it’s not as cheap as you think

• Throughout the die casting industry, it is customary for the die cast operators to keep a putty knife handy to scrape flash off the die faces. Many operators even keep them in their hip pockets, for convenience. Sometimes, I have found it almost impossible to even find a putty knife in the possession of an operator.

• While flash may be a fact of life of our process, it is not acceptable in the levels we often tolerate.

• Some misconceptions concerning flash:• 1. It helps vent the die. Wrong!, it allows uncontrolled loss of cavity pressure. It

increases the thickness of the casting, the gates, and the overflows adding weight that is not in the quote.

• 2. It doesn't cost anything. Wrong!, Flash is only 20% recoverable, if that. On a recent automotive casting for example, we "spotted the die" to correct the flash problem. The machine had an auto ladle, so it poured the same amount of metal after the repair as it did before. However when we started back up, the biscuit was 1 inch longer than it was before we "spotted the die". We were running a 3 1/2" tip. Therefore, we were loosing 9.621 cubic inches of metal every shot. That is .9621 lbs. of metal every shot! The casting was scheduled to run 8,000 pieces. /wk. at 4 cavity (which we were seldom able to do during that period, because of the problems caused by flash.) 8,000 pcs./4 cavity = 2,000 shots X 50 weeks =100,000 shots/year. X .9621 lbs. /shot =96,210 lbs./ year X $.75/lb. metal = $72,157 /year in metal X .80 (80% lost metal) = $57,726 net loss /year due to flash on this single job.

• A die that flashes .025 inch with a projected area of 94 square inches like this automotive casting, has 3.4 cubic inches of excess material. That is .34 lbs. per shot excess material. Let's say that the die runs 100,000 shots per year like this casting. That is an extra 34,000 lbs. of metal which we pay for and is not included in the quote. 34,000 lbs. X $.75/lb. =$25,500 /year!

• Trim thickness can also be a major cause of quality problems. If a slide flashes, such as happened frequently on another automotive component, it can result in trim shear. If die repair relieves the trim die while the die is flashing, then when the slide blow condition is corrected, there will be excessive trim burr remaining.

• In addition, scrap generated from lost cavity pressure results in internal porosity that is not visible from the surface. Generally, it is revealed when the casting is machined. There may be other processes prior to machining which add additional cost to the casting. These might include such operations as vibratory finish, as for the another automotive valve, shot blasting as performed on a variety of castings, or painting as done to other castings, etc. The painting on some castings nearly doubles the cost of the casting to that point! Machining occurs as a final operation. At this time 2/3 of the selling price is in the outside operations of paint and machining.

• If the defects reach the customer before they are detected, the cost of correction is further increased. We must pay travel costs, plus wages for sorting or rework in a customers facility. Travel cost alone to a nearby customer can run at least $250. Time away from the die casting plant to address the problem could be a minimum of 1 1/2 days. In addition, there are corrective action meetings to explain the cause of the defect, and to describe procedures for prevention in the future. The meetings add additional cost to the defective castings.

• 3. "I can't afford to fix it". See above. The direct cost to correct the flashing problem described above, was one person, one shift. $15./hr X 8 hrs. =$120. "If you can't afford to fix it, you can't afford to run it".

• Other problems resulting from flash:• 1. On dies with slides, flash accumulates under the slides, keeps the slide from fully seating,

and results in dimensional problems. Example, on an automotive control valve. When the slide is backed out, the valve seat has excessive machine stock, and we give away metal. The cam lock holds the die open allowing additional flash, which accelerates the accumulation of flash under the slide.

• 2. Flash accumulation under a slide can lead to compound damage on the die. One example of this is broken horn pins. This can also damage both the slide carrier and the key ways.

• 3. Safety: We have all experienced the discomfort of being burned by flying flash. In most cases, this is minor. However, worst case can result in lost time injuries and lost time.

• 4. Housekeeping: Much of the "trash" beneath the dies and on the floors around the die cast machines is flash. It becomes contaminated with die lube, tip lube, hydraulic fluid, die heater fluid, and water. This makes it virtually unusable as remelt. This is where a lot of the 80% number comes from in item 1. above.

• 5. Lost time from restarts: Accumulated over a 24 hour period, this can be a huge impact on quality, and productivity. Each restart can result in at least one and sometimes as many as three cold "start up" shots. They are either thrown out by the operator, which is the correct procedure, or later at machining after adding additional value to the casting.

• 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 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.

•

Choice of Machine• Pressure Die Casting machines are

identified by the closing force that is applied on die that is enough or sufficient

• Based on this capacity, the machine structure, hydraulic pressures and casting parameters such as casting shadow area, die size, etc. are ascertained. For the purpose, the designer must avail the machine manual.

Guiding Formula

• Closing Force required (tons) =

shadow area (sq. cm) * specific pressure (tons per sq. cm)

Computation of Shadow Area

• Shadow area is the total area of casting on moving half (includes runner, no. of cavities and overflows.)

• Specific pressure is an assumed value by experience for Aluminum Alloys:

• Assume 1 ton/sq. cm. for castings of premium grade with high strength and surface treatments

• Assume 0.75ton/sq. cm. for castings requiring high mechanical strength during field function.

• Assume 0.5ton/sq. cm. for castings for low engineering and high aesthetic performance.

Example of Shadow Area

Dosing of metalfrom the top

Die openingejection of die casting

Die fillingreal-time controlled

Venting slots

Conventional die casting

The conventional die casting is one of the most economical casting

process

The conventional die casting is one of the most economical casting

process

Production Of Castings by

HPDC

Castings are produced by

injecting Liquid Metal into the Mold

at High Pressure!

But the Injection proceeds in PHASES!

Three Phases are Basic

• Taking the liquid metal from Sleeve to the in-gate at slow speed! Called “First Phase “

• Filling the mold at high speed! Called “Second Phase “

• Compacting the liquid metal of the mold at high Pressure, during solidification! Called “Third Phase “

Objective of First Phase

• Although many die casters ignore the importance of this phase, it is the most vital. Ironically, most state of the art die casting machines have attempted technological improvement to ensure high efficiency of this phase of injection. The first phase is also termed as the "primary venting phase."

• One must understand that entrapped air in the casting system is the nemesis. Air removal (or venting) from the mold minimizes porosity in the casting. It is essential to figure out that in a closed die, air is present from shot sleeve to the tip of the mold cavity. When the plunger moves the liquid metal towards the mold, the air occupying vacant space of the mold is pushed ahead by the moving metal towards the mold. Most of the air then finds the exit through the air vents (overflows) till the liquid metal fully occupies the runner and in-gate profile. This brings about an ideal situation. In the absence of air, the runner is full of liquid metal limiting the air content to the cavity space only. With the incorporation of adequate venting, that is efficient to drive out balance air during the mold filling 2nd phase. However, there are other sources like vapor from die coatings and lubricants that render included porosity in castings..

Setting of the first phase• From the function the first phase performs, it is clear that two

parameters are important• 1. The distance to which the liquid metal needs to be carried

before the 2nd or mold filling phase is effective.• Speed at which the liquid metal needs to be dragged• Although theoretical calculation of both the above aspects is

possible, on the shop floor it is not really difficult to ascertain the limits by quick trials and minimum error. Of the two, #1 is set as follows:

• Close the heated die• Pour the liquid metal into the heated sleeve• Give the "shot" impulse and observe the plunger travel with a

hand on the • "Emergency Stop-switch” of the machine• As soon as the cam accompanying the plunger rod approaches the

limit switch, switch off the machine• Allow the dragged metal to solidify• Open the machine and observe the flow pattern with respect to

the in-gal profile.

Setting of the first phase contd.)

• If the metal has frozen short of in-gate, reset the position of the limit switch so as to increase the traverse (towards the machine platen)

• If the metal has frozen into the cavity profile, reset the position of the limit switch so as to decrease the traverse (away from the machine platen

• Repeat the trial till the condition is satisfied.• In modern machines, this setting is micro-processor based and is

much simpler and faster to establish the parameter.• This position of the switch marks the end of 1st phase and commencement of

the 2nd or mold filling (high speed) phase. It must be borne in mind that the reading on the scale fitted on the machine is specific to that machine. It can be repeated for all future production series, as long as the same machine is used. Any change of machine requires establishment of this parameter afresh following the same principle.

• To set the speed of the 1st phase, a full casting cycle is essential. For this purpose

• Set the speed of the second phase and the intensification pressure on the machine

• The heated die is closed after accurately setting the traverse of the plunger (see the steps mentioned above).

• Liquid metal of accurate shot weight dosed into the heated sleeve• Shot switch is actuated and the entire cycle is allowed• Repeat 10 to 15 shot cycles before inspecting the surface finish of the casting• Finally, heat the casting to about 350°C for 5 to 10 minutes and observe the

surface finish• How to conclude on speed results:

Setting of the first phase (Contd.)

• If the casting shows non-filling in the farthest region from the gate, the speed of 1st phase is too slow. Yet do not speed it up. Attempt pouring the metal at higher temperature, first (without changing the speed) and then decide on increasing the speed.

• If the casting is acceptable to visual inspection, conduct the heating test mentioned above. If the surface is smooth, the set speed is accurate. If there is appearance of blisters on the casting skin (also called "chicken-pox"), reduce the speed and repeat the trial till satisfied.

• The lower the filling ratio (shot volume to sleeve volume) the lower should be the speed of 1st phase.

• Conclusion:

Objective of Second Phase:

• This is the mold filling phase. It happens at high speed of injection plunger. Speeds up to 6-7m per second are also desired in some cases where surface finish is of paramount importance. Castings that are subsequently electroplated are typical cases. Not in all cases is this true. Castings for automotive application require higher strength than finish. Here 3-5 m per second is sufficient.

Relation between Second Phase and Air Venting:

• When the metal is dragged to the in-gate plane by the first phase what lies ahead is the air of the entire mold cavity. The second phase is actuated at high plunger (effectively, metal speed). The flowing metal must drive away this air completely to occupy the mold contours. When the trapped air cannot escape one of the following defects results (in ascending order of volume of trapped air):

• Blunt corners of the contour• Flow lines on surface of casting• Dispersed porosity in cut sections• Large blow-holes• Unfilled sections• Adequate provision for venting must be made in die

design itself. If needed, more venting must be incorporated even at later stages of production if there is incidence of rejection. Venting ensures both, better surface finish and internal soundness.

• Different types of venting methods are

Second Phase and Air Venting:

1. Overflow “wells”2. Straight hatches

3. Provision of ejector pins4. Serration vents

5. Combination of above methods to suit.

• All overflows must be brought with ejector pins. This ensures that the air also escapes through the sliding gap provided between the ejector hole and the pin itself. Placement of ejector pin below also ensures that the overflow bean is thrown out with the casting every single time. The length of the ejector pins must be deliberately kept shorter so that a button is formed that pushes the bean out and due to limited catching area releases the profile together with the casting. Should the pin be longer, it penetrates the overflow bean and the bean remains stuck on the pin and severed while the casting is removed from mold. In such cases most die operators find it tedious to remove it and allow the bean to remain as it is and the purpose of venting is defeated for all subsequent shots.The success of mold filling phase (second phase) is thus unthinkable without adequate venting!

Objective of Third Phase:

• This is the pressure intensification phase and is actuated by the intensifier of the machine. When the mold is completely filled and the metal starts solidify from mold-wall inward, an intensified pressure tends to push the liquid metal through the liquid core canal and feeds the portion of solidifying sections. The extent of this “feeding” is, however, limited and for this reason the casting layout must be such that the thicker sections of the casting are nearer to the in-gate. This is true of castings that have average wall thickness of 2 to 3 mm and above, having combination of thick bosses. For thin walled castings (1.5 to 2 mm) third phase actuation is neither effective nor employed! In these cases it is important that all thick sections are provided with core pins and maximum fillet radii are brought at the intersection of thin sections.

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