Ah yes, that is the question. In the early days of die casting it was not the question. Castings on the runner had plenty of time to cool in the baskets that carried the from the die casting machine to the trim press. These days most die casting plants have one trim press

per die casting machine to minimize the in process inventory. The challenge is to cool a shot of castings from the 425 deg C (800 F)

down to room temperature, so that it is cool enough to trim, It is also useful to have a few shots of buffer so that the trim operation does not have to be perfectly in sync with casting.

This discussion is colored by the weight of the shot. Shots of castings made by HPDC machines that are larger than 1100 tones are too heavy to be safely carried by an operator. Heavy shots are normally carried by robots. The cost of these robots becomes a significant factor in choosing an optimum process arrangement. This discussion will not touch on optimum arrangements for large casting cells because they are mostly OEM opetations

Starting with the smallest machines and lowest investment, we have the cooling chute. Shots are manually removed from the casting machine and stacked 10 high. Three stack of ten are usually enough to cool enough for trimming. A chute with a down slope makes transport easier.

If you develop your dies to the point where 100% of the aluminum is ejected every time a automated extractor can replace the man. This extractor removes castings and either quenches them or sets them on a cooling conveyor.. Cooling conveyors are either horizontal or vertical. Horizontal conveyors can have an exit chute that provides some uncoupling of cast and trim. I use quench tanks with a circulating pump to hold level constant and prevent concentrrating hot water at the top. A commercial aluminum oxidation inhibitor is also helpful. Sodium Silicate can be added to the water if you are manufacturing leak tight castings. Swishing the casting in the water speeds up the cooling if a short cycle time is needed. Casting are dry by trim if the time in the quench is carefully chosen.

I have also used quench tanks with dunk baskets when cooling castings without an automated extractor. If the cell has an automatic ladle and a sprayer a man can extract cool and trim.

Virtually all HPDC dies are made with thermal passages. Usually the die is cooled by flowing water through these passages. (this discussion is not about hot oil or compressed air cooling) Most die cast machine installations include ball valves to start and stop water flow through the thermal passages. These ball valves prevent waster from continuing to flow while the die is being changed. Good die casters know that it is possible to reduce start up scrap by delaying the start of water flow until after a few warm up shots have occurred. If water start up is delayed too long the in rush of cold water may split the die inserts.

Die castings is a thermal process. The best result occur if every shot is the same. I wish to remove exactly the same amount of heat every cycle. Heat removal occurs when the water is flowing. The method is flow or no flow (it is not water or no water – this splits inserts) Water flows in my die thermal passages for a constant amount of time after each shot once warm up is complete

In this computer age I prefer to build in intelligence. The first step is finding a computer controlled valve to replace the ball valve. I normally do not recommend specific brands but ASCO diaphragm valves are the only one I have found suitable for the task. (Other seasoned HPDC practitioners are welcome to chime in if they have found other good brands) This may be related to the fact that were designed for fire protection sprinkler systems (ASCO is a contraction of American Sprinkler Company) Yes they cost twice as much as the knock offs) You may not able to run the valve with the machine control but that does not prevent adding cover and ejector ASCO valves run by old fashioned timers in the panel.

My best installations are run by the machine PLC and set using the HMI. They include program features to

1. Avoid boiling in the die thermal passage
2. Run dies with split inserts
3. Allow 15 minute stops without incurring start up scrap
4. Replace hot oil units
5. Accelerate die warm up

Some machine builders like Buhler will configure new machines to include water timer control. Usually you need to specify that this is an option that you wish to purchase. I recommend a minimum of three (cover ejector & slides) separate valves.

Aluminum has an affinity for most metals. In aluminum high pressure die casting this shows up as solder build up. One useful exception is the black oxide ceramic surface layer that form on H13 (or H11) when it has a grain structure related to R 44 to 50 hardness. The high pressure die casting process is based on using this exception. (A matte surface finish to hold release is also required)

In my displayed picture I have shown a new plastic injection mold tool next to a new die cast tool. I will leave you to determine which example is the die cast tool. If you can succeed in making the correct determination (the die cast tool is on the right) you can do receiving inspection on incoming new die cast tools. Tools without the black oxide matte surface finish were made using the significantly less costly steps used to create a plastic injection mold

The only question when a “completed” die is found without a Rc 44-49 black oxide matte surface is whether the purchase specification required it.

FACADE

Unicorns only exist in our imagination. There currently seems to be a competition among businesses to portray an even more marvelous result. It is like winning the lottery. One winner sparks the imagination of millions.

I might be old fashioned. I prefer to purchase from die shops with a good track record of creating what I need. In this era of hype, I would rather have a system to evaluate the capability of a shop before I waste their time and mine quoting on dies. My evaluation system is based on asking die shops to put their best foot forward. I ask them to allow me to inspect a die which meets my die standard. Given the long lead time for designing and building a die, you cannot afford to discover deficiencies after die build is complete.

This is like being in school. I make no secret about what is going to be on the test.

Question 1 Are all insert blocks of steel tested for proper grain structure and

cleanliness prior to cutting at the die shop

Question 2 Are all insert surfaces that contact molten aluminum between Rc 44 and Rc 49 using hardness testing files to confirm

Question 3 Are inserts uniformly proud to the holder by .04 to .06 mm

Question 4 Do thermal passages leak

Question 5 Do aluminum contact surfaces have a matte black oxide finish.

Suitable die build shops pass my survey with ease. They have good working relationships with their heat treaters and steel suppliers.

Recesses are added to eliminate blind holes

I suspect that a blind person would rather not be blind. Threaded holes in die castings don’t want to be blind either. Blind threaded holes in die castings have more quality issues than through holes

STRIPPED THREADS

Cast bosses for creating blind threaded holes must be larger. This makes them more likely to be porous. It is more difficult to achieve a strong thread in a porous boss.

LEAKERS

Form taps without vent grooves like to hydraulically punch out the bottom of blind threaded holes

BOTTOMED BOLTS

Blind holes have the potential for creating a joint where the bolt is not tightened properly. For example a dog point can rest on the bottom of the drilled hole

EXTRA MACHINING OPERATIONS

A bottoming tap may be needed to created good thread that is deep enough

Die casting is a versatile process. I have pictured an example of a front engine cover that originally had blind threaded holes. Pockets were added behind the bosses to expose the back end of the threaded holes. In most cases this type of design change must be requested within a week of being awarded the casting business. This improvement also saved a CNC machining tool change. I no longer required two different lengths of step drills. The icing on the cake was that 100% leak testing went away.

Most die casters rely on spray to establish the correct die temperature to make good parts. This is not surprising. Evaporation of water quickly removes a lot of heat. A skilled manual die cast operator effectively targets and cools the hot spots with his spray wand. Most of the spray is used is water. Release agents are mixed with the water and form a thin coating on the die surface when the water evaporates. A typical release agent contains bactericide that prevents bacteria growth when it is still a concentrate.

Die casters who specialize in high volume parts have discovered that automatic sprayers have multiple spray heads. When properly implemented they reduce the amount of cycle time accomplishing spray. This assumes that the spray heads keep spraying. (plugged spray heads do not cool their portion of the die) Many shop try to mitigate spray nozzle plugging by flooding the die. I agree that the nozzles stay working longer running wide open at the expense of porosity in the casting and wasted lube.

I prefer to attack the causes of plugged nozzles. One of the big causes is bacteria growth in the spray. Water based die lubes are designed to be biodegradable when diluted with water. While this helps the environment, it causes a lot of sprayer plugging. As a minimum dumping a gallon of chlorine bleach into your central mixing system monthly is recommended. I prefer using the silver bullet. (silver ions inhibit bacteria growth) Systems that electrically add silver ions to the central lube mixer feed water, are a more effective answer. (these are sold for treating swimming pools) Bacteria growth prevention is best applied to every casting made.

Many real castings require cast holes that are not aligned with the mould direction. If the feature can be created on a pin that is less than 60mm in diameter a subcore is a good design choice. This is especially true if axis of this core needs to be skew; Many die caste struggle with subcores because they are not any easy to operate as regular core pins

ISSUES

1. HEAT REMOVAL

Subcores are effectively separated from the insert thermal passages. My subcores have 25mm rear diameters such that they contain a cascade The core cycle is timed to pull out of the casting as soon as possible. I add a second PLC to run the cores when faced with casting machines with OEM program locks

2. LUBRICATION

The subcore quickly seizes to the operating bushing if the running gap is not lubtivcated. Installing a pressurized lube pump to force grease into the gap only results in stained castings. The back side of my running fit is atmospheric vented. My lube reservoir in the die is filled once per day

3. SEIZING

I have not been able to eliminate the wear that causes a subcore to seize. My designs enable quick removal of the subcore cartridge and bushing without die removal. The bushing is 2/3 pre split on the back side to make removal and recovery of the subcore pin easier

4. BLOW BACK

Some brands of die cast machines do not maintain core hydraulic pressure during the shot. I manifold mount pilot check valves directly on the subcore cylinder to fix this. Even though many machines come with pilot checks on the core valve stack the hoses flex too much for this to be useful.

Creating a high pressure casting die design for the new 8000 ton machines would be a lot easier if you could just upscale a die design for a 4000 ton machine. The ejector pins would have twice the diameter and length. Unfortunately they would also have twice the sliding fit gap. Molten aluminum has the same properties independent of machine tonnage. It happily penetrates gaps that are twice as large. Aluminum in the gap around ejector pins effectively seizes them. This limits the maximum ejector pin diameter when casting 380 alloy to 11mm. Less fluid primary alloys like 356 can work with pins up to 15mm diameter.

Ejector pins in 8000 ton dies must deliver the same force per pin as pins in 4000 ton casting dies. Because buckling limits the maximum force that a pin can deliver. They cannot be longer than the pins used in a 4000 tin die. This strategy works well for the 8000 ton castings with shallow relief. Taller parts require more complicated designs to keep the ejector pins from buckling.

I like the position to win strategy used by Bear Bryant football coach of the University of Alabama. Because heat check increases the force on ejector pin, a pin that worked on a new die will buckle unexpectedly. Adding set screws to enable rear removal is no help whatsoever. Rear set \screws only work removing unbent pins (never the case) Whats worse is the replacement pin will buckle again almost immediately because the heat check that caused the pin failure is still there. Getting back to position to win. I initially drill the ejector pin clearance holes in the holder for threading such that a brass head hex head bolt can be easily added as a guide bushing. A column guided in the center can deliver 4 times the force. Good die designs have features that enable quick compensation for wear issues.

The discussion of the effect of scale on the die cast process begins with considering the reasons that we design in thermal passages in the die. Benefits of thermal passages can include

Cycle time reduction

Ordering of solidification

More effective die preheating

Refinement of grain structure

Transfer of heat around die

Reduction of poor fill / knit lines

Die solder control

Vent freeze off

Even though it is possible using die spray technique to run many dies without thermal passages, the yield reduction usually makes this an uneconomic choice.

As a young die cast engineer it took a while to understand why a die that started making good parts deteriorated into making scrap. Die casting is a thermal process. Scale build up in the thermal passages within the die upsets that process. And no, you do not need fancy instrumentation to measure the impact of scale build up. I have seen cores for making the fine grain structure needed to create castings with dry seal pipe threads accumulate enough scale to stop the flow in one shift. This was because boiling within the passage deposited the accumulated tower water minerals. Ostrich engineering applies. If you stick your head in the sand, you will not look for the scale build up within the thermal passages in the die.

Once you admit that your die cast process creates scale, it is possible to implement improvements. I have found that the a major reduction in scale formation is possible. Next I rank ordered a list of thermal strategies from worst to best

Tower water- steel pipes-atmospheric outlet pressure

City water- steel pipes-atmospheic outlet pressure

Hot oil-steel pipes-pressurized outlet

Closed loop hot water- stainless pipes- pressurized outlet

Closed loop de-ionized water- stainless pipes-pressurized outlet

(stainless pipes reduce rust build up)

(pressurized outlets control boiling within the die)

Do not feel bad if your plant currently uses a less favourable strategy. I can make a list because I have been there.

One of the early lessons taught to mechanical engineers is that materials deflect under applied loads. I can assure you that there is no shortage of applied loads in high pressure die casting. I thought that a 1000 ton press was massive as a new die cast engineer. Now we see 10,000 ton high pressure die casting presses. When you see the blocks of steel used to make high pressure die cast platens, it is hard to picture them flexing

This discussion is about the flexing of the ejector platen. On a 400ton die casting machine you can pretty much ignore ejector platen flexing. The ejector die itself is rigid enough to bridge across the platen face. On a 4000 ton die cast machine platen flex is an important consideration. The style of clamping mechanism is also a factor. There are four common styles.

Vertical pin axis book links

Horizontal pin axis book links

4 corner links

Hydraulic cylinder 2 platen

It is usually necessary to design the ejector die to match the style of machine clamp. Dies run in machines that they were not designed for are well known for flashing.

Large dies with slides will flash. Even if you are able to blue the die to shut off at steady state temperature, the first shot is not at steady state. Die require 30 to 100 shots to reach steady state even with preheat. Properly designed dies eject all flash every shot. This avoids die damage related to closing on flash. Remember the machine could be applying 10,000 tons on a small piece of flash