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.