What Is Metal Casting?
I run a die casting cell at ABIS in Shenzhen. Been doing this since 2007. We make transmission housings and EV motor cases, mostly A380 aluminum.
Metal casting means pouring liquid metal into a mold and letting it freeze. Die casting is one way to do it. We use steel molds and push the metal in with a lot of pressure. The parts come out almost ready to ship.
Dies
The die is two steel blocks that bolt to the machine. One side stays put. The other side moves back and forth on tie bars. When they close up, there's a cavity inside shaped like the part.
Our dies come from our own tooling shop here in Shenzhen. A new transmission housing die costs around $180,000. Takes four months to build. The steel is H13, heat treated to about 44 Rockwell. We get 150,000 shots out of a housing die before the parting line starts washing out and we need to send it for repair.
Machines
We run 80T up to 1600T machines. Cold chamber. The shot sleeve sits outside the furnace because aluminum eats the steel if you leave it submerged. A robot ladles the metal into the sleeve, the plunger shoves it into the die. The whole shot takes maybe 40 milliseconds.
Hot chamber is different. The gooseneck stays in the pot. Works for zinc because zinc doesn't attack steel the way aluminum does. I worked at a zinc shop back in Dongguan in 2004. The machines were smaller, the cycle was faster, but the parts were tiny door handles and zipper pulls. Not like what we do now.
The Shot
Everybody talks about three phases. Slow shot, fast shot, intensification. In reality, you spend most of your time messing with the transition point. Move it too early, you get cold shuts. Too late, the metal's already starting to skin over in the runner.
Our 1600T machine runs at 1,850 psi on intensification for the motor case. The process engineer set that up when we launched the job. I've never touched it. The slow shot is 0.15 m/s, fast shot hits 4.2 m/s. Those numbers are locked in the recipe.
Spraying
Between every shot, a robot sprays the die with lube. Water-based stuff, milky white. The nozzles hit all the deep pockets and cores. The spray cools the die and leaves a film so the casting releases clean.
If the spray heads clog, you'll know on the next shot. The part sticks. Then you're digging it out with a pry bar and grinding the solder off the die face. I've done that more times than I want to count.
Trimming
The part comes out with a biscuit, runners, and overflows attached. We run it through a 200 ton trim press. The trim die cuts everything off in one stroke. The scrap goes in a bin, back to the furnace.
Some plants use saws. We tried that on a prototype job. Slower, but you don't need to build a trim die. For production volume, trim press is the only way.
What Comes After
Most of our parts go to CNC. We leave stock on the mounting faces and bore locations. The machining cell takes it from there.
Leak testing happens after machining. The housing gets plugged and pressurized. If it holds 3 bar for 30 seconds, it passes. Porosity is the killer. Even a pinhole path through the wall fails the test. We impregnate about 8% of our castings to seal the microporosity.
Defects
Cold shuts look like lines on the surface where two metal fronts met and didn't fuse. Usually means the die is too cold or the fill time is too slow.
Porosity shows up on x-ray as little dark spots. Hydrogen in the melt causes one kind. Shrinkage in thick sections causes another. We degass with argon and keep the metal around 1,280°F. Still get some porosity. Always do.
Die soldering is when aluminum welds itself to the steel. Happens in hot spots where the lube burns off. You'll see it on cores that stick into the cavity. The only fix is to pull the die and polish it out.
Why Die Casting
We ship 1,400 motor housings a day off two cells. Cycle time is 72 seconds gate to gate. The parts weigh 14 pounds each. Try making that many parts that size with any other process. Forging can't do the internal geometry. Sand casting is too slow and the surface is rough. Machining from billet would waste 80% of the metal.
Die casting makes sense when you need a lot of complex aluminum parts. The tooling costs real money. The machines cost real money. But once you're running, the piece price is hard to beat.