High-Pressure Die Casting (script)
Sadly, porosity in pressure die castings is just a fact of life, so, NO, you can’t not have any porosity in your pressure die castings. What we are going to discuss today, however, is how to minimise porosity, and we’re going to explain the difference between short-shots, entrained air, and vacuum bubbles.
So, take a deep breath of air, hold it in, and enjoy this high-pressure episode of Serious Engineering for Serious Engineers.
The first type of porosity is called a short shot. Although it is technically not porosity, it is a real pain to deal with, and it is made up of air, but arguably it’s the easiest to deal with. Short shots usually occur when there’s not enough pressure or speed to force the molten, or ‘slush’, aluminium out into the farthest reaches of the cavity. The solution is to play around with the injection parameters, and maybe even add some additional overflows and venting. So, that’s the easy one out of the way.
The second type is entrainment of air. Entrainment is the enveloping of air that is already in the die, or even in the shot chamber, during injection. It’s a bit like when you’re trying to drink a glass water quickly and you gulp down a load air at the same time – very annoying. The way to avoid this is to ensure that there are no obstructions in the die, as I mentioned in our previous video. Another way is to ensure that you position the gate such that the material stands the best chance of flowing through the whole part without jetting over holes in the die or jetting perpendicular to heat exchanger cooling fins, for example. If you work closely with your PDC supplier, they can run a simulation tool such as Magmasoft, which is the PDC equivalent of Moldflow, which is for plastic injection molding. They can then tell you where you’re going to have problems long before the design is fixed.
How else can you get rid of entrained air? Well, in the past people experimented by filling the die with oxygen and therefore all the entrained gasses would be oxygen, and that would react violently with the molten aluminium and form all sorts of oxides and other compounds, and thereby turn the gasses into solids, leaving a vacuum which then collapses pulling molten aluminium in on itself. It did work, but as you can imagine it is extremely dangerous, and to quote a famous Online CNC Star from America – BOOM – and that’s why very few companies still offer that. Presumably because they burned to the ground.
One technique that is still used, is to pull a vacuum in the die, which also helps to suck the air out as the aluminium advances. Again, this is highly impractical and frankly it’s better to just redesign the part and the die to avoid entrainment in the first place, or push the problem to a place on the casting where it doesn’t matter. Again, consult your PDC supplier during the design phase.
Now let’s talk about number 3, vacuum bubbles. Vacuum bubbles occur once the die is filled, and during the Holding Stage of injection, the surfaces of the part have already frozen, but between those surfaces you still have liquid aluminium, and later, slush phase aluminium that is still crystallizing and shrinking. And don’t forget that this all happens in a matter of milliseconds. Once the hold pressure is not enough to backfill the slush, the pressure drops as the material on the inside tries to shrink, and there comes a point where it cannot pull the walls together, or cannot pull material in from the surrounding casting, and eventually that pressure drop is so great that vacuum bubbles form out of nothing. Those vacuum bubbles do however fill up with gasses, such as Nitrogen, from the surrounding metal, so technically, they are not wholly vacuous.
[Vacuous looking expression]
If we do nothing to mitigate these vacuum bubbles, you’re going to get around 8% porosity by volume. The way that you reduce vacuum porosity is to add silicon to the molten material, the more you add, the less vacuum porosity you get, but there is a practical limit. Now the problem with silicon is that it makes the casting more brittle the more you add. So, yes, you could get the porosity down very very low, but the casting would easily break. The sweet spot is adding somewhere between 9% and 11% of silicon and that will reduce vacuum related porosity down to around 3.5%. So please don’t ask for zero percent porosity unless you want a box filled with aluminium chips.
Here now are the two most popular and highly recommend aluminium alloys that will help you to avoid vacuum bubbles. And yes, I am pretty much repeating what I said in the last video on this subject:
ADC-10, and its equivalent A380, have between 7.5% to 9.5% silicon, and this alloy is one of the most popular and widely used throughout industry. Its close cousin is ADC-12, and its equivalent A383, which has between 9.5% to 11.5% silicon, so the silicon is right in the sweet spot and will give you around 3.5% vacuum bubbles. ADC-12 offers slightly better filling characteristics and resistance to hot cracking compared to ADC-10. So, ADC-12 is pretty much the go to for almost all applications worldwide.
So, there we have it. The big takeaways from this video are:
Number 1: Short-shots are definitely not acceptable and relatively easy to fix.
Number 2: Entrained air can be managed, and if not completely avoided, you can aim to get the entrained air into areas of the casting where the structural integrity is not a problem
And Number 3: You cannot vacuum bubbles if you want strong castings, but if you use ADC-12 or A383, the very best you can ask for is about 3.5% porosity by volume.
So, that’s all we have time for today, don’t forget to subscribe, share the Engineering Luv, and punch like, or Barry will kick your ass. And don’t forget, we’re the people that do Serious Engineering for Serious Engineers.