Pressure, speed, temperature - the "impossible triangle" of die casting process
The molten metal is filled into the mold cavity at high speed under high pressure and solidified under pressure.This sentence is a condensation of the entire die-casting working principle. Taken apart, it is the "three highs" - high speed, high temperature and high pressure.
The inner gate speed is generally 30~50 meters/second, which is converted into a speed of 108~180 kilometers/hour - the speed of racing on the highway. It only takes 0.01 to 0.1 seconds for the molten metal to enter the cavity and fill it up. In the blink of an eye, the molten metal has run through the entire mold.
In actual production, the inner gate speed is best not to exceed 60 meters/second - too fast will corrode the mold. But sometimes the product is large and thin, and the speed isn’t enough to fill it up, so the only option is to increase it. There is a priority issue here: make the product first, and then talk about mold life optimization.
The pouring temperature of aluminum alloy is 580~650°C, while the working temperature of the mold is only 120~220°C, and the temperature difference is as high as three to four hundred degrees. There is a "moderate principle" in temperature control: if it is too high, the molten metal will easily absorb air and stick to the mold; if it is too low, it will solidify before it is filled. It is necessary to be able to flow and fill while controlling the risk of air entrainment - this is the temperature window.
The pressure range is generally 30~100 MPa, and conventional products are about 50 MPa. What is the concept of 50 MPa? 1 MPa ≈ 10 kg/cm², 50 MPa means 500 kg per square centimeter. An area the size of a fingernail bears half a ton of weight.
Die casting, the core is "pressing". Where does the pressure come from? First understand the three core components:
Three things are connected to form a pressure transmission chain: hydraulic energy → kinetic energy of the punch → metal hydraulic pressure
There is another hardware detail: there are two energy accumulators on the die-casting machine - the fast injection accumulator is responsible for filling (requiring speed), and the booster energy accumulator is responsible for feeding (requiring force). Two systems, each performing its own duties.
The pressure isn’t constant during the entire injection process. It is divided into two stages:
The timing of switching is very critical: if you cut too early, the cavity isn’t full, if you cut too late, it will already be solid. This is a very important link in process debugging.
The indicator that truly determines the filling effect. Specific pressure = injection force ÷ punch cross-sectional area (MPa)
The large tonnage of your machine doesn’t mean that the force on the molten metal is large. The key is to look at the specific pressure - the force per unit area. For the same injection force, if the diameter of the punch is doubled, the specific pressure drops to a quarter. Therefore, when choosing the diameter of the punch, it isn’t only necessary to consider whether it can match the pressure chamber, but also whether the specific pressure is sufficient.
This is one of the core contents of the entire course. After the molten metal enters the mold cavity, it doesn’t flow randomly. Its flow method directly determines whether the product has pores and cold insulation.
The essential contradiction of the die-casting process: the speed must be fast enough to prevent the molten metal from solidifying before it is filled; but if the speed is too fast, it will change from laminar flow to jet flow, entraining air. Designers and machine tuners are looking for this balance every day.
Injection doesn’t start at high speed. The method is low speed first, then high speed, with a switching point in the middle:
- Low speed section: Allow the molten metal to enter the flow channel smoothly, and push out the air in the pressure chamber and the sprue in an orderly manner.
- High-speed section: When the air is almost exhausted near the gate, switch to high speed. After the gate cross-section sharply shrinks, the speed instantly soars to 30~50 m/s, quickly filling the mold cavity.
Then why not slow down the entire journey? Because the molten metal will cool down. If you use low speed to fill the cavity, it will solidify halfway through and it will not be filled at all. Therefore, cavity filling must be completed within a few tenths of a second.Switching too early → Air isn’t exhausted → Air entrainment | Switching too late → Metal cooling → Cold isolation
The mold designer's "flow control valve". Although the gate is small, it has a huge effect:First, speed up. The cross-sectional area of the gate is much smaller than that of the runner, like a funnel - wide in and narrow out, and the speed increases sharply. The cross-sectional area of punch and gate are generally 15~25 times, punch 2 m/s × 15 times = gate 30 m/s.
Second, control the direction. Where the gate is placed, in what direction, how big and how many gates are opened directly determine where the molten metal enters and where it flows.A good gate design: the molten metal spreads evenly, the cavity is filled almost at the same time, and the gas is pushed to the exhaust position in an orderly manner. A poor gate design: a certain area is filled first to block the exhaust, so the gas can’t escape from behind and the product has pores.
The cavity is filled and solidification is next. This process determines whether the product has shrinkage cavities and whether the structure is dense or not.
As soon as the molten metal hits the inner wall of the mold (the temperature difference is three to four hundred degrees), the surface layer is instantly "sucked" away heat and immediately solidifies into a chill layer with a thickness of only 0.3~0.8 mm. The characteristics of the chilled layer are fine grains, dense structure and high hardness. Then the solidification advances from the outside to the inside, the internal molten metal cools and shrinks, and the volume becomes smaller, resulting in shrinkage cavities.
After the cavity is full, the punch can’t retreat immediately - it must continue to push forward and apply high pressure. Push the still liquid metal at the gate into the shrinking position of the mold cavity to fill the shrinkage holes. It's like a syringe injection - when the medicine is missing, push the plunger and the medicine will go in. The punch is this piston.
Why must the gate solidify last? Because the gate is a feeding channel. The gate solidifies first, the channel is cut off, the cavity is still shrinking, and material is still needed, but the material can no longer enter - the shrinkage cavities remain in the product.
Therefore, when designing the mold: the area away from the gate is cooled first, and the area near the gate is cooled last. The material handle must also be thick enough - it is the last "ammunition magazine" of the punch. The processing allowance can’t be too large!
The surface quench layer is only 0.3~0.5mm dense, and the cooling slows down further, the grains become coarse, and there may be shrinkage and porosity. If you process the chill layer away, all you will see are sand holes - the more you process the sand holes, the more obvious they will be.
Processing allowance experience value: 0.3mm per side for small products, 0.3~0.5mm for large products. Just enough to ensure that it can be processed, but not too much - too much will increase the probability of sand holes.
The molten metal will shrink as it solidifies, and when it shrinks, it will hug the mold core tightly. Without the draft angle, the product can’t be pushed out. In severe cases, the product may be torn and even can’t be pushed out of the mold. Die-casting isn’t like plastic - even if it is 1mm deep, draft must be added. Generally 1~3 degrees, at least 1 degree.
Exhaust is just as critical. There is air + release agent volatile gas in the cavity. If it can’t be discharged, it will repeatedly impact the mold surface like an explosion under high temperature and pressure, causing erosion.
We talked about pressure, speed and temperature respectively before, but they are never separate, they always influence each other.
Different results:
The key is - these three parameters affect the whole body:
- Increase the temperature → the fluidity becomes better → faster filling at the same speed → the speed should be adjusted accordingly
- Increase the speed → the filling time becomes shorter → the temperature drop becomes smaller → the temperature window changes
- Pressure is increased → filling behavior changes → speed and temperature have to be re-matched
The essence of process debugging: In the space composed of the three dimensions of pressure, speed and temperature, find the "process window" - the temperature is high enough to prevent premature condensation, the speed is fast enough not to entrain air, and the pressure is high enough to enable feeding. If all three conditions are met at the same time, that point is your process window.
The molten metal fills the mold cavity at high speed driven by the punch and completes solidification under pressure.
The four key links are connected to form a complete die-casting work cycle:
- ① Generate pressure - hydraulic energy → kinetic energy of the punch → metal hydraulic pressure, three components plus two energy storage devices.
- ② Driven filling - low-speed exhaust first, then high-speed filling, laminar flow vs. jet flow, gate acceleration and direction control.
- ③ Pressure feeding - filling doesn't mean it is over. Pressurization continues to apply pressure to "squeeze" the shrinkage holes until solidification.
- ④ Cooling forming - solidification from outside to inside, the gate is finally solidified to ensure the feeding channel, and the surface dense layer is 0.3~0.5mm.
1) Filling pressure (first-stage injection): low pressure, pushing the alloy liquid forward slowly to exhaust, and then speeding up to fill the cavity;
2) Boosting pressure (two-stage injection/pressure holding): high pressure, huge mechanical force is applied instantly during metal solidification, the feeding melt shrinks, and shrinkage cavities and porosity are eliminated;
3) Clamping force: The clamping force exerted by the mold clamping cylinder on the mold prevents the mold from being expanded and material running out during high-pressure injection.
Temperature: The aluminum liquid is slightly low and the mold temperature is medium;
Speed: Reduce the high-speed injection speed to avoid air entrainment;
Pressure: Significantly increase the boost pressure holding pressure and extend the pressure holding time for feeding.