Low pressure die casting: the “hidden champion” in the field of precision manufacturing
In modern industrial manufacturing, casting technology has become one of the core processes for the production of metal parts due to its high efficiency and precise forming capabilities.
Compared with the well-known high pressure die casting, low pressure die casting plays an irreplaceable role in the field of high-end manufacturing with its unique process advantages.
From automobile wheels to aerospace structural parts, low pressure die casting has achieved a leap-forward improvement in casting quality by relying on the core principle of “low-pressure filling and high-pressure shrinkage”. This article will systematically analyze this “hidden champion” of precision manufacturing from the dimensions of technical principles, equipment composition, application scenarios and future trends.
1.The core definition and principle of low pressure die casting: Casting precision in low pressure
(1) What is low pressure die casting?
Low pressure die casting is a casting process in which molten metal is steadily pressed from the bottom to the top of the riser at the bottom of a sealed crucible into the mold cavity through a low-pressure gas (such as compressed air or nitrogen) of 0.01-0.06 MPa, and solidified under pressure. Its core feature is “low-speed filling and continuous pressure maintenance”, which avoids the turbulence defects of traditional gravity casting. It is also different from the high-speed impact of high-pressure die casting. It is a “golden mean” between gravity casting and high-pressure die casting.
Press into the mold cavity from bottom to top
(2) Working principle: “defect-free casting” is achieved in three steps
Step 1: Steady liquid filling. The molten metal flows along the liquid riser at a low speed of 0.1-0.5 m/s under low pressure drive, filling the cavity evenly like “pouring water into a cup”, avoiding splashing and oxidation slag inclusions (the filling speed of traditional gravity casting can reach 5-10 m/s, which is prone to turbulence).
Step 2: Continuous pressure maintenance After the metal liquid fills the cavity, the pressure is maintained for 2-30 minutes (depending on the thickness of the casting) to form “directional solidification”: the metal liquid at the thick wall continuously shrinks to the thin wall through the runner, eliminating shrinkage holes, and making the density of the casting reach more than 98% (close to the level of forgings).
Step 3: Depressurization and demolding After solidification is completed, the pressure is released, and the unsolidified metal liquid in the riser flows back to the crucible to avoid waste of pouring and riser (waste rate <5%). After the mold is opened, high-precision castings can be obtained.
2.Introduction to key equipment for low pressure die casting
The core equipment of low pressure die casting is the carrier for achieving “precise pressure control”, which mainly includes four modules:
(1) Sealed crucible and insulation system
Function: Store molten metal and maintain temperature (about 680-730℃ for aluminum alloy and about 640-680℃ for magnesium alloy). Common materials are heat-resistant alloys or graphite. Some are equipped with electromagnetic stirring devices to even out the melt.
Technical points: Sealing directly affects pressure stability. Minor air leakage will cause filling failure, which requires real-time monitoring through pressure sensors.
(2) Riser and filling system
Core components: The riser is made of refractory material (such as corundum ceramic), with an inner diameter of 50-150 mm and a length designed according to the mold height, taking into account both the flow resistance of the molten metal and the shrinkage compensation efficiency.
Innovation direction: The new “variable cross-section riser” can optimize the filling speed and reduce eddy currents, which is especially suitable for complex structural parts.
(3) Metal mold and temperature control system
Mold material: The mainstream is H13 hot working die steel, with nickel plating or nitriding treatment on the surface to improve wear resistance, and the mold preheating temperature is 150-250℃ (controlled by electric heating or circulating water).
Design key: The cavity must follow the “sequential solidification principle”, with risers set at thick walls and guide grooves set at thin walls to ensure smooth pressure shrinkage compensation channels.
Metal molds for low pressure casting
(4) Pressure control system
Core components: including air source (air compressor/nitrogen cylinder), proportional valve, PLC controller, to achieve precise control of the entire process of “pressurization speed – pressure holding pressure – pressure relief timing” (pressure fluctuation needs to be <±0.001 MPa).
Intelligent upgrade: Introduce PID algorithm to automatically adjust the pressure curve, combined with infrared temperature measurement real-time feedback, reduce the scrap rate to less than 1%.
Low pressure casting process demonstration
3.Process advantages of low pressure die casting: Why is high-end manufacturing inseparable from low pressure die casting?
(1) Casting quality “rolling grade” advantage
High density: continuous pressure holding and shrinkage compensation eliminate shrinkage holes, tensile strength is 20%-30% higher than gravity casting, fatigue life is increased by 50%, suitable for parts that withstand alternating loads (such as automobile wheels).
Heat treatment compatibility: Due to the low porosity (<0.5%), T6 heat treatment (solid solution + aging) can be performed, and the strength of aluminum alloy castings is increased from 200 MPa to more than 350 MPa.
High dimensional accuracy: The tolerance can reach CT6-7 level (±0.1-0.3 mm), the surface roughness Ra 1.6-3.2, and it can be assembled without machining.
automobile wheels
(2) Wide adaptability of materials and structures
Applicable materials: aluminum alloy (accounting for 80%, such as A356, ADC12), magnesium alloy (AZ91D, used for lightweight parts), and even copper alloy and zinc alloy can be formed (high pressure die casting is difficult to achieve).
Structural capabilities: It can produce parts with a wall thickness of 1.5-50 mm, from thin-walled shells (such as laptop magnesium alloy shells) to large thick-walled parts (such as the lower shell of a car battery pack, with a size of more than 1.5 meters), and complex ribs and boss structures can be formed in one go.
(3) Green manufacturing and cost advantages
Material utilization rate: up to 95% or more (the metal of the riser can be recycled, and there is no waste of pouring and riser), which is more environmentally friendly than high-pressure die casting (70%-85%) and sand casting (60%-70%).
Mold life: under low-pressure environment, the mold load is small, the thermal fatigue damage is low, and the life of complex molds can reach 300,000-500,000 times (the life of complex molds in high-pressure die casting is about 100,000-200,000 times), and the long-term maintenance cost is reduced by 40%.ach 300,000-500,000 times (the life of complex molds in high-pressure die casting is about 100,000-200,000 times), and the long-term maintenance cost is reduced by 40%.
4.Typical application scenarios of low pressure die casting: “Quality responsibility” from the automotive industry to aerospace
(1) Automotive industry: dual choice of lightweight and safety
Aluminum alloy wheels: more than 70% of aluminum alloy wheels in the world are made of low pressure die casting because of their high fatigue strength (can withstand 1 million cycles of load) and meet the safety requirements of high-speed driving.
New energy vehicles: battery pack shell (needs to be impact-resistant and leak-proof), motor end cover (needs high thermal conductivity), integrated body structure (such as some parts of the rear floor of Tesla Model Y are made of low-pressure die-casting + heat treatment strengthening).
New energy vehicle battery pack housing
(2) Aerospace: The first choice for performance in harsh environments
Aircraft structural parts: wing ribs, door frames, landing gear brackets, which need to be heat treated with T6 to achieve high strength and corrosion resistance. Low-pressure die-casting is the only casting process that can meet mass production requirements.
Aerospace engine parts: compressor casings and bearing seats, which work in high temperature and high vibration environments. The dense structure of low-pressure die-casting makes its reliability far superior to other casting methods.
Aircraft door frame
(3) Consumer electronics and high-end equipment
3C products: high-end laptop magnesium alloy shell (wall thickness 1.2 mm, taking into account both lightness and strength), drone racks (needs to be drop-resistant and impact-resistant).
Precision machinery: robot joints (accuracy ±0.05 mm), hydraulic valve bodies (complex internal flow channels, no leakage required), optical lens brackets (high dimensional stability requirements).
5.Current technical bottlenecks and breakthroughs of low pressure die casting
(1) Current challenges
Production efficiency: Single mold cycle 2-10 minutes (high-pressure die casting only 10-30 seconds), not suitable for large-scale production of simple parts.
Process sensitivity: Pressure, temperature, and filling speed must be precisely matched. Minor parameter deviations may lead to insufficient pouring or shrinkage (such as a 30% increase in defect rate when the pressure fluctuation is >0.005 MPa).
Dependence on large equipment: The production of ultra-large castings (such as integrated automobile chassis) requires 10,000-ton clamping force equipment, with an initial investment of more than 100 million yuan, which is difficult for small and medium-sized enterprises to afford.
(2) Technological innovation direction
Vacuum low pressure die casting: Vacuum the cavity (pressure <10 kPa) to further reduce pores and enable magnesium alloy castings to be anodized (such as Apple laptop shells).
Semi-solid die casting: Cool the molten metal to a semi-solid state (solid phase ratio 30%-70%) to improve filling stability. It is suitable for high-melting point alloys (such as copper alloys and titanium alloys).
6.How to choose low pressure die casting? Three core decision points
Performance first: If the casting needs heat treatment strengthening, high fatigue strength or harsh environment service (such as aviation, new energy vehicles), low pressure die casting is the only choice.
Batch and cost: For small and medium batches (<30,000 pieces) or large and complex parts (such as single piece weight >50 kg), low pressure die casting has significant advantages in mold cost and material utilization.
Structural complexity: For parts with deep cavities, multiple ribs, and uneven thickness, the stable filling and shrinkage capacity of low pressure die casting far exceeds gravity casting, and the cost is lower than precision forging.
7.Conclusion: Low pressure die casting opens a new dimension of high-end manufacturing
From “usable” to “easy to use”, low pressure die casting has achieved a leap from casting to “precision casting” by virtue of its exquisite control of “pressure”. Under the trend of “lightweight” of new energy vehicles, “high performance” of aerospace, and “precision” of consumer electronics, this precise and strong process is ushering in a golden period of development.
