The pressure casting process incorporates three main elements: machines, molds and alloys. It unifies pressure, speed and time into a single process and is mainly used for hot machining. The presence of pressure differentiates die casting from other casting methods.
Die casting is a rapidly developing technique in modern metalworking processes and is a special casting method that requires minimal cutting.
It involves filling a mold with molten metal under high pressure and speed, and then the metal crystallizes and solidifies under this high pressure to form the casting. High pressure and high speed are the main characteristics of pressure casting.
The commonly used pressure is tens of megapascals, the filling speed (inner channel speed) is approximately 16 to 80 meters per second, and the time for the molten metal to fill the mold cavity is extremely short, approximately 0.01 to 0.2 seconds.
Definition
The method of manufacturing products in this way has become an essential part of our country's casting industry due to its high production efficiency, simplified processes, superior casting tolerance levels, good surface roughness, high mechanical strength and the ability to eliminate a large number of machining procedures and equipment, thus saving raw materials.
Die casting is a process that organically combines and comprehensively applies the three main elements of a die casting machine, die casting mold and alloy. During pressure casting, the process of filling the mold cavity with metal is a unified process that involves factors such as pressure, speed, temperature and time.
Simultaneously, these factors interact and restrict each other, complementing and supporting each other. Only through the correct selection and adjustment of these factors to achieve harmony and consistency can the desired results be obtained.
Therefore, not only the processability of the casting structure, the advancement of the die casting mold, the performance and structural excellence of the die casting machine, and the adaptability of the chosen die casting alloy and the standardization of the die casting process should be emphasized during the die casting process, but the significant role of process parameters such as pressure, temperature and time on casting quality should also be taken into consideration. Effective control of these parameters must be prioritized during the pressure casting process.
Pressure
Injection force
Injection force is the force that drives the movement of the injection piston in the injection mechanism of the die casting machine. It is a main parameter that reflects the functions of the die casting machine. The magnitude of the injection force is determined by the cross-sectional area of the injection cylinder and the pressure of the working fluid.
The injection force calculation formula is as follows:
P injection force =P injection cylinder × π × D²/4
Where:
- P injection force – Injection force (N)
- P injection cylinder – Working fluid pressure in the injection cylinder (Pa)
- D – Injection cylinder diameter (m)
- π = 3.1416
Specific Pressure
The pressure exerted on the molten metal in the pressure chamber per unit area is called specific pressure. Specific pressure is also the result of converting the relationship between the injection force and the cross-sectional area of the pressure chamber.
Its calculation formula is as follows:
P specific pressure =P injection force /F cross-sectional area of the pressure chamber
Where:
- P specific pressure – Specific pressure (Pa)
- P injection force – Injection force (N)
- F cross-sectional area of the pressure chamber – Cross-sectional area of the pressure chamber (m²)
In other words, F cross-sectional area of the pressure chamber =πD²/4. Here D(m) is the diameter of the pressure chamber.
- π = 3.1416
Effect of pressure
(1) Impact of specific pressure on the mechanical properties of castings
As the specific pressure increases, the crystalline structure becomes finer, the thin crystalline layer becomes thicker, the surface quality improves due to better filling characteristics, the impact of air holes is reduced, and thus , tensile strength increases but elongation decreases.
(2) Impact on filling conditions
When the alloy melt fills the mold cavity under high specific pressure, the alloy temperature increases, the fluidity improves, which is beneficial to the improvement of casting quality.
Specific pressure selection
(1) Consideration based on strength requirements of castings
Divide castings into those with strength requirements and those with general requirements. For those with strength requirements, they must have good densification. This requires a high specific boost pressure.
(2) Consideration based on wall thickness of castings
In general, in the pressure casting of thin-walled castings, the resistance to flow in the mold cavity is greater, and the passage system is also thinner in thickness, so it has greater resistance.
Therefore, a higher specific filling pressure is required to ensure the required firing speed. For thick-walled castings, on the one hand, the selected injection speed is lower and the metal solidification time is longer, therefore, a lower specific filling pressure can be used; on the other hand, in order for the casting to have a certain densification, a sufficient specific reinforcing pressure is necessary.
For complex shaped castings, a higher specific filling pressure should be used. In addition, factors such as alloy type, channel speed size, power of die casting machine clamping capacity and mold strength should be considered appropriately. The size of the specific filling pressure is calculated mainly based on the selected firing speed.
As for the size of the specific boost pressure, it can be selected based on the alloy type by referring to the values in the table below. When ventilation conditions in the mold cavity are good and the ratio of the thickness of the passage system to the wall thickness of the casting is appropriate, a lower specific boost pressure can be used.
However, the worse the ventilation conditions and the lower the ratio between the thickness of the gate system and the wall thickness of the casting, the higher the specific reinforcement pressure must be.
Recommended specific boost pressure range table
| Part type | aluminum alloy | Zinc Alloy | Brass |
| Parts under light load | 30-40 MPa | 13-20MPa | 30-40MPa |
| Parts under heavy load | 40-80MPa | 20-30MPa | 40-60MPa |
| Parts with Large Sealing Surface and Thin Walls | 80-120MPa | 25-40MPa | 80-100MPa |
Relevant Forces
Definition
During the pressure casting process, at the end of the filling phase and in the transition to the pressure increase phase, the relative pressure (boosting pressure) acting on the solidifying metal, transmitted through the metal (pressure casting system), casting, overflow system) to the mold cavity wall surface, is known as mold expansion force (also known as backpressure).
Mold clamping force (also known as closing force) is an important parameter to be determined first when choosing a die casting machine.
Calculation method
When the mold expansion force acts on the parting surface, it is called parting surface expansion force. When it acts on the various side walls of the mold cavity, it is known as side wall expansion force.
The mold expansion force can be expressed as follows:
P expansion force =P Increase pressure × The projected area
Where:
- P expansion force – the expansion force of the mold (unit: N – Newton)
- P Increase the pressure – the boost pressure (unit: Pa – Pascal)
- The projected area – the projected area that supports the expansion force of the mold (unit: m2 – square meter)
Under normal circumstances, the mold clamping force should be greater than the calculated mold expansion force.
Otherwise, during the hydraulic ejection of molten metal, the mold separation surface will expand, causing metal spatter and preventing the establishment of pressure in the mold cavity. This leads to difficulty in guaranteeing the dimensional tolerance of the casting, or even difficulty in forming.
The mold clamping force generally must meet the requirements of the following formula:
P clamping force ≥ K × P expansion force
Where:
- P clamping force – The clamping force of the die casting machine (N – Newton)
- K Factor of safety (usually takes K = 1.3)
- P expansion force – Mold expansion force (N – Newton)
Injection speed
1. Normally, there are two types: punching speed and inner channel speed.
2. For slow injection, the punch pushes the molten metal into the inner channel at 0.3 meters/second.
3. For fast injection, the internal channel fills the mold cavity at 4-9 meters/second. Increasing the injection speed can convert the function into thermal energy, improving fluidity, which is beneficial to eliminate flow marks and cold laps, and improve mechanical properties and surface quality.
Selection factors and consideration for injection speed:
1. Thermal conductivity and specific heat, solidification temperature range.
2. If the mold temperature is low, the speed may be slow; otherwise, the speed may be high.
3. Complex castings use high injection speed.
The speed of the internal sprue is 15 to 70 meters/second (for molten metal).
4. The relationship between the punch injection speed and the internal sprue speed: the higher the punch injection speed, the higher the molten metal sprue speed.
Speed selection
1. The speed of straight sprue is 15 to 25 meters/second.
2. The speed of the cross-inlet channel is 20 to 35 meters/second.
3. The speed of the inner sprue is 30 to 70 meters/second for a wide gate.
4. For thin castings less than 3 millimeters, the internal sprue speed is 38-46 meters/second.
5. For castings with a thickness of 5 millimeters, the inner runner speed is 46-40 meters/second.
6. For thicker castings greater than 5 millimeters, select an internal feed speed of 47 to 27 millimeters/second.
Adjustment methods: adjust the punch injection speed, change the chamber diameter, change the cross-sectional area of the inner channel.
Testing and Analysis
1. Pressure casting parameter tester, first stage, second stage and transition point time increase.
2. Impact of impulse starting point on casting quality: After the first stage begins to fill 80%, switch to the second stage and increase the initial transition point time, and finally maintain pressure, otherwise it will affect the quality.
3. The impact of punch wear on die casting parameters;
4. Analysis of the causes of wear in the injection chamber and punch: The gap between the injection chamber and the punch is less than 0.1 millimeters, the back and forth friction between the punch and the chamber generates high temperatures, leading to easy damage.
The chamber diameter increases, the punch becomes smaller, the punch becomes stuck with aluminum chips, affecting the chamber transmission speed and pressure.
Therefore, the punch must use high temperature resistant lubricating oil, the injection rod must have cooling water, and at the same time, the punch material must be chosen, generally choosing ductile iron or beryllium bronze.
