In die-casting production, the improvement and optimization of the die casting process play a great role in promoting the quality of die-casting parts. This article is about how the vacuum die casting method can reduce the pores inside die casting parts and enhance the mechanical properties of die-casting parts. Vacuum die casting is the necessary production condition for current high-quality aluminum die-casting parts.
Table of Contents
Die casting technology status analysis
Die-casting technology constantly pursues technical progress and constantly pursues high-quality production. But because of its process characteristics, porosity exists, and at the same time, there are some shortcomings. The direction of today’s technology continues to explore how to avoid these shortcomings. Developing and applying new die-casting technology will bring opportunities to the die-casting industry. In recent years, with the rapid development of new energy vehicles, the automotive industry in various countries began to lightweight design. The good quality of the product is the guarantee of safety. Continuously improving the quality to reduce product porosity is a relentless road exploration.
Die casting process advantages and disadvantages
Die casting process and its advantages:
The essence of die-casting is under the action of high pressure, so that the liquid or semi-solid metal as well as high speed filling into the die-casting mold, and under the action of pressure molding and cooling solidification and get a casting of a forming process. It can see that high pressure and high-speed filling die casting die are the two main characteristics of the die casting process. Therefore, the main characteristics of die casting are.
(1) Molding under high pressure, high product denseness, high mechanical strength, and surface hardness, but the product elongation is low.
(2) Fast product filling, short cooling time, high production efficiency, and low cost when mass production.
(3) The product surface is rough, even up to Ra1.6-6.3.
(4) It is possible to produce parts with thin wall thickness.
(5) Fast filling, more internal gas entrapment, and product porosity.
(6) Do not heat treat. Heat treatment will expand the internal gas, resulting in product defects such as bulging or cracking.
The existence of gas hole problems also constantly promotes improving the die-casting process to meet the increasing requirements for die-casting quality. Still, vacuum die-casting technology is one of the methods to expand the scope of the die-casting process.
Vacuum die casting method
Vacuum die-casting technology is based on the traditional pressure casting technology supplemented by the cavity vacuum technology. That is, the first use of a vacuum extraction device will die-casting die chamber and cavity gas extraction so that the molten metal in relative vacuum conditions fills the cavity. In the die-casting process, vacuum die-casting technology is used to extract the gas in the death chamber and cavity, which can eliminate or significantly reduce the dissolved gas and porosity in the die casting and improve the density of the internal organization of the casting to enhance the surface quality and comprehensive mechanical properties of the die casting.
To obtain the ideal die casting alloy components, it is necessary to minimize the contact between the hot metal injected and the air and fumes in the cavity; in other words, it is to reduce the gas pressure of the air and fumes in the cavity as much as possible during the whole process from the beginning to the end of the die casting and to reduce the gas content of the whole process to a near-ideal condition. A vacuum is to be used to solve this exhaust problem.
Characteristics of vacuum die casting
The vacuum die-casting method will extract one of the many process methods at present, mainly the gas in the mold cavity so that the metal liquid in the vacuum state fills the cavity, so the die-casting process involved in the gas less, the mechanical properties of the casting is significantly improved, and the high vacuum method is in the ordinary vacuum die-casting based on the development of new technology, its characteristic is the mold cavity vacuum is less than 90 bar. The die-casting process removes the gas in the die cavity. It eliminates or significantly reduces the die-casting pores and dissolved gas to improve the mechanical properties of die castings and the surface quality of the advanced die-casting process. The application of the vacuum die-casting method to produce aluminum and magnesium alloy die castings have been proven to use welding, heat treatment, and other processing means. Room temperature performance also has a certain degree of improvement.
The vacuum die-casting method has the following advantages:
(1) Vacuum die casting method can eliminate or reduce the pores inside the die casting, improve the surface quality of the die casting and facilitate the production process casting size stability.
(2) Vacuum die-casting method greatly reduces the cavity back pressure. The specific pressure can be reduced by 40%. It is possible to die-cast larger castings with small machines.
(3) Improved filling conditions, ability to produce thin wall thickness and the larger structure of the casting.
(4) Increase the heat treatability and weldability of die castings.
The disadvantages of the vacuum die casting method are:
(1) complex mold sealing structure, manufacturing, and installation are more difficult, and therefore the cost is higher.
(2) Vacuum die-casting method, the effect is not very significant if not properly controlled.
(3) The vacuum valve needs maintenance and spare parts.
Compare the above advantages and disadvantages. With proper control, the vacuum die-casting method can improve the die-casting performance. And mold sealing and pressure chamber sealing caused by the cost of upgrading, with the popularity of technology and mass production brought about by the cost-sharing, will be well resolved.
Vacuum die casting technology vs die casting
Conventional Die Casting
Die casting is the abbreviation of pressure casting, which is the molten state or semi-molten state alloy poured into the pressure chamber of the die casting machine, with very high-speed filling and high pressure to complete the casting molding cooling high-efficiency casting method. High pressure and high speed is die-casting filling forming process of two major features also die-casting and other casting methods of the most fundamental difference.
Die-casting and other casting methods have the following advantages compared to each other.
(1) the accuracy and surface quality of the casting are high, and it can die-cast complex shapes of thin-walled parts or inlay parts.
(2) High production efficiency, easy-to-achieve mechanization, and automation.
(3) Castings are generally processed in small quantities or used directly without further mechanical processing.
(4) Threads, lines, words, patterns, and symbols can be cast.
In the die-casting production process, the metal liquid to high-speed jet state filling the cavity, the majority of the gas in the cavity can not be discharged, and high-speed aluminum liquid involved and wrapped; as the casting cools and solidifies, the gas begins to precipitate enrichment, and finally in the form of pores remain in the casting, the main form of expression is the processing of subcutaneous pores, such as Figure 1.
Their mechanical properties often limit the application range of aluminum alloy dies castings. To improve the mechanical properties of aluminum alloy die castings, heat treatment and other methods can be taken. Still, the high gas content of the casting limits the heat treatability of aluminum alloy die castings. The same casting that contains too much gas also affects the weldability of the casting; according to Figure 2, the ordinary die-casting parts contain gas. This deficiency in a large range of die castings in important or large complex force parts on the application.
Three types of reasons mainly cause castings in the air hole defects: First, the pressure injection process is too late to discharge the cavity and the air inside the pressure chamber by the high-speed aluminum liquid encapsulation (generally filling time in Los-100ms); Second, the mold coating, punch lubricating oil in heat evaporation reaction generated by gas; Third, the dissolved aluminum alloy contains a large amount of hydrogen, precipitation in the solidification process.
The following measures can be taken to avoid the three reasons for producing gas holes.
For paint evaporation gas, which can be significantly improved by choosing environmentally friendly, less-gas-generating paint and reducing the amount of paint used.
For dissolved hydrogen gas from aluminum alloys, the gas content can be greatly reduced by controlling retort materials and aluminum refining to remove gas, reducing the risk of porosity in castings.
Only the aluminum liquid roll gas, the traditional exhaust design is difficult to play a role, the high-speed filling process only by the aluminum liquid on the gas extrusion effect is difficult to exclude, at the same time the air pressure is too large will also hinder the aluminum liquid filling caused by gas resistance. Because the traditional die-casting process is difficult to solve the roll gas problem, the cavity and the air inside the pressure chamber become the most important source of air holes in die castings.
The passive exhaust has not worked to solve the pressure chamber and mold cavity gas problems. The die-casting manufacturers began to consider the use of active exhaust way to improve the quality of castings and reduce the rate of casting porosity. From the last century’s sixties, foreign manufacturers began to use the hood vacuum device to achieve vacuum die-casting; through decades of development in recent years, more applications of two types of vacuum die-casting methods have been. One type is the excitation and cooling exhaust tank method; the other is the vacuum valve method. This form of structure is simple and low-cost and is widely used. However, to increase the chance of metal condensation, the top gap is usually designed to be 0.2 mm, so the venting capacity is much less than expected, the vacuum level fluctuates greatly, and the stability is poor.
The vacuum valve method uses a proprietary mechanical vacuum valve mounted on the parting surface of the mold to close the spool by the impact of the alloy flow, as shown in Figure 5. It has the advantages of high vacuum, stability, and flexibility in setting the exhaust path. The disadvantages are that the vacuum valve must be maintained regularly, the vacuum spool is more worn and consumed, the cost is higher, the use of the system requires high-quality personnel, and the purchase of equipment and maintenance costs are higher. v1diecast has professional technicians and high-tech equipment to meet your needs.
Vacuum die casting has the following advantages over conventional die casting
(1) It can further reduce the bubbles inside the die-cast product, increase the metal solution’s density and improve the product’s strength, toughness, and other properties.
(2) A further increase in the metal solution’s flow rate facilitates the filling of difficult parts to cast.
(3) A greater casting pressure reduction, enabling large castings on small machines and extending the life of the molds and the press chamber ejector bars.
(4) Very good improvement of the solder properties of the die casting, which also allows processes such as electroplating, oxidation coloring, heat treatment, etc., to be facilitated.
Necessary conditions for vacuum die casting
Vacuum die-casting is dominated by the form of evacuating the internal gas of the cavity. Only in this form, to pump the air in the mold cavity clean to get a better vacuum effect, then there is a vacuum valve installed in the mold and exhaust channel connection (Figure 3), there is also a vacuum valve installed in the pressure chamber sprue material mouth, through the pressure chamber to pump out the cavity gas, which will install in the vacuum valve on the mold method is relatively simple and practical, its biggest advantage is that the mold design and the same as conventional die casting, only in Mold sealing needs to be considered, to protect the seal ring, mold design step parting surface, etc. (Figure 4).
Sealing of the punch and press chamber
The design of the material tube is the central part of the entire press injection kit. Using a sealed ring punch is best to use an integrated stock tube. The advantages are:
- Long life.
- High process stability.
- Easy barrel/machine/die alignment.
- Built-in pipe temperature control.
A successful barrel design is very important for seal ring punches.
When producing thin-wall die castings, the vacuum in the cavity needs to be controlled to <90 mbar and requires the following conditions to be met.
a. High sealing of the press chamber of the punch to reduce air leakage (Figure 5).
b. Can not spray too much mold release agent to avoid residual moisture.
c. The atomization of the sprayer should be good, preferably with a micro nozzle.
d. The amount of mold release agent sprayed should be small to avoid the accumulation of residue in the exhaust channel, which will affect the action of the vacuum valve or reduce the area of the exhaust channel.
e. Use less wax and high-purity refined mold release agents. Avoid too much residue on the mold surface, which will lead to burrs, damage the seal, and affect production.
f. Produce thin-walled parts with high-pressure injection speed, which requires good punch lubrication.
g. Avoid putting punch oil directly into the press chamber, which will affect the air content of the casting. It should use a linkage pulse type spraying method.
h. The punch must be cooled for a long time, keeping the water pressure and flow rate (Figure 6).
l. The tolerance and temperature control of the material tube is critical, requiring careful calculation and good temperature control.
Large-capacity vacuum tank and vacuum pump
To ensure high vacuum level, large capacity vacuum tank and vacuum pump must be equipped, such as (Figure 7) and (Figure 8) for 2000 tons die-casting machine with vacuum machine.
Selection of vacuum valve
A vacuum valve is a core of vacuum die-casting. Currently, a vacuum valve has a mechanical structure, a hydraulic solenoid control valve, and a cooling exhaust piece structure.
a. The mechanical valve works by using the kinetic energy of the alloy fluid to close the mechanical valve at the final filling point (Figure 9).
b. Hydraulic type solenoid control valve works on the principle of valve inlay into the die casting mold. Its advantages are large exhaust capacity and strong anti-pollution ability; the disadvantage is the need for electromagnetic control and hydraulic source, closing response time by the solenoid valve. And high requirements for the die-casting unit control system, the valve working process is controlled by the die-casting equipment. (Figure 10)
c. Cooling exhaust block, according to the structure, is also divided into a flat exhaust sheet and a three-dimensional exhaust sheet, of which a three-dimensional exhaust sheet has a three-dimensional rubbing board, and a flat exhaust sheet has a larger cross-sectional area of the pumping channel to improve the pumping efficiency. The cooling method is divided into water-cooled and self-cooling, where the water-cooled type usually uses hot work mold steel plus a cooling water channel; the principle of self-cooling is to rely on their heat conduction to cool the alloy liquid in the exhaust block, beryllium copper alloy is the most suitable production material.
The choice of which vacuum valve depends on the equipment interface and the requirement of casting quality.
With the development of die-casting industry, vacuum equipment is used by more and more die-casting manufacturers. Using vacuum equipment to produce castings mainly has those advantages:
- Reduce product porosity.
- Improve the product qualification rate.
- Reduce the front-end pressure of the metal liquid, which is conducive to filling and forming.
The main advantage of a vacuum is to reduce the product air content and thus improve the casting qualification rate, as well as the production of thin-walled parts that are difficult to fill and other products which have a high value in actual production. Of course, the vacuum system is affected by various aspects, first of all, the mold should be sealed, and the mold design should be reasonable. At the same time, the system should be maintained regularly during the die-casting process, considering all aspects’ influence. So it is not with the completely omnipotent vacuum system.
The quality of die-casting products depends on all aspects to support. The vacuum system, as more advanced technology, is now widely utilized. The vacuum die-casting process requires good maintenance and care, and the use of valves also requires replacing wearing parts, which increases the cost of die-casting products. Therefore, for die castings with no special requirements for mechanical properties, mature product process, and easy filling, vacuum production is generally not required; for products with special requirements for performance and porosity, such as high-quality thin-walled parts and structural parts, vacuum equipment is a necessity.
V1 Diecast has advanced vacuum die casting equipment to ensure high-quality thin wall die casting parts.