Die-casting process is a relatively common radiator molding process and occupies a place among various radiator molding processes. Especially some components that dissipate heat through shell, as well as heat dissipation of LED lamps.
Today I will share commonly used materials and production problems of die-casting process.
When mechanical engineers choose die-casting materials, they should reasonably select correct die-casting material based on various factors such as product performance, process performance, production conditions, economy, and characteristics of die-casting materials. Commonly used die-casting materials for mechanical parts include aluminum alloys, zinc alloys and magnesium alloys.
Aluminum alloy is currently the most widely used die-casting material and is widely used in automotive industry, motorcycle industry, aerospace, etc.
Characteristics of aluminum alloy are as follows:
(1) Density of aluminum alloy is small, only about 1/3 of iron, copper, and zinc. Its outstanding advantages are high specific strength and specific stiffness.
(2) Aluminum alloy has good electrical and thermal conductivity.
(3) Aluminum alloy has good resistance to oxidation and corrosion. In air, a dense disulfur trioxide oxide film is easily formed on the surface of aluminum, which can prevent further oxidation.
(4) Aluminum alloy has good die-casting properties. Aluminum alloy die-casting has a simple process, good forming and cutting performance, high mechanical properties and corrosion resistance, and is the most potential alloy to replace steel castings.
(5) High-temperature mechanical properties of aluminum alloys are very good, and they also maintain good mechanical properties when working at low temperatures.
(6) Disadvantage of aluminum alloy is that it is easy to produce large concentrated shrinkage cavities at final solidification point. In addition, aluminum alloy has a strong affinity with iron and is easy to stick to mold, so it should be pressed on a cold chamber die casting machine.
Zinc alloy is also relatively common. Characteristics of zinc alloy are as follows:
(1) Zinc alloy has excellent casting properties, mechanical properties, and toughness, and is widely used in traditional mechanical parts, hardware, locks, toys and other industries.
(2) Zinc alloy has excellent electrical and thermal conductivity properties, good vibration damping properties, and good electromagnetic shielding properties. Its application in electronics, telecommunications, and home appliances continues to grow.
(3) Zinc alloy is a versatile, reliable, low-cost material that is easy to produce by die-casting. Zinc alloy has good die-casting properties, so it is easier to die-cast products with complex shapes, thin walls, and high dimensional accuracy. Due to thin-wall casting properties of zinc alloy, requirements for product lightweight and cost reduction can be achieved.
(4) Compared with aluminum alloys and magnesium alloys, zinc alloys have higher tensile strength, yield strength, impact toughness and hardness, and better elongation.
(5) Surface of zinc alloy die-casting parts is very smooth and can be used directly without surface treatment. It is also easier to carry out various surface treatments, such as polishing, electroplating, spraying, etc., to obtain better surface quality.
(6) Zinc alloy has a low melting point and melts at 385℃. Compared with aluminum alloy and magnesium alloy, zinc alloy is the easiest to die-cast.
(7) Poor corrosion resistance. When impurity elements lead, cadmium, and tin in zinc alloy exceed standard, it will gradually age and deform, manifesting as volume expansion, significant decrease in mechanical properties (especially plasticity), and it will break over time.
(8) Timeliness. If used for a long time, shape and size of zinc alloy die castings will change slightly.
(9) Zinc alloy is not suitable for use in high and low temperature working environments. Zinc alloy has good mechanical properties at room temperature, but its tensile strength at high temperatures and impact properties at low temperatures decrease significantly. Zinc alloys are prone to aging, which is main reason why application scope of zinc alloys is limited. Working temperature range of zinc alloy is narrow. When temperature is lower than -10℃, its impact toughness decreases sharply. When temperature rises, mechanical properties decrease and creep is prone to occur. Therefore, temperature of stressed parts generally does not exceed lOOC. Strictly control purity of zinc alloy raw materials and smelting process. Adding a small amount of Mg and an appropriate amount of Cu to zinc alloy can reduce or eliminate aging phenomenon and improve cutting performance.
Another type is magnesium alloy. Characteristics of magnesium alloy are as follows:
(1) Magnesium alloy is called "green engineering material of the 21st century". Its density is 2/3 of aluminum alloy and 1/4 of steel, but its specific strength and specific stiffness are both better than aluminum alloy and steel, much higher than engineering plastics. It is an excellent and lightweight structural material.
(2) Magnesium alloy has good energy absorption and vibration absorption properties. When used in product casings, it can reduce noise transmission. When used in moving parts, it can absorb vibration and extend service life of parts.
(3) It has good electromagnetic shielding properties and can provide electromagnetic radiation protection for electronic products.
(4) Good rigidity and impact resistance.
(5) It has good ductility and is easy to form, which allows flexibility in product design and improves product quality.
(6) Melting point of magnesium alloy is low, which results in small low-temperature deformation and high dimensional accuracy, which is conducive to one-time mold opening; it has low affinity with iron and small adhesion to mold, which is conducive to improving productivity and mold life; good flow properties of magnesium alloy are beneficial to production of complex and small parts.
(7) Mechanical processing performance is the best, with small cutting force required, good cutting effect and long tool service life.
(8) Good heat dissipation, second only to aluminum alloy.
(9) Dimensional stability is good, changes in ambient temperature and time have little impact on size.
(10) 100% regenerable.
(11) Magnesium alloys are easily oxidized in air. After forming, magnesium alloy die-casting parts must undergo surface treatment to improve corrosion resistance and improve surface quality of parts. Commonly used surface treatment methods include electroplating, spraying, anodizing, etc. Similarly, magnesium alloy has disadvantages of high temperature brittleness and high tendency of hot cracking.

 

Frequently asked questions and solutions:

To prevent hammer head from getting stuck, you can start from two aspects: 1. During production, measure temperature frequently to avoid hammer head and cylinder becoming stuck due to excessive temperatures; 2. When using materials, high-quality alloy materials should be selected to avoid infiltration of impurities. When putting in recycled materials, be careful not to let impurities mix in, so as to avoid impurities sticking to hammer head and causing hammer head to get stuck.

If you encounter problem that hammer head is stuck, first wait until equipment is at normal temperature and try to turn hammer head. If it can't rotate, solve problem by replacing barrel and take out hammer head. If you want to solve jamming problem quickly, the fastest way is to change material pot.

Generally when encountering this kind of situation, it should be that nozzle head or body is blocked. At this time, observe whether there are no bright spots on the top of cutting head. If cross section is all gray, it means nozzle is blocked. Solutions are as follows: 1. Adjust temperature of nozzle to a higher temperature; 2. Reduce nozzle separation time by 0.1 to 0.2 seconds; 3. Turn down cooling water of fixed mold slightly.

It can be analyzed from following points:
1> There may be problems with materials. When using die casting materials, try to control proportion of waste materials not to exceed 30%:
2> Mold is not opened well, firstly, ejection force is unbalanced; secondly, cooling water is unreasonably opened, resulting in unbalanced mold temperature, and thirdly, filling flow is unreasonable.
3> Improper selection of process parameters. Main problem with process parameters is errors in mold retention time and ejection delay time. Mold retention time should not be long, about 3s per mm wall thickness; ejection delay cannot be long, generally 0.5-2s.

To solve this problem, you can look at following points:
1>The most important thing is that no impurities are allowed in composition of material, that is to say, for hard parts, it is best to use new original materials instead of recycled materials, especially quality of S1 elements;
2> Gem blades specially produced for these relatively hard alloys should be used. General service life is about 7 days.

There are several reasons: It could be that silicon oxide or aluminum oxide ingot is solving problem. But the biggest possibility comes from release agent, spraying too much release agent or organic content of release agent is too high. At temperature of hot-melting aluminum, some of these organic compounds are reduced to carbon elements, and some become organic macromolecular polymers. Carbon molecules are mixed with polymers and become contained on the surface as dark spots we see when aluminum casting is formed. Therefore, reduce concentration of spray agent and use other spray agents; or extend blowing time after spraying.

Why do metal splashes sometimes occur? There may be several reasons: mold clamping between movable and fixed molds is not tight, and gap between the two is large; or clamping force is insufficient, causing metal splashes; or it may be caused by non-parallel installation of die-casting machine and fixed mold; or span of support plate is large, and injection force causes sleeve plate to deform, resulting in spray material. If you encounter above reasons, you can solve problem of splashing through following debugging: reinstall mold; increase clamping force: adjust die-casting machine to keep movable and fixed mold mounting plates parallel to each other; add support plates to movable mold and add sleeve plates stiffness.

In order to be suitable for die casting, people add a lot of silicon to aluminum alloy used for die casting. When aluminum alloy solidifies in mold, silicon will float to the surface to form a silicon film that is very hard and wear-resistant. Some OEM designers take advantage of this feature to directly design inner surface of die casting hole as a bearing surface. This silicon surface layer is generally only 0.2 to 0.9mm thick. Too much machining will shorten life of this bearing surface.

1> At present, main problem is that spraying and injection oil are spread unevenly and gather locally, which leads to abnormal composition of surface aluminum alloy or appearance of cold spots, so phenomenon of oxidized patterns appears; texture of oxidized patterns is basically same as that on die casting.
2> Due to improper high-speed switching position, under-casting occurs, and patterns will appear after oxidation.
3> Textbook says that if copper ion content in oxidation tank is too high, patterns will also occur, but this is rare.

What should I do if die-casting mold is stuck?
1> First check whether mold temperature is normal and reduce alloy liquid pouring temperature and mold temperature;
2> Check whether ratio of release agent is abnormal, try to replace release agent, adjust spraying position and polish surface. Polish nitrided mold carefully to prevent damage to nitrided layer on the surface, causing it to become sticky the more you polish it;
3>Improve design structure of gating system to prevent alloy liquid from continuously eroding cavity wall or core;
4>Modify mold cooling system;
5>Adjust die-casting process parameters, appropriately reduce injection speed, and shorten second-speed stroke.